In recent years, the field of homogeneous catalysis has seen the emergence of new classes of robust and versatile catalysts featuring ambiphilic stable carbenes. Not only have carbenes successfully challenged the stronghold of phosphine ligands in industrially relevant organometallic systems (e.g. for olefin metathesis), they can also be potent organocatalysts in their own right. Recently, carbenes have even been shown to undergo oxidative addition and reductive elimination of enthalpically strong E-H bonds (E = B, N, Si, P), reactions conventionally reserved for transition metals. This suggests that carbenes could eventually replace transition metals altogether. Since oxidative addition and reductive elimination at carbon are still in their infancy, much remains to be understood from a structure vs. reactivity standpoint. Here, we provide an experimental process to compare the Brönsted basicity of carbenes through exchange reactions. From this study, we demonstrated that the most basic type of carbenes identified, the cyclic-bentallenes, can be more basic than Verkade proazaphosphatrane and even Schwesinger phosphazene P<sub>4</sub>(<sup>t</sup>Bu). Additionally, this ranking can be viewed as a comparison of the strength of the σ-donor character of the carbenes since the exchange process is based on the nucleophilic attack of the electron pair of a free carbene. That allows us to understand the impact of structural motifs for E-H bond activation, an aspect overlooked by other methods.

Experimental comparison of carbene Brönsted basicity, a proxy for σ-donating properties

ACS Spring 2022

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