Ir-Allylpalladium complexes

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

This reaction is quite general and a variety of ir-allylpalladium complexes have been prepared by it (86). 

Treatment of ir-allylpalladium chloride with CO in EtOH affords ethyl 3-butenoate (321)[284]. f3, p-Unsaturated esters, obtained by the carbonylation of 7r-allylpalladium complexes, are reactive compounds for Tr-allyl complex formation and undergo further facile transformation via ir-allylpalladium complex formation. For example, ethyl 3-butenoate (321) is easily converted into 1-carboethoxy-7r-allylpalladium chloride (322) by the treatment with Na2PdCl4 in ethanol. Then the repeated carbonylation of the complex 322 gives ethyl 2-... 

Allylic metal compounds useful for further transformations can be prepared by Pd-catalyzed reactions of allylic compounds with bimetallic reagents. By this transformation, umpolung of nucleophilic ir-allylpalladium complexes to electrophilic allylmetal species can be accomplished. Transfer of an allyl moiety from Pd to Sn is a typical umpolung. 

The stereoselectivity is explained by a change in the mode of acetate attack on the intermediate ir-allylpalladium complex. The coordination of acetate to palladium is effectively blocked by chloride ions, thus hindering cis-migration. As a result, path c is operative (Scheme 9.13). In the absence of chloride both cis and trans attacks (paths a and b) can occur depending on acetate concentration. 

Regioselectivity in the Addition of Soft Carbon Nucleophiles to ir-Allylpalladium Complexes 627... 

In addition, the palladium will preferentially position itself on the sterically less demanding face of an allyl system. Both stereoselectivities are neatly demonstrated by reacting either geometric isomer of 2-ethylidenenorpinane (1) with PdCh, where only a single ir-allylpalladium complex (2) that has a syn configuration is observed and has the Pd trans to the gem-dimethyl bridge (equation 2).32... 

Alkynyl- or vinyl-palladiums can be generated in situ from the corresponding organomercury compounds and added to alkenes to form ir-allylpalladium complexes after H-rearrangement (equation 9).50... 

The addition of potassium enolates to preformed ir-allylpalladium complexes has also been reported.111... 

Stereocontrol (vide infra) can be efficiently engineered in the addition of acetate to ir-allylpalladium complexes (equation 49).168-171 The regioselective addition of acetate of allylpalladium complexes to prepare y-acetoxy-( )-a,0-unsaturated esters172 and y-acetoxy-a.P-unsaturated sulfones has also been achieved (equations 50 and 51).173-174... 

Other synthetically useful additions of carboxylate anions to ir-allylpalladium complexes have been demonstrated, as illustrated in equations (52-54).175-179 Harder oxygen nucleophiles, such as phenox-ides175,176 and alkoxides,175-180 can also be added to ir-allylpalladium complexes (equation 55). Finally, the addition of NO2 to allylpalladium chloride dimer results in the production of ally nitrite (equation 56).18 ... 

Isocyanides react similarly with ir-allylpalladium complexes to generate P,y-un saturated im-inoethers.240"243 Comparable to the CO reaction, isocyanide insertion occurs preferentially in the less... 

The insertion of 1,3-dienes into a ir-allylpalladium complex is believed to proceed via an intermediate in which the metal is complexed to the less hindered double bond of an unsymmetrical diene, followed by an electrocyclic rearrangement which links the more substituted allyl terminus with the more substituted alkene (equation 77).246-251 Electron-withdrawing substituents on the ir-allyl fragment generally increase the rate of insertion,248 whereas substituents on the diene generally slow the rate.268... 

Allenes insert into ir-allyl complexes so as to generate new ir-allyl species (equation 78).248 The insertion of 2-r-butyl-1,3-butadiene into ir-allylpalladium complexes proceeds normally, but is then followed by an unusual cyclization reaction, presumably due to the disposition of the butenyl frag-... 

Finally, the reaction of hydrazones with ir-allylpalladium complexes may involve insertion of C=N into the allyl ligand.265-266... 

Treatment of ir-allylpalladium complexes with excess PdCl2,267-270 or MnCh267 results in the foima-tion of oc,(3-unsaturated ketones or aldehydes (equation 86). 

