2-, 3-, and 4-Arylalkanol Radical Cations
J. Am. Chem. Soc., Vol. 121, No. 28, 1999 6627
larger reactivity in acid medium is observed for the other
2-arylalkanol radical cations 5•+ and 6•+; however, product
studies clearly indicate that both undergo unimolecular C-C
bond cleavage and not C-H deprotonation.21
enon studied in detail, first by Whitten23 and then by Schanze24
and their associates for the radical cations of 2-(4-N,N-
dimethylaminophenyl)-1-phenylethanol and of a series of
2-(phenylamino)-1,2-diphenylethanols, respectively, where the
positive charge mainly resides on the nitrogen atom. These
studies were carried out in MeCN as the solvent and under
conditions where the rate constants for reaction of the radical
cation with the base were generally below the diffusion limit.
It was proposed that OH deprotonation and C-C bond cleavage
are concerted, but more recently Schanze also discussed the
possibilty that OH deprotonation precedes C-C bond cleavage
(E1cB mechanism).24
In our case, a similar distinction is made difficult by the
diffusion control of the initiating -OH reaction. Accordingly,
as already noted, all â-OH-substituted radical cations, 1•+, 4•+,
5•+, and 6•+, react with -OH at almost the same rate, although
the scissible C-C bond is made weaker, particularly in 5•+ and
6•+ by the presence on the â-carbon of alkyl or phenyl
substituents.25
Significant information in this respect is, however, provided
by the nature of the products formed in the -OH-induced decay
of 6•+. Indeed, for this radical cation, different products are
expected depending on whether or not an alkoxyl radical is
formed as reaction intermediate. Namely, if deprotonation in
the encounter complex or the intramolecular electron transfer
in the zwitterion are coupled to C-C bond cleavage, only CR-
Câ bond cleavage should be observed with formation of
phenylacetone and products deriving from the 4-methoxybenzyl
radical. On the other hand, if the alkoxyl radical 15 forms, in
addition to the above products we should also find 4-methox-
yphenylacetone and products deriving from the benzyl radical
since two energetically very similar C-C â-cleavage reactions
are possible for 15, as shown in Scheme 4.
When the decay of 1•+ and 4•+ is studied in the presence of
-OH, a completely different situation holds. The -OH promoted
decay of 1•+ and 4•+ occurs at a very fast rate (k-OH ) 8.3 and
7.6 × 109 M-1 s-1, respectively), which is similar to that of
4-methoxybenzyl alcohol radical cation (k-OH ) 1.2 × 1010 M-1
s-1)4 and very close to the diffusion limit. Moreover, in basic
solution 4•+ is about 80 times more reactive than its corre-
sponding methyl ether 7•+. Thus a shift from carbon to oxygen
acidity on moving from acidic to basic aqueous solution can
be reasonably suggested also when the OH group is in the
â-position. Such a shift leads to reaction products different than
those observed at pH ) 4. Accordingly, under steady-state
γ-radiolysis conditions at pH ) 10, 1•+ and 4•+ undergo C-C
bond cleavage, leading to products deriving from the 4-meth-
oxybenzyl radical as already discussed (Scheme 2, R1 ) H, R2
) H, Me). Products of C-C bond cleavage are also observed
with 5•+ and 6•+, which react with -OH at practically the same
rate as 1•+ and 4•+ (Table 1). Clearly, in basic media also 5•+
and 6•+ exhibit oxygen acidity.
Thus, for the -OH-promoted reactions of 2-arylalkanol radical
cations, a mechanism resembling the one described in Scheme
1 for 1-arylalkanol radical cations can be proposed (Scheme 3:
An ) 4-MeOC6H4).
Scheme 3
Scheme 4
An encounter complex between the radical cation and -OH
is first formed, which may lead to the formation of an alkoxyl
radical, either directly (path f) or via a radical zwitterion (paths
a and b), which then undergoes C-C bond cleavage forming
the 4-methoxybenzyl radical (the precursor of 4-methoxybenzyl
alcohol in the steady-state γ-radiolysis experiments) and CH2O
(path c). Alternatively, O-H deprotonation in the complex can
be coupled to C-C bond cleavage (Grob-type fragmentation,
path e).
The fact that the rate for reaction of 2-arylalkanol radical
cations with -OH is close to the diffusion limit is noteworthy
as it indicates that in the radical cation the acidity of the â-OH
group is higher than that of H2O (pKa ) 15.7) and hence
significantly higher than that of the parent compound (the pKa
of benzyl alcohol is around 17).22 This relatively high O-H
acidity in the radical cation, even when the OH group is
separated by 2 carbon atoms from the positively charged
aromatic ring, is very remarkable.
Since the base-induced decay of 6•+ leads only to phenyl-
acetone and products deriving from the 4-methoxybenzyl radical
(see results), a reasonable conclusion is that alkoxyl radicals
are not involved in the base-catalyzed fragmentation of â-OH-
substituted alkylaromatic radical cations. Very likely the
encounter complex decomposes through path e or paths a and
d in Scheme 3.
Surprisingly enough, the mechanistic dichotomy between
C-H and O-H acidity appears to extend also to substrates
bearing the OH group in the γ-position, which we would expect
to be significantly less acidic than the â-OH group in 1•+. Thus,
2•+ in acid medium undergoes CR-H deprotonation, as indicated
by the products study (see results) and by the reactivity which
We have already mentioned that, when 1•+ and 4•+ react with
-OH, C-C bond cleavage and not C-H deprotonation occurs.
That the presence of a â-OH group favors side-chain C-C bond
cleavage in aromatic radical cations is a well-known phenom-
(23) Ci, X.; Whitten, D. G. J. Am. Chem. Soc. 1989, 111, 3459-3461.
See also: Gaillard, E. R.; Whitten, D. G. Acc. Chem. Res. 1996, 29, 292-
297.
(24) Burton, R. D.; Bartberger, M. D.; Zhang, Y.; Eyler, J. R.; Schanze,
K. S. J. Am. Chem. Soc. 1996, 118, 5655-5664 and references therein.
(25) For example, the difference in BDE between 1•+ and 4•+ can be
estimated to be around 5 kcal mol-1, on the basis of the difference in
reduction potentials of +CH2OMe and +CH(CH3)OEt.26
(26) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem. Soc.
1988, 110, 132-137.
(20) Gilbert, B. C.; Warren, C. J. Res. Chem. Intermed. 1989, 11, 1-17.
(21) The C-C bond cleavage reactivity of arylalkanol radical cations in
acid media is discussed elsewhere.10
(22) Estimated on the basis of a pKa value of 17.4 for 2,4,6-trimethy-
benzyl alcohol: Deno, N. C.; Jaruzelski, J. J.; Schriesheim, A. J. Am. Chem.
Soc. 1955, 77, 3044-3051.