Si-H insertion

Silanes can react with acceptor-substituted carbene complexes to yield products resulting from Si-H bond insertion [695,1168-1171]. This reaction has not, however, been extensively used in organic synthesis. Transition metal-catalyzed decomposition of the 2-diazo-2-phenylacetic ester of pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) in the presence of dimethyl(phenyl)silane leads to the a-silylester with 80% de (67% yield [991]). Similarly, vinyldiazoacetic esters of pantolactone react with silanes in the presence of rhodium(II) acetate to yield a-silylesters with up to 70% de [956]. 

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Scheme 10.79 Rh-catalysed intermolecular Si-H insertions of methyl phenyldiazo-acetate and vinyldiazoacetates into allylsilyl ether with sulfonamide ligand.

Given the ability of 14 electron fragments [(dtbpm)Pt(O)] and [(dcpm)Pt(O)] to activate C-H and C-Si bonds of inert organosilanes under very mild reaction conditions, it was of course no big surprise that Si-H activation reactions of silanes are possible as well. Hydrido-silyl complexes were formed in practically quantitative reactions if 14 or IS were used as precursors for the [(dtbpm)Pt(O)] fragment. Examples of Si-H insertion products, all stable, isolable compounds which could be fully characterized, are 25 - 27, and others have been made. 

The potential of C-Si and Si-H insertion processes of platinum fragments like [(dtbpm)Pt(O)] and [(dcpm)Pt(O)] with respect to stoichiometric or catalytic applications is being evaluated in our group now. The chemistry of the nickel and palladium analogs of these intermediates and of their complexes [28, 29], in accord with theoretical expectations, has turned out to be comparably unusual and exciting. This will be reported elsewhere. 

Dirhodium(II) carboxylate catalysts have been used extensively for the catalysis of carbene insertions. In many cases, impressive selectivities have been achieved (19-21). In an effort to find selective catalysts for carbenoid insertions, Moody screened a series of dirhodium(II) carboxylate catalysts for their ability to catalyze carbenoid Si-H insertion (22). The authors surveyed the commercially available carboxylic acids, -10,000 of which are chiral. The members of this group that contained functionality that is incompatible to the reaction were culled out. The remaining chiral carboxylic acids (-2000 compounds) were then grouped into 80 different clusters. There is no discussion presented for the criteria used in the grouping of the acids. A representative acid from each cluster was then chosen for... 

In addition to insertions into polar X-H bonds by means of ylide intermediates, carbe-noids are capable of inserting into nonpolar bonds such as Si-H and C-H. The Si-H insertion by vinylcarbenoids has been developed as a novel method for the synthesis of allylsilanes 166 and 167 of defined geometry as illustrated in Eqs. (17) and (18) [28]. The alkene geometry of the vinyldiazoacetate is not altered during carbenoid formation or the subsequent Si-H insertion. 

Considerable interest has been shown in developing asymmetric variants of the Si-H insertion. The chiral auxiUary (Jl)-pantolactone has performed quite well in this chemistry, as illustrated in the formation of 169 in 79% diastereomeric excess (Eq. 19) [28]. A wide variety of chiral catalysts have been explored for the Si-H insertion chemistry of methyl phenyldiazoacetate [29, 117-119]. The highest reported enantioselectivity to date was obtained with the rhodium prolinate catalyst Rh2(S-DOSP)4, which generated 170 with 85% enantiomeric excess (Eq. 20) [120]. 

Recently, similar experiments have been carried out with MeS2SiCl2, t-BuMesSiCL and t-BuPhSiCl2- The Si—H insertion product was obtained in all of these cases, although the yields reported were smaller than with t-Bu2SiCl2- E. P. Black, Ph.D. Thesis, North Dakota State University, 1995. 

In a related strategy, Helquist and coworkers used thioether-substituted Fp complexes and applied the intermediate iron-carbene complexes to cyclopropana-tion (Scheme 1.17), C—H insertion and Si—H insertion reactions [48]. 

Rh -mediated hetero-H insertion may proceed by initial coordination of the intermediate metal-car-bene complex with the nonbonding electrons of the heteroatom. Proton transfer would then give the observed product. This can be an efficient process both for forming C—N bonds, as in the cyclization of (152 equation 62), and for C—O bonds, as illustrated by the construction of ether (155 equation 63). ° Si—H insertion of (156) was shown to proceed with retention of absolute configuration, as would be expected for a concerted transition metal carbene mediated process (equation 64). ... 

Analogously, the gas-phase thermal decomposition of trifluoro(l,l,2,2-tetrafluoroethyl)silane (at 140-200 C) or trimethyl(l,l,2,2-tetrafluoroethyI)silane (at 300-370°C) generated di-fluoromethyl(fluoro)carbene which underwent addition to alkenes to give 1-difluoromethyl-l-fluorocyclopropane 8. ° (Z)- and ( )-But-2-ene were cyclopropanated stereospecifically, and allylic C-H insertion was not observed. With dimethyl(vinyl)silane or allyl(dimeth-yl)silane, however, the carbene underwent both double bond addition and Si-H insertion. ... 

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