Matrix isolation and photochemistry of 1- and 2-naphthylchlorocarbene

Abstract

We have for the first time characterized the 1- and 2-naphthylchlorocarbenes by IR and UV/vis spectroscopy in N₂ matrices at 10 K. Although evidence suggests the presence of predominantly only one geometric isomer in the case of the 1-naphthylchlorocarbene, the IR and UV/vis spectra of the 2-naphthylchlorocarbene indicate two distinct conformations. With selective irradiation, the longer-wavelength absorbing s-Z-isomer of 2-naphthylchlorocarbene can be photochemically driven to the alternate s-E conformation. Irradiation of the 1- and 2-naphthylchlorocarbenes induces cyclization to 7-chloro-4,5-benzobicyclo[4.1.0]hepta-2,4,6-triene and 7-chloro-2,3-benzobicyclo[4.1.0]hepta-2,4,6-triene, respectively. The cyclization can be reversed by irradiation at shorter wavelengths.

Our results on the singlet naphthylchlorocarbenes photochemical ring-closures parallel observations of Chapman, McMahon, and co-workers on the parent triplet 1- and 2-naphthylcarbenes (P. R. West, A. M. Mooring, R. J. McMahon and O. L. Chapman, J. Org. Chem., 1986, 51, 1316; S. W. Albrecht and R. J. McMahon, J. Am. Chem. Soc., 1993, 115, 855). In particular, under matrix isolation conditions, the naphthylcarbenes appear to photochemically only cyclize to the corresponding cyclopropenes, which in turn only photochemically ring-open back to the carbenes. Ring-expansion in these systems to benzocycloheptatetraenes is not detected.