SCHEME 2
cyclization occurs in at least 6-fold higher yield in acetonitrile
as a solvent as compared to that in benzene, as monitored by
GC and 1H NMR analyses. Thus, the photolyses of all ketones
3-8 were conducted in N2-purged CH3CN solutions in a
photoreactor fitted with λ ca. 350 nm lamps (cf. Supporting
Information). The photolysate in each case was monitored by
TLC and GC analyses. After photolysis, the solvent was
removed in vacuo, and the residue was subjected to silica-gel
column chromatography to isolate the photoproducts, viz.,
fluorenone (Fl) and the dehalogenated benzophenone (Bp). For
some ketones, the reactions were run for longer durations to
ensure high conversions, as the colored fluorenone products
severely interfered with the absorption of UV radiation by their
precursor 2-bromobenzophenone reactants.
A perusal of the results in Table 1 suggests that the
photochemical route does indeed lead to respectable yields of
cyclization products depending on the nature of substituents and
their pattern of substitution; the conversion and mass balance
are reasonable for most cases. While the photolysis of parent
2-bromobenzophenone 3a and the corresponding 4′-trifluorom-
ethyl analogue 3c yielded fluorenones in only ca. 32-34% yield
(Table 1, entries 1 and 3), that of the methoxy analogues 3b,
4a, and 4b led to relatively higher yields of fluorenones, ca.
48% (Table 1, entries 2, 7, and 8); of course, the other major
product in all these cases is the dehalogenated benzophenone.
All other ketones with the exception of 6a, 7a, and 8c afforded
the corresponding fluorenones in 67-98% isolated yields (Table
1, entries 4-6, 9-11, 13, and 15-17); 6a and 7a (Table 1,
entries 12 and 14) were found to be unreactive, while 8c yielded
the corresponding fluorenone in 53% yield (Table 1, entry 18).
The photoinduced C-Br/I bond homolysis of Br/I-phenylke-
tones has been sufficiently investigated. In particular, Wagner
and co-workers have elucidated the mechanistic details of
photoinduced C-Br/I bond homolysis of bromo- and iodoac-
etophenones and benzophenones.7 Accordingly, the C-Br
cleavage is endothermic by 6-12 kcal/mol for triplet-excited
benzophenones, while it is exothermic for the corresponding
iodo analogues, due to the difference in Ph-Br (78-80 kcal/
mol) and Ph-I (64 kcal/mol) bond dissociation energies; the
triplet-excited energies of various benzophenones are 68-70
kcal/mol.8 It is assumed that for bromobenzophenones, the
lowest triplet-excited n,π* states convert into π,π* states before
cleavage. Further, Wagner et al. have shown that the triplets of
o-bromophenylketones undergo cleavage 600-fold faster than
those of meta- and para- analogues. Thus, photoinduced
homolytic C-Br/I bond cleavage is a facile pathway to generate
aroylphenyl radicals.
constants for various pathways of the parent 2-benzoylphenyl
radical.6 They have shown that a simple substitutent such as a
methyl group may modify the cyclization rates (kc and k′) of
c
the 2-benzoylphenyl radical, and it is rearranged one by 1-2
orders of magnitude.6a Thus, the photochemical outcome of
ketones 3-8,8 which depends on the partitioning of derived
2-aroylphenyl radicals among various pathways, should be
subject to the influence of substituents present in both phenyl
rings of benzophenone.9 From the analysis of preparative
photochemical results consolidated in Table 1, some generaliza-
tions emerge with regard to the reactivity of the 2-aroylaryl
radical in general.
For the sake of discussion of the influence of substituents on
the photoinduced cyclization of 2-bromobenzophenones 3-8,
we shall term the halogen-containing ring that leads to the aryl
radical as a radical ring and the other as a non-radical ring.
First, the results in Table 1 reveal that all ketones in which the
non-radical ring contains substituents that stabilize the incipient
hydrofluorenyl radical afford good to excellent yields of
fluorenones. That is, for ketones 3d,e, 4c,d, 5, 6b, 7b, and 8b
(Table 1, entries 4, 5, 9-11, 13, 15, and 17), the initially formed
2-aroylphenyl σ-radical may stabilize via cyclization to the
corresponding cyclohexadienyl π-radical. This is in line with
the observation by Karady et al.3b that the substituents that
stabilize the cyclohexadienyl radical accelerate ring closure (kc,
Scheme 2), thereby minimizing hydrogen migrations (kt).
Accordingly, the formation of the regioisomeric fluorenone
product (Fl′, Scheme 2) was not observed from ketones 5 and
6b upon photolysis. Thus, it may be concluded that the ketones
for which the hydrofluorenyl radical, formed subsequent to the
attack of the 2-aroylaryl radical on the non-radical ring, is
stabilized strongly by the substituents due to their nature as well
Mechanistically, various pathways that determine the fate of
the initially formed 2-aroylaryl radical, viz., radical translocation
(kt), hydrogen abstraction (kH), recombination (krc), and cycliza-
tion (kc), are shown in Scheme 2. The cyclization of translocated
radical may, in principle, lead to two regioisomeric fluorenones.
Indeed, Karady et al. showed that the photolysis of deuterated
methyl-substituted iodobenzophenone does lead to the isomeric
fluorenones.3a From their studies on 2-benzoylbenzene diazo-
nium salts, Hanson and co-workers have determined rate
(7) (a) Wagner, P. J.; Sedon, J.; Waite, C.; Gudmundsdottir, A. J. Am.
Chem. Soc. 1994, 116, 10284. (b) Wagner, P. J.; Waite, C. I. J. Phys. Chem.
1995, 99, 7388. (c) Wagner, P. J.; Sedon, J. H.; Gudmundsdottir, A. J. Am.
Chem. Soc. 1996, 118, 746.
(8) Triplet energies are less likely to vary considerably for all ketones
3-8, see: Leigh, W. J.; Arnold, D. R. J. Chem. Soc., Chem. Commun.
1980, 406.
(9) State-switching for substituted benzophenones does not occur except
for dialkylamino substituents, see: Wagner, P. J.; Park, B.-S. In Organic
Photochemistry; Padwa, A., Ed.; Marcel Dekker: New York, 1991; Vol.
II, Ch. 4.
J. Org. Chem, Vol. 72, No. 25, 2007 9787
Moorthy, Jarugu Narasimha
