Structural effects on the β-scission reaction of alkoxyl radicals. Direct measurement of the absolute rate constants for ring opening of benzocycloalken-1-oxyl radicals

Article abstract of DOI:10.1021/jo048026m

(Chemical Equation Presented) The absolute rate constants for β-scission of a series of benzocycloalken-1-oxyl radicals and of the 2-(4-methylphenyl)-2-butoxyl radical have been measured directly by laser flash photolysis. The benzocycloalken-1-oxyl radicals undergo ring opening with rates which parallel the ring strain of the corresponding cycloalkanes. In the 1-X-indan-1-oxyl radical series, ring opening is observed when X = H, Me, whereas exclusive C-X bond cleavage occurs when X = Et. The factors governing the fragmentation regioselectivity are discussed.

Full text of DOI:10.1021/jo048026m

Structural Effects on the â-Scission Reaction of Alkoxyl Radicals.
Direct Measurement of the Absolute Rate Constants for Ring
Opening of Benzocycloalken-1-oxyl Radicals
Massimo Bietti,*,1a Osvaldo Lanzalunga,1b and Michela Salamone1a
Dipartimento di Scienze e Tecnologie Chimiche, Universita` “Tor Vergata”, Via della Ricerca Scientifica,
1 I-00133 Rome, Italy, and Dipartimento di Chimica, Universita` “La Sapienza”, P.le A. Moro,
5 I-00185 Rome, Italy
bietti@uniroma2.it
Received November 5, 2004
The absolute rate constants for â-scission of a series of benzocycloalken-1-oxyl radicals and of the
2-(4-methylphenyl)-2-butoxyl radical have been measured directly by laser flash photolysis. The
benzocycloalken-1-oxyl radicals undergo ring opening with rates which parallel the ring strain of
the corresponding cycloalkanes. In the 1-X-indan-1-oxyl radical series, ring opening is observed
when X ) H, Me, whereas exclusive C-X bond cleavage occurs when X ) Et. The factors governing
the fragmentation regioselectivity are discussed.
SCHEME 1
C-C â-scission leading to a carbonyl compound and
an alkyl radical is one of the most important reactions
of alkoxyl radicals (Scheme 1), and accordingly, consider-
able attention has been devoted to the study of this
reaction.2-7
One of the main conclusions of these studies has been
that with tertiary alkoxyl radicals cleavage generally
leads to the most stable possible alkyl radical, even
though it has also been shown that in the reactions of
1-alkylcycloalkoxyl radicals other factors such as the
release of ring strain associated to ring opening may play
an important role.6,8-10 On the other hand, kinetic studies
leading to the determination of absolute rate constants
for â-scission of alkoxyl radicals in solution are still
limited,11-23 most of these studies concerning the reactiv-
ity of the tert-butoxyl15-17 and cumyloxyl18-23 radicals. In
particular, when dealing with the ring-opening reactions
of cycloalkoxyl radicals the only data available have been
obtained for the cyclopentoxyl and cyclohexoxyl radicals
by means of competitive kinetics as k ) 4.7 × 108 and
1.1 × 107 s-1, respectively.24 Results which are in line
with those obtained in relative reactivity studies,10 and
(12) Horner, J. H.; Choi, S.-Y.; Newcomb, M. Org. Lett. 2000, 2,
3369-3372.
(13) Nakamura, T.; Busfield, W. K.; Jenkins, I. D.; Rizzardo, E.;
Thang, S. H.; Suyama, S. J. Org. Chem. 2000, 65, 16-23.
(14) Mendenhall, G. D.; Stewart, L. C.; Scaiano, J. C. J. Am. Chem.
Soc. 1982, 104, 5109-5114.
(1) (a) Universita` “Tor Vergata”. (b) Universita` “La Sapienza”.
(2) Nakamura, T.; Watanabe, Y.; Suyama, S.; Tezuka, H. J. Chem.
Soc., Perkin Trans. 2 2002, 1364-1369.
(3) Wilsey, S.; Dowd, P.; Houk, K. N. J. Org. Chem. 1999, 64, 8801-
8811.
(4) Walling, C. Pure Appl. Chem. 1967, 15, 69-80.
(5) (a) Bacha, J. D.; Kochi, J. K. J. Org. Chem. 1965, 30, 3272-
3278. (b) Kochi, J. K. J. Am. Chem. Soc. 1962, 84, 1193-1197.
