Hydroesterification of Alkynes

The hydroesterification of alkynes (Equation 17.45) was first conducted with Ni(CO) as catalyst to produce acrylate esters as products. Much more active catalysts for this reaction have now been generated from a combination of palladium acetate and 2-pyridylphosphines. The use of these ligands for alkyne hydroesterification was conducted by Drent at Shell. 

Ligand type Temperature fC) Average rate (moi/moi Pd h) Selectivity (%MMA) 

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Hydroesterification of alkynes proceeds smoothly using PdCl2(PPh3)2 as a standard catalyst, offering a convenient synthetic method of a, jS-unsaturated esters 1. Carbonylation of 1-alkynes and asymmetric alkynes yields a mixture of regioisomers. 

Two mechanisms for the hydroesterification of alkenes have been considered. One pathway—the "alkoxide cycle"— begins with the insertion of CO into a metal alkoxide (Scheme 17.18) and the other—"the hydride cycle"— begins with the insertion of an alkene into a metal hydride (Scheme 17.19). The relative importance of the different pathways depends on the identity of the dative ligand. However, hydroesterification of ethylene with the bis(di-ferf-butylphosphinomethyl)benzene ligand is now generally accepted to occur through a palladium hydride. The mechanism of the hydroesterification of alkynes is less established, but is likely to occur by a sequence that shares some steps with the mechanism for the hydroesterifcation of alkenes. 

Interestingly, it was found that the reactivity of formate esters is opposite to that of the alcohols described in the previous system (Eq. 23). Regioselective hydroesterification of alkynes and alkynols using formate esters and the catalytic system Pd(OAc)2-dppb-PPh3 in the presence of p-TsOH at 20 atm of CO gave unsaturated esters in good selectivity (Eqs. 23and24).t 2]... 

The presence of catalytic amounts of halide salts (e.g. BU4NI) was found to enhance dramatically the efficiency of hydroesterification of alkenes (e.g. cyclohexenone) and alkynes catalysed by [(CO)i2Ru3] in the presence of the chelating 2-pyridylmethyl formate (2-C5H4NCH2OCHO). On the basis of IR and NMR studies, the halide effect... 

The insertion of alkynes into metal-hydride bonds occurs during a number of catalytic processes, including alkyne hydrogenation, hydrosilylation, silylformylation, hydroesterification and dimerization. This insertion chemistry is more complex mechanistically than the insertions of olefins into metal hydrides. In some cases, ds addition products have... 

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. 

Subsequently, palladium-based complexes were used for car-bonylation of aryl halides,hydroesterification and hydroamidation of olefins," hydroamidation of alkynes, Heck reaction, and oxidation and hydrogenation of olefins. i° ... 

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... 

Alkynes undergo simple hydroesterification under mild conditions to give a,B-unsatuiated esters. Addition of the ester function to terminal alkynes frequently occurs at the substituted carbon atom to give branched products, but linear esters may be obtained witfi high selectivity using palladium-tin catalysts (equation 36). Under oxidative carbonylation conditions cir-diesters are formed. ... 

In the presence of various functional groups in an appropriate distance from the alkenyl or alkynyl moieties, substituted alkenes or alkynes may undergo tandem reactions comprising hydroesterification as one of the individual steps. The bicylic cis-fused lactone core of plakortones, a novel class of activators of cardiac SR-Ca -pumping ATPase, was efficiently accessed by a palladium(II)-mediated cyclization-intramolecular hydroesterification reaction sequence. Cyclization of the starting enediols was carried out using either a small excess of Pd(II) [47] or a catalytic amount of Pd(II) and an excess of Cu(II) as the reoxidant [48],... 

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