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Studies on Rh(III)-catalyzed[5+2]/[3+2]cycloadditions of 2-hydroxystyrenes with alkynes
Received date: 2015-04-02
Revised date: 2015-04-13
Online published: 2016-01-15
Recently, Seoane et al. reported two new types of Rh(III)-catalyzed cycloadditons:1) the hetero-[5+2] cycloaddition of 2-hydroxystyrenes with alkynes leading to benzoxepines and 2)the carbo-[3+2] cycloaddition of 2-alkenylphenols with alkynes leading to spirocyclic products. Interestingly,in the two reactions the same catalyst and similar substrates were used but completely different products were obtained. To gain insight into the catalytic mechanisms and to unravel the origins of chemoselectivity differences, we herein present a comparative DFT mechanistic study on the two representative reactions. The results show that both cycloadditions consist of four steps,O—H deprotonation, C—H bond activation, alkyne migratory insertion, and reductive elimination. The reductive elimination step controls the chemoselectivity. When the steric effect is greater than the dearomatization effect, the reaction leads to the five-membered spirocyclic product. Otherwise the reaction produces the seven-membered heterocyclic product. We also found that the regioselectivity of C—H bond activation is determined by the ring tension in the transition states of C—H bond activation and alkyne insertion.
Key words： cycloaddition; DFT computation; Rh-catalysis
TAO Yuan , DANG Yanfeng , WANG Zhixiang . Studies on Rh(III)-catalyzed[5+2]/[3+2]cycloadditions of 2-hydroxystyrenes with alkynes[J]. Journal of University of Chinese Academy of Sciences, 2016 , 33(1) : 57 -64 . DOI: 10.7523/j.issn.2095-6134.2016.01.009
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