Generation and reactivity toward oxygen of carbon-centered radicals containing indane, indene, and fluorenyl moieties

Article abstract of DOI:10.1021/jo026666o

Resonance-stabilized radicals containing indane, indene, and fluorenyl moieties exhibit attenuated reactivity toward oxygen. Rate constants of ～10⁵ M^-1 s^-1 were observed for the most stabilized radicals. The dependence of kox (rate constant for radical trapping by oxygen) on the corresponding bond dissociation energies revealed that stereoelectronic effects are more important than steric effects in determining the low radical reactivity with oxygen. Scavenging by the nitroxide TEMPO was also examined, and revealed that in this case steric effects are more important than in the case of oxygen. The rate constants for the hydrogen abstraction by cumyloxyl and tert-butoxyl radicals generated thermally and photochemically have been determined in benzene, and were in the range of ca. (1-13) × 10⁶ M^-1 s^-1, showing that benzylic stabilization has a modest effect on substrate reactivity as a hydrogen donor toward alkoxyl radicals.

Full text of DOI:10.1021/jo026666o

Gen er a tion a n d Rea ctivity tow a r d Oxygen of Ca r bon -Cen ter ed
Ra d ica ls Con ta in in g In d a n e, In d en e, a n d F lu or en yl Moieties
Enrique Font-Sanchis, Carolina Aliaga, Elena V. Bejan, Raecca Cornejo, and J . C. Scaiano*
Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
tito@photo.chem.uottawa.ca
Received November 5, 2002
Resonance-stabilized radicals containing indane, indene, and fluorenyl moieties exhibit attenuated
5
-1 -1
reactivity toward oxygen. Rate constants of ∼10 M
s
were observed for the most stabilized
radicals. The dependence of kOX (rate constant for radical trapping by oxygen) on the corresponding
bond dissociation energies revealed that stereoelectronic effects are more important than steric
effects in determining the low radical reactivity with oxygen. Scavenging by the nitroxide TEMPO
was also examined, and revealed that in this case steric effects are more important than in the
case of oxygen. The rate constants for the hydrogen abstraction by cumyloxyl and tert-butoxyl
radicals generated thermally and photochemically have been determined in benzene, and were in
6
-1 -1
the range of ca. (1-13) × 10 M
s , showing that benzylic stabilization has a modest effect on
substrate reactivity as a hydrogen donor toward alkoxyl radicals.
In tr od u ction
the R-H bond strength is less than the appropriate C-H
bond strength in an alkane”.10
Since Gomberg prepared the persistent and stabilized
triphenylmethyl radical in 1900 diverse examples of
stabilized carbon-centered radicals have been reported.1
Thus, it is known that the introduction of bulky substit-
uents increases the persistency of the radicals; for
example, perchlorotriphenylmethyl and perchloro-9-
phenylfluorenyl radicals are rather persistent and un-
reactive toward oxygen in solid form.5,6 However, the
reaction of triphenylmethyl radicals with oxygen has
been found to be reversible and, at room temperature and
under air the equilibrium, is largely displaced toward the
peroxyl radical form.7 Further, cyanobis(pentachloro-
phenyl)methyl and R-isopropoxycarbonylbis(pentamethyl-
phenyl)methyl radicals, the substituted analogues of the
diphenylmethyl radical, are persistent and do not react
with oxygen. The persistency of these radicals has been
We have recently reported on the importance of
electronic effects to explain the low reactivity with oxygen
of some lactone and nitrile-derived carbon-centered radi-
-4
3
-1
cals (oxygen quenching rate constants e5 × 10
M
-
1
11-13
s
). Further, the reactivity enhancement for the
hydrogen abstraction from these compounds with tert-
butoxyl radicals (especially 2-coumaranone and 9-cyano-
fluorene) has showed the stabilizing effect of the lactone
and cyano group on the radical generated.
