A R T I C L E S
Unruh and Birney
7-one (2, ∆Gq ) 31.9 ( 0.5 kcal/mol)10,14 and of bicyclo[2.2.1]-
hepta-2,5-dien-7-one (3, ∆Gq ) 15 kcal/mol)18-20 are both
constrained to disrotatory pathways. These remarkably low
barriers certainly reflect the significant exothermicities of these
reactions, but have also been interpreted as evidence that the
disrotatory pathway is allowed.20 On the other hand, the
experimental activation energy for the thermal decarbonylation
of 1 is quite high. It has been measured as 51.3 ( 0.2 kcal/
mol11 or 46.4 ( 2.4 kcal/mol.16
critical; upon consideration of the high barrier for decarbonyl-
ation (vide supra), it was apparent that the pyrolysis temperatures
required to effect the decarbonylations of stereochemically
labeled 3-cyclopentenones would be sufficiently high to isomer-
ize the product diene(s) as in eq 1.25
In the work reported here, multiphoton infrared (MP-IR)
irradiation was used to carry out the themal pyrolysis of cis-
2,5-dimethyl-3-cyclopentenone (4) while avoiding subsequent
thermal isomerization of the product hexatriene(s). We refer to
this as MP-IR photolysis/thermolysis; it is a photolysis because
of the absorption of IR photons, but it is a thermolysis because
the chemistry occurs via thermally excited ground-state mol-
ecules. MP-IR photolysis/thermolysis is initiated by the sequen-
tial absorption of IR photons from a pulsed IR laser.26
Absorption is accompanied by intramolecular vibrational energy
redistribution (IVR), which in large organic molecules is very
rapid.26,27 Thus, MP-IR produces highly vibrationally excited
molecules in the ground electronic state with an effectively
randomized energy distribution, similar to that obtained in
conventional pyrolysis. There is, however, a significant differ-
ence. By tuning the IR laser and by judicious choice of reactants,
only one component of a gas mixture may absorb the light and
is heated. The overall rate of a reaction will depend on the rate
of IR absorption as well as the competition between collisional
cooling of the hot molecule and the rate of the unimolecular
reaction. The important point for this work is that collisions
can cool hot product molecules before they can react further.
This amounts to a pyrolysis in a room-temperature environment.
MP-IR decompositions of a number of systems have been
reported.28-38 Of the most relevance to the current work, MP-
IR thermolysis of cyclobutyl acetate yields cyclobutene, while
pyrolysis gives only butadiene as the product of secondary
thermolysis of cyclobutene.39 Thus, we anticipated that the
reaction scheme for the MP-IR photolysis/thermolysis of 4
would be as in eq 2. MP-IR would sufficiently heat 4 to give
4* and induce decarbonylation. This would generate 5, possibly
Information on the mode of departure of the carbon monoxide
is quickly lost after a decarbonylation; however, there have been
a number of studies designed to examine the fate of the CO.
Elegant pump-probe studies have determined the vibrational,
rotational, and translational energy distributions in the CO
immediately following gas-phase photodissociation of 1.22,23
Their results were interpreted in terms of a concerted fragmenta-
tion,23 with both C-C bonds breaking simultaneously as the
CO bends out of the molecular plane. In contrast to more
familiar pericyclic reactions, both thermal and photochemical
decarbonylations are allowed via the linear, disrotatory path-
way.4 However, the decarbonylation of 1 has been shown to
occur from the triplet state.15 In any case, the details of the
photochemical reaction are not necessarily transferable to the
thermal reaction. In shock-tube experiments by Simpson et al.,
fragmentation occurs from the ground state.21 These authors
suggest that the observed energy partitioning indicates that, at
the transition state, the CO bond length is closer to that of the
product CO than to that of the carbonyl in 1. This qualitative
prediction is borne out in the calculated transition state
geometries7-9 and is consistent with the disrotatory pathway in
Figure 1.
There have been numerous theoretical studies of the ground
state of 1 and of transition states for its cheletropic decarbon-
ylation using both semiempirical17,24 and ab initio methods.7-9,17
Recently, Quirante et al. have reported that a synchronous
transition state for this reaction is found at the MP2(FU)/6-
31G* level,9 and we have reported single point energies at the
MP4(SDTQ)/D95**//MP2(FC)/6-31G* level.8 The geometry of
the transition state (as in Figure 1) and the displacements of
the imaginary frequency correspond to a disrotatory motion on
the butadiene fragment.9 The barrier to thermal decarbonylation
of 1 is calculated to be 49.0 kcal/mol,8 which is in good
agreement with the experimental activation energy (vide supra).
The overall reaction is calculated to be 18.9 kcal/mol endo-
thermic.8
(25) Hess, B. A., Jr.; Baldwin, J. E. J. Org. Chem. 2002, 67, 6025-6033.
(26) Bagratashvilli, V. N.; Letokhov, V. S.; Makarov, A. A.; Ryabov, E. A.
Multiple Photon Infrared Laser Photophysics and Photochemistry; Harwood
Academic Publishers: Chur, Switzerland, 1985.
(27) Seder, T. A.; Weitz, E. Chem. Phys. Lett. 1984, 104, 545-551.
(28) Hintsa, E. J.; Wodtke, A. M.; Lee, Y. T. J. Phys. Chem. 1988, 92, 5379-
5387.
(29) Farneth, W. E.; Thomsen, M. W.; Beck, T. L. J. Phys. Org. Chem. 1990,
3, 567-574.
(30) Goodale, J. W.; Evans, D. K.; Ivanco, M. Can. J. Chem. 1990, 68, 1437.
(31) Kumar, A.; Chowdhury, P. K.; Rama, K. V. Z. Chem. Phys. Lett. 1991,
182, 165.
(32) Yamamoto, S.; Suzuki, M.; Sueishi, Y. Bull. Chem. Soc. Jpn. 1992, 65,
3112.
In view of the intense and long-standing interest in the
decarbonylation of 3-cyclopentenone, it may be surprising that,
over the past 30 years, the stereochemistry of the thermal
decarbonylation of unconstrained 3-cyclopentenones has not
previously been studied. There are presumably two reasons for
the lack of progress in this regard. The first is that the synthesis
of a stereochemically labeled 3-cyclopentenone, for example,
4, has appeared to be difficult. The second reason was more
(33) Coronado, E. A.; Ferrero, J. C. J. Photochem. Photobiol., A: Chem. 1998,
114, 89-94.
(34) Meco, D.; Lasorella, A.; Riccardi, A.; Servidei, T.; Mastrangelo, R.;
Riccardi, R.; Samoudi, B.; Daz, L.; Oujja, M.; Santos, M. J. Photochem.
Photobiol., A: Chem. 1999, 125, 1-11.
(35) Pola, J.; Urbanova, M.; Daz, L.; Santos, M.; Bastl, Z.; Subrt, J. J.
Organomet. Chem. 2000, 605, 202-208.
(36) Rubio, L.; Santos, M.; Torresano, J. A. J. Photochem. Photobiol., A: Chem.
2001, 146, 1-8.
(37) Santos, M.; Daz, L.; Pola, J. J. Photochem. Photobiol., A: Chem. 2002,
152, 17-24.
(38) Chowdhury, P. K. J. Photochem. Photobiol., A: Chem. 2003, 154, 259-
265.
(24) Quirante, J. J.; Enriquez, F. Theor. Chim. Acta 1994, 89, 251-259.
(39) Nguyen, H. H.; Danen, W. C. J. Am. Chem. Soc. 1981, 103, 6253-6255.
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8530 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003