Dynamics of α-CH Deprotonation and α-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 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 k_H/k_D 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 k_H/k_D 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).