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Transition State Models for Probing Stereoinduction in Evans Chiral Auxiliary-Based Asymmetric Aldol Reactions

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Title Transition State Models for Probing Stereoinduction in Evans Chiral Auxiliary-Based Asymmetric Aldol Reactions
 
Creator SHINISHA, CB
SUNOJ, RB
 
Subject activation strain model
density-functional theory
molecular-orbital methods
effective core potentials
gaussian-type basis
titanium enolate
diastereofacial selectivities
thermochemical kinetics
adiabatic connection
facial selectivity
 
Description The use of chiral auxiliaries is one of the most fundamental protocols employed in asymmetric synthesis. In the present study, stereoselectivity-determining factors in a chiral auxiliary-based asymmetric aldol reaction promoted by TiCl(4) are investigated by using density functional theory methods. The aldol reaction between chiral titanium enolate [derived from Evans propionyl oxazolidinone (la) and its variants oxazolidinethione (1b) and thiazolidinethione (1c)] and benzaldehyde is examined by using transition-state modeling. Different stereochemical possibilities for the addition of titanium enolates to aldehyde are compared. On the basis of the coordination of the carbonyl/thiocarbonyl group of the chiral auxiliary with titanium, both pathways involving nonchelated and chelated transition states (TSs) are considered. The computed relative energies of the stereoselectivity-determining C C bond formation TSs in the nonchelated pathway, for both 1a and 1c, indicate a preference toward Evans syn aldol product. The presence of a ring carbonyl or thiocarbonyl group in the chiral auxiliary renders the formation of neutral TiCl(3)-enolate, which otherwise is energetically less favored as compared to the anionic TiCl(4)-enolate. Hence, under suitable conditions, the reaction between titanium enolate and aldehyde is expected to be viable through chelated TSs leading to the selective formation of non-Evans syn aldol product. Experimentally known high stereoselectivity toward Evans syn aldol product is effectively rationalized by using the larger energy differences between the corresponding diastereomeric TSs. In both chelated and nonchelated pathways, the attack by the less hindered face of the enolate on aldehyde through a chair-like TS with an equatorial disposition of the aldehydic substituent is identified as the preferred mode. The steric hindrance offered by the isopropyl group and the possible chelation are identified as the key reasons behind the interesting stereodivergence between Evans and non-Evans products normally reported for the title reaction. The application of an activation strain model on the critical TSs has been effective toward rationalizing the origin of stereoselectivity. Improved interaction energy between the reactants is found to be the key stabilizing factor for the lowest energy IS in both chelated and nonchelated pathways. The present study provides newer insights on the role of titanium(IV) toward modulating stereoselectivity in aldol reactions.
 
Publisher AMER CHEMICAL SOC
 
Date 2011-07-15T03:19:02Z
2011-12-26T12:49:13Z
2011-12-27T05:35:01Z
2011-07-15T03:19:02Z
2011-12-26T12:49:13Z
2011-12-27T05:35:01Z
2010
 
Type Article
 
Identifier JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 132(35), 12319-12330
0002-7863
http://dx.doi.org/10.1021/ja101913k
http://dspace.library.iitb.ac.in/xmlui/handle/10054/4142
http://hdl.handle.net/10054/4142
 
Language en