Tool of increasing electron demand

The nature of electronic effects in cationic reactions has been probed by application of the Gassman-Fentiman tool of increasing electron demand. An aryl-substituted cationic center can be made more electron demanding, that is, electrophilic, by introduction of electron-withdrawing substituents onto the aryl ring. 

When a cationic center becomes sufficiently electrophilic, it may draw on electrons from neighboring n and a bonds and thus delocalize the positive charge. The onset of participation of n and a bonds can be detected as a departure from linearity in a Hammett-type plot as the electron-withdrawing ability of the aryl substituent is increased. 

In stable carbocation studies, NMR chemical shifts are generally used as a structural probe reflecting the charge density at the cationic center. In closely related homologous cations other factors that may affect chemical shift may be assumed to be constant. 

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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,... 

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. 

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 5-norbomen-2-yl system (421) is relatively well understood. Homoallylic participation is weak so that the double bond actually causes a rate retardation342-. Participation is not detected by the tool of increasing electron demand (Section 7.2.2.)244,266. Yet the exo-tosylate (421), X=OTs, acetolyzes 7000 times faster than the mfo-tosylate (425), X=OTs, to give the tricyclic acetate (424) as the principal product (ca. 90%)343. The hypothetical homoallylic intermediate (422) is... 

Famum and Olah s groups have extended the so-called Gassman-Fentiman tool of increasing electron demand coupled with H and NMR spectroscopy as the structural probe under stable ion conditions to show the onset of... 

On the basis of several studies, including the application of the tool of increasing electron demand,ion 167 can be best described as a nonclassical symmetrical bridged 27i-bishomocyclopropenium cation as opposed to a rapidly equilibrating pair of cyclopropylmethyl cations, for example 171. The observed unusually large coupling constants at the C(7) position of 167... 

Gassman and Fentiman proposed the so-called tool of increasing electron demand of the developing cation centre For their example of the solvolysis of 7-syn-aryl-7-anti-norbomenyl-p-nitroteizoates they confirmed the -participation by three criteria ... 

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