(6) Greene, F. D.; Savitz, M. L.; Osterholtz, F. D.; Lau, H. H.; Smith,
W. N.; Zanet, P. M. J. Org. Chem. 1963, 28, 55-64.
(7) Gray, P.; Williams, A. Chem. Rev. 1959, 59, 239-328.
(8) Aureliano Antunes, C. S.; Bietti, M.; Lanzalunga, O.; Salamone,
M. J. Org. Chem. 2004, 69, 5281-5289.
(15) Weber, M.; Fischer, H. J. Am. Chem. Soc. 1999, 121, 7381-
7388.
(16) Tsentalovich, Y. P.; Kulik, L. V.; Gritsan, N. P.; Yurkovskaya,
A. V. J. Phys. Chem. A 1998, 102, 7975-7980.
(17) Erben-Russ, M.; Michel, C.; Bors, W.; Saran, M. J. Phys. Chem.
1987, 91, 2362-2365.
(18) Baciocchi, E.; Bietti, M.; Salamone, M.; Steenken, S. J. Org.
Chem. 2002, 67, 2266-2270.
(19) Banks, J. T.; Scaiano, J. C. J. Am. Chem. Soc. 1993, 115, 6409-
6413.
(20) Avila, D. V.; Brown, C. E.; Ingold, K. U.; Lusztyk, J. J. Am.
Chem. Soc. 1993, 115, 466-470.
(21) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Ingold, K. U.;
Lusztyk, J. J. Am. Chem. Soc. 1989, 111, 9269-9270.
(22) Neta, P.; Dizdaroglu, M.; Simic, M. G. Isr. J. Chem. 1984, 24,
25-28.
(9) Walling, C.; Clark, R. T. J. Am. Chem. Soc., 1974, 96, 4530-
4533.
(10) Walling, C.; Padwa, A. J. Am. Chem. Soc. 1963, 85, 1593-1597.
(11) Newcomb, M.; Daublain, P.; Horner, J. H. J. Org. Chem. 2002,
67, 8669-8671.
(23) Baigne´e, A.; Howard, J. A.; Scaiano, J. C.; Stewart, L. C. J.
Am. Chem. Soc. 1983, 105, 6120-6123.
10.1021/jo048026m CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/14/2005
J. Org. Chem. 2005, 70, 1417-1422
1417

Bietti et al.
SCHEME 2
CHART 1
Results and Discussion
Product Studies. Argon-saturated CH2Cl2 solutions
containing 1-methylindanol (3a) or 1-ethylindanol (4a)
(10 mM), DIB (11 mM), and I2 (10 mM) were irradiated
with visible light (λmax ≈ 480 nm) at T ) 20 °C. The
irradiation time was chosen in such a way as to avoid
complete substrate conversion. After workup of the
reaction mixture, the reaction products were identified
by GC-MS and 1H NMR and quantitatively determined,
together with the unreacted substrate by GC and 1H
NMR, using bibenzyl as internal standard.
The reaction of 3a led to the formation of 2-(2-
iodoethyl)acetophenone as the exclusive reaction product.
1-Indanone was instead the exclusive product observed
with 4a. Formation of these products can be explained
in terms of the â-scission reaction of the intermediate
1-alkylindan-1-oxyl radicals 3• and 4• as described in
Scheme 2.
which have been rationalized in terms of the greater ring
strain associated to a five-membered ring as compared
to a six membered one.
The recent discovery that arylcarbinyloxyl radicals
display an absorption band in the visible region of the
spectrum25 makes possible the direct measurement of
alkoxyl radical reactivity by laser flash photolysis.18-20,26
Along this line, to obtain kinetic information on the ring
opening reaction of cycloalkoxyl radicals, we have carried
out a time-resolved kinetic study on the reactivity of a
series of benzocycloalken-1-oxyl radicals (1•-6•) whose
structures are shown in Chart 1. As a matter of com-
parison, the study has been extended to the 2-(4-methyl-
phenyl)-2-butoxyl radical (7•),27 a tertiary alkoxyl radical
which is structurally related to 3• and 4•, lacking however
the rigidity imposed by the presence of the five-membered
ring.
With the 1-methylindan-1-oxyl radical (3•) exclusive
ring-opening occurs followed by rapid reaction of the ring-
opened carbon centered radical with iodine32,33 to give
2-(2-iodoethyl)acetophenone (path a). The 1-ethylindan-
1-oxyl radical (4•) undergoes instead exclusive C-ethyl
bond cleavage to give 1-indanone (path b).
