Photochemical reactions of transition metal organyl complexes with olefins XIV. Molecular structure of an intermediate of the light-induced [6+4]-cycloadditions of dienes to tricarbonyl( η6-1,3,5-cycloheptatriene)chromium (0)

Abstract

Tricarbonyl( η 6-1,3,5-cycloheptatriene)chromium ( 1) yields upon UV irradiation in tetrahydrofuran at 203 K solvent stabilised dicarbonyl( η 6-1,3,5-cycloheptatriene)(tetrahydrofuran)chromium ( 2). Complex 2 reacts with 2,3-dimethylbutadiene ( 3) in a substitution reaction with successive CC bond formation to give the 1:1 adduct dicarbonyl( η 5:3-6-(2′-3′-dimethyl-3′butene-1′,2′-diyl )-cylphepta-2,4-dien-1-yl)chromium ( 4). Treatment of 4 with carbon monoxide at ambient temperature causes the formation of a second CC bond and yields tricarbonyl( η 4:2-8,9-dimethylbicyclo[4.4.1]undeca-2,4,8-triene)chromium ( 5), the formal [6+4]-cycloaddition product of 1 and 3. The thermolabile complex 2 was studied by IR; complexes 4 and 5 were characterised by IR and NMR spectroscopy. A formation mechanism for 4 and 5 is proposed. Complex 4 crystallises in the orthorhombic space group Pna2 t, a = 1416.2(2), b = 788.5(3), c = 116.2(2) pm, V = 1.2464 nm 3, Z = 4. The crystal and molecular structure of 4 was determined by X-ray structure analysis.