Molybdenum catalysis

Trost and Hachiya [140] studied asymmetric molybdenum-catalyzed alkylations. Interestingly, they noticed that the regioselectivity of this transformation performed with a non-symmetric allylic substrate varied according to the nature of the metal Pd-catalyzed substitutions on aryl-substituted allyl systems led to attack at the less substituted carbon, whereas molybdenum catalysis afforded the more substituted product. They prepared the bis(pyridylamide) ligand 105 (Scheme 55) and synthesized the corresponding Mo-complex from (C2H5 - CN)3Mo(CO)3. With such a catalyst, the allylic... 

ADMET condensation of 17 is completed using molybdenum catalysis to give the unsaturated polymer 18, which is reduced to 19 using a variant of hydrazine reduction chemistry. Complete saturation of the polymer backbone has been demonstrated and is illustrated by the absence of olefin protons in the 13C NMR of 19a shown in Fig. 5. 

DR. TAUBE A problem with the molybdenum catalysis is that the species which reacts readily with CIO may be an unstable, possibly mononuclear, species rather than one of the condensed forms which we ordinarily encounter. Paffett and Anson [Paffett, M. T. Anson, F. C. Inorg. Chem. 1981 20, 3967] have shown that mononuclear Mo(V) reacts moderately rapidly with C10. My guess is that mononuclear Mo(IV) would be even more reactive. Mononuclear Mo(IV) is probably a typical yl ... 

The synthesis of quaternary amino acids 86 have been shown using azlac-tones 85 as nucleophiles and the Trost ligand 39 under palladium [179] or molybdenum catalysis (Scheme 8) [180]. The allylic alkylation of glycine imino esters under biphasic conditions has also been achieved using a chiral phase-transfer catalyst in combination with an achiral Pd catalyst producing the unnatural amino acid derivatives [181]. 

Ring closing metathesis is a fundamental part of the strategy for the synthesis of Brevetoxin subunits employed by Clark and Kettle <97TL123>. This approach, complementary to that of Nicolaou and co-workers <96JA1565>, involves cyclisation of 26 to 27 by molybdenum catalysis. Yields of over 90% have been achieved The approach may also useful in the preparation of the 6,6-cyclic ethers, but not for 6,8-cyclic ethers of the brevetoxins. An alternative system for the latter has been proposed <97TL127>. 

On the other hand, molybdenum catalysis results in predominant attack at the more-hindered position when non-bulky nucleophiles are employed (Schemes 9.22 and 9.23). While the use of the palladium-phosphine system gives the product under steric control, with the molybdenum carbonyl system, attack is now under charge control. The carbon better able to support positive charge, i.e. the more substituted carbon, is the site of attack. More-hindered nucleophiles, however, again favour attack at the less hindered terminus, ... 

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

J. Haber, The Role of Molybdenum in Catalysis, Climax Molybdenum Co. Ltd., London, 1981. 

Cobalt in Catalysis. Over 40% of the cobalt in nonmetaUic appHcations is used in catalysis. About 80% of those catalysts are employed in three areas (/) hydrotreating/desulfurization in combination with molybdenum for the oil and gas industry (see Sulfurremoval and recovery) (2) homogeneous catalysts used in the production of terphthaUc acid or dimethylterphthalate (see Phthalic acid and otherbenzene polycarboxylic acids) and (i) the high pressure oxo process for the production of aldehydes (qv) and alcohols (see Alcohols, higher aliphatic Alcohols, polyhydric). There are also several smaller scale uses of cobalt as oxidation and polymerization catalysts (44—46). 

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