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Stereoselective Photochemical Electrocyclic Valence Isomerizations of a-Phellandrene Conformational Isomers' John E. Baldwin2 and Spencer M. Krueger3
Contribution from the Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, Illinois 61801, and the Department of Chemistry, University of Oregon, Eugene, Oregon 97403. Received June 25, 1969 Abstract : The primary products from the unsensitized photolysis of a-phellandrene are the cis,trans- and cis,cis3,7-dimethylocta-l,3,5-trienes;the trans,trans and trans,cis isomers are formed through photoisomerizations
of the primary products. The temperature dependence of the ratio of the two primary products, when plotted as log (ratio) us. T+, gives a linear correlation consistent with the postulate that the a-phellandrene conformer with a pseudoaxial isopropyl substituent is photoisomerized to the cis,cis-triene, while the conformer with a pseudoequatorial isopropyl group is converted only to the cis,trans-triene. The conversion of the pseudoaxial to the pseudoequatorial conformational isomer of a-phellandrene occurs with A H o = -0.46 kcal/mol. The data and interpretation correctly predict the sign reversal in the molecular rotation of a-phellandrene noted in independent ord and cd studies.
M
olecules such as 3-substituted cyclobutenes (1) or 5-substituted cyclohexadienes (2) would be allowed, according t o the theoretical principles codified in the Woodward-Hoffmann rules,4 t o give either cis or trans electrocyclic ring-opened valence isomers through concerted thermal or photochemical reactions.
later publication defined structure 5 for the bicyclic olefin reached by extended irradiation. lo
I I A CH,CH-CH=CH-CH=C-CH=CH,
..-L
4
I
1
CH3
CH3
I R dl-5
2
\
R
Four cyclobutenes of type 1 have been pyrolyzed and ~ compatible found to give the trans p r o d u ~ t , ~a- result with the simple notion that, of the two competitive activated complexes, the one minimizing steric interactions involving bulky substituents would be of lowest energy. Analogous investigations of the stereochemistry of the thermal rearrangements of cyclohexadienes, and of the photorearrangements of cyclobutenes and cyclohexadienes, have been wanting. a-Phellandrene (3), the molecule selected for study in this work, was investigated earlier by de Kock, Minnaard, and H a ~ i n g a ,and ~ found t o give a mixture of geometrical isomers of 3,7-dimethylocta-l,3,54riene(4) which reacted further on prolonged irradiation. A
Results Photoproducts. Irradiation at 253.7 nm of a 0.03 M solution of a-phellandrene in hexane until a 3 0 z conversion of diene had been achieved gave a mixture of four isomeric 3,7-dimethylocta-1,3,5-trienes. These products were isolated by glpc and identified through spectroscopic data as, in order of increasing retention time on an analytical Carbowax 20M column, the cis,cis- (6), trans,cis- (7), cis,truns- (a), and trans,truns- (9) isomers.
6
7
(1) Supported in part by Public Health Service Research Grant
No. GM-14381 and GM-16576.
s
&HJ
(2) Alfred P. Sloan Foundation Fellow; inquiries should be directed
to the University of Oregon.
(3) University Fellow (1966-1967) and Sun Oil Fellow (1967), University of Illinois; National Institute of General Medical Sciences Predoctoral Fellow (1968-1969). (4) R. Hoffmann and R. B. Woodward, Accounts Chem. Res., 1, 17 (1968), and unpublished work. ( 5 ) E. Gil-Av and J. Shabtai, J . Org. Chem., 29,257 (1964). (6) H. M. Frey, Trans. Faraday Soc., 60, 83 (1964). (7) H. M. Frey, D. C. Marshall, and R. F. Skinner, ibid., 61, 861
(1965). 18) M. Pomerantz and P. H. Hartman, Tetrahedron Lett., 991 (1968). (9) R. J. de Kock, N. G. Minnaard, and E. Havinga, Rec. Trao. Chim. Pays-Bas, 79, 922 (1960).
Journal of the American Chemical Society
91:23
i-C,H-
8
9
The mass spectra of the trienes had molecular ion peaks at mje 136, and abundant ions at 121 (M+ CHI), 106 (M+ - 2CH3), and 93 (M+ - CaH7); they (10) J. Meinwald, A. Eckell, and K. Erickson, J . Amer. Chem. Soc., 87, 3532 (1965).
/ November 5, 1969
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were identical but for differences in relative ion abundances. Isomers 8 and 9 had bands at 965 and 964 cm-l, respectively, appropriate t o a trans-disubstituted double bond. Isomers 6 and 8 had absorptions for C(2)-H at 6 6.88 and 6.87 ppm, respectively, while the corresponding C(3)-C(4)-trans isomers 7 and 9 showed this proton at much higher field (
