Nickel(0)-mediated [3+2+2] and [2+2+2+1] cyclization reactions of chromium Fischer carbene complexes and alkynes

Dramatic changes in Fischer carbene reactivity occur in the presence of [Ni(cod)2]. Thus, instead of Dötz [3+2+1] benzannulation giving substituted phenols, alkenyl carbene complexes incorporate two equivalents of alkyne in a [3+2+2] cyclization, while simple chromium carbene complexes undergo a [2+2+2+1] cyclization (see scheme). These cyclizations are regio- and diastereoselective. cod = 1,5-cyclooctadiene.