Spectral properties and absolute rate constants for β-scission of ring-substituted cumyloxyl radicals. A laser flash photolysis study

Article abstract of DOI:10.1021/jo0163041

A laser flash photolysis study of the spectral properties and β-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after β-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and σ⁺ substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H₂O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for β-scission of the cumyloxyl radicals, which varies between 7.1 × 10⁵ and 1.1 × 10⁶ s^-1, a result that suggests that cumyloxyl radical β-scission is not governed by the stability of the resulting acetophenone. Finally, κ_β increases on going from MeCN to the more polar MeCN/H₂O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for β-scission through increased solvation of the incipient acetophenone product.

Full text of DOI:10.1021/jo0163041

2266
J . Org. Chem. 2002, 67, 2266-2270
Sp ectr a l P r op er ties a n d Absolu te Ra te Con sta n ts for â-Scission of
Rin g-Su bstitu ted Cu m yloxyl Ra d ica ls. A La ser F la sh P h otolysis
Stu d y
Enrico Baciocchi,1a Massimo Bietti,*,1b Michela Salamone,1b and Steen Steenken1c
Dipartimento di Chimica, Universita` “La Sapienza”, P.le A. Moro, 5 I-00185 Rome, Italy,
Dipartimento di Scienze e Tecnologie Chimiche, Universita` “Tor Vergata”, Via della Ricerca Scientifica, 1
I-00133 Rome, Italy, and Max-Planck-Institut fu¨r Strahlenchemie, D-45413 Mu¨lheim, Germany
bietti@uniroma2.it.
Received November 21, 2001
A laser flash photolysis study of the spectral properties and â-scission reactions of a series of ring-
substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad
absorption band in the visible region of the spectrum, which decays on the microsecond time scale,
leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to
the corresponding acetophenone formed after â-scission of the cumyloxyl radicals. The position of
the visible absorption band is red-shifted by the presence of electron-donating ring substituents,
while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting
that + R ring substituents promote charge separation in the excited cumyloxyl radical through
stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion.
Along this line, an excellent Hammett-type correlation between the experimentally measured
energies at the visible absorption maxima of the cumyloxyl radicals and σ+ substituent constants
is obtained. A red-shift is also observed on going from MeCN to MeCN/H2O for all cumyloxyl radicals,
pointing toward a specific effect of water. The ring substitution does not influence to a significant
extent the rate constants for â-scission of the cumyloxyl radicals, which varies between 7.1 × 105
and 1.1 × 106 s-1, a result that suggests that cumyloxyl radical â-scission is not governed by the
stability of the resulting acetophenone. Finally, kâ increases on going from MeCN to the more polar
MeCN/H2O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization
of the transition state for â-scission through increased solvation of the incipient acetophenone
product.
Sch em e 1
Alkoxyl radicals are important intermediates that play
a major role in lipid peroxidation,2 in the photooxidation
of hydrocarbons in the atmosphere,3 and in several
autoxidation processes occurring in industrial and bio-
logical systems.2 Among these radicals, tert-butoxyl and
cumyloxyl have attracted considerable attention, and
their reactivity, which is mainly related to hydrogen atom
transfer and C-C â-scission reactions (Scheme 1), has
been the subject of several studies.4,5
A recent and important discovery was that arylcarbin-
yloxyl radicals display a broad absorption band in the
visible region of the spectrum whose position is signifi-
cantly red-shifted by the presence of electron-donating
ring substituents,6,7 an observation that led to the sug-
gestion that such radicals are characterized by a certain
degree of internal charge transfer, with the aromatic ring
bearing a partial positive charge.
(1) (a) Universita´ “La Sapienza”. (b) Universita´ “Tor Vergata”. (c)
Max-Planck-Institut.
(2) Halliwell, B.; Gutteridge, J . M. C. Free Radicals in Biology and
Medicine, 3rd ed.; Oxford University Press: 1999.
