Thermal Rearrangements of Spiro[2.4]hepta-1,4,6-trienes W. E. Billups,* ,† Rajesh K. Saini, † Vladislav A. Litosh, † Lawrence B. Alemany, † William K. Wilson, ‡ and Kenneth B. Wiberg* ,§ Department of Chemistry and Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005, and Department of Chemistry, Yale University, New Haven, Connecticut, 06520 billups@rice.edu Received December 13, 2001 Thermolysis of spiro[2.4]hepta-1,4,6-triene (1a) at 50 °C yielded bicyclo[3.2.0]hepta-1,3,6-triene (5), which dimerized in two different fashions to form cyclobutanes. The 1,2-dimethyl and 1-propyl derivatives of 1a also rearranged at 50 °C, but at a faster rate, each yielding a pair of cyclobutane dimers. The structures of these symmetrical dimers were investigated by 1D and 2D NMR and NOE difference spectroscopy. Ab initio calculations indicated that the two strained olefins 1a and 5 had comparable energies about 50 kcal/mol lower than norborna-1(7),2,5-triene, which was thus excluded as a reaction intermediate. The possibility that double bonds arranged perpen- dicularly in space might interact by conjugation has generated considerable interest in the synthesis of mol- ecules with geometry suitable for this interaction. The spiro[2.4]hepta-1,4,6-trienes are of interest in this regard since ground-state stabilization through spiroconjugation has been predicted on the basis of theoretical calcula- tions. 1-6 The high strain energy of the cyclopropene ring suggests that facile low-temperature rearrangements might be expected. In this paper, we show that spirenes 1a-c readily undergo [1,5]-sigmatropic reactions to yield bicyclo[3.2.0]hepta-1,3,6-trienes as reactive intermedi- ates. The synthesis of spiro[2.4]hepta-1,4,6-triene 1a is illustrated in Scheme 1. Photolysis of a solution of diazocyclopentadiene 7 in (2-bromovinyl)trimethylsilane yielded the desired precursor 2 as a mixture of trans and cis isomers. 8,9a A coproduct identified as 3 was isolated by preparative gas chromatography. The origin of 3, although formally the HBr addition product of the silane, has not been determined. Conversion to the spirene 1a was accomplished by treating 2 with CsF in dimethyl sulfoxide. 9 Low-temperature distillation provided 1a in 68% yield. Spirenes 1b and 1c were prepared by pho- tolysis of diazocyclopentadiene in the presence of the appropriate alkyne (Scheme 1). 8 When 1a or its alkylated derivatives 1b or 1c was heated to 50 °C, a pair of dimeric products was formed cleanly in high yield. In formulating a pathway leading from spirene 1a to the dimers, we considered the pathway shown in Scheme 2, a [1,5]-sigmatropic shift in which a cyclopropenyl carbon of 1a migrates to an adjacent carbon of the cyclopentadienyl ring, leading to bicyclo[3.2.0]- * To whom correspondence should be addressed. Fax: +1-713-348- 6355. † Department of Chemistry, Rice University. ‡ Department of Biochemistry and Cell Biology, Rice University. § Department of Chemistry, Yale University. (1) Goldstein, M. J.; Hoffmann, R. J. Am. Chem. Soc. 1971, 93, 6193. (2) Simmons, H. E.; Fukunaga, T. J. Am. Chem. Soc. 1967, 89, 5208. (3) Hoffmann, R.; Imamura, A.; Zeiss, G. D. J. Am. Chem. Soc. 1967, 89, 5215. (4) Hoffmann, R.; Imamura, A.; Hehre, W. J. J. Am. Chem. Soc. 1968, 90, 1499. (5) Tajiri, A.; Nakajima, T. Tetrahedron 1971, 27, 6089. (6) Reviews: (a) Du ¨ rr, H.; Gleiter, R. Angew. Chem., Int. Ed. Engl. 1978, 17, 559. (b) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1. (c) Gleiter, R. Angew. Chem., Int. Ed. Engl. 1974, 13, 696. See also: (d) Bischof, P.; Gleiter, R.; Du ¨ rr, H.; Ruge, B.; Herbst. P. Chem. Ber. 1976, 109, 1412. (e) Computational interpretation of the salient features of the PE spectrum of spiroheptatriene has also been reported. See: Galasso, V. J. Mol. Struct. (Theochem) 1992, 257, 181. (7) (a) Weil, T.; Cais, M. J. Org. Chem. 1963, 28, 2472. (b) Doering, W. v. E.; DePuy, C. H. J. Am. Chem. Soc. 1953, 75, 5955. (8) The procedure of Du ¨ rr et al. was followed: Du ¨ rr, H.; Ruge, B.; Schmitz, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 577. (9) Billups, W. E.; Luo, W.; Gutierrez, M. J. Am. Chem. Soc. 1994, 116, 6463. See also: (b) Mitsuhashi, T.; Jones, W. M. J. Chem. Soc., Chem. Commun. 1974, 103. (c) Amaro, A. H.; Grohmann, K. J. Am. Chem. Soc. 1975, 97, 5946. (d) Brown, W. T.; Jones, W. M. J. Org. Chem. 1979, 44, 3090. Scheme 1 Scheme 2 4436 J. Org. Chem. 2002, 67, 4436-4440 10.1021/jo011144e CCC: $22.00 © 2002 American Chemical Society Published on Web 05/29/2002