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Asymmetric Dual-Catalytic Cascade by Chiral N-Heterocyclic Carbene and Quinuclidine: Mechanism and Origin of Enantioselectivity in Benzofuranone Formation

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Title Asymmetric Dual-Catalytic Cascade by Chiral N-Heterocyclic Carbene and Quinuclidine: Mechanism and Origin of Enantioselectivity in Benzofuranone Formation
 
Creator REDDI, Y
SUNOJ, RB
 
Subject INTRAMOLECULAR STETTER REACTION
LONE-PAIR
COOPERATIVE CATALYSIS
REACTION INSIGHTS
PI INTERACTIONS
3+2 ANNULATION
BETA-LACTONES
ACYL AZOLIUMS
ENALS
ACID
asymmetric catalysis
transition state
N-heterocyclic carbenes
noncovalent interactions
Stetter reaction
dual catalysis
reaction mechanism
 
Description Contemporary literature offers a number of interesting examples for asymmetric multicatalytic reactions using chiral N-heterocyclic carbenes (NHCs) in conjunction with other catalysts. One of the very recent examples demonstrated a convenient strategy toward realizing chiral benzofuranones from salicylaldehyde and dimethyl acetylenedicarboxylate (DMAD). In this article, we report the mechanism and insights on the origin of asymmetric induction as obtained through a comprehensive density functional theory (M06-2X and mPW1K) investigation. Different likely catalyst-substrate combinations as well as the timing/sequence of activation of different substrates are carefully examined so as to identify the most preferred pathway. In the lowest energy path, the activation of DMAD by quinuclidine occurs first; the resulting zwitterionic intermediate then undergoes a Michael addition with a salicylate ion to yield a salicylate-DMAD adduct, which, in turn, is intercepted by the chiral NHC. In the next crucial step, an enantioselective C-C bond formation via an intramolecular Stetter reaction furnishes the benzofuranone framework bearing a chiral carbon atom. Two transition state models, with and without an explicitly bound catechol (an additive employed in the reaction that resulted in enhanced enantioselectivity), are considered. A distinct energetic advantage, of the order of 3.4 kcal/mol, for the addition of the re face of the Breslow intermediate (derived from the chiral NHC and the salicylate-DMAD adduct) to the re face of the dimethyl maleate moiety is noticed in the stereocontrolling C-C bond formation step. The Gibbs free energy difference between the diastereomeric transition states for (re,re) and (re,si) modes of addition is traced to the differential nonbonding interactions (O-H center dot center dot center dot pi, lone pair (lp)center dot center dot center dot pi, and C-H center dot center dot center dot O). The predicted enantioselectivity is in good agreement with the experimental observations.
 
Publisher AMER CHEMICAL SOC
 
Date 2016-01-15T06:23:04Z
2016-01-15T06:23:04Z
2015
 
Type Article
 
Identifier ACS CATALYSIS, 5(3)1596-1603
2155-5435
http://dx.doi.org/10.1021/cs502006x
http://dspace.library.iitb.ac.in/jspui/handle/100/17922
 
Language en