Palladium-catalysed reactions transmetallation

Palladium-catalysed reactions of dimetallic compounds 358 such as X2B—BX2, R3Sn—SnR3, R3S11—SiR3 or R3Si—SiR3 with halides via oxidative addition and transmetallation are useful for the preparation of carbon main group metal bonds 359. 

The coupling of anilines with arylboronic acids has been described in a process catalysed by palladium acetate without the need for ligands, bases, or salts. The mechanism shown in Scheme 17, where S represents solvent, involves in situ formation of the dia-zonium salt from the aniline, and formation of an arylpalladium alkoxo complex which allows the transmetallation step with arylboronic acids. It has been shown that a free amine may be used as a DG in a palladium-catalysed reaction promoted by soluble silver salts. The latter aid the formation of intermediates, (136), which may undergo cyclopalladation followed by transmetallation with an arylboronic acid and reductive elimination. Related work has shown thatbiaryl-2-amines may react with aryl iodides in the presence of palladium acetate and silver acetate to give mono- or di-arylated species such as (137). 

There are also palladium-catalysed procedures for allylation. Ethyl 3-bromo-l-(4-methylphenylsulfonyl)indole-2-carboxylate is allylated at C3 upon reaction with allyl acetate and hexabutylditin[27], Ihe reaction presumably Involves a ir-allyl-Pd intermediate formed from the allyl acetate, oxidative addition, transmetallation and cross coupling. 

Although the palladium catalysed conversion of allylsilanes bearing a pendant hydroxyl group to fiirans (3.64.), reported by Szabo, bears remarkable similarity to the reaction depicted in 3.63., it has also some distinctive features. The formation of the allylpalladium complex is achieved in a transmetalation step and therefore requires the presence of a... 

Transition-metal-catalysed reactions of glycals with carbon nucleophiles provide an important route to C-glycosides.24 These reactions may proceed by two different pathways depending on the oxidation state of the transition metal used and the nature of the carbon nucleophile (Scheme 3.5c). For example,25 transmetallation of Pd(OAc)2 with pyrimi-dinyl mercuric acetate results in the formation of a pyrimidinyl palladium complex, which adds to the double bond of glycal derivatives. The resulting intermediate can undergo different transformations depending on the reaction conditions. 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

Many of the applications of organotin compounds are specific to one class of compounds (e.g. tin hydrides or allylstannanes) and these topics are covered in the corresponding chapters. On the other hand, reactions involving tin/lithium transmetallation, or palladium-catalysed coupling, can be carried out with a variety of types of organostannanes, and these reactions are collected together in this chapter. These topics have an extensive literature that is beyond the scope of this book, and the treatment here is limited to an outline of the fields, and a guide to the literature. 

The mechanism of palladium-catalysed cross-coupling starts, as in the Heck reaction, with oxidative addition of the halide or triflate to the initial palladium(O) phosphine complex to form a palladium(ll) species. But the next step is new it is a transmetallation, so-called because the nucleophile (Ri) is transferred from the metal in the organometallic reagent to the palladium and the counterion (X=hallde or triflate) moves in the opposite direction. The new palladium(ll) complex with two organic ligands undergoes reductive elimination to give the coupled product and the palladium(O) catalyst, ready for another cycle. 

The three basic steps in the palladium-catalysed Suzuki-Miyaura reaction involve oxidative addition, transmetalation, and reductive elimination. A systematic study of the transmetalation step has found that the major process involves the reaction of a palladium hydroxo complex with boronic acid, path B in Scheme 3, rather than the reaction of a palladium halide complex with trihydroxyborate, path A. A kinetic study using electrochemical techniques of Suzuki—Miyaura reactions in DMF has also emphasized the important function of hydroxide ions. These ions favour reaction by forming the reactive palladium hydroxo complex and also by promoting reductive elimination. However, their role is a compromise as they disfavour reaction by forming of unreactive anionic trihydroxyborate. A method for coupling arylboronic acids with aryl sulfonates or halides has been developed using a nickel-naphthyl complex as a pre-catalyst. It works at room temperature in toluene solvent in the presence of water and potassium carbonate. ... 

There has been a wide-ranging review of transmetalation reactions of arenes concentrating on the use of boron-containing compounds. The reaction of arylboronate esters and related compounds with alkyl halides is catalysed by copper(I) formation of an aryl—copper intermediate followed by an 5N2-type reaction with the alkyl electrophile is likely. Palladium complexed with a diimine is an excellent catalyst in the phenylation reaction of Michael acceptors with phenylboronic acid so as to yield products such as (16). The nickel-catalysed reaction of phenylboronic acid with styryl epoxides has been shown to yield a-substituted alcohols such as (17). 

Transmetallation of the organic group from zirconium to another metal opens up possibilities. The palladium-catalysed coupling reactions can be found in Section 2.4. Addition of dimethyl cuprate results In transmetallation to copper. The resulting cuprate then displays typical cuprate reactivity, such as addition to enones. More economically, small amounts of copper can catalytically activate the zirconium complex towards this kind of chemistry, although the precise mechanism is unclear. Additions to enones can also be achieved directly using nickel catalysis (Scheme 5.64). Transmetallation to zinc has also been demonstrated. ... 

A study of palladium-catalysed conjugate addition of diorganozincs to various enone types indicated that both Pd(0) and Pd(II) complexes could catalyse the reaction. Phosphine ligands such as PPhj or PBuj were effective, but only at a 1 1 Pd P ratio a 1 2 ratio caused yields to collapse. The observation is consistent with a mechanism computed for the Pd(0) case, in which the enone is simultaneously coordinated to Pd(0) and R2Zn this undergoes oxidative addition to palladium with simultaneous transmetalation from Zn to Pd, followed by reductive elimination. 

The arylation of carbon-hydrogen bonds in polycyclic aromatic hydrocarbons, using aryl boron compoimds or aryl silanes, may be achieved with a palladium acetate/o-chloranil catalyst. The Suzuki-Miyaura reaction involves palladium-catalysed coupling of an arylboronic acid with an aryl hahde in the presence of base. After oxidative addition of palladium to the hahde, reaction with base may form intermediates such as (105). Transmetalation with the boronic acid followed by reductive elimination yields... 

The coupling of terminal alkynes with aryl or alkenyl halides catalysed by palladium and a copper co-catalyst in a basic medium is known as the Sonogashira reaction. A Cu(I)-acetylide complex is formed in situ and transmetallates to the Pd(II) complex obtained after oxidative addition of the halide. Through a reductive elimination pathway the reaction delivers substituted alkynes as products. 

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