F34. Stereochemistry of the Thermal Retro Diels-Alder Reactions of Deuterium-Labeled Cyclohexene, Norbornene and Bicyclo[2.2.2]oct-2-ene

David K. Lewis,a David A. Glenarb, Steven Hughesa , Bansi L. Kalrac, Jessica Schliera, Rajesh S. Shuklad and John E. Baldwind

a Department of Chemistry, Connecticut College, New London, Connecticut 06320

b Code 693, NASA Goddard Space Flight Center, Greenbelt, MD 20771

c Department of Chemistry, Hollins University, Roanoke, VA 24020

d Department of Chemistry, Syracuse University, Syracuse, New York 13244

Recent experimental and theoretical studies on retro Diels-Alder fragmentations of bicyclo[2.2.1]hept-2-ene (1), cyclohexene (2), and bicyclo[2.2.2]oct-2-ene (3) have addressed whether two carbon-carbon single bonds are broken simultaneously or sequentially, and what factors determine the relative importance of concerted versus stepwise reaction channels [1-4].

 

Gas phase DHf data for 1, 2, and 3 coupled with theoretical estimates of DHf for the corresponding diradicals 4, 5, and 6 suggest that the transition structures reached via "concerted" retro Diels-Alder pathways are of lower energy than the alternative diradical structures formed via cleavage of a single C-C bond, in all three fragmentations [5]. The expected energy difference (concerted vs. diradical transition structure) is largest for 1, smaller for 2, and smallest (close to zero) for 3.


Earlier experiments have shown that norbornene (1) fragments in a stereochemically conservative fashion: cis,exo-5,6-d2-bicyclo[2.2.1]hex-2-ene (1-d2) gave only cis-d2-ethylene [6, 7]. Cyclohexene 2-d2, however, gave rise to some trans-1,2-d2-ethylene along with the predominant cis isomer at 821 oC and 907 oC, with the percentage of trans isomer increasing with temperature, evidence implicating diradical and vinylcyclobutane intermediates [8, 9]. More recently, under the gas-phase pump-probe reaction conditions of femtosecond-resolved dynamic studies, 1, 2, and 3 all gave transient species thought to be the diradicals 4, 5, and 6. Stepwise processes as well as concerted fragmentations appeared to be involved [1, 2]. Internal energies of transition state structures formed via photo-excitation are likely to be greater than energies of transition states formed in thermal excitation experiments, possibly accounting for the apparently larger contribution of diradical processes in the photo-excitation experiments.

 

From this experimental evidence and the calculated thermochemical trends one can predict that 3-d2 would fragment with the greatest loss of stereochemistry, reflecting a more important role played by a diradical intermediate. This prediction has now been tested experimentally.

In the present study, Retro Diels-Alder fragmentations of 1-d2 and 3-d2 were conducted using a 2.54 cm i.d. single-pulse shock tube, and the cis/trans ratios of the 1,2-d2-ethylenes formed were measured by tunable diode laser (TDL) infrared absorption spectroscopy.

A sample of 1-d2, heated to 576 oC gave only cis-d2-ethylene, as expected from the thermochemical analysis and earlier work [6, 7]. A sample of 3-d2 heated to 665 oC gave only unlabeled ethylene and cis-d2-ethylene. No trans-d2-ethylene was detected. This outcome is inconsistent with the expectation that the stepwise pathway would play a larger role in this fragmentation.

It seems that thermal Diels-Alder and retro Diels-Alder reactions may involve families of trajectories over the transition region of the potential energy surface. Some of those trajectories may lead to nonconcerted, stereochemically nonconservative processes, as in the cyclohexene to butadiene plus ethylene fragmentation [8, 9]; but these trajectories may be less available in the more highly constrained molecules 1 and 3. It is likely that conformational opportunity, rather than orbital symmetry or thermochemistry, is the dominant factor controlling reaction stereochemistry in these isomerizations [10].

 

References

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10. Compare the considerations of "energy landscapes" and "landscape dynamics" in (a) Zewail, A. H. Angew. Chem., Int. Ed. 2000, 39, 2586-2631. (b) Zewail, A. H. J. Phys. Chem. A 2000, 104, 5660-5694.