Role of Donor–Acceptor Cyclopropenes in Metal Carbene Reactions. Conversion of <i>E</i>-Substituted Enoldiazoacetates to <i>Z</i>-Substituted Metallo-Enolcarbenes

Abstract

The influence of geometrical isomers of silyl-protected γ-substituted enoldiazoacetates has been examined in transition-metal-catalyzed vinylcarbene cycloaddition reactions. These reactions often occur with the intervention of donor–acceptor (D-A) cyclopropenes that can serve as metal carbene sources. Pathways to cycloaddition products that occur with and without D-A cyclopropene involvement have been identified. E-γ-Substituted enoldiazoacetates do not undergo cycloaddition reactions unless they first form D-A cyclopropene intermediates. When cycloaddition reactions occur from the metallocarbene only after formation of the D-A cyclopropene, E-γ-substituted enoldiazoacetates are converted to Z-γ-substituted metallo-enolcarbenes, and both geometrical isomers of silyl-protected γ-substituted enoldiazoacetates result in the same product selectivity.