Chemistry and kinetics of dipropylcarbene in solution

The photochemistry of 2-methoxy-2-methyl-5,5-dipropyl-Delta(3)-1,3,4-oxadia zoline (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 corresp onding azine. Fragmentation to form 4-heptanone is also observed. Yields of 4-diazoheptane in CH2Cl2 are much larger than those in pentane. 4-Diazohep tane 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 inefficie ntly photolyzed to dipropylcarbene (DPC), which can be trapped with piperid ine or with pyridine in laser flash photolysis experiments. Analysis of the piperidine and pyridine data indicates that the lifetime of DPC in cyclohe xane, methylene chloride, or Freon-113 (CF2ClCFCl2) solution at ambient tem perature 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, whi ch is about 20-fold shorter than that of dimethylcarbene in perfluorohexane at ambient temperature. Upon photolysis (254 nm) of oxadiazoline 1a in arg on, 4-diazoheptane and 1-methoxydiazoethane are formed. These diazo compoun ds undergo subsequent photolysis that revealed the formation of methoxy(met hyl)carbene and E- and Z-3-heptene. It was not possible to detect DPC in ar gon at 14 K.