Molybdenum hexacarbonyl oxidant

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. 

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. 

If photochemical apparatus is not available, the cycloisomerization reaction can be conducted using trimethylamine N-oxide to promote oxidative decarbonylation of molybdenum hexacarbonyl in a mixture of EtjN and EtgO, followed by addition of 1-phenyl-3-butyn-1-ol (1). In the submitters hands, this procedure required somewhat higher loading of molybdenum hexacarbonyl, and purification of the 2-phenyl-2,3-dihydrofuran (2) product required silica gel chromatography. 

Jeong and co-workers reported the activation of molybdenum hexacarbonyl in situ by the aid of dimethylsulf-oxide. A mixture of the substrate and Mo(CO)6 (l.Oequiv.) together with the excess of DMSO (lOequiv.) in toluene and/or benzene was heated to give the corresponding products. 

An unanticipated catalytic reaction of olefinic hydrocarbons was described in 1964 by Banks and Bailey.1 2 They discovered that C3-C8 alkenes disproportionate to homologs of higher and lower molecular weight in the presence of alumina-supported molybdenum oxide [Eq. (12.1)], cobalt oxide-molybdenum oxide, molybdenum hexacarbonyl, or tungsten hexacarbonyl at 100-200°C, under about 30 atm pressure ... 

The experimental data indicate that molybdenum compounds, including oxides, sulfides, halides, acids, salts, heteropolymolybdic acids, salts of heteropolymolybdic acids, esters of heteropolymolybdic acids, and molybdenum coordination compounds are catalysts for the epoxida-tion reaction. However, certain compounds such as sodium silicomolyb-date, esters of heteropolymolybdic acids, and the coordination compounds (molybdenum hexacarbonyl and molybdenum oxyacetylacetonate) were more effective catalysts. 

Additional evidence that reduction is not the role of R AlCla.. in catalyst formation is provided by the observation that the complexes [Bu4N] [Mo(CO)5X] and [R4N] [Mo(CO)5COR L in which the molybdenum is in a low oxidation state, require an organoaluminum reagent for catalytic activity (44, 45). In these examples, the function of the organoaluminum is most likely the removal of CO ligands to make available sites for olefin coordination. Molybdenum hexacarbonyl alone is reported to be a disproportionation catalyst in this case expulsion of the CO groups is attained thermally at 98 °C (46). 

Other molybdenum complexes able to catalyze the selective oxidation of secondary alcohols are ammonium molybdate in the presence of H202,30 benzyltrimethylammonium tetrabromooxomolybdate in the presence of r-BuOOH31 and molybdenum hexacarbonyl in the presence of catalytic cetylpyridinium chloride and stoichiometric t-BuOOH.32... 

The first catalysis of an olefin metathesis reaction was reported by Banks and Bailey in 1964 (56). They reported that activated molybdenum hexacarbonyl on alumina converted propylene, for example, into ethylene and 2-butene at 150°C and 30 atm. Oxides of rhenium are also powerful heterogeneous catalysts. 

The reaction, of metal carbonyls with 1,3-diketones generally results in a complete displacement of carbon monoxide accompanied by oxidation of the metal to yield 1,3-diketonato complexes. For example, iron pentacarbonyl, chromium hexa-carbonyl, and molybdenum hexacarbonyl afford FefCgHjOOs,1 Cr(CsHr02)8,2 and Mo(CgH702)s,2,s respectively, when allowed to react with 2,4-pentanedione. 

Alternative oxidation reagents, fm-buty I hydroperoxide, molybdenum hexacarbonyl, or lead tetraacetate oxidation of the corresponding glycol, were tried in order to avoid dilactone formation, but the yields were not satisfactory [98]. 

