Chromium hexacarbonyl preparation

Chromium carbene complexes like 13, which are called Fischer carbene complexes, can conveniently be prepared from chromium hexacarbonyl 11 and an organolithium compound 12, followed by an O-alkylation step ... 

Other miscellaneous imines that underwent photoreaction with chromium alkoxycarbenes include iminodithiocarbonates [33],the mono-N-phenylimine of benzil and the bis-JV-phenyl imine of acetoin [20]. By preparing the chromium carbene complex from 13CO-labeled chromium hexacarbonyl, /J-lactams with two adjacent 13C labels were synthesized [34]. 

Disodium pentacarbonylchromate is prepared by adding a solution of sodium naphthalenide (88 mmol) in THF (160 mL) to chromium hexacarbonyl (40 mmol) in THF (240 mL). The resulting solution of disodium pentacarbonylchromate is transferred to a 500-mL graduated cylinder (to estimate the concentration). This solution can be stored at -20 °C under argon for at least two months. 

In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dbtz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocycli-zations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 -I- 2] cycloreversion) and pyrrolidine to yield a substituted benzene. 

Experimental Procedure 2.1.1. Preparation of a Chromium Carbene Complex from Chromium Hexacarbonyl [Methoxy(methyl)carbene]pentacarbonylchro-... 

Chromium hexacarbonyl is used as an additive to gasoline to increase the octane number as a catalyst in isomerization and polymerization reactions and in the preparation of chromium mirror or plate. 

Chromium hexacarbonyl is prepared by the reaction of anhydrous chromi-um(lll) chloride with carbon monoxide in the presence of a Grignard reagent. A 60% product yield may be obtained at the carbon monoxide pressures of 35 to 70 atm. Other chromium salts may be used with carbon monoxide and Grignard reagent in the preparation. The compound may also be obtained by the reaction of a chromium salt with carbon monoxide in the presence of magnesium in ether or sodium in diglyme. 

The synthesis of unnatural chiral amino acids is a considerable challenge to the organic chemist. One approach is via chiral organochro-mium(0) carbenes (56). The starting material is easily prepared from chromium hexacarbonyl and organolithium reagents (Fig. 5), and the resulting carbenes are air-stable yellow to red solids. 

Many complexes of conjugated ketones are also known, such as the iron tricarbonyl complexes of substituted cyclopentadienones 30), although reaction with chromium hexacarbonyl occurs only if phenyl substituents are available for tt complexing 31). A common difficulty of preparing complexes of heterocyclics is the ability of the heteroatom to form o bonds with the metal. 

The chromium(II) complex of mesoporphyrin IX dimethyl ester (276) can be prepared by metal insertion with an excess of chromium hexacarbonyl under nitrogen.1242 After removal of solvent and extinction with toluene, Crn(MPDME) is crystallized by the addition of n-pentane. Presumably the weakly acidic NH protons of the porphyrin oxidize Cr° to Cr11 and form H2. 

A special highlight of this chemistry is the possibility to construct compounds with two adjacent - C-labeled carbon atoms [17], which are of great interest for the investigation of biological systems by means of H NMR spectroscopic methods. For this purpose, the starting complexes (i.e. 36) are prepared from C-labelled chromium hexacarbonyl (Scheme 11). 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

A number of substituted benzenes, naphthalenes, indans, pyridmes, and indoles form arene(tricarbonyl)chromium complexes upon thermolysis under an inert atmosphere, usually in a high boiling ether, or by irradiation of the arenes in the presence of chromium hexacarbonyl. The complexes are relatively air-stable and can usually be stored for long periods in the absence of light. Somewhat milder conditions can be used by transfer of the chromium tricarbonyl group from preformed naphthalene(tricarbonyl)chromium, tris(L)tricarbonyl chromium (L = acetonitrile, ammonia, pyridine), or tricarbonyl( -l-methylpyrrole)chromium. Enan-tiomerically pure arene(tricarbonyl)chromium complexes having two different substituents, either ortho or meta can be prepared conveniently by classical resolution of racemic... 

Tris(l,l,l-trifluoro-2,4-pentanedionato)chromium(III) has been prepared by heating chromium(III) chloride with l,l,l-trifluoro-2,4-pentanedione (trifluoroacetylacetone) in an aqueous solution of urea - t and also by the reaction between chromium hexacarbonyl [Cr(CO)6] and 1,1,1-tri-fluoro-2,4-pentanedione. ... 

The chromium carbene may be prepared by addition of a vinyllithium to chromium hexacarbonyl followed by alkylation with a reactive electrophile such as trimethyloxonium tetrafluoroborate (Meerwein s salt, MegO+BF -). 

