Intermediate -allylpalladium complexes

Palladium-catalyzed allylation using nucleophiles with allylic halides, acetates, carbonates, etc. via intermediate allylpalladium complexes, and typically with overall retention of stereochemistry. 

The intramolecular nucleophilic attack of a nitrogen atom on an allylpalladium complex was also used to construct a five and a six membered heterocycle in the same step. TV-substituted 2-iodobenzamides bearing an allene function in the appropriate distance from the iodine underwent cyclization through the carbopalladation of the allene moiety by the arylpalladium complex, formed in the first step of the catalytic cycle. The intermediate allylpalladium complex, part of a nine membered ring, cyclized readily to give the pyrroloisoquinolone derivative in excellent yield (4.23.). The nature of the added ligand and the solvent both had a marked influence on the efficiency of the transformation.26... 

The Tsuji-Trost reaction is the Pd-catalyzed allylation of nucleophiles [105] with allylic halides, acetates, carbonates, etc. This transformation proceeds via intermediate allylpalladium complexes (e.g. 110), and typically proceeds with overall retention of stereochemistry. In addition, the trapping of the intermediate allylpalladium complex usually occurs at the least hindered carbon. A representative example of this transformation is shown below in an application to the formation of an 7V-glycosidic bond. Treatment of 2,3-unsaturated hexopyranoside 109 with imidazole in the presence of a Pd(0) catalyst... 

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

An active catalytic species in the dimerization reaction is Pd(0) complex, which forms the bis-7r-allylpalladium complex 3, The formation of 1,3,7-octa-triene (7) is understood by the elimination of/5-hydrogen from the intermediate complex 1 to give 4 and its reductive elimination. In telomer formation, a nucleophile reacts with butadiene to form the dimeric telomers in which the nucleophile is introduced mainly at the terminal position to form the 1-substituted 2,7-octadiene 5. As a minor product, the isomeric 3-substituted 1,7-octadiene 6 is formed[13,14]. The dimerization carried out in MeOD produces l-methoxy-6-deuterio-2,7-octadiene (10) as a main product 15]. This result suggests that the telomers are formed by the 1,6- and 3,6-additions of MeO and D to the intermediate complexes I and 2. 

Formation of a bis-allylated product of 4-nitrobenzoyl chloride by the reaction with allyltrimethyltin in the presence of a benzylpalladium(ll) complex was observed by Stille and co-workers in 1983.4 Trost and King also reported allylation of aldehydes by allyltin reagents in 1990.456 However, the precise mechanism was unclear until the extended studies were performed by Yamamoto and co-workers since 1995.4S7,4S7a 4S7 Aldehydes and imines react with allyltin reagents in the presence of a palladium catalyst (Equations (95) and (96)), and imines are chemoselectively allylated in the presence of aldehydes (Equation (97)).4S7,4S7l 4 b Mechanistic studies using NMR spectroscopy proved that bis-7t-allylpalladium complex 203 is a key nucleophilic intermediate (Figure 3). 

Both enantiomers of racemic 2-propenyl acetate can be formed from meso-type 7r-alkyl palladium intermediates by oxidative addition. 7r-Allylpalladium complexes with two alkyl substituents at the 1- and 3-positons are known to... 

A mechanism for the asymmetric induction for Pd-catalyzed allylic alkylations using chiral ligands such as 23 was proposed on the basis of stereochemical results and the X-ray structure of the intermediate Pd complex 24 <2004T2155>. The enantioselectivity of the alkylations, an example of which is shown in Equation (8), was rationalized by a conformational equilibrium that favored one of two possible 7i-allylpalladium complexes due to steric interference between the aryl substituent on the sulfmyl group of 24 and the phenyl of the 7i-allyl system. 

The stereochemical outcome can be rationalized by the mechanism illustrated in Scheme 14.22. The formation of an enantiomeric pair of allylpalladium complexes (Sp)/(RP)-99 offers two possibilities for the attack of the nucleophile in the subsequent addition leading to the formation of the stereoisomers (R)- and (S)-101. It should be mentioned that the structure of intermediate 102, prepared from a-allenic phosphate 91, could be proved by both NMR spectroscopy and single-crystal X-ray analysis and therefore serves as evidence for the formation of intermediate 100 (Scheme 14.22 and Eq. 14.12) [49]. 

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. 

On the other hand, the reaction of bisallenes with Bu3SnSnBu3 may generate cis-bisallene Pd(SnBu3)2 (C) and/or a chelated cj-allylpalladium complex D reversibly. The fast carbocydization of these intermediates would give rise to a vinylpalladium complex E, which then yields a ds-compound by reductive elimination and/or a cis-bicyclodiene through (T-bond metathesis as a kinetically controlled product (Scheme 16.66). 

The reaction mechanism (Scheme 6.25) involves formation of a cationic 7t-allylpalladium complex by the oxidative addition of the substrate onto the catalyst. In case of a dimethylallyloxycarbonyl protecting group this step is disfavoured compared to Alloc and therefore the removal of dimethylallyl groups is slower or requires more catalyst. Accordingly, in homogeneous CH3CN/H2O solutions deprotection of (allyl)phenylacetate proceeded instantaneously with 2 mol % [Pd(OAc)2]/TPPTS while it took 85 min to remove the dimethylallyl group (cinnamyl is an intermediate case with 20 min required for complete deprotection). The reactivity differences are... 

A new elegant stereoselective synthesis of humulene (192) has been achieved by a route (Scheme 22) in which the 11-membered-ring framework [cf. (191)] is produced by cyclization of the 11-allylpalladium complex derived from intermediate (190). Buddledin-A (193), -B (194), and -C (195) are new piscicidal sesquiterpenoids which have recently been isolated from the root bark of Buddleja davidiC The caryophyliane framework of these compounds has been established by spectroscopic data and X-ray analysis of the mono-bromohydrin (196) derived from buddledin A (193). An extension of previous studies on the cyclization of the epoxy-ketone (197) derived from caryophyllene has shown that the base-catalysed cyclization of the isomeric epoxy-ketones (198) and (199) provides compounds... 

I.1.3 Reactions with 1,2-. 1,3-, and 1,4-dienes. The reaction of conjugated dienes with aryl and alkenyl halides can be explained by the following mechanism. Insertion of a conjugated 1,3-diene into an aryl or ulkenylpalladium bond gives the T-allylpalladium complex 243 as an intermediate, which reacts further... 

The 7r-allylpalladium 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 7r-allylpalladium, which is trapped by malononitrile to give 264. o-Iodophenylmalonate (265) adds to 1,4-cyclohexadiene to form a 7r-allylpalla-dium intermediate via elimination of H—Pd—X and its readdition, which is trapped intramolecularly with malonate to form 266[176],... 

The palladium catalysed substitution reaction of allylic systems has also been utilised in the formation of five membered rings. Intramolecular nucleophilic attack of the amide nitrogen atom on the allylpalladium complex formed in the oxidative addition of the allyl acetate moiety on the catalyst led to the formation of the five membered ring (3.63.). In the presence of a copper(II) salt the intermediate pyrroline derivative oxidized to pyrrole.80... 

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. 

Dienes underwent facile chloropalladation producing an unstable cr-vinylpalladium(II) intermediate, which inserted another mole of allene to give an isolable 2-substituted r)3-allylpalladium complex. 

Ketoacids126,127 form the same intermediates as the allyl 3-ketoesters by nucleophilic addition of the carboxylate to a n-allylpalladium complex. Decarboxylation generates the allylpalladium enolate, which again yields Pd° and allylated ketone. Enol silyl ethers have also been employed with allyl arsenites93 to provide allylated ketones. 

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