Experiments towards the synthesis of bicyclo (3.1.0) hex-1,5-ene (I) and bicyclo (4.1.0) hept-1,6-ene (II) were carried out in the solution and gas phases. These and smaller members of the class of 1,2-bridged cyclopropenes were also studied using theoretical techniques.

The synthetic approaches to I and II utilized the corresponding bridgehead dibromides and diodides. The previously unknown diiodides were synthesized in modest yield (30-50%) from the diacids using known techniques.

The reaction of 1,6-diiodobicyclo(4.1.0) heptane (III) with methyllithium in ether at -105$\sp\circ$C, followed by quenching of the methyllithium and addition of 1,3-diphenylisobenzofuran produced the Diels-Alder adduct of II, providing the first indication that II is stable in solution. Similar conditions did not produce the adduct of I from 1,5-diiodobicyclo(3.1.0) hexane (IV). The reaction of III and IV with methyllithium at -78$\sp\circ$C in the presence of 1,3-diphenylisobenzofuran gave adducts of I and II. In the absence of the diene, the reaction of III with methyllithium gave a tetrameric product in 51.2% yield, while the reaction of IV produced a complex mixture.

The gas phase reaction of dihalide precursors to I and II with potassium atoms or solid-supported methyllithium produced the rearrangement products 3-methylenecyclopentene and 3-methylenecyclohexene, respectively, suggesting the intermediacy of the cyclopropenes. The potassium reactions also yielded substantial quantities of the parent alkanes of I and II.

The theoretical work concerning I, II and smaller homologs provided structures, frequencies and electron density distributions for these compounds. At the 6-31G* level of theory, II was determined to be planar at the double bond, while I and smaller homologs are non-planar. The C = C stretch in the infrared moves dramatically to lower wavenumbers as the bridging-ring size decreases. Calculated atomic populations (via the techniques of Bader) and molecular orbitals were also obtained. These two formalisms lead to a surprisingly similar view of the changes in bonding on bridging-ring contraction and olefin pyramidalization.