Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 14, Issue 35, Pages 11076-11081Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200801568
Keywords
ligand design; pH-dependent catalyst; substituent effects; transfer hydrogenation; water chemistry
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Funding
- Sumitomo Foundation
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Transfer hydrogenation catalyzed by an iridium catalyst with 4,4'dihydroxy-2,2'-bipyridine (DHBP) in an aqueous formate solution exhibits highly pH-dependent catalytic activity and chemoselectivity. The substantial change in the activity is due to the electronic effect based on the acid-base equilibrium of the phenolic hydroxyl group of DHBP. Under basic conditions, high turnover frequency values of the DHBP complex, which can be more than 1000 times the value of the unsubstituted analogue, are obtained (up to 81000 h(-1) at 80 degrees C). In addition, the DHBP catalyst exhibits pH-dependent chemoselectivity for alpha,beta-unsaturated carbonyl compounds. Selective reduction of the C=C bond of enone with high activity are observed under basic conditions. The ketone moieties can be reduced with satisfactory activity under acidic conditions. In particular, pH-selective chemoselectivity of the C=O versus C=C bond reduction was observed in the transfer hydrogenation of cinnamaldehyde.
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