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Iron-Catalyzed Borylation of Alkyl, Allyl, and Aryl Halides: Isolation of an Iron(I) Boryl Complex
Abstract
Activation of B 2 pin 2 with t BuLi facilitates the Fe-catalyzed borylation of alkyl, allyl, benzyl, and aryl halides via the formation of Li[B 2 pin 2 (t Bu)] (1). The reaction of 1 with a representative iron phosphine precatalyst generates the unique iron(I) boryl complex [Fe(Bpin)(dpbz) 2 ] (2). A lkylboronic acid derivatives are of great synthetic interest and can be prepared by a variety of methods. 1 A recent and highly attractive methodology, developed originally by Marder and co-workers, 2 relies on the borylation of alkyl halides with diboron esters, in particular bis(pinacolato)-diboron (B 2 pin 2) (Scheme 1). Originally, this method required Cu catalysts, 2,3 but the scope has been expanded to include Pd-, 4 Ni-, 5 and, most recently, Zn-based catalysts. 6 The equivalent iron-catalyzed process is highly desirable due to iron's low cost and toxicity. 7 We report that both phosphine-containing and coligand-free iron-based catalysts can be employed in the reaction to excellent effect. Furthermore, we show that a unique iron(I) boryl complex can be isolated under catalytically relevant conditions. From optimization studies (Table 1) it was clear that the barrier to success lay with the transfer of the boryl group from B 2 pin 2 to the iron center. Initial attempts at coupling
... Therefore, organometallic reagents are usually employed as additives to afford the low-valent iron catalyst via reductive elimination. [57][58][59][60] During our investigation on ironcatalyzed borylation and silylation of phenol derivatives, interestingly, we found that iron catalysts might possess some unique reactivity in the functionalization of inert bonds. [40][41][42][43] Inspired by these findings, we envisioned that when special oxygen-based electrophiles, such as aryl carbamates, are used as coupling partners, the ironcatalyzed cross-electrophile coupling reactions might proceed under a suitable catalytic system. ...
... A cationic (dppbz) 2 -Fe(II)-Bpin complex was observed by high-resolution mass spectrometry (see Supporting Information Figures S7 and S8). In Bedford's work, 60 the diboron reagent could reduce the Fe(II) catalyst to an Fe(I)-boryl species. Therefore, a known Fe(I)-boron complex was synthesized to promote this transformation (see Supporting Information Figure S9). ...
... To our delight, the coupling product was obtained in 5% yield in the presence of Fe(I)-boron complex 1 (Scheme 3d1 and Supporting Information Table S21). The very low efficiency of this reaction may be caused by the high stability of dppbz-ligated iron(I)-Bpin complex 1. 60 Moreover, an important dppe-ligated Fe(I)-Cl complex was synthesized to perform this reaction, and 22% yield of the coupling product 1 was provided (Scheme 3d2). Additionally, Fe(0) complexes such as Figures S10 and S11). ...
... Although a recent report of iron catalysis yielding alkylboronic esters through hydroborylative cyclization 16 has been an interesting pathway, the reports of Fe catalysts for alkyl halide activation have been scarce. 17 In 2014, Cook 18 and Bedford 19 independently reported a direct cross-coupling of alkyl halides with bis(pinacolato)diboron, catalyzed by iron salts. These methods need a reactive organometallic reagent, EtMgBr and t BuLi, respectively, to facilitate this transformation (Scheme 1). ...
... The tertiary bromide substrate required higher catalyst loading and reaction temperature (see SI). Reaction with 1-bromo adamantane (1l), gave the desired product (3l) in moderate yield (54%). Since the previous reports 18,19 showed limitation toward alkyl chlorides, we were interested in investigating the scope of this reaction to the inexpensive and easily accessible alkyl chlorides, using 3-phenylpropyl chloride (1m) as model reacting partner. A better result was obtained with 12 mol % of A, 1.8 equiv of 2a and NaOEt, using MTBE as the solvent at 80°C for 22 h (3a, 82%; see SI). ...
The mild borylation of alkyl bromides and chlorides with bis(neopentylglycolato)diborane (B2neop2) mediated by iron-bis amide is described. The reaction proceeds with a broad substrate scope and good functional group compatibility. Moreover, sufficient catalytic activity was obtained for primary and secondary alkyl halides. Mechanistic studies indicate that the reaction proceeds through a radical pathway.
... [20][21][22][23][24][25][26] Palladium, [27][28][29][30][31][32][33][34][35][36][37][38][39] rhodium, [40][41][42] and iridium [43][44][45][46][47][48][49] are typical catalysts for these reactions. In recent years, several inexpensive and more environmentally friendly transition metals, such as iron, [50][51][52] cobalt, [53][54][55] nickel, [56][57][58] and copper, [59][60][61][62][63][64] have been found to be attractive alternatives for precious metal catalysts. ...