Time-Resolved Studies. Benzocycloalken-1-oxyl radi-
cals (1•-6•) were generated by ns 248 or 266 nm laser
flash photolysis (LFP) of the parent tert-butyl-benzo-
cycloalken-1-yl peroxides (1p-6p: between 2.0 and 5.6
× 10-3 M) in argon or oxygen saturated MeCN solution
(eq 1).
With radicals 3•, 4•, and 7• it is possible, at least in
principle, to obtain two different C-C bond fragmenta-
tions. Previous studies have shown that the 2-phenyl-2-
butoxyl radical undergoes exclusive C-ethyl bond cleav-
age,28 and an analogous behavior can be reasonably
expected for 7•. No information is instead available for
3• and 4•. Thus, to obtain information on the fragmenta-
tion regioselectivity of 3• and 4• we have also carried out
product studies generating the radicals photochemically
by visible light irradiation of CH2Cl2 solutions containing
the parent 1-alkylindanols, (diacetoxy)iodobenzene (DIB),
and I2.8,29
All benzocycloalken-1-oxyl radicals displayed an ab-
sorption band in the UV region of the spectrum between
(24) (a) Beckwith, A. L. J.; Hay, B. P. J. Am. Chem. Soc. 1989, 111,
2674-2681. (b) Beckwith, A. L. J.; Hay, B. P. J. Am. Chem. Soc. 1989,
111, 230-234.
(29) It is well established that under these conditions the DIB/I2
reagent converts alcohols into intermediate hypoiodites which are then
photolyzed to give alkoxyl radicals, precursors of the observed reaction
products (see, for example, refs 30 and 31).
(25) (a) 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.
(b) Avila, D. V.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1992,
114, 6576-6577.
(30) (a) Sua´rez, E.; Rodriguez, M. S. In Radicals in Organic
Synthesis; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim, 2001;
Vol. 2, pp 440-454. (b) Togo, H.; Katohgi, M. Synlett 2001, 5, 565-
581.
(26) Banks, J. T.; Scaiano, J. C. J. Phys. Chem. 1995, 99, 3527-
3531.
(27) The tert-butyl-2-(4-methylphenyl)-2-butyl peroxide (7p) was
chosen because, as compared to the ring-unsubstituted one, the
presence of a methyl group ensured a significantly higher absorption
at the laser excitation wavelength (266 nm). As discussed previously
for cumyloxyl radicals (see ref 18), the reactivity should not be affected
by the presence of the ring-substituent.
(31) Courtneidge, J. L.; Lusztyk, J.; Page`, D. Tetrahedron Lett. 1994,
35, 1003-1006.
(32) Minisci, F.; Recupero, F.; Gambarotti, C.; Punta, C.; Paganelli,
R. Tetrahedron Lett. 2003, 44, 6919-6922.
(33) Hook, S. C. W.; Saville, B. J. Chem. Soc., Perkin Trans. 2 1975,
589-593.
(28) Hawkins, E. G. E. J. Chem. Soc. 1949, 2076-2077.
1418 J. Org. Chem., Vol. 70, No. 4, 2005

Structural Effects on the â-Scission Reaction of Alkoxyl Radicals
FIGURE 1. Time-resolved absorption spectra observed after
248 nm LFP of an oxygen-saturated MeCN solution containing
4 × 10-3 M tert-butyl-1-tetralyn peroxide (5p) at 70 (filled
circles), 150 (empty circles), 300 (filled squares), and 700 ns
(empty squares) after the 20 ns, 20 mJ laser flash.
FIGURE 2. Time-resolved absorption spectra observed after
248 nm LFP of an oxygen-saturated MeCN solution containing
4 × 10-3 M tert-butyl-1-indanyl peroxide (1p) at 35 (filled
circles), 75 (empty circles), and 350 ns (filled squares) after
the 20 ns, 20 mJ laser flash.
320 and 350 nm, and a broad absorption band in the
visible region between 500 and 530 nm depending on
radical structure.25 Both bands were found to be unaf-
fected by oxygen and were observed to decay following
first-order kinetics. A more intense band centered around
280 nm was also observed which however was found to
undergo a much slower decay and which is assigned to
the tert-butoxyl radical on the basis of the comparison
with literature data.16,18,34 Decay of the benzocycloalken-
1-oxyl radical bands led in all cases to a corresponding
strong increase in absorption in the UV region of the
spectrum which, again, was found to be unaffected by
oxygen and which is assigned to the ring-conjugated
carbonyl product formed after â-scission (see below).
