Exploratory photochemistry of alkylbromo‐ and alkylfluorodiazirines. Excited‐state hydrogen migration and carbene formation

Laser flash photolysis of a series of alklylbromo‐ and alkylfluorodiazirines in pentane at ambient temperature generates alkylhalocarbenes by decomposition of the diazirine excited states. The halocarbenes can be intercepted with pyridine to form ylides. The ylides absorb intensely between 350 and 400 nm and are fairly long lived (τ » 10 μs), making them convenient probes of the yield and dynamics of the carbene. The yield of the ylides increases with increasing pyridine concentration up to 1·5 M . At pyridine concentrations > 1·5 M the yield of ylide is saturated, signifying that every carbene generated in a laser pulse is captured by pyridine prior to reaction with solvent or intramolecular rearrangement. The yield of trappable carbene generated from alkylbromodiazirines closely tracks the bond dissociation energy of the CH bond adjacent to the diazirine moiety. The data indicate that the excited states of the alkylbromodiazirines suffer CH migration (or CC migration with cyclobutylbromodiazirine) and nitrogen extrusion in competition with carbene formation. The yield of trappable carbene derived from the alkylfluorodiazirines is independent of the bond dissociation energy of the adjacent CH bond. This is probably a consequence of the great thermodynamic stability of α‐fluorocarbenes.