C—Pd bond

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

The most characteristic feature of the Pd—C bonds in these intermediates of both the stoichiometric and catalytic reactions is their reaction with nucleophiles, and Pd(0) is generated by accepting two electrons from the nucleophiles as exemplified for the first time by the reactions of 7r-allylpalladium chloride[2] or PdCl2-COD[3] complex with malonate and acetoacetate. It should be noted... 

Another feature of the Pd—C bonds is the excellent functional group tolerance. They are inert to many functional groups, except alkenes and alkynes and iodides and bromides attached to sp carbons, and not sensitive to H2O, ROH, and even RCO H. In this sense, they are very different from Grignard reagents, which react with carbonyl groups and are easily protonated. 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

The alka-l,2,4-trienes (ailenylaikenes) 12 are prepared by the reaction of methyl propargyl carbonates with alkenes. Alkene insertion takes place into the Pd—C bond of the ailenyipailadium methoxide 4 as an intermediate and subsequent elimination of/3-hydrogen affords the 1,2,4-triene 12. The reaction proceeds rapidly under mild conditions in the presence of KBr. No reaction takes place in the absence of an alkali metal salt[4j. 

Organometallic Compounds. Mononuclear carbon monoxide complexes of palladium are relatively uncommon because of palladium s high labihty, tendency to be reduced, and competing migratory insertion reactions in the presence of a Pd—C bond (201). A variety of multinuclear compounds... 

Palladium(O)- and Pd(II)-catalyzed carbonylation reactions have been the subject of several recent articles 32, 116, 119, 124, 173, 228, 232). In one case, the attending CO insertion into a Pd—C bond was shown to proceed with retention of configuration at carbon 131a, 218). 

A plausible mechanism proposed for this reaction involves migratory insertion of an olefin into the Pd-Si bond of a paUadium-silyl intermediate I followed by migratory insertion of the pendant olefin into the resulting Pd-C bond of II forming palladium-alkyl intermediate III. Reaction of Iff with hydrosilane releases the carbocy-cle to regenerate the palladium-silyl complex I (Scheme 3-21) [61]. 

The other mechanism (B) includes (b) insertion of the C=C triple bond of the enyne into the Pd-H bond with Pd bound to the internal carbon and H to the terminal carbon to give 30, (c) insertion of CO into the Pd-C bond, and (d) reductive elimination of 29 under regeneration of Pd(0)Ln (Scheme 7-8). 

The reaction between [PdCl2(MeCN)2] and ethylene diamine-A, 3-ethylguanine yields the analogous C8-cyclometalated compound [PdCl(en-Et-A-C< )]+ 28 (Pd-C bond length 1.974 A) (56) (Fig. 29). Again, proton transfer to the adjacent N7 accompanies C8-metalation and this is observed downfield in the NMR spectrum... 

With the aid of a normal coordinate analysis involving different isotopomers a linear structure of the Pd-Si-0 molecule is deduced. The results of ab initio MP2 calculations (Tab. 4) confirm the experimentally obtained IR spectra and their interpretation. The Pd-C bond in PdCO is similar to the Pd-Si bond in PdSiO which means, that the donor bond is strengthened by x acceptor components. This conclusion is in line with the high value of the Pd-Si force constant (exp. f(PdSi) = 2.69, f(SiO) = 8.92 mdyn/A) as well as with the energy of PdSiO (Pd + SiO —> PdSiO + 182 kJ/mol for comparison Pd + CO —> PdCO + 162 kJ/mol, MP2 level of theory). 

The insertion of allenes in the Pd-C bond of cyclopalladated 3-arylisoquinoline derivatives 327 afforded compounds 328, derived from the berberinium cation (Scheme 71). This reaction takes place via the formation of an intermediate (r/ -allyOpalladium complex <2003JOM313>. This chemistry has been extended to the preparation of other cationic N-heterocycles, including naphtho[r+/ ]( uinolizinium derivatives <2004EJ01724>. 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Figure 4. Structural data obtained for the cationic r 3-allyl complexes 26 (R = Me or CH2Ph), illustrating the differing Pd-C bond lengths which result from unfavorable steric interactions.

It is evident that steric repulsion occurs between one of the allylic substituents and the syn-disposed oxazoline substituent, causing a significant lengthening of the corresponding Pd-C bond and a slight rotation (ca. 15 °) of the allyl moiety. Comparison of the -intermediate 26 with the absolute configuration of the product indicated that nucleophilic... 

Electronic differentiation - the expected electronic influence of the unsymmetrical ligand is clearly displayed in the differing lengths of the two Pd-C bonds, with Pd-C(l) (Xrans to phosphorus) be-... 

The proposed mechanism involves the usual oxidative addition of the aryl halide to the Pd(0) complex affording a Pd(II) intermediate (Ar-Pd-Hal), subsequent coordination of allene 8 and migratory insertion of the allene into the Pd-C bond to form the jt-allylpalladium(II) species 123. A remarkable C-C bond cleavage of 123 leads by decarbopalladation to 1,3-diene 120 and a-hydroxyalkylpalladium species 124. /8-H elimination of 124 affords aldehyde 121 and the H-Pd-Hal species, which delivers Pd(0) again by reaction with base (Scheme 14.29). The originally expected cyclization of intermediate 123 by employment of the internal nucleophilic hydroxyl group to form a pyran derivative 122 was observed in a single case only (Scheme 14.29). 

Similarly, a double functionalization can be reached when an activating group is present in close vicinity to the triple bond. Tsuji et al. have discovered that with a diphosphine palladium(O) complex, a carbonate function in the a-position of the alkyne provides by decarboxylation a palladium methoxy species on which the alkyne moiety can be isomerized into an al-lenyl a -bonded group. CO insertion in the Pd - C bond, reductive elimination with the methoxy group and further cyclization with incorporation of a second CO molecule give rise to the corresponding cyclopentenone as shown in Scheme 21 [127]. 

Because of its nature, DEK must form via a hydride mechanism. Up to the formation of a Pd-acyl intermediate, the paths leading to MP or DEK are similar. DEK forms if the insertion of a second molecule of ethene into the Pd-acyl bond is followed by protonolysis of the Pd - C bond of the resulting Pd-alkylacyl intermediate. 

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. 

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. 

An interesting reaction course was followed in the case of dipropargyl amides and dipropargyl sulfide. With these diynes, in fact, triple bond insertion into the Pd - C bond of an alkoxycarbonylpalladium complex was followed by insertion of the second triple bond and alkoxycarbonylation (Scheme 35) [303,... 

The mechanism is the same as a regular Stille coupling, except that coordination of CO and insertion into the Pd-C bond intervenes between the oxidative addition and transmetallation steps At some point the TfO- group on Pd is exchanged for a Cl- group. 

As the insertion of the carbon monoxide retains the configuration of the Pd-C bond, a modification of the procedure permits the conversion of /ra/w-cinnamic acid into the cis-isomer (Scheme 8.11) [5]. 

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