Lifetimes of α-Dialkoxy Carbocations Produced via Radical Cations Generated by Electron Transfer and Photoionization in Aqueous Solution

Article abstract of DOI:10.1021/ja00195a061

One-electron oxidation of acetals PhCH2CR(OR')2 by reaction with photolytically produced SO4^.- or by biphotonic photoionization (λ = 248 nm) in aqueous solution leads to the benzyl radical PhCH2^. and to α-dialkoxymethyl carbocations RC(+)(OR')2.The reaction proceeds via a radical cation intermediate which undergoes rapid (k >/= 7*1E7 s^-1) heterolytic C-C fragmentation.The cations thus formed react with water with rate constants k(H2O) which have been measured by time-resolved conductance (R = H, alkyl, Ph) and optical (R = Ph) methods.A total of 24 acyclic and cyclic cations have been kinetically characterized, with values of k(H2O) ranging from 4.6*1E7 s^-1 (4,4,5,5-tetramethyl-1,3-dioxolan-2-ylium) to 8.6*1E7 s^-1 (2-phenyl derivative of the above cation).Replacement of R = H by methyl leads to an increase in the lifetime by a factor of about 1000.However, the substitution of R = Me by bulkier alkyl groups (e.g. tBu) does not further kinetically stabilize (in the case of the cyclic cations) or it even destabilizes the cations (with the acyclic systems).The effect of R = Ph depends on whether the cation is cyclic or acyclic: In the acyclic case steric interaction between the ortho hydrogens of the phenyl ring and the OR' substituents prevents complete coplanarity at C_α as a result of which charge delocalization to the phenyl ring becomes less efficient, i.e. the kinetic stability of the cation is not increased relative to the cation with R = Me.In the case of the sterically uncongested cyclic systems, Ph leads to an increase of stability compared to Me.A heterolytic C-C fragmentation of an electronically excited neutral compound to a carbanion and a carbocation has also been characterized, with the acetals 2-phenyl-2-(x-nitrobenzyl)-1,3-dioxolane (x = para or meta).