SCHEME 3
oxide (1p), as shown in Figure 2. The indan-1-oxyl radical
(1•) visible absorption band was red-shifted by 10 nm,25a
and relatively less intense as compared to 5•, and
underwent a first-order decay accompanied by a corre-
sponding buildup of optical density at 245 nm assigned
•
to 2-HCOC6H4CH2CH2 .
As an example, Figure 1 shows the time-resolved
absorption spectra obtained after 248 nm LFP of an
oxygen saturated MeCN solution containing tert-butyl-
1-tetralyn peroxide (5p). No significant spectral difference
was observed when the experiment was carried out in
argon-saturated solution. The spectrum recorded 70 ns
after the laser flash (filled circles) shows two absorption
bands centered at 320 and 520 nm which, in agreement
with previous studies, are assigned to the tetralyn-1-oxyl
radical (5•).25a This species undergoes a first-order decay
accompanied by a corresponding buildup of optical
density at 240 nm assigned to the carbonyl product
2-HCOC6H4CH2CH2CH2• formed by C-C â-scission of 5•
(Scheme 3).35
The 2-(4-methylphenyl)-2-butoxyl radical (7•) was in-
stead generated by 266 nm ps LFP of tert-butyl-2-(4-
methylphenyl)-2-butyl peroxide (7p) in MeCN (eq 2).
A relatively more stable absorption band centered at
280 nm is also observed which, as described above, is
assigned to the tert-butoxyl radical.16,18
An analogous behavior was observed after 248 nm LFP
of a MeCN solution containing tert-butyl-1-indanyl per-
A visible absorption band centered at 510 nm appeared
immediately after the 20 ps laser pulse (Figure 3a), which
can be reasonably assigned to the 2-(4-methylphenyl)-2-
butoxyl radical (7•).
This radical was observed to undergo a first-order
decay on the nanosecond time-scale (Figure 3b) which,
on the basis of product studies,28 is assigned to C-ethyl
bond cleavage (Scheme 4).
(34) An extinction coefficient ꢀ280 ) 560 M-1 cm-1 has been deter-
mined for the tert-butoxyl radical in MeCN solution (see ref 16). Since
identical amounts of the tert-butoxyl and benzocycloalken-1-oxyl
radicals 1•-6• are produced after LFP of peroxides 1p-6p, the
extinction coefficients of the UV and visible absorption bands of radicals
1•-6• are smaller than this value.
(35) The 240 nm band is assigned to the radical 2-HOCC6H4CH2-
CH2CH2• which is expected to react rapidly with oxygen. However, since
this band is associated to the ring-conjugated carbonyl chromophore,
it is not expected to be influenced by the reaction with oxygen.
The experimental rate constants for decay of radicals
1•-7• were measured spectrophotometrically by monitor-
ing the decrease in absorption at the visible absorption
maximum of the radicals and/or the increase in absorp-
tion in the UV region around 240 nm and are collected
J. Org. Chem, Vol. 70, No. 4, 2005 1419

Bietti et al.
TABLE 1. Visible Absorption Band Maximum
Wavelengths for Radicals (1•-7•) and Kinetic Data for
Their Decay by â-Scission (kâ) in MeCN
λmax
radical
indan-1-oxyl
1-deuterioindan-1-oxyl
1-methylindan-1-oxyl
1-ethylindan-1-oxyl
(vis)/nma
kâ/s-1b
T/°C
1•
2•
3•
4•
530
530
520
520
1.3 × 107
1.3 × 107
1.3 × 107
g8 × 107c
1.2 × 107
8.2 × 105d
1.5 × 106d
2.5 × 108
g8 × 107c
1.1 × 107
7.1 × 105e
22
22
22
0
-15
22
tetralyn-1-oxyl
5•
6•
520
500
510
benzosuberyl-1-oxyl
22
2-(4-methylphenyl)-2-butoxyl 7•
22
0
-15
22
4-methylcumyloxyl
510
a Visible absorption band maximum of the radical. b Determined
by following the decay of the radical at the visible absorption
maximum and/or the buildup of the carbonyl product formed after
C-C â-scission at 240 nm. c Under these conditions, the radical
was too reactive to be studied by ns LFP, and on the basis of the
buildup at 240 nm, only a lower limit could be determined. d A
dependence of the decay rate on laser energy was observed, and
the rate constants were extrapolated from the intercepts of the
rate constant vs laser energy plots (see the Supporting Informa-
tion). e Taken from ref 18.
