Pd-mediated reaction

These findings expand on extensive development of ligandless catalysis for a variety of Pd-mediated reactions. For reviews see (a) Beletskaya, I. P. Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Trans .) 1990, 39, 2013 (b) Beletskaya, I. P. J. Organomet. Chem. 1983, 250, 551. 

Preparation of 2,6-Disubstituted Purines (65) Using Pd-Mediated Reactions Involving Amines and Boronic Acids (Fig. 11)33... 

Several other examples of the utility of Pd-mediated reactions in synthesis of aryl and vinyl derivatives of pyrrole and indole were reported. Schmidt and coworkers examined arylation of 1-vinylpyrroles under Heck conditions. Reaction took place at the A-vinyl group. While the parent compound gave a mixture of -and -arylation, 2,3-dialkyl-1-vinylpyrroles preferred -substitution. <95RCB767> Grieb and Ketcha used Suzuki coupling conditions to prepare several 1-... 

Palladium chemistry of heterocycles has its idiosyncrasies stemming from their different structural properties from the corresponding carbocyclic aryl compounds. Even activated chloroheterocycles are sufficiently reactive to undergo Pd-catalyzed reactions. As a consequence of a and y activation of heteroaryl hahdes, Pd-catalyzed chemistry may take place regioselectively at the activated positions, a phenomenon rarely seen in carbocyclic aryl hahdes. In addition, another salient peculiarity in palladium chemistry of heterocycles is the so-called heteroaryl Heck reaction. For instance, while intermolecular Pd-catalyzed arylations of carbocyclic arenes are rare, Pd-catalyzed arylations of azoles and many other heterocycles readily take place. Therefore, the principal aim of this book is to highhght important Pd-mediated reactions of heterocycles with emphasis on the unique characteristics of individual heterocycles. 

General procedures for the displacement of a chloro function from purines Some halide substituents on nitrogen heterocycles are difficult to substitute by means of nucleophilic aromatic substitution. In order to overcome the lack of reactivity at the C2 position of purines, Pd-mediated reactions are used for C-C and C-N bond formations (Scheme 12) [36, 91],... 

As well as Pd-mediated reactions, there are other processes in which electrophiles are bound to the polymeric support. These may involve Hnear chain elongation as well as cyclization. 

The isomerization reaction appears to be a major drawback, especially of the Pd-mediated reaction [8]. 

Non-carbenoid, ruthenium-catalyzed intramolecular enyne metathesis shares a mechanistic resemblance to the Pd-mediated reactions above. 

Pyrimidine nucleosides are readily attacked by electrophiles, such as Hg(II) acetate in the pyrimidine 5-position. The anion is subsequently changed to halide. Thus 2 -deoxyuridine has been mercurated by Hg(OAc)2 in aqueous methanol and coupled with iodobenzene or substituted derivatives on heating with diglyme in a Pd-mediated reaction with formation of 5-aryl derivatives (366) (Scheme 86). The acetate is used for solubility reasons (84TL2431). 

Regioselective carboxylation of thiophene by Pd-mediated reaction with CO in the presence of mercuric chloride, mercuric acetate or thallium(III) acetate has been reported <90JMOC23>. It is preferable to carry out the reaction in two steps— iiiitial mercuration, followed by reacting the thienylmercury derivative with CO in the presence of palladium acetate. 

This conversion of an acyclic bisdiene to a cychzed enediene constitutes a new Pd-mediated reaction mode, which can be rationalized as outlined in Scheme 7. Com-plexation of bisdiene 17 following oxidative cyclization (i.e., oxidative addition with intramolecular coupling of the diene moieties) affords palladacycle 20. Protonation in an... 

In the Pd-mediated reaction chloroallenes also gave inversion. 

A Pd-mediated reaction of a vinyl epoxide with an isocyanate offers synthetically useful transformation (Scheme 4). A diastereoselective vicinal hydroxyamination is established, which involves trapping an initial zwitterion with an isocyanate and subsequent nucleophilic attack of a nitrogen anion on a rr-allylpalladium intermediate (Scheme 4)... 

Pd-mediated reaction of allylic alcohols with phosphines affords phosphonium salts, which react with aldehydes to form conjugated dienes by a Wittig-type reaction (Scheme 14).[62H64] -pjjg reaction of an allylic alcohol, phenyl isocyanate, and PBur produces a phosphonium salt under mild conditions, in which an allylic phenylcarbamate... 

Multiple, successive insertion of isonitriles into the palladium-carbon o--bond has also been found with organopalladium(II)bis(phosphine) complexes. However, unlike the Ni- catalyzed polymerization, only single, double, and triple insertion reactions of isonitriles have been reported for Pd-mediated reactions. For instance, reaction of methylpalladium(II) complex 2 with cyclohexyl isonitrile in ratios of 1 1, 1 2, and 1 3 afforded selectively single (3), double (4), and triple (5) insertion products, respectively (Scheme 3). However, no further insertion of cyclohexyl isonitrile to 5 took place. No reactivity of the triple insertion complex 5 was explained by the intramolecular coordination of the nitrogen atom of the third imino group, which formed a stable five-mem-bered chelating palladium complex. 

Figure 1. Types of functionalized polystyrene resins used for Pd-Catalyzed and Pd-mediated reactions on solid support (for reviews and explanations, see Ref. [1]).

Direct 2-arylation (i.e., 2-C-H functionalization) of benzoxazole has been achieved (a) by Cu-mediated reaction with aryl iodides in the presence of Na2C03 [281] and (b) by Pd-mediated reaction with aryl chlorides or bromides in the presence of CS2CO3 [282]. 

Stable palladium nanopartides were also prepared using thermosensitive VE star polymers. The obtained partides were shown to work as a catalyst for the Heck coupling of iodobenzene and ethyl acrylate. A feature of this system is nonnecessity of expensive and toxic phosphine ligands. In mechanistic point of view, this zero-valent Pd-mediated reaction would shed new light on how Heck coupling reactions proceed. 

Another interesting Pd-mediated reaction with a Cu(ii) oxidant comes from a study by the You group in 2011, in which they were able to crosscouple heterocycles in high yields with good regioselectivity (Scheme 6.11). ... 

This survey has shown the usefulness of Pd-catalyzed oxycarbonylation in numerous natural product syntheses. In many cases, the chemo-, regio-, and stereoselectivities of Pd-mediated reactions have succeeded in building a complex structure. Construction of oxa-heterocyclic rings has been realized under mild reaction conditions and in the presence of several functional groups. 

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