Synthetic Application of Silyl Anions

Certain silyl anions are useful silylating reagents in organic synthesis. Since some leading reviews have covered the synthetic applications Ud,e/,2), only a few typical examples are mentioned here. 

Me3SiLi undergoes smoothly conjugate addition to a,/3-unsaturated ketones in the absence of copper salt [Eq. (48)] (29). 

By use of Me3SiK, some mono-, di-, and trisubstituted epoxides can be deoxygenated stereospecifically, with inversion of stereochemistry [Eq. (49)1 (96, 97). Thus, reaction of cis and trans epoxides with Me3SiK in 

HMPA at 65 C for 3 hours affords the corresponding trans and cis olefins, respectively. Backside attack of Mc3SiK on the cis (and trans) epoxides generates the threo- (and erythro-) jS-alkoxysilanes, respectively, followed by syn elimination (Peterson elimination) to give the olefin with inverted stereochemistry. 

Disilanyllithium le3SiMe2SiLi also undergoes conjugate addition to an a,/8-unsaturated ketone just like Me3SiLi 68a-c). In comparison with Me3SiLi, Me3SiMe2SiLi shows a better stereoselectivity in the addition [Eq. (50)]. 

---

The chemistry of metalated organosilicon compounds has been the subject of several reviews1, the most recent ones by Lickiss and Smith13 and Tamao and Kawachilb, which cover the literature up to the year 1994. This chapter will now take into account the developments in the chemistry of metalated silanes up to the middle of 1996 however, for completeness there will be some overlap with former reviews. The emphasis of this review is on the synthesis and structure of these metalated silanes. However, some examples of their utilization for synthetic purposes will also be given where appropriate. For more information about synthetic applications of silyl anions the reader is referred to some leading references in this fieldla,b h k. 

A metal-free trimethylsilyl anion is formed from hexamethyldisilane by cleavage with TBAF in HMPA in equilibrium concentration, as revealed by H and F NMR analysis of the reaction mixture. Treatment of trisilane with TBAF provides Me3SiMe2Si /NBu4 and Me3SiF the fluoride anion attacks at the terminal silicon of trisilane [Eq. (21)] (59). Synthetic application of this species is described in Section VII. Other metal-free silyl anions are described in Section V. 

Meyers, A. I. Heterocycles in Organic Synthesis Wiley-Interscience, 1974 (332 pages). DegITnnocenti, A. Pollicino, S. Capperucci, A. Synthesis and Stereoselective Functionalization of Silylated Heterocycles as a New Class of Formyl Anion Equivalents Chemical Communications 2006, 4881—4893. Yus, M. Najera, C. Foubelo, F. The Role of 1,3-Dithianes in Natural Product Synthesis Tetrahedron 2003, 59, 6147-6212. Albright, J. D. Reactions of Acyl Anion Equivalents Derived from Cyanohydrins, Protected Cyanohydrins and a-Dialkylaminonitriles Tetrahedron 1983, 39, 3207-3233. Seebach, D. Corey, E. J. Generation and Synthetic Applications of 2-Lithio-l,3-dithianes /. Orpi. Chem. 1975, 40, 231-237. 

I n 1993, the first cinchona-catalyzed enantioselective Mukaiyama-type aldol reaction of benzaldehyde with the silyl enol ether 2 of 2-methyl-l -tetralone derivatives was achieved by Shioiri and coworkers by using N-benzylcinchomnium fluoride (1, 12 mol%) [2]. However, the observed ee values and diastereoselectivities were low to moderate (66-72% for erythro-3 and 13-30% ee for threo-3) (Scheme 8.1). The observed chiral inductioncan be explained by the dual activation mode ofthe catalyst, that is, the fluoride anion acts as a nucleophilic activator of the silyl enol ethers and the chiral ammonium cation activates the carbonyl group of benzaldehyde. Further investigations on the Mukaiyama-type aldol reaction with the same catalyst were tried later by the same [ 3 ] and another research group [4], but in all cases the enantioselectivities were too low for synthetic applications. 

Early transition-metal silyl compounds have received much less attention, largely due to the fact that general, straightforward synthetic routes have not been available. Syntheses based on oxidative additions are less applicable, given the more electropositive character of the early metals. Particularly for d° silyl complexes, most syntheses are based on nucleophilic displacement of halide by a silyl anion reagent, usually an alkali metal derivative. 

Diaryliodonium salts have found synthetic application as arylating reagents in reactions with various organic substrates under polar, catalytic, or photochemical conditions. Typical examples of arylations of nucleophiles under polar, non-catalytic conditions are shown in Scheme 3.271 and include the reactions of diaryliodonium salts with thiosulfonate anions 675 [861], fluoride anion [862,863], malonates 676 [864] and silyl enol ethers 677 [865]. 

---