Journal of Physical Chemistry p. 13413 - 13418 (1993)
Update date:2022-08-04
Topics: Chemical reactions Kinetics Data Analysis Product Formation Experimental Techniques Unsymmetrically substituted Reaction rate Competitive 1,2-Carbon Migrations Cyclopropylchlorocarbene Transition States
Moss, Robert A.
Liu, Weiguo
Krogh-Jespersen, Karsten
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.
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