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Refined Transition-State Models for Proline-Catalyzed Asymmetric Michael Reactions under Basic and Base-Free Conditions

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Title Refined Transition-State Models for Proline-Catalyzed Asymmetric Michael Reactions under Basic and Base-Free Conditions
 
Creator SHARMA, AK
SUNOJ, RB
 
Subject DENSITY-FUNCTIONAL THEORY
INTRAMOLECULAR ALDOL REACTIONS
ENERGY DIFFERENCES
ENAMINE
MOLECULES
ORGANOCATALYSIS
STEREOSELECTIVITY
THERMOCHEMISTRY
OXAZOLIDINONES
SELECTIVITIES
 
Description The stereocontrolling transition state (TS) models for C-C bond formation relying on hydrogen bonding have generally been successful in proline-catalyzed aldol, Mannich, alpha-amination, and alpha-aminoxylation reactions. However, the suitability of the hydrogen-bonding model in protic and aprotic conditions as well as under basic and base-free conditions has not been well established for Michael reactions. Through a comprehensive density functional theory investigation, we herein analyze different TS models for the stereocontrolling C-C bond formation, both in the presence and absence of a base in an aprotic solvent (THF). A refined stereocontrolling TS for the Michael reaction between cyclohexanone and nitrostyrene is proposed. The new TS devoid of hydrogen bonding between the nitro group of nitrostyrene and carboxylic acid of proline, under base-free conditions, is found to be more preferred over the conventional hydrogen-bonding model besides being able to reproduce the experimentally observed stereochemical outcome. A DBU-bound TS is identified as more suitable for rationalizing the origin of asymmetric induction under basic reaction conditions. In both cases, the most preferred approach of nitrostyrene is identified as occurring from the face anti to the carboxylic acid of proline-enamine. The predicted enantio- and diastereoselectivities are in very good agreement with the experimental observations.
 
Publisher AMER CHEMICAL SOC
 
Date 2014-10-15T08:31:21Z
2014-10-15T08:31:21Z
2012
 
Type Article
 
Identifier JOURNAL OF ORGANIC CHEMISTRY, 77(23)10516-10524
http://dx.doi.org/10.1021/jo3023654
http://dspace.library.iitb.ac.in/jspui/handle/100/14689
 
Language en