Alkoxycarbonylpalladium species

Succinic diesters and acrylic esters are formed through insertion of the olefin into the Pd-C bond of an alkoxycarbonylpalladium species X - Pd - C02R (ensuing from the reaction between PdX2, CO and an alcohol R OH used as an external nucleophile, Scheme 3). Further carbon monoxide insertion, followed by nucleophilic displacement by R OH, then leads to the succinic diester (Scheme 3, path a). /J-H elimination may also take place to give the a./J-unsaluralcd ester (Scheme 3, path b). This latter pathway is followed at low carbon monoxide partial pressures. 

The preparation of dialkyl oxalates by oxidative carbonylation of alcohols was first described by Fenton et al. in the early 1970s [72-74]. For example, the reaction can be carried out at a temperature around 125 °C and a pressure of about 70 bar in the presence of PdCl2 and iron or copper salts. Water is formed as a by-product and has to be removed from the reaction mixture by the addition of water-binding agents such as trialkyl orthoformates. Instead of oxygen benzo-quinone can also be used for the reoxidation of the catalyst system. Ammonia or amines seem to have a positive influence on selectivity and efficiency of the reae-tion. For some more examples, cf. [77-80, 117]. Mechanistic studies give some indication that alkoxycarbonylpalladium species occur as intermediates [52, 75, 76] (eq. (12)). 

The elementary steps implicated in Mechanism I were extensively studied by Norton and co-workers, who were able to independently synthesize, isolate, and characterize all the organopalladium intermediates involvedJ This elegant and thorough work unequivocally showed that intramolecular insertion of the (alkoxycarbonyl)palladium species on the unsaturated bond in 3-en-l-ols, 3-yn-l-ols, and 4-yn-l-ols occurs exclusively in a syn-exo fashion (path a). It is important to point out, however, that in a catalytic reaction Pd(0)-initiated mechanisms like II-IV cannot be ruled out Clearly, in the model system used by Norton there was no alternative to Mechanism I, since an alkoxycarbonylpalladium complex was synthesized first and the proton was added at the end of the sequence. 

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