Development and application of selective olefin synthesis utilizing the unique features of the silicon atom

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(1)The Peterson reaction between (t-BuO)Ph_2SiCH_2CN and not only aliphatic aldehydes but also aromatic aldehydes proceeded with high Z-selectivity in the range of 92:8 to >99:1.(2)In the Horner-Wadsworth-Emmons reaction between (PhO)_2P(O)CH_2C(O)NRR' and aldehydes, it was found that high Z-selectivity could be achieved in some cases. Furthermore, it was found that amide reagent (PhO)_2P(O)CH_2C(O)NHR bearing a free hydrogen upon the nitrogen atom could be used for olefination in contrast to the Peterson reagent Ph_3Si CH_2C(O)NHR.(3)It was found that β-monosubstituted-α,β-unsaturated Weinreb amides, prepared from either (CF_3CH_2O)_2P(O)CH_2C(O)N(OMe)Me or (o-TolylO)_2P(O)CH_2C(O)N(OMe)Me, react with organocerium reagents with hardly any isomerization of the double bond to efficiently yield the corresponding β-monosubstituted-α,β-unsaturated ketone. For the preparation of β-monosubstituted-α,β-unsaturated Weinreb amides, the Still type reagent was more appropriate for aromatic aldehydes whereas the Ando type reagent was better for aliphatic reagents.(4)In order to elucidate the factors leading to high Z-selectivity in the reaction of (t-BuO)Ph_2SiCH_2CN, a mechanistic examination was carried out using model compounds RCH(OSiR'_3)CHMeCN. The anti compounds gave the z-olefins exclusively upon treatment with base whereas the syn compounds lead to mixtures. The former results imply that the olefin ratio is determined primarily by the first addition step, while the latter suggests that sterics plays a major role in determining the favored conformation for deprotonation in E1cb reactions.