Structural Effects in Solvolytic Reactions. 38. Carbon-13 Nuclear Magnetic Resonance Studies of Carbocations. 6. < sup > 1 < /sup > Effect of Increasing Electron Demand on the Chemical Shifts of the α-Carbon Atoms in Substituted Aryldialkyl Carbocations. Correlation of the Data with a New Set of Substituent Constants, σ < sup > αC+ < /sup >

Abstract

With increasing electron demand, the 13C NMR shifts of the α-carbons, the carbon atoms adjacent to the cationic center (C+), undergo significant downfield shifts. However, the substituent chemical shifts, ∆δCα, for the α-carbon atoms (CH3) of substituted tert-cumyl cations are not correlated either by the σ+ or the σc+ constants. Plots of ∆δCα against σ+ and of ∆δCα against σC+ reveal that the points for the para derivatives deviate systematically from the lines defined by the data for the meta derivatives. These deviations indicate that a new set of para substituent constants, one intermediate in value between the σp+ and σpC+ constants, is needed to correlate satisfactorily the -carbon chemical shifts for the para derivatives. Accordingly, such a set of substituent constants, σαC+, was estimated by using the slope (-4.1) of the line defined by a plot of ∆δCα for the meta derivatives σm+ to satisfy the Hammett-type equation ∆δCα = ραC+ σαC+. The utility of these new σαC+ constants to correlate chemical shift data for the a positions of substituted benzylic cations was tested with the data for the α different systems. Indeed, very good correlations were observed when the chemical shifts for these systems are plotted against these σαC+ values: 2-aryl-2-butyl, r = 0.997, ρ(CH2)αC+ = -4.1, r = 0.998, ρ(CH3)αC+ = -4.0; 3-aryl-3-pentyl, r = 0.999, ραC+ = -3.9; 4-aryl-4-heptyl, r = 0.994, ραC+ = -3.6; 1-aryl-1-cyclopentyl, r = 0.999, ραC+ = -4.9; 1-ary 1-1-cyclohexyl, r = 0.998, ραC+= -4.9; 1-ary-1-1-cycloheptyl, r = 0.999, ραC+ = -4.7; 9-aryl-exo-5,6-trimethylene-9-norbornyl, r = 0.999, ραC+ = -4.1; 9-aryl-endo-5,6-trimethylene-9-norbornyl, r = 0.998, ραC+ = -4.4; 2-aryl-2-adamantyl, r = 0.993, ραC+ = -5.9. The trend in the ραC+ values in these cations is discussed. © 1981, American Chemical Society. All rights reserved.