,8
We have proposed five parameters to explain the
diminished reactivity toward oxygen of carbon-centered
radicals: (a) benzylic resonance stabilization; (b) favor-
able stereoelectronic effects; (c) unpaired spin delocal-
ization on heteroatoms, particularly oxygen; (d) electron-
withdrawing effects; and (e) steric effects.12 Thus, the
present studies were undertaken in an attempt to
understand better the structural parameters that can
attenuate the reactivity of carbon-centered radicals with
molecular oxygen, and to test the validity of the paradigm
of points a-e for a group of structurally related hydro-
carbon radicals.
ascribed mainly to steric factors, rather than electronic
effects.9
We use the term persistent as defined by Griller and
Ingold to describe radicals that have “a lifetime signifi-
cantly greater than methyl under the same conditions”,
while the term stabilized is reserved for radicals “when
(1) Gomberg, M. J . J . Am. Chem. Soc. 1900, 22, 757-771.
(2) Koelsch, C. F. J . Am. Chem. Soc. 1957, 79, 4439-4441.
(3) Ziegler, K.; Ochs, C. Chem. Ber. 1922, 55, 2257-2277.
(4) Sabacky, M. J .; J ohnson, C. S.; Smith, R. G.; Gutowsky, H. S.;
Martin, J . C. J . Am. Chem. Soc. 1967, 89, 2054-2058.
5) Ballester, M.; Riera-Figueras, J .; Castaner, J .; Badia, C.; Monso,
J . M. J . Am. Chem. Soc. 1971, 93, 2215-2225.
6) Ballester, M.; Castaner, J .; Pujadas, J . Tetrahedron Lett. 1971,
(
(
(10) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1976, 9, 13-19.
(11) Scaiano, J . C.; Martin, A.; Yap, G. P. A.; Ingold, K. U. Org. Lett.
2000, 2, 899-901.
2
0, 1699-1702.
(
7) Howard, J . A.; Ingold, K. U. Can. J . Chem. 1968, 46, 2655-2660.
(8) J anzen, E. G.; J ohnston, F. J .; Ayers, C. L. J . Am. Chem. Soc.
(12) Bejan, E. V.; Font-Sanchis, E.; Scaiano, J . C. Org. Lett. 2001,
3, 4059-4062.
1
967, 89, 1176-1183.
9) Clarke, L. F.; Hegarty, A. F.; O’Neill, P. J . Org. Chem. 1992, 57,
(
(13) Font-Sanchis, E.; Aliaga, C.; Focsaneanu, K.-S.; Scaiano, J . C.
Chem. Commun. 2002, 15, 1576-1577.
3
62-366.
1
0.1021/jo026666o CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/25/2003
J . Org. Chem. 2003, 68, 3199-3204
3199

Font-Sanchis et al.
CHART 1
SCHEME 1
In this context, we now report the generation and
reactivity with oxygen of radicals derived from the
aromatic compounds in Chart 1. We chose these mol-
ecules to establish the importance of resonance stabiliza-
tion and steric effects to explain the unreactivity of
certain carbon-centered radicals with oxygen. As part of
this work, we have also examined the reactivity of these
substrates toward alkoxyl radicals.
F IGURE 1. Determination of the rate constant for the
reaction of tert-butoxyl radicals with 9-phenylfluorene (9) and
9
-tert-butylfluorene (8) in benzene at room temperature.
TABLE 1. Ra te Con sta n ts for th e Rea ction of
ter t-Bu toxyl Ra d ica ls w ith Su bstr a tes a t Room
Tem p er a tu r e in Ben zen e
Resu lts
concn of
substrates, rate constant,
monitoring
wavelength,
nm
Hyd r ogen Don or Ability. The reactivity of com-
pounds 2, 4, 6, 8, and 9 toward alkoxyl radicals was
determined by studying their reaction with tert-butoxyl
radicals with use of laser flash photolysis techniques.14
The radicals produced in these reactions have convenient
absorptions that can be used to monitor directly their
formation.
a
6
-1 -1
substrate
M
10 M
s
diphenylmethane15
0.91
2
4
6
8
9
0.03-0.06
0.02-0.14
0.01-0.04
0.01-0.44
0.01-0.06
8.1
<5.8
6.2
340
340
500
490
340
b
1.8
9.9
Thus, tert-butoxyl radicals were produced by 355-nm
laser excitation of a deaerated solution of the peroxide
in benzene 50% (v/v) (reaction 1, Scheme 1). The growth
a
Errors for rate constants were determined to be less than 5%.
