Competing Thermal Electrocyclic Ring-Closure Reactions of (2Z)-Hexa-2,4,5-trienals and Their Schiff Bases. Structural, Kinetic, and Computational Studies

Abstract

The electrocyclic ring closure reactions of (2Z)-hexa-2,4,5-trienals (vinylallenals) to alkylidene-2H-pyrans and of the corresponding Schiff base derivatives to alkylidenepyridines can be concurrent. Rates of vinylallenal cyclization and imine formation upon addition of n-butylamine determine the extent of the competition. The activation energies for the electrocyclization of a series of 6-substituted-(2Z)-4-tert-butyl-3-methylhexa-2,4,5-trienals 2 and trienimines 4 depend on the steric interactions between the substituents at C6 and the tert-butyl group at C4. Mixtures of alkylidene-2H-pyrans 3 and alkylidenepyridines 5 are obtained with bulky groups at C6, whereas only the latter is obtained with a C6-t-Bu and only the former with substituents of moderate size at C6. The reaction rates were indirectly derived from the empirical observations using a global optimization study based on differential evolution. The cyclizations are torquoselective, and the kinetically favored (E)-alkylidene heterocycles evolve by electrocyclic ring opening/ring closure toward the thermodynamic Z isomers upon extended reaction times. Density functional theory (DFT) calculations of the electrocyclizations helped in their characterization as monorotatory processes with pseudopericyclic features and made it possible to rationalize the reactivity trends and the torquoselectivity.