Palladium hydride, reversible elimination

An important modification of the initial protocol has been introduced by Larock ef al. to afford indanones. The saturated ring results from the termination of the process by protonolysis of palladium enolate, which is likely to be generated from reversible palladium hydride elimination-addition. Proton source required for protonolysis hypothetically comes from adventitious water coming with hygroscopic chloride or solvent (Scheme 13). ... 

In the absence of base, vinylmercury reagents and lithium tetrachloropalladate(II) react with alkenes to form ir-allylpalladium complexes arising from addition of the vinylpalladium chloride to the alkene followed by palladium hydride elimination, a reverse readdition and rr-allyl formation (equation 10).31... 

Vinyl substitution of primary or secondary allylic alcohols with aryl halides usually produces 3-aryl aldehydes or ketones, respectively. The reaction is believed to involve an addition of the intermediate arylpalladium halide to die double bond, placing the aryl group mainly on the more distant carbon from the hydroxy group, followed by palladium hydride elimination, a reverse readdition and another elimination with a hydrogen atom on the carbon bearing the hydroxy group. The product is probably a ir-com-plex of the enol which ultimately either dissociates or collapses to a a-complex with palladium on the... 

Another potential complication is palladium-promoted double-bond isomerization of the alkene product, which can also destroy a newly formed stereogenic center [10]. This process involves re-addition of palladium hydride to the alkene in the initially generated (alkene)palla-dium hydride complex (the reverse of P-hydride elimination). Double-bond isomerization results when re-addition occurs with different regioselectivity than the original P-hydride elimination. 

It is reasonable to assume that butene formation starts with a palladium hydride. This inserts ethylene twice (the first insertion is probably reversible) and then terminates by / -hydride elimination to regenerate the palladium hydride and form butene. Thus, butene formation shows that olefin insertion (in a Pd-alkyl bond) and y6-elimination are intrinsically rapid reactions. However, the copolymer produced in the same experiment shows neither double olefin insertion errors nor the unsaturated end-groups indicative of -elimination. 

The q -(Si-H)Pd(0) 43 was found to catalyze the hydrocarboxylation reaction of allenes, indicating that 43 worked as the palladium hydride complex 13 in solution via reversible oxidative addition/reductive elimination of the Si-H bond after dissociation of PPhj (Scheme 9.12) [21]. This result prompted us to investigate a... 

Hydride elimination is the step of the Mizoroki-Heck reaction yielding the product (Figure 3.1, step 4). For this process to occur, the insertion complex must be able to rotate to a position where a /3-hydrogen is aligned syn to the palladium(ll) centre. The elimination will then result in formation of a reconstituted alkene and a palladium hydride species. The j8-H-ehmination is reversible (see Figures 3.5 and 3.6) and the preferred formation of the thermodynamically more stable tram-products is thus explained [12]. There is today no precise knowledge of how palladium(ll) is reduced back to catalytically... 

We do not know if the vinylic alcohol is actually an intermediate or whether a hydride-71 complex of it rearranges directly to the aldehyde as probably happens in the palladium-catalyzed oxidation of ethylene to acetaldehyde. The formation of 4% 2-methyl-2-phenylpropanal is unexpected. This product must arise from a reversed addition of the phenylpalladium group followed by a hydrogen transfer from the hydroxyl-bearing carbon to the palladium, followed by reductive elimination of a hydridopalladium group. An alkyoxypalladium intermediate has been proposed (39). 

Similarly, vinylstannanes can also yield products of cine-substitution (Scheme 8.17), specially if tin and an electron-withdrawing or aryl group are bound to the same carbon atom [40, 137-141]. It has been suggested that formation of these products proceeds via intermediate formation of a palladium carbene complex [138, 140] or via reversible /3-hydride elimination [141], and can be avoided by addition of Cu(I) salts [142], which increase the rate of Stille coupling, or by protecting vinylic C-H groups by transient silylation [143]. 