This value may incorporate some double bond addition that
cannot be readily separated with laser photolysis techniques.
b
of the radical signal reflects reaction 2 (k
any other form of reaction between tert-butoxyl and RH
even when the product is undetectable). The term k
2
) and includes
(Scheme 2).16 In eq 7, ka is the rate constant for the
(
0
incorporates other forms of decay of tert-butoxyl that do
not depend on substrate concentration, such as reaction
with the solvent and â-cleavage. The experimental rate
constant for the growth is given by eq 3. Figure 1 shows
two representative plots according to eq 3 from which
hydrogen atom abstraction, k is the rate constant for the
â
â scission, and k ′ incorporates the hydrogen atom
0
abstraction from the solvent and from dicumyl hypo-
nitrite.
This method was used to determine k for 1, 3, and 5.
a
the values of k
2
are obtained. The rate constants deter-
Thermolysis of dicumyl hyponitrite (0.0027 M) was
mined are given in Table 1.
carried out in deaerated benzene at 30 °C for 12 h in the
presence of various substrate concentrations (0-70 mM).
The relative yields of 2-phenyl-2-propanol (A) and aceto-
phenone (K) were determined by gas chromatography.
Unfortunately, radicals formed from 1 and 3 did not
produce good signals that could be used to monitor their
formation at different concentrations of substrate. More-
over, 5 absorbs at 355 nm, so it was impossible to
The plot of A/K vs [RH] yields k
the contribution of k ′ has been neglected in eq 7.
The values of k for 1, 3, and 5 (see Table 2) were
obtained by using k determined for Ciba Irganox HP-
a â
/k from the slope, where
2
calculate k for this compound with this method. These
0
problems led us to use another method. Thus, thermolysis
of di-R-cumyl hyponitrite has been used to calculate the
reactivity of cumyloxyl radicals toward hydrocarbons
a
a
(15) Arends, I. W. C. E.; Mulder, P.; Clark, K. B.; Wayner, D. D. M.
J . Phys. Chem. 1995, 99, 8182-8189.
(16) Avila, D. V.; Brown, C. E.; Ingold, K. U.; Lusztyk, J . J . Am.
Chem. Soc. 1993, 115, 466-470.
(
14) Paul, H.; Small, R. D.; Scaiano, J . C. J . Am. Chem. Soc. 1978,
1
00, 4520-4527.
3
200 J . Org. Chem., Vol. 68, No. 8, 2003

Reactivity toward Oxygen of Carbon-Centered Radicals
SCHEME 2
F IGURE 2. Transient absorption spectra recorded following
3
3
55-nm laser excitation of a sample containing 10.4 mM
-phenyl-1-indene (4) in benzene/di-tert-butyl peroxide (50/50)
under oxygen, (b) 0.72 and (2) 10.1 µs after the laser pulse.
Inset: Transient kinetic traces recorded at 420 nm under
nitrogen (0) and oxygen (O).
TABLE 2. Ra te Con sta n ts for th e Rea ction of
Cu m yloxyl Ra d ica ls w ith Su bstr a tes a t 30 °C in Ben zen e
a
rate constant,
6
-1 -1
substrate
10 M
s
Ciba Irganox HP-136
12.4b
1
3
5
5.40
6.80
13.0
a
Errors for rate constants are estimated at 10%. b Calculated
by laser flash photolysis of dicumyl peroxide in the presence of
different amounts of HP-136 in benzene at room temperature.
1
3611 (k
dicumyl peroxide as source of cumyloxyl radicals (see eq
).
In general, the rate constants were not unusual for
a
HP-136), using the first method (see eqs 1-3) and
8
F IGURE 3. Transient absorption spectra recorded following
3
55-nm laser excitation of a sample containing 4 mM 1,3-
diphenyl-1-indene (5) in benzene/di-tert-butyl peroxide (50/50)
under nitrogen, (b) 5.28 and (2) 9.52 µs after the laser pulse.
benzylic C-H bonds. Higher rate constants were ob-
served for those compounds that generated more reso-
nance-stabilized radicals.
