Structural Effects in Solvolytic Reactions. 37. Effect of Increasing Electron Demand on Carbon-13 Nuclear Magnetic Resonance Shifts in the 9-Aryl-exo-5,6-trimethylene- and 9-Aryl-endo-5,6-trimethylene-9-norbornyl Carbocations. Effect of the Cationic Carbon Center on Remote Carbon Atoms in the Skeleton As Revealed by Increasing Electron Demand

Abstract

The 13C NMR shifts of a series of meta- and para-substituted 9-phenyl-exo-5,6-trimethylene-9-norbornyl and 9-phenyl-endo-5,6-trimethylene-9-norbornyl carbocations were measured in SbF5/FSO3H/SO2ClF at -80 °C. Excellent linear correlations were realized in the plots of ∆δC+ against the σc+ constants. The 9-aryl-exo-5,6-trimethylene-9-norbornyl system yields a pc+ value of -16.3 with a correlation coefficient of 0.999. The 9- aryl-endo-5,6-trimethylene-9-norbornyl system gives a ρc+ value of -15.9 with a correlation coefficient of 0.999. In addition, the data reveal some unusual effects of the cationic center on the remote carbon atoms in the skeleton. For example, the Cδ carbon in the 9-aryl-endo-5,6-trimethylene-9-norbomyl system undergoes an upfield shift of 1.5 ppm from the p-OCH3 to the p-CF3 substituent, as compared to the downfield shifts (or no shifts) observed for the Cα, Cβ, and Cγ carbons in simple aryldialkyl carbocations. Possible reasons for such upfield shifts with electron demand are discussed. © 1981, American Chemical Society. All rights reserved.