Bond dissociation energies for radical dimers derived from highly stabilized carbon-centered radicals

Article abstract of DOI:10.1021/ol049111j

(Matrix Presented) The temperature dependence of the dissociation of dimers formed from highly stabilized carbon-centered radicals has been examined. Analysis of the data yields the bond dissociation energy (BDE) for the central head-to-head C-C bond in t

Full text of DOI:10.1021/ol049111j

ORGANIC
LETTERS
2004
Vol. 6, No. 15
2579-2582
Bond Dissociation Energies for Radical
Dimers Derived from Highly Stabilized
Carbon-Centered Radicals
Mathieu Frenette, Carolina Aliaga, Enrique Font-Sanchis, and J. C. Scaiano*
Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie,
Ottawa, Ontario K1N 6N5, Canada
tito@photo.chem.uottawa.ca
Received May 12, 2004
ABSTRACT
The temperature dependence of the dissociation of dimers formed from highly stabilized carbon-centered radicals has been examined. Analysis
of the data yields the bond dissociation energy (BDE) for the central head-to-head C−C bond in these compounds. For example, for the dimer
derived from 3-phenyl-2-coumaranone, BDE is 23.6 kcal/mol and the C−C bond length 1.596 Å, a rather long value for a σ bond.
Lactone-based antioxidants, introduced by Ciba a few years
ago, have stimulated interest in the understanding of carbon-
centered radicals with greatly attenuated reactivity toward
oxygen.1-5 We have proposed that the lack of reactivity
toward oxygen can be rationalized on the basis of five
parameters, all influential, but where their relative importance
depends on the particular molecule under study. These
parameters are as follows: (a) benzylic resonance stabiliza-
tion; (b) favorable stereoelectronic effects, (c) unpaired spin
delocalization on heteroatoms (particularly oxygen), (d)
electron-withdrawing effects, and (e) steric effects.
Our recent studies have included lactones,1,2 phthalides,4
hydrocarbons,5 and substituted nitriles.3 Among these sys-
tems, radicals 1-4 (Figure 1) are either unreactive or have
greatly attenuated reactivity toward oxygen, in contrast with
the high reactivity of most carbon-centered radicals.6,7
Figure 1. Structure of the radicals studied.
(1) Scaiano, J. C.; Martin, A.; Yap, G. P. A.; Ingold, K. U. Org. Lett.
2000, 2, 899-901.
In radicals 1-4, steric effects may contribute, but all
indications are that they are not the dominant reason for the
lack of reactivity toward oxygen. For example, all form head-
(2) Bejan, E. V.; Font-Sanchis, E.; Scaiano, J. C. Org. Lett. 2001, 3,
4059-4062.
(3) Font-Sanchis, E.; Aliaga, C.; Focsaneanu, K.-S.; Scaiano, J. C. Chem.
Commun. 2002, 15, 1576-1577.
(4) Font-Sanchis, E.; Aliaga, C.; Cornejo, R.; Scaiano, J. C. Org. Lett.
2003, 5, 1515-1518.
(6) Maillard, B.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc. 1983,
105, 5095-5099.
(7) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1976, 9, 13-18.
(5) Font-Sanchis, E.; Aliaga, C.; Bejan, E. V.; Cornejo, R.; Scaiano, J.
C. J. Org. Chem. 2003, 68, 3199-3204.
10.1021/ol049111j CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/29/2004

