Thiocarbonyl Ylides and Electrophilic Azo Compounds; Sterically Hindered Cyclic Hydrazodicarboxylic Esters

摘要

The cycloadduct 5, prepared from adamantanethione S-methylide with dimethyl azodicarboxylate, exists is solution in two conformations P and Q (70:30 in CDCl_3), which are separated by a barrier of #DELTA#G not= 18.3 +- 0.6 kcal mol~(-1); according to NMR spectra, both forms are chiral. Steric hindrance to 3'N-CO_2CH_3 rotation by the adamantane system generates C=O outside and C=O inside conformations. The barrier to enantiomerization (AB -> A_2 for 5'-H_2) is higher (> 20.1 kcal mol~(-1)). There combinations of rate processes are discussed. Except for the 5'-methyl compound 19a, the chiral cycloadducts of four other thiocarbonyl ylides to dimethyl azodicarboxylate do not show diastereoisomeric conformations in their NMR spectra at ambient temperature.