Régine Amardeil’s research while affiliated with Université Bourgogne Europe and other places

We describe the synthesis and characterization of two classes of hybrid phosphino ligands functionalized with amino ester or amino acid groups. These compounds are built either on a rigid planar phenyl platform or on a functionalized – conformationally controlled – rotational ferrocene backbone. Modifications at the −PR2 phosphino groups (R=aryl and alkyl, with various steric bulk, Ph, Mes, i‐Pr, Cy) and at the amino acid/amino ester functions are reported, showing a valuable high modularity. The coordination chemistry of these compounds regarding palladium and gold was investigated, in particular with respect to the coordination mode of the phosphino groups and the preferred interaction with metals for the amino ester and amino acid functions. For all the hybrid ligands, based either on ferrocenyl or phenyl platforms, the (P,N)‐chelating effect dominates in solution for coordination to Pd(II), while linear P−Au(I) complexes without interaction with the amino groups are assumed. The investigation of the catalytic activity of these new ligands in the demanding palladium‐catalyzed Suzuki–Miyaura coupling of o‐dibromoarenes with fluorophenylboronic acid underlined the importance of the amino ester dicyclohexylphosphinoferrocene for avoiding the deleterious homocoupling and arene oligomerization side‐reactions that were otherwise observed with the other phosphine ligands. Hybrid 1‐amino ester‐ or 1‐amino acid‐1′‐phosphinoferrocenes and some corresponding 1,2‐amino ester‐phosphinobenzenes were synthesized from reductive amination of carbonyl precursors. Their coordination modes to Pd(II) and Au(I) are reported, and the superior activity of a dicyclohexylphosphinoferrocene amino ester in a challenging Suzuki coupling of dihaloarenes is illustrated.

Ferrocene is unique among organometallic compounds, and serves notably as a versatile platform towards the production of ligands useful to promote transition metals chemistry. A general limiting aspect of the synthesis of ferrocene derivatives is the efficient access to sophisticated highly functionalized polysubstituted ferrocenes, i.e. bearing four or more substituents replacing hydrogen atoms on the cyclopentadienyl rings. These ferrocene derivatives can bear various functional or/and structuring spectator substituents. Their preparation involves synthetic difficulties resulting from the need of multiple functionalizations coexisting altogether, and satisfying functional group compatibility and high selectivity issues. In the last decades, our group initially designed highly functionalized polyphosphines and hybrid ligands (1,1',3,3'‐tetrafunctionalized Fc) using dialkylated 1,1'‐tert‐butylferrocene as a scaffold, which opened the way to various new classes of hybrid compounds. Some of these original ferrocenes were used as ligands, promoting metal catalysed C–C and C–X bond formation (X = O, S, N, etc.). Thus, highly functionalized ferrocenes, which include notably (P,P,P,P)‐, (P,P,N,N)‐, (P,P,P')‐, (P,P,B)‐, (P,B)‐ and (N,B)‐compounds were developed in which the heteroatoms coexist in a close proximity on a common ferrocene platform with a controlled conformation. The present review details the concepts attached to the synthesis of these highly functionalized ferrocene species, and illustrates their main features and applications related to organometallic chemistry directed towards organic synthesis by metal catalysis.

A general challenging issue in the synthesis of hemilabile ferrocene ligands is the access to highly functionalized ferrocene starting materials. These can bear donor/acceptor atoms, additional functional and structuring spectator substituents, but the preparation of such species poses general synthetic difficulties. We report herein alternative synthetic routes to hybrid ferrocene donors such as tert‐butylated phosphanylcarboxylic acids and their corresponding aldehydes. These hybrid ambiphilic species that combine Brønsted‐acidic and Lewis‐basic functional moieties in their structure were characterized by multinuclear NMR and single‐crystal X‐ray diffraction analysis. In the solid state, the mutual arrangement of functional groups at the ferrocene unit is highly dependent on hydrogen bonding interactions despite the structural hindering effect of the tert‐butyl groups introduced. The ligands were converted to gold(I) complexes and phosphane selenides to further monitor the properties and mutual influence of the functional groups in the prepared compounds.

