Molybdenum hexacarbonyl reaction

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. 

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 6.47 Palladium-catalyzed carbonylation reactions yielding acids, esters, and lactones 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-... 

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. 

In a similar manner, Brummond et al. demonstrated the first total synthesis of 15-deoxy-A12,14-prostaglandin J2 (162) that was completed using a silicon-tethered allenic Pauson-Khand reaction to obtain the highly unsaturated cyclopentenone substructure [36]. Treatment of alkynylallene 160 with molybdenum hexacarbonyl and dimethyl sulfoxide affords the desired cycloadduct 161 in 43% yield (Scheme 19.30). Trienone 161 was obtained as a 2 1 Z E mixture of isomers in which the Z-isomer could be isomerized to the desired E-isomer. The silicon tether was cleaved and the resulting product converted to 15-deoxy-A12,14-prostaglandin J2 (162). 

When sodium ethoxide is used in place of sodium hydroxide in the carbonylation reaction of benzyl halides with dicobalt octacarbonyl, ethyl esters are produced instead of the acids [15], Esters are also produced directly from iodoalkanes through their reaction with molybdenum hexacarbonyl in the presence of tetra-/i-butylammo-nium fluoride [16]. Di-iodoalkanes produce lactones [16]. The reaction can be made catalytic in the hexacarbonyl by the addition of methyl formate [16]. t-Butyl arylacetic esters are produced in moderate yield (40-60%) under phase-transfer catalytic conditions in the palladium promoted carbonylation reaction with benzyl chlorides [17]. 

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. 

Tetrakis-/i-(carboxylato)-dimolybdenum(II) complexes have been obtained by only one general route, namely by the direct interaction of carboxylic acids with molybdenum hexacarbonyl.8 This reaction requires elevated reaction temperatures and prolonged reaction times. These same compounds are obtained in comparable or better yields by the brief reaction of tetrachloro- or tetrabromotetrakis(tributylphosphine)dimolybdenum(II) with alkyl- or aryl-carboxylic acids in refluxing benzene. The bis-/i-(arylcarboxylato) complexes... 

Bromobenzyl alcohol and its derivatives were converted to phthalides by the palladium catalysed insertion of carbon monoxide and intramolecular quenching of the formed acylpalladium complex. 2-Hydroxymethyl-1-bromonaphthaline, for example, gave the tricyclic product in excellent yield (3.34.). An interesting feature of the process is the use of molybdenum hexacarbonyl as carbon monoxide source. The reaction was also extended to isoindolones, phthalimides and dihydro-benzopyranones 42... 

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