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A Mechanistic Survey of the Oxidation of Alcohols and Ethers with the Enzyme Laccase and Its Mediation by TEMPO
Author(s) -
d'Acunzo Francesca,
Baiocco Paola,
Fabbrini Maura,
Galli Carlo,
Gentili Patrizia
Publication year - 2002
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(200212)2002:24<4195::aid-ejoc4195>3.0.co;2-x
Subject(s) - chemistry , laccase , mediation , organic chemistry , enzyme , combinatorial chemistry , political science , law
Abstract The oxidation of alcohols and ethers by O 2 with the enzyme laccase, mediated by the stable N ‐oxyl radical TEMPO, affords carbonylic products. An ionic mechanism is proposed, where a nucleophilic attack of the oxygen lone‐pair of the alcohol (or ether) onto the oxoammonium form of TEMPO (generated by laccase on oxidation) takes place leading to a transient adduct. Subsequent deprotonation of this adduct α to the C−O bond leads to the carbonylic product. Additional mechanistic considerations for the laccase‐mediated oxidation of ethers and thioethers are offered. The proposed mechanism is supported by: (i) investigating the inter‐ and intramolecular selectivity of oxidation with appropriate substrates, (ii) thermochemical considerations, and (iii) attempting a Hammett correlation for the oxidation of a series of 4‐X‐substituted benzyl alcohols, wherein a shift of the rate‐determining step as a function of the 4‐X‐substituent results. Based on the above points, the lack of mediation efficiency of another stable N ‐oxyl radical (viz., IND‐O · ) can be explained. (© Wiley‐VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2002)