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Diarylcarbenium ions (benzhydryl cations) are generated from diarylmethyl chlorides by 20-11s laser pulses (248 nm) in acetonitrile solution at 20 OC. The second-order rate constants for their reactions with nand wnucleophiles (anions, alcohols, water, alkenes, allylsilanes, alkyl and silyl enol ethers) are determined by monitoring the decay of the UV-vis transients at variable nucleophile concentrations. Only reactive nucleophiles (k2 > 106-107 L mol-' s-I) can be investigated by this method because of the concomitant reactions of the carbenium ions with the solvent acetonitrile and the chloride ions roduced in the photoheterolysis. The largest observed values of k2 are -2 X 1Olo for reactions with anions and (2-4) X IO8, mol-' s-I for reactions with neutral nucleophiles. Alkoxy-substituted ethylenes are 300-1 Os times more reactive than the corresponding alkyl-substituted ethylenes. The reactivities of structurally analogous alkyl and silyl enol ethers differ by less than 1 order of magnitude. In sharp contrast to the situation previously observed for the reactions of benzhydryl cations with alkenes, the nucleophilic reactivities of the enol ethers correlate with their ionization potentials and not with the stabilities of the carbenium ions produced in the rate-determining step. The rate constants measured for the reactions of the flash photolytically generated benzhydryl cations with alkenes and allylsilanes agree well with those extrapolated from the reactivities of these nucleophiles toward less electrophilic benzhydryl cations, which have previously been determined by conventional techniques. Combination of the two sets of data yields a nucleophilicity scale with respect to the reference electrophile @-H3CC6H4)$H+. Introduction Although reactions of carbenium ions with alkyl and silyl enol ethers represent key steps of many synthetically important transformations,' kinetic data for these reactions have, to our best knowledge, not yet been reported. Quantitative information on the nucleophilicity of these donor-substituted ethylenes has previously been derived from kinetic investigations of their acidcatalyzed hydrolyses,2 their cycloaddition rates toward electron deficient alkenes' and dienes? and their low ionization potentials determined by photoelectron spectroscopy5 or UV-vis spectroscopy of CT complexes.6 We report now on the kinetics of the reactions of laser-flash photolytically generated diarylcarbenium ions7** toward alkyl and silyl enol ethers. For a comparison, we have also investigated some alkenes, allylsilanes, and n-nucleophiles under identical conditions. Since rate constants for the reactions of alkenes and allylsilanes with diarylcarbenium ions have previously been determined conventionally by monitoring the reaction sequence depicted in Scheme I,9 the two sets of data can be connected to give a more comprehensive nucleophilicity scale for mystems with respect to carbenium electrophiles. Metbod Diarylchloromethanes were photolyzed with 20-11s laser pulses (248 nm) in acetonitrile solution as described previously," to yield benzhydryl radicals and benzhydryl cations, which were detected by their UV-vis signals. In the presence of nucleophiles, the decay of the transient carbenium ion follows a firstsrder rate law (eq I) , and the second-order rate -d[Ar2CHt] /dr = kOb[Ar2CH+] (1) (2) constant k2 can be derived from the slopes of kob vs [nuc] plots (Figure I ) . According to eq 2, kob incorporates ko. the term for the reaction of Ar2CHt with the solvent acetonitrile, and k2[nuc], the pseudo-first-order term for the reaction of Ar2CHt with the nucleophiles ([nuc] >> (Ar2CH+]). Since the values of k, range from 3 X IO5 to 3 X IO6 s-' for kob = k, + k2[nuc] LQbeck. * MQlheim. Scheme I X Ar2CH Y Ar2CH Y slow fast Ar2CH+ MX, + or Table I. Second-Order Rate Constants ( k 2 , L mol-' s-I) for the Reactions of Benzhydryl Cations with Anions in Acetonitrile at 20 i 1 O C Ph2CH+ @-H,COC6Hd)2CHt F 2.0 x 10'0 1.4 X IO'O CI2.0 x 10'0 1.5 X IO'O Br2.1 x 10'0 1.6 X IO'O J1.9 X IO'O 1.6 X IO'O SCN2.7 X IO 'O 1.4 X IO'O SH2.4 X 1O'O HzPOc 2.1 x 10'0 1.3 X IO'O NOT 2.2 x 10'0 NO,2.1 x 10'0 2.5 x 109 the carbenium ions used in this investigation? only reactive nucleophiles with k2[nuc] 2 IO'S-' can be studied by this method. (1 ) (a) Reetz, M. T. Angew. Chem. 1982,94,97; Angew. Chem., Inr. Ed. Engl. 1982, 21, 96. (b) Mukaiyama, T. Org. React. (N.Y.) 1982, 28, 203. (c) Mukaiyama, T.; Murakami, M. Synthesis 1987, 1043. (d) Effenberger, F. Angew. Chem. 1969,81,374; Angew. Chem., Int . Ed. Engl. 1969,8, 295. (e) Meenvein, H. Houben Weyl, Methoden der Organischen Chemie; Thiemc: Stuttgart, 1965; Vol. VI-3, p 199. (0 Povarov, L. S. Russ. Chem. Reu. (Engl. Trans!.) 1965, 34,639. (g) Mathieu, J.; Weill-Raynal, J . Formation o/C-C Bonds; Thieme: Stuttgart, 1979; Vol. Il l , p 196. (h) Makin, S. M. Russ. Chem. Reo. (Engl. Trans!.) 1969, 38, 237. (i) Makin, S. M. Pure Appl. Chem. 1976, 47, 173. (2) (a) Knittel, P.; Tidwell, T. T. J . Am. Chem. Soc. 1977, 99, 3408. (b) Chwang, W. K.; Nowlan, V. J.; Tidwell. T. T. J . Am. Chem. Soc. 1977, 99, 7233. (c) Chwang, W. K.; Knittel, P.; Koshy, K. M.; Tidwell, T. T. J . Am. Chem. SOC. 1977, 99, 3395. (d) Kresge, A. J.; Chen, H. L.; Chiang, Y.; Murrill, E.; Payne, M. A.; Sagatys, D. S. J. Am. Chem. Soc. 1971,93,413. (e) Kresge, A. J.; Chwang, W. K. J . Am. Chem. SOC. 1978, 100, 1249. 0002-7863/91/1513-7710$02.50/0