Organopalladium species

The reaction mechanism differs from that of other catalytic hydrogenations that also are carried out in the presence of palladium as catalyst, e.g. that of olefins. Presumably an organopalladium species is formed as an intermediate, which then reacts with the hydrogen ... 

Organopalladium intermediates are also involved in the synthesis of ketones and other carbonyl compounds. These reactions involve acylpalladium intermediates, which can be made from acyl halides or by reaction of an organopalladium species with carbon monoxide. A second organic group, usually arising from any organometallic reagent, can then form a ketone. Alternatively, the acylpalladium intermediate may react with nucleophilic solvents such as alcohols to form esters. 

Addition of organopalladium species (aryl-, vinyl-, allylpalladium and palladium enolates) to C=0 and CN multiple bonds has enjoyed wide applications as the key step of various catalytic processes. 

The palladium(0)-catalyzed cyclization of amide-allenes via a carbopalladation has been developed by several groups. The reaction proceeds through the carbopalladation of the allene moiety with an organopalladium species (R-Pd-X), generated by oxidative addition of R-X to palladium(O), and subsequent reductive elimination of the resultant 7r-allylpalladium intermediate.47,47a 47f... 

Palladium-catalyzed coupling reactions of the Heck type have in many instances involved indole and pyrrole derivatives. Although the mechanisms are complex, organopalladium species are implicated (84H(22)1493). Vinylation of A-substituted-3 -iodoindoles with amidoacrylate groups provides a useful functionalization of indoles (Scheme 81) (90JOM(39l)C23). Yields are improved in intramolecular reactions, e.g. (406 — 407) and (408 — 409) <92H(34)219,91CPB2830). 

The Heck reaction involves the coupling of an organopalladium species formed by oxidative addition to an alkene followed by /S-hydride elimination. The product is an alkene in which a vinyl hydrogen on the original alkene is replaced by die organic group on palladium. Thus aryl and alkenyl halides can be coupled to alkenes. 

In a similar manner, Lu and Liu have more recently utilized the hetero-Michael addition of lithium propargylic alkoxides to alkylidene malonates in a synthesis of stereodefined allylidene tetrahydrofurans, based on the use of allylic chloride as coupling partner [98]. In this case, the cydization reaction is initiated by a catalytic amount of palladium salt [Pd(OAc)2] rather than by an organopalladium species as mentioned above. 

For mechanistic investigations on Pd/Ag-catalyzed enyne or arylyne synthesis, Pale et al. mixed various alkynylsilvers with vinyl Inflates or aryl iodides in the presence of palladium salts or complexes (Scheme 10.66).105 This reaction clearly demonstrated that organosilvers can be transmetalated to organopalladium species, even if no halide was present, as demonstrated when starting from vinyl triflates. 

The organopalladium addition reactions to produce substituted olefinic compounds are very useful laboratory syntheses since a wide variety of substituents and functional groups can be present in both the organopalladium species and the olefin. The only groups which may inhibit the reaction to some extent are ones which form stable complexes with the palladium salt, such as unhindered amines. Lower yields are generally... 

Another interesting tandem Michael initiated sequence was developed in our laboratory by combining the conjugate addition of unsaturated alkoxides to alkylidene malonates with a palladium-mediated coupling reaction with an organic halide. In this cydization reaction, an organopalladium species acts as the electrophilic partner of the cydization. This reaction results in the trans addition of the organopalladium species and of the nucleophile across the unsaturation, and therefore, in overall difunctionalization of the unsaturated substrates [66,67]. 

The activation of sp2- or sp-hybridized CH bonds (for reviews on metal-mediated CH bond activation in catalytic processes see [132-137]) generating an organopalladium species suitable for subsequent transformation is the key step for sequences initiated by CH activation. 

Eberson has compared the isomer ratios for the acetoxylation of aromatic substrates at the anode with those obtained from different oxidants (Table 1) [26]. The anodic reaction proceeds by an initial electron transfer, thus forming a radical cation, which is then attacked by acetate ion. The homogeneous reactions, which come closest to the results of the anodic oxidation, are the Co(III)Wi2O40 and the Ag(II) oxidation. These and also the Ce(IV) oxidant, which still exhibits some similarities in the isomer distribution, have been characterized as electron transfer oxidants, whereas the Pd(II)-catalyzed acetoxylation has been shown to proceed via organopalladium species [27]. The isomer distributions in the latter, where predominantly meta orientation is being observed, are drastically different from those of ET-mediated reactions. 

Arylidene butyrolactones were also accessible by addition of nucleophiles onto a Pd(II)-activated alkyne. Thus, the oxidative addition of Pd(0) to 617 in the presence of a base gave 618 in 70% yield (Scheme 102) (96T11463). The proposed mechanism of this reaction was suggested to involve an oxidative insertion of Pd(0) into the Ar-I bond to give the organopalladium species 619. A subsequent addition of the pendant carboxylate to the activated alkyne provides metallocycle 620 that undergoes reductive elimination to furnish 618. Similarly, subjecting 621 to the same reaction conditions produced 622 in 77% yield. 

Organopalladium species with two organic units attached to the metal, R PdR, are generally nnstable extrusion of the metal, in a zero oxidation state, takes place, with the conseqnent linking of the two organic units. Because this is again a concerted process, stereochemistry in the organic moieties is conserved. 

The organopalladium species generated from coupling reaction of ArB(OH)2 with an allene is readily trapped by a properly distanced carbonyl group. Accordingly, 5,6-alkadi-enals are transformed into cw-2-(a-styryl)cyclopentanols. Adding (5)-SEGPHOS to complex the Pd salt has the desirable effect of asymmetric induction. ... 

Exchange reactions between stable organometallics and palladium salts are useful for preparing isomerically pure organopalladium species to be used in situ for insertion reactions. Exchanges have been made with organocobalt , -iron , -phosphorus , -silicon , -lead , -tin , -boron , -thallium , -zinc , -magnesium and -mercury . Of these, mercurials are most often used, e.g. ... 

By use of aryl compounds prepared in this way, the arylation of alkenes and a variety of other organic unsaturated compounds can be achieved.52 A palladium compound, usually Li2PdCl4, is used as a transfer agent and organopalladium species are believed to be unstable intermediates by use of air and a Cu" salt the reaction can be made catalytic, since the Pd metal formed in the reaction is dissolved by Cu11 and the Cu1 so produced is oxidized by air (cf. the Wacker process, Section 24-B-5). A typical reaction is... 

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