Research of the Stereoselectivity and Mechanism of the Hydroboration Reaction Between B(C6F5)3/Ammonium Chloride Systems with Terminal Alkyne

Sun, G.; Min, S.; Fang, J.; Borzov, M.; Nie, W.

Авторы: Sun Guofeng, Min Su, Fang Jie, Borzov Maxim, Nie Wanli
Журнал: Acta Chimica Sinica
Том: 75
Номер: 8
Год издания: 2017
Издательство: SCIENCE PRESS
Местоположение издательства: 300 WEST CHESNUT ST, EPHRATA, USA, PA, 17522
Первая страница: 824
Последняя страница: 830
DOI: 10.6023/A17040141
Аннотация: Stereoselective hydroboration reaction of alkynes has been considered as one of the most important organic reaction. To date a handful of metal-catalyzed systems have been demonstrated to achieve trans-hydroboration of alkynes. This paper describes the first non-metal-catalyzed systems which could stereoselectively hydroborate the terminal alkynes in a trans-configuration. The Lewis acid B(C6F5)3 and ammonium chloride have been used as the reaction substrates, and phenylsilane as the hydride source. The hydroboration reaction could be performed in a one-pot procedure by mixing of B(C6F5)3, ammonium chloride and silane together in an equivalent amount. But this one-pot reaction is not so nice since there is always mixed with the ammonium hydroborate[R2NH2]+[H-B(C6F5)3]- intermediates products. A series of ammonium hydroborates prepared from the corresponding primary, secondary, tertiary and quaternary amine hydrochlorides have been isolated, and used in the directly hydroboration with terminal alkynes. To our surprise the ammonium hydroborate[R2NH2]+[H-B(C6F5)3]- could not react with the alkynes alone. When using[R2NH2]+[H-B(C6F5)3]- to react with alkynes, trace amount of catalytic Lewis acid B(C6F5)3 is necessary to firstly activate the carbon-carbon triple bonds and form the crucial zwitterionic σ-complexes. The mechanism study has shown that different from the typical Lewis acid/Lewis base FLPs system reacted with alkynes, in this B(C6F5)3/ammonium chloride system the ammonium chloride plays an important role on the stereoselective control of the reaction. The week interaction between the Cl ion and B(C6F5)3 in the σ-complexes has not only slowed down the unfavorite 1,1-carboboration reaction, but also stabilized the σ-complexes which has offer the chance for the nucleophilic reagent to attack the reaction center in a cis-or trans-mode. In our experiment the bulky ion[H-B(C6F5)3]-could only attach the active alkynes from the trans-side and form the Z-hydroboration product. This work demonstrates that the combination of the ammonium halides with the Lewis acid B(C6F5)3 could act as a novel "frustrated Lewis pair" to activate alkynes, and will enable the development of even more sophisticated FLP and related catalyzed reactions.
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