Tr-allylpalladium

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

Preparation of Tr-Allylpalladium Complexes from Alkenes and Their Reactions with Carhon Nucleophiles... 

In the reaction of aryl and alkenyl halides with 1,3-pentadiene (248), amine and alcohol capture the 7r-allylpalladium intermediate to form 249. In the reactions of o-iodoaniline (250) and o-iodobenzyl alcohol (253) with 1,3-dienes, the amine and benzyl alcohol capture the Tr-allylpalladium intermediates 251 and 254 to give 252 and 255[173-175]. The reaction of o-iodoaniline (250) with 1,4-pen tadiene (256) affords the cyclized product 260 via arylpalladiuni formation, addition to the diene 256 to form 257. palladium migration (elimination of Pd—H and readdition to give 258) to form the Tr-allylpalladium 259, and intramolecular displacement of Tr-allylpalladium with the amine to form 260[176], o-Iodophenol reacts similarly. 

The TT-allylpalladiLim complexes formed as intermediates in the reaction of 1,3-dienes are trapped by soft carbon nucleophiles such as malonate, cyanoacctate, and malononitrile[ 177-179). The reaction of (o-iodophenyl-methyl) malonate (261) with 1,4-cyclohexadiene is terminated by the capture of malonate via Pd migration to form 262. The intramolecular reaction of 263 generates Tr-allylpalladium, which is trapped by malononitrile to give 264. o-[odophenylmalonate (265) adds to 1,4-cyciohexadiene to form a Tr-allylpalladium intermediate via elimination of H—Pd—X and its readdition, which is trapped intramolecularly with malonate to form 266)176]. 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

Allylic carbonates are most reactive. Their carbonylation proceeds under mild conditions, namely at 50 C under 1-20 atm of CO. Facile exchange of CO2 with CO takes place[239]. The carbonylation of 2,7-octadienyl methyl carbonate (379) in MeOH affords the 3,8-nonadienoate 380 as expected, but carbonylation in AcOH produces the cyclized acid 381 and the bicyclic ketones 382 and 383 by the insertion of the internal alkene into Tr-allylpalladium before CO insertion[240] (see Section 2.11). The alkylidenesuccinate 385 is prepared in good yields by the carbonylation of the allylic carbonate 384 obtained by DABCO-mediated addition of aldehydes to acrylate. The E Z ratios are different depending on the substrates[241]. 

It is known that tr-allylpalladium acetate is converted into allyl acetate by reductive elimination when it is treated with CO[242,243]. For this reason, the carbonylation of allylic acetates themselves is difficult. The allylic acetate 386 is carbonylated in the presence of NaBr (20-50 mol%) under severe conditions, probably via allylic bromides[244]. However, the carbonylation of 5-phenyl-2,4-pentadienyl acetate (387) was carried out in the presence of EtiN without using NaBr at 100 °C to yield methyl 6-phenyl-3,5-hexadienoate (388)[245J. The dicarbonylation of l,4-diacetoxy-2-butene to form the 3-hexenedioate also proceeds by using tetrabutylphosphonium chloride as a ligand in 49% yield[246]. 

The regioselective and stereospecific construction of C-20 stereochemistry is explained by the following mechanism. The Pd(0) species attacks the ( )-/3-carbonate 616 from the a-side by inversion to form the Tr-allylpalladium species 620, which has a stable syn structure[392]. Then concerted decarboxylation-hydride transfer as in 621 takes place from the a-side to give the unnatural configuration in 617. On the other hand, the Tr-allylpalladium complex 622... 

It is widely recognized that an allyl group is a useful protecting group for acids, amines, and alcohols. Facile formation of a Tr-allylpalladium complex from... 

Scheme 16.22 A proposed reaction mechanism via an nnti-Tr-allylpalladium complex.

Hard carbon nucleophiles of organometallic compounds react with ir-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal Tr-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

When butadiene is treated with PdCl2 the l-chloromethyl-Tr-allylpalladium complex 336 (X = Cl) is formed by the chloropalladation. In the presence of nucleophiles, the substituted 7r-methallylpalladium complex 336 (X = nucleophile) is formed[296-299]. In this way, the nucleophile can be introduced at the terminal carbon of conjugated diene systems. For example, a methoxy group is introduced at the terminal carbon of 3,7-dimethyl-l,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the 7r-allyl complex 339 after the cyclization[288]. 

