An investigation of the mechanism of the rhodium-catalyzed [5 + 1 + 2 + 1] cycloaddition reaction of vinylcyclopropanes, terminal alkynes and carbon-monoxide

Abstract

Spectroscopic investigations on the [RhCl(CO)[subscript 2]][subscript 2] catalysed [5 + 1 + 2 + 1] cycloaddition of VCP, CO and 4-ethynylbiphenyl were performed and the existence of an off-cycle species was revealed. This species that forms at high Rh-catalyst loading mitigates cycloadduct formation even at elevated temperatures and long reaction periods. Furthermore, the mechanism for the conversion of the inactive species to an active species was hypothesised, as informed by previous computational studies. The starting material for the spectroscopic study, VCP, was synthesised by a modified procedure. The reaction of 2-methoxyethanol with 1,3-butadiene in toluene was followed by the isolation of the resulting brominated intermediate. Subsequently, purification of the intermediate, followed by an elimination reaction afforded 2-(2-methoxyethoxy)-1,3-butadiene. A modified vacuum distillation method was used to purify the diene and subsequently, TLC monitored Simmons-Smith cyclopropanation was performed on the diene. The resulting VCP was also purified by the same method employed for the diene.