Theoretical study on the mechanism and chemoselectivity in gold(i)-catalyzed cycloisomerization of β,β-disubstituted ortho-(alkynyl)styrenes

Abstract

The mechanism and chemoselectivity of gold(I)-catalyzed cycloadditions of β,β-disubstituted ortho-(alkynyl)styrenes were explored by DFT calculations. Two different ortho-(alkynyl)styrenes, namely β,β-diaryl-o-(alkynyl)-styrene (Series A) and β-methyl-β-phenyl-o-(alkynyl)-styrene (Series B), were selected as model reactants. The obtained calculation results indicate that both model o-(alkynyl)styrenes undergo a similar 5-endo-dig cyclization to generate the gold carbocationic intermediate, whereas different pathways were identified after the formation of this intermediate. In Series A, the proton of the cyclopentene backbone is eliminated to support the crucial benzofulvene intermediate. Finally, benzofulvene undergoes the iso-Nazarov cyclization to accomplish the cascade reaction with a release of the gold(I) catalyst and the formation of the final dihydroindeno[2,1-a]indene product. For Series B, the proton elimination occurs on the methyl group rather than the cyclopentene backbone, resulting in the vinylidene indene. Subsequently, the iso-Nazarov cyclization would give the final product upon addition of an additional Brønsted acid (PTSA: p-toluenesulfonic acid). Furthermore, the effect of the substituent in Series A and the detailed mechanism of the Brønsted acid-catalyzed iso-Nazarov cyclization of Series B were also analyzed. The obtained theoretical results not only well rationalize the experimental observations, but also provide an insight into important details of the title reaction.