(3) See for example: (a) Me´reau, R.; Rayez, M.-T.; Caralp, F.; Rayez,
J .-C. Phys. Chem. Chem. Phys. 2000, 2, 3765-3772. (b) Fittschen, C.;
Hippler, H.; Viskolcz, B. Phys. Chem. Chem. Phys. 2000, 2, 1677-1683.
(c) Atkinson, R. Int. J . Chem. Kinet. 1997, 29, 99-111.
(4) Leading references on the reactivity of tert-butoxyl radicals: (a)
Weber, M.; Fischer, H. J . Am. Chem. Soc. 1999, 121, 7381-7388. (b)
Tsentalovich, Y. P.; Kulik, L. V.; Gritsan, N. P.; Yurkovskaya, A. V. J .
Phys. Chem. A 1998, 102, 7975-7980. (c) Paul, H.; Small, R. D., J r.;
Scaiano, J . C. J . Am. Chem. Soc. 1978, 100, 4520-4527. (d) Walling,
C. Pure Appl. Chem. 1967, 15, 69-80.
(5) Leading references on the reactivity of cumyloxyl radicals: (a)
Avila, D. V.; Ingold, K. U.; Lusztyk, J .; Green, W. H.; Procopio, D. R.
J . Am. Chem. Soc. 1995, 117, 2929-2930. (b) Banks, J . T.; Scaiano, J .
C. J . Am. Chem. Soc. 1993, 115, 6409-6413. (c) Avila, D. V.; Brown,
C. E.; Ingold, K. U.; Lusztyk, J . J . Am. Chem. Soc. 1993, 115, 466-
470. (d) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Ingold, K. U.;
Lusztyk, J . J . Am. Chem. Soc. 1989, 111, 9269-9270. (e) Baigne´e, A.;
Howard, J . A.; Scaiano, J . C.; Stewart, L. C. J . Am. Chem. Soc. 1983,
105, 6120-6123.
On the basis of theoretical calculations, the visible
absorption band has been attributed to a π f π* transi-
(6) Avila, D. V.; Ingold, K. U.; Di Nardo, A. A.; Zerbetto, F.; Zgierski,
M. Z.; Lusztyk, J . J . Am. Chem. Soc. 1995, 117, 2711-2718.
(7) Avila, D. V.; Lusztyk, J .; Ingold, K. U. J . Am. Chem. Soc. 1992,
114, 6576-6577.
10.1021/jo0163041 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/01/2002

â-Scission of Ring-Substituted Cumyloxyl Radicals
J . Org. Chem., Vol. 67, No. 7, 2002 2267
tion, creating an increase in the electron density on the
oxygen atom. On the basis of such calculations, it was
also predicted that electron-withdrawing ring substitu-
ents should produce a blue-shift in the position of the
visible absorption band, but no experimental evidence of
this was provided.6
In this context, it seemed worthwhile to study the
influence of both electron-releasing and electron-with-
drawing ring substituents on the spectral properties of
cumyloxyl radicals. Moreover, since the internal charge
transfer discussed above might also influence the aryl-
carbinyloxyl radical ground-state energy, as well as that
of the transition state for the â-scission reaction described
in Scheme 1, a study of the influence of ring substitution
upon the rate constant for â-scission of cumyloxyl radicals
was also carried out.
Thus, in this paper we report on a laser flash photolysis
study of the spectral properties and reactivities of ring-
substituted cumyloxyl radicals.
F igu r e 1. Time-resolved absorption spectra observed after
248 nm LFP of tert-butyl 3,4-dimethoxycumyl peroxide (1.1
mM) in an Ar-saturated MeCN solution at 0.13 µs (filled
circles), 0.38 µs (empty squares), 1 µs (filled diamonds), and 7
µs (empty circles) after the 20 ns, 40 mJ laser flash. Insets:
(a) First-order decay of the 3,4-dimethoxycumyloxyl radical
monitored at 670 nm and (b and c) the corresponding first-
order buildup of absorption at 270 and 300 nm, respectively,
due to the formation of 3,4-dimethoxyacetophenone.
