Palladium intermediates, carbonyl-inserted

Substituted propargylic alcohols were found to undergo direct carbonylation to the corresponding butenolides in 67-98% yield (Eq. 9.120) [86]. This reaction requires a catalytic amount of Pd2(dba)3-CHC13 (4%) and l,4-bis(diphenylphosphi-no)butane (8%) in CH2C12 under an atmosphere of CO (600 psi) and H2 (200 psi) at 95 °C for 36 h. The cyclocarbonylation reaction is believed to proceed via an allenyl-palladium intermediate, which is formed by initial insertion of Pd(0) into the C-O bond of the alkynol followed by rearrangement (Scheme 9.25). 

The mechanism of the indenone synthesis (Scheme 3) seems to involve (1) oxidative addition of the aryl iodide to Pd(0) (2) arylpalladium coordination to the alkyne and subsequent insertion of the alkyne to form a vinylpalladium intermediate (8), (3) then either the vinylic palladium intermediate adds to the carbonyl group and subsequently undergoes a /3-hydride elimination (path A) or the alde-hydic C-H bond may oxidatively add to the palladium to produce an organopalla-dium(IV) intermediate (six-membered ring palladacycle) which subsequently undergoes rapid reductive elimination of the indenone and palladium (path B). The actual mode of ring closure of the vinylic palladium intermediate to the inde-... 

Carbonylation of the three-coordinated cationic palladium(II) complex of reaction (j) gives the carbonyl complex, which is an intermediate of the subsequent carbonyl inserted product ". ... 

This alternative to the Friedel-Crafts reaction, extensively developed by Stille and coworkers, is particularly important, since the reaction conditions are essentially neutral, and so provides a method for acylation of compounds containing an acid-sensitive functionality which would preclude the use of the Friedel-Crafts reaction. Reaction temperatures are often below 100 C, and high (1000-fold) turnovers of the catalyst have been achieved. Solvents employed include chloroform, toluene, and, on occasions, HMPA. Some reactions have been carried out under an atmosphere of carbon monoxide to prevent excessive decarbonylation of the acyl palladium intermediate. Indeed, carbonylative coupling of alkenylstannanes with allyl halides in the presence of carbon monoxide ca. 3 atm or greater 1 atm =101 kPa) offers an alternative to the Friedel-Crafts acylation, ketones being formed by the reaction of the stannane with the acyl species formed by carbon monoxide insertion into the allyl palladium intermediate. ... 

Insertions of vinylic palladium halides with olefins not conjugated with carbonyl groups is more complex. Rearrangement of the initial insertion product to a i-allyl-palladium derivative usually occurs as in Eq. (p). Since the a -allylic complexes are relatively stable, catalytic reactions to form dienes with tertiary amines as bases are slow and inefficient . A useful catalytic reaction occurs, however, if a nucleophilic secondary amine is used as base instead of the tertiary amine. The a-allylic palladium intermediates are attacked by the amine to form tertiary allylic amines and regenerate the catalyst ... 

Further studies revealed that there are two courses leading to amide from the carbony-lation of benzylpalladium complexes depending on the amount of secondary amine employed. One involves the formation of benzyl(carbamoyl)palladium intermediate, from which amide is produced on reductive elimination when neat amine is used. The other is composed of the CO insertion into benzyl-palladium bond to give phenylacetylpalladium intermediate, which may further react with amine to liberate the single carbonylation product. The latter process takes place when a limited amount of amine is employed. The a-keto amides were found to be produced in the reaction of carbon monoxide with trans- and di-methylpalladium carbamate complexes, PdMe(OCONRR )L2, accompanied by formation of amides (Eq. 20). The process was assumed to involve decarboxylation of the carbamate ligand associated with CO insertion and attack of the amide entity on the coordinated CO. 

On the other hand, when the oxidative carbonylation of a ,a -disubstituted propynylamines was carried out in the presence of an excess of CO2, the intermediate carbamate species could undergo cyclization with incorporation of CO2 into the five-membered cycle, either by direct nucleophilic attack of the carbamate oxygen to the triple bond coordinated to Pd(II) (Scheme 33, path a) or through the intermediate formation of a palladium carbamate complex followed by triple bond insertion (Scheme 33, path b). Carbon monoxide insertion into the Pd - C bond of the resulting stereoisomeric vinylpalladium intermediates then led to the final oxazolidi-none derivatives. 

Figure 15.8 a simple example is presented of a subsequent insertion of CO and methanolysis of the palladium acyl intermediate [14], This is not a very common reaction, because both the ligand requirements and the redox conditions for Wacker and carbonylation chemistry are not compatible. For insertion reactions one would use cis coordinating diphosphines or diimines, which makes the palladium centre more electron-rich and thus the nucleophilic attack in the Wacker part of the scheme will be slowed down. In addition, the oxidants present may lead to catalytic oxidation of carbon monoxide. 

Ini Hally, oxidaiive addition of the 2-chloroquinolme 9 to a Pd° species leads to complex 30, which contains Pd11 Carbonylation then occurs through CO insertion into the palladium-aryl bond, giving intermediate 31. finally, methanolysis releases methyl ester 10, and the Pd° species is regenerated. 

The cycle is started with the formation of a Pd-alkoxy complex that reacts with CO to an alkoxycarbonyl intermediate. In the next step, the approach of the olefin and insertion into the carbonyl palladium bond is predicted. In the last step, the starting complex is rebuilt by the addition of an alcohol and the cleavage of the hydroesterification product [59]. 

Copper(I) triflate was used as a co-catalyst in a palladium-catalyzed carbonylation reaction (Sch. 27). The copper Lewis acid was required for the transformation of homoallylic alcohol 118 to lactone 119. It was suggested that the CuOTf removes chloride from the organopalladium intermediate to effect olefin complexation and subsequent migratory insertion [60]. Copper(I) and copper(II) chlorides activate ruthenium alkylidene complexes for olefin metathesis by facilitating decomplexation of phosphines from the transition metal [61]. 

An important development in Heck chemistry arose from the recognition that a-palladium(II) intermediates can be exploited for transformations other than y3-hydride elimination [50]. Examples of potential reaction pathways include insertion of a second olefin (path A), anion capture (path B), or carbonylation (path C) (Scheme 6-22). Due to the... 

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