38092-03-4Relevant articles and documents
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).
Kinetics and Mechanism of the Hydrolysis of Anilide Acetals
McClelland, Robert A.,Patel, Geeta
, p. 6908 - 6911 (2007/10/02)
A kinetic study of the hydrolysis of anilide acetals ArC(OMe)2NMeAr' is reported.At high pH (>10) a pH independent C-O cleavage reaction is observed producing methanol and benzimidatonium ion; the latter is further hydrolyzed to amide and ester products.Substituent and solvent effects support a simple ionizaiton mechanism with a transition state near the ion.At pH + catalysis attributable to pre-equilibrium protonation followed by rate-determining cleavage.Estimation of the pKa of the protonated anilide acetal (-1) provides an estimate of its rate of C-N cleavage (ca.108 s-1) which is near the diffusion limit.At pH 7 there is a change in the rate-determining step in product formation in this C-N cleavage reaction, and what is observed kinetically is the decomposition of the hemiorthoester ArC(OMe)2OH, the product of hydration of the dialkoxy carbocation.The principal piece of evidence for the changeover is the observation that the rate constants in acid solution are independent of the aniline moiety of the anilide acetal.
