[13939-06-5]  · C6MoO6  · Hexacarbonylmolybdenum  · (MW 264.00)

(starting material for many organomolybdenum compounds and reagents;1 can be used as catalyst for catalytic allylations2 and epoxidations)

Alternate Name: molybdenum hexacarbonyl.

Physical Data: mp 150 °C (dec); d 1.96 g cm-3.3

Solubility: only very slightly sol most organic solvents; slightly sol THF, diglyme, MeCN.

Form Supplied in: white crystals; commercially available.

Handling, Storage, and Precautions: indefinitely stable at rt under nitrogen or argon. The crystals can be handled in air for short periods with no significant decomposition. Prolonged exposure to air produces some oxidation to a blue oxide. The compound readily sublimes and purification is effected conveniently by sublimation under static vacuum, which yields large crystals of pure material. In most cases, the material can be used as supplied. Owing to the high vapor pressure of the solid, caution should be taken to avoid accidental breathing of the vapor or ingestion of this potentially toxic material; however, Mo(CO)6 is relatively innocuous compared to the high toxicity of liquid metal carbonyls. Many reactions involve displacement of carbon monoxide; hence they should be carried out in an efficient fume hood.

Organomolybdenum Complexes.

Hexacarbonylmolybdenum is the starting material for many organomolybdenum compounds and reagents. It is more readily available and less expensive than its other Group 6 counterparts. Although (arene)molybdenum carbonyl complexes are known, the chemistry of (arene)chromium carbonyl complexes has been better developed. The chemistry of importance to organic synthesis often involves molybdenum carbonyl complexes with cyclopentadienyl or other ligands and this chemistry has been more fully developed than that of chromium or tungsten. Although direct reactions of potential ligands occur with Mo(CO)6, a typical organomolybdenum synthesis would begin by displacing three of the carbonyl ligands with a donor solvent while heating under vigorous reflux (eq 1) to yield a more reactive tricarbonyl intermediate. Displacement of carbonyl groups by other ligands occurs much more readily with Mo(CO)6 than with Hexacarbonyltungsten. For example, the reaction depicted in eq 1 is typically complete within 2 h, whereas the tungsten analog may still only be partially substituted after several days. UV irradiation or use of higher boiling nitriles also increases the rate of displacement.

Oxidative additions of allylic halides occur to yield MoII species which usually contain h3-allylic moieties.3 Typical reagents and catalysts are then formed by the addition of lithium cyclopentadienide3 or bisphosphines4 (eqs 2-4).

Another convenient starting material is the dimer [CpMo(CO)3]2, which can be obtained by heating Mo(CO)6 with high-quality cyclopentadiene dimer under reflux,5 or the air oxidation of CpMo(CO)3H (not very reproducible),6 or quite reliably by the oxidation of CpMo(CO)3- with Iron(III) Sulfate (eqs 5 and 6).7 Although the CpMo(CO)3- anion, prepared as in eq 5, is often used directly in the preparation of CpMo(CO)3R by reaction with alkyl halides, it is more often prepared by reduction of dimer (5) with Sodium Amalgam in THF or by treatment with Lithium Triethylborohydride.8

Regiochemistry of Allylic Alkylations.

The reaction of the dimethyl malonate anion with an allylic acetate is catalyzed by Mo(CO)6 and several complexes which are readily derived from it. Trost and Lautens2 have carried out an extensive study of the regiochemistry of these catalytic reactions. A number of these reactions can be carried out stoichiometrically on isolated h3-allylmolybdenum complexes.2,4 The mechanism of the catalytic reaction is generally presumed to involve nucleophilic attack on an h3-allyl intermediate formed via oxidative addition to a Mo0 complex.9 One of the classic examples is the reaction of crotyl acetate with an enolate (eq 7).10

The regiochemistry of the reaction can be drastically influenced by the ligands on molybdenum and this can provide useful syntheses for regioisomers that might not be available via the more commonly used palladium catalysts. For example, with some nucleophiles the preferred regioisomer for Mo(CO)6 is the reverse of that obtained with a Pd0 catalyst in eq 8.2 The Mo(CO)6 catalyst yields a ratio of (6):(7) of 15:85, whereas Tetrakis(triphenylphosphine)palladium(0) yields a ratio of 100:0. The Mo(CO)4(2,2-bipyridyl) catalyst shows better regioselectivity than Mo(CO)6, however, and yields exclusively (7). Generally the palladium routes give additions at the least substituted terminus, whereas Mo(CO)6 will often give a greater fraction of addition to the more substituted terminus. This generalization for molybdenum catalysis can vary substantially with nucleophiles, as seen in eq 7.

Another reaction which shows this reversal of regiochemistry between Mo and Pd reagents is the conversion of allylic acetates to allyltrimethylsilanes (eq 9).11 The palladium-catalyzed reaction gives a ratio of (8):(9) of 61:39; whereas the Mo(CO)6-catalyzed reaction gives a 0:100 ratio. The molybdenum-catalyzed reaction is also effective with carbonates and provides a route for a-allylation of ketones (eq 10).12

Although the reaction is stoichiometric, the Mo-catalyzed alkylation of an allyl sulfide results in desulfenylative allylation. The regiochemistry of the addition correlates with the original position of sulfur (eqs 11 and 12).13

Allylic alkylations catalyzed by Pd0 complexes14,15 and Mo(CO)62,16 occur with net retention of configuration. This is consistent with double inversion, i.e. inversion in forming the h3-allyl complex and nucleophilic attack trans to the metal. The use of chiral nonracemic allylic acetates in Pd-catalyzed reactions has shown that the first step involves inversion.17 Thus the double inversion mechanism appears to be general for Pd0. Net retention is also consistent with a double retention mechanism and this may occur in some molybdenum-catalyzed reactions, as suggested by the observation that addition of (R)-(E)-4-acetoxy-5-methyl-2-hexene to Mo(CO)3(MeCN)3 occurs with retention.18 On the other hand, net inversion has been observed in a stoichiometric reaction.18

Hydrogenations and Reductions.

