Theoretical Insight into the Au(I)-Catalyzed Intermolecular Condensation of Homopropargyl Alcohols with Terminal Alkynes: Reactant Stoichiometric Ratio-Controlled Chemodivergence.

ABSTRACT

The mechanisms and chemoselectivities on the Au(I)-catalyzed intermolecular condensation between homopropargyl alcohols and terminal alkynes were investigated by performing DFT calculations. The reaction was indicated to involve three stages transformation of the homopropargyl alcohol (R1) via intramolecular cyclization to the cyclic vinyl ether (R1'), formation of the C-2-arylalkynyl cyclic ether (P1) via hydroalkynylation of R1' with phenylacetylene (R2), and conversion from P1 to 2,3-dihydro-oxepine (P2). The results revealed the origin of the reaction divergence and rationalized the experimental observations that a 13 reactant stoichiometric ratio affords P1 as the major product, whereas the 11.1 ratio results in P2 in high yield. The reactant stoichiometric ratio-controlled divergent reactivity is attributed to different catalytic activities of the gold catalyst toward different reaction stages. In the 13 situation, the excess R2 induces the Au catalyst toward its dimerization and/or hydration, inhibiting the conversion of P1 to P2 and resulting in product P1. Without excess R2, the Au catalysis follows a general cascade reaction, leading to product P2. Theoretical results described a general strategy controlling the reaction divergence by a different reactant stoichiometric ratio. This strategy may be enlightening for chemists who are exploring various synthesis methods with high chemo-, regio-, and enantioselectivities.