above. The remarkably narrow distributions (see Fig. 2) suggest
the presence of discrete, well defined sites.
The lifetime enhancement observed opens exciting possibil-
ities for novel nucleophilic substitution chemistry; essentially
Grignard-type chemistry, triggered photochemically in supra-
molecular systems.
species within the zeolite is probably Na+2; this is consistent
with the reactivity – and increased lifetime – observed.
Conceivably, interaction between the cations and the carbonyl
may assist this reaction. Use of Mg2+ as a counterion may be
interesting and is the subject of current studies.
From a mechanistic point of view these results are quite
important since they demonstrate the potential of zeolites to
host carbanion mediated reactions. Further, in the last decade
many new zeolitic materials have been synthesised and
characterized, including a range of mesoporous materials with
large cages (such as MCM-41). In addition, a variety of
methodologies have been developed to BtuneB the acid–base
and redox properties of zeolite supercages. We believe that the
implementation of these methodologies and the use of new
materials with reduced acidity will lead to improved synthetic
applications in the future. Our current research is exploring
some of these possibilities.
Product studies were performed in perfluorohexane slurries.
Irradiation of zeolite inclusion complexes in slurries is common
practice; however, these experiments run the risk that actual
light exposure occurs when the substrate is in the liquid phase.
Use of fluorous solvents, where most organic molecules are
insoluble, readily eliminates this concern. Irradiation of 1 at 254
nm in the absence of added quencher reveals 3-ethylbenzophe-
none (3) as the main reaction product, as has been observed in
solution. Addition of typical nucleophilic scavengers leads to
observable lifetime reductions. For example, exposure of the
zeolite to small quantities of water reduces the lifetime of the
long-lived component by two to three times. The reduced
lifetime is still an order of magnitude longer than what is
observed in aqueous solution but this is reasonable considering
the reduced mobility and concentration as compared to
solution.
Generous financial support from NSERC (J. C. S) and the
Spanish DGES (H. G., Grant 97-1016-CO2) is gratefully
acknowledged. G. C. is the holder of an Ontario Graduate
Scholarship and M. N. C. thanks the Ontario Graduate
Scholarship in Science and Technology for funding.
Similar lifetime reductions were observed using acet-
aldehyde and bromoalkanes as quenchers. In both cases
preparative experiments were also conducted by loading 1@Y
with the desired quencher by vapour diffusion and irradiating
the resulting solid as a perfluorohexane slurry (dry conditions).
Acetaldehyde was absorbed to a loading of 415 molecules per
supercage. Bromoethane was absorbed to a much lesser extent
resulting in a loading of only ~ 2 molecules per supercage. In
both cases we detected the peak characteristic for the product of
SN2 reaction (Scheme 2) by GCMS following Sohxlet extrac-
tion of the zeolite with ethyl acetate, however, the SN2 reaction
on bromoethane was inefficient while in the case of acet-
aldehyde the alcohol product 4 was obtained in 82% yield and
with excellent conversion (MS m/z 254 (M+), 209, 177, 105,
77). Protonation to give 3 was largely suppressed in the
presence of acetaldehyde, accounting for about 8–9% of the
products. The remainder ( < 10%) corresponded to a chromato-
graphic peak that was tentatively assigned to the dimer of the
radical produced by electron loss from 2; the presence of
electron acceptor sites in the zeolites is well established.20,21
Notes and references
1 H. L. Casal and J. C. Scaiano, Can. J. Chem., 1985, 63, 1308.
2 N. J. Turro, A. McDermott, X. Lei, W. Li, L. Abrams, M. F. Ottaviani,
H. S. Beard, K. N. Houk, B. R. Beno and P. S. Lee, Chem. Commun.,
1998, 697.
3 T. Hirano, W. Li, L. Abrams, P. J. Krusic, M. F. Ottaviano and N. J.
Turro, J. Org. Chem., 2000, 65, 1319.
4 J. C. Scaiano and H. García, Acc. Chem. Res., 1999, 32, 783.
5 M. L. Cano, M. N. Chrétien, H. García and J. C. Scaiano, Chem. Phys.
Lett., 2001, 345, 409.
6 F. Boscá, M. L. Marín and M. A. Miranda, Photochem. Photobiol.,
2001, 74, 637.
7 A. Alomar, Contact Dermatitis, 1985, 12, 112.
8 L. L. Constanzo, G. De Guidi, G. Condorelli, A. Cambria and M. Fama,
Photochem. Photobiol., 1989, 50, 359.
9 M. C. Marguery, N. Chouini-Lalanne, J. C. Ader and N. Paillous,
Photochem. Photobiol., 1998, 68, 679.
10 F. Boscá, M. A. Miranda, G. Carganico and D. Mauleón, Photochem.
Photobiol., 1994, 60, 96.
11 C. F. Chignell and R. H. Sik, Photochem. Photobiol., 1995, 62, 205.
12 D. de la Peña, C. Martí, S. Nonell, L. A. Martínez and M. A. Miranda,
Photochem. Photobiol., 1997, 65, 828.
13 S. Monti, S. Sortino, G. De Guidi and G. Marconi, J. Chem. Soc.,
Faraday Trans., 1997, 93, 2269.
14 S. Monti, S. Sortino, G. De Guidi and G. Marconi, New J. Chem., 1998,
22, 599.
15 L. J. Martínez and J. C. Scaiano, J. Am. Chem. Soc., 1997, 119,
11066.
16 G. Cosa, L. J. Martínez and J. C. Scaiano, Phys. Chem. Chem. Phys.,
1999, 1, 3533.
Scheme 2
17 F. Wilkinson and C. J. Willsher, Appl. Spectrosc., 1984, 38, 897.
18 F. Wilkinson and G. Kelly, Diffuse Reflectance Laser Flash Photolysis,
ed. J. C. Scaiano, CRC Press, Boca Raton, 1989, vol. 1, pp. 293.
19 J. C. Scaiano, M. Tanner and D. Weir, J. Am. Chem. Soc., 1985, 107,
4396.
20 M. A. OANeill, F. L. Cozens and N. P. Schepp, J. Phys. Chem. B, 2001,
105, 12746.
21 A. Corma, V. Fornés, H. García, V. Martí and M. A. Miranda, Chem.
Mater., 1995, 7, 2136.
The product from reaction with bromoethane was only
obtained as a minor component in the photolysis of 1@Y, the
protonated compound, i.e. 3-ethylbenzophenone, was the major
product; this contrasts with the efficiency of the acetaldehyde
reaction and may be a combined consequence of the lower
loading and the increased reactivity of typical Grignard-type
intermediates towards carbonyls as compared to halogenated
alkanes. By analogy to metal alkyls and Grignard reagents, the
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