by hydrogen abstraction at the allylic position and
addition to the double bond.15 Addition to the double bond
is expected to occur at the 2-position in 4, and addition
to the 1-position is less likely and results in the formation
of a secondary alkyl radical, which is not detectable in
the UV-visible region. Thus, the transient at 340 nm,
which was quenched by oxygen, could be assigned to the
Rea ctivity tow a r d Oxygen . The first two molecules
chosen for study, indane (1) and 1-phenylindane (2),
present only benzylic resonance stabilization. Photolysis
of di-tert-butyl peroxide in the presence of 2 shows the
formation of a radical, where the UV band, centered
around 340 nm, is virtually identical with that for the
diphenylmethyl radical.17 In an oxygen-saturated sample,
1
-phenyl-2-tert-butoxyindanyl radical; and the absorption
the signal for radical 2 was completely suppressed. The
same high reactivity toward oxygen was observed for the
radical from indane 1.
The study of the second series of molecules based on
the indene structure (3-5) should reveal the importance
of the stereoelectronic effects due to the presence of the
indene ring and also phenyl substituents. Photolysis of
di-tert-butyl peroxide in the presence of 3-phenyl-1-indene
bands at 340, 420, and 500 nm that were not affected by
oxygen are assigned to the 1-phenylindenyl radical. Note
that rate determinations with laser flash photolysis
techniques include all modes and sites of reaction,
regardless of which species is monitored.
The radical generated from laser flash photolysis of di-
tert-butyl peroxide in the presence of indene 3 was readily
quenched by oxygen, as expected. Finally, the laser flash
photolysis of di-tert-butyl peroxide in the presence of 1,3-
diphenyl-1-indene (5) yielded a transient with absorption
bands at 360 and 500 nm (Figure 3); this transient could
be ascribed to the 1,3-diphenylindenyl radical. The same
spectrum was obtained when the experiment was carried
out under an oxygen atmosphere; moreover, the growth
at 360 nm was only slightly affected by oxygen. No
benzylic product-radical arising from addition to the
double bond was generated in this case. Steric effects due
to the presence of a second aromatic ring in the indene
(4) yielded a transient with an absorption band at 340
nm together with weak signals at 420 and 500 nm.
Similar experiments under oxygen (Figure 2) produced
the same spectra but with a diminished band at 340 nm.
Monitoring at 340 and 420 nm, the comparison of
nitrogen- and oxygen-saturated samples showed that
oxygen only affected the signal at 340 nm, indicating that
two different radicals were generated. It has been previ-
ously reported that tert-butoxyl radicals react with indene
(
17) Scaiano, J . C.; Tanner, M.; Weir, D. J . Am. Chem. Soc. 1985,
1
07, 4396-4403.
J . Org. Chem, Vol. 68, No. 8, 2003 3201

Font-Sanchis et al.
F IGURE 5. Transient absorption spectra recorded following
3
55-nm laser excitation of a sample containing 0.5 M 9-tert-
butylfluorene (8) in benzene/di-tert-butyl peroxide (50/50)
under nitrogen 0.8 µs after the laser pulse. Inset: Transient
kinetic traces recorded at 490 nm under nitrogen (0) and
oxygen (4).
TABLE 3. Ra te Con sta n ts for th e Rea ction of Oxygen
w ith th e Ra d ica ls Gen er a ted fr om Differ en t Su bstr a tes
a t Room Tem p er a tu r ea
F IGURE 4. Top: Transient kinetic traces recorded at 500 nm
following 355-nm laser excitation of a sample containing 0.134
M fluorene 6 in benzene/di-tert-butyl peroxide (50/50). Bot-
tom: Transient kinetic traces recorded at 340 nm following
rate constant,
monitoring
wavelength, nm
radicals fromb
5
-1 -1
10 M
s
diphenylmethane17
6300
4
5
7
8
9
8.2
2.8
920
166
7.6
500
380
500
490
340
3
9
55-nm laser excitation of a sample containing 0.067 M
-phenylfluorene (9) in benzene/di-tert-butyl peroxide (50/50).
c(same radical as 6)
Traces in both panels recorded under nitrogen (2) and oxygen
0).
