The Breslow catalytic cycle describing the benzoin condensation promoted by N-heterocyclic carbenes (NHC) as proposed in the late 1950s has since then been tried by generations of physical organic chemists. Emphasis has been laid on proofing the existence of an enaminol like structure (Breslow intermediate) that explains the observed umpolung of an otherwise electrophilic aldehyde. The present study is not focusing on spectroscopic elucidation of a thiazolydene based Breslow intermediate but rather tries to clarify if this key-intermediate is indeed directly linked with the product side of the overall reaction. The here presented EPR-spectroscopic and computational data provide a fundamentally different view on how the benzoin condensation may proceed: a radical pair could be identified as a second key-intermediate that is derived from the Breslow-intermediate via an SET process. These results highlight the close relationship to the Cannizarro reaction and oxidative transformations of aldehydes under NHC catalysis.

Within the past few decades, it has become apparent that the description of chemical reactions based on statistical models like classical transition state theory (TST) or Rice–Ramsperger–Kassel–Marcus theory (RRKM) does not always sufficiently meet reality when it comes to product selectivity and reaction rates. Besides quantum effects like tunnelling events, there are so-called non-MEP (non-minimum energy path) reaction mechanisms involving reaction path bifurcations, roaming and non-statistical dynamic effects. These might lead to product ratios that are not governed by relative barrier heights of competing pathways and therefore pose a challenge for TST-based theories. This perspective will give an overview of tools—experimental and theoretical ones—which have been developed and utilized so far to identify and analyze such dynamic factors in reaction mechanisms. The tools and methods are explained by an eclectic choice of examples—not providing a full review, of the important and ground-breaking work on dynamic effects on various chemical reactions that has been conducted by the groups of Hase, Singleton, Carpenter, Houk and others.

The Breslow catalytic cycle describing the benzoin condensation promoted by N-heterocyclic carbenes (NHC) as proposed in the late 1950s has since then been tried by generations of physical organic chemists. Emphasis has been laid on proofing the existence of an enaminol like structure (Breslow intermediate) that explains the observed umpolung of an otherwise electrophilic aldehyde. The present study is not focusing on spectroscopic elucidation of a thiazolydene based Breslow intermediate but rather tries to clarify if this key-intermediate is indeed directly linked with the product side of the overall reaction. The here presented EPR-spectroscopic and computational data provide a fundamentally different view on how the benzoin condensation may proceed: a radical pair could be identified as a second key-intermediate that is derived from the Breslow-intermediate via an SET process. These results highlight the close relationship to the Cannizarro reaction and oxidative transformations of aldehydes under NHC catalysis.