405
potassium r-butoxide in r-butyl alcohol-pyridine. After hydrolysis and careful work-up to ensure isolation of all hydrocarbon product, nmr analysis showed signals only of starting compounds and no evidence for the trans stereoisomers.
Acknowledgment. We are grateful to the National Science Foundation and the s, Army Research Office (Durham) for financial support of this research.
Cycloaddition. X. Reversibility in the Biradical Mechanism of Cycloaddition. Tetrafluoroethylene and 1,1-Dichloro-2,2-difluoroethylenewith 2,4-Hexadiene Paul D. Bartlett, Carol J. Dempster, Lawrence K. Montgomery, Kathleen E. Schueller, and Gunter E. H. Wallbillich Contribution from the Converse Memorial Laboratory of Harvard University, Cambridge, Massachusetts 02138. Received September 3, 1968 Abstract: The four stereoisomeric cyclobutanes from thermal cycloaddition of 1,l-dichloro-2,2-difluoroethylene (“1122”) to the stereoisomeric 2,4-hexadienes1at 80” have been isolated by preparative vapor phase chromatography and their configurations as previously assigned have been confirmed by nuclear magnetic resonance. Tetrafluoroethylene at 120” adds to trans,trans-2,4-hexadieneto yield only the two stereoisomeric trans-propenylcyclobutanes in a translcis ratio in the ring of 4.2, and with no isomerization of the recovered diene. At this same temperature the addition of 1122 to trans,trans-2,4-hexadieneis attended by isomerization of the diene and formation of minor amounts of cis-propenylcyclobutanes along with major amounts of the trans-propenylcyclobutanes. The isomerization is interpreted as due to reversal of the initial biradical formation, which may occur after rotation in the biradical. The similar amounts of loss of configuration with tetrafluoroethylene and 1122 lead to the suggestion that the rate of ring closure is controlled, not by the rate constant of radical-radical combination, but by the rate of rotations about the three single bonds of the biradical to bring it into a conformation suitable for ring closure from an original distribution of non-cis conformations. he thermal addition of 1,l-dichloro-2,2-difluoro- incomplete rotational equilibration of an intermediate biradical 3 (Chart I); from the relative amounts of ethylene (“1 122”) to the geometrical isomers of 4-tt and 4 4 from 1-tt and 1-ct when X = C1, it was 2,4-hexadiene yields mixtures of cyclobutanes of iden-
T
Chart I
\+ 1-tt
F2C=CXz
(-)
2
34 t
44 t
lt 3-ct
4.ct
I Cc
4.cc
I-ct
tical orientation but differing in configuration at the ring.’ The proportions of the products correspond to (1) L. K. Montgomery, K. Schueller, and P.D. Bartlett, J . Am. Chem. SOC.,86, 622 (1964).
possible to conclude that 3-ct is converted by internal rotation into 3-tt with a rate constant ten times that with which it closes to 4 4 . The same figure could be arrived at by using the4-tcj4-ccratios from 1-ct compared to 1-cc. Bartlett, et al.
ReversibiIity in Cycloaddition
406 T l l e I. Cycloadditions of truns,lruns2,4-Hexadiene, 120", 24 hr
-
Recovered dienes,
5'
) 'i ..
CFZ=CFZ CCIz=CF*
0.06
-*
..
18.2 1.4
-
0.73
..
Cycloadducts, 4-tt
4-tc
50.4
4-cc
12.1
... (61.6) 35.6
2.50
(14.8) 12.2
(36.5)
(2.55)
(0.40)
0.07
Upper figure, per cent conversion; lower figure, in parentheses, yield based on unrecovered diene. present,
