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TiCl(4)-Promoted Baylis-Hillman Reaction: Mechanistic Rationale toward Product Distribution and Stereoselectivity

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Title TiCl(4)-Promoted Baylis-Hillman Reaction: Mechanistic Rationale toward Product Distribution and Stereoselectivity
 
Creator PATEL, C
SUNOJ, RB
 
Subject electron-deficient alkenes
asymmetric aldol reactions
density-functional theory
titanium(iv) chloride
lewis-acids
ab-initio
basis-set
ketones
aldehydes
complexes
 
Description The mechanism of TiCl(4)-prornoted Baylis-Hillman reaction between methyl vinyl ketone (MVK) and acetaldehyde, in the absence of any base, is studied using the mPW1K density functional theory. The study focuses on several mechanistic intricacies as well as selectivity issues at each step of the reaction. The minimum energy pathway for this reaction involves three major steps such as a chloride transfer resulting in a chloro-enolate, titanium-mediated aldol reaction, and elimination of HCl or HOTiCl(3). Both s-cis and s-trans conformers of MVK are considered along with various modes of chloride transfer involving different complexes between TiCl(4), aldehyde, and MVK. Chloride transfer is found to be kinetically more favored For s-cis-WK than for s-trans-MVK. The diastereoselectivity in the next step, i.e., Ti-mediated aldol reaction between the enolate and aldehyde, is found to be dependent on the geometry of the enolate, wherein anti and syn BH products are predicted for Z and E enolates, respectively. An interesting secondary orbital iriteraction between the oxygen atoms of the enolate and aldehyde moieties in the transition states for the C-C bond formation is identified as one of the contributing factors toward the predicted diastercoselectivity in the formation of the alpha-chloromethyl aldol product (P2). It has earlier been reported that Under different experimental conditions, any of the three products such as (i) a normal BH product (P1), (ii) 2-(chloromethyl)vinyl ketones (M), and (iii) alpha-chloro methyl aldol could be generated (Scheme 1). The present study offers valuable insights toward rationalizing the observed product distribution as well as diastereoselectivity in TiCl(4)-promoted BH reaction under base-free conditions. The computed energetics indicate that when MVK is employed as the Michael acceptor, the formation of 2-(choromethyl)vinyl ketone is the preferred product rather than the corresponding normal BH product, consistent with the known experimental reports.
 
Publisher AMER CHEMICAL SOC
 
Date 2011-07-15T02:57:57Z
2011-12-26T12:49:12Z
2011-12-27T05:35:00Z
2011-07-15T02:57:57Z
2011-12-26T12:49:12Z
2011-12-27T05:35:00Z
2010
 
Type Article
 
Identifier JOURNAL OF ORGANIC CHEMISTRY, 75(2), 359-367
0022-3263
http://dx.doi.org/10.1021/jo902123x
http://dspace.library.iitb.ac.in/xmlui/handle/10054/4136
http://hdl.handle.net/10054/4136
 
Language en