FIGURE 3. (a) Transient absorption spectrum observed after
266 nm LFP of an air-saturated MeCN solution containing 1.5
× 10-2 M tert-butyl-2-(4-methylphenyl)-2-butyl peroxide (7p)
at 100 ps after the 20 ps laser pulse. (b) Decay kinetics
monitored at 510 nm.
SCHEME 4
cleavage was observed, with 4• the fragmentation regio-
selectivity is essentially governed by the greater stability
of the ethyl radical as compared to the methyl one and
by the entropy gain associated to the C-ethyl cleavage,
whereas the release of ring strain associated to ring
opening plays a minor role.
in Table 1, together with the corresponding data for the
4-methylcumyloxyl radical.
The spectral data displayed in Table 1 show that the
position of the visible absorption band of benzocycloalken-
1-oxyl radicals is not influenced to a significant extent
by the ring size and by the presence of an alkyl substi-
tuent in position 1, in agreement with previous studies.25a
For what concerns instead the 2-(4-methylphenyl)-2-
butoxyl radical (7•), the visible absorption band is cen-
tered at 510 nm in line with the value observed for the
4-methylcumyloxyl radical,18,25 again indicating that the
position of the visible absorption band is not influenced
by small modifications in side-chain structure.
Table 1 also shows the values of the rate constants for
â-scission of the radicals 1•-7•. In the indan-1-oxyl series,
with the exclusion of 4• (see below), the value of kâ is not
influenced by the nature of the group in position 1
(H, D, Me), the three radicals (1•-3•) undergoing ring
Unfortunately, the weak visible absorption band of 4• 34
did not allow LFP studies on the picosecond time-domain,
and due to the high reactivity of this radical, kâ was
determined by nanosecond LFP only at T ) -15 °C,
whereas at T ) 0 °C only a lower limit (kâ g 8 × 107 s-1
)
could be determined.36
Quite interestingly, the observation that 4• undergoes
exclusive C-ethyl bond cleavage whereas ring opening
was the only fragmentation pathway observed for the
1-ethylcyclopentoxyl radical8 suggests that, due to the
rigidity imposed by the presence of the fused phenyl ring,
the release of ring strain associated to ring opening is
relatively less important in the former radical as com-
pared to the latter one. Another factor which may also
account for this behavior is represented by the fact that,
as compared to the 1-ethylcyclopentoxyl radical, in 4•
C-ethyl bond cleavage can be assisted by the overlap
between this bond and the aromatic π system.25a
opening with the same rate constant: kâ ) 1.3 × 107 s-1
.
This result is apparently in contrast with those obtained
recently by Newcomb and Horner for a series of alkoxyl
radicals,11 showing that the fragmentation rate constant
increased as a function of the carbonyl compound pro-
duced in the fragmentation reaction in the order CH2O
< MeCHO < Me2CO. Clearly, the observation of identical
rate constants for 1• and 3• suggests that with these
radicals the release of ring strain associated to the five
member ring opening is significantly more important
than the stability of the carbonyl product formed.
Comparison between the rate constants for ring open-
ing of the benzocycloalken-1-oxyl radicals37 points toward
the importance of ring strain in governing the reactivity
of these radicals. The observed reactivity order, kâ (1•) >
kâ (6•) > kâ (5•), to which correspond the following relative
reactivities, 15.8:1.8:1, parallels the ring strain of the
corresponding cycloalkanes.38 Interestingly, an analogous
The comparison between the rate constants for 1• and
2• shows that, as expected, replacement of H by D does
not influence the transition state for â-scission.
The 1-ethylindan-1-oxyl radical (4•) was instead found
to undergo â-scission with a significantly higher rate
constant than those measured for radicals 1•-3•, in a
reaction which, on the basis of product studies, is
assigned to C-ethyl bond cleavage. An observation which
indicates that in contrast with 3•, where no C-methyl
(36) As a matter of comparison, also the reactivity of radical 7• was
studied at T ) -15 and 0 °C (see Table 1).
