The mechanism and origins of and selectivity of cross-benzoin reactions between furfural and α-amino aldehydes, catalyzed by a triazolium-based NHC, were investigated using density functional theory calculations. -Boc-α-amino aldehydes were found to react with selectivity, while -Bn--Boc-α-amino aldehydes react with selectivity. We find that the product is more thermodynamically favored than the product for reactions with -Boc-α-amino aldehydes, and that the formation of the product for reactions involving -Bn--Boc-α-amino aldehydes is kinetically favored. The switch in selectivity is a result of an intramolecular hydrogen bond in the -Boc-α-amino aldehyde, whereas switching to -Bn--Boc-α-amine removes the hydrogen bond. The steric and electronic interactions in the transition state are rationalized by a Felkin-Anh model.
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