Alkyne–Carbene Chelate Complexes of Chromium: Arrested Intermediates in the Benzannulation Reaction and Precursors of Densely Functionalized Centrosymmetric Chrysenes

Abstract

AbstractTetracarbonyl‐π2‐alkyne–carbene chromium(0) complexes 3a–c stabilized by a rigid C2‐arene unit bridging the alkyne and the carbene ligands were synthesized from their pentacarbonyl precursors 2a–c by low‐temperature photodecarbonylation. The chelates 3a–c are the first stable alkyne–carbene complexes containing a Group VIB metal carbonyl unit, and can be regarded as arrested intermediates in the chromium‐mediated benzannulation reaction. Comparative X‐ray and 13C NMR studies indicate only weak coordination of the alkyne, both in the solid state and in solution. Decomposition of chelates 3a–c as well as the thermal decarbonylation of their pentacarbonyl precursors 2a–c results in dimerization of the alkynylcarbene ligands to give densely substituted oxygenated chrysene derivatives. A modification of the substitution pattern in the central chrysene rings results in a gradual distortion of the arene system across the central CC arene bond.