Higher-order cyclization reactions of alkenyl Fischer carbene complexes: a new selective all-carbon [8 + 2] cyclization with 8-methoxyheptafulvene and computational mechanistic analysis

Abstract

A new higher-order cyclization reaction of alkenyl Fischer carbene complexes is described. Chromium and tungsten alkenyl Fischer carbene complexes react toward 8-methoxyheptafulvene through an all-carbon formal [8 + 2] cycloaddition reaction with complete regio- and stereoselectivity. Tetrahydroazulene compounds bearing four consecutive stereocenters are generated. The reaction mechanism is rationalized based on computational calculations. It was found that this transformation proceeds through a concerted process. The nature of the observed stereo- and regioselectivity can be attributed to both steric and electronic factors.

Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher‐Order Cycloaddition Reactions

“…The metal fragment plays an important role in the stabilization of the transition state. Hence, steric and electronic factors apparently induce the observed regio‐ and stereoselectivity [155] …”

“…FCCs containing group VI transition metals [29,31–33] are very versatile in cyclization and cycloaddition reactions with an α,β‐unsaturated system under different conditions, [29] resulting in carbocycles, [34–39] heterocycles [29,40–43] and a diversity of other compounds. Chromium carbene complexes have shown particularly suitable reactivity, compared to the greater stability of tungsten and the extreme reactivity of molybdenum complexes [44–47] .…”