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 α-CH deprotonation and α-desilylation reactions with bases and silophiles. Laser excitation (308 nm) of a 60:40 MeOH:MeCN solution of PhNMe2 (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 × 1010 and 1.3 × 1010 M−1 s−1 (25 °C). The decay rate is lowered (ca. 4-fold) by the inclusion of salts (ca. 0.1 M) such as nBu4NCO4, LiClO4, nBu4NCl, nBu4NBF4, and nBu4NO3SCF3 in MeOH-MeCN and by changing the solvent from MeCN to MeOH and to EtOH. The cation radical of PhNMeCH2(TMS) (480 nm) and the simultaneously generated DCB anion radical undergo second order decay in MeCN with respectiv rate constants of 1.2 × 1010 and 9.9 × 109 M−1s−1 (25 °C). The silylamine cation radical decay rate was found to be governed by the concentration of silophiles (MeOH, H2O, and nBu4NF) in MeCN solutions. The observations are consistent with a silophile-induced desilylation process with second-order rate constants of 8.9 × 105 (MeOH), 1.27 × 106 (H2O), and 3.1 × 109 M−1 s−1 (nBu4NF). The rate of DMA cation radical decay is a function of base concentration. Both nBu4NOAc and nBu4NO2CCF3 react with the DMA cation radical (in 60:40 MeOH:MeCN containing 0.1 M nBu4NClO4) with second-order rate constants for α-CH deprotonation of 3.1 × 105 and 8 × 104 M−1 s−1 (25 °C), respectively. Measurements with PhN(CD3)2 and nBu4NOAc gave a kH/kD for α-CH deprotonation of 3.6 (60:40 MeOH:MeCN, 25 °C). Para-substituents have a pronounced effect on the rate of α-CH deprotonation by nBu4NOAc; second-order rate constants of 2.3 × 104,1.1 × 105, and 2.5 × 106 M−1 s−1 were determined for the p-OMeC6H4NMe2, p-MeC6H4NMe2 and p-CF3C6H4NMe2 cation radicals. Studies with Ph2NMe demonstrated that its cation radical (645 nm) can be generated by SET to DCB and that its decay through -CH deprotonation by nBu4NOAc has a second-order rate constant of 9.5 × 105 M−1 s−1 and a kH/kD value of 2.8 (25:75 MeOH:MeCN, 25 °C). Finally, the effects of -substituents on the rates of nBu4- NO Ac-induced α-CH deprotonation of tertiary amine cation radicals were evaluated by use of the amines Ph2NCHR1R2. The second-order rate constants (25 °C, 25:75 MeOH:MeCN) are 2.3 × 105(R1 = Me, R2 =), 1.7 × 105(R1 = R2= Me), 3.2 × 106 (R1 = Ph, R2 = H), 2.6 × 106 (R1 = CH═CH2, R2 = H), and 7.0 × 107 M−1 s−1 (R1 = C≡CH, R2 = H).
ASJC Scopus subject areas
- Colloid and Surface Chemistry