Chemistry and kinetics of dipropylcarbene in solution

The photochemistry of 2-methoxy-2-methyl-5,5-dipropyl-Delta(3)-1,3,4-oxadiazoline (1a) and 2,2-dimethoxy-5,5-dipropyl-Delta(3)-1,3,4-oxadiazoline (1b) was investigated. Photolysis (300 nm) of these compounds in solution leads to fragmentation to 4-diazoheptane (major), which slowly forms the corresponding azine. Fragmentation to form 4-heptanone is also observed. Yields of 4-diazoheptane in CH2Cl2 are much larger than those in pentane. 4-Diazoheptane can be trapped with 1-pentene to form a pyrazoline or with methanol to form 4-methoxyheptane. The pyrazoline can be decomposed photochemically to form 1,1,2-tripropylcyclopropane. In solution, 4-diazoheptane is inefficiently photolyzed to dipropylcarbene (DPC), which can be trapped with piperidine or with pyridine in laser flash photolysis experiments. Analysis of the piperidine and pyridine data indicates that the lifetime of DPC in cyclohexane, methylene chloride, or Freon-113 (CF2ClCFCl2) solution at ambient temperature is controlled by 1,2 hydrogen migration to form Z- and E-3-heptene. The lifetime deduced under these conditions is approximate to 300 ps, which is about 20-fold shorter than that of dimethylcarbene in perfluorohexane at ambient temperature. Upon photolysis (254 nm) of oxadiazoline 1a in argon, 4-diazoheptane and 1-methoxydiazoethane are formed. These diazo compounds undergo subsequent photolysis that revealed the formation of methoxy(methyl)carbene and E- and Z-3-heptene. It was not possible to detect DPC in argon at 14 K.