Jt-allylpalladium complex

Palladium can be the leaving atom if the substrate is a Jt-allylpalladium complex (an T complex). Ions of ZCHZ compounds react with such complexes in the presence of triphenylphosphine, for example. 

Shimizu and Tsuji [4] reported the first highly regioselective synthesis of 1,2-di-substituted allylic amines through capture of a Jt-allylpalladium complex by pyrrolidine (Scheme 16.2). This methodology has since been extended to a wide range of amines and allenes [5]. 

The reaction was further applied to the synthesis of spiro heterocycles (Scheme 16.4) [8], The oxidative addition of an iodide to a Pd(0) species generates an ArPdl species, into which an internal olefin inserts to form an alkylpalladium complex otherwise difficult to access. Allene participates in the reaction at this stage to provide a jt-allylpalladium complex, which is attacked by the amine intramolecularly to afford the procuct. 

A one-pot synthesis of 3,3-disubstituted indolines was achieved by taking advantage of a sequential carbopalladation of allene, nucleophile attack, intramolecular insertion of an olefm and termination with NaBPh4 (Scheme 16.6) [10]. First, a Pd(0) species reacts with iodothiophene selectively to afford ArPdl, probably because the oxidative addition step is facilitated by coordination with the adjacent sulfur atom. Second, the ArPdl adds to allene, giving a Jt-allylpalladium complex, which is captured by a 2-iodoaniline derivative to afford an isolable allylic compound. Under more severe conditions, the oxidative addition of iodide to Pd(0) followed by the insertion of an internal olefm takes place to give an alkylpalladium complex, which is transmetallated with NaBPh4 to release the product. 

The palladium-catalyzed reaction of iodobenzene and an allenyl malonate provided vinylcyclopropane in a highly regioselective manner (Scheme 16.7) [11, 12]. A jT-allylpalladium complex, generated by the addition of PhPdl to a 2-allenyl malonate, can be trapped by an internal malonate anion to afford a vinylcyclopropyl derivative. The site selectivity in this cyclization is dependent on the nature of the entering RX groups, catalytic systems involving phosphine ligands, solvents and bases. 

The possibility of Jt-allylpalladium complex formation through carbopalladation is excluded from the observation that no four- and/or six-membered rings are produced. The reaction apparently proceeds via an alternative pathway which involves a sequence of Jt-coordination of PhPdl to an allenic terminal double bond, oxypallada-tion and ensuing reductive elimination (Scheme 16.9). 

First, oxidative addition of 2-iodophenol to a Pd(0) species gives rise to an arylpal-ladium complex, which in turn undergoes carbonylation followed by insertion of allene to generate a 2-acyl-jt-allylpalladium complex. Attack by an internal hydroxyl group gives an a-exo-methylene ketone (Scheme 16.11). 

An a-allenic sulfonamide undergoes Pd-catalyzed carbonylative cyclization with iodobenzene, affording a mixture of isomeric heterocycles (Scheme 16.12) [17]. The coupling reaction of an allene with a PhCOPdl species takes place at the allenyl central catrbon to form a 2-acyl-Jt-allylpalladium complex, which is attacked by an internal sulfonamide group in an endo mode, affording a mixture of isomeric heterocycles (Scheme 16.13). 

Homoallylic alcohols are provided by Pd-catalyzed reaction of iodobenzene, allene and aldehydes (Scheme 16.15) [19, 20]. A nucleophilic allylindium intermediate is generated through transmetallation of a Jt-allylpalladium species with indium. Such a Jt-allylpalladium complex can alternatively be provided through carbopalladation of ArPdl to a proximate acetylene followed by insertion of allene. 

The reaction of an allenylcyclobutanol with ArPdl would provide four possible jT-allylpalladium complexes, that is, two anti- and two syn-isomers. The rearrangement would, however, take place more favorably via two anti-isomers, A and B, which are equilibrated. Consequently, the products can be provided in a highly dia-stereoselective manner via thermodynamically more stable intermediate A (Scheme 16.22) [26],... 

Three-component assembly of allenes, organic halides and arylboronic acids has been reported in which Suzuki coupling of a Jt-allylpalladium complex with an orga-noboronic acid is utilized (Scheme 16.26) [31], Addition of phosphorus ligands to the reaction mixture greatly decreases either the product yields or E/Z ratios. The decrease in E/Z ratio may be explained based on the fact that donor ligands readily promote anti-syn rearrangement of a Jt-allylpalladium species via a cr-allylpalladium intermediate. 

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

Scheme 16.90 Ene reaction of an allene with a Jt-allylpalladium complex.

A bromoallene was demonstrated to act as an allyl dication equivalent. When treated with Pd(0) in an alcoholic solvent, an ei-hydroxybromoallene provides a mediumsized heterocycle (Scheme 16.101) [106]. The oxidative addition of a bromoallene to Pd(0) generates an allenylpalladium species, which is successively transformed into a Jt-allylpalladium complex through the attack of the hydroxyl group on the sp carbon followed by the protonation of the resulting Pd-carbene complex. Finally, the products are provided as a mixture of regioisomers by the nucleophilic attack of the external methanol. 

In 1998, Yamamoto et al. reported the first catalytic enantioselective allylation of imines with allyltributylstannane in the presence of a chiral 7i-allylpalladium complex 23 (Scheme 9) [15]. The imines derived from aromatic aldehydes underwent the allylation with high ee values. Unfortunately, the allylation reaction of aliphatic imines resulted in modest enantioselectivities. They proposed that a bis-Jt-allylpalladium complex is a reactive intermediate for the allylation and reacts with imines as a nucleophile. The bis-Jt-allylpalladium complex seemed the most likely candidate for the Stille coupling [16]. Indeed, the Stille coupling reaction takes place in the presence of triphenylphosphine even if imines are present, whereas the allylation of imines occurs in the absence of the phosphine [17]. They suggested the phosphine ligand played a key role in controlling the... 

