Molybdenum, hexacarbonyl

Olefin isomerization can be catalyzed by a number of catalysts such as molybdenum hexacarbonyl [13939-06-5] Mo(CO)g. This compound has also been found to catalyze the photopolymerization of vinyl monomers, the cyclization of olefins, the epoxidation of alkenes and peroxo species, the conversion of isocyanates to carbodiimides, etc. Rhodium carbonylhydrotris(triphenylphosphine) [17185-29-4] RhH(CO)(P(CgH )2)3, is a multifunctional catalyst which accelerates the isomerization and hydroformylation of alkenes. 

Finally, the bimolecular cycloaddition of alkynes with 2-phenylazirines in the presence of molybdenum hexacarbonyl has been studied (79TL2983). The pyrrole derivatives (294) obtained appear to arise from an initial [2 + 2] cycloaddition followed by a ring opening reaction. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Molybdenum hexacarbonyl [Mo(CO)6] has been vised in combination with TBHP for the epoxidation of terminal olefins [44]. Good yields and selectivity for the epoxide products were obtained when reactions were performed under anhydrous conditions in hydrocarbon solvents such as benzene. The inexpensive and considerably less toxic Mo02(acac)2 is a robust alternative to Mo(CO)6 [2]. A number of different substrates ranging from simple ot-olefms to more complex terpenes have been oxidized with very low catalytic loadings of this particular molybdenum complex (Scheme 6.2). The epoxidations were carried out with use of dry TBHP (-70%) in toluene. 

Mdleleni et al. [67] have reported a simple sonochemical route to generate nanos-tructured molybdenum sulfide (M0S2) using molybdenum hexacarbonyl and sulfur. 

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). 

A more complicated type of reaction leading to 2-styrylindoles is observed when 2-arylazirines are treated with the rhodium complexes,70 [(Ph3P)2 Rh(CO)Cl] or [Rh(CO)2Cl]2, or with dicobalt octacarbonyl71 (Scheme 42). In contrast, 2-arylazirines with molybdenum hexacarbonyl give pyrazines and dihydropyrazines, and with diiron enneacarbonyl give pyrroles (see Sections V,C,2 and IV,A,1, respectively). The use of relatively low molar ratios of 2-arylazirine to rhodium catalyst (2 1) causes the formation of 2,5-diarylpyrroles. 

The formation of pyrazoles from reactions of suitably substituted 2-arylazirines and molybdenum hexacarbonyl has been discussed earlier in this section (see Schemes 89, 90)47 an analogous procedure depicting the transformation of 2-formyl-3-phenyl-2H-azirine into 3-phenylisoxazole is illustrated in Scheme 109.47... 

The ring cleavage of 3-aryl-2-substituted-2//-azirines by molybdenum hexacarbonyl has been described earlier in regard to the synthesis of pyrroles, pyrazoles and isoxazoles. In contrast to this behavior, analogous reactions of 2-unsubstituted derivatives lead to the formation of mixtures of 2,5-diarylpyrazines (139) and isomeric 3,6- and 1,6-dihydropyrazine derivatives (140,141) (Scheme 163).47,53 It is possible that the pyrazine products are formed by an intermolecular nitrene mechanism akin to the intramolecular processes described earlier (see Scheme 22 in Section IV,A,1). 

A 1,3-oxazepine derivative (158) has been isolated in low (2-3%) or unspecified yield by treatment of the Z-ketovinylazirine 157 with diiron nonacarbonyl50 or molybdenum hexacarbonyl,51 respectively (Scheme 182) the major products of these reactions are pyrrole derivatives (see Scheme 23 in Section IV,A,1). There is no preparative value in this type of oxazepine synthesis (Scheme 182) since the transformation can be affected efficiently in a thermal reaction at 100°C.52... 

Scheme 6.46 Palladium-catalyzed aminocarbonylations using molybdenum hexacarbonyl as a solid source of carbon monoxide.

An interesting series of ring-closing alkyne metathesis reactions (RCAM) has recently been reported by Fiirstner and coworkers (Scheme 6.72) [152], Treatment of biaryl-derived diynes with 10 mol% of a catalyst prepared in situ from molybdenum hexacarbonyl and 4-(trifluoromethyl)phenol at 150 °C for 5 min led to a ca. 70% iso-... 

In a related study, the same group investigated molybdenum-catalyzed alkylations in solution and on a solid phase [35], demonstrating that microwave irradiation could also be applied to highly enantioselective reactions (Scheme 7.15). For these examples, commercially available and stable molybdenum hexacarbonyl [Mo(CO)6] was used to generate the catalytic system in situ. The reactions in solution provided good yields (see Scheme 6.50). In contrast, the conversion rates for the solid-phase examples were rather poor. However, the enantioselectivity was excellent (>99% ee) for both the solution and solid-phase reactions. 

In a more recent study, Wannberg and Larhed reported solid-supported aminocar-bonylations employing molybdenum hexacarbonyl as a solid source of carbon monoxide [37]. Carbon monoxide is smoothly liberated at the reaction temperature upon the addition of the strong base l,8-diazabicyclo[2.2.2]octane (DBU). In this transfor-... 

The molybdenum hexacarbonyl complex was recently introduced as a condensed source of carbon monoxide for Heck carbonylations [29]. This easily handled and inexpensive solid delivers a fixed amount of carbon monoxide when heated to approxi-... 

Equation 11.15 Carbonylative coupling with molybdenum hexacarbonyl. 

An efficient synthetic route to (10Z)- and (10 )-19-lluoro-la,25-dihydroxy vitamin D3 has been developed (488). The key feature of this pathway is the introduction of a 19-fluoromethylene group to a (5 )-19-nor-10-oxo-vitamin D derivative. The 10-oxo compound 445 has been obtained via a 1,3-dipolar cycloaddition reaction of (5 )-la,25-dihydroxyvitamin D with in situ generated nitrile oxide, followed by ring cleavage of the formed isoxazoline moiety with molybdenum hexacarbonyl. Conversion of the keto group of (5 )-19-nor-10-oxo-vitamin D to the E and Z fluoromethylene group has been achieved via a two-step sequence, involving a reaction of lithiofluoromethyl phenyl sulfone, followed by the reductive de-sulfonylation of the u-lluoro-j3-hydroxysulfone. The dye-sensitized photoisomerization of the (5 )-19-fluorovitamin D affords the desired (5Z)-19-fluorovitamin D derivatives, (10Z)- and (10 )-19-fluoro-la,25-dihydroxy-vitamin D3. 

Scheme 22 illustrates a special application of the azide-tetrazole ring closure described by Ponticelli et al. <2004JHC761>. The diazido compound 84 exists as an azide valence bond isomer. When this compound, however, is subjected to reduction by molybdenum hexacarbonyl, one azido group undergoes reduction selectively to an... 

Molybdenum disulfide electrodes, sloping discharge curve, 3 414 Molybdenum ditelluride, 17 26 Molybdenum enzymes, 17 321 Molybdenum hexacarbonyl, 7 594 ... 

With the silyl-substituted difluoroallene 245 and molybdenum hexacarbonyl, the expected [2+ 2+ 1] cycloaddition is not observed. CO is not incorporated and the cyclobutene derivatives 246 can be isolated in good yields (Scheme 15.77) [148]. 

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