Side-Chain Fragmentation of Arylalkanol Radical Cations. Carbon−Carbon and Carbon−Hydrogen Bond Cleavage and the Role of α- and β-OH Groups

A product analysis and kinetic study of the one-electron oxidation
of a number of 1-arylpropanols, 1,2-diarylethanols, and some of their methyl ethers by potassium
12-tungstocobaltate(III) (abbreviated as Co(III)W)
in
aqueous acetic acid was carried out and complemented by pulse
radiolysis experiments. The oxidations occur via
radical cations which undergo side-chain fragmentation involving the
Cα−H and/or Cα−Cβ bond.
With 1-(4-methoxyphenyl)-2-methoxypropane (1), only deprotonation of
the radical cation is observed. In contrast, removing
the ring methoxy group leads to exclusive C−C bond cleavage in the
radical cation. Replacing the side-chain β-OMe
by β-OH, the radical cation undergoes both C−C and C−H bond
cleavage, with both pathways being base catalyzed.
C−C bond breaking in the radical cation is also enhanced by an
α-OH group, as shown by 1-(4-methoxyphenyl)-2,2-dimethyl-1-propanol (7), where this pathway, which is
also base catalyzed, is the only one observed.
Interestingly,
α- and β-OH groups exhibit a very similar efficiency in assisting
the C−C bond cleavage route in the radical cations,
as is evident from the kinetic and products study of the oxidation of
1-phenyl-2-(4-methoxyphenyl)ethanol (5)
and
1-(4-methoxyphenyl)-2-phenylethanol (6) by Co(III)W,
and from pulse radiolysis experiments on 5 and 6.
C−C
bond cleavage is the main reaction for both radical cations which
exhibit a very similar rate of fragmentation (k
=
2.0 and 3.2 × 104 s-1, respectively).
In both fragmentation reactions a small solvent isotope effect,
k(H2O)/k(D2O)
(1.4 for 5•+ and 1.2 for
6•+) and negative activation entropies are
observed. These data suggest that a key role in
the assistance by α- or β-OH groups to C−C bond cleavage is
played by hydrogen bonding or specific solvation of
these groups. The kinetic study of the oxidations promoted by
Co(III)W has also shown that when only one group,
OH or OMe, is present in the side chain (either on Cα or
Cβ), the fragmentation step or both the electron transfer
and
fragmentation steps contribute to the overall oxidation rate.
However, with an OH group on both carbons of the
scissile bond, as in 1-(4-methoxyphenyl)-1,2-propanediol
(9), the rate of C−C bond cleavage is so fast that
the
electron transfer step becomes rate determining.