D. Madhavan, K. Pitchumani / Tetrahedron 58 (2002) 9041–9044
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The presence of only triplet product and the absence of any
singlet-derived products under these conditions ruled out
any reaction in the liquid phase of these clay slurries.
Among the substituted acetophenone and benzophenone
sensitizers used, the 40-substituted sensitizers were found
to be more effective than 20-substituted sensitizers in
the reaction. This is in accordance with their efficiency in
solution indicating that the 20 and 40-isomers behave
differently in their sensitization efficiency.
sensitizers are their reusability with only a marginal
decrease in their efficiency and the easier removal of the
sensitizer from the reaction medium. This approach of
ionizing and then anchoring the sensitizer onto the clay
surface, removes restrictions on the choice of the sensitizers
and those sensitizers which are not soluble in the reaction
medium can also be used. The results are thus significant,
since clean triplet–triplet energy transfer is demonstrated
for the first time in a photochemical reaction in the layered
clay microenvironment, materials which in spite of their
huge natural abundance are of less interest to photochemists
so far and studies in them are restricted mainly to
photophysical events.
Similarly, anchoring through the other side of the sensitizer,
as in a-aminoacetophenone hydrochloride also resulted in
inefficient energy transfer. Anionic hydrotalcites clays were
also used as microreactors. 40-Hydroxyacetophenone and
40-hydroxybenzophenone anchored onto anionic hydro-
talcites by ionic exchange were also found to be efficient
sensitizers to myrcene. Though the overall conversion was
only around 20% with these clay-bound sensitizers, this
disadvantage was offset by their other advantages such as
reusability, simpler work-up, etc. When they were used
for the second time (after a simple filtration, washing and
drying), there was only a marginal decrease in the
percentage conversion (Table 1).
2. Experimental
Bentonite clay obtained from Aldrich, was used after
repeated exchange with sodium chloride. Mg–Al-HTlc was
prepared as per reported procedure.17 A typical example for
5:1 HTlc is described below: Al(NO3)3·9H2O (0.01 mol)
and Mg(NO3)2·6H2O (0.05 mol) were dissolved in deio-
nised water (100 mL), and a second solution (60 mL) of
Na2CO3 (0.03 mol) and NaOH (0.07 mol) was prepared.
After the first solution was slowly added to the second one,
the resulting mixture was heated at 658C for 18 h with
vigorous stirring. The white slurry was then cooled to room
temperature, filtered, washed well with deionised water and
dried overnight at 1108C.
Using these clay-bound sensitizers, an improved yield of
triplet product was observed compared to the corresponding
(Table 1) free sensitizer in solution. This is ascribed to
decreased two-dimensional mobility within the clay layers
and consequent increase in energy transfer between the
immoblised sensitizer and the substrate. Another interesting
observation which would also have contributed to the
increase in energy transfer is, when these sensitizers are
converted to their ionic forms, the consequent shift in
their lmax brings them closer to that of 2-acetylnaphthalene
(lmax 281 nm), the sensitizer with a triplet energy
(59.5 kcal mol21) which is the most efficient0 for myrcene.
For example, the lmax values of the free 4 -aminoaceto-
phenone (314 nm) and 40-aminobenzophenone (332 nm)
have decreased to 278 and 293 nm upon treatment with an
equimolar quantity of hydrochloric acid. Thus, it is likely
that ionization and consequent anchoring of the sensitizer
onto clay interlayer decreases the energy difference between
the triplet states of the sensitizer and myrcene and had
ensured efficient energy transfer. Attempts to identify the
nature of the interaction of the sensitizer with clay by
recording the UV diffuse reflectance spectrum was not
successful, as we were unable to record a good clean
spectrum. This may be due to the low amount of sensitizer
and also the poor sensitivity of UV absorption to diffuse
reflectance measurements.
The sensitizer anchored clays were prepared by stirring 1 g
of Naþ-exchanged bentonite/hydrotalcite with 50 mg of
sensitizer in 10 mL of AR acetone for 6 h. Evaporation
of the solvent at reduced pressure gave a powder, which
was washed repeatedly with diethyl ether, to remove any
surface adsorbed sensitizer. The clay sensitizer matrices
were characterized by X-ray powder diffraction patterns. A
comparison of the data for Naþ-bentonite (A) with that
of 40-aminoacetophenone-bentonite (B), 20-aminoaceto-
phenone-bentonite (C), a-aminoacetophenone-bentonite
(D), 40-aminobenzophenone-bentonite (E) and 20-amino-
benzophenone-bentonite (F) revealed substantial differ-
ences. The ratio of the major peaks (at d values of 2.7631
þ
˚
and 1.9729 A) in Na -bentonite at 2.03 had changed to
1.82, 1.80, 1.80, 1.77 and 1.87, respectively, in (B)–(F).
˚
A prominent new peak at ,10–12 A was observed in
allþsensitizer-anchored clays, which was absent in
in (A) now had altered intensities. For example, the peak
˚
Na -bentonite. In addition the three peaks at 3.0–3.2 A
˚
at d,3.3 A in (A) now had increased intensity in (B)–(F).
In the case of (B), new peaks were observed at d values of
˚
6.8099, 6.6075 and 6.4168 A. These observations indicate
Thus the present approach provides an useful route to
achieve triplet state chemistry of organic substrates within
the layered clay microenvironment. Besides acting as a
constrained microreactor, they also help in anchoring the
sensitizer of choice, thus opening up the possibility of
extension of this approach to other more complex and
difficult systems. Unlike zeolites10,11 (where only nonpolar
solvents are used to confine the substrates inside the cages),
in the present study a wide range of solvent systems may be
employed (more polar solvents can also used) and size
restrictions8 on the substrate are no longer the constraints or
limiting factor. The other advantages of using clay-bound
that the sensitizers are indeed all anchored onto clay surface.
Myrcene was purified by distillation under vacuum and
stored under cooling in a refrigerator.
In a typical photolysis experiment, 0.1 mL (0.588 mmol) of
myrcene and 200 mg of sensitizer anchored clay were
irradiated for the specified time as a slurry in dry ether under
a nitrogen atmosphere. All the sensitized irradiations were
carried out at 365 nm and the direct irradiation reactions at
254 nm for 20 h with stirring. After irradiation, clay samples
were filtered, extracted with chloroform and the solvents
Madhavan