Resu lts
Cumyloxyl radicals (ArC(CH3)2O•) were generated at
room temperature by 248 nm laser flash photolysis (LFP)
of dicumyl peroxide or tert-butyl cumyl peroxides (be-
tween 1.1 and 5.5 × 10-3 M) bearing a variety of ring
substituents (ArC(CH3)2OOC(CH3)3: Ar ) 4-MeC6H4,
4-MeOC6H4, 3,4-(MeO)2C6H3, 2,5-(MeO)2C6H3, 3-ClC6H4,
4-ClC6H4, 4-CF3C6H4), in MeCN and MeCN/H2O (eqs 1
and 2, respectively).
hν
C6H5C(CH3)2O-OC(CH3)2C6H5 248 nm8
2C6H5C(CH3)2O• (1)
hν
ArC(CH3)2O-OC(CH3)3 248 nm8
ArC(CH3)2O• + (CH3)3CO• (2)
However, 248 nm laser irradiation of 2,5-(MeO)2C6H3-
C(CH3)2OOC(CH3)3, both in MeCN and MeCN/H2O, did
not lead to the formation of the expected cumyloxyl
radical 2,5-(MeO)2C6H3C(CH3)2O•. A relatively long-lived
transient was formed instead, showing absorption maxima
around 310 and 455 nm, which was attributed to the
aromatic radical cation8 formed by photoionization of the
parent peroxide. This assignment is supported by the
observation that 248 nm irradiation of 2,5-dimethoxy-
benzyl alcohol in H2O led to the simultaneous formation
of the solvated electron and of a species characterized
by absorption maxima at 310 and 455 nm, identical to
those observed for 2,5-dimethoxybenzyl alcohol radical
cation generated by pulse radiolysis in aqueous solution.8
All cumyloxyl radicals displayed a broad absorption
band in the visible region of the spectrum, which is
unaffected by oxygen.6 This band was observed to decay
on the microsecond time scale, leading in all cases to a
corresponding strong increase in absorption in the UV
region of the spectrum that, again, is unaffected by
oxygen. For example, in the case of tert-butyl 3,4-
dimethoxycumyl peroxide, the time-resolved spectra ob-
tained after 248 nm LFP in MeCN (Figure 1) show after
F igu r e 2. Time-resolved absorption spectra observed after
248 nm LFP of tert-butyl 4-trifluoromethylcumyl peroxide (5.5
mM) in an Ar-saturated MeCN solution at 55 (filled circles),
150 (empty circles), 350 (filled diamonds), 1500 (empty dia-
monds) and 7000 ns (filled squares) after the 20 ns, 40 mJ
laser flash. Insets: (a) First-order decay of the 4-trifluorometh-
ylcumyloxyl radical monitored at 425 nm and (b) time-resolved
absorption spectra observed after 248 nm LFP of di-tert-butyl
peroxide (150 mM) in an Ar-saturated MeCN solution at 75
ns (filled circles), after the 20 ns, 40 mJ laser flash.
0.13 µs (filled circles) an absorption band centered at 670
nm, which is attributed to the 3,4-dimethoxycumyloxyl
radical.
This species undergoes a first-order decay (inset a) that
is accompanied by a simultaneous buildup of optical
density at 300 and 270 nm (insets b and c, respectively),
assigned to 3,4-dimethoxyacetophenone, which shows two
absorption maxima at these wavelengths.
Figure 2 displays the time-resolved spectra obtained
after 248 nm LFP of tert-butyl 4-trifluoromethylcumyl
peroxide in MeCN. The spectrum recorded after 55 ns
(filled circles) shows a broad absorption band in the
visible (around 425 nm), which is assigned to the 4-tri-
fluoromethylcumyloxyl radical, an intense band centered
(8) Baciocchi, E.; Bietti, M.; Gerini, M. F.; Manduchi, L.; Salamone,
M.; Steenken, S. Chem. Eur. J . 2001, 7, 1408-1416.

