Oxidative Carbonylation of Alkenes

Depending on the carbon monoxide pressure and on the nature of the reaction mixture (acidic or basic) the oxidative carbonylation of aUcenes can be directed toward unsaturated esters, diesters or ether-esters (Table XV). 

The formation of these different products can be explained mechanistically. 

In acidic or neutral medium the alcohol reacts with the coordinated olefin thus yielding an alkoxy palladium intermediate subsequent carbonylation leads to the corresponding ether-ester. 

Synthesis of ketones from dienes and aliphatic halides. 

Starting Material Product Catalyst Conditions Yield Ref. 

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Reactions of another class are catalyzed by Pd(II) compounds which act as Lewis acids, and are treated in Chapter 5 and partly in Chapter 4. From the above-mentioned explanation, the reactions catalyzed by Pd(0) and Pd(II) are clearly different mechanistically. In this book the stoichiometric and catalytic reactions are classified further according to reacting substrates. However, this classification has some problems, viz. it leads to separate treatment of some unit reactions in different chapters. The carbonylation of alkenes is an example. Oxidative carbonylation of alkenes is treated in Chapter 3 and hydrocar-bonylation in Chapter 4. 

As a unique reaction of Pd(II), the oxidative carbonylation of alkenes is possible with Pd(ll) salts. Oxidative carbonylation is mechanistically different from the hydrocarboxylation of alkenes catalyzed by Pd(0), which is treated in Chapter 4, Section 7.1. The oxidative carbonylation in alcohol can be understood in the following way. The reaction starts by the formation of the alkoxy-carbonylpalladium 218. Carbopalladation of alkene (alkene insertion) with 218 gives 219. Then elimination of /3-hydrogen of this intermediate 219 proceeds to... 

The first report of oxidative carbonylation is the reaction of alkenes with CO in benzene in the presence of PdCh to afford the /3-chloroacyl chloride 224[12,206]. The oxidative carbonylation of alkene in alcohol gives the q, f3-unsaturated ester 225 and /3-alkoxy ester 226 by monocarbonylation, and succinate 111 by dicarbonylation depending on the reaction conditions[207-209]. The scope of the reaction has been studied[210]. Succinate formation takes... 

Addition of H and CO to alkenes and alkynes catalysed by transition metal complexes is called hydrocarbonylation, and is useful for the syntheses of carboxylic acids, their esters, aldehydes and ketones [1]. Oxidative carbonylation of alkenes and alkynes with Pd(II), treated in Section 11.1.5, differs mechanistically from hydrocarbonylation. Some carbonylation reactions occur at under 1 atm or low pressures, without using a high-pressure laboratory apparatus. Several commercial processes based on hydrocarbonylation have been developed. 

The dicarboxylation of cyclic alkenes is a useful reaction. Only cxo-methyl-7-oxabicyclo[2.2.1]heptane-2,3,5,6-tetracarboxylate (121) was prepared from the cyclic alkene 120 using Pd on carbon and CuCl in MeOH at room temperature with high diastereoselectivity [89]. Methyl nitrite 37 is used for efficient oxidative carbonylation of alkenes to produce the succinate derivatives 122 [la,90]. It was claimed that the... 

Palladium salts can bring about oxidative carbonylation of alkenes in the presence of copper(II) salts which can reoxidize Pd° to Pd . Oxidative carbonylation is favored over simple hydroesterification by the presence of bases and by low temperatures (25 C) and low pressures (3-15 bar). The products can be a, -unsaturated esters, dicarboxylic acid esters or -alkoxy esters. By careful optimization of the conditions (25 °C, 4 bar CO, methanol solvent, CuCh reoxidant and sodium butyrate buffer) high yields of diesters can be obtained (equation 35). ... 

P-Lactones can be obtained by oxidative carbonylation of alkenes in the presence of water. Ethylene, for example, is converted to p-propiolactone by carbonylation in aqueous acetonitrile at -20 C using a catalytic amount of PdCh and a stoichiometric quantity of copper(II) chloride (equation 37). Palladium-catalyzed carbonylation of halides can also be used to prepare p-lactones under mild conditions. The reaction takes place at room temperature and pressure in the presence of [PdCl2(PPh3)2] and has been applied to both bromides and chlorides (equations 38 and 39). 

