DYNAMICS OF ALPHA-CH DEPROTONATION AND ALPHA-DESILYLATION REACTIONS OF TERTIARY AMINE CATION RADICALS

Time-resolved laser spectroscopy has been used to generate and characterize a series of tertiary amine cation radicals and to determine the rates of their alpha-CH deprotonation and alpha-desilylation reactions with bases and silophiles. Laser excitation (308 nm) of a 60:40 MeOH:MeCN solution of PhNMe(2) (DMA) and 1,4-dicyanobenzene (DCB) promotes SET-induced formation of the DMA cation radical (460 nm) and DCB anion radical (340 nm), which undergo decay by back electron transfer at nearly equal rates and with respective second-order rate constants of 1.1 x 10(10) and 1.3 X 10(10) M(-1) s(-1) (25 degrees C). The decay rate is lowered (ca. 4-fold) by the inclusion of salts (ca. 0.1 M) such as nBu(4)NClO(4), LiClO4, nBu(4)NCl, nBu(4)NBF(4), and nBu(4)NO3SCF(3) in MeOH-MeCN and by changing the solvent from MeCN to MeOH and to EtOH. The cation radical of PhNMeCH(2)(TMS) (480 nm) and the simultaneously generated DCB anion radical undergo second order decay in MeCN with respective rate constants of 1.2 X 10(10) and 9.9 x 10(9) M(-1) s(-1) (25 degrees C). The silylamine cation radical decay rate was found to be governed by the concentration of silophiles (MeOH, H2O, and nBu(4)NF) in MeCN solutions. The observations are consistent with a silophile-induced desilylation process with second-order rate constants of 8.9 x 10(5) (MeOH), 1.27 X 10(6) (H2O), and 3.1 x 10(9) M(-1) s(-1) (nBu(4)NF). The rate of DMA cation radical decay is a function of base concentration. Both nBu(4)NOAc and nBu(4)NO(2)CCF(3) react with the DMA cation radical (in 60:40 MeOH:MeCN containing 0.1 M nBu(4)NClO(4)) with second-order rate constants for alpha-CH deprotonation of 3.1 x 10(5) and 8 x 10(4) M(-1) s(-1) (25 degrees C), respectively. Measurements with PhN(CD3)(2) and nBu(4)NOAc gave a k(H)/k(D) for alpha-CH deprotonation of 3.6 (60:40 MeOH:MeCN, 25 degrees C). Para-substituents have a pronounced effect on the rate of alpha-CH deprotonation by nBu(4)NOAc; second-order rate constants of 2.3 x 10(4), 1.1 x 10(5), and 2.5 x 10(6) M(-1) s(-1) were determined for the p-OMeC(6)H(4)NMe(2), p-MeC(6)H(4)NMe(2) and p-CF(3)C(6)H(4)NMe(2) cation radicals. Studies with Ph(2)NMe demonstrated that its cation radical (645 nm) can be generated by SET to DCB and that its decay through alpha-CH deprotonation by nBu(4)NOAc has a second-order rate constant of 9.5 x 10(5) M(-1) s(-1) and a k(H)/k(D) value of 2.8 (25:75 MeOH:MeCN, 25 degrees C). Finally, the effects of alpha-substituents on the rates of nBu(4)-NOAc-induced alpha-CH deprotonation of tertiary amine cation radicals were evaluated by use of the amines Ph(2)NCHR(1)R(2). The second-order rate constants (25 degrees C, 25:75 MeOH:MeCN) are 2.3 x 10(5) (R(1) = Me, R(2) = H), 1.7 x 10(5) (R(1) = R(2) = Me), 3.2 x 10(6) (R(1) = Ph, R(2) = H), 2.6 x 10(6) (R(1) = CH=CH2, R(2) = H), and 7.0 x 10(7) M(-1) s(-1) (R(1) = C=CH, R(2) = H).