Molybdenum complexes reduction

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

The carbanions take up 02 and these take up protons to give hydroperoxides in good yields. But because they are explosive in nature, they are not usually isolated and on reduction with sodium sulphite on trialkyl phosphite give alcohols. Alcohols can also be prepared via hydroperoxy molybdenum complexes and alkyl boranes. These reactions are summarized as follows ... 

For both complexes, reversible metal-centered one-electron oxidations and reductions have been observed. The products of such redox processes have been examined by monitoring the EPR and electronic spectra obtained by controlled-potential electrolysis and, in the case of the molybdenum complex, have been identified as [Mo(IV)0(TMTEC)]+ and [Mo(VI)0(TMTEC)] ions. 

On treatment with CsF, 74j yields the salt 75a 119). The same anion together with a cation containing a dioxaphospholane ring 75b has been synthesized from thermolysis of the aminoiminodiphosphorane 74n 229). Compound 74j forms a stable donor-acceptor complex with trimethylphosphane 119). Reduction of 74j with trimethylsilyldiphenylphosphane affords the 2 -dioxaphospholane 73a, which serves as a phosphane ligand in the molybdenum complex 76 229). The phosphane ligands are in cis-positions. 

Hydrazido and organohydrazido ligands are important intermediates in the reduction or utihzation of N2 in biological and chemical systems, and their molybdenum complexes are potential catalysts for aUcene polymerization and metathesis reactions. 

Simple chiral phosphines have already been mentioned (Section 3.1.3) and the macrocycle enantiomers are discussed below (Section 4.6). Current research in this area is concentrated on bidentate chiral phosphines, such as the ligands (24)-(27). Although their transition metal complexes are normally used for stereospecific synthesis, Whitmire and coworkers used the molybdenum complexes to resolve their racemic bisphosphines via flash chromatography. The phosphines were decomplexed by reductive cleavage at low temperatures (-78 °C) using sodium naphthalenide (Scheme 1). 

With the similar molybdenum complex the yield of ammonia reaches only ca 0.7 mol per metal atom, the remainder being evolved as free dinitrogen, with molybdenum being oxidized to molybdenum(III). Apparently, however, no such complexes have been observed as intermediates in dinitrogen reduction in protic media. 

The yields were found also to increase in the presence of phosphines, particularly trimethyl or tributyl phosphine. After all the improvements of the catalyst and reaction conditions the system became by far the most active of known non-biological catalytic systems for the reduction of dinitrogen at ambient temperature and pressure. The specific activity (the rate of N2 reduction per mole of the complex) reached and even exceeded that of nitrogenase. Up to 1000 turnovers relative to the molybdenum complex can be observed at atmospheric pressure and more than 10 000 turnovers at elevated N2 pressures. 

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