High Peri- and Regiospecificity of Phencyclone: Kinetic Evidence of the Frontier-Controlled Cycloaddition Reaction and Molecular Structure of the Cycloadduct

Article abstract of DOI:10.1021/jo01292a022

Phencyclone (2-oxo-1,3-diphenyl-2H-cyclopentaphenanthrene, 1a) demonstrates a high reactivity for dienophiles of the styrene family, as well as a number of cyclic olefins, to give endo <4+2>? cycloadducts specifically.The configuration of the adducts was assigned after the spectra and properties of the adducts were found to be comparable to those of one determined crystallographically.In these reactions, the substituent effects on the reaction rate were examined by using para-substituted styrenes 3a-h and 2-oxo-1,3-bis(para-substituted phenyl)-2H-cyclopentaphenanthrenes 1b,c.The pseudo-first-order rate constants of these reactions were obtained by following the decrease of the absorption band in the visible region by UV spectrometry.It is observed that the reaction was slowest for the unsubstituted styrene and faster when either electron-withdrawing or -donating substituents were present.We carried out LCAO MO calculations for the reactants and discuss the correlation between the kinetic data and the energies of the frontier orbitals of the reactants.Phencyclone is found to have a high reactivity toward dienophiles, and the spcificity is discussed in terms of the following controlling factors: (1) HOMO-LUMO control, (2) secondary orbital interaction, (3) steric interaction, (4) dipole-dipole interaction, (5) coplanarity.