Oxidative carbonylation of alkenes is a unique reaction of Pd(II). Three types of oxidative carbonylation to give -substituted acid derivatives 130, a, -unsaturated esters 132 and succinate derivatives 134 are known, which can be understood by the following mechanism. Palladation of alkenes with PdX2, followed by CO insertion, generates the acylpalladium intermediate 129 whose reductive elimination affords -substituted carboxylic acid derivatives 130 (path a). Reaction in alcohol in the presence of a base starts by the formation of the alkoxycarbonylpalladium 128. Carbopalladation of alkene with 128 generates 131. Then y3-H elimination of the intermediate 131 yields the a-unsaturated ester 132 (path b). Further CO insertion to 131 gives the acylpalladium intermediate 133 and its alcoholysis yields the succinate derivative 134 (path c). Formation of the jS-alkoxy ester 130 (X-OR) is regarded as nucleophilic substitution of Pd-X in 131 with alcohols. 

The oxidative carbonylation of alkenes, which is promoted by Pd(II), is described in Chapter 2.2.7. The Pd(0)-catalyzed carbonylation reactions, which are mechanistically different from the oxidative carbonylation, are treated in this section. 

Instead of using CuCl2 and BQ, butyl nitrite was also applied in the oxidative carbonylation of alkenes by Chauvin and colleagues. When PdCl2(PhCN)2 and PPha were used as the catalyst system, PdCl2(C02Bu)N0(PPh3) was isolated and proved to be one of the reaction intermediates. In the gas phase of the reaction, CO2, N2O, NO and N2 were found, but no N2O. 

In 1996 Castanet et al. developed a CO-free procedure for the oxidative carbonylation of alkenes [32]. Instead of a MeOH/CO mixture, methyl formate was used in the presence of a Pd VCu system and unsaturated esters were produced in one step. During the reaction, methyl formate acted as the source of both alcohol and CO, but an initial partial pressure of CO was required in order to obtain high yields. Moreover, they demonstrated that by the addition of lithium methoxide, the handling of CO could be avoided. 

Scheme 8.4 Palladium-catalyzed oxidative carbonylation of alkenes to branched products...

In this chapter we discussed the transition metal catalyzed oxidative carbonylation of alkenes, alkynes and organometallic reagents. In these types of reactions, an additional oxidant is needed to reoxidize the catalyst back to an active state after a reductive elimination step. The oxidants applied are normally Cu(OAc)2 or BQ, air or O2, as more green oxidants should be investigated and applied in oxidative carbonylation reactions. In contrast, carbonylative reduction reactions using CO as a reductant are also interesting. In the next chapter, the reduction of C-NO2 with CO will be discussed. 

It is also worth mentioning that methyl formate was found to be able to replace carbon monoxide in carbonylation. It was shown that oxidative carbonylation of alkenes to olefinic esters with the Pd-Cu system can be performed using methyl formate in the presence of carbon monoxide or even in its absence provided that LiOCH3, which favors the decomposition of methyl formate to methanol and CO, is added (Scheme 16). A mechanism involving alkoxycarbonylation was proposed. " ... 

Oxidative Carbonylation of Alkenes. Oxidative carbonylation of alkenes with PdCl2 in benzene affords /3-chloroacyl chlorides (eq 26). Oxidative carbonylation of alkenes in alcohol affords a,/3-unsaturated esters and 8-aIkoxy esters by monocarbonylation and succinate derivatives by dicarbonylation (eq 27). ... 

Oxidative Carbonylation of Aromatics. The oxidative carbonyla-tion of aromatic rings (Fig. 22), is much more difficult to achieve than oxidative carbonylation of alkenes. Earlier work was done under forcing conditions of temperature and pressure however, Pd(ll)/Cu(ll)... 

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