Generation and reactivity of a triplet 1, 4-biradical conformationally trapped in a crystalline cyclopentanone

Full text of DOI:10.1021/ja980112e

4540
J. Am. Chem. Soc. 1998, 120, 4540-4541
Scheme 1
Generation and Reactivity of a Triplet 1,4-Biradical
Conformationally Trapped in a Crystalline
Cyclopentanone
Krisztina Peterfy and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry
UniVersity of California
Los Angeles, California 90095
ReceiVed January 12, 1998
We have recently shown that the crystalline solid state offers
a promising vehicle for controlling highly reactive intermediates.
Carbenes, biradicals, and excited states may be confined to well-
defined reaction trajectories,1-4 and their selectivity can be
analyzed in terms of molecular and environmental parameters
determined by X-ray diffraction.5-8 We report here our initial
studies on the photochemical generation and reactivity of 1,4-
biradicals (1,4-BR) in crystals of their cyclopentanone precursors
(Scheme 1a).9 Our interest in 1,4-BR is motivated by their
chemical dichotomy in competing cyclization and cleavage
reactions (Scheme 1b) and their intermediacy in 2π + 2π
dimerizations10-12 and Norrish type-II reactions.13,14 Although
recent work on 1,4-BR has centered on the use of increasingly
faster techniques,13,15,16 controlling their reactivity in crystalline
media is also of great interest. We analyze three model
cyclopentanones to determine their suitability as precursors for
1,4-BR in crystals. Their photochemistry is characterized in terms
of disproportionation vs decarbonylation and/or cyclization vs
cleavage reactions. We also investigate whether the solid-state
reaction may be carried out with high yields and maintained
selectivity.
Scheme 2
The strategy devised for the generation of 1,4-biradicals in
crystals comes from our investigations on substituted cyclohex-
anones.2 The proposed mechanism involves electronic excitation
of the ketone followed by intersystem crossing (isc) and an
R-cleavage reaction in the triplet state (Scheme 1a). It is expected
that triplet acyl-alkyl biradicals (3BR-1) formed in this manner
may live long enough to undergo a spin-conserving decarbony-
lation (-CO) to yield a triplet 1,4-BR in competition with
intersystem crossing (isc′) to 1BR-1 and formation of the ground-
state ketone (dotted lines, Scheme 1a). The role of the R-sub-
stituents is to facilitate R-cleavage and decarbonylation, which
must compete with excited-state deactivation and intersystem
crossing (isc′), respectively. We have suggested that product
formation from 1BR-1 in crystals is unlikely due to rapid
regeneration of the ground-state ketone.2 In contrast, we expect
that 1,4-BR should not revert back to BR-1 and ketone, but give
cyclobutanes and/or olefins from their singlet manifold.
To test the above expectations, we analyzed the solution and
crystal photochemistry of compounds 1-3 (Scheme 2).17 These
were chosen to determine whether alkyl or aryl substituents may
satisfy the requirements of the solid-state reaction. In benzene,
compound 1 yields a 2:1 mixture of 2-benzyl-2,5-dimethyl-6-
phenyl-5-hexenals 4 with a combined quantum yield of 0.9. The
lack of cyclobutanes and/or alkenes indicates that decarbonylation
is slower than intersystem crossing. The lack of ketone isomer-
ization suggests that radical coupling is disfavored relative to
(1) Shin, S. H.; Cizmeciyan, D.; Keating, A. E.; Khan, S. I.; Garcia-Garibay,
M. A. J. Am. Chem. Soc. 1997, 119, 1859-1868.
(2) Choi, T.; Peterfy, K.; Khan, S. I.; Garcia-Garibay, M. A. J. Am. Chem.
Soc. 1996, 118, 12477-12478.
(3) Shin, H. S.; Keating, A. E.; Garcia-Garibay, M. A. J. Am. Chem. Soc.
1996, 118, 7626-7267.
(4) Choi, T.; Cizmeciyan, D.; Khan, S. I.; Garcia-Garibay, M. A. J. Am.
Chem. Soc. 1995, 117, 12893-12894.
(5) Schmidt, G. M. J. Solid State Photochemistry; Verlag Chmie: New
York, 1976.
(6) Scheffer, J. R.; Pokkuluri, P. R. In Photochemistry in Organized and
Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; pp 185-
246.
1
disproportionation in BR-1.
(7) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433-481.
