Mechanism study on rhodium(III)-catalyzed C[sbnd]H functionalization of o-vinylphenols with alkynes: Regioselectivity and chemoselectivity

Abstract

The mechanism of rhodium(III)-catalyzed CH functionalization of o-vinylphenols (1a) with alkynes has been investigated with DFT calculations. The results suggest the whole reaction is comprised with four stages: (I) OH deprotonation, (II) CH activation induced by Rh(III) catalyst interacting with o-vinylphenols, (III) alkyne coordination and (IV) regeneration of Rh(III) catalyst. The CH activation step proceeds under the concerted metalation deprotonation mechanism. The regioselectivity and chemoselectivity of the reaction also have been discussed, respectively. The regioselectivity for different CH activation sites is depended on the coordination structure of the related Rh(III) complex. In addition, when the substrates are substituted with alkyl (or alkenes) at the terminal position of the alkene, such as (E)-2-(prop-1-en-1-yl)phenol 1b and 2-allylphenol 1c, the apparent activation energies (ΔG) are obviously higher than that of 1a, which indicates that they are unfavorable to occur. The effect of CO2Me substituent on the para position to the hydroxyl (1d) is also investigated.