The application of a series of hydroxyl‐containing Schiff‐base ligands in the synthesis of zinc complexes is reported. The substituents on the benzene rings, the positions of the C=N double bonds, and the ratio of the ligands to ZnEt2 affect the nuclearities of the zinc compounds. Complexes of various nuclearities were prepared, ranging from trinuclear cluster and dinuclear complexes to mononuclear compounds. Treatment of HL¹ (HL¹=2‐(((1‐(2‐(dimethylamino)ethyl)‐1H‐pyrrol‐2‐yl)methylene) amino)phenol) with one equivalent of ZnEt2 generated a trinuclear compound [(L¹)ZnEt]3 ⋅ 0.8Tol (1 ⋅ 0.8Tol). Three dinuclear compounds [(L²)ZnEt]2 ⋅ Tol (2 ⋅ Tol), [(L³)ZnEt]2 (3), and [(L⁴)ZnEt]2 (4), and two mononuclear complexes [(L⁵)ZnEt] (5) and [(L⁶)ZnEt] (6) were formed by the reactions of HL²−HL⁶, respectively, with ZnEt2. The HL²−HL⁶ ligands were obtained by slightly modifying the backbone of the HL¹ ligand. Three homoleptic compounds [(L¹)2Zn] ⋅ THF (7 ⋅ THF), [(L⁴)2Zn] ⋅ Tol (8 ⋅ Tol), and [(L⁵)2Zn] (9) were afforded by changing the ratio of the ligands to ZnEt2 from 1 : 1 (compounds 1 ⋅ 0.8Tol, 4 and 5) to 1 : 0.5. The catalytic potential of all nine compounds for the borylation of aryl iodides by B2Pin2 was explored. The catalytic activity of 2 ⋅ Tol was the highest. The reactions catalyzed by 2 ⋅ Tol possess the features of high functional group tolerance and broad substrate scope.
... [7] While iron-catalyzed CÀ C bond-forming crosscouplings have been extensively explored, the CÀ B formation through the coupling of an electrophile with a boron reagent has met with limited success. In 2014, the groups of Cook [8] and Bedford [9] independently reported an elegant iron-catalyzed borylation of alkyl halides using ethyl magnesium bromide and tert-butyl lithium as activators, respectively. Later on, Nakamura, [10] Qu [11] and Feng [12] have further developed ironcatalyzed borylation of different electrophiles including aryl halides, allylic esters and alkyl chlorides with tert-butoxides as additives at elevated temperatures. ...
A novel iron‐catalyzed borylation of propargylic acetates leading to allenylboronates has been developed. The method allows the preparation of a variety of di‐, tri‐ and tetrasubstituted allenylboronates at room temperature with good functional group compatibility. Stereochemical studies show that an anti‐SN2’ displacement of acetate by boron occurs; this also allows transfer of chirality to yield enantiomerically enriched allenylboronates. The synthetic utility of this protocol was further substantiated by transformations of the obtained allenylboronates including oxidation and propargylation.
... 8 On the contrary, the transition-metal-catalyzed borylation reaction as an alternative approach to constructing C(sp2)-B bond has been developed from haloarenes and aromatic hydrocarbons as the corresponding boronic acids/esters. [9][10][11][12][13][14][15] These are robust methodologies to produce organoboron compounds. However, they require expensive transition metal catalysts and high temperatures. ...
This study investigates the photo-induced C–X borylation reaction of aryl halides by forming a halogen-bonding complex. The method employs 2-naphthol as a halogen-bonding acceptor and proceeds under mild conditions without a photoredox catalyst under 420 nm blue light irradiation. The method is highly chemoselective, broadly functional group tolerant, and provides concise access to corresponding boronate esters. Mechanistic studies reveal that forming the halogen-bonding complex between aryl halide and naphthol acts as an electron donor-acceptor complex to furnish aryl radicals through photo-induced electron transfer.
... The reaction is effective already at room temperature and leads to the corresponding aryloboronates in yields of 61%−83%. 394 Cobalt 395 and iron complexes 396,397 have also been found to be active in aryl halide borylation. As in palladium catalysis the formation of a metal-boryl complex is of key importance for catalysis with Ni, Cu, Fe, and Co complexes. ...
While the formation and breaking of transition metal (TM)–carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)–metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM–E and TM–H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid–hydrogen and metalloid–metalloid bonds to unsaturated compounds; activation and functionalization of C–H bonds and C–X bonds with hydrometalloids and bismetalloids; activation and functionalization of C–H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.
... At the same time, Bedford et al. reported an Fe(II)-catalyzed borylation of alkyl, allyl, benzyl, and aryl halides (Scheme 55). 277 In this process, the nucleophilic diboron adduct Li[B 2 pin 2 ( t Bu)], generated from t BuLi and B 2 pin 2 , reacted with electrophiles in the presence of an iron phosphine precatalyst and MgBr 2 to give the corresponding organo- boronate esters in moderate to good yields. While activated alkyl electrophiles, such as allyl, benzyl, and secondary alkyl halides, can be borylated in the absence of any ligand, simple primary alkyl halides required the presence of a ligand (using [FeCl 2 (dcpe)] as a precatalyst; dcpe = (1,2-bis-(dicyclohexylphosphino)ethane)). ...
... By applying this synthetic strategy, a large number of boryl complexes of W, Fe, Co, Rh, Ir and Pt have been isolated by various research groups [3]. This encouraged us to test the reactivity of a diborane (6) (PP = dppe, dpbz; X = halide, aryl) [60][61][62]. The Fe-B bond distance of 2.035(3) Å lies in the longer range of Fe-Bboryl distance and is comparable to that observed in the Fe(II) boryl complexes [9,37,61]. ...
Transition metal (TM) complexes of boron comprising two-center two-electron (2c–2e) bond/s are systematically developed over the decades. Among them, TM boryl complexes (i.e. LnM–BR2) featuring tri-coordinated boron have been studied extensively because of their unique bonding pattern and diverse reactivity towards organic substrates. These TM boryl complexes were found to be the key intermediates in many catalytic cycles, for example, diboration, hydroboration and C–H borylation reactions. As a result, this field has drawn immense attention in the recent years to develop new molecules. Synthesis and structural characterization of various TM boryl complexes implicitly directed to the isolation of TM complexes containing hitherto unknown structural motif, which could not be isolated in pure main group chemistry. The present review gives a summary of the contemporary progresses for the synthesis and reactivity of various TM boryl complexes reported by others and us. This review offers a detailed discussion on the structure and bonding of the TM boryl complexes. In addition, some of the potential applications of these species in various organic transformations are discussed.
