Molecular electrostatic potentials-II. Mechanistic aspects of electrophilic interactions of some five-membered heterocycles
The interactions of furan, pyrrole and N-methyl pyrrole with electrophiles have been investigated by means of the calculated molecular electrostatic potentials, obtained from INDO wave functions, which indicate the most attractive sites and pathways of attack for an approaching electrophile. The very distinct preference of these heterocycles for α-substitution can be interpreted quite satisfactorily in the case of furan if it is assumed that the α-hydrogen moves out of the plane of the molecule, forming a quasi-tetrahedral carbon, as an initial step in the reaction. For pyrrole and N-methyl pyrrole it is found to be necessary to invoke out-of-plane "bending" of the N-H, N-CH3 and Cα-Hα bonds to explain both the preference for α-substitution, and also the relative selectivities of furan, pyrrole and N-methyl pyrrole. The energies, atomic charges and bond orders of various forms of these heterocycles are also discussed. © 1975.
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