Molybdenum oxide, stereochemistry

Table 23.2 Oxidation states and stereochemistries of compounds of chromium, molybdenum and tungsten...

Molybdenum and tungsten are similar chemically, although there are differences which it is difficult to explain. There is much less similarity in comparisons with chromium. In addition to the variety of oxidation states there is a wide range of stereochemistries, and the chemistry is amongst the most complex of the transition elements. 

Table 18-C-i Oxidation States and Stereochemistry of Molybdenum and Tungsten... 

The anti addition of amines to the double bond of cationic (alkene)(cyclopentadienyl)di-carbonyliron complexes and to the analogous molybdenum and tungsten complexes has been reported31 33. The adducts underwent carbonyl insertion-cyclization to give chelate complexes, which were then oxidized to /8-lactams. For example, from the Fp complexes of ( )- and (Z)-2-butene the corresponding /8-lactams were obtained diastereoselectively in 10-15% yield by the direct oxidation of the benzylamine adducts with chlorine at low temperature33. The stereochemistry was determined by H-NMR spectroscopy. 

In summary, a 6-substituted pterin was first identified as a structural component of the molybdenum cofactor from sulfite oxidase, xanthine oxidase and nitrate reductase in 1980 (24). Subsequent studies provided good evidence that these enzymes possessed the same unstable molyb-dopterin (1), and it seemed likely that 1 was a constituent of all of the enzymes of Table I. It now appears that there is a family of closely related 6-substituted pterins that may differ in the oxidation state of the pterin ring, the stereochemistry of the dihydropterin ring, the tautomeric form of the side chain, and the presence and nature of a dinucleotide in the side chain. In some ways the variations that are being discovered for the pterin units of molybdenum enzymes are beginning to parallel the known complexity of naturally occurring porphyrins, which may have several possible side chains, various isomers of such side chains, and a partially reduced porphyrin skeleton (46). 

Poraij-Koshits MA, Atovmyan LO (1974) Crystal chemistry and stereochemistry of oxide compounds of molybdenum. Nauka, Moscow (in Russian)... 

Electrophilic addition of aldehydes to [CpMo(CO)2(Tl -6-R-cyclohexadien-l-yl)] (R = H, Me, Ph) catalysed by boron trifluoride was reported to afford isolable (Tj -diene) cationic salts of molybdenum. Demetallation by trimethylamine-N-oxide afforded functionalised cyclohexadiene compounds in good yields. The stereochemistry of the carbon-carbon bond forming reaction was clarified. 

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