Chromium alkoxides

In the nickel(ll)-catalyzed NHK reaction, the first step is the reduction of Ni " to Ni that inserts into the halogen-carbon bond via an oxidative addition. The organonickel species transmetallates with Cr " to form the organochromium(lll) nucleophile, which then reacts with the carbonyl compound. To make the process environmentally benign, a chromium-catalyzed version was developed where a chlorosilane was used as an additive to silylate the chromium alkoxide species in order to release the metal salt from the product. The released Cr " is reduced to Cr " with manganese powder. 

A chromium-catalyzed version has been developed by Fiirstner which makes this process environmentally benign. The key feature of this process uses chlorotrimethylsilane as an additive for the silylation of the chromium alkoxide species 5 in order to release the metal salt from the product 6. The liberated Cr(III)Cl2X can then be reduced to the active species Cr(II)Cl2 by means of a stoichiometric and nontoxic reducing agent as a manganese(O) metal. 

Numerous modifications of chromium-based catalysts have been made through the introduction of various additives, the most effective of which are titanium alkoxides (38,39). These additives apparentiy reduce surface silyl chromate moieties to chromium titanates, which are then oxidized to titanyl chromates. These catalysts offer a better control of the resin molecular weight (39). 

The multi-component procedure is also effective for the chromium-catalyzed addition of organic halides to aldehydes (the Nozaki-Hiyama-Kishi reaction) [73]. The active Cr(II) species is recycled by redox interaction with Mn powder as the stoichiometric co-reductant in the presence of MesSiCl (Scheme 34), which mainly liberates the chromium catalyst from the alkoxide adduct. The chemo- and diastereo-selective addition reaction is performed with a variety of organic halides and alkenyl triflates. In the case of crotyl bromide, the addition is highly stereoconvergent, i.e., the respective anti-... 

Reaction of haloafene chromium tricarboriyl complexes [Cr(CO)3ArX] with alkoxide anions... 

Also non-heteroatom-substituted tungsten [440,443,444,451,452,461], molybdenum [437], and chromium carbene complexes [440] have been prepared by a-abstraction of alkoxide. 

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

Scheme 12 Intramolecular O2 insertion into chromium phenyl bond yield 0x0 alkoxide...

Again several alkyls add—molybdenum, chromium, iron, cobalt, nickel, the alkali metal alkyls and aluminum alkyls react. A tin alkoxide has recently been studied by Russian workers and found to add to acetylenes. Mercury chloride, of course, adds and two cobalt—cobalt bonded compounds add to acetylene. The second is questionable because it dissociates in solution and the reaction may be a radical reaction, one cobalt adding to each end of the triple bond. 

A method which gives a chromium(II)-substituted alkoxide ([Cr(OCBu3)2 LiCl(THF)2], Section 35.3.5.2) directly from CrCl3 is to add a solution of the alcohol previously treated with BuLi to a slurry of the halide in THF.77 How the reduction to chromium(II) takes place has not been clarified. 

The complex [Cr(OCBu3)2 LiCl(THF)2], which was obtained according to Scheme 15,77 has a unique structure. The metal ion is three-coordinate, and the donor atoms outline a distorted T (22). It is not obvious how the reduction occurs because the BuLi is added in stoichiometric quantity to the alcohol and this solution then added to the CrCl3 suspension. Since LiCl is eliminated on the addition of hexane to give [Cr(OCBu3)2(THF)2], Scheme 15 may provide a simpler route to chromium(II) alkoxides than that from the very air- and moisture-sensitive complex [Cr N(SiMe3)2 2(THF)2]. 

The insertion of various isocyanates into chromium(lll) alkoxide M—O bonds has been reported.737 The complexes are prepared by refluxing the isocyanates with a suspension of the alkoxide in benzene. No structural data were given for the products. Unusual bimetallic alkoxides have recently been prepared738 by the reaction of Cr[Al(OPr )4]3 with alcohols and acetylacetone (166). A wide range of spectroscopic methods were used to study them. In general, the results were in accord with a monomeric formulation similar to (166) below Cr[Al(OMe)4]3 was grossly insoluble the small size of the methyl groups may permit extensive polymerization. 

Many polymeric chromium(III) complexes in this general class exist. For the general formula [Cr(L)(0PRR 0)2] , compounds have been prepared involving hydroxide, perfluorocarboxy-late, alkoxide, aryloxide and carboxylates.849,850... 

The stability problems with the tetraalkyls also apply to the tetraalkoxides in addition, the high oxidative power of Cr means that alcoholysis with primary alcohols and most secondary alcohols leads to oxidation to the aldehyde or ketone and the formation of a chromium(III) alkoxide. Relatively stable chromium(IV) alkoxides are obtained only from tertiary alcohols and some heavily substituted secondary alcohols. 

These methods, particularly (2), have been used in the preparation1316 of other chromium(IV) tertiary alkoxides (Table 108). These are blue liquids or low melting solids which can be distilled in vacuum and are monomeric in cyclohexane. Their magnetic moments are close to 2.8 BM and that of Cr(OBu )4 is almost independent of temperature as expected for tetrahedral Cr1 " (d2, pm — 2.83 BM). The electronic absorption spectrum of blue Cr(OBu )4... 

Table 108 Chromium(rV) Alkoxides and Amides Preparation and Properties...

We could show that the modification of transition metal alkoxides is a versatile tool to adjust the reactivity of precursors for the needs in lyotropic crystalline templating processes. In case of high surfactant concentrations where the liquid crystalline template is formed prior to the addition of the precursor the use of a modifier may become unnecessary. The synthesis of nanostructured rhenium dioxide and the utilization of MTO as precursor for this purpose clearly shows that in some cases the use of unusual specialized compounds is imperative. First promising results in the synthesis of nanostructured chromium oxide surfactant composites have been displayed although hydrolysis of the precursor seems to be still uncompleted within the nanostructure. The possibility of tailoring the d-values in a desired way besides the synthesis of certain particle morphologies encourages for further work in the future. 

As indicated in Scheme 27, indoles may be alkylated by their acid-catalyzed reaction with alcohols. Similarly, r-butylation of pyrroles has been effected by the acid-catalyzed reaction with t- butyl acetate (B-77MI30502), and the diarylmethylation of 1-methylpyrrole from the acid-catalyzed reaction with the chromium trichloride complex of the diarylcarbinol has been described (78JA4124). The alkylation of indoles by alcohols in the presence of the aluminum alkoxide and Raney nickel appears to be efficient for the synthesis of 3-substituted indoles, but is less successful in the alkylation of 2-methylindole (79JHC501). The corresponding isopropylation of pyrrole produces 2,5-diisopropylpyrrole and 1-isopropylpyrrolidine, as the major products (79JHC501). 

The active site concentration on the organochromium catalysts may be higher than that of the oxide catalysts. The activity usually assumes a more linear increase with chromium loading than on the oxide catalysts, at least up to 2% Cr. Yermakov and Zakharov, studying allyl-Cr(III)/silica catalysts, stopped the polymerization with radioactive methanol, and found that the kill mechanism is different from that on the oxide catalysts (59). The proton of the methanol, and not the alkoxide, became attached to the polymer. This suggests a polarity opposite to that of the oxide catalysts, with the site being more positive than the chain. 

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