BF3-catalyzed oxa-Diels-Alder reaction of ethyl vinyl sulfide and beta-methyl-alpha-phenylacrolein: a molecular electron density theory study

摘要

Uncatalyzed and BF3-catalyzed oxa-Diels-Alder reaction of ethyl vinyl sulfide (EVS) and beta-methyl-alpha-phenylacrolein (ACR) experimentally explored by Ishihara and coworkers is investigated by employing molecular electron density theory. Based on their report, the titled reaction resulted in the formation of two cis and trans cycloadducts with a ratio of 86:14. Conceptual density functional theory analysis indicated that EVS and ACR should act as nucleophile and electrophile, respectively. Coordination of the BF3 LA increases significantly the electrophilicity power of ACR. Regioselectivity in the studied reaction was investigated by calculation of the Parr functions, and a good agreement was observed between both theoretical and experimental results. Furthermore, the diastereoselectivity of both catalyzed and uncatalyzed reactions was studied by using potential energy surface analysis, and a satisfactory agreement was observed with the experimental outcomes. Reduction of the activation barriers for the catalyzed reaction, relative to the uncatalyzed one, rationalized why the reaction should be performed experimentally at a very low temperature (-40 degrees C). Mechanistic studies indicated that in contrast to the uncatalyzed reaction, the catalyzed reaction takes place in two steps in dichloromethane and involves formation of a stable zwitterionic intermediate. QTAIM analysis indicated that the interaction of the BF3 LA with ACR during the reaction has non-covalent character with partial covalent one. Finally, NCI analysis of TSs explained satisfactorily the preference formation of the cis cycloadduct.