The traditional method for the preparation of alkylboronic esters involves transition metal catalysis or transition metal‐free methods relying on single electron transfer. Each method relies on the suitable choice of catalyst or electron transfer mediator. In this study, we have developed a new method for the synthesis of alkylboronic esters from alkyl chlorides and diboron(4) compounds, without the need for additional metal or non‐metal catalysts. We discovered that under 400 nm visible light irradiation, alkyl chlorides can be activated in the presence of bis(neopentyl glycolato)diboron and an alkoxide base to form alkyl radical intermediates, which then react with diboron(4) compounds, resulting in high yields of alkylboronic esters. The reaction is applicable to both primary and secondary alkyl chlorides. The simplicity of operation and the broad substrate scope of this method make it practical for the synthesis of valuable alkylboronic ester intermediates.
Borylation of haloarene is one of the most important methodologies to synthesize borylated arenes. Generally, borylation of haloarene occurs smoothly at sterically less-hindered para- or meta-position by using a transition metal, a photoredox catalyst, or basic conditions. This study reports on ortho-specific and chemoselective borylation of ortho-iodoarene possessing carboxamide under visible-light irradiation. When a haloarene containing both C–I and C–X bonds is employed as a substrate, another C–X bond (not ortho) remains intact during the reaction. Mechanistic studies revealed that the key to success of this reaction is to generate diboron-bridged five-membered ring as a transition state, in which the diboron-bridged five-membered ring and the benzene ring in the transition state are perpendicular to each other owing to steric repulsion by the iodine atom at the ortho-position. This chemoselectivity is suitable for the synthesis of borylated building blocks.
Over the past several decades, significant progress has been made in the field of transition-metal-catalyzed carbon–carbon bond formation. Among these advancements, iron-catalyzed reactions have garnered significant attention in organic chemistry due to their economic and environmental advantages. Iron complexes have emerged as highly promising catalysts for carbon–carbon coupling reactions. They possess several advantageous features, such as low cost, widespread availability, and lower toxicity compared to other transition metals commonly used as catalysts. Through the utilization of iron catalysts, the coupling of organometallic reagents with diverse substrates has been achieved, leading to the synthesis of a wide range of important organic compounds. In this review, the main focus is on iron-catalyzed cross-coupling reactions involving alkyl electrophiles and various organometallic reagents. These organometallic reagents include those based on organomagnesium, organozinc, organoaluminum, and organoboron nucleophiles. The objective is to create C—C bonds through these reactions. Iron salts, when combined with suitable additives or ligands, have been employed as efficient catalysts in these processes.
We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate. We use DFT studies to interrogate spin states, connecting this to experimentally identified diamagnetic and paramagnetic species. The extreme conformational flexibility of the salan system appears connected to challenges associated with crystallization of likely on-cycle species.
We found that bis(neopentylglycolato)diboron (B2nep2) served as a bidentate ligand and a one-electron reducing agent for early transition metal chlorides to afford MCl4(B2nep2). Treatment of B2nep2 with MCl5 (M = Nb and Mo) produced MCl4(B2nep2) via two successive reactions, coordination of B2nep2 to the metal center and one-electron reduction from M(V) to M(IV), while coordination of B2nep2 to MCl4 (M = Zr, Ti) was observed without reduction of the central metals. DFT studies for the reduction of NbCl5 by B2nep2 clarified the initial formation of seven-coordinated and B2nep2-ligated Nb(V) species, NbCl5(B2nep2), and one chloride on niobium(V) moves to the Lewis acidic boron center to generate NbCl4[(B2nep2)Cl]. The chloride on the boron atom of NbCl4[(B2nep2)Cl] is trapped by the second B2nep2 to give [NbCl4(B2nep2)][ClB2nep2]. After the formation of [ClB2nep2]- as an anionic sp2-sp3 diboron adduct, one-electron reduction of the niobium(V) center produces NbCl4(B2nep2) along with [ClB2nep2]˙ as a plausible diboron species, whose decomposition affords ClBnep and B2nep2. The reduction of metal halides in the presence of B2nep2 was exemplified by green LED irradiation of TiCl4(B2nep2), producing chloride-bridged titanium(III) species, (B2nep2)TiCl2(μ-Cl)2TiCl2(B2nep2).
Iron-bisphosphines have attracted broad interest as highly effective and versatile catalytic systems for two- and three-component cross-coupling strategies. While recent mechanistic studies have defined the role of organoiron(II)-bisphosphine species as key intermediates for selective cross-coupled product formation in these systems, mechanistic features that are essential for catalytic performance remain undefined. Specifically, key questions include the following: what is the generality of iron(II) intermediates for radical initiation in cross-couplings? What factors control reactivity toward homocoupled biaryl side-products in these systems? Finally, what are the solvent effects in these reactions that enable high catalytic performance? Herein, we address these key questions by examining the mechanism of enantioselective coupling between α-chloro- and α-bromoalkanoates and aryl Grignard reagents catalyzed by chiral bisphosphine-iron complexes. By employing freeze-trapped 57Fe Mössbauer and EPR studies combined with inorganic synthesis, X-ray crystallography, reactivity studies, and quantum mechanical calculations, we define the key in situ iron speciation as well as their catalytic roles. In contrast to iron-SciOPP aryl-alkyl couplings, where monophenylated species were found to be the predominant reactive intermediate or prior proposals of reduced iron species to initiate catalysis, the enantioselective system utilizes an iron(II)-(R,R)-BenzP* bisphenylated intermediate to initiate the catalytic cycle. A profound consequence of this radical initiation process is that halogen abstraction and subsequent reductive elimination result in considerable amounts of biphenyl side products, limiting the efficiency of this method. Overall, this study offers key insights into the broader role of iron(II)-bisphosphine species for radical initiation, factors contributing to biphenyl side product generation, and protocol effects (solvent, Grignard reagent addition rate) that are critical to minimizing biphenyl generation to obtain more selective cross-coupling methods.