Hydrocarbonation of enyne proceeds via hydropalladation of enyne with 16, which affords the exo methylene Jt-allylpalladium complex. Reductive elimination would lead to formation of 5. 

In the hydrocarbonation of methylenecyclopropanes 8 with nitriles, the hydro-palladation of 8 with 16 gives the alkylpalladium complexes 23 and/or 24 (Scheme 5). The complex 23 would undergo rearrangement by distal bond cleavage to give the Jt-allylpalladium 25 (route A). The reductive elimination of Pd(0) from 25 produces 9. The palladium complex 24 would isomerize to the Jt-allylpalladium complex 27 via proximal bond cleaved ring-opened intermediate 26 (route B). The reductive elimination of Pd(0) from 27 gives 10. 

A nucleophilic allylation-heterocyclization via bis-Jt-allylpalladium complexes with allyltributylstannane and an o-chloroallyl benzaldehyde generates allyl-vinyl-substituted phthalans in good yields (Equation 144) <2002CL158>. No Stille coupling products have been observed in these reactions. 

Intermolecular Reactions of jt-Allylpalladium Complexes Preparation of Amines... 

The ambiphilic character of JT-allylmthenium complexes is in remarkable contrast to palladium chemistry [29]. A series of (jt-C3H5)RuX(CO)3 (X = Br, OAc or OTf) complexes prefer the attack of electrophiles such as aldehydes as well as the attack of nucleophiles such as NaCH(C02Me)2, while Jt-allylpalladium complexes react exclusively with nucleophiles. Thus, stoichiometric reactions of Jt-allylmthenium complex with benzaldehyde and the sodium salt of diethyl malonate afford the corresponding homoallyl alcohol and allylmalonate, respectively (Scheme 5.1). The carbonyl ligand plays a very important role, and ambiphilic reactivity is realized only in ruthenium complexes bearing a carbon monoxide ligand. 

Scheme 3-54). This transformation constitutes a cascade of an intramolecular Heck insertion and subsequent heterocyclization. The initially formed arylpalladium species attacks the bridgehead position of the diene functionality in 238 to foim a JT-allylpalladium complex which is trapped by the internal nucleophilic phenol moiety (cf. Scheme 3-26). Since the starting diene 238 can be prepared in both enantiomeric forms by asymmetric reduction of a ketone, this sequence allows the preparation of both the natural morphine and its unnatural enantiomer. 

Kamijo and Yamamoto developed a palladium-catalyzed three-component synthesis of N-cyanoindoles (131) (Scheme 5.39) [81]. The reaction of a 2-alkynyliso-cyanobenzene (132), allyl methyl carbonate, and trimethylsilylazide in the presence of Pd2(dba)3 (2.5 mol%) and tri(2-furyl)phosphine (10 mol%) at 100 °C afforded the N-cyanoindoles 131 in good yield. Key steps of this reaction involved the formation of jt-allylpalladium complex 133, its Curtius-like rearrangement to intermediate 134, and subsequent isomerization to the Jt-allylpalladium cyanamide complex 135. A wide range of functional groups is tolerated at the para, meta, and even ortho positions of the aromatic ring. 

Many common ligands are shown there. It is seen that 361 was converted to the Ji-allyl complex with various ligands and reacted with methyl methanesulfonyl sodioacetate (359) to form a mixture of 362 and 363. A variety of carbanion nucleophiles can be reacted with jt-allylpalladium complexes.221... 

This transmetalation changes polarity of allyl ligand the starting jt-allylpalladium complex undergoes nucleophilic attack at the terminal carbons of the allyl ligand, while the produced Fe complex reacts with an electrophile to form a carbon-carbon bond. 

The Pd-catalysed allylation of carbon nucleophiles with allylic compounds via Jt-aUylpaUadium complexes is called the Tsuji-Trost reaction [32]. Typically, an allyl acetate or carbonate (54) reacts with a Pd-catalyst resulting in displacement of the leaving group to generate a Jt-allylpalladium complex (55) that can undergo substitution by a nucleophile (56) (Scheme 4.14). In 1965, Tsuji reported the reaction of ti-aUylpaUadium chloride with nucleophiles such as enamines and anions of diethyl malonate and ethyl acetoacetate. A catalytic variant was soon reported thereafter in the synthesis of allylic amines [33]. In 1973, Trost described the alkylation of alkyl-substituted 7i-aUylpalladium complexes with methyl methylsulfonylacetate... 

Ma has developed a three-component allene carboamination reaction for the stereoselective synthesis of 2,5-as-disubstituted pyrrolidine derivatives [54]. A representative transformation involving allene 58, 4-iodoanisole, and imine 59 that generates 60 in 90% yield is shown below (Eq. (1.28)). The reaction is believed to proceed through the intermediate Jt-allylpalladium complex 62, which is formed by carbopalladation of the alkene to give 61 followed by addition of the malonate anion to the activated imine. Intramolecular capture of the allylpalladium moiety by the pendant nitrogen nucleophile affords the pyrrolidine product. A related asymmetric synthesis of pyrazolidines that employs azodicarboxylates as one of the electrophilic components has also been reported [55]. The pyrazolidine products are obtained with up to 84% ee when chiral bis oxazolines are employed as ligands. 

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