Substituent effects on chemical shifts and geminal coupling constants of benzylic protons.

Abstract

Four series of rigid molecules were synthesized, namely, 6-substituted-phthalides, 5-substituted-3-phenylphthalans, 2-substituted-5H-dibenzo[a,d]cycloheptenes and 2-substituted-10, 11-epoxy-10,11-dihydro-5H-dibenzo[a,d]cycloheptenes. The nuclear magnetic resonance spectrum of each compound was measured and the benzylic proton absorptions analyzed. The chemical shifts, geminal coupling constants and in some cases the vicinal and the long range coupling constants obtained from these analyses were plotted against the Hammett sigma values in each series. The rho values obtained from these plots show that the chemical shifts of the benzylic protons depend partially on the conformation of the methylenic protons with respect to the benzene ring. Other factors also contribute to the observed rho values. The rho values for the geminal coupling constants depend entirely on the orientation of the two protons with respect to the benzene ring. Evidence on the conformation of 10,11-epoxy-10,11-dihydro-5H-dibenzo [a,d]cycloheptene was provided by variable temperature NMR measurements and an examination of Dreiding models. It is concluded that the epoxy group occupies a quasi-equatorial conformation. Assignments of configuration of the protons in position 5 with respect to the epoxy group in 2-methoxy-10,11-epoxy-10,11-dihydro-5H-dibenzo[a,d]cycloheptene were made from a determination of their intramolecular nuclear Overhauser effects.