Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11801
Even though this result was unexpected, it must be pointed
out that it could be predicted, at least on thermodynamic grounds.
Accordingly, on the basis of appropriate thermochemical cycles,
the ∆G° value for reaction 2 should be e-29 kcal mol-1 16
, i.e.,
largely more negative than the value (∆G° ) -16.8 kcal mol-1
)
calculated for the proton transfer reaction (eq 1).18 Thus, in this
system, hydrogen atom transfer is thermodynamically favored over
proton transfer.
The possibility that the carbocation is formed by reaction 1,
followed by the electron transfer reaction of the carbon radical
(E° < 0.16 V vs NHE)16,17b with ground-state chloranil (E° )
0.254 V vs NHE)19b is considered unlikely as no change in the
formation rate of the carbocation was observed by increasing the
CA concentration. Moreover, the diarylmethyl cation is formed
with the same efficiency both in the absence and in the presence
of oxygen.20 If the carbocation would be formed from the carbon
radical, the efficiency of its formation should be affected by the
presence of oxygen, which is expected to trap the carbon radical
(vide infra). Finally, CAH• and its decay should be observed in
the spectrum, which is not the case.
Figure 2. Time-resolved absorption spectra of CA (4.0 × 10-3 M) and
(4-MeOC6H4)2CH2 (1.0 × 10-2 M) in Ar-saturated CH2Cl2 recorded 0.1
(O), 0.4 (∆), 2.6 (]), and 8 (b) µs after the laser pulse (λexc ) 355 nm,
pulse width of 7 ns and laser energy of ca. 3 mJ pulse-1). Insets: kinetics
recorded at 350, 450, and 515 nm.
state CA.22 In line with this conclusion, the formation of the
carbocation at 515 nm was no longer observed in the presence
of O2, which traps the carbon radical (the shape of the absorption
band centered at 350 nm also changed). A linear plot between
the first-order rate constant for the formation of the carbocation
and the CA concentration was obtained, from which a second-
order rate constant of 6 × 107 M-1 s-1 for the reaction between
the carbon radical and CA was calculated.
Very interestingly, a different situation was found when the
laser photolysis of (4-MeOC6H4)2CH2 was performed in CH2Cl2.
The time-resolved spectra obtained in this solvent are reported
in Figure 2. The formation of CA•- and (4-MeOC6H4)2CH2
,
•+
both absorbing at 450 nm, was again observed. Even though the
absorptions of these two species are difficult to distinguish, the
first-order decay of the 450 nm absorption (k ≈ 3 × 107 s-1; see
inset of Figure 2) is fully consistent with the radical ions CA•-
and (4-MeOC6H4)2CH2•+ decaying as a bound pair. The absorp-
tion at 450 nm decreases and is replaced (after about 3 µs) by
that of CAH• (λmax ) 360, 420, and 435 nm)11a,13 and (4-
MeOC6H4)2CH• (λmax ) 350 nm).14 Thus, in CH2Cl2, the radical
cation undergoes a proton transfer reaction (eq 1), and not a
hydrogen atom transfer reaction (eq 2), as observed in MeCN.
However, the decrease in absorption at 350-360 nm was
accompanied by the buildup of an absorption at 515 nm, attributed
to the diarylmethyl cation. In line with this attribution, water
accelerated the decay rate of the 515 nm absorption. First-order
kinetics were observed, and the rate was found to depend on the
concentration of CA, which suggests that the carbocation is
formed by the reaction between the carbon radical and ground-
Thus, a mechanistic changeover, from homolytic to heterolytic
C-H bond cleavage in the radical cation, is observed on going
from MeCN to CH2Cl2. This is probably due to the much stronger
basicity of CA•- in the latter solvent where, moreover, (4-
•+
MeOC6H4)2CH2 is deprotonated within a radical-ion pair. In
CH2Cl2, therefore, reaction 1 may become favored over reaction
2. An increase in the reduction potential of (4-MeOC6H4)2CH+
on going from MeCN to CH2Cl2 might also play a role.
In conclusion, it has been clearly shown, for the first time,
that an alkylaromatic radical cation can undergo homolytic
breaking of the CR-H bond when it reacts with CA•- in MeCN.
The low basicity of CA•- in MeCN (pKa ) 6.8)18 and the
substantial spin density on the oxygen atoms probably play a
fundamental role in this respect, as well as, of course, the
reduction potential of the formed carbocation. Thermochemical
calculations indicate that in MeCN the homolytic process remains
thermodynamically favored with respect to the heterolytic one
as long as the reduction potential of the formed carbocation is
<0.7 V vs NHE. Thus, the homolytic process may be much more
frequent than it was hitherto thought,23 and chloranil-sensitized
photolyses in MeCN might represent an attractive new method
for the production and the study of a significant number of aryl-
methyl cations. We are presently working to test this possibility.
