Transition structures and energetics for the Cope rearrangement of cis-1,2-divinylcyclobutane: an ab initio study

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Abstract

Transition structures and energetics for the Cope rearrangements of cis-1,2-divinylcyclobutane, obtained by RHF/6-31G* and MP2(full)/6-31G*//RHF/6-31G* calculations, are reported. Three transition structures (endo-boatlike, chairlike and exo-boatlike) have been located, giving rise to formation of cis,cis-, cis,trans- and trans,trans-1,5-cyclooctadienes, respectively. cis-1,2-divinylcyclobutane rearranges via an endo-boatlike transition structure and yields the corresponding cis,cis product with higher exothermicity. Formation of severely strained cis,trans- and trans,trans-1,5-cyclooctadienes from this rearrangement is also discussed.

Introduction

The thermal rearrangement of cis-1,2-divinylcyclobutane (1) to cis, cis-1,5-cyclooctadiene (3) was first reported by Vogel in 1958 (Scheme 1) [1], [2]. Subsequent extensive physical organic studies by Berson [3], [4], [5], [6], [7], [8], [9] and other investigators elucidated the important mechanistic features of the reaction [10], [11], [12], [13]. Driven by the release of ring strain, the Cope rearrangements of 1 often proceed at significantly lower temperatures than analogous reactions involving acyclic 1,5-dienes [14], [15]. It should be noted that charge-accelerated versions of these reactions proceed under unusually mild conditions [16], [17], [18]. It appears to be well accepted that the rearrangements of divinylcyclobutane 1 proceed in a concerted fashion via boat-like transition state 2, in which the vinyl groups lie over the four-membered rings to afford cyclooctadiene 3 (Scheme 1) [9]. The energy of activation Ea for the rearrangement of 1 to 3 has been reported to be about 24.0 kcal/mol [1], [10], [11], which is 4.0–5.0 kcal/mol higher than that for the rearrangement of cis-1,2-divinylcyclopropane to cis,cis-1,4-cycloheptadiene [19]. The application of the divinylcyclobutane rearrangement to the synthesis of functionalized cyclooctane derivatives began to receive serious attention in the early 1980's [2].

Although divinylcyclobutane rearrangement has been extensively used in organic synthesis, it has not been studied from a theoretical point of view. By elementary considerations, three transition states are possible for this rearrangement, which give rise to formation of cis,cis-, cis,trans- and trans,trans-1,5-cyclooctadienes. The formation of severely strained cis,trans- and trans,trans-1,5-cyclooctadienes from these rearrangements have not been examined in detail. The aim of this study is to contribute to a better understanding of transition structures and energetics of such processes [20], [21]. We report herein a detailed study of Cope rearrangement of cis-1,2-divinylcyclobutane (1) at the restricted Hartree–Fock [22] and second-order Møller–Plesset perturbation theory levels [22].

Section snippets

Computational methods

All ab initio molecular orbital calculations were performed at the restricted Hartree–Fock (RHF) level [22] by using gaussian 98 program package [23]. Initial geometries were prepared by using PC Spartan Pro program [24]. For HF calculations, Pople's 6-31G* split valence basis set was used [22]. It should be noted that geometries were optimized without constraint in the final run of the calculation. Vibrational frequencies were then computed to characterize each stationary structure as a

Results and discussion

The conformations of divinylcyclobutane 1 were first investigated. Depending upon the orientation of vinyl groups, three distinct conformations of 1 were located, as depicted in Scheme 2 and Fig. 1. These conformers easily convert into each other since the conformational energy barrier among them is less than 3.8 kcal/mol. It should be noted that no direct pathway between 1tt and 1cc could be located. In all conformers, the four-membered ring is slightly folded, which gives partial relief from

Conclusion

We investigated the Cope rearrangement of cis-1,2-divinylcyclobutane at the RHF/6-31G* and MP2(full)/6-31G*//RHF/6-31G* levels. We located three transition structures, i.e. endo-boatlike, chairlike and exo-boatlike transition structures, to afford cis,cis-, cis,trans- and trans,trans-1,5-cyclooctadiene. RHF/6-31G* calculations overestimated the activation barriers for the rearrangement of cis-1,2-divinylcyclobutane but MP2 calculations predicted activation energies which are closer to the

Acknowledgements

We thank the Scientific and Technical Research Council of Turkey (TBAG-2250), the State Planning Organization of Turkey (DPT-2000K120390), and the Research Board of Middle East Technical University (BAP-2002-01-03-06) for support of this research.

References (36)

  • J.J. Bronson et al.
  • G. Sartori et al.

    Tetrahedron Lett.

    (1966)
  • M. Kahn

    Tetrahedron Lett.

    (1980)
  • E. Piers
  • V. Barone et al.

    Chem. Phys. Lett.

    (1996)
  • E. Vogel et al.

    Ann. Chem.

    (1958)
  • J.A. Berson et al.

    J. Am. Chem. Soc.

    (1972)
  • J.A. Berson et al.

    J. Am. Chem. Soc.

    (1972)
  • J.A. Berson et al.

    J. Am. Chem. Soc.

    (1972)
  • J.A. Berson et al.

    J. Am. Chem. Soc.

    (1973)
  • J.A. Berson et al.

    J. Am. Chem. Soc.

    (1973)
  • J.A. Berson et al.

    J. Am. Chem. Soc.

    (1975)
  • J.A. Berson et al.

    J. Am. Chem. Soc.

    (1976)
  • G.S. Hammond et al.

    J. Am. Chem. Soc.

    (1964)
  • D.J. Trecker et al.

    J. Am. Chem. Soc.

    (1964)
  • C.A. Stewart

    J. Am. Chem. Soc.

    (1971)
  • S.J. Rhoads
  • W.E. Doering et al.

    Angew. Chem. Int. Ed. Engl.

    (1963)

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