Hexacarbonylchromium1

Cr(CO)6

[13007-92-6]  · C6CrO6  · Hexacarbonylchromium  · (MW 220.06)

(catalyst for alkene isomerization, hydrogenation of 1,3-conjugated dienes, oxidation of allylic and benzylic positions; reagent for preparation of tricarbonyl(arene)chromium and Fischer carbene complexes)

Alternate Name: chromium hexacarbonyl.

Physical Data: sublimes at 50-80 °C; mp 154-155 °C (in sealed tube); d 1.77 g cm-3.

Solubility: insol water, ethanol; slightly sol ether, CHCl3.

Form Supplied in: white solid; widely commercially available.

Handling, Storage, and Precautions: highly volatile and must be stored and used in a well ventilated fume hood.

Oxidation.

Treatment of alkenes with t-Butyl Hydroperoxide in the presence of 0.5 equiv of Cr(CO)6 or Cr(CO)x(MeCN)y species in refluxing MeCN results in the oxidation of allylic methylene groups to give a,b-unsaturated ketones selectively in the presence of certain alcohols (eq 1).2 Benzylic positions are also oxidized to phenyl ketone derivatives3 under the same conditions (eq 2).

Hydrogenation and Isomerization of Dienes.

The catalytic behavior of (arene)Cr(CO)3 complexes has been exploited in the homogeneous regioselective 1,4-hydrogenation of dienes to cis-alkenes.4 Only 1,3-dienes that can easily achieve the less stable s-cis configuration to undergo this catalytic hydrogenation (eq 3). Similar catalytically active species Cr(CO)3H2(diene) (1) can be generated photochemically from Cr(CO)6 or from kinetically thermally labile p-bonded ligated complexes (naphthalene, anthracene, MeCN) in coordinating solvents (THF, acetone, dioxane) under milder conditions (eq 4).5 This regioselective hydrogenation of dienes can be applied to the total synthesis of complex natural products (eq 5) such as carbacyclin, cyanopcarbacyclin, and deplanchein.6 1,4-Hydrosilylation of dienes is also achieved to produce (Z)-allyltrimethylsilanes7 under these conditions. The (arene)chromium complexes are superior catalysts for stereoselective hydrogenation of alkynes to (Z)-alkenes, and in addition, cisoid a,b-unsaturated enones and imines can be reduced.8

Chromium hexacarbonyl offers a highly regioselective method for the isomerization of cisoid 1,3-dienes to transoid dienes (eq 6).9 (Naphthalene)chromium tricarbonyl catalyzes isomerization of silyloxymethylbutadiene derivatives to silyl dienol ethers in quantitative yields via a U-shaped pentadienyl intermediate (eq 7).10

(Arene)Cr(CO)3 Complexes.

Complexation of an arene ring to the chromium tricarbonyl unit is easily accomplished by simple heating with Cr(CO)6 or by ligand transfer with Cr(CO)3L3.1,11 The significant properties of (arene)chromium complexes in organic synthesis are as follows: nucleophilic substitution to arene ring; enhancement of acidity of aromatic hydrogen; enhancement of acidity of benzylic hydrogen; enhancement of solvolysis at the benzylic position; and steric hindrance of the Cr(CO)3 group.

Nucleophilic Substitution.1

Some carbon nucleophiles add to tricarbonyl(arene)chromium complexes to yield anionic h5-cyclohexadienyl complexes (2) (eq 8), which give the substituted arenes via decomplexation by oxidation with iodine. Protolysis of the intermediate cyclohexadienylchromium complexes (2) generate cyclohexadienes, and reaction with electrophiles generates either the arene chromium complexes or produces the acylated species.