Although precious metal catalysts are more often used for hydrogenations, Mo(CO)6 can function as a hydrogenation19 or transfer hydrogenation catalyst.20 However, some of the more synthetically useful reactions are those hydrogenations effected by hydrosilanes using catalytic amounts of Mo(CO)6 that provide selective reductions of unsaturated ketones, esters, amides, and nitriles (eq 13).21

The low oxidation state of molybdenum in the hexacarbonyl complex gives it reductive capabilities which provide a route for dehydrosulfurization (eq 14).22 The reaction of alkyl or aryl sulfonyl chlorides with excess (10-30%) Mo(CO)6 in anhydrous tetramethylurea provides a route to disulfides (eq 15).23

The oxidative addition of halides to Mo0 provides an alternate route for dehalogenation of a-halocarbonyl compounds.24 For a-halo ketones, partitioning between the methyl ketone and the aldol product (eq 16) depends upon the catalyst.25 For Mo(CO)6 in 1,2-dimethoxyethane the methyl ketone is formed as the major, if not the only, product. Use of Mo(CO)3(NCMe)3 with 2-chloroacetophenone allows stabilization and isolation of h3-allylmolybdenum adducts of condensation of two and three acetophenone units (eq 17).26

Epoxidations and Oxidations.

Molybdenum carbonyl can be used as an alkene epoxidation catalyst in conjunction with t-Butyl Hydroperoxide.27,28 It provides a convenient route for monoepoxidation of 1,5-cyclooctadiene (eq 18).27 This approach can be useful for epoxidation of enol ethers,29 which complements approaches using peracids or Tetracarbonylnickel, a far more toxic metal carbonyl.30 This approach also provides a route to larger ring lactones, such as 6-ketononanolide (eq 19).31 The molybdenum is oxidized to a peroxy species in these reactions. In the absence of t-BuOOH, however, epoxides can rearrange to aldehydes in the presence of catalytic amounts of Mo(CO)6 (eq 20).32 The Mo(CO)6/t-BuOOH system also allows selective oxidation of a secondary alcohol to a ketone in the presence of a primary alcohol (eq 21).33 The selectivity is improved upon addition of cetylpyridinium chloride.

Ring Formation and Opening.

Ring closures in (10) have been effected by Mo(CO)6 (eq 22)34 to form benzocyclobutenones in a stoichiometric reaction. Stabilization of a metallocyclobutene intermediate provides a route to a-naphthols from arylcyclopropenes (eq 23) in a catalytic reaction; however, nearly molar equivalents of Mo(CO)6 increase the yield and the rate substantially.35

There are other examples of stabilization of carbene intermediates by molybdenum which are useful in heterocyclic synthesis. Pyrroles have been prepared from a-azidostyrene (eq 24)36 and 2,5-pyrrolidones have been prepared from azirines and diethyl sodiomalonate (eq 25).37 Although the reaction can be carried out with SbF5, Mo(CO)6 also catalyzes cyclopropane formation for diazo additions to electron-deficient alkenes (eq 26).38 The (E):(Z) ratio ranges from 0.7 to 2.2, depending on R1 and R2.

The N-O bond is easily cleaved by Mo(CO)6 and this provides a route for conversion of oxime acetates to ketones (eq 27).39 Ring openings of isoxazoles40 and isoxazolines41 provide routes to b-amino-a,b-unsaturated ketones (eq 28) and b-hydroxy ketones (eq 29). This is also a key step in an asymmetric synthesis of a-aminocarboxylic acids from 1-alkoxy-3-siloxy-2-aza-1,3-dienes which has been published recently (eq 30).42 The acyl nitroso intermediate is generated in situ via oxidation of a hydroxylamine.

Other Organomolybdenum Reagents.

Recent reviews43,44 have covered some of the extensive chemistry of organomolybdenum reagents derived from Mo(CO)6, which allows stoichiometric regioselective and stereoselective C-C bond formation. Control of regiochemistry by differences in electronic character of the nitrosyl and carbonyl ligands can be observed in nucleophilic additions to [CpMo(NO)(CO)(allyl)]+ complexes.45 The stereogenic center at molybdenum has been resolved, which allows the preparation of chiral alkenes in high enantiomeric purity.45 [CpMo(CO)(dienyl)]+ systems, such as Dicarbonyl(cyclohexadiene)(cyclopentadienyl)molybdenum Tetrafluoroborate, provide routes for stereocontrolled multiple functionalization of cyclohexene rings.43,46 CpMo(CO)2(2H-pyran) cations derived from the chiral pool have allowed enantioselective transformations for replacement of the pyran carbonyl group by two different substituents.47 CpMo(NO)(halide)(crotyl) complexes, such as Bromo(crotyl)(cyclopentadienyl)(nitrosyl)molybdenum, are useful reagents for stereocontrolled synthesis of b-methyl homoallyl alcohols via crotyl addition to aldehydes.48 These reagents and their methallyl analogs have been resolved and yield products of high enantiomeric purity, comparable to those obtained with crotyltitanium, crotylboron, and crotylboronate reagents. The molybdenum reagents have advantages in ease of separation of products and resistance to moisture.

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John W. Faller

Yale University, New Haven, CT, USA

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