Die Reaktion von 1‐Halogeno‐1‐Lithiocyclopropanen mit CuCl2: Konkurrenz zwischen ‘Carben‐Dimerisierung’ und oxidativer Kupplung

Abstract

The Reaction of 1‐Halogeno‐1‐lithiocyclopropanes with CuCl2: Competition between ‘Carbene Dimerization’ and Oxidative CouplingThe 1‐chloro‐1‐lithiocyclopropanes 2a–d react at low temperature with CuCl2 to give diastereoisomeric mixtures of oxidative‐coupling products 5a–d and of ‘carbene dimers’ 6a–d. The relative amount of 5a–d increases with CuCl2 concentration and reaction time. Diastereoselectivity of the reaction seems to be low, and separation as well as spectroscopic structure assignment of single diastereoselectivity of the reaction seems to be low, and separation as well as spectroscopic structure assignment of single diastereoisomers are difficult. The conformational behavior of 1,1′‐dichloro‐1,1′‐bi(cyclopropyls) 5c and 5d is discussed. Contrary to 2a–d, 1‐bromo‐1‐lithiocyclopropanes normally react with CuCl2 to give ‘carbene dimers’ 6 and no coupling products 5. So far the only exception is 1‐bromo‐1‐lithio‐2‐phenylcyclopropane 2e which in the presence of CuCl2 gives some percents of coupling products 5e besides carbene dimers 6b as main products. An X‐ray structure analysis of the predominant diastereoisomer 5e was performed.