1,3-Dipolar character of six-membered aromatic rings. Part XXI. Thermal cycloadditions of 1-(5-nitro-2-pyridyl)- and 1-(4,6-dimethylpyrimidin-2-yl)-3-oxidopyridinium with 2, 4, and 6 π-electron components

Abstract

The regio- and stereo-selectivities of reactions of the title compounds at the 2- and 6-positions with mono- and disubstituted ethylenes are elucidated and rationalised by FMO theory including secondary orbital overlap and taking into account steric and dipolar effects. For monosubstituted ethylenes, FMO theory correctly predicts that electron donor, electron acceptor, and conjugated olefins all add in the same regioselective sense. Stereoselectivity is shown to depend rationally on the structures of the betaine and the addend.Buta-1,3-diene, its 2,3-dimethyl and 1-monomethyl derivatives, and cyclopentadiene add as 4π-electron addends at the 2- and 4-positions of the betaines. Such reactions are favoured kinetically, but under thermodynamic control reaction also occurs with the diene acting as a 2π-addend at the 2- and 6-positions of the betaine. The peri-selectivities and stereoselectivities are discussed in terms of FMO theory: the reversal of electron demand leading to easier reaction with electron-rich dienes is explained by PNIFLS.Fulvenes add at the betaine 2- and 6-positions. Peri-, regio-, and stereo-selectivity are successfully rationalised in terms of FMO theory.