Journal Article Mechanistic Study on Highly Efficient Direct 1,2‐Carboboration of Alkynes with 9‐Borafluorenes

庄子, 良晃  ,  Shoji, Yoshiaki  ,  竹ノ内, 久美子  ,  Takenouchi, Kumiko  ,  福島, 孝典  ,  Fukushima, Takanori

Description
We recently reported a new one‐pot transformation reaction of alkynes into 9,10‐diarylphenanthrene derivatives, which proceeds through efficient catalyst‐free 1,2‐carbobration of alkynes with 9‐chloro‐9‐borafluorene that yields a chlorodibenzoborepin, followed by oxidative deborylation/C‐C coupling of the resultant chlorodibenzoborepin. Based on new experimental observations for the catalyst‐free 1,2‐carboboration using diphenylaceylenes and 1Br or 1OTf as well as results from theoretical investigations, here 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 diphenylaceylene with the borafluorenes can be described as a second‐order reaction. The reaction rates became larger with increasing the acceptor numbers of the borafluorenes, evaluated by the Gutmann‐Beckett method. Interestingly, thermodynamic parameters indicated that activation entropy term, rather than activation enthalpy term, largely contributes to the reaction rate. This result was also supported by DFT calculations. Overall, among the borafluorenes examined, 1Br exhibited the highest reactivity toward a wide variety of substituted diarylacetylenes. Similar to the case of chlorodibenzoborepin, when the dibenzoborepin obtained from 1Br or 1OTf was oxidized using FeCl3, an efficient deborylation/C‐C coupling took place to give the corresponding 9,10‐diarylphenanthrene derivatives in high yields.

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