(
a
Rate constants were estimated by monitoring the decay of the
structure might explain the absence of addition at the
alkene moiety.
radicals in nitrogen-, air-, and oxygen-saturated samples. Errors
for rate constants were calculated to be less than 10%. The
b
Clearly, the planarity of the radical caused by the five-
member ring, while important, is not sufficient to induce
lack of reactivity toward oxygen, and the π-stabilization
of the phenyl substituent at the benzylic position is an
important factor.
radicals were generated from the corresponding compounds by
laser excitation of tert-butyl peroxide in benzene, using 355-nm
pulses. c Excitation at 266 nm was used for direct photolysis of
haloalkane 7, leading to the fluorenyl radical.
In the third series of selected molecules, containing the
five-member ring of fluorene and substituted at the
bilization of the phenyl substituent or steric effects are
the major cause of the reduced reactivity toward the
oxygen of radical 9, the substrate 9-tert-butylfluorene (8)-
was studied. The photolysis of di-tert-butyl peroxide in
the presence of 8 shows the formation of a transient with
maxima at 340, 460, and 490 nm (Figure 5). This
transient was assigned to the 9-tert-butylfluorenyl radical
by comparison with the fluorenyl radical. Further, in an
oxygen-saturated sample, the signal for radical 8 was
totally quenched, with barely a fast spike indicating its
presence in the early stages following the laser pulse.
Thus, the failure to observe any reactivity of radical 9
toward oxygen, while radical 8 is reactive, suggests that
the electronic effects due to the presence of the phenyl
ring are more important in determining the lack of
reactivity with oxygen. That is, 8 and 9 would not be
expected to be that different if steric effects dominated.
To gather accurate data on the reactivity of these
stabilized carbon-centered radicals with oxygen, the rate
constants for reaction at room temperature were mea-
sured at different concentrations of oxygen, using laser
flash photolysis (see Table 3). The rate constants for
radicals derived from 4, 5, and 9 were at least ten
thousand times slower that those for typical carbon-
centered radicals. However, the presence of a bulky tert-
butyl group at the 9-position of the fluorenyl ring only
decreased the reactivity five times. On the other hand,
9
-position provides benzylic resonance stabilization, fa-
vorable stereoelectronic effects, and in most cases sig-
nificant steric hindrance at the radical center.
The photolysis of di-tert-butyl peroxide in the presence
of fluorene 6 shows the formation of the fluorenyl radical
whose spectra has already been reported,18 with a sharp
maximum around 500 nm. Comparing the laser photoly-
sis results for nitrogen- and oxygen-saturated samples,
we observed that, not surprisingly, the signal for the
fluorenyl radical was totally suppressed (Figure 4, top).
Employing 9-chlorofluorene (7) as a photochemical source
of the fluorenyl radical, the high reactivity toward oxygen
was confirmed. Part of the motivation to study 7 is
related to the determination of the rate constant for the
reaction of the radical with oxygen, vide infra.
Two other molecules based on the fluorene moiety were
also examined, 9-phenylfluorene (9) and 9-tert-butyl-
fluorene (8). The photolysis of di-tert-butyl peroxide in
the presence of 9-phenylfluorene shows that the growth
at 340 nm due to the 9-phenylfluorenyl radical is unaf-
fected by oxygen (Figure 4, bottom).19 To know if π-sta-
(
18) Wong, P. C.; Griller, D.; Scaiano, J . C. J . Am. Chem. Soc. 1981,
03, 5934-5935.
19) Siskos, M. G.; Zarkadis, A. K.; Steenken, S.; Karakostas, N.;
Garas, S. K. J . Org. Chem. 1998, 63, 3251-3259.
1
(
3
202 J . Org. Chem., Vol. 68, No. 8, 2003

Reactivity toward Oxygen of Carbon-Centered Radicals
TABLE 4. Ra te Con sta n ts for th e Rea ction of
Ca r bon -Cen ter ed Ra d ica ls w ith TEMP O
rate constant,
monitoring
wavelength, nm
5
-1 -1
radical from
10 M
s
Ciba Irganox HP-136b
1.35
6.8
2200
73
1.46
310
<1
<1
540
550
9
1
3
5
6
8
9
-carbomethoxyfluorene
a
a
b
a
b
b
380
500
340
a
See ref 15. b The radicals were generated from the correspond-
ing compounds by laser excitation of tert-butyl peroxide in benzene,
using 355-nm pulses.
