Siloxy compounds alcohols

A procedure for alkylation of C=0 double bonds in the presence of (metal-free) organocatalysts and non-metallic nucleophiles has been reported by the Iseki group for trifluoromethylation of aldehydes and ketones [185]. On the basis of a previous study of the Olah group [186, 187] which showed the suitability of non-chiral phase-transfer catalysts for trifluoromethylation of carbonyl compounds, Iseki et al. investigated the use of N-benzylcinchonium fluoride, 182, as a chiral catalyst. The reaction has been investigated with several aldehydes and aromatic ketones. Trifluoromethyltrimethylsilane, 181, was used as nucleophile. The reaction was, typically, performed at —78 °C with a catalytic amount (10-20 mol%) of 182, followed by subsequent hydrolysis of the siloxy compound and formation of the desired alcohols of type 183 (Scheme 6.82). 

A mixture of cyclohexanone (0.96 g. 10 mmol) and CF,TMS (1.7 g, 12 mmol) in THF (10 mL) cooled 10 0 C was treated with a catalytic amount of TBAF 3H2O (ca. 20 mg). Instantaneously, a yellow color developed with the initial evolution of TMSF, and the mixture was brought to rt and stirred. The resulting siloxy compound was then hydrolyzed with aq IICl. The mixture was extracted with EtjO (75 niL), dried (MgSO, ) and concentrated, The residue was either distilled or crystallized from a suitable solvent to give the product this alcohol, a colorless oil, solidified in the receiver during distillation yield 1.28 g (77 %) bp 72 73 C/40 Torr mp 59-61 C. 

As described in the sections above, it is well established that reactions of Lewis acid-activated aldehydes and ketones with silyl enolates afford -hydroxy or /7-sil-oxy carbonyl compounds (Mukaiyama aldol reactions). Occasionally, however, ene-type adducts, that is /-siloxy homoallyl alcohols, are the main products. The first example of the carbonyl-ene reaction of silyl enolates was reported by Snider et al. in 1983 [176]. They found that the formaldehyde-MesAl complex reacted smoothly with ketone TMS enolates to give y-trimethylsiloxy homoallyl alcohols in good yield. Yamamoto et al. reported a similar reaction of formaldehyde complexed with methylaluminum bis(2,6-diphenylphenoxide) [177]. After these early reports, Kuwajima et al. have demonstrated that the aluminum Lewis acid-promoted system is valuable for the ene reactions of several aldehydes [178] and for-maldimine [179] with silyl enolates bearing a bulky silyl group. A stepwise mechanism including nucleophihc addition via an acyclic transition structure has been proposed for the Lewis acid-promoted ene reactions. 

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