Benzannulation and Cyclopentannulation Reactions of Electron-Poor Amino-Stabilized Alkenyl Fischer Carbene Complexes

The alkenyl(amino)carbene complexes of chromium and tungsten bearing an electron-withdrawing group at the Cβ-position 5–6 were prepared by condensation of methyl(amino)carbene complexes 1–2 and esters of glyoxylic acid. The complexes 5–6 were readily α-metallated with n-BuLi affording the deuteriated metal carbene complexes 7–8. The chromium complexes 5, 7 display a remarkable reactivity toward different types of alkynes. These complexes reacted with both unactivated alkynes and terminal electron-poor alkynes to give exclusively the phenol products 11–13 via the benzannulation reaction (Dötz reaction). In contrast, the reaction of carbene complexes 5 with internal electron-poor alkynes yielded solely the cyclopentadiene derivatives 15 via the cyclopentannulation reaction. The tetracarbonyl carbene intermediates 16 were isolated and fully characterized.