66822-18-2Relevant academic research and scientific papers
EPR and computational studies of the formation and β-scission of cyclic and acyclic dialkoxyalkyl radicals
Feilding,Franchi,Roberts,Smits
, p. 155 - 163 (2007/10/03)
EPR spectroscopy and density functional theory have been applied to study the formation and subsequent β-scission of a series of dialkoxyalkyl radicals. Abstraction of hydrogen by photochemically-generated tert-butoxyl radicals from acyclic acetals R1O(R2O)CHR3, and from cyclic analogues derived from diols, takes place mainly from the acetal carbon atom to give radicals of the type R1O(R2O)CR3 and relative rates of abstraction have been determined in competition experiments. When R3 = phenyl or vinyl, the activating influence of these substituents on hydrogen-atom abstraction is smaller than might be expected, probably because delocalisation of the unpaired electron on to the unsaturated group comes at the expense of planarisation at Cα, in opposition to the natural pyramidalising tendency of the two α-alkoxy groups. Absolute rate constants and Arrhenius activation parameters for β-scission of R1O(R2O)CR3 have been determined by a steady-state EPR method and the results can be understood in terms of angle-strain and stereoelectronic effects. β-Scission of selected cyclic dialkoxyalkyl radicals that carry a phenyl or vinyl substituent at the radical centre has been investigated using density functional theory at the UB3LYP/6-31G(d,p) level. Computed activation parameters are in good agreement with the experimental results, where comparison is possible. Both experiment and theory indicate that benzylic 2-phenyl-1,3-dioxan-2-yl radicals undergo β-scission more readily than the corresponding allylic 2-vinyl-1,3-dioxan-2-yl radicals.
Lifetimes of α-Dialkoxy Carbocations Produced via Radical Cations Generated by Electron Transfer and Photoionization in Aqueous Solution
Steenken, S.,McClelland, R. A.
, p. 4967 - 4973 (2007/10/02)
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 Cα 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).
