Alkynes hydrocarboxylation

According to results obtained from cyclic alkenes or from alkynes hydrocarboxylation proceeds in a cis manner. Thus, cis addition from the less hindered exo side is found in the deuterocarboxylation of norbornene with Ni(CO)414. 

In the metal-carbonyl catalysed hydrocarboxylation of alkynes ( Reppe reaction ) nearly exclusive cia-addition of H—COOH is found (Ohashi et al., 1952). 

K. Tani and Y. Kataoka, begin their discussion with an overview about the synthesis and isolation of such species. Many of them contain Ru, Os, Rh, Ir, Pd, or Pt and complexes with these metals appear also to be the most active catalysts. Their stoichiometric reactions, as well as the progress made in catalytic hydrations, hydroal-coxylations, and hydrocarboxylations of triple bond systems, i.e. nitriles and alkynes, is reviewed. However, as in catalytic hydroaminations the holy grail", the addition of O-H bonds across non-activated C=C double bonds under mild conditions has not been achieved yet. 

In this process, double bonds were found to be less reactive than triple bonds. Thus norbornene or styrene were hydrocarboxylated in low yields (10-40%) [121]. In unconjugated as well as conjugated ene-ynes, only the alkyne moiety was carboxylated with regio- and stereoselectivity similar to that observed for alkynes [122]. 

Carboxylic acids and their derivatives like esters, amides, anhydrides, and acyl halides are formally synthesized from olefins, carbon monoxide, and compounds represented by Nu-H such as H2O, ROH, RNH2, RCOOH (Equations (4) and (5)). Alkynes also react under similar conditions to afford the corresponding unsaturated carboxylic acid derivatives. These reactions have been named hydrocarboxylation, hydroalkoxycarbonylation, and hydroaminocarbonylation. 

Hydrocarboxylation of Alkynes Intramolecular addition of carboxylic acids (weak nucleophiles) to alkynes led to lactones, which were first reported by Schmidbaur et al. in the reaction of acetic add with 3-hexynes to obtain, in addition to enol ester, 3-hexanone. Traces of water were probably present in the solvent to enable the process to be carried out [99]. 

The hydrocarboxylation reaction of alkenes and alkynes is one which utilizes carbon monoxide to produce carboxylic acid derivatives. The source of hydrogen is a protic solvent (equation 35) dihydrogen is not usually added to the reaction. There are a number of variations to this reaction, since the solvent can be water, alcohols, amines, acids, etc. The catalysts can be Group VIII-X transition metals, but cobalt, rhodium, nickel, palladium and platinum have found the most use. 

The hydrocarboxylation reaction of simple alkenes and alkynes in the presence of primary or secondary amines or ammonia yields amides (equations 48 and 49). The fact that formamides can be used in place of amines suggests that a key intermediate in the reaction is the hydride metal carboxamide (20). 

The 1,2-addition of H and CO2H, or CO2R to alkenes and alkynes is called hydrocarboxylation, or hydroesterification, and proceeds with catalytic amounts of... 

When applied to triple bonds, hydrocarboxylation gives a,p-unsaturated acids under very mild conditions. Triple bonds give unsaturated acids and saturated dicar-boxylic acids when treated with carbon dioxide and an electrically reduced nickel complex catalyst. Alkynes also react with NaHFe(CO)4, followed by CuCl2 2 H2O, to give alkenyl acid derivatives. A related reaction with CO and palladium catalysts in the presence of SnCE also leads to conjugated acid derivatives. Terminal alkynes react with CO2 and Ni(cod)2, and subsequent treatment with DBU (p. 1132) gives the a,p-unsaturated carboxylic acid. ... 

Additions. Hydrocarboxylation of alkenes" and alkynes with HCOOH under CO is accomplished by Pd catalysis. A similar reaction undergone by a propar-gylic carbonate after an S 2 process leads to an itaconic diester segment.The intermediate is converted into a cyclopentenone when the propargyl carbonate... 

