Theoretical Insight into Ligand- and Counterion-Controlled Regiodivergent Reactivity in Synthesis of Borylated Furans: 1,2-H vs 1,2-B Migration

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Abstract

Divergent catalytic reactions provide quick access to diverse molecular scaffolds through controlled reaction pathways by tuning the catalytic conditions, especially changing auxiliary ligands and/or counterions. This study presents a computational study of Au(I)-catalyzed regiodivergent cycloisomerizations of boron-containing alkynyl epoxides toward C2- and C3-borylated furans. The calculations clarified the mechanistic details of the reaction and the regiodivergence induced by two gold catalysts. The proposed catalytic cycle involves two major stages: ring expansion of alkynyl epoxide accompanied by B(MIDA) group migration to form a m-boron furyl heterocyclic intermediate and a formal 1,2-H or a 1,2-B migration to form a C3- or C2-borylated furan. The former is identified as the bottleneck of the reaction, which proceeds via σ activation of the oxirane moiety rather than π activation of the alkynyl moiety proposed in experimental work, and the latter controls the regiodivergence. Calculations show that the counterion and ligand in the gold catalyst play a less important role in the ring expansion, but they are crucial for the divergent formation of borylated furans: an OTf– counterion with an (ArO)3P ligand favors the 1,2-H migration, leading to the formation of C3-borylated furan, whereas an SbF6– anion with an IPr ligand promotes the 1,2-B migration, supporting the predominant formation of C2-borylated furan. The theoretical results rationalize the observed regioselectivity and provide key insights into the mechanism of the migratory cycloisomerization of boron-containing alkynyl epoxides.

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