F IGURE 6. Plot of the logarithm of the rate constants for
the reaction of various radicals with oxygen against the
difference between the CH bond strength in toluene and in
the substrate. Numbering according to Chart 1.
we have reported that the oxygen quenching rate con-
stant for the 9-carbomethoxyfluorenyl radical should be
3
-1 -1 13
lower than 5 × 10 M
s
,
suggesting the importance
of electronic effects in this system.
In extreme cases where other parameters come into play
R a t e Con st a n t for t h e Tr a p p in g of Ca r b on -
Cen t er ed R a d ica ls b y TE MP O. The reactions of
carbon-centered radicals with nitroxides have been ex-
(e.g., steric hindrance and resonance stabilization) the
radicals are sufficiently stable to be isolated, and in cases
such as hindered nitroxides, galvinoxyl and diphenyl-
picryl hydrazyl (DPPH) are commercially available.
amined in considerable detail.2
0-22
In general these
reactions are quite fast, but below the diffusion-controlled
limit. The rate constants for trapping are higher for
aliphatic radicals, largely reflective of the reduced sta-
bilization of these radicals. Thus, it seems interesting to
investigate the reactions of some carbon-centered radicals
toward 2,2,5,5-tetramethylpiperidin-1-oxyl (TEMPO). The
rate constants were obtained from the transient decay
at different TEMPO concentrations (see Table 4). Those
radicals with extremely low reactivity toward oxygen
9
-1
The reaction of benzyl radicals with oxygen (∼10 M
s- ) is irreversible at room temperature; only the cases
involving high-resonance delocalization, such as tri-
phenylmethyl radical, lead to reversible reaction with
1
23
8
oxygen. For the triphenylmethyl radical steric effects
force all rings into a propeller-like conformation, where
the rings are not in the preferred orientation for maxi-
mum electron delocalization. Thus, the oxygenation rate
with benzylic radicals can reflect aspects related to the
spin density at the benzylic position, the radical stabili-
zation through spin delocalization, stereoelectronic ef-
fects, and steric effects in the reaction with oxygen.
(radicals generated from Ciba Irganox HP-136, 9-carbo-
methoxyfluorene, and 1,3-diphenylindene) showed the
lowest rate constants. Interestingly, the 9-tert-butyl-
fluorenyl radical and the 9-phenylfluorenyl radical were
unreactive with TEMPO, even at TEMPO concentrations
as high as 0.1 M. It is clear that steric effects due to the
presence of the tert-butyl group in 8 are enough to control
the lack of reactivity in the case of the hindered TEMPO
radical. Naturally, radical 9 was also unreactive.
The C-H bond dissociation energy (BDE) is a good
measure of the stabilization energy of benzyl radicals:
it is clear that a stronger bond generates a less stable
radical. In this way, the relationship between kOX and
the corresponding bond dissociation energies can give us
information about the importance of the stereoelectronic
effects. Thus, the logarithm of kOX was plotted against
C-H BDE relative to toluene previously reported by
Discu ssion
The five-member ring moiety of all the structures in
this paper leads to an enhancement of the hydrocarbon
reactivity toward alkoxyl radicals by approximately a
factor of 6, as judged from the comparison of values for
24-26
Bordwell et al. (Figure 6).
Radicals generated from diphenylmethane, fluorene,
3
-phenylindene, 1,3-diphenylindene, and phenylfluorene
showed a good correlation with log kOX, thus, no other
factors appear relevant in determining the reactivity with
oxygen than the stability of these radicals. In the case of
the benzyl radical the value of kOX approaches the
diffusion limit. Interestingly the reactivity of the radical
from 4 is very modest; here spin delocalization into the
π system must be responsible for the low reactivity with
oxygen.
1
and 6 with those for diphenylmethane (see Tables 1
and 2). Further stabilization by additional phenyl groups
only led to an enhancement of about a factor of 2, as
based on the comparisons of 5 with 4 and of 9 with 6.
Interestingly, steric effects (compare 6 and 8) are com-
parable with those for reaction of radicals with oxygen
(see Table 3).
