Molybdenum complexes reaction with phenols

Molybdenum is essential to the formation and activity of assimila-tory nitrate reductases. Cells must assimilate molybdate from the environment, metabolize molybdenum in some manner to form active molybdenum cofactor, and then incorporate it into a large molecular weight protein so that it can perform a reversible redox reaction with nitrate. Investigations on the aqueous Mo (III) model systems for nitrate reduction and the coordination of molybdate by naturally produced phenolates will hopefully lead to an understanding of the complex process of molybdenum acquisitions by and molybdenum function in nitrate reductases. 

The gas-phase equilibrium between 2-hydroxypyridine and 2-pyridone favours the hydroxy-form, but in the equilibrium between 2-hydroxypyridine iV-oxide and N-hydroxy-2-pyridone, the major tautomer is the hydroxy-pyridone. Bicyclic adducts between 2-pyridones and dimethyl acetylene-dicarboxylate, unobtainable at atmospheric pressure, have been obtained at 10—15 kbar. A novel route to iV-hydroxy-2-pyridone involves the trimethyl-silylation of 2-pyridone followed by oxidation of the resulting 2-(trimethyl-silyloxy)pyridine with the DMF complex of molybdenum pentoxide. p-Nitro-phenols (45) and nitro-acetamides (46) are formed from the reaction of 3,5-dinitro-2-pyridones (43) with the sodium salts of /3-keto-esters (44) (Scheme 20). ... 

In these reactions the tested organic substance and an inorganic salt give a more or less stable strong color due to a complex salt. In Part 2 of this monograph a series of such reactions is described, such as, for example, the reaction of hydroxamic acids with ferric salts (p. 276), the reaction of phenols with ferric chloride (p. 188), the reaction of molybdenum with o-dihydro-xybenzenes (p. 191), of diacetyl dioxime with nickel salts (p. 227), of phenols with Millon s reagent (p. 196), alcohols with ceric ammonium nitrate (p. 170), alcohols with vanadium-hydroxy quinoline complex (p. 171), and the reaction of cis-enols of )5-dicarbonylic compounds and a-dicarbonyl compounds with ferric chloride, (p. 294.)... 

The first examples of catalytic RCAM for the synthesis of functionalized macrocycles were reported by Ftirstner and Seidel [7]. Two catalyst systems were employed in this study the instant system prepared by reaction of catalytic amounts of Mo(CO)6 1 with stoichiometric quantities of phenols, and the well-defined tungsten-alkylidyne 2 first prepared by Schrock in 1981 [5, 8]. Subsequent applications of RCAM, particularly in natural product synthesis, have either utilized these two systems or a molybdenum system developed by Fiirstner that employs in situ reaction of trisamido molybdenum complex 3 [9] and CH2CI2 (Figure 7.1) [10]. 

For a number of years, phenolic substances were dosed by colorimetric techniques, based on redox reactions usually known as Folin Ciocalteau methods, even if a number of adjustments were developed to fit different matrix characteristics. The Folin Cioalteau reagent is a mixture of phosphomolybdic and phosphotingstic acids, with molybdenum in the 6+ oxidation state and, when the reaction takes place, it is reduced to form a complex called molybdenum blue and tungsten blue. In this complex, the mean oxidation state is between 5 and 6 and the formed complex is blue so it can be read spectrophotometrically at 750 nm. 

Peroxide complexes of molybdenum(VI) are intermediates in industrially important epoxidation reactions. The so-called Mimoun-type complexes [Mo 0(02)2LaxLeq]° " (12) generally exhibit pentagonal-bipyramidal structures, with an axial 0x0 group trans to Lax and Leq, and two peroxo groups in the equatorial plane. These complexes are effective reagents for the selective oxidation of alcohols to aldehydes, amines to nitroso compounds, sulfldes to sulfoxides and then to sulfones, phenols to o-quinones, and in the sulfoxidation of thianthrene-5-oxide. 

Reaction of a molybdenum(O) complex Mo(PMe3)6 with an excess amount of phenol gives tetraphenoxomolybdenum(IV) complex accompanied by evolution of molecular hydrogen (Eq. 3.59) [196]. 

Monomeric molybdenum(v) complexes of the type [M0OCI3L] (L = bipy or phen), [M0OCIL2] (L = 8-hydroxy- or 8-mercapto-quinoline), and [MoOCl-(MeOH)L] [L = 0-(salicyIidenimino)phenol] reduce NO3" in a one-electron step (in DMF) producing NO 2 and the corresponding Mo complex. The kinetics are complex with some deviations from linearity under first-order conditions. In the presence of added Cl" and excess NO3", however, linearity to 87 % reaction is observed. The rate law is of the form... 

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