Other Carbonyl Catalysts

Four mechanisms, which are not mutually exclusive, have been proposed to account for this (Dakshinamurti and Lai, 1992 Dakshinamuiti, 2001)  

Central effects on blood pressure regulation as a result of decreased synthesis of brain GABA and serotonin (5-hydroxytryptamine). Glutamate decarboxylase activity in the nervous system is especially sensitive to vitamin Bg depletion, possibly as a result of mechanism-dependent inactivation by transamination. Although there is no evidence that aromatic amino acid decarboxylase activity is reduced in vitamin Bg deficiency, there is reduced formation of serotonin in the central nervous system. 

Increased sympathetic nervous system activity. There is evidence of elevated plasma concentrations of adrenaline and noradrenaline in vitamin Bg-deficient animals. 

Increased uptake of calcium by arterial smooth muscle, leading to increased muscle tone, and hence increased circulatory resistance and blood pressure. This could reflect increased sensitivity of vascular smooth muscle to calcitriol (vitamin D) action in vitamin Bg deficiency the membrane calcium-binding protein is regulated by vitamin D, and vascular tissue has calcitriol receptors. 

Increased end-organ responsiveness to glucocorticoids, mineralocorti-coids, and aldosterone (Section 9.3.3). Oversecretion of (and presumably also enhanced sensitivity to) any of these hormones can result in hypertension. Vitamin Bg supplementation would be expected to reduce end-organ sensitivity to these hormones, and thus might have a hypotensive action. 

Four mechanisms, which m e not mutually exclusive, have heen proposed to 

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Although not as popular as with other carbonylation catalysts, biphasic conditions (particularly with phase transfer catalysts) may be employed to accomplish carbonylation under very mild conditions for benzylic and vinyl bromides389. 

Acetylene is condensed with carbonyl compounds to give a wide variety of products, some of which are the substrates for the preparation of families of derivatives. The most commercially significant reaction is the condensation of acetylene with formaldehyde. The reaction does not proceed well with base catalysis which works well with other carbonyl compounds and it was discovered by Reppe (33) that acetylene under pressure (304 kPa (3 atm), or above) reacts smoothly with formaldehyde at 100°C in the presence of a copper acetyUde complex catalyst. The reaction can be controlled to give either propargyl alcohol or butynediol (see Acetylene-DERIVED chemicals). 2-Butyne-l,4-diol, its hydroxyethyl ethers, and propargyl alcohol are used as corrosion inhibitors. 2,3-Dibromo-2-butene-l,4-diol is used as a flame retardant in polyurethane and other polymer systems (see Bromine compounds Elame retardants). 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

In either the acid catalysis or the niekel carbonyl (or other metallic catalyst) method, if alcohols, thiols, amines, etc. are used instead of water, the product is the corresponding ester, thiol ester, or amide, instead of the carboxylic acid. 

Rhodium and cobalt carbonyls have long been known as thermally active hydroformylation catalysts. With thermal activation alone, however, they require higher temperatures and pressures than in the photocatalytic reaction. Iron carbonyl, on the other hand, is a poor hydroformylation catalyst at all temperatures under thermal activation. When irradiated under synthesis gas at 100 atm, the iron carbonyl catalyzes the hydroformylation of terminal olefins even at room temperatures, as was first discovered by P. Krusic. ESR studies suggested the formation of HFe9(C0) radicals as the active catalyst, /25, 26/. Our own results support this idea, 111,28/. Light is necessary to start the hydroformylation of 1-octene with the iron carbonyl catalyst. Once initiated, the reaction proceeds even in the... 

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

King and coworkers—formate intermediates and a kinetic approach over Cr and other Group VIb carbonyl catalysts. Like Pettit and coworkers,18 34 King et a/.25,3 J 42,43,54,59,138,155 also studied water-gas shift on mononuclear carbonyls of iron and, in a detailed investigation, indicated that Scheme 7 of Pettit et al,34 was... 

Catalyst solutions of Fe and Ru or Fe/Ru mixed metal carbonyls were found to exhibit two orders of magnitude higher activity than either the Ru or Fe carbonyl catalysts alone. A weaker synergistic behavior was observed for Fe/Ir mixed metal carbonyls, which were 50% more active. Rh carbonyl catalysts were the most active ( 590 mol H2/mol catalyst per day), but did not display a synergism in combination with other metal carbonyls. 