Chromium(III) oxide, Cr203, is used as a protective coating on metal surfaces and can be prepared by the chemical vapor deposition of Cr(acac)3 (acac 6) in an oxygen atmosphere [44]. Chromium hexacarbonyl, Cr(CO)6 (9) also can be converted into... 

Substitution m- to the methoxy group, established earlier for arene/chromium tricarbonyl systems (ref. 119), has been used to prepare the 1-(2-dithianyl) derivative of estrone (ref.120). A solution of 2-lithio-1,3-dithian in tetrahydrofuran at -78°C treated with the chromium tricarbonyl complex of HO-protected estrol (from the protected steroid with chromium hexacarbonyl by refluxing in... 

Preparation. This chromium complex is usually prepared by the reaction of benzene with chromium hexacarbonyl under high pressure. Rausch has recently published a simple synthesis based on a patent procedure in which equi-volume amounts of benzene and 2-picoline are refluxed for an extended period under nitrogen. Carbon monoxide is evolved, and little sublimation of chromium hexacarbonyl occurs. Yields are 90-94% for reactions conducted for 96 hr. or more. The method is presumably successful because of formation of the 2-picoline complex (A), which reacts with benzene to form (1). 

This organometaUic substance is prepared by the reaction of chlorobenzene with chromium hexacarbonyl in a special apparatus that allows recycling. ... 

Although the chromium hexacarbonyl necessary for this preparation may be obtained by the method described in an earlier synthesis, several improved procedures have been developed recently. Of these, the carbonylation of a mixture of chromium (III) acetylacetonate [2,4-pentanedionato-chromium(III)], magnesium, and pyridine in the presence of catalytic quantities of iodine, or the carbonylation of a mixture of chromium(III) chloride, aluminum chloride,... 

The sodium sand is treated with excess of freshly cracked cyclopentadiene (see preparation of cyclopentadienylvana-dium tetracarbonyl for details) to prepare a red solution of cyclopentadienylsodium. To the resulting solution is added 8.8 g. (0.04 mol) of solid chromium hexacarbonyl, and the mixture is then refluxed with stirring until no unreacted chromium hexacarbonyl, or very little, remains in the reflux condenser (3 to 6 hours). The resulting yellow solution of... 

Chromium-carbene complexes can be prepared from 2- and 3-substituted indolyllithium compounds and chromium hexacarbonyl, followed by methylation. These reactive species combine with alkynes to give cyclization products, which are oxygenated carbazoles. Oxidation to the quinone can be effected with ceric ammonium nitrate (can) where possible, otherwise the product is a cyclohexadiene (Scheme 66) <89JOC3249>. 

Conjugate reduction of enones. The alkali metal carbonylchromates reduce a, -unsaturated carbonyl compounds to the corresponding saturated carbonyl compounds in 4d-807o yield. They are comparable to potassium hydridotetra-carbonylferrate (6, 483-486), but are simpler to prepare because chromium hexacarbonyl is a stable solid and less toxic than iron pentacarbonyl. Examples ... 

Chromium hexacarbonyl has been prepared by the reaction of chromium (III) chloride with phenylmagnesium bromide in the presence of carbon monoxide. " Unsatisfactory yields have been reported and difficulty experienced in repeating the synthesis. Unfortunately, no other methods for the preparation of this substance are recorded in the literature. 

Chromium carbenes can also be prepared by the so-called Semmelhack-Hegedus route. Chromium hexacarbonyl is first reduced to a nucleophilic pentacarbonyl dichromate dianion 23 with sodium naphthalenide or potassium carbide. Reaction of this dianion species with an acid chloride gives a metal alkoxide that can be quenched with an electrophile to provide the desired chromate ester 24. Alternatively, the dianion can be added to an amide carbonyl to give a tetrahedral intermediated which collapses to the chromate amide 25 on treatment with trimethylsilyl chloride. 

One frequently used solvent for the alkali metal reduction of metal carbonyls is liquid ammonia, in which alkali metals form blue solutions. These blue solutions are useful for some of the more drastic reductions, such as that of the very stable octahedral chromium hexacarbonyl to the anion [Cr(CO)s] ". Unfortunately, the reactivity of liquid ammonia toward most of the halide derivatives of interest for reaction with metal carbonyl anions has seriously limited its use in their preparation. Behrens and co-workers, however, have made an extensive study of the reactions of the [Cr(CO)5] anion prepared by the reduction of chromium hexacarbonyl with sodium metal in liquid ammonia these reactions will be discussed later. 

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