Laser flash photolysis alkene cyclopropanation

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

Ab initio and RRKM calculations indicate that the reactions of C, CH, and (H2C ) with acetylene occur with no barrier." Laser flash photolysis of the cyclopropanes (69) and (70) was used to generate the corresponding dihalocarbenes. The absolute rate constant for the formation of a pyridine ylide from Br2C was (4-11) x 10 lmoP s. The rates of additions of these carbenes to alkenes were measured by competition with pyridine ylide formation and the reactivity of BrClC was found to resemble that of Br2C rather than CI2C . 

Insight into the nature of the intermediate in the cyclopropanation of electron-rich and electron-poor alkenes was given by laser flash photolysis (LFP) studies of phenyl-bromodiazirine in the presence of various amounts of tetrabutylammonium bromide.23 Electron-rich alkenes react exclusively with the carbene (11) leading to (12). Electron-poor alkenes yield cyclopropanes (13) only slowly with the carbene (11) and more rapidly with the carbanion (14) arising from the addition of the bromide to the carbene. 

Laser flash photolysis has, as usual, illuminated the problem. Jones and Rettig photodecomposed 9-diazofluorene (188) in hexafluorobenzene and cw-4-methyl-2-pentene mixtures, and showed that the degree of stereoselectivity in the cyclopropane products depended on the concentration of the alkene. Laser flash photolysis showed that the first detectable intermediate in the photolysis reaction is the triplet carbene, and suggests that the product studies are consistent with initial formation of a singlet fluorenylidene which has an extremely short life (less than 5 ns) before forming the triplet. The singlet can be trapped only by high alkene concentrations while the more stable triplet is easily trapped. 

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