Solvent effects on the o-neophyl rearrangement of 1,1-diarylalkoxyl radicals. A laser flash photolysis study

Article abstract of DOI:10.1021/jo0518755

A laser flash photolysis study has been carried out to assess solvent effects on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. The rearrangement rate constant k decreases by increasing solvent polarity and an excellent correlation with negati

Full text of DOI:10.1021/jo0518755

SCHEME 1
Solvent Effects on the O-Neophyl
Rearrangement of 1,1-Diarylalkoxyl
Radicals. A Laser Flash Photolysis Study
Massimo Bietti* and Michela Salamone
Dipartimento di Scienze e Tecnologie Chimiche,
Universita` “Tor Vergata”, Via della Ricerca Scientifica,
1, I-00133 Rome, Italy
bietti@uniroma2.it
Received September 7, 2005
decreases on going from the initial 1,1-diarylalkoxyl
radical to the transition state.1 These observations were
rationalized in terms of a reactant-like transition state.
In particular, the indication of a decrease in the extent
of charge separation clearly suggests that solvent effects
should play a role on the O-neophyl rearrangement of
1,1-diarylalkoxyl radicals.
Solvent effects on alkoxyl radical reactivity have been
studied in detail for hydrogen atom abstraction,5-12
â-scission,11-18 and 1,2-hydrogen atom shift19 reactions;
no information in this respect is instead available for the
O-neophyl rearrangement. Along this line, and in order
to gain additional support to the mechanistic picture
presented above, we have carried out a laser flash
photolysis study on the reactivity of the 1,1-diphe-
nylethoxyl radical 1• in different solvents. As a matter
of comparison, some experiments have been also carried
out for the R-phenyl-R-(4-trifluoromethylphenyl)cyclo-
propylmethoxyl radical 2•.
A laser flash photolysis study has been carried out to assess
solvent effects on the O-neophyl rearrangement of 1,1-
diarylalkoxyl radicals. The rearrangement rate constant k
decreases by increasing solvent polarity and an excellent
correlation with negative slope is obtained between log k and
the solvent polarity parameter ETN. These evidences are in
full agreement with the previous indication that the extent
of internal charge separation decreases on going from the
starting 1,1-diarylalkoxyl radical to the transition state.
We have recently provided evidence that the O-neophyl
rearrangement of 1,1-diarylalkoxyl radicals proceeds
through a concerted mechanism (Scheme 1, path a)1 and
not, as previously suggested,2,3 through a stepwise mech-
anism via formation of a bridged 1-oxaspiro[2,5]octadienyl
radical intermediate (Scheme 1, paths b and c).
In this study it was proposed that the rearrangement
transition state is characterized by a limited extent of
carbon radical character.1 Moreover, following the indica-
tion of the existence of a certain degree of internal charge
transfer in arylcarbinyloxyl radicals, with the oxygen
atom bearing a partial negative charge and the partial
positive charge delocalized over the aromatic ring,4 it was
(5) Aliaga, C.; Stuart, D. R.; Aspe´e, A.; Scaiano, J. C. Org. Lett. 2005,
7, 3665-3668.
(6) Snelgrove, D. W.; Lusztyk, J.; Banks, J. T.; Mulder, P.; Ingold,
K. U. J. Am. Chem. Soc. 2001, 123, 469-477.
(7) de Heer, M. I.; Mulder, P.; Korth, H.-G.; Ingold, K. U.; Lusztyk,
J. J. Am. Chem. Soc. 2000, 122, 2355-2360.
(8) MacFaul, P. A.; Ingold, K. U.; Lusztyk, J. J. Org. Chem. 1996,
61, 1316-1321.
(9) Valgimigli, L.; Banks, J. T.; Ingold, K. U.; Lusztyk, J. J. Am.
Chem. Soc. 1995, 117, 9966-9971.
(10) Avila, D. V.; Ingold, K. U.; Lusztyk, J.; Green, W. H.; Procopio,
D. R. J. Am. Chem. Soc. 1995, 117, 2929-2930.
(11) Avila, D. V.; Brown, C. E.; Ingold, K. U.; Lusztyk, J. J. Am.
Chem. Soc. 1993, 115, 466-470.
(12) Walling, C. Pure Appl. Chem. 1967, 15, 69-80 and references
therein.
(13) Bietti, M.; Gente, G.; Salamone, M. J. Org. Chem. 2005, 70,
6820-6826.
(14) Newcomb, M.; Daublain, P.; Horner, J. H. J. Org. Chem. 2002,
67, 8669-8671.
(15) Baciocchi, E.; Bietti, M.; Salamone, M.; Steenken, S. J. Org.
Chem. 2002, 67, 2266-2270.
also suggested that the extent of charge separation
(16) Weber, M.; Fischer, H. J. Am. Chem. Soc. 1999, 121, 7381-
7388.
(1) Aureliano Antunes, C. S.; Bietti, M.; Ercolani, G.; Lanzalunga,
O.; Salamone, M. J. Org. Chem. 2005, 70, 3884-3891.
(2) Grossi, L.; Strazzari, S. J. Org. Chem. 2000, 65, 2748-2754.
(3) Banks, J. T.; Scaiano, J. C. J. Phys. Chem. 1995, 99, 3527-3531.
(4) Avila, D. V.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1992,
114, 6576-6577.
(17) Tsentalovich, Y. P.; Kulik, L. V.; Gritsan, N. P.; Yurkovskaya,
A. V. J. Phys. Chem. A 1998, 102, 7975-7980.
(18) Bacha, J. D.; Kochi, J. K. J. Org. Chem. 1965, 30, 3272-3278.
(19) Konya, K. G.; Paul, T.; Lin, S.; Lusztyk, J.; Ingold, K. U. J.
Am. Chem. Soc. 2000, 122, 7518-7527.
10.1021/jo0518755 CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/12/2005
J. Org. Chem. 2005, 70, 10603-10606
10603

