Photolysis of 10,10-difluorophenanthren-9(10H)-one. Evidence for solvent-assisted α-cleavage

Article abstract of DOI:10.1016/S0040-4039(03)00899-2

The photolysis of 10,10-difluorophenanthren-9(10H)-one 1 in different solvents shows that the major competing reaction of the diradical formed by α-cleavage: recombination and hydrogen atom abstraction depends on the hydrogen atom donating ability of the solvent. Photolysis of 1 in cyclohexane in the presence of air or oxygen leads mainly to α-cleavage while photoreduction with the formation of 10-fluoro-9-phenanthrol occurs when the solution is deaerated prior to irradiation. In acetonitrile, a poor hydrogen donor, recombination of the diradical back to starting compound 1 is the sole process.

Full text of DOI:10.1016/S0040-4039(03)00899-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4247–4250
Photolysis of 10,10-difluorophenanthren-9(10H)-one.
Evidence for solvent-assisted a-cleavage
&
Boris Sket,* Berta Kosˇmrlj, Maja Harej and Darko Dolenc
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Asˇkercˇeva 5, SI-1000 Ljubljana, Slovenia
Received 7 February 2003; revised 26 March 2003; accepted 4 April 2003
Abstract—The photolysis of 10,10-difluorophenanthren-9(10H)-one 1 in different solvents shows that the major competing
reaction of the diradical formed by a-cleavage: recombination and hydrogen atom abstraction depends on the hydrogen atom
donating ability of the solvent. Photolysis of 1 in cyclohexane in the presence of air or oxygen leads mainly to a-cleavage while
photoreduction with the formation of 10-fluoro-9-phenanthrol occurs when the solution is deaerated prior to irradiation. In
acetonitrile, a poor hydrogen donor, recombination of the diradical back to starting compound 1 is the sole process. © 2003
Elsevier Science Ltd. All rights reserved.
The a-cleavage of ketones1 was first described in 1907
and is generally known as the Norrish Type I process.
a-Cleavage is a fundamental reaction of electronically
excited carbonyl compounds and for several systems
the reactivity of the triplet (np*) state is greater than
that of the corresponding singlet excited state by some
orders of magnitude.
ing by perturbing the population of the equilibrated
conformers. This means that some individual transi-
tion-state geometries are more favoured than others.4
Several investigators have observed solvent effects5–7 on
the a-cleavage reactions. Quantum yields and often
chemical yields are very low in benzene, whereas they
are respectable to good in alcohols. No explanation for
this poorly understood effect is currently known. It is
possible that further decomposition of untrapped kete-
nes is responsible for low yields.
In cyclic ketones, the reaction pathways available to the
acyl–alkyl diradicals,2,3 resulting from the a-cleavage,
are recombination to give the original compound, a
second a-cleavage (decarbonylation), internal dispro-
portionation and rearrangements to an oxacarbene.
In cycloalkanones, the major competing triplet reaction
is photoreduction by the solvent;8,9 this proceeds with a
pseudounimolecular rate constant of 106 s−1 in hexane10
and 107 s−1 in ethanol. The value is somewhere between
that of methanol11 and is surprisingly low in acetonitrile
(k <104 s−1).
As may be expected, the rate coefficients of the indivi-
dual processes have an appreciable effect on which of
the pathways is preferred. Since the competing reac-
tions require different diradical conformations, suffi-
ciently long lifetimes of the triplet-derived diradicals,
that allow them to attain conformational equilibration
prior to reaction must also be considered. The diradi-
cals produced from cyclic ketones usually follow more
efficient modes of reaction than the second a-cleavage.4
In this letter we present the results of the photolysis of
10,10-difluorophenanthren-9(10H)-one 112 in cyclohex-
ane and acetonitrile, showing that the efficiency of
a-cleavage is not simply affected by solvent polarity,
but is rather controlled by the hydrogen atom donor
ability of the solvent.
Generally, the diradicals formed by a-cleavage undergo
either recombination or internal disproportionation
reactions. The disproportionation processes depend on
the ring size of the cyclic ketone, and position and size
of alkyl substituents that influence diradical partition-
There are two main reasons for our choice of 10,10-
difluorophenanthren-9(10H)-one for these studies.
“ The internal disproportionation of the diradical
formed by a-cleavage is not possible.
“ There is no active hydrogen atom for internal hydro-
gen atom abstraction from the np* triplet state of
the carbonyl group.
* Corresponding author. Tel.: +386-1-241-9240; fax: +386-1-241-9273;
e-mail: sket.boris@email.si
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00899-2

