Organochromium compound

This is a rapidly developing area of Cr chemistry, and the two main classes discussed here are the chromium(0) pentacarbonyl carbenes (56) and the chromium(III) alkyls (57). 

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. 

Irradiation of these complexes with visible light results in CO insertion to produce a species with ketene-like properties that can react with olefins, imines and alcohols (Fig. 6). When the photolysis is performed in a CO atmosphere, Cr(CO)6 is regenerated. 

To obtain amino acids, amino carbenes (X = NH2 in Fig. 6) must be used, and if chirality is required, the asymmetry of the new a-carbon center must be controlled. Higedus and co-workers have achieved good stereochemical control by using a chiral oxazolidine chromium carbene. 

The resulting product is, however, an N-substituted oxazolidine amino acid, but this can easily be removed by either reductive or oxidative (Fig. 7) procedures. These and similar reactions can be used to prepare y-hydroxy-a-amino acids, arylglycines, a-alkyl-a-amino acids, and a variety of nonproteinogenic dipeptides (Fig. 7). The review by Muzart (58) is an excellent source of further information on the use of Cr compounds in organic synthesis. 

---

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

Around 1800, the attack of chromite [53293-42-8] ore by lime and alkaU carbonate oxidation was developed as an economic process for the production of chromate compounds, which were primarily used for the manufacture of pigments (qv). Other commercially developed uses were the development of mordant dyeing using chromates in 1820, chrome tanning in 1828 (2), and chromium plating in 1926 (3) (see Dyes and dye intermediates Electroplating Leather). In 1824, the first chromyl compounds were synthesized followed by the discovery of chromous compounds 20 years later. Organochromium compounds were produced in 1919, and chromium carbonyl was made in 1927 (1,2). 

Halide always appears in the coordination sphere of Cr(m) immediately following reduction. Castro and Kray propose that the organochromium compound is formed by subsequent attack of R- upon further Cr(ll). 

Sneedon, R. P. A. Organochromium Compounds, Academic Press New York, 1975. 

Organochromium compounds with various coordination modes of the ligand, namely CpCr(CO)2S2P(OPr )2 (bidentate), CpCr[S2P(OPr )2]2 (one bident-ate and one unidentate) and Cr[S2P(OlV)2]3 (bidentate) are formed in reactions of [CpCr(CO)3]2 with (Pr 0)2(S)PSSP(S)(0Pr )2. Variable temperature NMR shows rapid interchange between bi- and unidentate ligands in solutions of CpCr[S2P(OPr )2]2.61... 

Hexavalent chromium (Cr+6) is the most biologically active chromium chemical species, although little is known about the properties of organochromium compounds, water-soluble species, or their interactions in complex mixtures... 

Nucleophilic addition of organochromium compounds to carbonyls or imines has been well investigated.212-214... 

Introduction.—The reference literature for organochromium compounds has been supplemented. 

Gmelin s Handbook of Inorganic Chemistry, supplementary work, vol. 3. Organochromium Compounds, 8th Edn., Gmelin Institute, 1971. 

The extensive organometallic chemistry of chromium, i.e. the hexacarbonyl and its derivatives, organochromium compounds without carbonyl ligands, cyanide and isocyanide complexes, alkene, allyl, diene, cyclopentadiene and arene derivatives, and complexes of a-donor carbon ligands, has been recorded in Chapters 26.1 and 26.2 of Volume 3 of Comprehensive Organometallic Chemistry .1 In the present section, chromium complexes... 

It is curious that during 30 years of interminable debate about valence, almost no mention has been made of organochromium compounds that also make active catalysts. As early as 1961 Walker and Czenkusch at Phillips showed that diarene-Cr(O) compounds polymerize ethylene when deposited on silica or silica-alumina (51). We now suspect that the Cr(0) is oxidized by silanol groups to Cr(I), implying that Cr(I) is also an active valence. Such catalysts, however, do not resemble Cr(VI)/silica. The kinetics and polymer obtained are entirely different. 

Many other organochromium compounds have since been synthesized and found to be active, including those with chromium exhibiting every valence up to Cr (IV). Chromocene is a well-studied example of an active divalent compound (52-55). Pentadiene-Cr(II) (56) is another, along with allyl-Cr(II) (52, 57). Allyl-Cr(III) is also active (52, 57-61). -Stabilized alkyls of Cr(II) and Cr(IV) such as trimethylsilylmethyl-Cr(IV), which also polymerizes ethylene when supported on an oxide carrier, have been synthesized and tested in this laboratory (57,62). All these organochromium catalysts are comparable in activity to the Cr(VI)/silica standard. 

It is incorrect to regard only one particular valence state of chromium as the only one capable of catalyzing ethylene polymerization. Active catalysts have been made from organochromium compounds with every valence from Cr(I) to Cr(IV). On the commercial Cr(VI)/silica catalyst the predominant active valence after reduction by ethylene is probably Cr(II), but other states, particularly Cr(III), may also polymerize ethylene under certain conditions. 

A number of organochromium compounds also form highly active polymerization catalysts when deposited on an oxide carrier. Usually the carrier does play an essential role, because without it such compounds rarely exhibit any activity. In most respects the organochromium catalysts are quite different from their oxide counterparts. 

Although organochromium catalysts are not well characterized, organochromium compounds are thought to bind to the support by reaction with surface hydroxyls as other types do. When Cr(allyl)3 or Cr(allyl)2 is used, propylene is released (59,60). Chromocene loses one ring (52-55), and / -stabilized alkyls of chromium lose the alkane (81). 

Many of the organochromium compounds exist as dimers, e.g. diallyl-Cr(II), and one exists as a tetramer i.e., Cr(II)4tmsm8 (82,83). This is interesting in view of the assertion by some (2) that paired chromium is necessary for polymerization. In fact, neither species is very active for ethylene polymerization until it has been supported on a carrier. The monomeric organochromium compounds behave in about the same way. It seems likely that these polymeric chromium compounds react with the support to form isolated monomeric surface species, thus becoming coordin-atively unsaturated. When monomeric compounds react with the surface the loss of a ring or other multicoordinate ligand probably also leaves vacancies in the coordination sphere. 

The valence of the starting organochromium compound has been varied from Cr(0) to Cr(IV), but seems to make little difference. All species are quite active, and all initiate polymerization rapidly in comparison to the oxide catalysts. There is no induction time, since the chromium is already reduced, and no gradual rise in rate. Polymerization usually starts immediately on contact with ethylene and either holds steady or slowly declines during a 1 hr run. 

The earlier synthesis of chromium(III) chloride1 has been used by graduate students at Chapel Hill for a decade with varying degrees of success. The alternate synthesis, herein described, has been found to be more reliable in the hands of graduate students. In particular the product has been used as the starting material in satisfactory syntheses of organochromium compounds. 

---