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On the [2 + 21 Thermal Cyclodimerization of cis,trans-1,3-Cyclooctadiene1 Albert Padwa,*2a*b William Koehn,2b*c Joseph Masaracehia,2b C. L. O ~ b o r nand , ~ D. J. Trecker3
Contribution from the Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214, and the Research and Development Department, Union Carbide Corporation, So. Charleston, West Virginia 25303. Received October 30, 1970 Abstract: When heated, neat or in hydrocarbon solvent, cis,trans-1,3-cyclooctadieneunderwent both dimerization and isomerization. The structures of the 2 2 cyclodimers were assigned as the trans,cis, trans,trans, and cis,& dimers. The ratio of the three dimers was found to be independent of the reaction temperature. The reaction followed good second-order kinetics and has AH* = 19 kcal/mol and AS* = -10 eu. The results have been rationalized on the basis of a stepwise mechanism for dimer formation.
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decomposition involving an intermediate tetramethylene he concerted, suprafacial, thermal fusion of two diradica1.22-28 It seems that the thermodynamically olefins t o form a cyclobutane ring is a symmetryunfavorable transition state needed for concerted fragforbidden process. In accord with this prediction, mentation is too difficult to be attained with simple cythermal “ 1,2” cycloadditions of simple olefins have been clobutanes. In contrast, it has been argued that the shown t o involve diradical intermediates. Thermal high stereospecificity observed in the pyrolytic cleavage cycloaddition can, in theory, proceed in a concerted of P - l a c t o n e ~ ~and ~ - P-lactams27 ~~ is the result of a con+a) combinasymmetry-allowed fashion by a (a% certed retrogression where the developing carbonyl tion of the two r b o n d s 4 The high degree of stereogroup functions as the antarafacial site. It would ap2 cycloaddition of olefins with specificity in the 2 21 cycloallenes,6-12 ketenes, 1 3 - l 9 and reactive isocyanatesZ0Nz1 pear as though the majority of thermal [ 2 points toward the possibility that these reactions are addition and retrogression reactions of simple systems proceeds through a stepwise mechanism involving 1,4concerted. It appears that a cumulative n-bond system diradical intermediates. Presumably this is because can function more readily as a n z a donor than an isosteric hindrance and angle strain factors develop t o prolated double bond. Orbital symmetry theory applied hibitive levels as the two n bonds attempt to attain the to the concerted fragmentation of cyclobutane demands that the process occur via the (a% aza) p a t h ~ a y . ~ requisite geometry for a ( n 2 s nza)process. While most of the experimental literature seems to Relevant studies on the pyrolysis of simple cyclobutanes, 21 cycloaddifavor a nonsynchronous process for [2 however, have been interpreted in terms of a stepwise tion of olefins, there is one notable case where the evi(1) For a preliminary report of this work see C. L. Osborne, D. J. dence seemingly points t o a concerted mechanism. Trecker, A. Padwa, W. Koehn, and J. Masaracchia, Tetrahedron Lett., The very interesting work of Kraft and K o l t z e n b ~ r g ~ ~ 4653 (1970). (2) (a) Alfred P. Sloan Foundation Fellow, 1968-1970; (b) State on the stereoselective dimerization of bicyclo[4.2.2]University of New York at Buffalo; (c) NSF Cooperative Fellow, 1968deca-trans-3,cis-7,9-triene (1) has been put forward by 1970. Woodward and Hoffmann4 as an example of a concerted (3) Union Carbide Corp. (4) R. B. Woodward and R. Hoffmann, Angew. Chem., 81,797 (1969); ( n 2 s nza) cycloaddition.
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Angew. Chem., Int. Ed. Engl., 8,781 (1969). ( 5 ) (a) P.D. Bartlett and L. K. Montgomery, J . Amer. Chem. Soc., 86, 628 (1964); (b) P. D. Bartlett, Science, 159. 833 (1968); (c) J. S. Swenton and P. D. Bartlett, J . Amer. Chem. Soc., 90, 2056 (1968); (d) P. D. Bartlett and G. E. H. Wallbillich, ibid., 91, 409 (1969). (6) E. F. Kiefer and M. Y . Okamura, ibid., 90,4187 (1968). (7) R. Huisgen, L. Feiler, and G. Binsch, Angew. Chem., 7 6 , 892 (1964). (8) R. Montaigne and L. G. Ghosez, ibid., 80, 194 (1968). (9) W. R. Dolbier and S. H.Dai, J . Amer. Chem. Soc., 90, 5028 (1968). (10) J. E. Baldwin and J. A. Kapecki, ibid., 91,3106 (1969); 92,4874 ( 19 70), (11) W. R. Moore, R. D. Bach and T. M. Ozretich, ibid,, 91, 5918 (1969). (12) J. E. Baldwin and U. V. Roy, Chem. Commun., 1225 (1969). (13) W. T. Brady, E. F. Hoff, R. Roe, Jr., and F. H.Parry, J . Amer. Chem. Soc., 91,5679 (1969). (14) T.DoMinh and 0. P. Strausz, ibid., 92, 1766 (1970). ( 1 5 ) R. Huisgen, L. A . Feiler, and P. Otto, Tetrohedron Lett., 4485 (1969). (16) R. Huisgen and P.Otto, ibid., 4491 (1968). (17) W. T. Brady and H. R. O’Neal, J. Org. Chem., 32,612 (1967). (18) W. T. Brady and E. F. Hoff, J . Amer. Chem. Soc., 90, 6256 (1968). (19) N. S. Isaacs and P. S. Stanbury, Chem. Commun., 1061 (1970). (20) E. J. Mariconi and J. F. Kelly, Tetrahedron Lett., 1435 (1968). (21) H. Bestian, H. Biener, K. Clauss, and H. Heyn, Justus Liebigs Ann. Chem., 718, 94 (1968).
