Kinetics of the Competitive 1,2-Carbon Migrations of an Unsymmetrically Substituted Cyclopropylchlorocarbene

Article abstract of DOI:10.1021/j100152a056

Photolysis of 3-(2,2-dimethylcyclopropyl)-3-chlorodiazirine (4) at 25 deg C in pentane affords 81 percent of the 1,2-C migration products 1-chloro-3,3-dimethylcyclobutene (5) and 2-chloro-3,3-dimethylcyclobutene in a 4.8:1 distribution, as well as 19 percent of the fragmentation products, isobutene, and chloroacetylene.Experiments in the presence of the carbene trap trimethylethylene point to product formation from the electronically excited diazirine (4^*), which affords 19-20 percent of fragmentation products, ca. 17 percent of 5 and 6 (3.8:1), and ca. 63 percent of 2,2-dimethylcyclopropylchlorocarbene (3).The carbene ring expands to 5 and 6 (5.2:1).Laser flash photolytic studies afford absolute rate constants for 3 -> 5 (1.3 * 10⁶ s^-1) and 3 -> 6 (2.5 * 10⁵ s^-1).The 5-fold preference of 3 for CH2 migration (to 5) over CMe2 migration (to 6) is attributed to differential steric affects.Ab initio calculations afford structures and energies for 3 and the CH2 and CMe2 transition state (8-CH2 and 8-CMe2).The calculations agree with the experimental findings in that ΔG^* for CH2 migration is found to be 1.4 kcal/mol less than ΔG^* for CMe2 migration, corresponding to a computed 10-fold kinetic preference for CH2 migration.The expected additional steric congestion is apparent in the 8-CMe2 transition state.