(8) Hollingsworth, M. D.; McBride, J. M. AdV. Photochem. 1990, 15, 279-
379.
(9) The reactivity of 1,4-BR in crystals has been documented in studies of
the Norrish type-II reaction, e.g.: Ariel, S.; Evans, S. V.; Garcia-Garibay,
M.; Harkness, B. R.; Omkaram, N.; Scheffer, J. R.; Trotter, J. J. Am. Chem.
Soc. 1988, 110, 5591-5592.
(10) Schuster, D. I. In The Photochemistry of Enones; Patai, S., Pappoport,
Z., Eds.; John Wiley & Sons Ltd.: New York, 1989; pp 623-756.
(11) Andrew, D.; Weedon, A. C. J. Am. Chem. Soc. 1995, 117, 5647-
5663.
(17) Cyclopentanone 1 was prepared by hydrogenation of 2,6-ben-
zylidenecyclopentanone followed by reaction with tBuOK and MeI. Com-
pounds 2 and 3 were prepared from 2,2-diphenylcyclopentanone (Easton, N.
R.; Nelson, S. J. J. Am. Chem. Soc. 1953, 75, 640) by reaction with MeI and
tBuOK, and by reaction with pentaphenyl bismuth (Barton, D. H. R.; Charpiot,
B.; Pan, E. T. H.; Motherwell, W. B.; Pascard, C.; Pichon, C. HelV. Chim.
Acta 1984, 67, 586), respectively.
(12) Jones, G. Org. Photochem. 1981, 5, 1-122.
(13) Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252-258.
(14) Wagner, P. J.; Park, B.-S. Org. Photochem. 1991, 11, 227-366.
(15) Pedersen, S.; Herek, J. L.; Zewail, A. H. Science 1994, 266, 1293.
(16) Caldwell, R. A. In Kinetics and Spectroscopy of Carbenes and
Biradicals; Platz, M. S., Ed.; Plennum: New York, 1990; pp 77-116.
S0002-7863(98)00112-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/28/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 18, 1998 4541
Figure 1. Time course of the solid-state irradiation of cyclopentanones
1 (squares), 2 (circles), and 3 (diamonds).
Figure 2. DSC traces of partially reacted samples of compound 3
(conversion: top 0%, middle 40%, and bottom 65%).
The quantum yield of reaction from 2 in benzene is only one
twentieth of that of 1 (Φ ) 0.04). Low conversions (10-20%)
yield pentenal 5, cyclobutane 6, and 1,1-diphenylethene 7 in a
5:2:3 ratio as determined by 1H NMR. The formation of aldehyde
5 indicates that R-cleavage occurs at the side of the phenyl
substituents.
The solution photochemistry of ketones 1 and 2 is characterized
by low yields of decarbonylation and different tendencies for
cyclization and disproportionation. In contrast, the solution
photochemistry of tetraphenyl cyclopentanone 3 is dominated by
decarbonylation with Φ ) 0.2. As reported by Barton et al.,18
1,1-diphenylethene 7 and tetraphenylcyclobutane 9 were obtained
in a 3:1 ratio.18
contrast, since conformational relaxation in crystals of 3 is
unlikely, 1,4-BR produced by decarbonylation should remain
trapped in high energy syn or gauche conformations to give
products with a selectivity that reflects the rate constants for
product formation in this medium.
Interestingly, while both reactions may in principle occur from
syn (or gauche) conformers, only cyclization was observed.
Although this preference may come from intrinsically faster
cyclization rates, it may also come from environmental restrictions
associated with differences in reaction and activation volumes.
While cyclization has negative reaction and activation volumes,
cleavage of the 1,4-biradical to yield two molecules of 1,1,-
diphenylethylene has activation and reaction volumes that are
large and positive.25 McBride et al.26 have shown that internal
pressure in crystals may reach extremely high values, which may
control the chemoselectivity observed.
