Density Functional Study on the Preactivation Scenario of the Dötz Reaction: Carbon Monoxide Dissociation versus Alkyne Addition as the First Reaction Step

Abstract

Two different mechanistic proposals have been investigated theoretically for the initial steps of the chromium-assisted Dötz benzannulation reaction at the nonlocal density functional level of theory. The energy requirements needed for the reaction to start through the usually assumed CO-dissociative mechanism are calculated to be 144.7 kJ mol-1 for (CO)5CrC(OH)(C2H3) (1). Under the mild thermal experimental conditions, the decarbonylation step may be sometimes a serious bottleneck for the reaction to proceed, even to the extent of completely blocking the formation of the subsequent tetracarbonyl compound (CO)4CrC(OH)(C2H3) (5), which is supposed to add an alkyne molecule in the second step. An alternative path is suggested, where the alkyne reacts directly with the saturated metal−carbene complex 1 instead of competing with a CO molecule for a position in the coordination sphere of the metal center in 5. Our calculations reveal that if alkyne addition takes place in 1 before CO loss, then the initial process becomes clearly exothermic (−163.4 kJ mol-1). Moreover, this alternative path makes it easier for CO dissociation to occur as the second step because at this point the complex has more internal energy than 1 to expel a CO ligand. From a comparison of both mechanistic routes, it is concluded that the Dötz reaction better proceeds initially through an associative step.