Radicals 1• and 2• have been generated in MeCN,
MeCN/H2O 1:1, CF3CH2OH (TFE), and CH2Cl2 by 266
nm laser flash photolysis (LFP) of the parent tert-butyl
peroxides 1p and 2p (method 1) as described in eq 1:
TABLE 1. Visible Absorption Band Maximum
Wavelengths and Decay Rate Constants for
1,1-Diarylalkoxyl Radicals 1• and 2• Measured in
Different Solvents at T ) 22 °C under Argon
method of
radical
1•
solvent
MeCN
MeCN
MeCN
MeCN/H2Oh
TFE
CH2Cl2
benzene
CCl4
generationa λmax (vis)/nmb
kV/s-1
c
1
2
2
1
1
1
2
2
530
e
e
2.8 × 106 d
2.9 × 106 f
2.9 × 106 f,g
2.3 × 106
1.5 × 106
3.3 × 106
4.1 × 106 f
4.8 × 106 f
5.0 × 106 d
4.5 × 106
2.2 × 106
7.3 × 106
540
540
530
e
e
2•
MeCN
1
1
1
1
520
540
540
520
MeCN/H2Oh
TFE
The spectral properties and reactivities of radicals 1•
and 2• in MeCN have been described previously.1 Both
radicals display a broad absorption band in the visible
region of the spectrum, centered at 530 and 520 nm,
respectively, which is unaffected by oxygen. This band
was observed to undergo a first-order decay, leading in
both cases to a corresponding strong increase in absorp-
tion in the UV region of the spectrum, assigned to the
rearranged carbon centered radical formed by aryl shift
in the 1,1-diarylalkoxyl radical (Scheme 1).
As compared to MeCN, in MeCN/H2O 1:1 and TFE a
red shift in the position of the visible absorption band
maximum of 10-20 nm was observed for both radicals,
in line with that observed previously for related aryl-
carbinyloxyl radicals.13,15
CH2Cl2
a Method 1: 266 nm LFP of peroxides 1p and 2p (eq 1). Method
2: 355 nm LFP of 4-nitrobenzenesulfenate ester 1e (eq 2). b Visible
absorption band maximum of the 1,1-diarylalkoxyl radical. c Mea-
sured following the decay of the 1,1-diarylalkoxyl radical at the
visible absorption band maximum. d Reference 1. e Under these
conditions, the superimposition of the 4-nitrobenzenethiyl radical
(4-NBS•) and 1,1-diphenylethoxyl radical (1•) absorption bands (see
text) did not allow the determination of λmax (vis). f Measured
following the fast component of the double exponential decay in
the 440-570 nm range. g Oxygen saturated. h MeCN/H2O 1:1
(v/v).
The decay of radicals 1• and 2• generated by method 1
in MeCN, MeCN/H2O 1:1, TFE, and CH2Cl2 was mea-
sured spectrophotometrically by monitoring the decrease
in optical density at the corresponding visible absorption
maxima and was found to follow first-order kinetics. The
corresponding rearrangement rate constants thus ob-
tained are reported in Table 1 together with the visible
absorption band maximum wavelengths.
However, because method 1 could not be employed for
alkoxyl radical generation in solvents such as benzene
and CCl4, radical 1• was also generated in MeCN,
benzene, and CCl4 by 355 nm LFP of the parent 4-ni-
trobenzenesulfenate ester 1e (method 2) as described in
eq 2:14,20
FIGURE 1. Time-resolved absorption spectra observed after
355 nm LFP of 1e (8.0 × 10-5 M) in an argon-saturated MeCN
solution at 96 ns (b), 319 ns (O), and 2 µs (9) after the 8 ns,
10 mJ laser pulse. Inset: decay of absorption monitored at
480 nm (O) and 530 nm (b).
played in Figure 1. The spectrum recorded after 96 ns is
characterized by a broad absorption band between 410
and 600 nm, which on the basis of the comparison with
literature data can be reasonably assigned to the super-
imposition of the 4-nitrobenzenethiyl radical (4-NBS•:
λmax ) 480 nm)14,21 and 1,1-diphenylethoxyl radical
(1•: λmax ) 530 nm)1,3,22 absorption bands. This assign-
ment is also supported by the observation that the
spectrum is not influenced by oxygen, in agreement with
literature data.1,3,21,22
The time-resolved absorption spectra observed after
355 nm LFP of an argon-saturated MeCN solution
containing 4-nitrobenzenesulfenate ester 1e are dis-
The spectrum was observed to undergo a double
exponential decay in the 440-570 nm range (see, for
(21) Alam, M. M.; Ito, O. J. Org. Chem. 1999, 64, 1285-1290.
(22) 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.
(20) Horner, J. H.; Choi, S.-Y.; Newcomb, M. Org. Lett. 2000, 2,
3369-3372.
10604 J. Org. Chem., Vol. 70, No. 25, 2005