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B. Sket et al. / Tetrahedron Letters 44 (2003) 4247–4250
4248
Scheme 1. Photoproducts obtained after irradiation of 1.
To determine the effect of the solvent, we used cyclo-
hexane as a nonpolar solvent with good hydrogen atom
donor ability and the polar acetonitrile as a weak
hydrogen atom donor.
additional signals at l=28.68, 27.67, −124.72, −157.03
and −180.31 ppm.
The signals at 27.67 and 28.68 ppm correspond to the
fluorine atom of acyl fluorides 2 and 3, the signal at
−124.72 ppm corresponds to the fluorine atom bonded
to the sp2 carbon of compound 6, the signal at −157.03
is in agreement with the known chemical shift for the
fluorine atom14 in compound 9, while a doublet with
the coupling constant 2JFH=45 Hz at −180.31 ppm
corresponds to the fluorine atom in compound 7. The
relative ratio of fluorine signals for the mentioned
compounds is very similar to the ratio of products
obtained by GC analysis.
Results
Irradiation of a 0.013 M solution of 10,10-difluoro-
phenanthren-9(10H)-one 1 (Scheme 1) for 4 h in cyclo-
hexane at u=360 nm (umax of 1 in cyclohexane=345.5
nm) resulted in a 10% conversion to a mixture of
products: 2, 3, 4, 6, 7, 9 and 10. The crude reaction
mixture was analyzed by GC/MS and 19F NMR.13 The
product distributions and the relative % yields are given
in Table 1.
When the photolysis of 1 was carried out as a cyclohex-
ane solution saturated with oxygen (acting as triplet
quencher), lower conversion (3%) with the formation of
products formed by a-cleavage was observed. On the
other hand, when a cyclohexane solution of 1 was
deaerated before irradiation, compound 9 was formed
The structures of the products were determined on the
basis of their mass and the 19F NMR spectra of the
crude reaction mixture, which shows besides the signal
at l=−103.80 ppm, that corresponds to the gem
difluoromethylene group of starting compound 1, five
Table 1. Conversion and product distributiona in the photolysisb of 1 in cyclohexane

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B. Sket et al. / Tetrahedron Letters 44 (2003) 4247–4250
4249
between the carbonyl carbon and that of the
difluoromethylene group is very short (r(CO:CF2)=
as the main product by hydrogen atom abstraction on
the np* triplet state of the carbonyl group.
6
6
,
1.57 A). On the other hand, the acyl group of the triplet
diradical can abstract a hydrogen atom from the sol-
vent thus forming a difluorobenzyl radical derivative
(abstraction is much more convenient from the twisted
conformer of the diradical), which diffuses from the
solvent cage and reacts with cyclohexyl radicals (path
b) and with oxygen (path a) when the photolysis was
carried out in the presence of air or when the reaction
mixture was purged with O2. In the latter case the
peroxyl radicals formed are transformed into acyl
fluoride 2, which is readily further oxidized into car-
boxylic acid 3. After a Friedel–Crafts type intramolecu-
lar acylation of 3 the product 4 was formed. The
reaction is probably catalyzed by hydrogen fluoride
liberated during the formation of product 2 and the
conversion of 5 to 6 and 8 to 9. The relative % yield of
product 4 in comparison with product 3 increased by
increasing the irradiation time.
With longer irradiation time (24 h) the conversion of 1
increases up to 50% but at the same time the reaction
mixtures become more complex.
Irradiation of 1 in acetonitrile for 4 h at u=360 nm
resulted in no conversion. The 19F NMR spectrum of
the crude reaction mixture showed only a signal corre-
sponding to the gem CF2 group of starting compound
1. GC analysis using diphenylacetylene as an internal
reference showed no additional signals. We also deter-
mined that compound 1 is photostable in acetonitrile at
longer irradiation times (24 h and 72 h).
Discussion
The np* excited state of 1, formed after absorption of
one photon, can further react by two competing pro-
cesses: (1) a-cleavage and (2) abstraction of a hydrogen
atom.
Reaction of a cyclohexyl radical with the difluorobenzyl
radical derivative results in the formation of product 5,
which after the elimination of hydrogen fluoride gener-
ates compound 6. An acid catalyzed rearrangement of
the double bond in 6 produces the compound 7.
a-Cleavage produces a diradical in a solvent cage that is
capable of C–C bond rotation about the biphenyl bond
and equilibrium of different rotational conformers is
established. The calculated enthalpy of formation (DHf)
for the planar structure is −148.7 kJ mol−1 and −160.4
kJ mol−1 for a twisted conformer where the phenyl
rings are orthogonal to each other15 (Fig. 1).
Hydrogen atom abstraction also proceeds from the np*
excited state of the carbonyl group of starting ketone 1
and therefore, competes with a-cleavage, but is less
expressed in aerated solutions (12%, Table 1) and is
completely suppressed in solutions purged with O2. On
the other hand, when the reaction solution was
degassed this reaction became more favourable and the
major product 9 was obtained by elimination of hydro-
gen fluoride from the first formed reduced product 8
(91%).
In cyclohexane, a good hydrogen donor, the triplet
diradicals formed by a-cleavage can be first trans-
formed by intersystem crossing into singlet diradicals16
that recombine to form the starting compound 1. We
believe that the process proceeds as a result of the
planar conformation of the diradicals since the distance
Photolysis in acetonitrile, a poor hydrogen atom donor,
led to no conversion of starting compound 1. The only
process that took place even over prolonged time (t=72
h) was recombination of initially formed diradicals. It is
possible that the solvent polarity has an influence on
the formation of the pp* and the np* triplet states. In
non-polar cyclohexane the reactive np* triplet is formed
because it is lower in energy that the unreactive pp*
triplet. In polar acetonitrile the position of the two
triplet states is reversed. Here an unreactive pp* triplet
is formed. To prove or eliminate this possibility we
tested another polar solvent methanol, which also acts
as a good hydrogen donor. Preliminary results on the
photolysis of 1 in methanol show a similar reaction
pathway to that observed in cyclohexane. The products
are derived from a-cleavage but the overall conversion
is higher than in cyclohexane (20%).
Conclusion
The results obtained in the photolysis of 1 in different
solvents show that the major competing reactions of the
diradical formed by a-cleavage recombination and
hydrogen atom abstraction, depend on the hydrogen
donor ability of the solvent. A higher chemical yield of
products was obtained from the diradical in methanol
in comparison with that in cyclohexane. This is likely to
Figure 1. Ball-and-stick models for (a) planar and (b) twisted
diradical; (c) planar and (d) twisted difluorobenzyl radical.

