A STUDY OF THE VACUUM PYROLYSIS OF 6,6-DIHALOBICYCLO[3.1.0]HEXANES WITH HEI ULTRAVIOLET PHOTOELECTRON-SPECTROSCOPY

To establish the utility of using a high-power CW (continuous wave) CO2 laser as a directed heat source for preparing organic transients targeted for gas-phase ultraviolet photoelectron spectroscopic studies, we studied the vacuum pyrolysis of 6,6-dibromobicyclo[3.1.0]hexane (1a), 6,6-dichlorobicyclo[3.1.0]hexane (1b), and exo-6-bromo-endo-6-chlorobicyclo[3.1.0]hexane (1c). While it has already been established that 1a and 1b rearrange to the 2,3-dihalocyclohexenes 3a and 3b, respectively, when heated in the condensed phase, upon vacuum pyrolysis 1a and 1b readily eliminate HX - not the dihalogens - and give the 2-halo-1,3-cyclohexadienes 2a and 2b in high yield. That 2,3-dibromocyclohexene (3a) does not eliminate HBr at the laser power used to pyrolyze 1a and 1b and that bromochloro compound 1c selectively loses HCl to form 2a suggests that the 1,3-elimination of HX occurs in a concerted fashion without isomerization of the cyclopropane.