Increasing electron demand

Gassman, P.G. Fentiman Jr., A.F. J. Am. Chem. Soc., 1969, 91, 1545 1970, 92, 2549. For a discussion of the use of the tool of increasing electron demand to probe neighboring-group activity by double bonds, sigma bonds, and aryl rings, see Lambert, J.B. Mark,... 

Tertiary 1-aryl-l-cyclopropylethyl cations 27 show the expected deshielding of the cationic center chemical shift with increasing electron demand, as measured by aryl rings58. Thus, <513C(C+) values of227.8,247.6 and 251.1 were observed for thep-OMe,p-H and p-CF3 substituents, respectively. 

The nature of electronic effects in cationic reactions has been probed by application of the Gassman-Fentiman tool of increasing electron demand.67 Aryl-substituted cationic centers can be made more electron demanding (i.e., electrophilic) by introduction of electron-withdrawing substituents into the aryl ring. 

A series of aryl-substituted cycloalkyl cations 38 and 39 and other aryl-substituted cyclic systems (40,41,42) have been studied in connection with the application of the tool of increasing electron demand.67,153-156... 

Brown, H. C. Rao, C. G. Ravindranathan, M. Structural effects in solvolytic reactions. 23. New cr+ constants for activating substituents. The solvolysis of 1-aryl-l-cyclopropyl 3,5-dini-trobenzoates containing activating substituents in the aryl group. The tool of increasing electron demand and I-strain./. Am. Chem. Soc. 1977, 99, 7663-7667. 

Tertiary 1-aryl-l-cyclopropylethyl cations 27 show the expected deshielding of the cationic center chemical shift with increasing electron demand, as measured by or... 

The twist angles of the aryl groups from coplanarity in the propeller conformation of a,a-diarylethyl cation [31C ] are comparable with those of [29C (X,Y)]. The dependence of the selectivity parameters upon conformation therefore should be as significant as in the above [29C (X,Y)] system. The substituent effects on the protonation equilibrium of diarylethylenes [31(X,Y)] can be interpreted by assigning a preferred conformation to the cations. It also seems evident in this system that the p value for the P-conformers does not vary so significantly with a series of Y substituents. Any processes involving a.a-diarylethyl cation [31C ] as an intermediate should reflect the dependence of selectivity upon conformations. Small p values for the solvolyses of 1,1-diphenylethyl-OPNB [13(X,Y)] cannot be interpreted by the tool of increasing electron demand . 

Brown et al. (1977) applied the tool of increasing electron demand to this bicyclic system with the solvolysis of a series of 2-aryl-2-bicyclo[2.1.1]hexyl p-nitrobenzoates in 80% aqueous acetone. The excellent log k rj a+ relationship, gave a p+ of —4.31, a value more negative than for the related 2-aryl-2-e rfo-norbornyl system where a-participation is believed to be absent. Also no support for u-participation in secondary bicyclo[2.l.l]hexyl derivatives could be found. 

Brown and Ravindranathan (1977) tested whether the tool of increasing electron demand could establish the presence of Tea-participation in the pentacyclo[4.3.0.0. 0 ". 0 " ]nonyl system. They found that both the application of the tool of increasing electron demand and low methyl/hydrogen and phenyl/hydrogen ratios unambiguously support the earlier conclusion that the solvolysis of this system proceeds with Tea-participation. 

The solvolyses of either exo- or cn-2-aryl-2-norbornyl-OPNBs gave good Brown correlations with nearly identical p values, -3.83 for exo- and -3.75 for endo-isomQvs, which are identical to that of 1-arylcyclopentyl-OPNB (Takeuchi and Brown, 1968 Brown and Takeuchi, 1968). Electron demand at the electron-deficient centre of the two systems should be the same, and obviously cr-participation cannot be a significant factor in the predominant exo-substitution in these derivatives. In the solvolysis of 2-aryl-2-norbornenyl-OPNB [8], there is also no difference in the p values between exo- and endo-isomevs which are -4.21 and -4.17, respectively (Brown and Peters, 1975). Similarly, in 2-arylbenzonorbornen-2-yl OPNB [9] (Brown et ai, 1969) and its 6-methoxy derivatives (Brown and Liu, 1969), the increasing electron demand cannot be detected at all. It is therefore remarkable that the solvolysis of cxo-2-aryl-fenchyl-OPNB [10] gives an rvalue close to 1.0, and nevertheless, a distinctly lower p value of —2.27, the lowest extreme of p values for tertiary dialkylbenzyl solvolyses (Fujio et al., unpublished). 

Fig. 11, Effect of increasing electron demand on the rates of solvolysis of 7-arylnor-bornyl (.303) and 7-aryl-anfi-norbornenyl p-nitrobenzoates (304). [Reproduced from Brown, H. G The nonclassi-cal ion problem. New York Plenum Press 1977.1...

---