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

Time-resolved laser spectroscopy has been used to generate and charact erize 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 ne arly 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) i n 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 r espective 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 b e 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 concen tration. Both nBu(4)NOAc and nBu(4)NO(2)CCF(3) react with the DMA cati on radical (in 60:40 MeOH:MeCN containing 0.1 M nBu(4)NClO(4)) with se cond-order rate constants for alpha-CH deprotonation of 3.1 x 10(5) an d 8 x 10(4) M(-1) s(-1) (25 degrees C), respectively. Measurements wit h PhN(CD3)(2) and nBu(4)NOAc gave a k(H)/k(D) for alpha-CH deprotonati on of 3.6 (60:40 MeOH:MeCN, 25 degrees C). Para-substituents have a pr onounced effect on the rate of alpha-CH deprotonation by nBu(4)NOAc; s econd-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)N Me demonstrated that its cation radical (645 nm) can be generated by S ET to DCB and that its decay through alpha-CH deprotonation by nBu(4)N OAc 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 e ffects 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 (2 5 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).