Hetero-Diels-Alder Cycloadditions of α,β-Unsaturated Acyl Cyanides,Part 4 , Substituent Effects in Reactions with p-Substituted Styrenes

Cycloadditions of α,β-unsaturated acyl cyanides (=2-oxonitriles) 1 – 6 to styrene and its p-substituted derivatives 7a – f , h are of inverse electron demand and provide, under mild conditions, regio- and stereoselectively 2-aryl-3,4-dihydro-2H-pyran-6-carbonitriles 8 – 13 , generally in good yield. Rates for the cycloaddition of acryloyl cyanide 1 to p-substituted styrenes, determined in competition reactions of substrate pairs relative to that of styrene, increase in the order of electron-donating ability NO₂p-substituent. Linear correlation of log (k_X/k_H), and σ_p⁺ substituent constants (a Hammett-type plot), gives a reaction constant ρ_p⁺ of −1.47±0.17, supporting a concerted mechanism.