Chromium, allylic

Substituents in the allyl group of a catalyst have a marked effect on the polymerization efficiency (9,12). This is shown in Table IV for the polymerization of ethylene with chromium and zirconium allyls and for the polymerization of methyl methacrylate with chromium allyls. Introducing a methyl group into the allyl ligand increases the activity by a factor of 2 to 7. In some polymerizations of ethylene Cr(2-Me-allyl)3 compounds are ten times more effective than the simple allyl derivatives. The introduction of... 

Zr(allyl)3Br gives mainly, but not exclusively, a center of type (XV). This follows from the observation that reaction of Zr(allyl)3Br with silica gives two molecules of propene per metal atom and no halogen is liberated. Addition of excesses of n-butanol to the SiCb/Zrfallyl Br reaction product however gives one molecule of propene per metal atom and one molecule of HBr per metal atom is liberated with excess benzoic acid solution. The structure of (XVI) was determined in a similar manner. Chromium allyls give transition metal centers with structure (XVII). 

The initial rate of polymerization of methyl methacrylate initiated by chromium allyls (12) in toluene showed identical dependences on monomer and catalyst concentrations, as Zr(benzyl)4 initiated polymerization of styrene. Some data for the monomer dependence are shown in Fig. 14. 

The mechanism proposed for the polymerization of styrene initiated by Zr (benzyl) 4 differs from that proposed for the polymerization of methyl methacrylate initiated by chromium allyls (12, 44)- In these papers it was considered that the concentration of complex (I) was comparable to the catalyst concentration, a fact which now seems unlikely in view of the... 

One is left to ponder initiation by other organochromium catalysts. Chromium allyls or 2,4-dimethylpentadienylchromium(II) could conceivably rearrange into p-l coordination upon addition of ethylene. However, chromocene must initiate the first chain in some other way, because the site must retain the ring. Thus, for chromocene catalysts, the initiation problem is similar to that described for chromium oxide. The diarene-chromium(O) and Cr(0)(CO)6 catalysts may also have this problem. Perhaps this is why these catalysts sometimes initiate polymerization more sluggishly than the chromium alkyls. However, there is also some evidence that the Cr(0) compounds can be oxidized by surface OH groups to leave a Cr-H group, which could also be considered an alkylated species. 

The data of Table 55 show how the polymer composition varied with activation temperature. Such observations have been reported from this and other laboratories for catalysts made with several different organo-chromium compounds [301,640,644,654], and most recently by Bade et al. [311], who used chromium allyl to make their catalyst. Presumably, the calcination temperature of the silica resulted in the formation of two very different species. Cr(DMPD)2 reacted with silica treated at 250 and at 400 °C to yield di-attached or coordinated species, whereas it reacted differently with silica treated at 600 °C, because on that support only a single oxide attachment can form. Clearly, the higher OH group population has a major effect on the behavior of the site. 

However, more recently Bade and coworkers [49] reported that the deposition of Cr(2-Me-allyl)j onto silica forms an active catalyst for ethylene polymerization in which the silica dehydration temperature used to dry the silica prior to the deposition of the chromium allyl affects the catalyst activity, the molecular weight of the polyethylene and the density of the polyethylene produced with the finished catalyst. 

Bade et at postulate that the primary reaction products formed from the reaction of chromium allyls with the surface hydroxyl groups on dehydrated silica are shown in Figure 3.41 A and B. In addition, chromium allyls may also react with a siloxane linkage with silica dehydrated at 800°C, and this reaction is shown in Figure 3.41C. 

The formation of an epoxyketone (1) is generally favoured when the expected product of oxidation of an allylic alcohol is a cisoid enone. This type of reaction is promoted by acid conditions and may be prevented by using the chromium trioxide-pyridine reagent which gives only the unsaturated ketone (2) corresponding to the starting alcohol. ... 

Snatzke has found that a solution prepared from chromium trioxide and dimethylformamide with a small amount of sulfuric acid has similar chemical properties as the Sarett reagent. It is useful with acid sensitive compounds and oxidation occurs at such a moderate rate that selective oxidations are often possible. Although the position allylic to a A -double bond is not attacked, the 3-hydroxy-A -system cannot be oxidized satisfactorily to the cor-... 

Chromium-pillared clay, -BuOOH, CH2CI2, 10 h, 80% yield. Simple allyl ethers are cleaved to give ketones, and allylamines are also depro-tected (84-90% yield). 

Cephachlor (35) became accessible when methods for the preparation of C-3 methylenecephalosporins became convenient. The allylic C-3-acetoxyl residue characteristic of the natural cephalosporins is activated toward displacement by a number of oxygen- and sulfur-containing nucleophiles. Molecules such as can therefore be prepared readily. Subsequent reduction with chromium(II) salts leads to the desired C-3... 

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-... 

The condensation is usually carried out by adding a solution containing equimolar amounts of the allyl halide and the aldehyde or ketone to a solution of at least two equivalents of chromium-(II) chloride in THF at 0 5°C. Frequently, the less precious component is used in 50-100% excess. Although commercially available anhydrous chromium(II) chloride can be utilized (Method B), its in situ preparation from chromium(III) chloride and lithium aluminum hydride (Method A) is often preferred. The removal of chromium and aluminum hydroxide, which are formed on aqueous workup, can be accomplished by filtration in the presence of a filtration aid. 

Chromium(II) Chloride Mediated Addition of Allyl Bromides to Aldehydes General Procedure12 ... 

A mixture of 10 mmol of the allyl bromide and 10-15 mmol of the aldehyde, dissolved in 20 mL of THF, is added dropwise at — 5 to 0°C to the chromium(II) chloride solution in THF prepared by method A or B. The mixture is stirred for 36 h at this temperature and then 15 mL of sat. sodium hydroxide and 20 g of anhyd Na2S04 are added stirring is continued for 20 min at 201C. The mixture is filtered over a pad of Celite/Na2S04 (7 l). The filtrate is concentrated and the residue purified, usually by chromatography on silica gel with pentane/diethyl ether or hexane/ethyl acetate. 

A chiral nonracemic cyclic allyl iodide was shown to react with excess chromium(II) chloride and (4-methoxyphenylmethoxy)acetaldehyde to yield a single diastereomer, which was converted to la,25-dihydroxy vitamin D332. 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

Some data allow the realization of the second possibility to be proposed (8, 140) at least in the case of catalysts formed with the use of 7r-allylic compounds of chromium. In the reaction of Cr(7r-C3H3)2 with the Si02 surface, complexes are formed that may have vacant coordination sites... 

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