PHOTOLYSIS OF ALKYLHALODIAZIRINES AND DIRECT OBSERVATION OF BENZYLCHLOROCARBENE IN CRYOGENIC MATRICES

Photolysis of 3-benzyl-3-chlorodiazirine and its deuterated homologues 3-chloro-3-(phenyldideuteriomethyl)diazirine and 3-chloro-3-((pentade uteriophenyl)methyl)diazirine has been investigated by means of IR and UV-vis spectroscopy in argon and xenon matrices. Parent benzylchloroc arbene was produced in low yields and could be detected by UV-vis spec troscopy only, while among the photoproducts of the diazirine bis-deut erated at the benzylic positions, (phenyldideuteriomethyl)chlorocarben e was readily detected by IR spectroscopy. It still is a minor product , though, in a mixture of (Z)-beta-chlorostyrene, (E)-beta-chlorostyre ne, a distorted styrene, and traces of diazo compound. The benzylchlor ocarbenes are very photolabile and also decay thermally at temperature s as low as 10 K. The rearrangement of benzylchlorocarbene-d2 has been monitored using IR spectroscopy. The small temperature dependence of the rate and the curved Arrhenius plot indicate that tunneling is impo rtant at low temperatures. After photolysis at 10 K of any of the benz ylhalodiazirines, warming the matrix gives rise to a second thermal pr ocess which can also be followed by IR spectroscopy. This process is a ssigned to the relaxation of a beta-halostyrene which is produced from the diazirine in a distorted conformation. Its thermal relaxation, in duced by the softening of the matrix at temperatures above 20 K (Ar) o r 30 K (Xe), mainly produces (Z)-beta-chlorostyrene. The carbene rearr angement can be distinguished from the relaxation processes by kinetic analysis of the thermal reactions. For comparison, benzylbromodiaziri ne and methylchlorodiazirine have also been photolyzed in argon matric es. In these two cases, HX elimination is a side reaction.