Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-tert-butyliminoborane and Alkynes
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- 作者：Marius Schäfer ; Nicholas A. Beattie ; K. Geetharani ; Julian Schäfer ; William C. Ewing ; Mirjam Krahfuß ; Christian Hörl ; Rian D. Dewhurst ; Stuart A. Macgregor ; Christoph Lambert ; Holger Braunschweig
- 刊名：Journal of the American Chemical Society
- 出版年：2016
- 出版时间：July 6, 2016
- 年：2016
- 卷：138
- 期：26
- 页码：8212-8220
- 全文大小：530K
- 年卷期：0
- ISSN：1520-5126
文摘
Di-tert-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl(PiPr3)2]2, which gives a rhodium η4-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B–C bond coupling en route to the rhodium η4-1,2-azaborete complex and the selective oxidative cleavage of the B–N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene.