Reduction of pyridinoid heterocyclic compounds. A MINDO/3 study

Abstract

A MINDO/3 study has been carried out on the anions that can be formed by addition of H^– to a number of pyridinoid heterocyclic compounds. Salts of such anions are formed when the parent heterocyclic compounds are treated with a hydride-transferring reducing agent. Correlation with earlier experimental results suggests that the product obtained with mild reducing agents, like ZnH₂ or Zn(C₅H₆N)₂.2C₅H₅N, is the thermodynamically most stable one. The reduction of pyridine by the strong reducing agents MgH₂ and AlH₃ was studied by ¹H n.m.r. spectroscopy, and it was found that a mixture of the isomeric 1,2- and 1,4-dihydro-1-pyridyl anions is formed initially. The final product, however, was a pure 1,4-dihydro-1-pyridyl derivative. This is discussed in terms of hydride exchange between reduced and unreduced substrate molecules co-ordinated to the same metal ion.