Alkyl boronic acid and its derivatives are important motifs in organic synthesis and pharmaceuticals. Herein, we report a photoinduced CuCl2 mediated C(sp3)–H borylation of unactivated alkanes. This protocol features mild...
Novel boronyl borinic ester I was generated by quenching the B2pin2/secBuLi‐ate complex with trifluoroacetic acid anhydride (TFAA) via ring‐opening in the 1,3,2‐dioxaborolane moiety on ate‐boron. Detailed NMR studies of the B2pin2/secBuLi‐ate complex in solution and in solid state allowed us to assume its oligomeric nature in solids with only ate‐boron involved in the oligomerization process. The O‐trifluoroacetyl pinacolate residue on borinic ester I initially formed on quenching with TFAA undergoes an unusual intramolecular transesterification with the carbonyl group of trifluoroacetyl forming othroester moiety in a few hours at r. t. to give boronyl borinic ester II. A solution of these reagents I/II was proved to be efficient for borylation of (2‐fluoroallyl)pyridinium salts that are highly base sensitive.
Organoboron compounds are versatile synthons in synthetic chemistry, materials science, and medicinal chemistry. The preparation of them often requires either reactive organometallic reagents or transition metals. In recent years, transition‐metal‐free borylation methodologies using radical chemistry for C−B bond formation have attracted much attention and developed rapidly, which circumvents the use of preformed organometallic reagents. Various successful approaches from different radical precursors (such as carboxylic acids, amines and alcohols, which are abundant in nature) through either thermochemical, photochemical or electrochemical pathways have been developed. Therefore, in this review, we hope to provide an overview of recent advances in the discovery, development, and mechanistic study of transition‐metal‐free radical borylation processes.
The first nickel bis-boryl complexes cis-[Ni( i Pr2ImMe)2(Bcat)2], cis-[Ni( i Pr2ImMe)2(Bpin)2] and cis-[Ni( i Pr2ImMe)2(Beg)2] are reported, which were prepared via the reaction of a source of [Ni( i Pr2ImMe)2] with the diboron(4) compounds B2cat2, B2pin2 and B2eg2 ( i Pr2ImMe = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B2cat2 = bis(catecholato)diboron; B2pin2 = bis(pinacolato)diboron; B2eg2 = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB2 moiety in these square planar complexes, reminiscent of the bonding situation of "non-classical" H2 complexes. [Ni( i Pr2ImMe)2] also efficiently catalyzes the diboration of alkynes using B2cat2 as the boron source under mild conditions. In contrast to the known platinum-catalyzed diboration, the nickel system follows a different mechanistic pathway, which not only provides the 1,2-borylation product in excellent yields, but also provides an efficient approach to other products such as C-C coupled borylation products or rare tetra-borylated compounds. The mechanism of the nickel-catalyzed alkyne borylation was examined by means of stoichiometric reactions and DFT calculations. Oxidative addition of the diboron reagent to nickel is not dominant; the first steps of the catalytic cycle are coordination of the alkyne to [Ni( i Pr2ImMe)2] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)2(η2-cis-(Bcat)(R)C[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( i Pr2ImMe)2(η2-cis-(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( i Pr2ImMe)2(η2-cis-(Bcat)(H7C3)C[double bond, length as m-dash]C(C3H7)(Bcat))].
Herein, we describe an iron-catalyzed benzylic C-H borylation reaction. The reaction targets primary and secondary C(sp3)-H bonds to deliver high-value boronic esters under mild conditions with short (7-8 min) reaction times. Functional groups are well tolerated, and complete site selectivity is observed in the presence of multiple benzylic C-H bonds.
An efficient synthesis of alkylboronate esters by copper nanoparticle stabilised on nitrogen-doped carbon nanotubes (N-CNT) catalysed alkyl halides borylation is reported. This nanocatalyst provides a practical access to alkylboronate esters...
Organoboron compounds play an ubiquitous role in the field of organic synthesis and catalysis. They possess diverse reactivity, exhibit great functional group tolerance and hence can undergo useful transformations. Most of the iron catalysts are abundant, biocompatible, inexpensive and environmental-friendly. Thus, scientific community is putting more efforts in the design of iron-catalyzed borylation reactions, and boryl group could be successfully incorporated into various substrates. An overview of iron-catalyzed borylation reactions is given in this review covering literature up to 2021.