(37) Unfortunately, attempts to synthesize the tert-butyl benzo-
cyclobutenyl peroxide from the parent benzocyclobuten-1-ol failed.
(38) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds;
John Wiley & Sons: New York, 1994.
1420 J. Org. Chem., Vol. 70, No. 4, 2005

Structural Effects on the â-Scission Reaction of Alkoxyl Radicals
emission between 400 and 550 nm, λmax ≈ 480 nm). The reactor
was a cylindrical flask equipped with a water cooling jacket
thermostated at 20 °C. Irradiation times were chosen in such
a way as to avoid complete substrate consumption. In a typical
experiment, a solution of the alcohol 3a or 4a (10 mM) in CH2-
Cl2 (5 mL) containing (diacetoxy)iodobenzene (11 mM) and
iodine (10 mM) was irradiated for times varying between 5
and 10 min under Ar bubbling. The reaction mixture was then
poured into water and extracted with CH2Cl2 (2 × 10 mL).
The combined organic layers were washed with a 10% aqueous
thiosulfate solution (2 × 30 mL) and water (2 × 30 mL) and
dried over anhydrous sodium sulfate. Reaction products were
identified by GC-MS and 1H NMR and quantitatively deter-
reactivity order was observed for the nucleophilic ring-
opening reactions of cyclic dimethylammonium ions.39
It is also very interesting to compare the kâ value
measured for the 2-(4-methylphenyl)-2-butoxyl radical
(7•), kâ ) 2.5 × 108 s-1, with that measured previously
for the 4-methylcumyloxyl radical: kâ ) 7.1 × 105 s-1 18
.
The comparison shows that in tertiary arylcarbinyloxyl
radicals ethyl radical ejection occurs 350 times faster
than that of a methyl radical, a result which is in line
with those obtained in previous studies, based however
on relative reactivities.4,5b,13
Compounds 7•, 3•, and 4• can be considered structurally
related tertiary arylcarbinyloxyl radicals (Chart 1), the
former one lacking the rigidity imposed by the presence
of the five-membered ring. Thus, comparison between the
rate constants for â-scission measured for these radicals
may provide useful information on the effect of radical
structure on reactivity. As mentioned above, 3• undergoes
exclusive ring-opening whereas C-ethyl cleavage is the
exclusive reactive pathway for both 4• and 7•. The rate
constants collected in Table 1 show that 7• undergoes
â-scission ∼20 times faster than 3•, a result which
suggests that the entropy gain associated to the C-ethyl
cleavage in the former radical is relatively more impor-
tant than the release of ring strain associated to the five-
membered ring opening in 3•. For what concerns instead
the comparison between 4• and 7•, even though it was
not possible to measure the rate constant for â-scission
of 4• at room temperature, the data collected in Table 1
point toward similar reactivities for these two radicals.
This observation suggests that in the â-scission reactions
of arylcarbinyloxyl radicals which lead to the formation
of the same alkyl radical the stability of the carbonyl
product formed in the fragmentation reaction plays a
minor role. This result is in line with those obtained
recently for the â-scission reactions of a series of ring-
substituted cumyloxyl radicals.18
1
mined, together with the unreacted substrate by GC and H
NMR, using bibenzyl as internal standard. Good mass balances
(g85%) were obtained in all experiments.
The reaction of 3a led to the formation of 2-(2-iodoethyl)-
acetophenone as the exclusive reaction product. After workup,
2-(2-iodoethyl)acetophenone was isolated from the reaction
mixture by preparative TLC (silica gel, eluent hexane/ethyl
acetate 10:1) and identified by GC-MS, 1H NMR, and 13C
NMR. GC-MS m/z (relative abundance): M+ 146, 131, 127,
1
116, 115 (100), 103, 91, 89, 77, 63, 51, 50. H NMR (CDCl3):
δ 7.74-7.26 (m, 4H), 3.41 (s, 4H), 2.61 (s, 3H). 13C NMR
(CDCl3): δ 201.3, 140.7, 137.1, 132.0, 130.1, 127.1, 126.4, 38.8,
29.5, 6.2.
The reaction of 4a led to the formation of 1-indanone
(identified by comparison with an authentic sample) as the
exclusive reaction product. No effort was made to detect the
formation of ethyl iodide.