With substituted aromatics, the following regioselectivity for nucleophilic substitution is generally observed (eq 9); alkoxy substituents strongly direct to the meta position, and trimethylsilyl groups ensure para substitution.12 Alkylation of the chromium complexes of indoles and benzofurans takes place at the C-4 or C-7 position, depending on the steric effect of the C-3 substituent.13 Intramolecular nucleophilic alkylation can also be achieved; the products formed depend on chain length, reaction conditions, and other substituents on the aromatic ring (eq 10).14 With an (anisole)chromium complex, 3-substituted cyclohexenone derivatives can be obtained by the protolytic cleavage of the intermediates (eq 11).17 A combination of the intramolecular alkylation with protolysis gives an elegant synthesis of the spiro sesquiterpenoids acorenone and acorenone B (eq 12).15,16

Reaction with a range of electrophiles regenerates the (arene)chromium complexes under reversible conditions, but electrophilic attack upon the cyclohexadienyl complex formed under irreversible conditions from dithiane carbanions produces trans addition products (eq 13).17

Lithiation.

With anisole, fluorobenzene, and chlorobenzene chromium complexes, lithiation always occurs at an ortho position of the substituents under mild conditions (eqs 14 and 15).18 Protected phenol or aniline chromium complexes with sterically bulky substituents produce meta lithiation exculsively.19 The lithiated position of some (arene)chromium complexes (3) differs from that of the corresponding chromium-free compounds (4).20

Activation of the Benzylic Position.

Both chromium complexed carbanions and carbocations are stabilized at the benzylic position.21 Dialkylation of alkyl halides at the benzylic position occurs via stabilized carbanions under mild conditions (eq 16).22 Regio- and stereoselective products are obtained via the benzylic carbanions, depending on the conformation of the tricarbonyl group to the arene (eq 17).22 (Styrene)chromium complexes stabilize negative charges at the benzylic position by addition of nucleophiles to the b-position, giving bifunctionalization products by trapping with electrophiles (eq 18).23

Tricarbonylchromium stabilized benzylic carbocations can be captured by a large variety of nucleophiles,24 such as alcohols, amines, thiols, nitriles, trimethylsilyl enol ethers, allylsilanes, electron-rich aromatics, dialkylzincs, and trialkylaluminums (eq 19). The relative stereochemistry formed during these reactions via carbocations in acyclic systems proceeds with net retention.21b,c,25 Friedel-Crafts acylation of (styrene)chromium complexes has been explored via the benzylic cations (eq 20).26 Tricarbonylchromium-stabilized oxonium ions are also utilized for steroselective carbon-carbon bond forming reactions (eq 21).27

Steric Effect of the Cr(CO)3 Group.1

A feature of (h6-arene)chromium complexes is the steric interference offered to the approach of reagents, which occurs exclusively from the exo-face at the reactive center in cyclic chromium complexes (eq 22).28 Even in the (acyclic arene)chromium complexes, such as o-substituted benzaldehyde and phenyl alkyl ketones with electron-donating ortho substituents (OMe, Me, F), addition of nucleophiles to the chromium complexed benzylic carbonyl group proceeds stereoselectively, giving one diastereomeric complex predominantly (eq 23).29 A Friedel-Crafts reaction produces predominantly an exo-substituted complex (eq 24).30

A 1,2 or 1,3 unsymmetrically disubstituted arene is prochiral and therefore the corresponding chromium tricarbonyl compounds are chiral.31 (Substituted arene) complexes with amine, carboxyl, and formyl groups at the ortho position are resolved into optically active chromium complexes through corresponding diastereomeric adducts (eq 25).32 Biocatalysts also perform the kinetic resolution of racemic chromium complexes (eq 26).33 The optically active chromium complexes can be prepared by diastereoselective ortho lithiation34 of the chiral benzaldehyde or acetophenone acetal complexes, and diastereoselective chromium complexation35 of the chiral ortho-substituted benzaldehyde aminals (eq 27). Catalytic asymmetric cross-coupling of meso (1,2-haloarene)chromium complex produces chiral monosubstituted complexes.36 The chiral (arene)chromium complexes can be used as ligands in asymmetric reactions.37

Related Reagents.

(h6-Benzene)tricarbonylchromium; Dodecacarbonyltriiron; Pentacarbonyliron.


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Motokazu Uemura

Osaka City University, Japan



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