Iron(II) boryl complexes of the pyrrole-based pincer ligand, CyPNP (CyPNP = anion of 2,5-bis(dicyclohexylphophinomethyl)pyrrole) have been synthesized and their insertion reactivity interrogated. Compounds of the type [Fe(BE)(CyPNP)] (E = pinacholato or catecholato) can be generated by treatment of the precursors, [Fe(OPh)(py)(CyPNP)] or [FeMe(CyPNP)], with B2E2. The boryl complexes are meta stable, but permit additional reactivity with several unsaturated substrates. Reaction with alkynes, RCCR', leads to rapid insertion into the Fe-B bond to generate stable vinyl boronate complexes of the type [Fe(C{R}C{R'}BE)(CyPNP)] (R, R' = H, Me, Ph, -CCPh). Each of the compounds is five-coordinate in the solid state by virtue of coordination of one of the oxygen atoms of the boronate ester. Similar reaction with nitriles, RCN (R = Ph, Me), results in facile de-cyanation to produce the correpsonding hydrocarbon complexes, [FeR(CyPNP)]. In the case of the bulky nitrile 1-AdCN, the insertion intermediate, [Fe(C{Ad}NBpin)(CyPNP)], has been isolated and structurally characterized. Treatment of the boryl complexes with styrene derivatives results in initial insertion to give an alkylboronate complex followed by either β-H elimination or protonation to give the products of C-H borylation and hydroboration, respectively.
Carboxylic acids are cheap, abundant, and environmentally friendly and are widely employed as partners in cross-coupling reactions. Transition-metal-catalyzed carboxylic acid transformations are typically achieved at extreme temperatures (usually >150°C), significantly limiting their practicality. Here, iron-catalyzed decarbonylation of aryl and alkyl carboxylic acids has been developed, providing the corresponding borylated products with good efficiency. This protocol features excellent functional group compatibility, mild conditions, and the late-stage borylation of bio-relevant carboxylic acids, thus offering excellent opportunities for applications in medicinal chemistry and organic synthesis. Preliminary mechanistic studies suggest that the iron(I)-boryl species may be the crucial intermediate and that a decarbonylative process is involved in this catalytic transformation.
Defunctionalization of readily available feedstocks to provide alkenes for the synthesis of multifunctional molecules represents an extremely useful process in organic synthesis. Herein, we describe a transition metal‐free, simple and efficient strategy to access alkyl 1,2‐bis(boronate esters) via regio‐ and diastereoselective diboration of secondary and tertiary alkyl halides (Br, Cl, I), tosylates, and alcohols. Control experiments demonstrated that the key to this high reactivity and selectivity is the addition of a combination of potassium iodide and N,N‐dimethylacetamide (DMA). The practicality and industrial potential of this transformation are demonstrated by its operational simplicity, wide functional group tolerance, and the late‐stage modification of complex molecules. From a drug discovery perspective, this synthetic method offers control of the position of diversification and diastereoselectivity in complex ring scaffolds, which would be especially useful in a lead optimization program.
A tetra(o-tolyl) (μ-hydrido)diborane(4) anion 1, an analogue of [B2H5]- species, was facilely prepared through the reaction of tetra(o-tolyl)diborane(4) with sodium hydride. Unlike common sp2-sp3 diborane species, 1 exhibited a σ-B-B bond nucleophilicity towards NHC-coordinated transition-metal (Cu, Ag, and Au) halides, resulting in the formation of η2-B-B bonded complexes 2 as confirmed by single-crystal X-ray analyses. Compared with 1, the structural data of 2 imply significant elongations of B-B bonds, following the order Au > Cu > Ag. DFT studies show that the diboron ligand interacts with the coinage metal through a three-center-two-electron B-M-B bonding mode. The fact that the B-B bond of the gold complex is much prolonged than the related Cu and Ag compounds might be ascribed to the superior electrophilicity of the gold atom.
The literature on the synthesis of saturated and partially saturated (sp³-enriched) boronic acids, boronates, and trifluoroborates is surveyed comprehensively starting from the early 1960s to 2020, focusing mainly on results published in the last decade. The synthetic methods are subdivided into two large groups: those based on the boronate moiety introduction and those commencing from boron-containing substrates. In particular, borylations at various functional groups or even C–H bonds, hydroborations, inter- and intramolecular carboborations, cycloadditions, and metathesis of alkenyl boronates are discussed, together with other methods.
A practical and direct method was developed for the production of versatile alkyl boronate esters via transition metal‐free borylation of primary and secondary alkyl sulfones. The key to the success of the strategy is the use of bis(neopentyl glycolato) diboron (B2neop2), with a stoichiometric amount of base as a promoter. The practicality and industrial potential of this protocol are highlighted by its wide functional group tolerance, the late‐stage modification of complex compounds, no need for further transesterification, and operational simplicity. Radical clock, radical trap experiments, and EPR studies were conducted which show that the borylation process involves radical intermediates.
Transition metal–catalyzed cross-coupling reactions are some of the most widely used methods in chemical synthesis. However, despite notable advantages of iron (Fe) as a potentially cheaper, more abundant, and less toxic transition metal catalyst, its practical application in multicomponent cross-couplings remains largely unsuccessful. We demonstrate 1,2-bis(dicyclohexylphosphino)ethane Fe–catalyzed coupling of α-boryl radicals (generated from selective radical addition to vinyl boronates) with Grignard reagents. Then, we extended the scope of these radical cascades by developing a general and broadly applicable Fe-catalyzed multicomponent annulation–cross-coupling protocol that engages a wide range of π-systems and permits the practical synthesis of cyclic fluorous compounds. Mechanistic studies are consistent with a bisarylated Fe(II) species being responsible for alkyl radical generation to initiate catalysis, while carbon-carbon bond formation proceeds between a monoarylated Fe(II) center and a transient alkyl radical.
An electrochemically promoted decarboxylative borylation reaction is reported. The reaction proceeds under mild conditions in an undivided cell without use of transition metal- or photo-catalysts. The key feature of the reaction is the compatibility of diboron reagents with the electrochemical conditions. This reaction exhibits broad substrate scope, good functional group tolerability, and easy scalability.