Time-Resolved LFP Studies. Benzocycloalken-1-oxyl radi-
cals 1•-6• were generated at room temperature by direct laser
flash photolysis (LFP) of the corresponding tert-butyl peroxides
(1p-6p), 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.40 Argon- or oxygen-
saturated solutions of the peroxides (between 2.0 and 5.6 ×
10-3 M: A248 ≈ 0.3-0.6) in MeCN were flowed through a
2 mm (in the direction of the laser beam) × 4 mm (in the
direction of the analyzing light, 90° geometry) Suprasil quartz
cell. All experiments were carried out at 22 ( 2 °C. Rate
constants were obtained by averaging 6-12 values, each
consisting of an average of 10-50 laser shots, and were
reproducible to within 10%.
In conclusion, the absolute rate constants for ring-
opening of a series of benzocycloalken-1-oxyl radicals
have been measured directly by laser flash photolysis.
The radicals undergo ring opening with rates which
parallel the ring strain of the corresponding cycloalkanes.
In the 1-X-indan-1-oxyl radical series ring-opening is
observed when X ) H, Me, whereas exclusive C-X bond
cleavage occurs when X ) Et, a behavior which indicates
that the fragmentation regioselectivity is essentially
governed by the interplay between the stability of the
radical formed by C-X bond cleavage, entropic effects,
and the release of ring strain associated to ring opening.
Some experiments were also carried out employing a Nd:
YAG laser (266 nm), providing 8 ns pulses with energies
between 5 and 20 mJ/pulse (output power of the laser),
irradiating argon-saturated MeCN solutions of peroxides 4p
or 7p (1.5 and 3.8 × 10-3 M, respectively: A266 ≈ 1), contained
in a 3 mL Suprasil quartz cell (10 mm × 10 mm) under
magnetic stirring. The experiments were carried out at 0 and
-15 °C. As a matter of comparison, peroxide 1p was also
studied under these conditions at T ) 22 °C, and a value of kâ
for fragmentation of 1• identical to that measured employing
248 nm LFP (kâ ) 1.3 × 107 s-1) was obtained.
Experimental Section
Materials. MeCN of the highest available purity was used
as received. Commercial samples of indan-1-ol (1a) and tetral-
1-ol (5a) were used without further purification. Details on
the synthesis of the arylcarbinols (2a-4a, 6a, 7a) and of the
tert-butyl arylcarbinyl peroxides (1p-7p) are given in the
Supporting Information.
Product Studies. The reactions were carried out under an
argon atmosphere. Dichloromethane was purified prior to use
by column chromatography over basic alumina. Irradiations
were performed with visible light (10 × 15 W lamps with
The 2-(4-methylphenyl)-2-butoxyl radical (7•) was generated
by direct LFP of tert-butyl-2-(4-methylphenyl)-2-butyl peroxide
(7p), using 266 nm light (fourth harmonic of a Nd:YAG laser,
Continuum PY61C-10) providing 20 ps pulses with energies
between 2 and 3 mJ/pulse. A solution of the peroxide (1.5 ×
10-2 M: A266 ≈ 1) in MeCN was flowed through a 2 mm (in
the direction of the pump) × 4 mm (in the direction of the
(39) Di Vona, M. L.; Illuminati, G.; Lillocci, C. J. Chem. Soc., Chem.
Commun. 1985, 380-381. Mandolini, L. Adv. Phys. Org. Chem. 1986,
22, 1-111.
(40) Faria, J. L.; Steenken, S. J. Phys. Chem. 1993, 97, 1924-1930.
J. Org. Chem, Vol. 70, No. 4, 2005 1421

Bietti et al.
probe, 90° geometry) quartz cell. All experiments were carried
out at 22 ( 2 °C. The picosecond spectrometer and the methods
of data handling have been described in detail previously.41
Dr. Gagik Gurzadyan for helpful discussions and as-
sistance in the flash photolysis experiments.
Supporting Information Available: Details on the syn-
thesis and characterization of the precursor arylcarbinols and
tert-butyl arylcarbinyl peroxides. Plots of the decay rate
constants against laser energy for radicals 5• and 6•. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. Financial support from the Min-
istero dell’Istruzione dell’Universita` e della Ricerca
(MIUR) - Progetto FIRB (RBAU01PPHX_004) is grate-
fully acknowledged. We thank Prof. Steen Steenken and
(41) Gurzadyan, G. G.; Steenken, S. Chem. Eur. J. 2001, 7, 1808-
1815.
JO048026M
1422 J. Org. Chem., Vol. 70, No. 4, 2005