The reaction of an alkene (or alkyne), CO, and H2O to directly produce a carboxylic acid is called Reppe carbony-lation chemistry or, more recently, hydrocarboxylation see Reppe Reaction). An excellent review of palladium-catalyzed Reppe carbonylation systems has been published recently by Kiss, and coverage of this important material will not be repeated here. This catalytic reaction has been known for quite some time, although the stoichiometric Ni(CO)4-based carbonylation of acetylene was the first commercial carbonylation process implemented (equation 13). The extreme toxicity of Ni(CO)4, however, has limited practical applications see Nickel Organometallic Chemistr. Co, Rh, and Pd catalysts have certainly replaced Ni(CO)4 in smaller-scale laboratory reactions, though for historical reasons a number of the fundamental mechanisms discussed in this section are based on Ni(CO)4. 

The state of the art in catalytic intermolecular additions of carboxylic acids to terminal alkynes (Scheme 27) prior to 2000 has been reviewed by Dixneuf and Bruneau, who conclude that complexes [(p-cymene)RuCl2(PR3)] are catalysts for Markovnikov additions (toluene, 80-100°C), whereas [(dppb)Ru(methallyl)2] is the preferred catalyst for anti-Markovnikov hydrocarboxylations of alkynes with predominant selectivity for (Z)-enol esters (toluene, 50-60°C) [165, 166]. 

Rhenium A hydrocarboxylation with high selectivity for anti-Markovnikov addition and predominant (Z)-enol ester product is mediated by ReBr(CO)5 (1 mol%, 110°C, 15 h) [168]. The 7i-activation mechanism proposed by the authors does not fit to the observed anti-Markovnikov selectivity. Iridium The precursor complex [ IrCl(cod) 2] (1 mol%) combined with P(OMe)3 (4 mol%) and Na2C03 (2 mol%) produces a catalyst that adds carboxylic acids to terminal alkynes (toluene, 100°C, 15 h) to give a mixture isomers with variable selectivities, although the Markovnikov product is usually formed in excess (ca 5 1) [169]. The complex []IrCl(cod) 2] (1 mol%) in the presence of Na2C03 (0.6 equiv) is also a catalyst for the transvinylation of vinylacetate with diverse alcohols [170]. 

For alkynes (and in part, allenes), synthetically useful protocols for Markovnikov and anti-Markovnikov selective hydrations, hydroalkoxylations (mainly intramolecular), and hydrocarboxylations are available and find increasing applications in organic synthesis. In the past decade, the research focus on cationic gold(l) complexes has led to new additions to the catalysis toolbox. It can be predicted that a further refining of such tools for alkyne functionalization with respect to catalytic activity and functional group tolerance will take place. 

RXN20 Hydrocarboxylation, Hydrocarboalkoxylation and Hydrocarboamination of Alkenes and Alkynes... 

The hydrocarboxylation of alkynes R C=CR with CO2 and (EtO)3SiH, catalysed by CuF complexes of NHC ligands (e.g. IMes and Cl2lpr), produces the ( )-configured carboxylic acids R CH=C(R )C02H. ... 

Nickel A three-component hydrozincation of alkynes R C=CR with Et2Zn (3 equiv.), carried out under the atmosphere of CO2 (at normal pressure), has been shown to form ( )-conflgured hydrocarboxylation products R CH=C(R )C02H in the presence of (COD)2Ni as a catalyst (l-3mol%) and CsF (1 equiv.) in MeCN at 60 °C over 1.5h.i ... 

Like many carbonylation processes, the hydrocarboxylation and hydroesterification reactions were first reported by Reppe. These first reactions involved the hydrocarboxylation of alkynes. These reactions were conducted with nickel carbonyl as catalyst and occurred with very low turnover numbers. Hydrocarboxylation and hydroesterification have now been studied extensively in both academic and industrial laboratories. As a result of these investigations, commercialization of this chemistry as part of new industrial processes has occurred, and the mechanism of these processes is now generally accepted. This section of Chapter 17 presents the scope and industrial applications of hydrocarboxylation and hydroesterification, the types of catalysts that have been used for these processes, and the elementary steps that constitute the catalytic cycle for olefin and alkyne hydroesterification. 

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