As p-hydride elimination is reversible, hydropalladation with the opposite regiochemistry provides a mechanism for forming regioisomers of the alkene. This allows the most stable alkene that is accessible by the hydropalladation-dehydropalladation sequence to dominate. The only restriction is that all of these processes are syn. The migration can be prevented by the addition of bases like silver carbonate, which effectively removes the hydrogen halide from the palladium complex as soon as it is formed. This synthesis of a complex trans dihydrofuran involves the Heck reaction followed by alkene isomerization and then a Heck reaction without migration to preserve the stereochemistry. 

Palladium-catalyzed vinylations of aryl halides are generally referred to as the Heck reaction (for reviews on the Heck reaction see [34-40]), a versatile process that can be performed inter- and intramolecularly [41]. In the Heck reaction the carbon-carbon single bond forming step is an insertion of an al-kene into the aryl-Pd bond, i.e., a carbopalladation, giving rise to an alkyl-Pd species. If this insertion is terminated by /1-hydride elimination the expected vinylation product is the outcome of the classical Heck reaction. Likewise, reversible insertion of a highly strained olefin where the /1-hydride elimination is suppressed leads to an entry to multiple Pd-catalyzed bond forming processes. 

Other than palladium, rhodium is also known to perform similar migrations. Representative examples will also be briefly discussed. The palladium migration along alkyl chains via reversible P-hydride elimination/reinsertion processes will not be covered [32-36],... 

Rearrangement of the initial insertion product occurs in the reaction of various vinylic palladium halides with alkenes . The vinylic palladium halides are prepared in situ from vinyl mercurials and palladium halides. The initial insertion products undergo a jS-metal hydride elimination followed by a reverse readdition to form relatively stable n-allylic Pd complexes ... 

The catalytic cycle starts with oxidative addition of 73b to palladium(O) (77 78), followed by enantiofacial alkene-coordination to form the diastereomeric complexes (R)-79 and (S)-79 (78 (R)-79 and 78->(S)-79). In both cases, sy -selective migratory insertion R)-79— R)-S0 and (S)-79 (S)-80) and subsequent stereospecific P-hydride elimination furnishes a pair of diastereomeric alkene-hydridopaUadium(II) complexes (R)-81 and (S)-81 ((R)-80 (R)-81 and (S)-80—> (S)-81). The Ji-bound Pd—H fragment in complex (S)-81 dissociates rapidly ((S)-81—>77), whereas (R)-81 undergoes hydropaUadation but with reversed regjoselec-tivity ((R)-81—>(R)-82) P-hydride elimination and alkene dissociation then affords... 

Palladium Pd(II)-catalysed hydroalkylation of Al-protected allylic amines PG(R )N-CH(R )C=CH2 (PG = protecting group) by Bu"ZnBr and other alkylzinc reagents has been reported to afford anti-Markovnikov products PG(R )N-CH(R )CH2CH2-Bu". Mechanistic studies suggest that a reversible jS-hydride elimination/hydride insertion process furnishes the primary Pd-alkyl intermediate, which undergoes transmetallation followed by reductive elimination to form a new sp -sp carbon-carbon bond. ° DFT PBE/3z calculations have been employed to elucidate the solvent effect on hydroxymethoxycarbonylation of cyclohexene catalysed by (Ph3P)2Pd. ... 

The proposed catalytic cycle for the above-described conjugate reduction is outlined in Scheme 17. Initial coordination of the nucleophilic Pd(0)-phosphine complex to the electron-deficient olefin to form complex I is a reversible process that occurs rapidly at room temperature. Oxidative addition of the sihcon hydride moiety to complex I would result in the hydrido olefin complex II. Migratory insertion of the hydride ligand into the electrophilic /S-carbon of the coordinated olefin can result in the palladium enolate intermediate in. Reductive elimination of the silicon moiety and the enolate completes the catalytic cycle and forms the silyl enol ether IV. The latter is prone to acid-catalyzed hydrolysis to produce the saturated ketone. "" ... 

This reaction is reversible by the addition of hydride ion to [6-18]. Similar elimination reactions are observed for the palladium complex. An example of the use of hydrogen chloride in a reversible system is given in equation (6-32). 

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