Mechanistic Study of Highly Efficient Direct 1,2-Carboboration of Alkynes with 9-Borafluorenes

We recently reported a new one-pot transformation of alkynes into 9,10-diarylphenanthrene derivatives, which proceeds through efficient catalyst-free 1,2-carboboration of alkynes with 9-chloro-9-borafluorene (1(Cl)), which yields a chlorodibenzoborepin, followed by oxidative deborylation/C-C coupling of the resultant chlorodibenzoborepin. Herein, based on new experimental observations for the catalyst-free 1,2-carboboration by using diphenylacetylenes and 1(Br) or 1(OTf) as well as results from theoretical investigations, we show how the substituent on the boron atom of 9-borafluorene affects the reactivity toward alkynes. Kinetic studies indicated that the 1,2-carboboration of diphenylacetylene with the borafluorenes can be described as a second-order reaction. The reaction rates became larger with the increase in the acceptor numbers of the borafluorenes (1(Br) > 1(OTf) > 1(Cl)), which was evaluated by the Gutmann-Beckett method based on a Lewis acid/base complexation in solution. Interestingly, thermodynamic parameters obtained experimentally indicated that the term of activation entropy, rather than the term of activation enthalpy, largely contributes to the reaction rate. This experimental result was also supported by DFT calculations. Overall, among the borafluorenes examined, 1(Br) exhibited the highest reactivity toward a wide variety of substituted diarylacetylenes. Similar to the case of chlorodibenzoborepin, when the dibenzoborepin obtained from 1(Br) or 1(OTf) was oxidized by using FeCl3, an efficient deborylation/C-C coupling took place to give the corresponding 9,10-diarylphenanthrene derivatives in high yields.