This chapter describes the synthesis of symmetrical and unsymmetrical diboron compounds, and their synthetic applications for installing boron into organic molecules via activation of the boron–boron σ-bond under transition-metal/transition-metal-free catalysis. Because of the vast scale of these diboron-based boron-installing reactions, this chapter focuses mainly on the reactions with unsaturated carbon–carbon bonds such as alkynes and alkenes, α,β-unsaturated carbonyl componds, organic halides, and Caryl–H bonds. A discussion of the regio-, stereo-, and enantioselectivities of the boron-installing reactions is also included.
Iron-catalyzed functionalization of inert bonds has scarcely been documented. Herein, we report an iron-catalyzed method for the silylation and borylation of (hetero)aromatic fluorides. This protocol features a wide range of...
Diazabutadiene derivatives have been identified as a distinct class of reagents, capable of cleaving B-B bond of the diboron(4). The cleavage is accompanied by formation of a new C=C bond...
We report an operationally simple and neutral conditions for borylation of alkyl bromides and iodides to alkyl boronic esters under transition metal- and photo-free conditions. A series of substrates with...
Nano‐ferrite‐supported Cu nanoparticles (Fe‐dopamine‐Cu NPs) catalyzed anti‐Markovnikov‐selective hydroboration of alkenes with B2pin2 is reported under mild reaction conditions. This protocol can be applied to a broad range of substrates with high functional group compatibility. In addition, we demonstrated the use of Fe‐dopamine‐Cu NPs as a catalyst for the β‐borylation of α,β‐unsaturated ketones and ester, providing alkylboronate esters in up to 98% yield. Reuse of the magnetically recyclable catalyst resulted in no significant loss of activity in up to five reaction runs for both systems.
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A highly efficient photocatalytic protocol for borylation of alkyl bromides and chlorides with graphene supported Cu/Pd alloy nanoparticles as a heterogeneous catalyst is reported. This photocatalytic system operates with visible light and air, provding a wide range of primary and secondary alkyl halides with B2pin2 or B2neop2 in high yields at low temperatures, thereby demonstrating its broad utility and functional group tolerance. The high performance is attributed to a synergistic effect of localized surface plasmon resonance (LSPR) of Cu and charge transfer from Cu to Pd due to the alloy surface charge heterogeneity. Energetic electrons from Pd to electrophilic alkyl halides lead to the formation of the alkyl radicals, which fast react with nucleophilic adduct of diboron compound with base adsorbed on the positively charged Cu sites to form the corresponding borylation product.
The significant progress made in the context of iron-catalyzed cross-coupling reactions in the past several years has boosted competition with well-known nickel and palladium catalyst systems. Iron-catalyzed protocols not only benefit from economic and toxicology standpoints, but also exhibit a range of reactivities and tolerate many functional groups. To date several iron catalyst systems have been developed, ranging from the use of simple iron salts to more complex stoichiometric/catalytic modifiers. Effective N-heterocyclic carbenes and bisphosphine ligands have also been developed for the cross-coupling reactions of organic electrophiles using diverse organometallic reagents. Indeed, the use of bisphosphine ligands in the field of iron-catalyzed cross-coupling reactions is important for new applications in modern synthetic organic chemistry. This account summarizes the recent developments in practical and novel iron-catalyzed cross-coupling reactions employing bulky o-phenylene bisphosphine ligands and their mechanistic views.
Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles. Mechanistic studies revealed that while Fe(I) can be accessed on catalytically relevant time scales, lower average oxidation states are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis(diphosphine)iron(I) complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples were studied by Mossbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the presence of an S = 1/2 species consistent with the formation of a mono(diphosphine)iron(I) species with inequivalent phosphine donor environments. DFT analysis of model complexes allowed us to rule out a T-shaped Fe(I) structure, as this is predicted to be high spin.
Palladium‐ and nickel‐catalyzed cross‐coupling recations of unactivated alkyl bromides with diboron reagents have been developed as practical methods for the synthesis of primary and secondary alkylboronic esters. These reactions extend the concept and utility of Pd‐ and Ni‐catalyzed cross‐coupling of aliphatic electrophiles. They also show different substrate selectivity and ligand dependence as compared to the recently reported Cu‐catalyzed borylation reaction.
The use of low-cost iron(III) acetoacetate (Fe(acac)3) and tetramethylethylenediamine (TMEDA) enables the direct cross-coupling of alkyl halides with bis(pinacolato)diboron. This approach allows for the borylation of activated or unactivated primary, secondary, and tertiary bromides. Moreover, even the borylation of benzylic or allylic chlorides, tosylates, and mesylates are possible. The reactions proceed under mild conditions at room temperature and show broad functional-group compatibility and "robustness" as measured by a modified Glorius robustness screen.
While attractive, the iron-catalyzed coupling of arylboron reagents with alkyl halides typically requires expensive or synthetically challenging diphosphine ligands. Herein, we show that primary and secondary alkyl bromides and chlorides, as well as benzyl and allyl halides, can be coupled with arylboronic esters, activated with alkyllithium reagents, by using very simple iron-based catalysts. The catalysts used were either adducts of inexpensive and widely available diphosphines or, in a large number of cases, simply [Fe(acac)3] with no added co- ligands. In the former case, preliminary mechanistic studies highlight the likely involvement of iron(I)–phosphine intermediates.
