Hydridopalladium intermediate

An electron-rich metal can deprotonate the dicarbonyl derivative, affording the hydridopalladium intermediate 23, which can undergo a Tr-allyl 24 formation through diene insertion (which can be assimilated to a hydridopalladation of olefin) (Scheme 7). The attack of the enolate to the -jr-allyl species occurs with good enantioselectivity in the presence of the chiral ligand. The final product 21 is released and the palladium(O) complex 22 is regenerated. 

The reaction of 2-alkynoate esters and electron-rich phenols in the presence of formic acid and catalytic amounts of either Pd(OAc)2 or Pd2(dba)3 CHCl3 provides an efficient way to synthesize coumarins (Eq. 35) [76]. This chemistry has been utilized to synthesize the natural products fraxinol methyl ether, ayapin, herniarin, xanthoxyletin, and alloxanthoxyletin. The reaction appears to involve a Pd(0) catalyst and the formation of hydridopalladium intermediates. 

Pd(0) generated by the action of TPPTS may react in oxidative addition across the C-H bond of CH-acids, and the resulting hydridopalladium intermediate reacts with allylating agent with the formation of either a- or Jt-allyl-palladium intermediate. It is quite noteworthy that the use of aqueous solvent may help the formation of allylpalladium intermediate in an Snl-like process. 

Another useful class of palladium-catalyzed cycloisomerizations is based on the general mechanistic pathway shown in Scheme 13. In this chemistry, a hydridopalladium acetate complex is regarded as the catalytically active species.27b-29 According to this pathway, coordination of a generic enyne such as 59 to the palladium metal center facilitates a hydropalladation reaction to give intermediate 60. With a pendant alkene, 60 can then participate in a ring-form-... 

In analogy to the mechanism of the palladium-catalyzed enyne cyclization, it is postulated that exposure of palladium(O) to acetic acid promotes in situ generation of hydridopalladium acetate LnPd"(H)(OAc). Alkyne hydrometallation affords the vinylpalladium complex A-10, which upon r-carbopalladation of the appendant alkyne provides intermediate B-7. Silane-mediated cleavage of carbon-palladium bond liberates the cyclized product along palladium(O), which reacts with acetic acid to regenerate hydridopalladium acetate to close the cycle (Scheme 33). 

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). 

The alkyne insertion reaction is terminated by anion capture. As examples of the termination by the anion capture, the alkenylpalladium intermediate 189, formed by the intramolecular insertion of 188, is terminated by hydrogenolysis with formic acid to give the terminal alkene 192. Palladium formate 190 is formed, and decarboxylated to give the hydridopalladium 191, reductive elimination of which gives the alkene 192 [81]. Similarly the intramolecular insertion of 193 is terminated by transmetallation of 194 with the tin acetylide 195 (or alkynyl anion capture) to give the dienyne 196 [82], Various heterocyclic compounds are prepared by heteroannulation using aryl iodides 68 and 69, and internal alkynes. Although the mechanism is not clear, alkenylpalladiums, formed by insertion of alkynes, are trapped by nucleophiles... 

The transformation that has come to be known as the Heck reaction is broadly defined as the palladium(O)-mediated coupling of an aryl or vinyl halide or triflate with an alkene. The basic mechanism for the Heck reaction of aryl halides or trifiates (as outlined in more detail in the Key Chemistry), involves initial oxidative addition of the chiral palladium(O) catalyst to afford a a-arylpalladium(II) complex. Coordination of an alkene and subsequent carbon-carbon bond formation by syn insertion provide a a-alkylpalladium(II) intermediate, which readily undergoes P-hydride elimination to release the alkene product. Finally, the hydridopalladium(II) complex has to be converted into the active palladium(O) catalyst to complete the catalytic cycle. 

Trost and Toste explain their Pd(0)-catalyzed reaction by the following mechanism. First, reaction of Pd(0) and HCO2H generates hydridopalladium 38, and the vinylpalladium intermediate 39 is formed by insertion of alkyne, and either the... 

Although hydridopalladium species have often been postulated as important reactive intermediates in a number of reactions, their precise role in catalysis and/or organic chemistry remains a challenging research area. Due to their instability, identification of hydridopalladium species is rare and they have never been detected under true catalytic conditions. In this section, a brief account of the preparation and characterization of hydridopalladium complexes will be given followed by a discussion on their chemical behavior. 

The study of the involvement of hydridopalladium complexes under basic conditions is far more elusive. The involvement of these species in the /S-hydrogen elimination step has never been observed directly and their involvement is only implied based on mechanistic considerations. A good example was provided by studies of the reductive elimination step of the Heck arylation reaction. An alkylpalladium(II) intermediate releases the product methyl cinnamate, while simultaneously capturing a... 

Heck reactions of allylalcohols, with the involvanent of neutral intermediates and more strongly coordinating leaving groups, alternatively provided aldehydes or ketones, as a result of H /3-elimination or double bond migration by hydridopalladium leaddition/elimination to give unstable enols (Sect. B.vii). 

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