The NHC-catalysed benzoin condensation has been studied mechanistically by a combination of experimental and computational chemistry. The presented EPR-spectroscopic and computational data provide evidence for a radical pair as a potential second key-intermediate that is derived from the Breslow-intermediate via an SET process.

Mechanistic details of transformations catalyzed by N‐heterocyclic carbenes (NHC) are currently of great interest, targeting questions on the active catalyst in operation and the structure and reactivity of key intermediates. These mechanistic studies are driven by the need to understand the big impact of subtle changes on the catalyst system on its reactivity, as well as to clarify the situation around the elusive intermediates in the classical catalytic cycle. In the activation process of aldehydes the formation of an enaminol structure, coined the Breslow intermediate, is widely accepted. Previously, however, we could prove that this enaminol has, besides its ionic reaction pathway, also the option to follow a radical pathway, by undergoing redox processes with the starting material. In the present study we aim to elucidate if these radical pathways are a more general phenomenon, specifically in the formal hydroacylation reaction of alkynes. The observation of radical species and computational analysis of key elemental steps in the radical and ionic pathways may lead to an onward development of chemistry based on NHC‐stabilized carbon radicals.