The only examples of this synthesis employed isoxazolopyrazine substrates that were themselves made from pyrazines. Thus l-benzyl-5,6-dihydro-2(l//)-pyrazi-none 4-oxide (99) underwent addition by ethynylbenzene to give 5-benzyl-2-phemi-6,7-dihydro-3a//-isoxazolo[2,3-a]pyrazin-4(5//)-onc (100) (60%), which subsequently underwent ring cleavage by molybdenum hexacarbonyl in wet acetonitrile to afford l-benzyl-3-phenacyl-3,4,5,6-tetrahydro-2(l//)-pyrazinone (101) in 54% yield several analogues were made similarly.1539... 

The reaction between molybdenum hexacarbonyl and elemental fluorine at —65° results in the formation of Mo2F9, which upon thermal degradation produces molybdenum pentafluoride as one of the products.1 Other syntheses of molybdenum pentafluoride include the reduction of molybdenum hexafluoride with phosphorus trifluoride,2 tungsten hexacarbonyl, or molybdenum metal at high temperatures3 and the oxidation of powdered molybdenum metal with elemental fluorine at 900°.3 The present method consists in the reaction of molybdenum hexafluoride with powdered molybdenum metal at 60° and results in the formation of pure molybdenum pentafluoride in yields of 80% and greater. 

Reagents which effect epoxidation of the enol ether unsaturation effect a-hydroxylation comparable to the peracid approach. Thus a combination of molybdenum hexacarbonyl and r-butyl hydroperoxide converts the substrates to a-silyloxy derivatives. The peroxide generate in situ from benzonitrile, potassium carbonate and hydrogen peroxide can also perform the oxidation. Molybdenum-peroxy complexes, including MoOPH, could presumably also effect this transformation. Lastly, dimethyldioxirane has been used to epoxidize alkenes and it is likely that application of this useful, debris free, organic peroxide to these reactions will soon emerge. 

Methods for the oxidation of azo to azoxy compounds have been reviewed.Typical oxidants are MCPBA and other peroxy acids and hydrogen peroxide. r-Butyl hydroperoxide is also an effective oxidant for azobenzenes in the presence of molybdenum hexacarbonyl. Trifluoroperacetic acid has been used to oxidize perfluoroazobenzene and other fluorinated azobenzenes to the azoxy compounds in high yield It is also capable of oxidizing azoxyarenes, such as compound (47), to the di-A(-oxides. 4... 

Oxidation of molybdenum hexacarbonyl with acetic acid produces the carboxylate-bridged quadruple-bonded dimers, as shown inequation (35). This complex can undergo a number of substitution reactions that leave the metal metal bond intact. ... 

It was observed in the early 1960s that Mo(CO)6 supported on aluminum oxide yields a catalyst for what is now known as the Alkene Metathesis reaction. The reaction of molybdenum hexacarbonyl with the surface proceeds as in Scheme 1. ... 

Oxuziruttnes. Oxidation of Schiff bases (I) with HPTA catalyzed by molybdenum hexacarbonyl (2, 287, 3, 206-207 this volume) or by MoCly in benzene solution gives oxaziridines (2) generally in high yield. 

Formally, pentavalent neutral metallocorroles have been prepared by Murakami and coworkers.The first of these was the oxomolybdenum(V) corrole derivative 2.179. ° This complex was prepared by heating free-base corrole 2.82 with molybdenum pentachloride in oxygen-free decalin (Scheme 2.1.56). Alternatively, molybdenum hexacarbonyl (Mo(CO)e) could be used as the metal source. In both cases, oxidation to the oxomolybdenum complex 2.179 was believed to occur during workup (involving chromatography on neutral alumina followed by recrystallization). In this way, complex 2.179 was isolated in c. 40% yield. Similar yields of the oxochromium(V) complex 2.180 could be achieved via the reaction of 2.82 with anhydrous chromium(II) chloride in DMF. Here too, spontaneous oxidation during workup was used to afford the formally pentavalent oxo-complex 2.180. 

As in the case of the cyclic arsenic oxides discussed above, it has been possible to obtain ordered cyclic (MeAsS) oligomers, where n = 3 or 4, by treating (MeAs), with elemental sulphur in the presence of molybdenum hexacarbonyl There is an isolobal relationship... 

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