Hydridopalladium complex

Morpholine also gives the allyhc amine in high yield. The reaction is thought to involve a known hydridopaUadium-catalyzed isomerization of alkynes to aUenes followed by reaction of the latter with the hydridopalladium complex to give 1-phenyl-substituted q -allylpalladium complexes. These complexes react with amines affording the allylic amines. Primary amines give the diallylic amines. An intramolecular version has been developed for the synthesis of 2-(2-phenyl)-pyrroUdines and -piperidines [319]. 

Many palladium-catalyzed reactions are initiated by the reaction of a palladium(O) complex with an acidic derivative.367 The catalytic cycle is considered to be induced by a hydridopalladium complex. When the acidic derivatives are strong acids (e.g., HBF4, HC1, CF3C02H, HOTs), the hydridopalladium formation may be regarded as the protonation of basic Pd° to afford complexes HPdL3 +368-374 or IIPdL2.S +,375 in which S = solvent (see Equation (1)) ... 

In the same research group the cationic hydridopalladium complex [Pd(H)(H20)(PCy3)2] [BF4] has been shown to catalyze the hydroxycarbony-lation of triple bonds. As a representative example the dehydration occurring to give the dienoic acid is displayed in Scheme 3 [35]. The same cationic complex is able to activate a carbon oxygen bond in a-allenic alcohols to provide dienoic acids but with the COOH group in the branched position (Scheme 3) [36]. 

The prototype of this sequence has been discovered by Brewis and Hughes as early as in 1965 in the carbonylation of 1,5-diene (Equation (26)), " though in this case the process is initiated not by oxidative addition of Pd(0) to C-X bond, but by the addition of hydridopalladium complex to double bond. ... 

Thus, inorganic palladium compounds often add to olefins, largely in the Markovnikov manner producing adducts which generally decompose rapidly by eliminating the elements of a hydridopalladium complex. 

In spite of the successful use of NHCs in a number of palladium-catalyzed reactions, no system for hydrogenation was reported until 2005. This can be easily explained as it had been observed that hydridopalladium-carbene species decompose due to attack of the hydride on the carbene, which results in its reductive elimination to yield the corresponding imidazolium salt [ 190]. However, Cavell and co-workers recently showed that the oxidative addition of imidazolium salts to bis-carbenic palladium complexes leads to isolable NHC-hydridopalladium complexes [191]. This elegant work evidenced the remarkable stabilizing effect of NHC ligands in otherwise reactive species and led to the development of the first NHC-palladium catalyst for hydrogenation. 

The oxidative addition to palladium reagents proceeds readily with 1-haloalkenes at room temperature. However, the oxidative addition reactions of halides other than vinyl or aryl usually are very sluggish. Moreover, alkyl-Pd(II)-X complexes in which the alkyl moiety contains an sp -bonded hydrogen at the [3-position may undergo rapid dehydropalladation by. syn- 3-hydrogen elimination, generating the hydridopalladium complex and a double bond. Thus, the substrates used for the oxidative addition reaction are usually restricted to vinyl and aryl halides and triflates. 

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. 

To date only a limited number of hydridopalladium complexes have been successfully isolated and characterized unequivocally. The different synthetic routes, as well as structural, spectral, and theoretical studies of discrete hydridopalladium species, have been reviewed extensively in 1996. " With the notable exception of the unusual zerovalent complexes (such as [PdHJ ), almost all of the hydridopalladium complexes reported so far have been monohydride complexes coordinated by phosphine ligands. 

The first hydridopalladium complexes to be isolated and characterized in pure form were the trawi-[(R3P)2Pd(H)(X)] complexes (where PR3 = tertiary phosphines, X = anionic ligands), and they still represent the largest class of isolated hydridopalladium complexes. 

The most common ways to prepare hydridopalladium complexes of the type trans-[(R3P)2Pd(H)(X)] are ... 

The formation of hydridopalladium complexes free of phosphine ligands (Figure 4) were observed when cyclometalated palladium chloride complexes were subjected to hydrogenolysis in DMF solutionsJ ... 

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... 

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