Cuᴵᴵ-katalysierte Kupplung von Cyclopropyl-carbenoiden

Abstract

Contrary to the reaction of 'Hückel anions' like cyclopentadienide or cyclononatetraenide, treatment of 1-bromo-1-lithio-carbenoids 2 with 1 mol-equiv. of CuCl₂ in THF does not result in an oxidative coupling 2 → 8 (see Scheme 2), but in a formal dimerization of two carbenes to give bi(cyclopropylidenes) 10. Reaction parameters influencing the yields are discussed in the case 2a → 10a (R=PhS). Yields of ca. 40% of 10a are obtained under 'kinetic control' at high concentrations of 2a and in THF/Et₂O mixtures. Besides CuCl₂, which is catalytically active, FeCl₃ has been found to be a coupling agent as well, while AgBF₄, AgCl, CoF₃, and CuCl were inactive in THF. Mixed couplings of two different carbenoids are possible, while diastereoselectivity of the active transition metal complex seems to be low. First experimental results obtained with 2b (R=BuO) and 2c (R=PhCH₂) hint at a considerable scope of the sequence 2 → 10.