Solvolyses of monosubstituted benzhydryl bromides. Nucleophilic solvent intervention and dependence of solvation on the extent of charge delocalization in cationic transition states

Solvolysis of benzylic chlorides with extended charge delocalization. ?-tert-butyl(2-naphthyl)methyl, 9-fluorenyl and monosubstituted benzhydryl chlorides

“…12,14 Interesting results were also observed from the solvolysis of 9-fluorenyl chloride (2) and monosubstituted benzhydryl chlorides (3). Like the corresponding bromides, 14,18,30 good linear relationships could not be found for log k vs Y Cl [equation (1)], as was shown in Table 2. Deviations from linear correlations with Y BnCl were observed for 2,3a-c and 3e.…”

“…Similarly to benzhydryl bromides, 14,18 excellent linear relationships were obtained from log k-log k(1) plots for 3a-d. Therefore, a new Y xBnCl scale of solvent ionizing powers based on log k values for a-tert-butyl(2-naphthyl)methyl chloride (1) can be developed.…”

“…A unimolecular mechanism has generally been accepted for interpreting the solvolysis of benzhydryl halides, 15 p-nitrobenzoates, 31 mesylates 32 and dimethylsulfonium ion, 33 and also the solvolysis of 9-fluorenyl bromide 34 and tosylate. 35 Our recent studies, however, indicated significant nucleophilic solvent intervention in the solvolysis of monosubstituted benzhydryl bromides containing a strong electron-withdrawing group, 14,18 and in the case of 9-fluorenyl bromide. 30 Moreover, a new Y xBnBr scale was found to be necessary to correlate the solvolytic reactivities of 2 and 3.…”

“…Solvolysis of secondary a-tert-butylbenzyl substrates was in general considered to proceed via a limiting S N 1 mechanism, 11,12,14,18,[25][26][27] with some exceptions such as phenolysis of a-tert-butyl(4-methoxy)benzyl p-nitrobenzoate 28 and acetolysis of the corresponding chloride. 29 Similarly to the corresponding bromide 14 and tosylate, 12 the solvolysis of a-tert-butyl(2-naphthyl)-methyl chloride (1) is also likely to involve a limiting mechanism with positive charge delocalization over both rings in the transition state.…”

Kinetic Evidence for the Solvent Intervention in the Solvolysis of Tertiary Benzylic Benzoates†

“…3 Later studies indicate that those Y X scales were not appropriate to the solvolysis of benzylic substrates 4 or substrates having extended charge delocalization in transition state, such as substituted naphthylmethyl tosylates 5 and benzhydryl bromides. 6 A series of new Y BnX scales for the former, 7 and Y xBnX scales for the latter substrates 8 were then developed. The advantage of using those new scales, rather than Y X scales, in mechanistic studies was realized.…”