Molybdenum-carbon bonds

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

Simple oxidation/reduction has also been studied for a wide range of isocyanide complexes. Again those complexes containing mainly molybdenum-carbon bonds have been reviewed in... 

Molybdenum carbenes, in ene—yne metathesis, 11, 271—272 Molybdenum-carbon bonds in Mb carbonyls, 5, 455 456 reactivity, 5, 397... 

There are two well-characterized examples of a naked carbon atom bound by a triple bond to a metal center (Fig 14.3.8). The molybdenum carbide anion [CMo N(R)Ar 3]- (R = C(CD3)2(CH3), Ar = C6H3Me2-3,5), an isoelectronic analog of NMo N(R)Ar 3, can be prepared in a multistep procedure via deprotonation of the d° methylidyne complex HCMo N(R)Ar 3. The Mo=C distance of 171.3(9) pm is at the low end of the known range for molybdenum-carbon multiple bonds. In the diamagnetic, air-stable terminal ruthenium carbide complex Ru(=C )C12(LL/)(L = L = PCy3, or L = PCy3 and L = l,3-dimesityl-4,5-dihydroimidazol-2-ylidene), the measured Ru-C distance of 165.0(2) pm is consistent with the existence of a very short Ru=C triple bond. 

Salient structural features of CpL2Mo(rj2-CRCR2) complexes include (1) the orientation of the two Cg substituents roughly orthogonal to the MCaCj3 plane (this is opposite to the location of Cp substituents of rj -vinyl ligands which lie in the MC C plane), (2) short M—Ca distances (1.94-1.96 A) appropriate for a molybdenum-carbon double bond (174),... 

Howes, B. D., Bray, R. C., Richards, R. L., Turner, N. A., Bennett, B., and Lowe, D. J., 1996, Evidence favoring molybdenum-carbon bond formation in xanthine oxidase action 0- and C-ENDOR and kinetic studies, Biochemistry 35 143291443. 

Paxton, R.R., Carbon, Graphite and Metal-Bonded Molybdenum Disulphide Solid Lubricant Bearings, Tribology Inti., 15, 285, (1982). 

Green, M. (1986) The road from alkynes to molybdenum-carbon multiple bonds. Organomet. Chem., 300, 93. 

Alloys of TiC with steel bond (or with a bond of carbon steel with an addition of 2,97o chromium and molybdenum in this case, the composition of the final alloy is 26 o Ti, 6.5% C, 1,8% Cr, 1.8% Mo the rest iron) The alloy is prepared by sintering followed by annealing in a [978] 1960... 

Although a number of reagents can be used to reduce an isoxazole ring, molybdenum hexacarbonyl31 was selected for use in this synthesis. The action of this reagent on 24 reduces the weak N-0 bond of the isoxazole ring and produces a //-amino-a,//-unsaturated aldehyde (i.e. a vinylogous formamide) (see Scheme 19). Intermediate 87 forms smoothly upon deprotection of the terminal acetylene carbon with basic methanol-THF. 

Metal-metal bonds and metal-carbon bonds in the chemistry of molybdenum and tungsten alkox-ides. M. H. Chisholm, Polyhedron, 1983, 2, 681-721 (90). 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

The formation of 2H-pyrroles (21) and a pyrrole derivative (22) from the reaction of 3-phenyl-2//-azirines and acetylenic esters in the presence of molybdenum hexacarbonyl is intriguing mechanistically (Schemes 24, 25).53 Carbon-nitrogen bond cleavage must occur perhaps via a molybdenum complex (cf. 23 in Scheme 26) but intermediate organometallic species have not yet been isolated.53 Despite the relatively poor yields of 2H-pyrrole products, the process is synthetically valuable since the equivalent uncatalyzed photochemical process produces isomeric 2H-pyrroles from a primary reaction of azirine C—C cleavage54 (Scheme 24). 

The total syntheses of these pepper alkaloids are not those of pyrrolidines but rather syntheses of their acid parts. Thus dihydrowisanidine (137) has been prepared by a series of reactions, the key step of which is the formation of the carbon-carbon double bond by a Wittig-Homer reaction (217, 218). Schemes 41 and 42 summarize two syntheses of okolasine from sesamolmethyl ether (279) of course, routes to okolasine also yield the corresponding piperidine alkaloid wisanine. Molybdenum-catalyzed elimination of allylic acetate (149) yielded (E,E)-diene ester 150 en route to trichonine (220) worthy of note is the use of an aluminum amide in the preparation of amide 143 from ester 150 (Scheme 43). 

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