Solvolytic reactivities of 4-nitrobenzhydryl bromides (2b-5b) and chlorides (2c-5c) were studied using single-and dual-parameter Grunwald-Winstein-type correlation analyses with Y BnX and Y xBnX scales, respectively. Extended charge delocalization over two aryl rings at cationic transition states were found for 3 and 5, but not for 2 or 4. Calculations of the charge distributions in 3c and in the corresponding cation 3a were performed using a Hartree-Fock approximation (RHF/6-31G* basis set) and density functional models (pBP/DN** and other basis sets), respectively, on Mulliken population analysis and on electrostatic potential analysis. The possible contribution of non-canonical resonance structure is discussed. a Referred to structural formula 3c and 3a; charges on hydrogen atoms were summed into the attached carbon atoms. b Mulliken population analyses. c Electrostatic potential analyses.

The solvolysis of 1-tert-butyl-1-(4-phenoxy)methyl bro mide (5b) and chlo ride (5c) in a va ri ety of solvents were car ried out. Lim it ing SN1 mech a nisms were sug gested from the ob ser va tion of lin ear re la tion ships us ing sin gle-parameter Winstein-Grunwald type cor re la tion be tween log a rithm of rate con stants with YxBnBr and Y xBnCl scales, re spec tively. This re sult in di cated pos i tive charge delocalization over both aryl rings and pro vided fur ther ev i dence for pos si ble con tri bu tion of non-canonical res o nance struc ture in the cationic transi tion state. IN TRO DUC TIONGrunwald-Winstein type cor re la tion anal y ses (Eqs 1 and 2), 1,2 in which Y is the sol vent ion iz ing power and N is the nucleophilicity, have long been a use ful tool in the study of solvolytic mech a nisms for or ganic re ac tions. 3 Dif fer ent YX scales have been de vel oped for aliphatic and alicyclic substrates (RX) con tain ing a dif fer ent leav ing group X. 4 In our pre vi ous stud ies, new Y BnX 5 and Y xBnX 6 scales were sug gested for deal ing with the solvolysis of benzylic sub strates with pos i tive charge delocalization over one and two aryl rings, respec tively. These new scales could also be ap plied to the solvolysis of cer tain ar o matic acyl halides, such as, Y BnCl to benzoyl chlo rides 7 and YBnBr to benzoyl bro mides.More in ter est ingly, ex tended charge delocalization over both phenyl rings in the cationic tran si tion state of the solvolysis of N,N-diphenylcarbamoyl chlo ride (1) was re alized based on the ob ser va tion of lin ear cor re la tions with YxBnCl scale, and ab in itio cal cu la tions.9 Pos si ble con tri bu tion of non-canonical res o nance struc ture (2) was thus pro posed (Scheme I). 9 Sim i larly, in the solvolysis of 4-nitro-4 -phenoxy benzhydryl bro mide (3b) and chlo ride (3c) a non-canonical res o nance struc ture (4) (Scheme II) was also con sid ered to be in volved in the tran si tion state. 10 More ev i dence is needed to con firm the gen er al iza tion of the sig nif i cance of non-canonical res o nance in the cationic tran si tion state of solvolysis.Since the solvolysis of 1-aryl-1-tert-butylmethyl de riva tives was gen er ally con sid ered to pro ceed via lim it ing S N 1 mech a nism, 5a,11-13 we un der took to ex am ine the sol vent effects on the solvolytic re ac tiv ity of 1-tert-butyl-1-(4-phen -