(16) Calculated from the equation ∆G°(reaction 2) ) ∆Ghom(Ar2CH-H•+
)
- ∆Ghom(CA-H-) (Ar ) 4-MeOC6H4). ∆Ghom(Ar2CH-H•+) is the Gibbs
energy for the homolytic cleavage of the C-H methylene bond in the substrate
radical cation. Its value was estimated at 25 °C by the equation ∆Ghom(Ar2-
CH-H•+) ) 2.3RTpKa(Ar2CH2•+) - 23.06E°(H+/H•) + 23.06E°(Ar2CH+/Ar2-
CH•); the value of pKa(Ar2CH2•+), -5.5, was calculated as already described5
but using the most recently reported E°(H+/H•) value in MeCN (-1.77 V vs
NHE).17a Since the oxidation potential for (4-MeOC6H4)PhCH• is 0.16 V,17b
a substantially lower value is expected for the oxidation potential of
(4-MeOC6H4)2CH•; thus, ∆Ghom(Ar2CH-H•+) should be <37 kcal mol-1. The
value of ∆Ghom(CA-H-), the Gibbs energy for the homolytic O-H bond
cleavage in CAH-, was taken equal to the value (66.3 kcal mol-1) evaluated
in DMSO.17c
Acknowledgment. Thanks are due to the National Council of
Research (CNR) and the Ministry for the University and Scientific and
Technological Research (MURST) for financial support.
(17) (a) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287-
294. (b) Johnston, L. J.; Kanigan, T. J. Am. Chem. Soc. 1990, 112, 1271-
1273. (c) Cheng, J.-P.; Handoo, K. L.; Xue, J.; Parker, V. D. J. Org. Chem.
1993, 58, 5050-5054.
•+
JA9820902
(18) Calculated from the equation ∆G°(reaction 1) ) 2.3RT[pKa(Ar2CH2
)
- pKa(CAH•)]. pKa(Ar2CH2•+) is as above, and pKa(CAH•) in MeCN, 6.8,
was calculated at 25 °C by the equation pKa(CAH•) ) (1/2.3RT)[∆G°(CA-
H•) + 23.06E°(H+/H•) - 23.06E°(CA/CA•-)]. ∆G°(CA-H) is estimated to
(21) (a) Sehested, K.; Holcman, J.; Hart, J. J. Am. Chem. Soc. 1977, 81,
1363. (b) Gan, H.; Leinhos, U.; Gould, I. R.; Whitten, D. G. J. Am. Chem.
Soc. 1985, 99, 3566.
be 56 kcal mol-1 [the BDE for (CA-H•) (64 kcal mol-1 19a minus the entropy
)
factor (8 kcal mol-1)],5 E°(H+/H•) is as above, and E°(CA/CA•-) is 0.254 V
(vs NHE in MeCN).19b
(22) (a) At present, the possibility that, at least in part, the carbocation is
also formed by reaction of CAH• and the carbon radical cannot be excluded.
(b) Even though the yield in carbocation from (4-MeOC6H4)2CH• is difficult
to estimate due to the overlap of several absorptions at 350 nm, it is quite
low. Therefore, it is possible that the reaction of the radical with CA competes
with radical-radical reactions leading to the adduct between MPM and CA,
(4-MeOC6H4)2CHOC6Cl4OH, which, accordingly, is the main reaction product
in steady-state photolyses in CH2Cl2.
(19) (a) Friedrich, L. E. J. Org. Chem. 1983, 48, 3851. (b) Penn, J. H.;
Deng, D.-L. Tetrahedron 1992, 48, 4823-4830.
(20) By considering that the ꢀ values (M-1 cm-1) are 105 for the
carbocation,14 104 for CA•- 12 and 103 for the radical cation,21a respectively,
,
it has been estimated that the yield in carbocation from CA•- and (4-
•+
MeOC6H4)2CH2 is around 13%. Probably, the hydrogen atom transfer
reaction competes with diffusive re-encounter of the radical cation with the
(23) The homolytic process may escape detection if a carbocation is formed
whose absorption falls outside the investigated region of the spectrum or
overlaps with other signals.
radical anion whose rate has been estimated to be 2 × 1010 M-1 s-1 21b
,
followed
by back electron transfer.