Padwa, et al. / [2
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The ( r 2 s n2a) combination of 7r bonds predicts that the stereochemical relationships at three of the four carbon atoms of the cyclobutane ring will be retained (22) H. M. Frey, Adcan. Phys. Org. Chem., 4, 147 (1966). (23) H. M. Frey and R.Walsh, Chem. Rea., 69, 103 (1969). (24) A. T. Cocks, H. M. Frey, and I. D. R.Stevens, Chem. Commun.. 458 (1969). (25) A. T.Cocks and H.M. Frey, J . Chem. SOC.A , 1671 (1969). (26) J. E. Baldwin and P. W. Ford, J . Amer. Chem. Soc., 91, 7192 (1969). (27) L. A. Paquette, M. J . Wyuratt, and G. R. Allen, hid., 92, 1763 (1970). (28) L. A. Paquette and J. A. Schwartz, ibid., 92, 3215 (1970). (29) D. S.Noyce and E. H. Banitt, J . Org. Chem., 31, 4043 (1966). (30) 0. L. Chapman and W. R. Adams, J . Amer. Cheni. Soc., 90, 2333 (1968). (31) M. U. S. Sultanbawa, Tetrahedron Lett., 4569 (1968). (32) K. Kraft and G. Koltzenburg, ibid., 4357, 4723 (1967).
+ 21 ThermaI Cyclodimerization of cis,trans-I ,3-Cyclooctadiene
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while one carbon will be inverted. The isolation of the trans,cis dimer 2 as the major product from the thermolysis of 1 is thus compatible with a concerted, symmetry-allowed, ( r 2 s r 2 a ) combination. It should be emphasized that although orbital symmetry control is implicated in this dimerization, the mere isolation of 2 does not prove such control, especially since two additional cyclodimers were also formed. The formation of dimer 2 could easily be attributed to a multistep process involving diradical intermediates. In order to shed 21 cycloadditions new light on the mechanism of [2 of distorted olefins, we have examined the thermal dimerization of ~is,trurts-1,3-cyclooctadiene(3). The trans double bond present in 3 is sufficiently strained to serve as a general model for the thermal dimerization of distorted r bonds devoid of activating substituents.
cyclic ring closure. We have found that cis,trans-1,3cyclooctadiene undergoes both isomerization and dimerization in the neat and that at 25" the major path involves combination of two trans double bonds to afford three [2 21 cyclodimers. The ratio of the dimers was found to be practically independent of the reaction temperature over the range -20 to $90" (see Table 11). The reaction followed
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Table 11. Per Cent Composition of Cyclodimers from the Thermolysis of cis,/ru~?s-l,3-Cy~looctadiene~~ dimer*-Temp, "C
-20.0 4.0 28.0 40.0 65.0 90.0
Results cis,trans-1,3-Cyclooctadiene(3) was prepared by the method of Liu33and isolated following the procedure described by Cope. 3 4 Heating 3 in a sealed tube under a nitrogen atmosphere in the dark gave a mixture of cis,cis-cyclooctadiene (4), bicycIo[4.2.0]oct-7-ene (S), and three cyclobutane dimers, 6 , 7, and 8.34a The niaterial balance of the reaction mixture after standing at room temperature for 40 hr was 97 so that no more than 3 of the cis,trans diene went to polymer.
I
8
22 21 21 22 22 24
12 10 8 8
66 69 71 70 70 69
7
7
Dimerization carried out in the neat. total dimer formed.
* Represents per cent of
good second-order kinetics in cyclohexane and the second-order rate constants were determined from three runs each at three different temperatures. Kinetic data on the dimerization of 3 are given in Table 111. An Table 111. Summary of Kinetic Parameters for the Thermal Dimerization of cis,truns-l,3-Cyclooctadiene in Cyclohexane"
5
3
4
6
1.
Temu, "C
kmol-l min-l
AH+, kcalimol
30.2 48.0 76.0
0.041 0.299 2.54
19.0
AS*, eu
- 10
I .O M cyclohexane. 7
6
8
Table I shows the results of two experiments in which the composition percentages of the product were deterTable I. Thermolysis of cis,tru~~s-l,3-Cyclooctadiene (3) Temp, "C
Time, hr
25 90
40 0.5
7 -
3
14 12
4
7 2
Composition, %---5 6 1
2 14
16 17
6 5
8
53 51
mined at different temperatures. It is interesting to note that an insignificant amount of bicyclo[4.2.0]oct7-ene ( 5 ) is formed at 25". Fonken and coworkers have reported that when cis,irans-1,3-cyclooctadieneis heated at temperatures of 80" or higher it is converted ~~ t o 5 in quantitative yield3; by a c ~ n r o t a t o r yelectro(33) R. S. Liu, J . Amer. Chewi. Soc., 89, 112 (1967). (34) A . C. Cope and C. L. Bumgardner, ibid., 78, 2812 (1956). (34a) NOTEADDEDIN PROOF. Dr. J. J. Bloomfield has informed us that he has also noted the formation of three dimers from the thermolysis of neat cis,rrans-cyclooctadiene. The structures of the dimers were not established but were suggested to be cyclobutane derivatives. See J. J. Bloomfield and J . S . McConaghy, Tetrahedron Lett., 3723 ( 1969). (35) I