An intramolecular version offers useful synthetic methods for heterocycles. The total syntheses of a- and 7-lycoranes (373 and 374) have been carried out by applying the intramolecular aminochlorination of the carbamate of 5-(2-aminoethyl)-l,3-cyclohexadiene (372) as a key reaction[312,313]. Interestingly, the 4,6- and 5,7-diene amides 375 and 377 undergo the intramolecular animation twice via tr-allylpalladium to form alkaloid skeletons of pyrrolizidine (376) and indolizidine (378), showing that amide group is reactive[314]. 

Allylic acetates are widely used. The oxidative addition of allylic acetates to Pd(0) is reversible, and their reaction must be carried out in the presence of bases. An important improvement in tr-allylpalladium chemistry has been achieved by the introduction of allylic carbonates. Carbonates are highly reactive. More importantly, their reactions can be carried out under neutral conditions, 14]. Also reactions of allylic carbamates(14], allyl aryl ethers[6,15], and vinyl epoxides[16,17] proceed under neutral conditions without addition of bases. 

Another reaction occurs by the attack of a soft nucleophile at the central carbon to form the 7r-allylpalladium complex 7, which undergoes further reaction with the nucleophile typical of Tr-allylpalladium complexes to form the alkene 8,... 

The intermolecular insertion of alkenes into tr-allylpalladium is unknown, except with norbomadiene[476]. On the other hand, the intramolecular insertion of alkene group in 766 into 7r-allylpalladium proceeds smoothly to give the... 

Allyl esters of acetoacetates7-8-9-11 react with Pd° catalysts to generate initially a bisphosphine allylpal-ladium cation, with the 3-ketocarboxylate serving as counterion. Under the reaction conditions the (3-ketocarboxylate decarboxylates, yielding a -Tr-allylpalladium ketone enolate complex. The required nucleophile is thus formed in situ and is capable of Pd-mediated alkylation. A wide spectrum of reactions have been based on this chemistry which will be discussed in later sections. 

The reactivity of -Tr-allylpalladium-phosphine complexes generated stoichiometrically or from alkenes allylically substituted with a leaving group, is essentially identical and, as a result, allyl species will be generally considered in this section without distinction as to the origin of the palladium complex. 

Oxygen nucleophiles can be added successfully to Tr-allylpalladium complexes by various methodologies. Those additions that are fostered by the use of classical oxidizing agents will be treated separately in Section 3.3.2.3.2. 

The most intense interest in the addition of oxygen nucleophiles to Tr-allylpalladium complexes has centered on the delivery of OAc. For example, in allylpalladium-OAc complexes, acetate can be induced to migrate to the allyl ligand by the addition of CO (equation 46).162-164 Rearrangements and isomerizations of allyl acetates can also be readily accomplished via Pd catalysis (equations 47 and... 

Oxa- Tr-allylpalladium complexes (10), which can also be envisioned as palladium enolates (11), are susceptible to (3-hydride elimination, and as such have been principally used in methodologies for the preparation of (3,(3-unsaturated carbonyl compounds. 

What may be characterized as an aza-Tr-allylpalladium complex has been proposed as an intermediate in the preparation of a, -unsaturated nitriles by decarboxylation-dehydration of allyl a-cyanocarboxy-lates (equation 144).369 These reactions have been summarized in the context of several reviews.89,117 If the oxa-7r-allyl species is blocked from the elimination of a (3-H, then it can undergo typical ir-allylpalla-dium chemistry, such as insertion by alkenes, dienes and CO (equation 145).370 371... 

It is clear that although substantial progress has been made in the understanding and control of regioselectivity in the addition of nucleophiles to Tr-allylpalladium complexes via attack at the metal, considerable variability still exists in these reactions that is not readily mechanistically rationalized. 

Acyliron and acylnickel anions, as well as NaCo(CO)4, acylate Tr-allylpalladium complexes selectively at the less substituted allyl terminus (equation 294).223... 

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