Organochromium

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. 

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

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). 

The normal preparation of organochromium(0) compounds is iadirect. First the organochromium (I) compound is formed... 

In 1983, Nozaki, Takai, Hiyama, and their coworkers disclosed that vinyl and aryl iodides or bromides are reduced with chromium(n) chloride, and that the resulting organochromium(in) species react smoothly with a host of aldehydes to give allylic or benzylic alcohols in excellent yields.6 As shown in Scheme 1, the chromium(n) chloride-mediated carbonyl addition can be conducted efficiently at... 

Allylic bromides can also serve as progenitors for nucleophilic organochromium reagents. An elegant example is found in Still and Mobilio s synthesis of the 14-membered cembranoid asperdiol (4) (see Scheme 2).7 In the key step, reduction of the carbon-bromine bond in 2 with chromium(n) chloride in THF is attended by intramolecular carbonyl addition, affording a 4 1 mixture of cembranoid diastereoisomers in favor of the desired isomer 3. Reductive cleav-... 

Homolytic and free radical pathways in reactions of organochromium complexes. J. H. Espenson, Prog. Inorg. Chem., 1983, 30,189-212 (85). 

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... 

A more elaborate organochromium method for the generation of 1-substituted isobenzofurans was also reported. As can be seen below, treatment of the o-alkynylbenzaldehyde with the Fischer chromium carbene complex provides the isobenzofuran-Cr(CO)3 complex 128 which can be trapped by the electron-deficient Ai-phenylmaleimide with excellent exo-selectivity. . 

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... 

Nozaki-Hiyama-Kishi (NHK) reactions215,216 are well known and often employed as a useful method for the synthesis of natural products by coupling of allyl, alkenyl, alkynyl, and aryl halides or triflates with aldehydes. The organochromium reagents are prepared from the corresponding halides or triflates and chromium(ll) chloride, and are employed in polar aprotic solvents (THF, DMF, DMSO, etc.). Subsequently, it was found that nickel salts exhibited a significant catalytic effect on the formation of the C-Cr bond217,218 (Equation (19)). 

Mercury(II) reacts with organochromium complexes by electrophilic substitution. Rate constants have been reported for Hg2+ attack at a series of alkylchromium complexes with the macrocyclic ligand 1,4,8,11-tetrazacyclotetradecane, CrR(H20)([14]aneN4). The Hammett relationship established for a series of meta and para substituted benzyl analogues is consistent with attack of the Hg2+ at a-carbon (89). 

Figure 1. Plot of AGM8 for the homolysis of organochromium cations vs. AG for the homolysis of the correspondingly substituted ethanes (with OH replaced by CHS). Key for organochromium ions O, CrC(RfR )OH2+ , CrCfRJt fOR"2 V, CrCH(CHs)t2 and +, CrCHJPh2. ...

Organochloroaluminate ionic liquids as catalysts, 42 495-496 Organochromium catalysis attachment to support, 33 92-93 kinetics of polymerization, 33 93 support effects, 33 94-95 termination mechanism, 33 93-94 Organochromium catalysts, 33 58, 92-95... 

Fractional orders such as 3/2 often hint at a chain mechanism. The autoxidation of (H20)5CrCH(CH3) + leads to a number of products. Log (inititd rate) vs log (initial concentration of organochromium cation) plots give a 3/2 slope. The rate is independent of [H+] and [O2] and the rate law is therefore... 

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. 

Organoboration, of iminoboranes, 31 155-156 Organoboron alJcyls, 2 75 Organochromium alkyl compounds, 44 354-357 carbenes, 44 352-354 compounds, 44 344... 

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