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Silyl Radicals
Abstract
Introduction
Organosilyl Radical Structure, Reactions and Mechanisms
Organosilanes as Reagents in Radical Chain Reactions
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Di- and trimethylsilyl radicals, generated by the reaction of H atoms with di- and trimethylsilane, react to produce three main products: 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane. These products are formed by both radical combination and radical disproportionation reactions. The disproportionation reactions form Me2Si which inserts into the Si–H bonds of the reactants. From a quantitative determination of the disilane products as a function of the reactant ratio, a value for the branching ratio of cross-disproportionation of di- and trimethylsilyl radicals relative to the branching ratio for the disproportionation of dimethylsilyl radicals can be extracted. Our results provide strong evidence that the ratio of the rate constants for hydrogen abstraction from di- and trimethylsilane by H atoms is larger than absolute rate measurements suggest. Analysis also shows that the geometric mean rule for cross-radical reaction is closely obeyed. Disproportionation reactions yielding silaethenes occur to a minor extent and are responsible for the formation of six trisilanes. Secondary reactions, mainly initiated by H-atom abstraction from tetra- and pentamethyldisilane by silyl radicals, also take place. The relative rate constants estimated for these reactions are in agreement with a previous determination.
Ab initio UMP2/DZP calculations on localized organic and organometallic radicals reproduce very well the experimental hyperfine splitting (hfs) constants and show that substituents largely influence spin distribution. In a-substituted alkyl, silyl and phosphonyl radicals there is no relation between the magnitude of the hfs constant and the geometry at the radical center as generally accepted. The sizeable variations observed experimentally in the hfs constant at the radical center upon a-substitution are due to the electronic effect of the α-substituents rather than to structural changes at the radical center. In β-substituted alkyl radicals adopting the eclipsed conformation electronegative substituents reduce sizeably the spin density at β-hydrogens so that the unexpectedly low β-proton splittings observed experimentally are due to the electronic effect of the β-substituents rather than to an asymmetric bridging as generally recognized.
Ab initio molecular orbital calculations using the triple-ζ 6-311G** basis set, with (MP2, QCISD) and without (UHF) the inclusion of electron correlation calculate that methyl radicals abstract hydrogen atom from silacyclopentadiene (3, R = H), 1-methylsilacyclopentadiene (3, R = CH3) and 1-silylsilacyclopentadiene (3, R = SiH3) via transition states (5) in which the attacking and leaving radicals adopt collinear (or nearly so) arrangements. Transition states (5) which have (overall) Si–C separations of about 3.17–3.19 Å are calculated at MP2/6-311G**; structures (5) appear to be sensitive to the nature of the substituent on silicon, with an earlier transition state calculated for the reaction involving the silyl-substituted silane (3, R = SiH3), while the methyl-substituted system (3, R = CH3) is calculated to proceed with a slightly later transition state at the MP2/6-311G** level of theory. Energy barriers (ΔE1‡) of 35–42 kJ mol–1 are calculated for the forward reactions, while the reverse reactions (ΔE2‡) are calculated to require about 119–124 kJ mol–1 at MP2/6-311G**. Inclusion of higher-order correlation correction (QCISD/6-311G**//MP2/6-311G**) has only a marginal effect on the calculated energy barriers.
The rate constants for the reaction of a cumylperoxyl radical with Bu3SnH and (TMS)3SiH were determined at 72.5 °C to be 1600 and 66 M–1 s–1, respectively, by using inhibited hydrocarbon oxidation methodologies.
The Bu3SnH- or (TMS)3SiH-mediated 5-endo-trig radical cyclisation of the N-(cyclohex-1-enyl)acetamide 10 gives a mixture of the cis-fused (3R*,3aS*,7aS*)- and trans-fused (3R*,3aS*,7aR*)-3-aryloctahydroindol-2-ones 11a and 11b, whereas the 5-exo-trig radical cyclisation of the N-(cyclohex-2-enyl)acetamide 17 proceeds in a stereoselective manner to give only 11a. The latter method has been applied to the synthesis of the 5,11-methanomorphanthridine derivative 30, a key intermediate for the synthesis of (±)-pancracine 1.
