Regio‐ and Stereoselectivity of Ene Reactions: The Dimerization of Bicyclic 1,3‐Fused 2‐(Trimethylsilyl)cycloprop‐1‐enes
Cycloalkenes proceed through bromination, dehydrobromination and dibromocarbene addition reactions to give tribromocyclopropanes 10 and 11. The treatment of tribromocyclopropanes 10 and 11 with 3 equiv. of methyllithium in diethyl ether at –78 °C followed by treatment with trimethylsilyl chloride produce 8-(trimethylsilyl)bicyclo[5.1.0]oct-1(8)-ene (4) and 9-(trimethylsilyl)bicyclo[6.1.0]non-1(9)-ene (5). Both 4 and 5 undergo ene dimerization via the same steric isomer and an endo transition state to generate the stable adducts 6 and 7, respectively, as the sole isomers. Compound 6, containing an unstable bicyclo[5.1.0]oct-1(8)-enyl group and produced in high yields, has been reported to be an unstable species. Both of the ene dimers, (trimethylsilyl)cyclopropenes 6 and 7, can be converted into cyclopropenes 8 and 9 by treatment with a fluoride salt followed by protonation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)