2268 J . Org. Chem., Vol. 67, No. 7, 2002
Baciocchi et al.
Ta ble 1. Visible Absor p tion Ba n d Ma xim u m
Wa velen gth s for th e Cu m yloxyl Ra d ica ls (Ar C(CH3)2O•)
a n d Kin etic Da ta for Th eir Deca y a n d th e Bu ild u p of th e
P r od u ct Acetop h en on e in MeCN a n d MeCN/H2O
λmax
Ar
solvent
(vis)/nma
kv/s-1 b
kV/s-1 c
4-CF3C6H4
MeCN
425
445
445
480
485
515
500
530
510
555
580
625
670
690
1.1 × 106
2.4 × 106
1.0 × 106
MeCN/H2Od
MeCN
3-ClC6H4
C6H5
MeCN/H2Od
MeCN
2.0 × 106 1.8 × 106
7.0 × 105 7.8 × 105
2.7 × 106 2.7 × 106
1.1 × 106 1.0 × 106
3.6 × 106 3.1 × 106
7.2 × 105 7.1 × 105
2.8 × 106 2.8 × 106
1.1 × 106 1.0 × 106
3.6 × 106 3.2 × 106
9.9 × 105 9.7 × 105
MeCN/H2Od
MeCN
4-ClC6H4
4-MeC6H4
4-MeOC6H4
MeCN/H2Od
MeCN
MeCN/H2Od
MeCN
MeCN/H2Od
3,4-(MeO)2C6H3 MeCN
F igu r e 3. Hammett-type correlation between the experimen-
tally measured energies at the visible absorption maxima and
σ+ substituent constants. r2 ) 0.994. Correlation does not
include the 4-chlorocumyloxyl radical (O).
MeCN/H2Oe
-
2.1 × 106
a
b
Visible absorption maximum of the cumyloxyl radical. De-
termined by following the buildup of the pertinent acetophenone
between 240 and 300 nm. Error e10%. c Determined by following
the decay of the cumyloxyl radical at the visible absorption
Since the visible absorption band results from a π f
π* transition,6 it is suggested that + R ring substituents
promote charge separation in the excited cumyloxyl
radical through stabilization of the partial positive charge
on the aromatic ring of the incipient radical zwitterion.
In agreement with theoretical predictions,6 the position
of the visible absorption band is blue-shifted by the
presence of electron-withdrawing ring substituents, 4-chlo-
rocumyloxyl radical representing, however, an exception
since for this radical λmax is red-shifted with respect to
the unsubstituted cumyloxyl radical by ca. 15 nm.
Interestingly, an excellent Hammett-type correlation
between the experimentally measured energies at the
visible absorption maxima of the cumyloxyl radicals and
σ+ substituent constants is obtained (Figure 3). The
4-chlorocumyloxyl radical is significantly outside the
correlation line, a result which is consistent with the
importance of the substituents + R effect in determining
the position of the visible absorption band.
Going from MeCN to MeCN/H2O 1:1 results in a red-
shift in λmax (between 20 and 45 nm) for all cumyloxyl
radicals. Since the position of the visible absorption band
of the cumyloxyl radical was found to be solvent inde-
pendent in a variety of solvents, including dipolar aprotic
(MeCN) and protic (AcOH, (CH3)3COH) ones,5c,6 the
specific effect of water is very remarkable. Probably, this
effect is due to the ability of H2O to solvate both positively
and negatively charged species, thus contributing to the
stabilization of the excited state, which has a partial
radical zwitterionic character.
d
maximum. Error e10%. MeCN/H2O 1:1 (v/v). e MeCN/H2O 4:1
(v/v).10
at 280 nm, and a weaker one at 310 nm. The band
centered at 310 nm, which undergoes a first-order decay
with the same rate as the decay of the 425 nm band (inset
a), is also attributed to the 4-trifluoromethylcumyloxyl
radical.