In this paper, the high reactivity of silyl macroradicals toward double bonds of olefinic compounds has been explained by means of quantum-mechanical calculations through their frontier orbital characteristics. In this way, the main orbital interaction corresponds to the overlapping between the SOMO of the disilyl radical and the LUMO of the olefin. In order to obtain more accurate results of differential reactivity, an orbitalic SOMO-HOMO interaction should be included in addition to the main SOMO-LUMO one. Also, we theoretically studied the regioselectivity of the addition of silyl radicals to double bonds obtaining similar results as for carbon centered radicals where the reaction takes place on the less hindered carbon of the olefin. Regarding to the geometrical and electronic parameters, it has been shown that carbon radicals have a sp(2) geometry and a negative charge on the radical center whilst silyl radicals have a sp(3) geometry and a positive charge. Both factors contribute to the enhanced reactivity of silyl radicals with respect to carbon ones.
The rates and selectivities of the hydrogen-atom abstraction reactions of electrically-neutral free radicals are known to depend on polar effects which operate in the transition state. Thus, an electrophilic species such as an alkoxyl radical abstracts hydrogen much more readily from an electron-rich C-H bond than from an electron-deficient one of similar strength. The basis of polarity-reversal catalysis (PRC) is to replace a single-step abstraction, that is slow because of unfavourable polar effects, with a two-step process in which the radicals and substrates are polarity-matched. This review explores the concept of PRC and describes its application in a variety of situations relevant to mechanistic and synthetic organic chemistry.
Roberts and Steel have proposed an empirical calculation for obtaining energies of activation for hydrogen abstractions by radicals, A(.) + H-B. The results of their calculations led them to: (a) conclude that, in identity exchanges (A = B and Delta H = 0), the bond dissociation enthalpy D(A-H) is a significant factor; and (b) suggest that antibonding between A and B is not an important factor, Both findings are in conflict with our conclusions from results of our model for hydrogen abstractions. It is shown here that an examination of experimental results for several identity hydrogen exchanges does not support (a) and (b). Antibonding is important and, in identity exchanges, D(A-H) is not an important factor.
Radical allylations with 2-functionalized allyl phenyl sulfones have been performed by using tris(trimethylsilyl)silane as radical mediator. Yields varied from moderate to good depending on the nature of the starting materials. EPR studies on the alkyl radical adducts of allyl sulfones have been performed and conformational information has been obtained for the tert-butyl and tetrahydrofuranyl adducts of 2-carboxy allyl tris(trimethylsilyl)silane.
Reactions of unsubstituted and 2-substituted allyl phenyl sulfides with tris(trimethylsilyl)silane provide the corresponding allyl tris(trimethylsilyl)silanes rapidly and in high yields while the related reactions with allyl phenyl sulfones occur more slowly and in moderate yields.
A short, enantiospecific synthesis of the methylglycoside 5, a precursor of the lactonic moiety present in all mevinic acids and analogues, starting with 1,6-anhydro-D-glucose is described.
Ab initio molecular orbital calculations using pseudopotential basis sets and electron correlation (MP2, QCISD) predict that homolytic substitution by hydrogen atom as well as methyl, silyl, germyl and stannyl radicals at the chlorine, bromine and iodine atoms in hydrogen halides and halomethanes proceeds smoothly and without the formation of hypervalent (9-X-3) intermediates.
Ab initio calculation of the vibrational frequencies and equilibrium geometry of SiH+ is carried out in this paper.In the computation of the 2−D PEF,only valence electrons are correlated in the CEPA−1 calculations and canonical molecular orbitals are employed in the construction of the electron pairs. (AIP)
A number of nucleoside substrates has been reduced under free radical conditions with tris(trimethylsilyl)silane. In one case a novel type of a β-(acyloxy)alkyl radical rearrangement has been observed, which leads through the generation of a C-1’ radical species to the stereoselective preparation of an α-ribonucleoside. The rate of the above 1,2-migration has been estimated, and a comparison with previously reported results has been made.
Collection of kinetic data for reactions of the group 14 hydrides with various radicals provides essential information to chemists in both the synthetic and physical communities. This chapter focuses on the kinetics of reactions of the silicon, germanium, and tin hydrides with radicals. The absolute kinetics of radical reactions is determined. The quantitation of the kinetics of radical reactions involving the Group 14 metal hydrides in condensed phase and the type of metal hydride are discussed. Kinetic studies have been performed with many types of silicon hydrides, and much of the data can be interpreted in terms of the electronic properties of the silanes imparted by substituents. A number of approaches based on calculations are applied to obtain information on the factors that influence the reactivity of radical reactions. The hydrogen-atom transfer, the applied methodologies have spanned from high-level ab initio to empirical calculations. The heats of reactions and the methods used for the kinetic determinations are reviewed. Some important bond dissociation energies followed by kinetic methods used for determining radical reaction rate constants are discussed—namely, bond dissociation energies and radical kinetic methods.
