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A N-Heterocyclic Carbene-Supported Zinc Catalyst for the 1,2-Regioselective Hydroboration of N-Heteroarenes
Sumana Mondal1, Tejender Singh2, Sudip Baguli1
1Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, West Bengal, 741246, India.
Abstract:
A fluorenyl-tethered N-heterocyclic carbene LH (LH=[(Flu)H-(CH2 )2 -NHCDipp ]) and its monoanionic version L- are explored in complexation with zinc towards the hydroboration of N-heteroarenes, carbonyl, ester, amide, and nitrile under ambient condition. The N-heteroarenes exhibit high 1,2-regioselectivity which is justified by computational analyses. The relative hydroboration rates of differently p-substituted (electron donating vs. withdrawing) pyridines are also addressed. The monodentate LH offers a better catalytic activity than the chelating L- for steric reasons despite both giving three-coordinate zinc complexes. The mechanism involves a Zn-H species at the heart of these catalytic processes which is trapped by Ph2 CO. Computational studies suggest that the barrier to form the hydride complex is comparable to the barrier required for the following hydride transfer to pyridine.
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