Mechanism of the Ti(III)-Catalyzed Acyloin-Type Umpolung: A Catalyst-Controlled Radical Reaction.
J Am Chem Soc
; 137(45): 14396-405, 2015 Nov 18.
Article
em En
| MEDLINE
| ID: mdl-26495878
ABSTRACT
The titanium(III)-catalyzed cross-coupling between ketones and nitriles provides an efficient stereoselective synthesis of α-hydroxyketones. A detailed mechanistic investigation of this reaction is presented, which involves a combination of several methods such as EPR, ESI-MS, X-ray, in situ IR kinetics, and DFT calculations. Our findings reveal that C-C bond formation is turnover-limiting and occurs by a catalyst-controlled radical combination involving two titanium(III) species. The resting state is identified as a cationic titanocene-nitrile complex and the beneficial effect of added Et3N·HCl on yield and enantioselectivity is elucidated chloride coordination initiates the radical coupling. The results are fundamental for the understanding of titanium(III)-catalysis and of relevance for other metal-catalyzed radical reactions. Our conclusions might apply to a number of reductive coupling reactions for which conventional mechanisms were proposed before.
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MEDLINE
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En
Ano de publicação:
2015
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Article