Some β- or γ-substituted α-methylenebutyrolactones are butylated with Bul and (Me3Si)3SiH to give cis-α,β- or -α,γ-disubstituted lactones in high selectivites, while the same reaction with Bu3SnH in the presence of bulky Lewis acid reverses the stereoselectivity to give a trans-disubstituted lactone as the major product.
Tris(trialkylsilyl)silane [(i-Pr)3Si]3SiH (1) was synthesized, and the molecule was found to have a nearly planar structure of the polysilane skeleton by X-ray crystallography. Hydrogen abstraction from 1 by tert-butoxyl radical gave the highly stable silyl radical [(i-Pr)3Si]3Si· (2), which also has a planar structure.
A persistent silyl radical, tris(t-butyldimethylsilyl)silyl radical, (t-BuMe2Si)(3)Si . (1), was produced by two new methods in addition to a conventional hydrogen abstraction from the corresponding hydrosilane: one-electron oxidation of (t-BuMe2Si)(3)SiNa (2) by NO+BF4- and one-electron reduction of (t-BuMe2Si)(3)SiBr (3) by sodium.
The intramolecular trapping of a stabilized intermediate allylic radical generated by the addition of tris(trimethylsilyl)silyl (sisyl) radical to a conjugated system was performed. The observed low stereoselectivity suggests thermodynamic rather than kinetic control in this cyclization process.
The successful application of free radical transformations in organic synthesis requires a basic knowledge of elementary radical reactions and an understanding of the principles of free radical chain reactions. Using illustrative examples from the recent literature as a framework, this review provides an introduction to the design and application of free radical reactions for use in organic synthesis. While a few nonchain reactions are discussed, the primary focus is on four of the most important and versatile methods to conduct radical chain reactions. These are: the tin hydride method, the fragmentation method, the thiohydroxamate ester method (the Barton method), and the atom-transfer method. Particular emphasis is placed on the selectivity requirements which must be met for the design of sequences of radical reactions. The review appears in two parts in consecutive issues. Part 1 contains Sections 1 (Introduction) and 2 (The Tin Hydride Method), and Part 2 contains Sections 3 (The Fragmentation Method), 4 (The Thiohydroxamate Ester Method), 5 (The Atom-Transfer Method), and 6 (Non-Chain Methods). Part 1 1.Introduction 1.1.Chain Reactions and Synthetic Planning 2.The Tin Hydride Method 2.1.Intramolecular Cyclizations 2.2.Intermolecular Additions 2.3.Combination Sequences Part 2 3.The Fragmentation Method 4.The Thiohydroxamate Ester Method 5.The Atom-Transfer Method 5.1.Hydrogen Atom Transfer Addition and Cyclization 5.2.Halogen Atom Transfer 5.2.1. Halogen Atom Transfer Addition 5.2.2. Halogen Atom Transfer Cyclization 5.3.3. Halogen Atom Transfer Annulation 5.3.Heteroatom-Halogen Donors 5.4.Organocobalt Group Transfer 6. Non-Chain Methods 7.Summary and Conclusions
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A new asymmetric radical cyclization of optically pure s-alkoxy vinyl sulfoxides afforded various tetrahydrofuran derivatives in high yields with good to excellent diastereoselectivities. The sense of chiral induction depended on the configuration of the double bond of the vinyl sulfoxide which reacted in the s-trans conformation during the cyclization.
Alkenes and ketones are easily hydrosilylated by tris(trimethylsilyl)silane (1) via a radical mechanism, initiated by photolysis or a radical initiator. In the case of 1,6-dienes or 1-en-6-ones, the intermediate silylalkyl or siloxyalkyl radical, respectively, is trapped intramolecularly to give substituted cyclopentanes.
