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-((pentadeuteriophenyl)methyl)diazirine has been investigated by means of IR and UV-vis spectroscopy in argon and xenon matrices. Parent benzylchlorocarbene was produced in low yields and could be detected by UV-vis spectroscopy only, while among the photoproducts of the diazirine bis-deuterated at the benzylic positions, (phenyldideuteriomethyl)chlorocarbene was readily detected by IR spectroscopy. It still is a minor product, though, in a mixture of (Z)-beta-chlorostyrene, (E)-beta-chlorostyrene, a distorted styrene, and traces of diazo compound. The benzylchlorocarbenes are very photolabile and also decay thermally at temperatures 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 important at low temperatures. After photolysis at 10 K of any of the benzylhalodiazirines, warming the matrix gives rise to a second thermal process which can also be followed by IR spectroscopy. This process is assigned to the relaxation of a beta-halostyrene which is produced from the diazirine in a distorted conformation. Its thermal relaxation, induced by the softening of the matrix at temperatures above 20 K (Ar) or 30 K (Xe), mainly produces (Z)-beta-chlorostyrene. The carbene rearrangement can be distinguished from the relaxation processes by kinetic analysis of the thermal reactions. For comparison, benzylbromodiazirine and methylchlorodiazirine have also been photolyzed in argon matrices. In these two cases, HX elimination is a side reaction.