to-head dimers, a far more sterically demanding process than
reaction with oxygen.
While initial yields were mediocre, optimized conditions gave
63% of 22, a satisfactory outcome for the creation of such
weakly bound dimers. A very convenient one-pot synthesis
of 42 afforded 51% yield from the addition of 9-phenyl-9-
fluorenol (4a) to stoichiometric TMS chloride and excess
NaI in acetone followed by subsequent workup of released
molecular iodine with 10% Na2S2O3(aq). The highly sym-
metric and practically insoluble 42 could be recovered from
the filtration of a two-phase system of dichloromethane and
aqueous Na2S2O3.
Radicals 1-4 show significant and characteristic absorp-
tions in the ultraviolet and visible regions. These absorptions
can be used to monitor their presence in equilibria such as
that illustrated in eq 1. At room temperature, the radical
absorptions observed are negligible in comparison with their
absorptions at the higher temperatures employed in our
studies, and thus the room-temperature spectrum can be taken
as a reference. It is noted that the absorbances of all samples
were similar before and after the temperature-dependent
experiments were conducted, indicating the reversibility of
dimer formation. The temperature dependence of the spectra
is shown in Figure 2 for dimer 12 in toluene.
The other effects mentioned above play an important role,
but their relative importance is strongly system-dependent.
Thus, electron-withdrawing effects due to the lactone or
nitrile moieties are important in 1-3, but not in 4. Stereo-
electronic effects, leading to optimal geometry at the radical
center assisted by the five-membered ring, are key to 1, 2,
and 4, but not for 3. Heteroatom delocalization is important
in 1 and 2 that possess some oxygen-centered character,
while for 3 spin on nitrogen also leads to some C-CN
double-bond character in the radical. Thus, while 1-4 all
show attenuated or no reactivity toward oxygen, the param-
eters that control this lack of reactivity have very different
weights in each system.
All radicals form head-to-head dimers, which at moder-
ately high temperatures (vide infra) are present in solution
in equilibrium with the radicals, as illustrated in eq 1 for
radical 2. Note also the nomenclature used for the dimers,
simply adding the subscript “2” to the radical abbreviation.
Two novel synthetic methods are introduced for the
formation of precursors to radicals 2-4 (Scheme 1). Dimers
Scheme 1. Synthetic Approach to Dimers 22, 32, and 42
Figure 2. Absorption spectra of 1 at different temperatures,
obtained by thermal dissociation of 12 in toluene under nitrogen.
We note that for dimers 12 and 22 where different
diastereoisomers are possible, the crystal X-ray structures
show that under our synthesis conditions (see the Supporting
Information) only the meso isomer is produced.
Similar experiments to that shown in Figure 2 were carried
out for the dimers using toluene as solvent (with the
exception of dimer 42 carried out in 1,3-dichlorobenzene)
and working under a nitrogen atmosphere. We note that in
the case of 4 prolonged exposure to oxygen under conditions
of thermal dissociation eventually leads to the formation of
the peroxide, probably the result of trapping of a small
concentration of peroxyl radicals by a large excess of the
carbon-centered radical in an excellent example of the
operation of the Fischer-Ingold persistent free-radical ef-
fect.7,8
The data obtained from the temperature dependence of
the dissociation equilibria can be used to obtain the ∆H for
22 and 32 were obtained from the photolysis of tert-butyl
peroxide at 350 nm in the presence of the corresponding
monomeric substrate (2-H or 3-H) under an inert atmosphere.
(8) Fisher, H. J. Am. Chem. Soc. 1986, 108, 3925-3927.
Org. Lett., Vol. 6, No. 15, 2004
2580

