The environmental concerns, low abundance, and economic constrains regarding noble metals have inspired the studies of earth-abundant and low-cost base-metal alternatives for some catalytic transformations. We have designed and synthesized a series of new PNN- and NNN-types of iron and cobalt complexes 1-4 (Chart 1). Among these complexes, the phosphinite-iminopyridine iron complexes 1 are active for anti-Markovnikov hydrosilylations of alkenes with primary, secondary, and tertiary silanes. The catalyst system exhibits functional group tolerance with reactive groups such as amides, esters, and ketones. In addition, we have prepared a PNN iron complex 2 using Milstein’s bipyridyl phosphine ligand. The electron-rich iron complex is highly active for anti-Markovnikov hydroboration of aliphatic terminal olefins with pinacolborane. Such iron system is far more efficient than noble metal systems for catalytic hydroboration of a-olefins. More recently, we developed the cobalt PNN analogue 3 which can catalyze the hydroboration of both vinylarenes and aliphatic a-olefins. The reactions occur with high turnover numbers (up to 19800), broad functional group tolerance, and excellent regio- and chemoselectivity. The hydroboration reaction is further extended to a one-pot, two-step hydroboration and Suzuki-Miura cross-coupling of alkylboronates with aryl chlorides. Finally, we report cobalt complexes of novel iminopyridine-oxazoline ligands (IPO) 4 for asymmetric hydroboration of 1,1-disubstituted aryl alkenes. The new catalysts afforded α-alkyl-b-pinacolatoboranes in high yields and with excellent enantioselectivity (up to 99.5% ee).

Pincer iron and cobalt complexes for cataltyic alkene hydrofunctionalizations

249th National Meeting (2015)

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