It-allylpalladium

The readily accessible dibenzothiapyrylium salt (62)m reacts with ethyl lithio diazoacetate 47) in a 1 1 mixture of ether and tetrahydrofuran at —120 °C to form the diazo compound (65). Treatment of 65 with 5 mol-% of it-allylpalladium chloride dimer in a 1 2 mixture of chloroform and carbon tetrachloride at 0 °C and... 

A variety of additional organozinc-mediated couplings to iT-allylpalladium complexes have been reported, including those of perfluoroalkylzinc iodides,150 vinylzinc bromides,151 allylzinc acetates152 and arylzinc bromides (equations 35-37).153,154... 

The palladium-catalyzed reaction of aryl- and vinyl-tin reagents with stereochemically defined allyl chlorides proceeds with overall retention of configuration, indicating that the second step, entailing interaction of the iT-allylpalladium complex and the organotin, proceeds by transmetallation and reductive elimination (attack at Pd, retention) (equations 166 and 167).142145 Comparable results were obtained with cyclic vinyl epoxides and aryltins.143... 

Alkenylzirconiums react with either preformed allylpalladium complexes155 or allyl acetates and Pd° catalysts156-395 via addition directly to the palladium, followed by reductive elimination. In the reaction with steroidal iT-allylpalladium complexes (equation 174) the natural configuration at C-20 is now obtained, in contrast with the addition of malonates (ligand attack) which yielded the unnatural configuration.74... 

Carbon monoxide, which has a strong propensity for coordination to a metal, can react with a it-allylpalladium intermediate. Although most efforts in the asymmetric carbonylation have... 

More importantly the telomers 133 and 134 are formed by the reaction of pronucleophiles (NuH), incorporating the Nu group [51,52,53]. The reaction can be explained by the amphiphilic nature of bis-it-allylpalladium 68. The first step is the nucleophilic attack of the amphiphilic bis-it-allylpalladium 68 to NuH (or protonation) as shown by 130 to generate n-allylpalladium 131=132, and their electrophilic attacks as shown by 131 and 132 afford 133 and 134 [53a]. The reaction, carried out in deuterated MeOD, affords l-methoxy-2,7-octadiene (135), deuterated at carbon 6 [54]. Pd(Ph3P)4 serves as the catalyst. Most conveniently, Pd(OAc)2 and Ph3P are used as the catalyst precursor. In this case, Pd(OAc)2 is reduced in situ to Pd(0), which is an active species. 

There have been a number of measurements of the NMR spectra of ir-allylic ligands attached to transition metals, especially palladium (see Table XX). The chemical shifts of the rr-allylic ligands appear to be quite characteristic. Terminal and central carbon atoms have shifts of 45 to 78 ppm, and 90 to 130 ppm, respectively. The chemical shifts of the iT-allylpalladium system have been interpreted in terms of a... 

Additives (cosolvents) which serve as ligands have great influence on the reducing power of Smlj. Allylic and propargylic derivatives are reduced via it-allylpalladium species, and the proton source has important effects on the generation of allenes or alkynes. Chiral allenic esters are obtained when pantolactone delivers a proton to racemic organosamarium species derived from 4-phosphato-2-alkynoic esters. ... 

Chapter 3 by Jie Jack Li presents a collection of very interesting total syntheses of naturally occurring indole alkaloids where palladium chemistry plays a central role in the syntheses. Five different types of palladium-mediated reactions are treated (I) oxidative cyclization reactions promoted by palladium (II) species (2) transmetallation reactions with organoboranes, organoslannanes, and organozinc reagents (3) inter- and intramolecular Heck reactions (4) reactions with it-allylpalladium as the intermediate and (5) reactions using C-N bond formation as the key step for the synthesis. 

Compared with well-established electrophilic it-allylpalladium chemisty, the catalytic asymmetric reaction via umpolung of jt-allylpalladium has received very limited exploration [93]. Zhou and co-workers investigated the Pd-catalyzed asymmetric umpolung allylation reactions of aldehydes [22a, 94], activated ketones [95], and imines [96] by using chiral spiro ligands (5)-18e, (S)-17c, and (5)-17a, respectively. One representative example is that of the Pd/(5)-18e-catalyzed umpolung allylation of aldehydes with allylic alcohols and their derivatives, which provided synthetically useful homoallylic alcohols from readily available allylic alcohols, with high yields and excellent enantioselectivities (Scheme 33). 

