KINETICS OF ALKYLHALOCARBENE REARRANGEMENTS - MODULATION BY FLUORINE SUBSTITUENTS

Rate constants were measured by laser flash photolytic methods for hydrogen and carbon 1,2-migrations in four different alkylchlorocarbenes and in the analogous alkylfluorocarbenes. The carbenes, products, and rate constants (s-1) were as follows: phenoxymethylchlorocarbene to (Z)- and (E)-1-chloro-2-phenoxyethene (Z/E approximately 4/1), k = 3.6 x 10(7); phenoxymethylfluorocarbene to (Z)-1-fluoro-2-phenoxyethene (Z/E > 10/1), k = 1.3 x 10(7); neopentylchlorocarbene to (Z)- and (E)-1-tert-butyl-2-chloroethene (Z/E = 1/12), k = 1.4 x 10(7); neopentylfluorocarbene to (Z)- and (E)-1-tert-butyl-2-fluoroethene (Z/E = 1/2.5), k = 2.6 X 10(6); cyclobutylchlorocarbene to 1-chlorocyclopentene (C migration) and chloromethylenecyclobutane (H migration), k(c) = 4.6 X 10(7), k(H) = 2.1 X 10(7); cyclobutylfluorocarbene to 1-fluorocyclopentene (C migration) and fluoromethylenecyclobutane (H migration), k(c) = 1.8 x 10(6), k(H) = 5.3 x 10(5); cyclopropylchlorocarbene to 1-chlorocyclobutene, k = 9 x 10(5); cyclopropylfluorocarbene to 1-fluorocyclobutene, k = 1.4 x 10(5). Activation energies for several of these processes were in the range of 2-4 kcal/mol, with A approximately 10(8)-10(9) s-1.