关键词: 有机胺盐酸盐, 三(五氟苯基)硼, 硅烷, 炔烃, 硼氢化反应

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.

Key words: ammonium chloride, B(C₆F₅)₃, silane, alkyne, hydroboration