This is a response to criticisms expressed by Roberts (B. P. Roberts and A. J. Steel, J. Chem. Soc., Perkin Trans. 2, 1994, 2155) and to reservations regarding the triplet repulsion term of our a priori method of calculating energies of activation for hydrogen abstractions by free radicals. It is shown here that this term is related to earlier approaches of London, Eyring and Polanyi, and that our calculation describes satisfactorily reactions for which the empirical approach of Roberts and Steel shows substantial discrepancies. We reply to criticisms and apply our calculation to the understanding of recent experimental results pertaining to the H18OH + ˙OH identity exchange. We attempt to place in perspective differing views on the factors controlling reactivity.
At the highest level of theory in this study (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), ab initio molecular orbital calculations predict that abstraction of hydrogen atom from silane (SiH4) by methylthiyl radical (CH3S.) proceeds with an energy barrier of 34.0 kJ mol(-1) and is endothermic by 18.3 kJ mol(-1).
Ab initio molecular orbital calculations using a (valence) double-ζ pseudopotential basis set (DZP) with (MP2, QCISD) and without (SCF) the inclusion of electron correlation predict that the transition states (12–14) involved in homolytic (1,2)-translocation reactions of silyl (SiH3), germyl (GeH3) and stannyl (SnH3) groups between carbon centres, between carbon and nitrogen, and between carbon and oxygen proceed via homolytic substitution mechanisms involving front-side attack at the group (IV) heteroatom. While migrations between carbons are predicted to be unlikely, with calculated activation barriers of 71–137 kJ mol–1, depending on the level of theory, migrations from carbon to nitrogen and from carbon to oxygen are predicted to be facile. For example, rearrangement of the (silylmethyl)aminyl radical (H3SiCH2NH˙) to the silylaminomethyl species (H3SiNHCH2˙) is predicted to proceed with a barrier of 50.8–63.2 kJ mol–1 when electron correlation is included, in excellent agreement with experimental data. In addition, the analogous translocation to oxygen in the silylmethoxyl radical (H3SiCH2O˙), the prototypical radical Brook rearrangement, is calculated to require only 19.9 kJ mol–1 at the MP2/DZP + ZPVE level. Somewhat unexpectedly, MP2/DZP calculations predict that the stannylmethoxyl radical (H3SnCH2O˙) rearranges to the stannyloxymethyl radical (H3SnOCH2˙) without barrier.
An empirical approach has been used to devise a simple relationship [eqn. (B)] between the activation energy for an elementary hydrogen-atom transfer reaction (A) and ground state properties A˙+ H–B → A–H + B˙(A), Ea=Eof+αΔH°(1–d)+βΔχAB2+γ(sA+sB)(B) of the reactants and products. The role of polar effects, which operate in the transition state, is emphasised and described quantitatively in terms of the difference in Mulliken electronegativities (ΔχAB) of the radicals A˙ and B˙. Eqn. (B) reproduces the activation energies for 65 reactions, taken from the literature, within a standard error of ±2.0 kJ mol –1 and with a correlation coefficient of 0.988. Reactions of widely differing types are included and no distinction is made between gas-phase reactions and those which take place in non-polar solvents. Examples of hydrogen-atom transfer reactions which are not treated satisfactorily by eqn. (B) are discussed.
Reactivity of tetraaryldisilanes as radical reducing agents of alkyl phenyl chalcogenides initiated by Et3B or AIBN was studied. Here, the reactivity of alkyl sulfide was poor; however, various alkyl phenyl selenides and tellurides were reduced to the corresponding hydrocarbons in good yields with 1,1,2,2-tetraphenyldisilane.
Alkenyloxy(diphenyl)silanes that contain a terminal double bond undergo radical-chain cyclisation at 60–65 °C, in the presence of di-tert-butyl hyponitrite as initiator and a thiol as a catalyst. The thiol acts as a polarity-reversal catalyst and promotes the overall abstraction of hydrogen from the Si–H group in the alkenyloxysilane by the cyclic carbon-centred radical, formed by intramolecular addition of the corresponding silyl radical to the CCH2 group. Allyloxysilanes give five-membered-ring products via 5-endo-trig cyclisation of the intermediate allyloxysilyl radical. Homoallyloxysilanes give mixtures of five- and six-membered heterocycles, but the intermediate silyl radicals undergo predominantly 6-endo cyclisation, in contrast to the corresponding carbon-centred radicals which cyclise preferentially in the 5-exo mode. An analogous pentenyloxysilane gives only the seven-membered-ring product via a 7-endo radical cyclisation. Steric effects play an important part in influencing the final-product stereochemistry when this is determined in the hydrogen-atom transfer reaction between the cyclic adduct radical and the thiol catalyst. Complementary EPR spectroscopic studies of the short-lived intermediate cyclic adduct radicals have been carried out in the absence of thiol and the structures and conformations of these species have been determined. It is emphasised that, for thiol catalysis of the overall cyclisation of alkenyloxysilanes to be successful, it is necessary for the addition of the chain-carrying thiyl radical to the CCH2 group to be reversible under the reaction conditions.
