Carboxylation of alkynes

Hoberg [1-10] first emphasized the synthetic potential of oxametallacyclopentenones, showing that they are useful starting materials or intermediates for the synthesis of unsaturated cyclic anhydrides (for instance, substituted maleic anhydrides, which can be obtained from the oxanickelacycles by reductive carbon-ylation with CO) or unsaturated carboxylic acids, which can be released upon protonolysis. The stoichiometric coupling reaction has also proved to be useful for the synthesis of unsaturated carboxylic acids through carboxylation and 

Catalytic carboxylation of internal alkynes with CO2 has been achieved in the presence of Ni [50] or Cu [51] catalysts, and through a hydrocarboxylative pathway, not involving the intermediacy of a metaUaoxacyclopentenone, but, as previously noted for the hydrocarboxylation of alkenes (see Sect. 5.3), implying the intermediate formation of an M—H catalyticaUy active species, which may easily be generated in the reactimi mixture using diethylzinc [50] or hydrosilanes [51] as hydride transfer agent. 

Electronic effects of the substituents on the acetylenic carbons, as well as the nature of the ancillary ligands coordinated to nickel center, also markedly affect the rate of the coupling reaction. Saito [58] compared the carboxylation rates of both Ph-C=CH and 4-MeO-C6H4-C=CH with CO2 in the presence of Ni(cod 2/DBU (2 equiv.) and found that ethynylbenzene was carboxylated faster than 4-methoxy-1-ethynylbenzene, whereas Dunach noted that the reaction rate increased with the basicity of the ancillary ligand [59]. 

Internal or terminal alkynes can be catalytically converted into 2-pyrones by reaction with CO2 in the presence of phosphine-Ni(O) complexes (5.7). In addition to 2-p5Tones, cyclo-oligomers may also form as by-products. 

The synthesis of 4,6-dimethyl-2-pyrone from 1-propyne and CO2 (6 MPa, 403 K) was achieved by Pillai et al. using Rh4- or Fe2Rh4-carbonyl cluster-derived catalysts, supported on selected metal oxides such as Ti02, AI2O3, and Zr02 [76, 77]. Formation of 2-pyrone was accompanied by homocoupling by-products such as 1,3,5- and 1,2,4-trimethylbenzene. In the absence of CO2 only the trimethyl-benzenes were produced. FTIR studies indicated that a key intermediate in the formation of pyrone may be a monodentate carbonate absorbed on the catalyst surface [77]. 

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Scheme 7-5 A possible path of Pt(PPh3)j-catalyzed hydrothio-carboxylation of alkyne...

The nickel-catalyzed electrochemical carboxylation of l-(perfluoroalkyl) aUcenes proceeds regioselectively to afford 4-fluoro-4-perfluoroalkyl-3-alkenoic acids via a Ni(II)-allyl complex and a final F elimination (Fig. 19) [107]. Electrochemical carboxylation of alkynes in the presence of... 

Scheme 7. Electrochemical carboxylation of alkynes catalyzed by nickel complexes...

Ni-alkyne bonding consists of contributions from both the 77, 7t- and cr,diyl tautomers. This bonding picture helps visualize the insertion reactions with alkynes, alkenes, and CO that result in the formation of metallacycles. Thanks to such insertion reactions, Ni-alkyne species are active intermediates in a number of catalytic applications such as alkyne oligomerization, carbonylation, and insertion of heterocumulenes such as CS2 and GO2. For example, a recent example of a C02-fixation reaction involved the stoichiometric, alkylative or arylative carboxylation of alkynes to give a,(3- and / ,/ -unsaturated carboxylic acids. Ni(0)-alkyne complexes have also been used as pre-catalysts in the addition of hydrosilanes to alkynes. In most cases, monoalkynes react to give the products of m-addition, whereas diynes produce enynes (1,2-addition), allenes (1,4-addition), or 1,3-butadienes (1,2,3,4-addition). ... 

Scheme 5.12 Alkylative or arylative carboxylation of alkynes proposed mechanism...

Six I (2003) Titanium-mediated carboxylation of alkynes with carbon dioxide. Eur J Org Chem 2003 1157-1171... 

Saito S, Nakagawa S, Koizumi T, HirayamaK, Yamamoto Y (1999) Nickel-mediated regio-and chemoselective carboxylation of alkynes in the presenee of earbon dioxide. J Org Chem 64 3975-3978... 

Labbe E, Dunach E, Perichon J (1988) Ligand-directed reactirai products in the nickel-catalyzed electrochemical carboxylation of alkynes. J Organomet Chem 353 C31-C36... 

The carboxylation of alkynes (Scheme 1.5 b) and dienes has been successful, with both cumulated and conjugated systems being used (Schemes 1.6 and 1.7). In all cases high TONs have been obtained. The carboxylation of butadiene... 

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