Cycloaddition Reactions of (η6-Tropone)tricarbonylmetal(0) Complexes of Group 6 Metals with Alkynes

Abstract

(η6-Cyclohepta-2,4,6-trien-1-one)tricarbonylmetal(0) complexes of chromium (2a), molybdenum (2b), and tungsten (2c) are formed when tris(acetonitrile)tricarbonylchromium(0), -molybdenum(0), and -tungsten(0), respectively, are treated photochemically with tropone (cyclohepta-2,4,6-trien-1-one) 1. UV photolysis of 2a-2c and RC≡CR' (R = R' = Ph, Me3Si, Et; R = Ph, R' = Me3Si) in toluene gives the [6+2] cycloadducts, (η4,η2-8,9-diphenylbicyclo[4.2.1]nona-3,5,8-trien-1-one)tricarbonylmetal(0) 3a-3c, (η4,η2-8,9-bis(trimethylsilyl)bicyclo[4.2.1]nona-3,5,8-trien-1-one)tricarbonylmetal(0) 4a-4c, (η4,η2-8,9-diethylbicyclo[4.2.1]nona-3,5,8-trien-1-one)tricarbonylmetal(0) 5a-5c, and (η4,η2-8-phenyl-9-(trimethylsilyl)bicyclo[4.2.1]nona-3,5,8-trien-1-one)tricarbonylmetal(0) 6a-6c. Compounds 2a-2c, 3a-3c, 4a-4c, 5a-5c, and 6a-6c were purified by chromatography, recrystallized and isolated as analytically pure crystalline solids in moderate yields and characterized by mass, IR, and NMR spectroscopy. Pure organic heterobicyclotriene ligands 7a, 8a, 9a, and 10a were isolated from 3a, 4a, 5a, and 6a, respectively, by treatment with cerium(IV) ammonium nitrate (R = R' = Me3Si; R = Ph, R' = Me3Si) or by heating in toluene (R = R' = Ph, Et). It was not possible to isolate the bicyclic complexes resulting from the reactions of 2a-2c with terminal alkynes like phenylacetylene.