The solvent process involves treating phthalonitrile with any one of a number of copper salts in the presence of a solvent at 120 to 220°C [10]. Copper(I)chloride is most important. The list of suitable solvents is headed by those with a boiling point above 180°C, such as trichlorobenzene, nitrobenzene, naphthalene, and kerosene. A metallic catalyst such as molybdenum oxide or ammonium molybdate may be added to enhance the yield, to shorten the reaction time, and to reduce the necessary temperature. Other suitable catalysts are carbonyl compounds of molybdenum, titanium, or iron. The process may be accelerated by adding ammonia, urea, or tertiary organic bases such as pyridine or quinoline. As a result of improved temperature maintenance and better reaction control, the solvent method affords yields of 95% and more, even on a commercial scale. There is a certain disadvantage to the fact that the solvent reaction requires considerably more time than dry methods. 

During hydroformylation the IR spectra of the rhodium species (diphosphine)Rh(CO)2H for ligands 26, 18, and 30 do not change. The respective ee ae equilibria are not influenced, and no other carbonyl complexes are observed thus these are the catalyst s resting state. The coupling constants in the NMR spectra reflect the equilibria between ae and ee species. 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

The ene reaction is strongly catalyzed by Lewis acids such as aluminum chloride and diethylaluminum chloride204 Coordination by the aluminum at the carbonyl group increases the electrophihcity of the conjugated system and allows reaction to occur below room temperature, as illustrated in Entry 6. Intramolecular ene reactions can be carried out under either thermal (Entry 3) or catalyzed (Entry 7) conditions 205 Formaldehyde in acidic solution can form allylic alcohols, as in entry 1. Other carbonyl ene reactions are carried out with Lewis acid catalysts. Aromatic aldehydes and acrolein undergo the ene reaction with activated alkenes such as enol ethers in the presence of Yb(fod)3 206 Sc(03SCF3)3 has also been used to catalyze ene reactions.207... 

Other products were dimethyl ether (DME), methane and carbon dioxide. The data in Table I show that high yields of carbonylated products were produced with nickel catalysts supported on activated carbon and carbon black. Other nickel catalysts gave mainly methane and dimethyl ether. It is clear that a carbonaceous carrier is essential for the appearance of carbonylation activity for the nickel catalyst. The role of the carbonaceous carrier will be discussed later. 

Studies of I /Ru stoichiometry previously discussed and shown in Fig. 20 suggest that these two complexes, or at least a catalyst composition of the same stoichiometry, are present during catalysis. Studies of active solutions during catalysis by high-pressure infrared spectroscopy have also confirmed the presence of these complexes (191). Under 544 atm of H2/CO at 230°C in sulfolane solvent, the infrared absorptions for the carbonyl ligands of both complexes are observed clearly. No other carbonyl absorptions are evident. Samples have also been withdrawn from catalytic reactions and cooled immediately to low temperature before analysis by infrared spectroscopy these solutions also are found to contain only [HRu3(CO)j J and [Ru(CO)3I3]. ... 

Other Ruthenium Catalysts. Ru3(CO)i2 readily dissolved in piperidine to give a solution effective for catalytic carbonylation of the amine. The uptake plots resemble those shown in Figure 1 (curves B-E), and the maximum rate given in Table I refers to the initial rate. Attempts to characterize the ruthenium complexes formed from reaction of the dodecacarbonyl with amines have been unsuccessful. 

Kinetic resolution of chiral, racemic anhydrides In this process the racemic mixture of a chiral anhydride is exposed to the alcohol nucleophile in the presence of a chiral catalyst such as A (Scheme 13.2, middle). Under these conditions, one substrate enantiomer is converted to a mono-ester whereas the other remains unchanged. Application of catalyst B (usually the enantiomer or a pseudo-enantiomer of A) results in transformation/non-transformation of the enantiomeric starting anhydride ). As usual for kinetic resolution, substrate conversion/product yield(s) are intrinsically limited to a maximum of 50%. For normal anhydrides (X = CR2), both carbonyl groups can engage in ester formation, and the product formulas in Scheme 13.1 are drawn arbitrarily. This section also covers the catalytic asymmetric alcoholysis of a-hydroxy acid O-carboxy anhydrides (X = O) and of a-amino acid N-carboxy anhydrides (X = NR). In these reactions the electrophilicity of the carbonyl groups flanking X is reduced and regioselective attack of the alcohol nucleophile on the other carbonyl function results. 

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