Decay of these two bands is accompanied by a corre-
sponding buildup of optical density at 235 nm attributed
to 4-trifluoromethylacetophenone. The decay of the ab-
sorption band centered at 280 nm is much slower. This
band is assigned to the tert-butoxyl radical on the basis
of the close analogy with the absorption spectrum of this
species, generated independently after 248 nm LFP of
an argon-saturated MeCN solution containing 150 mM
di-tert-butyl peroxide (inset b).4b
An interesting observation is that on going from MeCN
to MeCN/H2O 1:1 the absorption band in the visible
undergoes a red shift up to 45 nm for all cumyloxyl
radicals.9
The experimental rate constants for decay of the
cumyloxyl radicals, both in MeCN and MeCN/H2O 1:1,
were measured spectrophotometrically by monitoring the
decrease in optical density at the visible absorption
maximum of the cumyloxyl radicals and (when possible)
the corresponding buildup in the UV region due to the
formation of the pertinent acetophenone, and they are
reported in Table 1, together with the values of λmax for
the visible absorption band of the cumyloxyl radicals.
The absorption band due to the tert-butoxyl radical at
λ ) 280 nm is only observed after LFP of tert-butyl cumyl
peroxides bearing electron-withdrawing ring substituents
(4-CF3, 3-Cl, and 4-Cl, respectively) and is in these cases
significantly more intense than the cumyloxyl absorption
band in the visible. These results indicate that the
extinction coefficient for the visible absorption band of
3-chloro, 4-chloro, and 4-trifluoromethylcumyloxyl radi-
Discu ssion
Sp ectr a l P r op er ties. The data shown in Table 1
confirm that, in agreement with previous reports,6 the
position of the visible absorption band of cumyloxyl
radicals is red-shifted by electron-donating ring substit-
uents. A red-shift is also observed on going from MeCN
to MeCN/H2O. This shift was previously reported by some
of us for the 4-methoxycumyloxyl radical;9 we now see
that this phenomenon is general and holds for both
electron-donating and electron-withdrawing ring substit-
uents.
cals is smaller than 560 M-1 cm-1 11
an observation that
,
(10) At higher water concentrations, photoionization, leading to the
aromatic radical cation, starts to compete with the homolysis of the
peroxidic bond.
(11) The extinction coefficient at the UV absorption band maximum
(9) Baciocchi, E.; Bietti, M.; Lanzalunga, O.; Steenken, S. J . Am.
Chem. Soc. 1998, 120, 11516-11517.
of the tert-butoxyl radical (275 nm) has been measured in MeCN as
4b
ꢀ275 ) 560 M-1 cm-1
.

â-Scission of Ring-Substituted Cumyloxyl Radicals
J . Org. Chem., Vol. 67, No. 7, 2002 2269
is in line with the results of theoretical calculations,
indicating a decrease in the visible band intensity on
going from the 4-methoxy to the 4-trifluoromethylcumyl-
oxyl radical.6 In contrast, in the case of 4-methyl, 4-meth-
oxy, and 3,4-dimethoxycumyloxyl radicals, the UV ab-
sorption is stronger than the tert-butoxyl one.6,11 Moreover,
the corresponding acetophenones strongly absorb in the
260-300 nm region (see, for example, Figure 1), and their
formation in small amounts shortly after the laser pulse
(<100 ns) by â-scission of the corresponding cumyloxyl
radicals prevents the observation of the tert-butoxyl
radical absorption band in the same spectral region.11
Rea ctivity. As already noted, in all cases the rate
constants for decay of the cumyloxyl radical are es-
sentially identical (within the experimental error) to the
rate constants obtained following the buildup of the
product acetophenone. Moreover, it is known that under
these conditions hydrogen atom abstraction by the cu-
myloxyl radicals from the solvent (MeCN or MeCN/H2O)
or from the parent peroxide (used in concentrations
between 1.1 and 5.5 × 10-3 M) is negligible.5c Therefore,
the first-order decay of the visible absorption band can
in all cases be assigned to the unimolecular â-scission
reaction leading to a methyl radical and the correspond-
ing acetophenone as described in Scheme 1. The value
of kâ obtained for the unsubstituted cumyloxyl radical
in MeCN (kâ ) 7.4 × 105 s-1, averaging kv_ and kV_ from
Table 1) is almost identical to that obtained previously
by Ingold and Lusztyk in MeCN [kâ ) (7.5-7.6) × 105
s-1], where the radical was generated by 266 or 308 nm
light.5c
Solvent effects on the rate constants for â-scission of
the cumyloxyl radical have been studied in detail;5b,c,12
the rate of â-scission increases with solvent polarity, an
effect that has been attributed to the increased stabiliza-
tion of the transition state for â-scission through in-
creased solvation of the incipient acetophenone product.
