Organochromium reaction

Intramolecular alkenylchromium additions to aldehydes have been repotted and are outlined in Section 1.6.4.3. An intriguing example of an intramolecular organochromium reaction has been communicated by Gorques and coworkers. Treatment of (130 Scheme 5) with CrCh in HMPA elicits a reductive 1,5-acyl transposition and generates chromium phenoxide (131). Wodc-up with aqueous ammonium chloride affords phenol (132). However, when the reaction occurs in the presence of boron trifluoride etherate benzo[h]furan (133) is obtained directly in excellent yield. 

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). 

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

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... 

Many electron acceptors are able to oxidize transient complexes with metal-carbon -bonds. The reaction of organochromium(III) species ((H20)5Cr-R2 + and L(H20)Cr R2 +, L =1,4,8,12-tetraazacyclo-pentadecane) have been studied for the acceptors Ru(bpy) +, 2E-Cr (bpy) +, Ni([14]aneN4)3 +, and IrCl (132,133), and proceed according to the following general equation ... 

The [Cr(0H2)6]2+/H202 reaction is important in the synthesis of air-stable cationic organochromium(III) complexes (see Section III,B), as the initially produced OH radicals react rapidly with added organic substrates to form alkyl radicals, which, in turn, react with [Cr(OH2)6]2+ (15), (16). 

The observation that reaction (13) is the slowest step, and that the organochromium complexes are highly colored with peak maxima at 300 (2500) and —400 (—300), provides a means of determining rate constants for (13) (18). 

Reaction (49) can be suppressed by addition of [Cr(OH2)6]2+, or it can be driven at the limiting rate by adding a scavenger to eliminate [Cr(OH2)6]2+ or -R. As a consequence, the first-order rate constant for (49) remains independent of the nature and concentration of the scavenger. Thus, these organochromium(III) alkyls can be regarded as a source of stored free radicals, but practical applications such as polymerization initiation are only in their infancy. 

One of the more recent reactions of organochromium(IH) alkyls to be studied is oxidation (50) to a Cr(IV) analogue. 

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). 

In contrast, the other organochromium catalysts, which terminate mainly by -elimination, produce extremely broad MWD polyethylene. In fact, the range of products is so broad that in addition to high polymers, a good portion of the product is also oligomeric. The activity is not diminished by hydrogen. Side reactions must occur easily on such catalysts because the polymers frequently are considerably branched (all types) and have some internal unsaturation as well. 

Diaryliodonium salts 98 react with aldehydes 99 in the presence of chromium dichloride and nickel dichloride with the formation of benzyl alcohols 100 (Scheme 44) [74]. The mechanism of this reaction involves the generation of organochromium(III) species via reaction of iodonium salts with chromium dichloride, followed by their nucleophilic addition to aldehydes to yield alcohols. 

Once formed in reaction 8.52, CH3 will be captured quantitatively by Craq2+ to form the organochromium complex ... 

Organochromium species containing tt-bonded ligands were first prepared by Hein as early as 1919, but were not definitively characterized for another 35 years. Hein reported on the reaction of PhMgBr with CrCb to give Chromorganoverbindungen this synthesis was reproduced by Zeiss and Tsutsui in 1954, at which time they identified the products as bis(arene)chromium cations. Shortly thereafter, Fischer and Haftier published their route to neutral bis(benzene)chromium. This early work has been chronicled, most recently by Seyferth. ... 

The derivatives of chromium hexacarbonyl, Cr(CO)e (L)j , make up the single largest class of organochromium compounds, and a substantial number of these are Cr compounds as well. The most common synthetic route involves direct reaction of Cr(CO)6 and x L, but displacement of a weakly bound hgand from a carbonyl derivative is also frequently employed. Carbonyl dissociation is usually promoted with heating or UV irradiation. 

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