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Alcohol Oxidation by Dioxygen and Aldehydes Catalysed by Square‐Planar Cobalt(III) Complexes of Disubstituted Oxamides and Related Ligands
Author(s) -
Fernández Isabel,
Pedro José R.,
Roselló Antonio L.,
Ruiz Rafael,
Castro Isabel,
Ottenwaelder Xavier,
Journaux Yves
Publication year - 2001
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200104)2001:7<1235::aid-ejoc1235>3.0.co;2-4
Subject(s) - chemistry , catalysis , cobalt , medicinal chemistry , alcohol oxidation , substituent , acetonitrile , ligand (biochemistry) , bond cleavage , hydride , redox , photochemistry , organic chemistry , hydrogen , biochemistry , receptor
The square‐planar cobalt(III) complexes of o ‐phenylenebis( N′ ‐methyloxamidate) (Me 2 opba) and related oxamate (Meopba) and bis(oxamate) (opba) ligands catalyse the selective oxidation, by dioxygen and pivalaldehyde, of a wide range of secondary alcohols to the corresponding ketones, in good yields and under mild conditions in acetonitrile at room temperature. Thus, the oxidation of the series of α‐alkylbenzyl alcohols PhCH(OH)R (R = Me, Et, i Pr, t Bu) results in the exclusive formation of ketones as a product of C−H bond cleavage, and no C−C bond cleavage products are observed in any case. The modulation of catalytic activity by ligand substituents among this series of cobalt catalysts highlights the role of oxocobalt(IV) species as the putative intermediates in these aerobic alcohol oxidation reactions. Relative reactivities for the oxidation of 1‐phenylethanol increase in the order Me 2 opba < Meopba < opba, which correlates with the oxidising power of the postulated Co IV =O intermediate. In contrast, competitive reaction studies on the oxidation of the series of para ‐substituted 1‐phenylethanol derivatives XPhCH(OH)CH 3 (X = H, OMe, Br, CF 3 , NO 2 ) show little variation in relative rate ( k X / k H ) with the para substituent and no correlation with Hammett σ parameters. Overall, these results are more in line with a mechanism involving concerted hydride transfer in the oxidation of alcohols by high‐valent oxocobalt species (rather than a hydrogen atom transfer or an electron transfer mechanism).