Most radical reactions are conducted by chain methods that have bimolecular chain transfer steps. It is proposed that unimolecular chain transfer (UMCT) reactions should have advantages for conducting difficult radical transformations. The concepts of the UMCT method are illustrated with allyltributylstannane, and the first examples of UMCT reactions of silicon hydrides are introduced. Examples of bimolecular reactions conducted with silicon hydride UMCT reagents are provided, and the potential of the UMCT method is discussed.
Saturated primary, secondary and tertiary alkyl halides RX (X = Cl, Br or I) are reduced to the corresponding alkanes RH in essentially quantitative yield by triethylsilane in refluxing hexane or cyclohexane in the presence of a suitable initiator and an alkanethiol catalyst. Reduction proceeds by a radical chain mechanism and the thiol acts as a polarity reversal catalyst which mediates hydrogen-atom transfer from the Si–H group of the silane to the alkyl radical R˙. Triphenylsilanethiol and perfluorohexanesulphenyl chloride are also effective catalysts; the latter is probably reduced in situ to the corresponding fluorinated thiol. Other silanes R3SiH (R = Prn, Pri or Ph) also bring about reduction. The silane–thiol couple therefore serves as a useful replacement for tributylstannane as a homolytic reducing agent for alkyl halides. Reduction of 6-bromohex-1-ene, to give a mixture of hex-1-ene and methylcyclopentane, is more sluggush than reduction of saturated halides and this is attributed to removal of the thiol catalyst by addition across the CC bond. Ethyl 4-bromobutanoate is smoothly reduced to ethyl butanoate without interference from the ester function. Dialkyl sulphides are reduced to alkanes by triethylsilane in a radical chain reaction, but the effect of added thiol depends on the nature of the S-alkyl groups in the sulphide. The trialkylsilanethiol couple can also successfully replace trialkylstannane as the reducing agent in the Barton–McCombie deoxygenation of primary and secondary alcohols via their S-methyl dithiocarbonate (xanthate) esters. Good yields of deoxy compounds are obtained from octan-1-ol, octan-2-ol, octadecan-1-ol, 5α-cholestan-3β-ol, cholesterol and 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose.
Rate constants and activation energies of free-radical reactions of silanes and silyl radicals were analyzed in terms of the
parabolic model of the transition state. The kinetic parameters were estimated for 16 groups of reactions of silanes and silyl
radicals. These parameters were used to calculate the activation energies for 112 free-radical reactions and to estimate the
dissociation energies of the Si−H bond for 21 compounds and those of the C−Cl bond for 12 substituted benzyl chlorides. Triplet
repulsion, electronegativity, and radii of atoms of the reaction center of the transition state were shown to play an important
role in the formation of the activation barrier.
Photolyses of tert-butyl-substituted disilanes 1a−d with C60 result in the formation of 1,16-adducts 2. The unusual products 3, and 5−7, in which the silyl and phenyl groups are attached on the 1,2-positions of C60, also are obtained in the reactions of 1e and 4 with C60. A free silyl radical process is suggested for these conversions on the basis of experiments with other di- and oligosilanes.
9,10-Di-tert-butyl-9,10-dihydro-9,10-disilaanthracenes (trans-3 and cis-3) were synthesized, and their structures were determined by X-ray crystallography. The trans and cis isomers have a different conformation of the disilacyclohexadiene rings: trans-3 has a chair-like structure, while cis-3 has a boat-like structure. On irradiation of a solution of trans-3 or cis-3 in the presence of di-tert-butyl peroxide, trans-3 and cis-3 isomerized to each other and reached an equilibrium to give a mixture of cis-3 (81%) and trans-3 (19%). The isomerization shows the inversion of the radical center of the intermediate silyl radicals, which is rare in reactions of silyl radicals. Considering the reaction mechanism, the silyl radicals derived from trans-3 and cis-3 are found to be relatively stable, probably due to bulky tert-butyl substituents.
