Molybdenum , reaction + radicals

Recently, Stair and coworkers [10, 11] developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces [12] and on molybdenum oxide thin films [13]. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor locat inside an ulttahigh vacuum chamber. This method avoids the complications of co-adsorbcd halide atoms, it allows higher covraages to be reached, and it allows tiie study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

The condensation step is catalyzed by the glycosyltransferase SpcF. Further enzyme-catalyzed oxidation is probably needed for the introduction of the hemiketal linkage between cyclitol and sugar units. The SpcY enzyme, which has a similar counterpart, HygY, among the /lyg-cluster encoded proteins (see Section 2.2.4.3.1), is a candidate enzyme for this reaction. SpcY is a member of the radical SAM superfamily of proteins and relatives of SpcY have been found before all in connection with molybdenum-cofactor biosynthesis but to our knowledge no details of the mechanism involved is known for those. 

An inner-sphere hydrogen atom abstraction from the alcohol by a peroxo metal complex, thus forming a coordinated ketyl radical [(CH3)2—C —O—V(0)(00H)]" , has been proposed for the aerobic oxidation of alcohols catalyzed by peroxidic molybdenum and vanadium derivatives (Scheme 16). While in the case of Mo-catalyzed reaction the H2O2 produced is quantitatively converted to products (ketone and H2O), in the vanadium mediated process, hydrogen peroxide accumulates . In this latter case, the direct involvement of a vanadium monoperoxo species has been substantiated by ESI-MS data. 

The first step of the mechanism leading The electrochemical study of the seven-to the formation of 8 and free nitrite coordinate complex [Mo(N2RR )(dtc)3]+ from the reaction of 7 with O2 probably 9+ (R, R = alkyl or aryl, dtc = 5 2CNMe2) involved a single electron transfer. Sub- provided an example of electrode-induced sequent radical-radical coupling of the activation of a hydrazido(2—) ligand. Corn-products, to afford a molybdenum-bound plex 9+ was shown to reduce in two nitrate, followed by N—O bond cleavage separate diffusion-controlled one-electron would eventually lead to the observed steps, with the first one reversible on the products (Sch. 8) [27]. CV timescale at room temperature and... 

Again several alkyls add—molybdenum, chromium, iron, cobalt, nickel, the alkali metal alkyls and aluminum alkyls react. A tin alkoxide has recently been studied by Russian workers and found to add to acetylenes. Mercury chloride, of course, adds and two cobalt—cobalt bonded compounds add to acetylene. The second is questionable because it dissociates in solution and the reaction may be a radical reaction, one cobalt adding to each end of the triple bond. 

It will be noted that nitrogen atoms are a good example of atoms the disappearance of which is hindered by untreated molybdenum.44 Calculated data and simple considerations show that the curve for accumulation of secondary products yielded by a reaction of primary radicals with molecules is not different from the primary product curve. This is due to the fact that only one oxygen atom participates in the formation of secondary products. Since the reactivity of a primary radical is such that it is converted into a stable molecule before reaching the trap, its presence may be detected. It will be noted also that while the limiting concentrations of primary and secondary products (when these are the only ones) are equal to the initial concentration atoms, the maximum concentrations of quadratic and cubic products are not higher than one-half and one-third, respectively, of this concentration. 

Thus, depending on the metal complex used, cyclohexene oxidation can occur via one or more of at least three major pathways, as shown in Reaction 20 path A, radical initiated decomposition of cyclohexenyl hydroperoxide path B, metal catalyzed epoxidation of the olefin and path C, metal catalyzed epoxidation of an allylic alcohol. Ugo found that path B becomes more pronounced when molybdenum complexes are used to modify the oxidation of cyclohexene in the presence of group... 

A complicating feature proved to be the fact that some alkyl radicals also induced the blue color, although it appeared possible to allow for this by varying the distance of the molybdenum oxide layer from the front face of the reaction cell. The silica surfaces were at first thought to be perfect reflectors for H atoms, but corrections were applied subsequently by Allen et al. (2) when this was found not to be strictly correct. 

The ether elimination is also observed as the first step in thermal decomposition of these alkoxides. The same reaction appears also to be responsible for the observed difference in the hydrolytic behavior of molybdenum and tungsten alkoxides the hydrolysis of the W alkoxides leads as for the majority of other metal alkoxides to the formation of hydroxospecies, forming sols and gels on polycondensation, while in the case of Mo alkoxides the added water, being a stronger acid than alcohols, acts as a catalyst for the ether elimination reaction and causes the formation of individual isopolyanions (independently of the nature of the alkyl radical or quantity of water added) [1774] ... 

MJ. Chetcuti et al., Bis benzylbis(dimethylamido)molybdenum] and -tungsten (M.tplbond.M) compounds and their reactions with carbon dioxide and 1,3 -diaryltriazenes. A radical difference. J. Am. Chem. Soc. 104, 4684- 1686 (1982)... 

There are several reports concerning the thermal and photochemical reactivity of triorganolead metal complexes. In general they eliminate plumbylenes, R2Pb, with a concurrent transfer of the organic radical to the metal center. Such chemistry is reported for platinum, iron and molybdenum inter alia. A typical reaction is outlined in equation 195484.494,507,509... 

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