Basic alcohol solutions have been found to be capable of reducing ir-allylpalladium complexes to the corresponding alkene (equations 90 and 91 ).278-283 In similar fashion, aqueous NaOH media has been reported to effect a disproportionation of the allylpalladium chloride dimer (equation 92).284... 

Steroidal ir-allylpalladium complexes are reduced by L1AIH4 with high regio- and stereo-selectivity (equation 97).W2 Similar results were found with LiAlHCOBu1. 293... 

The reversal in mode of reactivity or umpolung of ir-allylpalladium complexes can be accomplished by two strategies. The first involves conversion of the ir-allyl species to an allyl-M species, where M is a... 

The photochemistry of ir-allylpalladium complexes has been studied to a limited extent. Two basic reactions have been observed. Irradiation at 366 nm of ir-allylpalladium complexes produced 1,5-diene dimers, reportedly via a radical coupling mechanism.334 333 Similar irradiations in the presence of species capable of trapping the presumed allyl radical intermediate, such as BrCCb, BrCH2Ph or allyl bromide, now yield alkylated and halogenated allyls, in addition to 1,5-diene dimer. This reaction fails for simple alkyl or aryl halides due to the instability of the associated radical (equations 130 and 131 ).336... 

Both of these methods could be included as examples of umpolung of ir-allyl species, as the electrophilic allylpalladium has now been transformed into a nucleophilic allyl moiety. Finally, Bn can also be added to ir-allylpalladium complexes (equation 134).342... 

Attack by nucleophiles on the electrophilic ir-allylpalladium complex can take place by two distinct mechanisms, which have opposite stereochemical consequences. Stereochemical inversion is achieved by attack of the nucleophile directly on the allyl ligand on the face opposite the palladium (equation 148). Retention is achieved by attack of the nucleophile at the metal center, followed by reductive elimination (equation 149). The reductive elimination step could proceed through an V-allylpalladium-Nuc species or directly from an i)3-allylpalladium-Nuc complex. Recent evidence strongly suggests the latter pathway.384... 

Soft carbanions, RCXY, as defined in Section 3.3.2.2.1, have been shown to add to ir-allylpalladium complexes directly on the allyl ligand, on the face opposite the Pd (inversion) (equations 150 and... 

Allyltin reagents were suggested to add to ir-allylpalladium complexes directly on the allyl ligand,137138 although a definitive stereochemical experiment could not be successfully carried out. The fact that these reactions proceed with allyl inversion of the stannane (equation 168) would require an unlikely, sterically unfavorable allylpalladium-R intermediate if metal attack was followed. 

In choosing between the two possible modes of addition to a -ir-allylpalladium complex, namely ligand or metal attack, the overwhelming majority of nucleophiles opt for one mode to the total exclusion of the other. Oxygen nucleophiles, specifically carboxylates, however, appear to enjoy a relatively unique position in being able to add by both mechanisms. 

The insertion of CO239 or alkenes253 into ir-allylpalladium complexes proceeds with complete retention of configuration with respect to the metal (equations 199 and 200). 

Addition of hydride to ir-allylpalladium complexes uniformly proceeds by attack at the metal, followed by reductive elimination, irrespective of the source of the hydride, e.g. NaBH4,286,287 UAIH4,292 R3SnH385 or NH4+ HCO2- (equation 203).385... 

The nature of the ligands on the palladium in ir-allyl complexes can influence the regioselectivity exhibited by soft carbon nucleophiles. ir-Allylpalladium complexes generated from methylenecycloalkanes provide an example of the effect of ligands on regiochemistry. The complexes derived from methylene-cyclopentane and methylenecycloheptane both exhibit exclusive exocyclic addition by the anion of methyl(methylsulfonyl) acetate with triphenylphosphine ligands on the Pd (equation 223). In contrast, the complex derived from methylenecyclohexane yields a 62 38 ratio of exocyclic endocyclic addition (equation 226). 

The addition of Grignards and organolithium reagents proceeds by attack at the metal center in ir-allylpalladium complexes. The regiochemical selectivity exhibited by these hard carbon nucleophiles with ir-allyl complexes substituted at the termini with alkyl or aryl groups is comparable to the soft carbon nucleophiles (ligand attack) in most cases, with addition occurring predominantly at the less substituted terminus (equations 248 and 249).1591387... 

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