It is well-known that several heteroatom-centered
radicals do not react with oxygen. Examples of this
situation are alkoxyl, phenoxyl, and hydrazyl radicals.
(
23) Maillard, B.; Ingold, K. U.; Scaiano, J . C. J . Am. Chem. Soc.
983, 105, 5095-5099.
24) An error of (3 kcal/mol has been associated with the C-H BDE
1
(
(
20) Bowry, V. W.; Ingold, K. U. J . Am. Chem. Soc. 1992, 114, 4992-
measurements. The BDE for 1,3-diphenylindene was assumed to be
equal to the C-H BDE in 9-phenylfluorene.
(25) Bordwell, F. G.; Harrelson, J . A.; Satish, A. V. J . Org. Chem.
1989, 54, 3101-3105.
(26) Bordwell, F. G.; Satish, A. V. J . Am. Chem. Soc. 1992, 114,
10173-10176.
4
996.
(
21) Chateauneuf, J .; Lusztyk, J .; Ingold, K. U. J . Org. Chem. 1988,
5
3, 1629-1632.
(22) Beckwith, A. L. J .; Bowry, V. W.; Ingold, K. U. J . Am. Chem.
Soc. 1992, 114, 4983-4992.
J . Org. Chem, Vol. 68, No. 8, 2003 3203

Font-Sanchis et al.
Finally, the rate constants for the trapping of the
stabilized carbon-centered radicals derived from Ciba
Irganox HP-136, 9-carbomethoxyfluorene, 5, 8, and 9
with the persistent nitroxide TEMPO were at least three
hundred times slower than those for benzyl radicals. It
is clear that in the case of radical 8, where no reaction
was observed, the lack of reactivity must be due to steric
hindrance; in contrast, reactions with oxygen and with
tert-butoxyl were slowed, but still observable. Resonance
stabilization for 5 and 9 radicals appears to be the most
important factor responsible for the highly reduced
reactivities toward TEMPO and oxygen.
butyl peroxide in benzene, using the 355-nm pulses from a
Continuum Nd:YAG Surelite. Excitation at 266 nm from a
Continuum Nd:YAG Surelite laser (fourth harmonic, <10 ns,
2
0 mJ /pulse) was used for direct photolysis of haloalkane 7.
Transient signals were captured with a digital oscilloscope that
was interfaced to a computer, which also controlled the
experiment. The system was operated with software written
in the LabVIEW 5.1 environment from National Instruments.
All experiments were carried out with use of static cells
constructed from 7 × 7 mm Suprasil quartz tubing. Samples
were purged with a slow stream of either nitrogen or oxygen,
as required.
Ack n ow led gm en t. J .C.S. thanks the Natural Sci-
ences and Engineering Research Council of Canada and
Imperial Oil Canada for generous support. E.F.S. thanks
the Spanish Ministry of Education for a grant.
Exp er im en ta l Section
Rea gen ts. TEMPO, indane, indene, fluorene, and 9-phenyl-
fluorene were commercially available. Ciba Irganox HP-136
was a gift from CIBA Specialty Chemicals. Di-R-cumyl hypo-
J O026666O
1
6
27
28
nitrite, 1-phenylindane, 3-phenyl-1-indene, 1,3-diphenyl-
2
9
30
31
indene, 9-chlorofluorene, and 9-tert-butylfluorene were
prepared by literature methods.
(28) Halterman, R. L.; Zhu, C. Tetrahedron Lett. 1999, 40, 7445-
7448.
(29) Casuscelli, F.; Chiacchio, U.; Liguori, A.; Romeo, G.; Sindona,
La ser F la sh P h otolysis. The radicals were generated from
the corresponding compounds by laser excitation of di-tert-
G.; Uccella, N. Tetrahedron 1993, 49, 5147-5152.
(
(
30) Hurd, C. D.; Mold, J . D. J . Org. Chem. 1948, 13, 339-346.
31) Streitwieser, A. J .; Chang, C. J .; Reuben, D. M. E. J . Am. Chem.
(27) Valyocsik, E. W.; Sigal, P. J . Org. Chem. 1971, 36, 66-72.
Soc. 1972, 94, 5730-5734.
3
204 J . Org. Chem., Vol. 68, No. 8, 2003