Vinylsilanes, synthesis

In addition to transition metals, recent work has demonstrated that strong Lewis acids will catalyze the addition of silanes to alkynes in both an intra- and an intermolecular fashion.14,14a-14c The formation of vinylsilanes from alkynes is possible by other means as well, such as the synthetically important and useful silylcupration15,15a of alkynes followed by cuprate protonation to afford vinylsilanes. These reactions provide products which can be complementary in nature to direct hydrometallation. Alternatively, modern metathesis catalysts have made possible direct vinylsilane synthesis from terminal olefins.16,16a... 

Numerous applications of vinylsilanes have appeared in the literature in which alkyne hydrosilylation is not the basis of the vinylsilane synthesis. In some cases, more direct routes exist for the synthesis of the required vinylsilanes. However, in many cases advances in alkyne hydrosilylation might provide more efficient access to the vinylsilane starting materials. Some noteworthy applications of vinylsilanes in contexts where the vinylsilanes were not produced by alkyne hydrosilylation are presented here, with an emphasis on those employing vinylsilanes which may themselves be accessible by alkyne hydrosilylation. 

Retro-Brook rearrangement is relatively common and enjoys widespread application in many facets of organic synthesis. For example, vinylsilane synthesis from aUyloxysUane (eqnation 103)"°, allylsilane synthesis (eqnation 104)"°, and lithium eno-late formation from silyl enol ether (equation 105)" were reported. 

For the preparation of divinyl ketones, as required for the Nazarov reaction, various synthetic routes have been developed. A large variety of substituted divinyl ketones, including vinylsilane derivatives, can thus be prepared. The Nazarov cyclization, and especially the vinylsilane variant, has found application for the synthesis of complex cyclopentanoids. 

In their synthesis of (+)-cerulenin, Mani and Townsend employed lithiated epoxysilane 157, which they trapped with (4E,7 )-nonadienal to give a 77% yield of 158, which was further manipulated to give the natural product (Scheme 5.37) [58], as-ot, 3-Epoxy-Y,S-vinylsilanes 159 are regioselectively lithiated at the a-silyl position, and can subsequently be stereo selectively trapped with a range of electrophiles to give a-substituted epoxyvinylsilanes 160, which can in turn be isomerized to a-silyl-P-vinylketones 161 (Scheme 5.38) [59]. 

The Ireland-Claisen reaction of ( )-vinylsilanes has been applied to the stereoselective synthesis of syn- and c/nti-2-substituted 3-silyl alkcnoic acids. a R-2-Alkyl-3-silyl acids are prepared by rearrangement of ( )-silyl ketene acetals which are generated in situ from the kinetically formed (Z)-enolate of the corresponding propionate ester40. Chelation directs the stereochemistry of enolization of heteroelement-substituted acetates in such a way that the syn-diastereomers are invariably formed on rearrangement403. 

Hydroalumination of terminal alkenes using EtjAl as the hydride source must be carried out with titanium catalysts [24], since zirconium compounds lead to the formation of alumacyclopentanes [60, 61] (Scheme 2-11) and carbometallated products [62]. Suitable substrates for hydroalumination include styrene, allylnaphthalene and vinylsilanes. Only one of the ethyl groups in EtjAl takes part in these reactions, allowing the synthesis of diethylalkylalanes, which are difficult to obtain by other methods. 

The regioselective ring opening of the silyl epoxides is facilitated by the stabilizing effect that silicon has on a positive charge in the -position. This facile transformation permits vinylsilanes to serve as the equivalent of carbonyl groups in multistep synthesis.149... 

The use of tri-tert-butylphosphine has produced still higher selectivities, allowing near total control in the synthesis of (A)-vinylsilanes, including alkoxysilanes and disiloxanes.38,39 In the context of a total synthesis of an HMG-CoA reductase inhibitor, hydrosilylation with a chlorosilane catalyzed by a platinum(O) olefin complex, Pt2 [(CH2=CH)Me2Si]20 3 (also known as Karstadt s catalyst), followed by coupling with a 2,6-disubstituted aryl iodide forged a key intermediate shown in Scheme 6.38... 

Interestingly, the [RuCl2(p-cymene)]2 catalyst used for selective synthesis of (Z)-vinylsilanes produces instead the a-vinylsilanes with appropriately positioned hydroxyl groups.57 For the homopropargylic system shown (Scheme 12), the selectivity is 98 2. For propargylic or bishomopropargylic systems, only small amounts (2-13%) of the a-product... 

Table 6 Selective synthesis of a-vinylsilanes with a cationic ruthenium complex...

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

Classically, vinylsilanes have served as nucleophiles in Friedel-Crafts acylation and related reactions with strong electrophiles.4,4a 89 These strategies have been employed in the synthesis of natural products.90,9011... 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

Besides the acetylene—acetylene coupling reactions shown above, acetylene—allene [40] and allene—allene coupling reactions [40] are also feasible (Eqs. 9.18 and 9.19). These reactions provide convenient methods for the synthesis of stereodefined olefinic skeletons. As an addendum, the coupling of vinylsilane with acetylenes was effected by 1 to give stereodefined homoallylsilanes, as shown in Eq. 9.20 [41]. 

Electroreductive coupling of ketones with silyl-substituted olefins promotes interesting reactions that are useful for organic synthesis. For example, coupKng of ketones with trimethylvinylsilanes affords /I-trimethylsilyl alcohols, which are easily transformed to the corresponding olefins (Scheme 40). This reaction is interesting from the synthetic point of view since vinylsilane behaves as the equivalent to a /I-trimethylsilyl group-substituted anion [77, 83]. 

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