For large paramagnetic molecules in solution, the paramagnetic and
diamagnetic contributions to the magnetic susceptibility are comparable
and may be determined by NMR (Evans' method) in the same conditions
(concentration, temperature, solvent). Theoretical considerations show
that the solvent correction similarly affects both measurements and
cancels in the combined equations calculating the magnetic
susceptibilities and the magnetic moments. These simplified equations
have been successfully applied for the determination of the magnetism of
self-assembled heterodinuclear 3d-4f supramolecular complexes in
acetonitrile.
The reactions of iron chlorides with mesityl Grignard reagents and tetramethylethylenediamine (TMEDA) under catalytically relevant conditions tend to yield the homoleptic "ate" complex [Fe(mes)3 ](-) (mes=mesityl) rather than adducts of the diamine, and it is this ate complex that accounts for the catalytic activity. Both [Fe(mes)3 ](-) and the related complex [Fe(Bn)3 ](-) (Bn=benzyl) react faster with representative electrophiles than the equivalent neutral [FeR2 (TMEDA)] complexes. Fe(I) species are observed under catalytically relevant conditions with both benzyl and smaller aryl Grignard reagents. The X-ray structures of [Fe(Bn)3 ](-) and [Fe(Bn)4 ](-) were determined; [Fe(Bn)4 ](-) is the first homoleptic σ-hydrocarbyl Fe(III) complex that has been structurally characterized.
A new catalytic system based on a Zn(II) NHC precursor has been developed for the cross-coupling reaction of alkyl halides with diboron reagents, which represents a novel use of a Group XII catalyst for CX borylation. This approach gives borylations of unactivated primary, secondary, and tertiary alkyl halides at room temperature to furnish alkyl boronates, with good functional-group compatibility, under mild conditions. Preliminary mechanistic investigations demonstrated that this borylation reaction seems to involve one-electron processes.
Electrochemical discharge machining (ECDM) is an emerging non-traditional processing technique that involves high-temperature melting and accelerated chemical etching under the high electrical energy discharged. However, there are still several obstacles to overcome. First, both machining time and hole entrance diameter were found to increase with increasing machining depth. In particular, the increase becomes drastic when machining depth exceeds 250μm. In addition, achieving both high efficiency and accuracy in drilling a through hole in hard and brittle materials by ECDM poses even greater difficulty. To solve the above problems, this study proposed using a tool electrode with a spherical end whose diameter (150μm) is larger than that of its cylindrical body (100μm). Experimental results show that the curve surface of the spherical tool electrode reduces the contact area between the electrode and the workpiece, thus facilitating the flow of electrolyte to the electrode end, and enables rapid formation of gas film, resulting in efficient micro-hole drilling. Moreover, the curve surface does not cause excessive concentration of current density; and hence, bubbles grow at a more uniform speed; thus, increasing the discharge frequency. Comparison between machining depth of 500μm achieved by conventional cylindrical tool electrode and the proposed spherical tool electrode shows that machining time was reduced by 83% while hole diameter was also decreased by 65%.
Any old iron? Iron catalysts based on the widely available diphosphine ligand bis(diphenylphosphino)ethane have not previously fared particularly well in iron-catalyzed cross-coupling processes. However, this turns out not to be due to any inherently poor performance associated with the ligand, but rather the need to form a bis-chelate complex, either before or during the formation of an active Fe(I) species.
Measured magnetic susceptibilities of paramagnetic substances must typically be corrected for their underlying diamagnetism. This correction is often accomplished by using tabulated values for the diamagnetism of atoms, ions, or whole molecules. These tabulated values can be problematic since many sources contain incomplete and conflicting data. This article presents an explanation for the origin of the diamagnetic correction factors, organized tables of constants compiled from many sources, a simple method for estimating the correct order of magnitude for the diamagnetic correction for any given compound, a clear explanation of how to use the tabulated constants to calculate the diamagnetic susceptibility, and a worked example for the magnetic susceptibility of copper acetate. Keywords (Audience): First-Year Undergraduate / General
A mild Pd-catalyzed process for the borylation of alkyl bromides has been developed using bis(pinacolato)diboron as a boron source. This process accommodates the use of a wide range of functional groups on the alkyl bromide substrate. Primary bromides react with complete selectivity in the presence of a secondary bromide. The generality of this approach is demonstrated by its extension to the use of alkyl iodides and alkyl tosylates, as well as borylation reactions employing bis(neopentyl glycolato)diboron as the boron source.
Herein we demonstrate both the importance of Fe(I) in Negishi cross-coupling reactions with arylzinc reagents and the isolation of catalytically competent Fe(I) intermediates. These complexes, [FeX(dpbz)(2)] [X = 4-tolyl (7), Cl (8a), Br (8b); dpbz = 1,2-bis(diphenylphosphino)benzene], were characterized by crystallography and tested for activity in representative reactions. The complexes are low-spin with no significant spin density on the ligands. While complex 8b shows performance consistent with an on-cycle intermediate, it seems that 7 is an off-cycle species.
Through the use of a catalyst formed in situ from NiBr(2)·diglyme and a pybox ligand (both of which are commercially available), we have achieved our first examples of coupling reactions of unactivated tertiary alkyl electrophiles, as well as our first success with nickel-catalyzed couplings that generate bonds other than C-C bonds. Specifically, we have determined that this catalyst accomplishes Miyaura-type borylations of unactivated tertiary, secondary, and primary alkyl halides with diboron reagents to furnish alkylboronates, a family of compounds with substantial (and expanding) utility, under mild conditions; indeed, the umpolung borylation of a tertiary alkyl bromide can be achieved at a temperature as low as -10 °C. The method exhibits good functional-group compatibility and is regiospecific, both of which can be issues with traditional approaches to the synthesis of alkylboronates. In contrast to seemingly related nickel-catalyzed C-C bond-forming processes, tertiary halides are more reactive than secondary or primary halides in this nickel-catalyzed C-B bond-forming reaction; this divergence is particularly noteworthy in view of the likelihood that both transformations follow an inner-sphere electron-transfer pathway for oxidative addition.
