A molecular electron density theory study of the asymmetric hetero-Diels-Alder cycloaddition reaction between ferrocenyl-substituted thiabutadiene and methyl propiolate

The present work includes a molecular electron density theory (MEDT) study on the regioselectivity and molecular mechanism of 6-ferrocenyl-substituted 4H-thiopyran (cycloadduct 6) synthesis over the course of an asymmetric hetero-Diels-Alder (HDA) cycloaddition reaction between ferrocenyl-substituted thiabutadiene (FTBD 4) and methyl propiolate (MP 5) at 298 K in the presence of chloroform. In agreement with the experimental findings, the obtained results obviously indicate that the reaction takes place in an entirely S1-C5 regioselective fashion to yield cycloadduct 6 as an almost equimolar enantiomeric mixture. The electron localization function (ELF) analysis of the most relevant points P1 through P5 located over the intrinsic reaction coordinate (IRC) profile of the energetically most preferred TS1-1s characterizes a non-concerted two-stage one-step molecular mechanism in which formation of the advanced S1-C5 single bond and delayed C4-C6 one at cycloadduct 6 takes place through S1- to -C5 and C4- to -C6 coupling of these pseudoradical centers.