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In this study, we provide a theoretical explanation for the experimentally observed decrease in the organocatalytic activity of
-aryl imidazolylidenes methylated at the C4/5-H positions in the benzoin condensation of aromatic aldehydes. A comparative quantum chemical study of energy profiles for the NHC-mediated benzoin condensation of furfural has revealed a high energy barrier to the formation of the IPr
-based furanic Breslow intermediate that can be attributed to the negative steric interactions between the imidazole backbone methyl groups and
-aryl substituents.