Palladium nanoparticles generated in PEG catalyze the reaction of bis( pinacolato)diboron with various
aryl/benzyl halides to afford aryl/benzyl boronates in high yield. Arylboronates thus prepared, have been
directly used in the Suzuki–Miyaura coupling reaction with different aryl halides and benzyl halides in a
convenient one-pot, two-step solvent free green synthesis of unsymmetrical biaryls and diarylmethanes.
In this article, an account is given of recent developments in the chemistry of transition metal complexes of boron. Particular emphasis has been placed on the fundamental reaction chemistry of these systems which leaves the metalboron bond(s) (MB) intact. Substitution and abstraction chemistries have been developed in recent years and exploited to give access to a number of novel ligand types. This article summarizes these advances in a previously little investigated area of boryl/borylene chemistry.
Borylation of alkyl halides with diboron proceeded in the presence of a copper(I)/Xantphos catalyst and a stoichiometric amount of K(O-t-Bu) base. The boryl substitution proceeded with normal and secondary alkyl chlorides, bromides, and iodides, but alkyl sulfonates did not react. Menthyl halides afforded the corresponding borylation product with excellent diastereoselectivity, whereas (R)-2-bromo-5-phenylpentane gave a racemic product. Reaction with cyclopropylmethyl bromide resulted in ring-opening products, suggesting the reaction involves a radical pathway.
Bases play an important role in organocatalytic boron conjugate addition reactions. The sole use of MeOH and a base can efficiently transform acyclic and cyclic activated olefins into the corresponding β-borated products in the presence of diboron reagents. Inorganic and organic bases deprotonate MeOH in the presence of diboron reagents. It is concluded, on the basis of theoretical calculations, NMR spectroscopic data, and ESI-MS experiments, that the methoxide anion forms a Lewis acid-base adduct with the diboron reagent. The sp(2) B atom of the methoxide-diboron adduct gains a strongly nucleophilic character, and attacks the electron-deficient olefin. The methanol protonates the intermediate, generating the product and another methoxide anion. This appears to be the simplest method to activate diboron reagents and make them suitable for incorporation into target organic molecules.
Easy access: An unprecedented copper-catalyzed cross-coupling reaction of the title compounds with diboron reagents is described (see scheme; Ts = 4-toluenesulfonyl). This reaction can be used to prepare both primary and secondary alkylboronic esters having diverse structures and functional groups. The resulting products would be difficult to access by other means.
In the presence of novel iron(II) chloride-diphosphine complexes and magnesium bromide, lithium arylborates react with primary and secondary alkyl halides to give the corresponding coupling products in good to excellent yields. High functional group compatibility is also demonstrated in the reactions of substrates possessing reactive substituents, such as alkoxycarbonyl, cyano, and carbonyl groups.
Molecular compounds featuring electron-precise coordination modes of two-center metal-boron bonds are studied. A five-coordinate boratrane complex is found to be structurally authenticated, with the Co-B distance (2.132(4) Å) being significantly shorter than those in borane complexes of the second and third-row transition metal elements. Studies also found that addition of K[TmtBu] to [Pd(OAc)2] liberates 1 equiv each of acetic acid and potassium acetate, forming the unsual bimetallic bis(borane) complex. Chloride abstraction from gold boratrane with GaCl3 is found to provide a salt, in which the Au-B distance has lengthened considerably (2.448 Å) and the 11B NMR signal has shifted almost 30 ppm downfield.
A simple but effective copper-catalyzed borylation of aryl halides, including electron-rich and sterically hindered aryl bromides, with alkoxy diboron reagents occurs under mild conditions (see scheme). Preliminary DFT studies of the mechanism suggest that sigma-bond metathesis between a copper-boryl intermediate and the aryl halide generates the aryl boronate product.
Iron nanoparticles, either formed in situ stabilized by 1,6-bis(diphenylphosphino)hexane or polyethylene glycol (PEG), or preformed stabilized by PEG, are excellent catalysts for the cross-coupling of aryl Grignard reagents with primary and secondary alkyl halides bearing beta-hydrogens and they also prove effective in a tandem cyclization/cross-coupling reaction.
Nucleophilic, anionic boryl compounds are long-sought but elusive species. We report that reductive cleavage of the boron-bromine
bond in N,N′-bis(2,6-diisopropylphenyl)-2-bromo-2,3-dihydro-1H-1,3,2-diazaborole by lithium naphthalenide afforded the corresponding boryllithium, which is isoelectronic with an N-heterocyclic carbene. The structure of the boryllithium determined by x-ray crystallography was consistent with sp2 boron hybridization and revealed a boron-lithium bond length of 2.291 ± 0.006 angstroms. The structural similarity between
this compound and the calculated free boryl anion suggests that the boron atom has an anionic charge. The 11B nuclear magnetic resonance spectrum also supports the boryl anion character. Moreover, the compound behaves as an efficient
base and nucleophile in its reactions with electrophiles such as water, methyl trifluoromethanesulfonate, 1-chlorobutane,
and benzaldehyde.
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During the preparation of this paper iron TMEDA systems were also found to be active, albeit at significantly higher catalyst loading
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Boryl anions are rare. For an example, see: Segawa
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Organometallics
Communication
dx.doi.org/10.1021/om500847j | Organometallics 2014, 33, 5940−5943