Palladium enolate, mechanism

In the prostaglandin synthesis shown, silyl enol ether 216, after transmetaJ-lation with Pd(II), undergoes tandem intramolecular and intermolecular alkene insertions to yield 217[205], It should be noted that a different mechanism (palladation of the alkene, rather than palladium enolate formation) has been proposed for this reaction, because the corresponding alkyl enol ethers, instead of the silyl ethers, undergo a similar cyclization[20I],... 

Addition of ketene silyl acetals to aldehydes and ketones is also mediated by achiral palladium(ll) acetate-diphosphine complexes (Equation (109)).46S,46Sa Although the precise mechanism is still unclear, high catalytic activity may be ascribed to the intermediacy of palladium enolates. 

The mechanism of the trimethylsilyl enol ether cyclization may involve formation of a palladium enol-ate which adds to the double bond. However, another mechanism is also possible involving attack of a palladium(II)-alkene complex upon the silyl enol ether double bond. 

NMR-spectroscopic analysis led to the postulation of a mechanism [13] (Scheme 7) involving the formation of a palladium enolate. 

Fujii, A., Hagiwara, E., Sodeoka, M. (1999) Mechanism of Palladium Complex-catalyzed Enantioselective Mannich-type Reaction Characterization of a Novel Bin-uclear Palladium Enolate Complex. J. Am. Chem. Soc. 121 5450-5458. 

Ricci and coworkers [64] studied oxazoline moiety fused with a cyclopenta[P]thio-phene as ligands on the copper-catalyzed enantioselective addition of Et2Zn to chalcone. The structure of the active Cu species was determined by ESI-MS. Evans and coworkers [65] studied C2-symmetric copper(II) complexes as chiral Lewis acids. The catalyst-substrate species were probed using electrospray ionization mass spectrometry. Comelles and coworkers studied Cu(II)-catalyzed Michael additions of P-dicarbonyl compounds to 2-butenone in neutral media [66]. ESI-MS studies suggested that copper enolates of the a-dicarbonyl formed in situ are the active nucleophilic species. Schwarz and coworkers investigated by ESI-MS iron enolates formed in solutions of iron(III) salts and [3-ketoesters [67]. Studying the mechanism of palladium complex-catalyzed enantioselective Mannich-type reactions, Fujii and coworkers characterized a novel binuclear palladium enolate complex as intermediate by ESI-MS [68]. 

The reaction of trimethylsilyl dienol ethers with PdCl2(NCMe)2 affords palladium 7 -allyls with an aldehyde functionality (Equation (38)). The actual mechanism of the reaction was not discussed but the rearrangement of a palladium enolate transferred from the siloxane moiety seems plausible. A related rearrangement has been observed in a Pd aryloxide that leads to a bis 77 -allylic Pd complex, as was discussed in Section 8.06.3. - ... 

Shibasaki, Sodeoka, and coworkers disclosed the first enantioselective variant following this mechanism and noticed that sUyl enol ethers 220 undergo enantioselective additions to benzaldehyde when catalyzed by diaqua palladium(ll) complexes 268 of BINAP and Tol-BINAP to give, after treatment with acid, aldol products in up to 92% ee. Activation by tetramethylurea was found to be beneficial to enantioselectivity. NMR studies revealed that palladium enolates 269 function as real intermediates and identified them as active nucleophiles. In a key step of the... 

Two competing chain-transfer mechanisms in copolymerization of CO and ethene catalyzed by Pd11 acetate/dppp complexes were found. One involves termination via an isomerization into the enolate followed by protonation with methanol the rate of this reaction should be independent of the concentration of the protic species. The second chain-transfer mechanism comprises termination via methanolysis of the acylpalladium species, and subsequent initiation by insertion of ethene into the palladium hydride bond.501... 

Silyl enol ethers of alkenyl methyl ketones can be efficiently cyclized to cyclopentenones and cyclohexenones by treating them with stoichiometric amounts of palladium acetate244 an example indicating the elaboration of this approach to the synthesis of a reduced benzoxepinone derivative, and the suggested244 mechanism of the reaction, are depicted in Scheme 174. 

The intermolecular Heck reaction of halopyridines provides an alternative route to functionalized pyridines, circumventing the functional group compatibility problems encountered in other methods. 3-Bromopyridine has often been used as a substrate for the Heck reaction [124-126]. For example, ketone 155 was obtained from the Heck reaction of 3-bromo-2-methoxy-5-chloropyridine (153) with allylic alcohol 154 [125]. The mechanism for such a synthetically useful coupling warrants additional comments oxidative addition of 3-bromopyridine 153 to Pd(0) proceeds as usual to give the palladium intermediate 156. Subsequent insertion of allylic alcohol 154 to 156 gives intermediate 157. Reductive elimination of 157 gives enol 158, which then isomerizes to afford ketone 155 as the ultimate product This tactic is frequently used in the synthesis of ketones from allylic alcohols. 

Scheme 2 shows a similar mechanistic pathway for a Heck reaction taking place on a Pd octahedral comer. This mechanism is based on that established for soluble Pd catalysts (ref. 5). Adsorption of the aryl halide (or aryl acid chloride after decarbonylation) gives the aryl Pd halide, 15, by way of the adsorbed intermediate, 14. Vinyl ether adsorption, as in 16, takes place as described in Scheme 1. Aryl insertion gives the halometalalkyl, 17, which on f) elimination to the available 4dxy orbital gives the aryl enol ether, 2 (or 1 depending on which hydrogen is eliminated in 17). The resulting halo palladium hydride, 18, then reacts with the tertiary amine to give the amine hydrochloride and regenerates the octahedral comer for further reaction.

The second Heck reaction involves a naphthyl iodide (Ar2 = 2-naphthyl) but the initial mechanism is much the same. However, the enol ether has two diastereotopic faces syn or anti to the aromatic substituent (Ar1) introduced in the first step. Palladium is very sensitive to steric effects and generally forms less hindered complexes where possible. Thus coordination of the palladium(II) intermediate occurs on the face of the enol ether anti to Ar1. This in turn controls all the subsequent steps, which must be syn, leading to the trans product. The requirement for syn p-hydride elimination also explains the regiochemical preference of the elimination. In this cyclic structure there is only one hydrogen (green) that is syn the one on the carbon bearing the naphthyl substituent is anti to the palladium and cannot be eliminated.. ... 

Relatively less acidic ketones compared to 1,3-dicarbonyl compounds are also suitable substrates for the palladium catalyzed coupling. a-Aryl ketones are obtained as products. In the early examples, masked ketone enolates such as silyl enol ethers [42] and enol acetates [43-45] were used in the presence of a tin source. These reactions involve tin enolates or acylmethyltins as intermediates and thus proceed by transmetalation (mechanism B in Scheme 1). 

Improved understanding of the mechanism of palladium-induced dehydrogenations has led to the development of significantly better catalysts and reaction conditions. In particular, mixtures of PdCl2(PhCN)2 and silver triflate in the presence of -methylmorpholine have allowed the efficient dehydrogenation of aldehydes under ambient conditions and in nonacidic media (equation 18). Ketones undergo a similar reaction, affording enones in 60-78% yield, but require prior formation of the tin enol-ate with tin(II) triflate.Under these conditions, however, 2 equiv. of palladium(II) chloride were used to effect conversion, which severely limits the usefulness of the method. 

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