Table 1. Bond Dissociation Energies and Central C-C Bond
Length for Dimers of 1-4
monitored
temp
C-C bond
length (Å)
∆Hdiss
(kcal mol-1
)
radical
λmax (nm)
range (°C)
1
2
3
4
346
346
336
320
48-111
37-100
48-102
80-135
1.586a
1.596b
1.608c
22.8
23.6
26.2
15.2
a From the crystal structure (see Figure S1, Supporting Information).
b Reference 10. c Reference 11.
the process, which corresponds to the bond dissociation
energy (BDE) for the central C-C bond. The data were
analyzed using a simplified form of the van’t Hoff equation,
i.e.
Figure 3. Van’t Hoff plot according to eq 4 for dimer 12 (]), 22
(O), and 32 (4) in toluene and 42 (0) in 1,3-dichlorobenzene under
nitrogen.
change for the dissociation of 12. From the intercept obtained
from the plot according to eq 4 we were able to estimate ∆S
∼ +28 gibbs/mol, a very reasonable value for a dissociation
of this type.
∆S ∆H
ln Keq
)
-
(2)
R
RT
where Keq ) [R]2/[D], and R and D represent the radical
and dimer, respectively. Using Beer’s law, the experimental
absorbance can be expressed as
The BDE found for 12 and 22 are very similar, showing
that the presence of alkyl groups in 1 does not play an
important role in controlling the stability of the radical
generated. In an attempt to provide more evidence to explain
this behavior, we obtained the X-ray structure for dimer 12
(Figure S1, Supporting Information) and compared it with
that reported for 3-phenyl-2-coumaranone dimer 22. In both
molecules the phenyl groups adopt a gauche conformation
to decrease the repulsion between the carbonyl groups;
however, the tert-butyl groups in 1 are oriented toward the
exterior of the molecule to minimize steric interactions near
the central bond. It is clear that in all cases the central C-C
bond is much longer than the typical 1.54 Å for single bonds
between sp3 carbons. The C-C bond dissociation energies
in these dimers are remarkably low; compare for example
with AIBN, a typical initiator for free radical polymerization
that decomposes with an activation energy of 31.3 kcal/mol
and is usually regarded as an unstable compound.12 While
the radicals possess significant resonance stabilization, this
is not enough to prevent their dimerization, and in this sense
tha radicals are not persistent. The apparent stability of the
dimers in Table 1 is probably due to the intrinsic lack of
reactivity of the radicals with oxygen, that makes the back
reaction to reform the dimer the preferred reaction path for
the radicals; thus, while the dimers are stable compounds at
room temperature, their stability reflects the tendency of any
radicals formed to recombine. The compounds are best
described as “dynamically stable”.
A ) ꢀ[R]l
Therefore, eq 2 converts to
(3)
ln(ꢀ2l2[D])
∆S ∆H
+ -
ln A )
(4)
2
2R 2RT
Equation 4 shows that a plot of ln A against the reciprocal
temperature will yield -∆H/2R from the slope, provided that
[D] can be approximated as constant throughout the tem-
perature range studied. This criterion is easily met by the
systems under study. For example, in the case of 12 (see
Figure 1), the absorbance at 111 °C (the highest temperature
used) is approximately 1.4 for an optical path of 1 cm; [D]
) 0.01 M and the use of a previously reported method9
estimated a radical extinction coefficient of 37 500 M-1 cm-1
at 346 nm. From this we evaluate a radical concentration of
3.7 × 10-5 M, or a dimer conversion of ∼ 0.19%, low
enough to assume that [D] is constant. Thus, under these
conditions it is not necessary to know the absolute value of
the radical extinction coefficients, which are frequently
subject to considerable uncertainty.
Figure 3 shows the plots according to eq 4 for dimers of
1-4, from which the corresponding BDE were estimated
and are listed in Table 1.
While all plots gave excellent correlation, with statistical
errors around 0.1 kcal/mol, we estimate that the true
uncertainty of the BDE values is probably (0.5 kcal/mol.
We also took advantage of the availability of an estimated
extinction coefficient for 1 in order to estimate the entropy
Finally, we note that while the dimers studied in this
contribution all show simple mechanistic behavior, with
head-to-head dimer formation, other systems may show a
(10) Mori, Y.; Niwa, A.; Maeda, K. Acta Crystallogr. 1995, B51, 61-
65.
(11) Lam, Y.; Lee, G.-H.; Liang, E. Bull. Chem. Soc. Jpn. 2001, 74,
1033-1034.
(9) Arends, I. W. C. E.; Mulder, P.; Clark, K. B.; Wayner, D. D. M. J.
Phys. Chem. 1995, 99, 8182-8189.
(12) Lewis, F. M.; Matheson, M. S. J. Am. Chem. Soc. 1949, 71, 747-
748.
Org. Lett., Vol. 6, No. 15, 2004
2581

preference for coupling at other positions in the ring, just as
triphenylmethyl radical does. In dimers 12-42 the BDE
values support the arguments about the high degree of radical
stabilization proposed earlier on the basis of their interactions
(or lack thereof) with oxygen and the excellent hydrogen
donor properties of their H-saturated precursors. With their
weak bonds, high radical stability, and inertness toward
oxygen, these dimers are promising compounds in the
potential generation of thermomagnetic or photomagnetic
materials.
support. E.F.-S. thanks the Spanish Ministry of Education
for a grant, and M.F. thanks Dr. Louis Barriault for helpful
discussions involving the synthesis of dimer 42 and Dr. Juan
Juarez with assistance in the preparation of 22.
Supporting Information Available: Experimental details
for the temperature-dependent measurements and for the
synthesis of dimers 12, 22, 32, and 42. ORTEP diagram for
12. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. J.C.S. thanks the Natural Sciences and
Engineering Research Council of Canada for generous
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