Selective Zinc-Catalyzed 1,2-hydroboration of N-heteroaromatics via a Non-Hydride Mechanism |
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Authors: | Dinmukhamed Shakhman Dr Anton Dmitrienko Prof Melanie Pilkington Prof Georgii I Nikonov |
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Institution: | Chemistry Department, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, L2S 3 A1 Canada |
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Abstract: | A novel bidentate amine-imine ligand precursor LH has been synthesized. This compound was reacted with ZnMe2 to generate the zinc methyl complex, LZnMe ( 4 ). The latter compound was fully characterized by NMR spectroscopy and single crystal X-ray diffraction. Compound 4 is a catalyst for the hydroboration and hydrosilylation of N-heterocycles, but with moderate catalytic activity. A more active catalyst, the zinc hydride complex LZnH ( 5 ) was synthesized by reacting the lithium salt LLi with ZnCl2 followed by sequential reaction with tBuOK and PhMeSiH2. Compound 5 catalyzes the selective 1,2-hydroboration of nitrogen heteroaromatics with decreased catalyst load and under mild conditions. Deuterium-labeling experiments and kinetic studies provided insight into the possible reaction mechanism. It is proposed that hydride transfer to the substrate proceeds directly from the reductant (borane) via a six-membered transition state facilitated by the catalyst, in which it plays an ambiphilic role, activating the substrate via coordination to the Lewis acidic zinc and enhancing the hydricity of the borane through coordination to the zinc hydride. |
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Keywords: | Catalysis Hydroboration Hydrosilylation Regioselectivity Zinc |
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