A smart steric control of the metallocene backbone in bis- and poly(phosphino)ferrocene ligands favors intramolecular aurophilic interactions between [AuCl] fragments in polynuclear gold(I) complexes. We synthesized and characterized by multinuclear NMR and X-ray diffraction analysis mono-, di-, and polynuclear gold complexes of constrained ferrocenyl diphosphines, which bear either bulky tert-butyl groups or more flexible siloxane substituents at the cyclopentadienyl rings. The complexes meso-1,1'-bis(diphenylphosphino)-3,3'-di-tert-butylferrocene (4-m), rac-1,1'-bis[bis(5-methyl-2-furyl)phosphino]-3,3'-di-tert-butylferrocene (5-r), and rac-1,1'-bis(diphenylphosphino)-3,3'-bis[(tri-iso-propylsilyl)oxy]ferrocene (6-r) were used to form dinuclear gold complexes. Coordination of tert-butylated ferrocenyl phosphines generated aurophilic interactions in the corresponding dinuclear gold complexes, contrary to gold(I) complexes reported with 1,1'-bis(diphenylphosphino)ferrocene. The structurally related tetraphosphine 1,1',2,2'-tetrakis(diphenylphosphino)-4,4'-di-tert-butylferrocene (11) also gave access to mononuclear, dinuclear, and the original trinuclear gold chloride aurophilic complexes in which 14e(-) to 16e(-) gold centers coexist. In such complexes, nonbonded ("through-space") (31)P-(31)P' nuclear spin couplings were evidenced by high-resolution NMR. In these interactions nuclear spin information is transferred between the lone-pair electron of an uncoordinated phosphorus P and a phosphorus P' that is involved in a σ covalent bond Au-P'. The dinuclear aurophilic complex displayed a concerted shuttling of its [ClAu···AuCl] fragment between the four phosphorus donors of the tetraphosphine ligand. Thus, an aurophilic Au···Au bond, which is assumed to be a weak energy interaction, can be conserved within a dynamic shuttling process at high temperature involving an intramolecular coordination-decoordination process of digold(I) at phosphorus atoms.

A survey highlighting the most recent palladium catalytic systems produced and their performances for progress in direct synthesis of indole backbones by heterocarbocyclization of reactive substrates is provided. The discussion is developed in relation with the principles of sustainable chemistry concerning atom and mass economy. In this respect, the general convergent character of the syntheses is of particular interest (one-pot, domino, cascade or tandem reactions), and the substrates accessibility and reactivity, together with the final waste production, are also important. This critical review clearly indicates that the development of ligand chemistry, mainly phosphines and carbenes, in the last few decades gave a significant impetus to powerful functionalization of indoles at virtually all positions of this ubiquitous backbone (118 references).

Ferrocene-based diphosphane and triphosphane ligands for metal catalysis were synthesized from novel cyclopentadienyl salts substituted with both congested branched alkyl groups and electron-donating or electron-withdrawing phosphanyl groups. The triphosphanes evidenced a large diversity of heteroannular “through-space” JPP couplings, which arises from their different conformations in solution, despite an identical ferrocenyl backbone. Good catalytic performances were found for electron-rich ferrocenyl diphosphane ligands in palladium-catalyzed direct arylation of n-butyl furan with aryl chlorides via C−H activation.

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Preparation and characterization of the first examples of copper(I) ferrocenylpolyphosphine complexes are reported. The molecular structure of complex {P,P′,P′′-[1,1′,2,2′-tetrakis(diphenylphosphino)-4,4′-di-tert-butylferrocene]iodocopper(I)} (1) was solved by X-ray diffraction studies, and its fluxional behavior in solution was investigated by VT-31P NMR; both revealed a net triligated coordination preference of the ferrocenyl tetraphosphine Fc(P)4tBu with copper. The tetradentate ligand is an active auxiliary in Sonogashira alkynylation; therefore the general question of copper as a competitive coordination partner in the Pd/Cu-catalyzed Sonogashira reaction was raised and discussed. Electronically neutral, activated, and deactivated aryl bromides were employed for coupling with phenylacetylene with various [(Pd)/(Cu)/(tetraphosphine)] systems. The catalytic investigations shown that 1 mol % of complex 1 in combination with palladium is far more effective and selective for Sonogashira coupling than 5 mol % of CuI and palladium in the coupling to phenylacetylene of the deactivated aryl bromide 4-bromoanisole. This system efficiently avoids the concurrent and deleterious consumption of phenylacetylene by formation of diyne or enynes. To our knowledge, this is the first time that this kind of high selectivity is induced in Sonogashira alkynylation by initial ligand complexation to copper instead of palladium. These results demonstrate that coordination of Cu halide cocatalyst is a factor that should no longer be neglected in mechanistic and applied studies of the Sonogashira reaction.

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This tutorial review devoted to ligand chemistry deals with the design and properties of ferrocenyl polyphosphines, an original class of multidentate ligands. The development of a varied library of ferrocenyl tetra-, tri- and diphosphine ligands is reviewed. The multidentate nature of these species has led to unique spectroscopic and catalytic properties, in which the spatial proximity of phosphorus atoms is crucial. Regarding their catalytic applications, the key issues of catalyst longevity and ultralow catalyst loadings are discussed. Another part is concerned with fundamental advances gained in physical chemistry for structure elucidation by the study of the intriguing "through-space" NMR spin-spin J couplings existing within several of these polyphosphines.