A mechanistic study was performed on a novel radical ring-enlargement reaction of (3-oxa-2-silacyclopentyl)methyl radicals into 4-oxa-3-silacyclohexyl radicals. Two pathways, one via a pentavalent silicon-bridging radical transition state (or intermediate), the other via β-elimination to give a ring-opened silyl radical, can be postulated. The radical reactions of 1 and 2, which are precursors for a (3-oxa-2-silacyclopentyl)methyl radical C‘ and a 4-oxa-3-silacyclohexyl radical D‘, respectively, showed that the ring-enlargement rearrangement of C‘ into D‘ is irreversible. 1H NMR analysis of the radical reactions of 8a and 8b, which have an asymmetric center at silicon, indicated that the configuration at the silicon atom is retained via a pentavalent silicon-bridging radical transition state (or intermediate) during the ring-enlargement reaction. Furthermore, examination of the radical ring-enlargement reaction with a deuterium-labeled substrate 12D showed that the ring-enlargement reaction did not involve β-elimination to give a ring-opened silyl radical. Based on these results, we conclude that the ring-enlargement reaction occurs via a pentavalent silicon-bridging radical transition state (or intermediate). This is the first experimental evidence for such a pentavalent silicon radical, which has been previously postulated to understand radical reactions of organic silicon compounds.
The photochemical reaction of C60 with disilane 1 affords the adduct 2 as a bissilylated product. The unique redox properties of 2 are reported by means of differential pulse voltammetry. The compound 2 was characterized by NMR, IR, and UV−vis spectroscopies. Spectroscopic and theoretical investigation strongly support the 1,16-addition structure having C2 symmetry which results from addition at the 1,16 positions in C60. The results are reasonably accounted for by the generation of a silyl radical which is responsible for the formation of 2.
Charge transfer absorptions of linear oligosilanes and silanorbornadienes, charge transfer induced oligomerization, polymerization and cycloaddition of tetrasilacyclooctadiyne and its germanium analogues are described. Photo-induced electron transfer reactions of various types of organosilicon compounds are discussed in detail, and include photo-induced chlorinative Si–Si bond cleavage, photo-induced nucleophilic Si–Si bond cleavage, fluorinative Si–Si bond cleavage via electron transfer, skeletal rearrangement via photo-induced electron transfer and the structure of a silyl radical cation.
Synthesis of spirodilactones is achieved by intramolecular free radical Michael addition of enol esters derivatized from tetronic acid. Synthesis of spirolactone-lactams and spirolactone-thiolactones is also covered by the scope of this new reaction.
When 5-exo6-endo type of vinyl radical cyclization onto aldimine NC bond was examined using tin hydride mediated radical reaction conditions, selective 6-endo cyclization took place to lead to 3-methylenepiperidines. On the other hand, vinyl radical cyclization onto ketimine NC bond did not take place.
A new method for the synthesis of keto spiro-γ-lactones and keto spiro-γ-lactams by intramolecular free radical cyclization is described.
A general method for synthesis of tri- and tetracyclic isoindolinones is achieved by intramolecular free radical cyclization.
The unprecedented radical β-elimination of vinylsulfoxides opens a new access to functionalized di- and trisubstituted allenes. The radical precursors are obtained in two steps from a carbonyl derivative and a vinylsulfoxide. The radical translocation trick can also be used to trigger the β-elimination of the sulfinyl radical.
The reduction of 8 with magnesium, chlorotrimethylsilane, and iodine to give 9 and the cyclization of 12 to 14 in the presence of tris(trimethylsilyl)silane and AIBN provide a convenient route to a triquinane carrying a two-carbon chain in each ring segment.
The feasibility of utilizing the little known photo-induced [1,3]-allylic phenylthio shift in the context of functionalized diquinane construction has been tested.
Free-radical cyclization of halocarbonyl compounds can be achieved using the organosilane reagents phenylsilane and tristrimethylsilylsilane. Both 6-exo-trig and 5-exo-trig cyclizations can be accomplished using aldehydes or ketones as radical traps.