example, the inset in Figure 1). From the double expo-
nential fit, the decay of the two components was mea-
sured as 2.9 × 106 and 3 × 105 s-1, respectively. The
former value is very similar to the rate constant mea-
sured for the rearrangement of 1•, generated by method
1 in MeCN (between 2.5 and 3.2 × 106 s-1).1,3,22,23 On the
basis of this observation, the fast component of the double
exponential decay observed in the 440-570 nm range can
be reasonably assigned to the same process, i.e., the
O-neophyl rearrangement of 1•. The slow component is
instead assigned to the decay of 4-NBS•.21
An analogous behavior was observed after 355 nm LFP
of 1e in benzene and CCl4, and also in these solvents the
rearrangement rate constants of 1• were determined from
the first-order fit to the fast component of the double
exponential decay observed in the 440-570 nm range.
The rearrangement rate constants thus obtained for 1•,
generated by method 2 in MeCN, benzene, and CCl4, are
also reported in Table 1.
The kinetic data collected in Table 1 clearly show that
for both 1• and 2• the rearrangement rate constant
decreases by increasing solvent polarity. As an example,
with 1• the rate constant decreases from 4.8 × 106 to 1.5
× 106 s-1 on going from CCl4 to TFE. This behavior is in
full agreement with the mechanistic picture presented
above indicating that the O-neophyl rearrangement of
1,1-diarylalkoxyl radicals involves a decrease in the
extent of internal charge separation on going from the
starting 1,1-diarylalkoxyl radical to the transition state.
However, because a decrease in the extent of charge
separation can be expected for both the concerted and
the stepwise mechanisms described in Scheme 1 (path a
and paths b-c, respectively),1 the kinetic solvent effect
data displayed in Table 1 do not provide any new
information that may contribute to distinguish between
these two mechanistic possibilities.24
FIGURE 2. Plot of log(k/s-1) for the rearrangement of radical
1• against the normalized Dimroth-Reichardt solvent polarity
N
parameter ET in different solvents. From the linear regres-
sion: slope ) -0.58, r2 ) 0.9944.
been interpreted in terms of the stabilization of the
transition state for cumyloxyl radical C-Me â-scission
through increased solvation of the incipient carbonyl
product. As it has been pointed out that the ETN param-
eter is mainly related to the solvent anion solvating
ability,26 the observation of a log kâ vs ET correlation
N
has been explained in terms of the development of
negative charge on the oxygen atom of the forming
carbonyl compound. Along this line, the negative slope
of the log k vs ETN correlation (Figure 2) observed in the
O-neophyl rearrangement of the 1,1-diphenylethoxyl
radical 1• reasonably reflects the decrease in the extent
of negative charge on the oxygen atom on going from the
starting radical to the transition state.
Quite interestingly, an excellent correlation is obtained
between the logarithm of the experimental rate constants
measured for the rearrangement of the 1,1-dipenylethox-
yl radical 1• (log(k/s-1)) and the normalized Dimroth-
In conclusion, laser flash photolysis experiments car-
ried out in different solvents have shown that the rate
constant for O-neophyl rearrangement of 1,1-diaryla-
lkoxyl radicals decreases by increasing solvent polarity.
This observation provides support for the previous hy-
pothesis that the O-neophyl rearrangement of 1,1-dia-
rylalkoxyl radicals involves a decrease in the extent of
internal charge separation on going from the starting
radical to the transition state. On the other hand, the
kinetic solvent effect data do not provide any new
information that may contribute to distinguishing be-
tween the two possible mechanisms described for the
O-neophyl rearrangement of 1,1-diarylalkoxyl radicals:
the concerted mechanism and the stepwise one that
proceeds through the formation of a bridged 1-oxaspiro-
[2,5]octadienyl radical intermediate.
Reichardt solvent polarity parameter ET (Figure 2).25
N
An analogous correlation (log kâ vs ETN) has been
reported for the â-scission reaction of the cumyloxyl
radical,11,13 where however, in contrast with the present
study, the fragmentation rate constant was observed to
increase by increasing solvent polarity. This behavior has
(23) Falvey, D. E.; Khambatta, B. S.; Schuster, G. B. J. Phys. Chem.
1990, 94, 1056-1059.
(24) As described previously (see ref 1), in a concerted mechanism
a reactant-like transition state that is close in structure to the 1,1-
diarylalkoxyl radical can be expected. On the other hand, in a stepwise
mechanism the transition state of the rate-determining step will be
close in structure to the 1-oxaspiro[2,5]octadienyl radical intermediate.
As compared to the latter mechanism, with the former one a signifi-
cantly smaller decrease in the extent of internal charge separation on
going from the starting 1,1-diarylalkoxyl radical to the transition state
should be expected. Accordingly, the observation of relatively small
kinetic solvent effects in the rearrangement of 1•, with k decreasing
from 4.8 × 106 to 1.5 × 106 s-1 on going from CCl4 to TFE, may be
more in line with the operation of a concerted mechanism. As a matter
of comparison, significantly larger kinetic solvent effects have been
observed for the â-scission reaction of the cumyloxyl radical, with k
increasing from 2.6 × 105 to 6.1 × 106 s-1 on going from CCl4 to TFE
(see refs 11 and 13), indicative of a relatively larger increase in the
extent of charge separation on going from the cumyloxyl radical to the
carbonyl-like transition state.
Experimental Section
Materials. Spectroscopic grade MeCN, CH2Cl2, CCl4, TFE,
and benzene were used as received. The synthesis of peroxides
1p and 2p has been described previously.1 4-Nitrobenzene-
sulfenate ester 1e was prepared by reaction of 1,1-diphenyle-
thanol (1 equiv) and freshly distilled triethylamine (2.5 equiv)
with 4-nitrobenzenesulfenyl chloride (1.1 equiv) in anhydrous
CH2Cl2 at - 78 °C, according to a previously described proce-
dure.20 The crude product was purified by column chromatog-
(25) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry,
3rd ed.; Wiley-VCH: Weinheim, 2003.
(26) See ref 25, pp 462-463.
J. Org. Chem, Vol. 70, No. 25, 2005 10605

