Active metals acetals

On the other hand, Sb203 and metal oxides such as Ge02 exhibit a good catalytic activity for the polycondensation step. This explains why the associations of metal acetates with Sb203 are often reported catalytic systems. The following order in catalytic activity was found for the PEN polycondensation step Ti(IV)... 

In addition, no DNA cleavage was observed with 19 under silver acetate activated conditions, also consistent with Nozaki s [78] observation that this metal acetate produced little of this dimerization product. The DNA cleavage observed with high concentrations of silver acetate and no cleavage with... 

The preparation of an activated metal for use in an organometallic synthesis can be effected in a separate sonication step. An early example of this was the preparation of finely dispersed mercury for the reduction of a,a -dibromoketone to a mixture of a-acetoxyketones which favours the tertiary product (Eq. 3.6) [71]. The procedure is simple in that the dibromocompound, dissolved in acetic acid is subjected to sonication in a bath in the presence of a small amount of mercury. In this particular case... 

Among other things, Pd-Au coated catalysts are extremely well suited to the catalysis of the gas phase oxidation of ethylene and acetic acid to give vinyl acetate. The catalytically active metals are deposited in the form of a shell on or in the outermost layer of the support. They are often produced by penetration of the support with metal salts into a surface region and subsequent precipitation by alkalis to form water insoluble Pd-Au compounds (5). 

Some experiments were conducted with p-toluenesulfonic acid and acetyl borate as catalysts, using the same concentration range as for the metal acetates, to test their catalytic action in the reaction of acetaldehyde with peracetic acid. The results, although they did not rule out some catalytic activity, clearly indicated that these acids are much less active catalysts than manganese and cobalt acetates. 

There are a number of alternative approaches to lability measurements, and three which involve different timescales are listed in Table 2.8. The level of labile electro-active metal species present in aqueous samples can be determined using ASV (see Section 2.7.4). The electrochemical response (i.e. peak area values) reflects the hydrated metal ion content together with contributions from any metal complexes present which rapidly dissociate in the diffusion layer (around the mercury cathode). The labile metal is measured either at the natural sample pH, or after pH adjustment (e.g. after addition of pH 5 acetate buffer). 

Effect of strong acids. The rates of oxidation of aromatic hydrocarbons by metal acetate oxidants are also dramatically enhanced in the presence of strong acids. It has recently been reported275 that Mn(III) and Co(III) acetates in the presence of strong acid activators, such as trichloroacetic or trifluoroacetic acid (TFA), rapidly and selectively oxidize aromatic side chains at 25°C. 

Enhancement by strong acids such as TFA is a general feature of oxidations with metal acetates. Metal trifluoroacetates in TFA are much more powerful oxidants (electrophiles) than the corresponding acetates in acetic acid. Activation of the metal oxidant in TFA has been observed with co-balt(III)217 249,259,27S 276 manganese(III),237,275 lead(IV),277-281 thallium-(III),282-287 cerium(IV),288 289 and copper(II).290 Similarly, the electrophilic properties of copper(I)291 and mercury(II)292 acetates are strongly enhanced by replacement of acetate by trifluoroacetate. It has been proposed217,276 that the potent oxidizing properties of Co(III) trifluoroacetate are due to ionization to the cationic Co(III) species,... 

The catalyst plays a crucial role in technology. Previously, catalysts were based on palladium of 1 to 5 wt% impregnated on silica with alkali metal acetates as activators. Modern catalysts employ as enhancers noble metals, mostly gold. A typical Bayer-type catalyst consists of 0.15-1.5 wt% Pd, 0.2-1.5 wt% Au, 4-10 wt% KOAc on spherical silica particles of 5 mm diameter [14], The reaction is very fast and takes place mainly inside a thin layer on the particle surface (egg-shell catalyst). 

A similar mechanism might operate in the activation of an azolium salt by a transition metal compound forming the metal carbene complex. However, since a basic substituent on the metal (acetate, alkoxide, hydride) usually reacts with the H -proton, the proton is removed from the reaction as the conjugate acid and reductive elimination does not occur. 

To further optimize the performance of metal catalysts, promoters are often added. Textural promoters work by separating metal particles from one another to minimize sintering, whereas electronic and structural promoters change the electronic or crystal structure of the active metal. For instance, a small amount of potassium acetate is added to... 

Raney nickel can be purchased in a form which is ready for use (W-2), or can be prepared by published procedures which furnish catalysts with a range of activities. The increase in selectivity of Raney Ni catalysts upon aging apparently results from the formation of CO by the decomposition of the ethanol under which the catalyst is usually stored. Highly regio- and stereo-selective catalysts are prepared as slurries by the reduction of the metal acetates, and are used in situ or stored for later use. P-2 nickel is prepared by reducing an aqueous alcoholic solution of Ni(OAc)2 with NaBH4. More recently, highly selective Ni and Pd catalysts were prepared by the reduction of Ni(OAc)2 or Pd(OAc)2 with NaH and f-pentyl alcohol in THF. ... 

Active metals readily cleave a-ketols and a-ketol acetates. Reductions can be carried out with the aid of lithium, barium or calcium in ammonia, or with zinc or tin, which are usually used in acidic media. Zinc is a relatively mild reducing agent and is therefore somewhat selective. Axial steroidal ketol acetates are reduced more readily than equatorial (equations 16 and 17). On the other hand, metal-ammonia systems are powerful reductants thus, calcium and barium reduce axial and equatorial isomeric ketol acetates with equal ease. Lithium is a more powerful reductant and frequently overreduces a-ketols... 

Yoda [59] showed that in a series of metal acetates, those salts of metals having an electronegativity [60] lying between 1.0 and 1.7 (that is, calcium, manganese, zinc, cadmium, lead and cobalt) were most catedytic-ally active. 

But how ubiquitous actually are alkalis in the promotion of reactions catalyzed at metal surfaces An examination of recent authoritative sources [6,7] shows that the majority of medium-to large-scale processes do not employ alkali promoters, even when one includes nonmetallic (i.e., metal oxide) catalysts. In a number of cases (e.g., steam reforming of naphtha) it seems clear that the role of alkali is simply to reduce the acidity of the oxide support. There are famous cases, of course, where the presence of alkali species on the catalytically active metal surface is critically important to the chemistry. Notable are ethene epoxidation (Ag-Cs), ammonia synthesis (Fe-K), acetoxylation of ethene to vinyl acetate (Pd, Pd/Au-K), and Fischer-Tropsch synthesis (Fe, Co, Ru-K). The first three are major industrial... 

Allyiic halides, alcohols, ethers, acetates, lactones, phosphates, epoxides, sulfides, sulfonium salts, se-lenides and ammonium salts undergo transition metal catalyzed coupling reactions with C(sp )—Li, —Mg, —B, —Al, —Sn, —Zt, —Cd and — Hg reagents. Table 1 summarizes the allyiic leaving groups, alkenyl and aryl metallic reagents, catalytically active metals and references and Table 2 the regio- and stereo-chemical aspects. 

In the present study, a series of composite photocatalysts were synthesized by stepwise ion exchange and intercalation of transition metal acetates into fibrous layered tetratitanate microcrystals (K2TL4O9) and subsequent calcination in different atmospheres. The highest photocatalytic activity obtained for the CuOx-composites was studied in relation to the microstructure. 

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