Alkali Metal Based Catalysts

While hydroamination catalysts based on transition metals have been studied intensively over the past two decades, only a limited number of reports on alkali metal based hydroamination catalysts have emerged, although the first reports date back 60 years [71]. In particular, the application of chiral alkali metal complexes in asymmetric hydroamination of nonactivated aminoalkenes has drawn little attention to date [72, 73]. Also, attempts to perform asymmetric hydroamination utilizing 

The chemistry of organometallic group 4 metal compounds is well developed, thanks to their importance in polyolefin synthesis. Hence, their application in catalytic asymmetric hydroamination reactions is highly desirable. Group 4 metal complexes are commonly less sensitive and easier to prepare than rare earth metal complexes. Most important of all, many potential precatalysts or catalyst precursors are com mercially available. 

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Crown Ether/Alkali Metal Base Catalysts. 155... 

However, certain problems occur in the utilization of alkali metal-based catalysts. Especially potassium is deactivated by reactions with silicates of the coal and the nondeactivated part remains hard to recover. But corrosion and fouling problems initiated with alkali metals are the main arguments that prevent alkali metals from commercial breakthrough [27,30]. 

The simple lithium amide LiHMDS catalyzes the addition of aliphatic and (notably) aromatic amines to vinyl arenes [40]. The catalytic activity is increased by addition of TMEDA and the reaction can be carried out in bulk without additional solvent. More reactive primary aliphatic amines also form a bis-hydroamination product, although the formatiOTi of the latter may be suppressed by using an excess of amine (21). Less reactive aromatic amines and a- and p-substituted styrenes give the monohydroamination adducts selectively [40]. Other readily available alkali metal-based catalysts include NaH [166], t-BuOK [164, 167, 168] and CsOH [169]. 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

Starch can be vinylated with acetylene in the presence of potassium hydroxide in an aqueous tetrahydrofuran medium.1 1 The mechanism possibly involves the addition of the potassio derivative of starch across the carbon-carbon triple bond of acetylene, with subsequent hydrolysis of the organometallic intermediate to give the vinyl ether. Such a mechanism has been postulated for the formation of vinyl ethers from monohydric alcohols and acetylene, in the presence of an alkali metal base as catalyst.1 2 The vinylation of amylose is very similar to the vinylation of amylopectin, except for the relative ratio of mono- to di-substitution. With amylopectin, the proportion of disubstitution is greater. In both starches, the hydroxyl group on C-2 is slightly more reactive than the hydroxyl group on C-6 there is little substitution at the hydroxyl group on C-3. 

The promoting effect of the addition of alkali on the catalytic performance of many transition-metal-based catalysts is experimentally well known, but there is no general agreement on its theoretical explanation. The same holds for the opposite effect the poisoning of catalysts by, e.g., the adsorption of sulphur. 

Scheme 11.6 Proposed mechanism for the hydroamination/cyclization of aminoalkenes using alkali, alkaline earth, and rare earth metal based catalysts.

Group 4 metal based catalysts have been studied intensively in hydroamination reactions involving alkynes and allenes [77 81], but (achiral) hydroamination reac tions involving aminoalkenes were only recently reported [82 84]. The reactivity of these catalysts is significantly lower than that of rare earth, alkali, and alkaline earth metal based catalysts. In most instances, gem dialkyl activation [37] of the aminoalk ene substrate is required for catalytic turnover. 

The anionic polymerization of cyclosiloxanes is a complex process. For the alkali metal silanolate catalysts the weight of experimental evidence supports a mechanism based on growth from the metal silanolate ion pair. The ion pair is in dynamic equilibrium with ion-pair dimers which, for the smaller alkali metal ions like lithium and sodium, are themselves in dynamic equilibrium with ion-pair dimer aggregates. The fractional order in catalyst which is observed is a direct result of the equilibria between ion pairs, ion-pair dimers and ion-pair dimer aggregates. Polar solvents break down the aggregates and increase the concentration of ion-pair dimers and hence the concentration of ion pairs. Species like crown ethers and the [2.1.1] cryptate which form strong complexes with the metal cation increase the dissociation of ion-pair dimers into ion pairs. In the case of the lithium [2.1.1] cryptate dissociation into ion pairs is complete and the order in catalyst is unity. 

The condensation catalyzed by a strong base is first order with respect to substrate and catalyst (74,75). Because of the high acidity of silanol, all the alkali metal base (MtOH) is usually transformed into the silanolate anion. In the rate-determining step, the silanolate anion attacks the silicon atom in the silanol end group (eq. 12 and 13). 

Pearson also reported that the reaction over silica-supported alkali metal hydroxide catalysts is promoted markedly by the presence of water in the range of water/HCHO molar ratio = 1 to 5. With an acetic acid/HCHO/water molar ratio of 4.9/1/2.7, an SV of 750 h and a temperature of 405 °C, the yield of acrylic acid reaches 41.4 mol% based on the charged HCHO (8.5 mol% based on acetic acid) at the conversion of 53% the selectivity to acrylic acid is 78 mol% based on HCHO. 

Yamazaki and Kawai reported a study on the reaction of HCHO with acetonitrile or propionitrile using silica-supported metal salts or hydroxides as catalysts. Formalin is used as the source of HCHO. The performances are summarized in Table 15. It is concluded that silica-supported alkali metal hydroxide catalysts show the best performances. The optimum loading of alkali metals is in the range of 0.01 to 0.1 mol/60 g of silica gel. The optimum reaction conditions are nitrile/HCHO molar ratio of 5, temperature of 500 °C, and contact time of 2.5 x 10 s-g-cat/mol. The single-pass yields of acrylonitrile and methacrylonitrile are 75 and 65 mol%, respectively, based on the charged HCHO (25 and 22 mol% based on the charged nitrile) with a nitrile/HCHO molar ratio of 3. The reaction rate is first order with respect to the concentrations of both nitrile and HCHO. 

By virtue of a deep understanding of his LnM3tris(BINOLate)3 complexes (Ln = rare-earth metal, M = alkali metal) based on evidence from X-ray analysis and other experiments, Shibasaki developed chiral heterobimetallic yttrium(in) lithium(i) tris(binaphtholate) complex 22, which can promote the catal) ic enantioselective aza-Michael reaction of metho g lamine to enones in excellent yields with up to 97% ee as a Lewis-acid-Lewis-acid cooperative catalyst (Scheme 2.17). Transformation of the 1,4-adducts 23 afforded the corresponding optically active aziridines 24 in high yields. 

Synthetic catalysts are mainly Ni-based, noble metal-based, alkali metal-based, or inorganic oxides (Cr, Fe, Cu, or Co) (Navarro et al., 2007 Huber et al., 2006 Basu,... 

The esterification reaction may be carried out with a number of different anhydrides but the literature indicates that acetic anhydride is preferred. The reaction is catalysed by amines and the soluble salts of the alkali metals. The presence of free acid has an adverse effect on the esterification reaction, the presence of hydrogen ions causing depolymerisation by an unzipping mechanism. Reaction temperatures may be in the range of 130-200°C. Sodium acetate is a particularly effective catalyst. Esterification at 139°C, the boiling point of acetic anhydride, in the presence of 0.01% sodium acetate (based on the anhydride) is substantially complete within 5 minutes. In the absence of such a catalyst the percentage esterification is of the order of only 35% after 15 minutes. 

A three-step process developed hy Snamprogetti is based on the reaction of acetylene and acetone in liquid ammonia in the presence of an alkali metal hydroxide. The product, methylhutynol, is then hydrogenated to methylhutenol followed hy dehydration at 250-300°C over an acidic heterogeneous catalyst. 

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