raphy (silica gel; eluent hexane/CH2Cl2 1:1) and identified by
1H NMR and 13C NMR. 1H NMR (CD3CN): δ 8.14-8.11 (m, 2H),
7.44-7.31 (m, 12H), 1.99 (s, 3H). 13C NMR (CD3CN): δ 152.6,
144.8, 128.2, 127.8, 126.7, 123.9, 120.8, 117.3, 89.7, 24.6.
Laser Flash Photolysis Studies. Laser flash photolysis
experiments were carried out with a laser kinetic spectrometer
using the fourth harmonic (266 nm) or the third harmonic (355
nm) of a Q-switched Nd:YAG laser. The laser energy was
adjusted to e10 mJ/pulse by the use of the appropriate filter. A
3-mL Suprasil quartz cell (10 mm × 10 mm) was used and all
experiments were carried out at T ) 22 ( 0.5 °C under magnetic
stirring. When method 1 was employed, argon saturated solu-
tions of the peroxides 1p and 2p (≈ 2 × 10-3 M, A266 ≈ 1.0) in
MeCN, MeCN/H2O 1:1, TFE, or CH2Cl2 were photolyzed with
266 nm laser light. When method 2 was employed, argon or
oxygen saturated solutions of 4-nitrobenzenesulfenate ester 1e
(≈ 8 × 10-5 M, A355 ≈ 1.0) in MeCN, CCl4, or benzene were
photolyzed with 355 nm laser light. Rate constants were
obtained by averaging at least eight values and were reproduc-
ible to within 10%. The stability of the solution to the experi-
mental conditions was checked spectrophotometrically by com-
paring the spectrum of the solution before irradiation with that
obtained after irradiation.
Acknowledgment. Financial support from the Min-
istero dell’Istruzione dell’Universita` e della Ricerca
(MIUR) is gratefully acknowledged. We thank Prof.
Osvaldo Lanzalunga for assistance in the synthesis and
characterization of 4-nitrobenzenesulfenate ester 1e and
Prof. Basilio Pispisa and Dr. Lorenzo Stella for the use
and assistance in the use of a LFP equipment.
JO0518755
10606 J. Org. Chem., Vol. 70, No. 25, 2005