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B. Sket et al. / Tetrahedron Letters 44 (2003) 4247–4250
4250
be an effect of the lower CꢀH bond strength in
methanol.17 Another factor may be the different popu-
lations of the equilibrated diradical conformers medi-
ated by solvent molecules leading to recombination or
hydrogen atom abstraction.
7. Dalton, J. C.; Turro, N. J. Annu. Rev. Phys. Chem. 1970,
21, 499–560.
8. Scaiano, J. C. J. Photochem. 1973, 2, 81–118.
9. Horspool, W. M. Photochemistry in Organic Synthesis;
Coyle, J. S., Ed.; The Royal Society of Chemistry, Special
Publication, 1986; Vol. 57, pp. 61–79.
In acetonitrile, a poor hydrogen atom donor, recombi-
nation of the diradical back to starting compound 1 is
the sole process.
10. Wagner, P. J. J. Am. Chem. Soc. 1966, 88, 5672–5673.
11. (a) Charney, D. R.; Dalton, J. C.; Hautala, R. R.;
Snyder, J. J.; Turro, N. J. J. Am. Chem. Soc. 1974, 96,
1407–1410; (b) Yip, R. W.; Siebrand, W. Chem. Phys.
Lett. 1972, 13, 209–212.
Acknowledgements
12. Zupan, M.; Iskra, J.; Stavber, S. Bull. Chem. Soc. Jpn.
1995, 68, 1655–1660.
13. 2: lF=27.67 ppm; 228 (M+, 18); 200 (23); 199 (28); 182
(18); 181 (100); 180 (16); 171 (16); 170 (31); 153 (16); 152
(28); 151 (16); 104 (19); 76 (24). 3: lF=28.68 ppm; 244
(M+, 17); 228 (13); 227 (71); 226 (22); 225 (100); 212 (19);
199 (30); 199 (18); 197 (88); 181 (39); 180 (23); 172 (22);
170 (44); 152 (14); 151 (15); 82 (23); 67 (32). 4: 224 (M+,
25); 181 (21); 180 (100); 152 (54); 151 (24); 150 (14); 126
(13); 76 (29). 6: lF=−124.72 ppm; 294 (M+, 5); 213 (15);
212 (100); 183 (27); 164 (5); 163 (5); 55 (7); 41 (6). 7:
lF=−180.31 ppm; 294 (M+, 2); 213 (15); 212 (100); 165
(3); 164 (3); 163 (3); 55 (9); 41 (7). 9: lF=−157.03 ppm;
212 (M+, 100); 184 (16); 183 (68); 164 (23); 163 (23); 106
(13); 92 (13); 81 (8). 10: lF=−103.26 ppm; 314 (M+, 6);
232 (17); 231 (31); 213 (20); 212 (100); 211 (30); 183 (42).
14. Zupan, M.; Iskra, J.; Stavber, S. Croatica Chem. Acta
1996, 69, 1437–1447.
The authors would like to thank Professor M. K.
Lakshman from the Department of Chemistry, City
College of New York, for his helpful discussion.
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