On the mechanism of the Diels-Alder reaction--dimerization of trans-phenylbutadiene

Publication Year:
1972
Usage 931
Downloads 889
Abstract Views 42
Repository URL:
https://pdxscholar.library.pdx.edu/open_access_etds/972
DOI:
10.15760/etd.972
Author(s):
McNicholas, Michael Ward
Publisher(s):
Portland State University Library
Tags:
Diels-Alder reaction; Inorganic Chemistry; Materials Chemistry; Physical Chemistry
report description
The Diels-Alder dimerization of trans-1-substituted butadienes is expected to yield a cyclohexene adduct with the substituents in the 3 and 4 positions cis to one another. This prediction is based on past observations of other Diels-Alder additions. The cis-isomer is the only one consistent with a two-stage mechanism proposed by Woodward and Katz.In the case of trans-phenylbutadiene, the expected adduct is cis-3-phenyl 1-4-(trans-styry1) cyclohexene. Alder, Haydn and Vogt, however, reported that the corresponding trans-isomer is the dimerization product. There is reason to believe that the observation of the trans-isomer may have been the result of product isomerization during purification. In view of the unexpected nature of the product and its implications with respect to the Diels-Alder mechanism, the dimerization reaction was reinvestigated.trans-Phenylbutadiene was heated at 1300 for six hours. Analysis by thin layer chromatography on silica gel impregnated with silver nitrate revealed two major components as well as several minor ones. The two major components were isolated by column chromatography on silica gel impregnated with silver nitrate. They were present in roughly equal proportions. One of them was purified and found to be identical with cis-3-pheny l~4-(trans-styry l) cyclohexene prepared by an independent method. The second major component, not yet isolated in pure form, is probably the corresponding trans-isomer on the basis of IR and NMR spectra as well as the fact that the trans-isomer was found by Alder et al.An evaluation of the significance of this reaction with respect to the Diels-Alder mechanism will have to await positive identification of the second component as well as a more careful determination of its relative proportion to the cis-isomer.