Desilylation of TBDMS Ethers

A wide variety of Arrhenius acids, Lewis acids, transition metal compounds, and Lewis bases have been used to cleave the Si-0 bond in TBDMSOR ethers for parent alcohol regeneration [ 148]. It does not appear, however, that the use of a nonionic base in such transformations has been reported. 

TBDMS ethers of primary, secondary, and tertiary alcohols and also phenolic TBDMS ethers are desilylated to their corresponding alcohols and phenols, respectively, in DMSO at 80 °C in 68 - 94 % yield in the presence of 0.2 - 0.4 equivalents of lb [ 148]. Using Id as the catalyst, 85 - 97 % yields of desilylated alcohols were obtained from TBDMS. Both catalysts were much less effective for the desilylation of TBDPS ethers (22 - 45 % yield) under the same conditions as was used for TBDMS ethers. Desilylations were faster in DMSO than in acetonitrile. 

The addition of cyanotrialkylsilanes to carbonyl compounds, and especially the development of catalysts for this transformation continues to attract considerable attention owing to the role of cyanohydrin trialkysilyl ethers and cyanohydrins as versatile intermediates in organic synthesis [149]. A variety of catalysts has been developed for such reactions including Lewis acids, transition metal complexes, 18-crown-6 complexes of alkali metals, tetracyanoethylene, Lewis bases, and alkali earth bases. Trimethylcyanation of ketones using Lewis bases does not seem to have been mentioned in the literature. 

Pro-azaphosphatrane lb is an effective catalyst for the addition of tri-methylsilyl cyanide to aldehydes and ketones under mild conditions, giving rise to cyanohydrins and cyanohydrin silyl ethers, respectively, in moderate to high yields, Eqs. (24) and (25) [149]. 

Trimethylcyanation of both aromatic and aliphatic ketones in the presence of lb proceeded smoothly at room temperature to give the corresponding cyanohydrin silyl ethers in moderate to high yields. a,j8-Unsaturated ketones gave 1,2-addition products regioselectively, and no 1,4-adducts were detected. With 4-t-butylcyclohexanone and (- )-menthone, yields were high but diastere-oselectivity was poor. With (ll )-(-F)-camphor, however, the yield was low but diastereoselectivity was excellent. 

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