MATRIX-ISOLATION STUDY INTO THE MECHANISM OF PHOTOINDUCED CYCLIZATIONREACTIONS OF CHROMIUM CARBENES

The complex (CO)(5)Cr[C(OMe)(Me)] is known to react photochemically wi th N-benzylidenemethylamine (CH3NC(Ph)H) to form a beta-lactam (1,3-di methyl-3-methoxy-4-phenyl-2-azetidinone), whereas our study slows that (CO)(5)W[C(OMe)(Me)] does not. Irradiation of the complexes in argon and nitrogen matrices (lambda > 390 nm) resulted in a geometrical isom erization and the trapping of the syn isomers. There was no evidence f rom the matrix isolation experiments for the formation of a metal-kete ne complex, a likely intermediate in the solution photochemistry of (C O)(5)Cr[C(OMe)(Me)] with respect to beta-lactam formation. Three diffe rences were observed in the matrix and solution photochemistry of (CO) (5)Cr-[C(OMe)(Me)] and (CO)(5)W[C(OMe)(Me)]. (i) (CO)(5)Cr[C(OMe)(Me)] undergoes CO loss upon irradiation with UV light more readily than it s tungsten analogue. (ii) Upon UV irradiation of (CO)(5)Cr[(C(OMe)(Me] in a nitrogen matrix bands were observed in the IR spectrum which ind icated that two (CO)(4)(N-2)Cr[C(OMe)(Me)] isomers were formed, wherea s irradiation of (CO)(5)W[C(OMe)(Me)] under the same conditions produc ed only one nitrogen adduct, (iii) Irradiation of (CO)(5)Cr[C(OMe)(Me) ] in solution and in a solid CO matrix showed that this complex underw ent lass of the carbene ligand more readily than the tungsten analogue , These findings are consistent with the proposal that, upon irradiati on of chromium carbenes, a ketene transient could be formed by a cleav age of the chromium-carbene sigma bond and intramolecular nucleophilic attack by the carbene on a metal carbonyl. The photochemistry of the complex (CO)(5)Cr(OMe)(biphenyl) was studied in both argon and nitroge n matrices, This complex underwent CO loss very readily, and it is lik ely that this step is involved in at least one pathway of its solution photochemistry.