Because of the formation of the carbonyl group, the
transition state should have a higher dipole moment than
the cumyloxyl radical and is therefore more strongly
solvated.13 In line with this hypothesis, we observe that
kâ increases on going from MeCN to the more polar
MeCN/H2O 1:1 for all cumyloxyl radicals. For example,
for the unsubstituted cumyloxyl radical, kâ increases from
7.4 × 105 s-1 in MeCN to 2.7 × 106 s-1 in MeCN/H2O 1:1
(i.e. by a factor 3.6). Interestingly, the same factor is
observed on going from MeCN/H2O 1:1 to H2O (where a
value kâ ) 1.0 × 107 s-1 has been determined).12
expected to influence the stability of the reaction product
(substituted acetophenone), the observation of a negli-
gible effect on kâ seems to be in contrast with the proposal
of a product-like transition state for â-scission of the
cumyloxyl radical, based on the temperature dependence
of the secondary R-deuterium isotope effect in methyl
radical formation.14 However, it has been suggested that
in alkoxyl radical â-scission to give a carbonyl compound
and an alkyl radical, the predominant driving force for
cleavage is the stability of the alkyl radical,4d,15,16 whereas
the stability of the carbonyl product plays a minor role.
Exp er im en ta l Section
Ma ter ia ls. MeCN (Aldrich) of the highest available purity
was used as received. Water was obtained from a Millipore-
Milli-Q system. Dicumyl peroxide (Aldrich) was recrystallized
twice from methanol.
3-Chloro, 4-chloro, 4-trifluoromethyl, 4-methyl, 4-methoxy,
3,4-dimethoxy, and 2,5-dimethoxycumyl alcohols were pre-
pared by reaction of the corresponding arylmagnesium bromide
with acetone in anhydrous tetrahydrofuran, purified by column
chromatography (silica gel, eluent petroleum ether/ethyl ac-
etate 3:1), and identified by GC-MS and 1H NMR.
tert-Butyl cumyl peroxides were prepared by reaction of the
ring-subtituted cumyl alcohols with tert-butyl hydroperoxide
in the presence of p-toluenesulfonic acid, according to a slight
modification of a previously described procedure.17 A 5.0-6.0
M solution of tert-butyl hydroperoxide in decane was added
to a solution of the cumyl alcohol (tert-butyl hydroperoxide/
cumyl alcohol ≈ 1.2) in CH2Cl2 (purified by filtration through
an alumina column) containing p-toluenesulfonic acid (alcohol/
acid ≈ 10). The reaction mixture was stirred at room temper-
ature and the reaction was followed by TLC. tert-Butyl cumyl
peroxides were purified by column chromatography (Florisil,
1
eluent pentane) and identified by H NMR.17
1
ter t-Bu tyl 3-ch lor ocu m yl p er oxid e: H NMR (CDCl3) δ
1.22 (s, 9H, C(CH3)3), 1.53 (s, 6H, ArC(CH3)2), 7.18-7.34 (m,
2H, ArH), 7.42-7.44 (m, 2H, ArH).
