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|Title:||Sequential catalytic role of bifunctional bicyclic guanidine in asymmetric phospha-Michael reaction|
|Citation:||Cho, B., Tan, C.-H., Wong, M.W. (2011-06-21). Sequential catalytic role of bifunctional bicyclic guanidine in asymmetric phospha-Michael reaction. Organic and Biomolecular Chemistry 9 (12) : 4550-4557. ScholarBank@NUS Repository. https://doi.org/10.1039/c1ob05186e|
|Abstract:||The catalytic mechanism and origin of enantioselectivity of bicyclic guanidine-catalyzed phospha-Michael reaction between diphenyl phosphine oxide and β-nitrostyrene were investigated by DFT calculations at M06-2X/cc-pVTZ//M06-2X/cc-pVDZ level in conjunction with the implicit SMD solvation method. The catalyst is found to be involved in all 3 steps of the proposed catalytic cycle, namely (1) tautomerization of phosphine oxide, (2) C-P bond formation and (3) concerted hydrogen transfer. The bifunctional role of the guanidine catalyst is clearly demonstrated in all 3 key steps. Due to the geometry of the bicyclic guanidine catalyst, the preferred orientation of the reactants in the transition state of enantioselective C-P bond forming step favours the R enantiomer, in excellent accord with the observed enantioselectivity. Analysis of various transition states suggests that the asymmetric C-P bond formation is controlled by the hydrogen bonding interaction and steric effect between the catalyst and substrate. Various weaker C-H⋯X (X = N, O and π) interactions also play a role in stabilizing the key transition states. © 2011 The Royal Society of Chemistry.|
|Source Title:||Organic and Biomolecular Chemistry|
|Appears in Collections:||Staff Publications|
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