STEREOSELECTIVE PROTONATION OF CARBANIONS .1. DIASTEREOSELECTIVE PROTONATION OF C-H-ACTIVATED CYCLOHEXANE DERIVATIVES

The diastereoselectivity of the kinetically controlled protonation of carbanions derived from 4-tert-butyl-1-X-cyclohexanes (1: X = CN, 2: X = CO2Me, 2-Si: X = C(OMe)OSiMe3, 3: X = COPh, 4: X = SO2Ph) is systematically investigated. By variation of (a) the base for deprotonation of 1-4, (b) OH-, NH-, and CH-proton sources, (c) added salts, Lewis bases, and acids, (d) solvents (cf. Tables 2-5) the cis/trans ratios of diastereomers could be influenced as follows: 1: 41/59 --> 85/15; 2: 26/74 --> 73/27; 3: 26/74 --> 93/ < 3; 4: 1/99 --> 37/63. The results are discussed with regard to structure and aggregation of 1Li to Li-4 and compared with relevant data from the literature. It is concluded that so far dn empirical approach to high diastereoselectivities cannot be avoided and that the results cannot be predicted from the ratios obtained by the much slower alkylation reactions.