1
ter t-Bu tyl 4-tr iflu or om eth ylcu m yl p er oxid e: H NMR
(CDCl3) δ 1.23 (s, 9H, C(CH3)3), 1.56 (s, 6H, ArC(CH3)2), 7.56
(s, 4H, ArH).
ter t-Bu t yl 3,4-d im et h oxycu m yl p er oxid e: 1H NMR
(CDCl3) δ 1.25 (s, 9H, C(CH3)3), 1.56 (s, 6H, ArC(CH3)2), 3.87
(s, 3H, OCH3), 3.90 (s, 3H, OCH3), 6.79-7.08 (m, 3H, ArH).
ter t-Bu t yl 2,5-d im et h oxycu m yl p er oxid e: 1H NMR
(CDCl3) δ 1.30 (s, 9H, C(CH3)3), 1.59 (s, 6H, ArC(CH3)2), 3.77
(s, 3H, OCH3), 3.79 (s, 3H, OCH3), 6.73-7.31 (m, 3H, ArH).
Tim e-Resolved LF P Stu d ies. The alkoxyl radicals of
interest were generated at room temperature by direct laser
flash photolysis (LFP) of symmetric and asymmetric perox-
ides, using a 248 nm excimer laser (KrF*, Lambda Physik
EMG103MSC), providing 20 ns pulses with energies between
5 and 60 mJ /pulse (output power of the laser). Optical detection
was employed, with a pulsed xenon lamp as analyzing light.
Wavelengths were selected using a monochromator. The time-
dependent optical changes were recorded with Tektronix 7612
and 7912 transient recorders, interfaced with a DEC LSI 11/
73+ computer, which also controlled the other functions of the
instrument and preanalyzed the data.18 Argon- or oxygen-
saturated solutions of the peroxides (A248 ≈ 0.3-0.6) in MeCN
or MeCN/H2O flowed through a 2 mm (in the direction of the
With respect to the substituent effects, the data in
Table 1 show that ring substitution does not influence
to a significant extent the rate constants for â-scission
of the cumyloxyl radicals. Thus, C6H5C(CH3)2O• and
4-MeC6H4C(CH3)2O• undergo â-scission with very similar
kâ values (7.4 × 105 and 7.1 × 105 s-1, respectively), while
a slightly higher reactivity is observed for 4-MeOC6H4-
C(CH3)2O•, 3,4-(MeO)2C6H3C(CH3)2O•, and for the cumyl-
oxyl radicals bearing electron-withdrawing ring substit-
uents (kâ ≈ 1.0 × 106 s-1).
Thus, it appears that no significant charge separation
(e.g., as that responsible for the spectral behavior) occurs
in the transition state for â-scission of the cumyloxyl
radicals. By considering that the ring substituents are
(14) Zavitsas, A. A.; Seltzer, S. J . Am. Chem. Soc. 1964, 86, 3836-
3840.
(15) Kochi, J . K. J . Am. Chem. Soc. 1962, 84, 1193-1197.
(16) Wilsey, S.; Dowd, P.; Houk, K. N. J . Org. Chem. 1999, 64, 8801-
8811.
(12) Neta, P.; Dizdaroglu, M.; Simic, M. G. Isr. J . Chem. 1984, 24,
25-28.
(13) We thank a reviewer for this suggestion.
(17) Hendrickson, W. H.; Nguyen, C. C.; Nguyen, J . T.; Simons, K.
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2270 J . Org. Chem., Vol. 67, No. 7, 2002
Baciocchi et al.
laser beam) by 4 mm (in the direction of the analyzing light,
90° geometry) Suprasil quartz cell. All experiments were
carried out at about 22 ( 2 °C. Rate constants were obtained
by averaging 6-12 values, each consisting of an average of
10-30 laser shots, and were reproducible to within 10%.
Ack n ow led gm en t. This work was carried out into
the framework of the EU project “Towards Efficient
Oxygen Delignification” (Contract No. QLK5-CT-1999-
01277).
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