Can Remote Substituent Effects Influence Reactivity and Stereoselectivity in the Diels-Alder Cycloadditions of p-Substituted 6-Phenyl-6-methylfulvenes?

• Published in: Journal of organic chemistry Vol. 59; no. 25; pp. 7723 - 7731
• Main Authors: Gugelchuk, M. M, Chan, P. C.-M, Sprules, T. J
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 01.12.1994; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Organic; Physical Sciences; Science & Technology; ISODICYCLOPENTAFULVENES; SYSTEMS; CONJUGATED MOLECULES; AM1; FACIAL SELECTIVITY
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo00104a031

The kinetics, activation parameters, and stereoselectivity in the Diels-Alder reaction of a series of p-substituted 6-phenyl-6-methylfulvenes with N-phenylmaleimide have been measured in benzene-d(6) to examine the consequence of remote electronic perturbation on the system. The variation in equilibrium endo:exo ratios as a function of substituent adheres very well to a LFER with sigma(+) (rho = 0.701) in contrast to the kinetic stereoselectivity which shows no linear relationship of the Hammett type. A reasonably good LFER with sigma(-) is observed for the reactivities, especially when the NMe(2) and OMe examples are excluded (rho = -0.684). Temperature and solvent effects on these properties have also been investigated. Substituent-induced changes in preferred conformation and pi-electron densities of the fulvenes have been studied by ab initio molecular orbital calculations. The optimum twist angle between the pi-subsystems remained relatively constant (54-55 degrees) across the series except in the case of p-NMe(2) substitution where it is substantially decreased (36 degrees). Relative reactivities and kinetic stereoselectivities are seen to have a connection with the calculated magnitude and direction of the fulvene molecular dipole moment. The importance of frontier MO energies was examined.