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Transfer hydrogenation of ortho-hydroxybenzophenone ketimines catalysed by BINOL-derived phosphoric acid occurs by a 14-membered bifunctional transition structure

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Abstract

Chiral BINOL-derived phosphoric acids catalyse the transfer hydrogenation of ketimines using Hantszch esters. In many cases the nitrogen on the imine binds to the catalyst through the catalyst hydroxyl group and the nucleophile forms a second hydrogen bond to the phosphoryl oxygen. DFT and ONIOM calculations show that the introduction of an ortho-hydroxyaryl group on the carbon atom of the ketimine leads the reaction to proceed through a 14-membered bifunctional mechanism. The transition states of these reactions involve both hydrogen bonding from the hydroxyl group on the imine and the nucleophile's proton to the phosphate catalyst. This mechanistic pathway is lower in energy than the conventional route, consistent with the experimentally observed increased rates of reaction relative to imines that are not derived from ortho-hydroxybenzophenone. To complement the high-level calculations, an accessible qualitative model has been developed that predicts the correct sense of stereoinduction for all examples.

Graphical abstract: Transfer hydrogenation of ortho-hydroxybenzophenone ketimines catalysed by BINOL-derived phosphoric acid occurs by a 14-membered bifunctional transition structure

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Publication details

The article was received on 02 Jun 2017, accepted on 28 Jul 2017 and first published on 28 Jul 2017


Article type: Paper
DOI: 10.1039/C7OB01345K
Citation: Org. Biomol. Chem., 2017, Advance Article
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    Transfer hydrogenation of ortho-hydroxybenzophenone ketimines catalysed by BINOL-derived phosphoric acid occurs by a 14-membered bifunctional transition structure

    J. P. Reid and J. M. Goodman, Org. Biomol. Chem., 2017, Advance Article , DOI: 10.1039/C7OB01345K

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