Absolute reactivity of arylallyl carbocations

Abstract

A series of α‐vinyl arylmethyl cations were generated and studied using nanosecond laser flash photolysis. Rate constants for the decay of the substituted α‐vinyl arylmethyl cations were determined under solvolytic conditions in pure solvents and solvent mixtures of 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) and 2,2,2‐trifluoroethanol (TFE). In addition the absolute reactivity of the carbocations with added nucleophiles were obtained. The reactivities of the α‐vinyl arylmethyl cations were then compared to the reactivities of the corresponding α‐methyl, α‐phenyl, and α‐cyclopropyl arylmethyl cations. Hammett σ+ plots of each of the series of carbocations were obtained and the substituent effects on carbocation reactivity analyzed. These data show that the influence of substituent on the reactivity of the α‐vinyl carbocations was different from the substituents effects on the reactivity of the α‐methyl, α‐phenyl, and α‐cyclopropyl series. Copyright © 2008 John Wiley & Sons, Ltd.

Solvent nucleophilicities of hexafluoroisopropanol/water mixtures

“…Literature values of k 1 for the decay of E6 + in HFIP range from "around 10 2 s À1 " (meticulously dried HFIP but inaccurate measurement) [25] to 2.2 Â 10 4 s À1 (commercial HFIP, >99.8%, used as received). [26] The latter value is slightly larger than the calculated rate constant for the reaction of E6 + with HFIP containing 1% water (k calc = 7.2 Â 10 3 s À1 ). It thus seems problematic to derive accurate quantitative information about the electrophilic reactivities of carbocations from their decay rate constants in neat HFIP as traces of water may strongly affect the results.…”

“…a), we did not attempt to determine N 1 and s N of pure HFIP. Literature values of k 1 for the decay of E6+ in HFIP range from “around 10 2 s −1 ” (meticulously dried HFIP but inaccurate measurement) to 2.2 × 10 4 s −1 (commercial HFIP, 99.8%, used as received) . The latter value is slightly larger than the calculated rate constant for the reaction of E6+ with HFIP containing 1% water ( k calc = 7.2 × 10 3 s −1 ).…”

Nucleophilic reactivities of tertiary alkylamines

“…The choice of photo‐leaving group becomes more critical for less stabilized benzhydryl cations because photoheterolysis gets less favorable with decreasing cation stability. For the generation of benzhydryl cations with E > −2, which cannot readily be isolated as stable salts, we usually use precursors which are known to have a high efficiency of photoheterolysis, such as chlorides ( 5 ),51–53 acetates ( 6 ),54–58 and phosphonium salts ( 4 ) 59–63. The concentrations of Cl − , AcO − , and R 3 P generated by photolysis, which can be calculated from the absorbances and known extinction coefficients51 of the benzhydrylium ions, are so small that the rate of external return with the photo‐leaving group is usually negligible.…”

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

“…Carbocations are key intermediates in many organic reactions. [1][2][3][4][5] They can be generated by photoinduced heterolysis of organic halides such as RÀCl [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] or other precursors. [6-8, 21, 23-26] The photolysis of these precursors can also lead to the generation of carbon-centered radicals.…”

“…0, see Table 1) can be obtained by photolysis of the corresponding benzhydryl chlorides in CH 3 CN but not in CH 2 Cl 2 [9] and the photogeneration of these benzhydryl cations in CH 2 Cl 2 requires other photoleaving groups, such as PPh 3 . [25,26] In view of the important role of photolytic techniques in the characterization of benzhydryl ions [19,20,25,26,[35][36][37] and other reactive carbocations, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] it is desirable to find out whether an inhibition of the electron transfer or an efficient depletion of the ion pairs by geminate recombination is responsible for the negligible quantum yields in solvents of low polarity. Herein, we present ultrafast broadband pump-probe experiments on the photolysis of a set of benzhydryl chlorides (shown in Table 1).…”