Transition metal promoted oxidation and reduction reactions
Two areas of organotransition metal chemistry and their potential application to organic transformations are discussed. The synthesis of cations of the type [Fe(η5-C5H5)(L)2(CO)]+, [Fe(η5-C5H4CH3)(L)2(CO)]+, [Fe(η5-C5(CH3)5)(L)2(CO)]+ (where (L)2=(CO)2, (PPh3)(CO), (PMe3)(CO), (PPh3)2,(diphos) and (PMe3)2) and [Mo(η5-C5H5)(L)3(CO)] (where (L)3=(PPh3)(CO)2, (diphos)(CO) and (triphos)), many of them novel, is described. Investigations into the site of nucleophilic attack on the cations using hydride as a probe and the effect of varying the overall charge distribution of the cation are discussed. Hydride attack on a carbonyl ligand leads to the formation of metal formyl moieties and their detection by low temperature 1H n.m.r. spectroscopy is described; furthermore, the fate of the metal formyls was found to be dependent upon the nature of the other ligands in the complex. A new criterion for establishing the stereoselectivity of nucleophilic attack on η5-C5H5 ligands is proposed. Hydride attack on an η5-C5H4CH3 ligand was discovered to be regioselective occurring at the carbon atom alpha to the methyl-bearing carbon. The direct oxidation of alkenes to epoxides by hydrogen peroxide was shown to be catalysed by some of the metal carbonyl cations. The use of organotitanium reagents to convert vic-dibromides and epoxides to alkenes is discussed. (C5H5)2TiCl2 was shown to catalyse both a sodium amalgam debromination of disubstituted vic-dibromides and a zinc debromination of mono and disubstituted vic-dibromides. The latter reaction was developed into a synthetically useful procedure. Reduction of (C5H5)2TiCl2 by both sodium amalgam and zinc dust gave reagents which deoxygenated epoxides. Investigations into the regioselectivity and chemoselectivity of these reagents are discussed. During the synthesis of molecules containing both vic-dibromide and epoxide functionalities, a novel cyclisation was discovered which may have relevance to the biosynthesis of certain marine natural products.