LASER FLASH-PHOTOLYSIS STUDIES OF OXYGEN AND SULFUR ATOM-TRANSFER REACTIONS FROM OXIRANES AND THIIRANES TO SINGLET CARBENES

Laser flash photolysis (UV-LFP) studies of benzylchlorodiazirine (la), phenylchlorodiazirine (Ib), phenylmethoxydiazirine (Ic), -dimethoxy-5 ,5-dimethyl-Delta(3)-1,3,4-oxadazoline (6a), 3,4-diaza-2,2-dimethoxy 1 -oxa[4.3]spirooct-3-ene (6b), iro[adamantane]-2,2'-[Delta(3)-1,3,4-oxa diazoline] (6c), and diazofluorene (DAF) in the presence of oxiranes a nd thiiranes are reported. These compounds, upon irradiation, afford b enzylchlorocarbene (2a), phenylchlorocarbene (2b), methoxyphenylcarben e (2c), dimethylcarbene (8a), cyclobutylidene (8b), adamantylidene (8c ), and fluorenylidene (FL), respectively. Absolute rate constants for the transfer of oxygen and sulfur atoms to these carbenes have been de termined in both acetonitrile and cyclohexane solvents. These carbenes abstract oxygen and sulfur atoms with bimolecular rate constants rang ing from 10(4) to 10(10) M-1 s(-1) at 22 degrees C. Ylides from attack of carbenes onto heteroatom donors were not observed for any of the h eteroatom transfer reactions. It was found that the magnitudes of the rate constants for heteroatom transfer are dependent on the philicity of the carbene intermediate, and trends in the kinetic data suggest th at oxygen and sulfur atom transfers occur by concerted mechanisms thro ugh ylide-like transition states. The reactions of dimethoxycarbene (1 4) with cyclohexene oxide and propylene sulfide have been studied by t he thermolysis of 6a, in benzene, at 110 degrees C. It was found that dimethoxycarbene also abstracts oxygen and sulfur atoms, albeit in low yields. It is concluded that, for singlet carbenes, carbene electroph ilicity is important in these heteroatom transfer processes.