Carboxylic acids transition metal catalysts

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

Addition of HCN to acetone to form the cyanohydrin is still the main route to methyl methacrylate. Hydrocyanins can be converted to amino acids as well. The nitrile group can be easily converted to amines, carboxylic acids, amides, etc. Addition to aldehydes and activated alkenes can be done with simple base, but addition to unactivated alkenes requires a transition metal catalyst. The methods of HCN addition have been discussed by Brown [2],... 

Intramolecular Kharasch -type cyclizations.2 Unsaturated a,a-dichloro esters or carboxylic acids undergo cyclization in the presence of various transition metal catalysts, particularly Cl2Ru[P(C6H5)3]3 or Cl2Fe[P(OC2H5)3]3.3 Thus ethyl 2,2-dichloro-6-heptenoate (2) in the presence of 1 cyclizes rapidly to the cis-l,2-disub-... 

The f-transition metal catalysts were first described by von Dohlen [98] in 1963, Tse-chuan [99] in 1964 and later by Throckmorton [100]. In the 1980s Bayer [14] and Enichem [101] developed manufacturing processes based on neodymium catalysts. The catalyst system consists of three components [102] a carboxylate of a rare earth metal, an alkylaluminum and a Lewis acid containing a halide. A typical catalyst system is of the form neodymium(III) neodecanoate/diisobutylaluminum hydride/butyl chloride [103]. Neodymium(III) neodecanoate has the advantage of very high solubility in the nonpolar solvents used for polymerization. The molar ratio Al/Nd/Cl = 20 1 3. Per 100 g of butadiene, 0.13 mmol neodymium(III) neodecanoate is used. With respect to the monomer concentration, the kinetics are those of a first-order reaction. 

Hydrocarboxylation is the formal addition of hydrogen and a carboxylic group to double or triple bonds to form carboxylic acids or their derivatives. It is achieved by transition metal catalyzed conversion of unsaturated substrates with carbon monoxide in the presence of water, alcohols, or other acidic reagents. Ester formation is also called hydroesterification or hydrocarb(o)alkoxylation . The transition metal catalyst precursors are nickel, iron or cobalt carbonyls or salts of nickel, iron, cobalt, rhodium, palladium, platinum, or other metals4 5. 

Reaction of Methanol with Carbonyl Compounds. - Similar to the reaction of methanol with carboxylic acid, esters, or nitriles shown in Sections 5.2 and 6.2, attempts were made to use the HCHO which is formed by dehydrogenation of methanol. Ueda et al. performed the reaction of methanol with acetone over various transition metal catalysts supported on MgO using an acetone/methanol molar ratio of 1/10. The best performances are obtained with a catalyst containing 3.1 wt% of Fe. The main products are methyl vinyl ketone, methyl ethyl ketone, and 2-propanol. The yields are 7.1, 2.8, and 2.8 mol%, respectively, based on the charged acetone at the conversion of 20.1% selec-tivities are 34.8, 13.9, and 13.9 mol%, respectively, based on acetone. The yield of methyl vinyl ketone is much lower than that achieved in the reaction with HCHO. Unfortunately, there is no information about the reaction of methanol that exists in the feed ten times greater than acetone. It is considered that methyl ethyl ketone and 2-propanol are formed by hydrogenation of methyl vinyl ketone and acetone, respectively, with methanol. 

In Scheme 1.2, all of the types of carbonylations that are discussed in the book are depicted. Alcohols, amines, ethers, carboxylic acids and halides can be converted to acids, amides, esters, ketones, alkynones, alkenones, anhydrides and acid halides with the assistance of transition metal catalysts in the presence of a CO source. The CO sources used can be carbon monoxide gas, Mo(CO)6, Co(CO>6, formic acid, aldehyde, etc. If the starting material is alcohols or amines, some additives for activation are needed, such as BuONO, TsCl, AcCl. If the substrate is (Hetero)ArH, additional oxidants will be necessary this is a so-caUed oxidative carbonylation. If an unsaturated compound is to be carbonylated, a nucleophile NuH that carries an acidic hydrogen has to be present. In the case of insertion reactions, this is not necessary. 

The loss of N2 is mediated by heat, light, or a transition-metal catalyst such as Ag20. Typically, the unstable ketene is not isolated but is trapped by water (to yield a carboxylic acid) or another molecule in the reaction medium. 

Carbonylation is a very important process for the preparation of carboxylic acids (and their derivatives), aldehydes and ketones. It was earlier carried out in presence of transition metal catalysts. 

In addition to ruthenium-catalyzed reactions, a range of other transition metal catalysts have shown activity toward the addition reaction. A series of air-stable gold compounds promoted the addition of carboxylic acids to alkynes (Scheme 2.93) [138]. A variety of gold and silver compounds were screened as catalysts for the reaction, and the most effective pair under the mildest conditions was (Ph3P)AuCl and AgPF. Under the reactions conditions, the reaction was highly selective for the formation of the Markovnikov addition product, and minimal or none of the anti-Markovnikov products were observed. 

As described earlier, a variety of transition metal catalysts have been used to promote the addition of B-H bonds across alkynes. The development of metal-free approaches has been challenging to design due to the unique reactivity profile afforded by transition metal catalysts. To this end, a metal-free catalyst system has been devised for the addition of pinacolborane to internal alkynes (Scheme 6.48) [91]. Surprisingly, only a carboxylic acid additive was needed to effect the hydroboration reaction. In addition to common alkynes, an alkynylboronate was successfully borylated using the carboxylic acid catalyst. The functional group tolerance was remarkably broad, and a host of alkynes were successfully functionalized. Given the wide substrate scope, availability of the precursors and catalysts, and the metal-free nature of the chemistry, this is a reasonable approach to selective hydroboration reactions. 

Transition Metal Catalysts for the Amidation of Carboxylic Acids... 

Most commercial sorbic acid is produced by a modification of this route. Catalysts composed of metals (2inc, cadmium, nickel, copper, manganese, and cobalt), metal oxides, or carboxylate salts of bivalent transition metals (2inc isovalerate) produce a condensation adduct with ketene and crotonaldehyde (22—24), which has been identified as (5). 

Guo et al. [70,71,73] recently attempted to hydrogenate NBR in emulsion form using Ru-PCy complexes. However, successful hydrogenation can only be obtained when the emulsion is dissolved in a ketone solvent (2-butanone). A variety of Ru-phosphine complexes have been studied. Crosslinking of the polymer could not be avoided during the reaction. The use of carboxylic acids or first row transition metal salts as additives minimized the gel formation. The reactions under these conditions require a very high catalyst concentration for a desirable rate of hydrogenation. 

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