Solid-state irradiations of 1, 2, and 3 were carried out at 20 °C
with polycrystalline samples. Excitation wavelengths were chosen
near the tail19 of the n,π* absorption (λ > 300 nm).20,21 In contrast
to solution photolysis where moderate to high reactivity was
observed in all cases, only tetraphenyl cyclopentanone 3 reacted
in the crystalline solid state (Figure 1). The efficient dispropor-
tionation of 1 in solution was fully suppressed and crystals of 1
and 2 (both have mp ) 80-82 °C) remained unreacted. We
assign their stability to slow decarbonylation rates which are
An interesting aspect of the crystal photochemistry of 3 comes
from the high yields of 9 attainable. Solid-to-solid reactions are
postulated to occur through solid solutions of the product in the
crystal phase of the reactant.21,27 Exceptionally, reactions may
continue along a given crystal phase in single crystal-to-single
crystal, or topotactic transformations. More commonly, product
phases separate at concentrations where their solubility is
exceeded.4,20,21 Differential scanning calorimetry (DSC) analysis
of partially reacted samples of 3 suggests a phase separation
mechanism. While melting of the pure reactant and product
phases occur at 183 and 121 °C, respectively, partially reacted
samples showed a eutectic transition at 108 °C (Figure 2). A
broad endotherm between melting of the reactant and the eutectic
suggests either a three phase system or a spatially heterogeneous
sample.28 Although intermediate phases between those of the
reactants and products may be possible, photochemical reactions
in crystals proceed along an irradiation front and we favor the
second explanation. That the chemoselectivity of the reaction is
maintained as the composition changes strongly suggests the
potential of solid-state photochemistry for preparative purposes.
Work in progress is aimed at the detection of the biradical
intermediates and at a more detailed characterization of their solid-
state chemistry.
1
unable to compete with isc′ and reaction of BR-1 back to the
starting ketone (Scheme 1).
Irradiations of polycrystalline 3 (mp ) 180-182 °C) proceeded
in a smooth solid-to-solid reaction with maintained chemoselec-
tivity to give cyclobutane 9 as the only solid-state product. The
influence of the crystalline solid state on the selectivity of
cyclization vs cleavage pathways is unprecedented for analogous
1,4-biradicals.22 Cyclization requires gauche conformations that
allow for bonding interactions between the two radical termini,
and cleavage may in principle occur from syn, gauche, or anti
conformers as long as overlap between the singly occupied
p-orbitals and the biradical 2,3-bond is allowed (Scheme 1b). The
yields of cyclization and cleavage in solution reflect a complex
interplay among conformational equilibrium, conformational-
dependent triplet to singlet isc rates, and the elementary rates of
cyclization and cleavage from singlet biradicals.11,16,23,24 In
(18) Barton, D. H. R.; Charpiot, B.; Ingold, K. U.; Johnston, L. J.;
Motherwell, W. B.; Scaiano, J. C.; Stanforth, S. J. Am. Chem. Soc. 1985,
107, 3607-3611.
(19) Enkelmann, V.; Wegner, G.; Novak, K.; Wagener, K. B. J. Am. Chem.
Soc. 1993, 115, 10390-10391.
(20) Garcia-Garibay, M. A.; Constable, A. E.; Jernelius, J.; Choi, T.;
Cizmeciyan, D.; Shin, S. H. Physical Supramolecular Chemistry; Kluwer
Academic Publishers: Dordrecht, 1996; pp 289-312.
(21) Keating, A. E.; Garcia-Garibay, M. A. In Molecular and Supramo-
lecular Photochemistry; Ramamurthy, V., Schanze, K., Eds.; Marcel Dekker:
New York, In press.
(22) Although high stereospecificity in the cyclization of singlet 1,4-
biradicals suggests that product formation competes with conformational
equilibration, 1,4-biradicals from azo precursors cyclize and cleave with similar
probabilities: Bartlett, P. D.; Porter, N. A. J. Am. Chem. Soc. 1968, 90, 5317-
5319.
Acknowledgment. Support by the National Science Foundation
(CHE-9320619 and CHE-9624950), and by the donors of the Petroleum
Research Fund, is gratefully acknowledged.
JA980112E
(25) le Noble, W. J.; Kelm, H. Angew. Chem., Int. Ed. Engl. 1980, 19,
841.
(26) McBride, J. M. Acc. Chem. Res. 1983, 16, 304-312.
(27) Schmidt, G. M. J. J. Chem. Soc. 1964, 2014-2021.
(28) (a) Ozawa, R.; Matsuoka, M. J. Crystal Growth 1989, 98, 411-419.
(b) Matsuoka, M.; Osawa, R. J. Crystal Growth 1989, 96, 596-604.
(23) Scaiano, J. C. Tetrahedron 1982, 38, 819-824.
(24) Griesbeck, A. G.; Mauder, H.; Stadtmuller, S. Acc. Chem. Res. 1994,
27, 70-75.