Polar Effects. Part 14. Inductive Charge Dispersal in Bicyclo[2.2.2]oct‐1‐yl Cations

Abstract

The rate constants (log k ) for solvolysis of 2-, 3-, and 4-substituted bicyclo[2.2.2]octyl p-nitrobenzenesulfonates 10, 11, and 12, respectively, correlate linearly with the corresponding inductive substituent constants u?. The formation ofthe ion pairs 9 is, therefore, controlled by the [effect of neighboring substituents. It follows from the corresponding reaction constants pI of -I .54, -1.12, and -1.22 that inductivity is highest at the positions a to C(1). It is lower and practically equal at the p-and y-positions. Therefore, charge dispersal is similar to that previously observed in the quinuclidinium ion 7.The observation that the solvolysis rates of many bi-and tricyclic arenesulfonates are controlled by the inductive ( I ) effect of neighboring substituents has led to a new method to gauge charge dispersal in carbocations [I].This method makes use of the Hammett-type equation log klk, = plop, where k and k, are the rate constants for the substituted and unsubstituted compounds, respectively. or are the inductive constants [2] for the substituents used as probes for charge dispersal, and pI are the derived reaction constants that gauge the inductivity of the C-atoms to which the substituents are attached.For instance, the pI values for the substituted C-atoms in the solvolysis of the 2-norbornyl, 2-bicycl0[2.2.2]octyl and 2-adamantyl p-toluenesulfonates (tosylates) 1, 2, and 3 are -2.00 [3], -1.50 [4], and -0.80 [5], respectively. Inductivity thus differs widely, although these tosylates possess the common partial structure 4'). It was, therefore, concluded that p I values reflect directional electron mobility, a property which is evidently subject to constraints imposed by the entire molecule [l]. In the solvolysis of substituted 2-ex0 -and 2-end0 -norbornyl tosylates 5 and 6, respectively, in 80 % (v/v) EtOH positive charge is mainly dispersed to C(6); but substantial charge is transmitted to other neighboring C-atoms [612). The same applies to the ')' )It was ascertained in each case that solvolysis proceeds by the unimolecular S,l mechanism, i.e. without appreciable nucleophilic solvent participation. The strong participation of the dorsal C(6) in the ionization of 5 and the lesser involvement of the dorsal C(7) in 6 are considered to be the reason for the high rxoiendo rate ratio for unsubstituted 5 and 6, respectively [l].

Chemical Consequences of Long-Range Orbital Interactions in Cholestane-3,7-diol Monosulfonate Esters. A Seven-Center Fragmentation

“…The presence of alkyl substituents, especially on the α-position, which results in enhanced σ-participation is also a prerequisite for allowing seven-center fragmentation. The occurrence of seven-center fragmentation also supports Grob's view that the enhanced solvolysis rates of the 4-substituted bicyclo[2.2.2]octylsulfonate esters mentioned in the Introduction may be due to double hyperconjugation. , The synthetic value of this fragmentation reaction is limited because other more favorable reaction pathways, i.e., elimination and rearrangement, are preferred.…”

“…Stabilization by σ-participation will be more effective in 3(A) than in 2(A) because the C(4) atom adjacent to the carbocationic center in 3(A) bears two alkyl substituents (Me groups), whereas in 2(A) only one alkyl substituent, i.e., the C(8)−C(14) bond, is present next to the carbocationic center. In addition, cationic carbon atoms are better stabilized by alkyl substituents located at the α-position (as in 3(A) ) than by alkyl substituents at the γ-position (as in 2(A) ) …”