By the isolation of nonracemic chiral it-allylpalladium complexes82,83, ss, it was unambiguously demonstrated that the oxidative addition step occurs with inversion of configuration. Loss of chirality is mainly due to anti attack of free palladium(O) species on the rr-allyl complex (sec Section 1.5.6.1.2.1.). When the chiral allyl complexes are subjected to nucleophilic addition with soft carbanions, inversion of configuration is observed and thus retention of configuration for the overall process results. 

In a further study [25], 1,2-dimethylcyclohexa-1,4-diene was reacted with bis(acetonitrile)palladium dichloride in aqueous acetone to form predominantly a trans-it-allylpalladium complex as well as some of the Wacker oxidation product 3,4-dimethylcyclohex-3-en-l-one (Eq. (9.22)). 

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 alkenylpalladium bond gives the 7r-allylpalladium complex 330 as an intermediate, which reacts further in two ways. As expected, nucleophiles such as carbon nucleophiles, amines, and alcohols attack the 7r-allylpalladium intermediate to form the 1,4-addition product 331 and 1,2-addition product 332. In the absence of the nucleophiles, y5-H elimination occurs to afford the substituted 1,3-diene 333. In some cases, the substituted 1,3-diene 333 reacts again with aryl halide to form the it -allylpalladium 334. Subsequent elimination affords the 1,4-diarylated 1,3-diene 335. 

Under mild reaction conditions, arynes react with bis-it-allylpalladium complexes leading to 1,2-diallylated benzene derivatives in moderate to good yields (Equation 12.75) [143]. 

Scheme 8.32 Intramolecular carbopalladation of an allene with subsequent nucleophilic trapping of the it-allylpalladium intermediate [154].

Pyrrolidine derivatives 152 are accessible by cascade carbopalladation of a diene 151 and intramolecular trapping of the intermediate it-allylpalladium species by a secondary amine moiety. In this case, an eflBcient enantiocontrol has been achieved using chiral phosphine ligands (Scheme 8.36) [340]. 

Palladium-catalyzed hydrostannation of isoprene was used for the in situ generation of aUylstannane 9, which was trapped by an aldehyde to give the alcohol 10 (Eq. (4)) [20]. It was suggested that an intermediate HPdSn(OAc)Cl2 is formed. The authors proposed two mechanisms for the hydrostannation one according to Scheme 11.1, where Nu = Sn(OAc)Cl2 and another where the double bond inserts into the palladium-tin bond, followed by reductive elimination from a it-allylpalladium hydride. 

Allylic Carbonates and Carbamates AUyhc carbonates are the most reactive of these derivatives [73]. Oxidative addition of the allyl carbonate A is followed by decarboxylation to afford the positively charged it-allylpalladium complex B and alkoxide, which acts as base for the deprotonation of the nucleophile (Scheme 12.39). The in situ formation of the alkoxide, which is a poor nucleophile, is the reason why no additional base has to be used. In addition, the decarboxylation makes the formation of the n-allyl complex an irreversible process, in contrast to the reactions of acetates. 

The first direct addition of a nucleophihc allylpalladium complex to aldehydes was published in 1996 by Yamamoto et al. [139]. Later on, Wallner and Szabo [7c, 140] have reexamined the regio- and stereoselectivities of the reaction employing density functional theory (DFT) computations. The intermediate it-allylpalladium complex 122 is generated by the reaction of an aUyl acetate (or chloride) 121 with hexamethylditin in the presence of allylpalladium chloride dimer (Scheme 12.58). The bis-aUylic system thus generated can then transfer a nucleophilic allyl fragment to the aldehyde 123. The reaction of bis-allylpalladium reagents generally yields branched homoallyUc alcohols 124 with prevalent anti diastereoselectivity. 

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