Aryl pseudohalides

The complex Cp3Ce(IV)Cl has been synthesized by reacting NaCp and bis(pyridinium) hexachlorocerate in THF. It may also be obtained [56] by the reaction of bis(pyridinium) hexachlorocerate and Cp4Ce. These compounds, Cp3CeCl may be used to prepare other compounds [57] like Cp3CeX, where X = alkyl, aryl, pseudohalide, mercaptide, carboxylate. 

Fiirstner reported the iron-catalysed crosscoupling of aryl-halides or aryl-pseudohalides with allgrl-Grignard reagents. Iron-catalysed biaryl bond formation could also be achieved from the coupling of aryl-halides or aryl-pseudohalides with aryl-Grignard reagents. ... 

X=halide. Triscyclopentadienyl cerium(IV) chloride has been prepared by reaction of sodium cyclopentadienide and bis(pyridinium)hexachlorocerate in THF or benzene, or by ligand redistribution between bis(pyridinium)hexachlorocerate and tetracyclopentadienyl cerium (Kalsotra et al. 1971). These brown compounds, described as stable in water, act as starting materials for Cp CeZ (Z = alkyl, aryl, pseudohalide, mercaptide, carboxylate) (Marks and Ernst 1982). 

Ligand A anionic (e.g., halides, pseudohalides, alkyl, aryl, thiolate, alkoxide) ligand N neutral (e.g., amines, imines, phosphines, carbenes, nitriles, isonitriles, NO, CO) ligand C cationic (e.g., NO+)... 

The palladium-catalyzed Heck carbonylation reaction is a powerful means of generating amides, esters, and carboxylic acids from aryl halides or pseudohalides [28]. The development of rapid, reliable, and convenient procedures for the introduction of carbonyl groups is important for the development of high throughput chemistry in general and high-speed microwave-mediated chemistry in particular. Unfortunately, the traditional method of introducing carbon monoxide into a reaction mixture via a balloon or gas tube is not practical because of the special requirements of microwave synthesis. 

Scheme 5. Palladium-catalyzed carbonylations of aryl halides and pseudohalides...

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

The Sandmeyer reaction utilizes a diazonium salt to produce an aryl halide. The process begins by converting an amine to a diazonium salt. Decomposition of the diazonium salt in the presence of a copper halide places the halide ion into the position originally occupied by the amine. The most useful copper halides are CuCl and CuBr in addition, the copper pseudohalides, such as CuCN, also works by placing a nitrile in the position originally occupied by the amine. Figure 13-27 shows an example of the Sandmeyer reaction. 

Efficient homocoupling of the aryl iodonium salt 827 using Zn is catalyzed by Pd(acac)2[708], Homocoupling of the arylsulfonyl chloride 828 as a pseudohalide takes place in the presence of 2 equiv. of Ti tetraisopropoxide[709]. 

This class of compound is represented by the dicyanoiodate(I) anion, involving a pseudohalide rather than an alkyl or aryl ligand. The linear ion, with I—C distance of 2.302 A, has been characterized in the compound K[I(CN)2]C6H12N202, resulting from reaction of an aqueous solution of KCN with an ethanolic solution of ICN. The diimino oxalic acid diethyl ester molecule apparently stabilizes the structure.56... 

R = alkyl, aryl, acyl X = halide or pseudohalide L = phosphine ligand... 

Tertiary phosphines form a wide range of complexes of the type PtX2(PR3)2 (X - halide, pseudohalide R = alkyl, aryl or mixed alkylaryl). Space limitations preclude this chapter becoming a compendium of known compounds, therefore for each individual complex the reader is directed to Chemical Abstracts, Gmelin or the book by McAuliffe which contains extensive tabulations of compounds.1225 In this chapter we outline the general methods of synthesis, the properties, and the structures and spectral features which we expect to be found with this class of compounds. 

Abbreviations X = halide or pseudohalide, R = alkyl or aryl, oep = octaethylporphyrin, pz = C3H3N2, HB(Me2pz)3 = tiiii Ivdimethylpyi a/olyl)borale... 

A number of different polar and nonpolar covalent bonds are capable of undergoing the oxidative addition to M( ). The widely known substrates are C—X (X = halogen and pseudohalogen). Most frequently observed is the oxidative addition of organic halides of sp2 carbons, and the rate of addition decreases in the order C—I > C—Br >> C—Cl >>> C—F. Alkenyl halides, aryl halides, pseudohalides, acyl halides and sulfonyl halides undergo oxidative addition (eq. 2.1). 

Reaction Patterns of Aryl, Alkenyl and Benzyl Halides and Pseudohalides... 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

Mesylates are used for Ni-catalysed reactions. Arenediazodium salts 2 are very reactive pseudohalides undergoing facile oxidative addition to Pd(0). They are more easily available than aryl iodides or triflates. Also, acyl (aroyl) halides 4 and aroyl anhydrides 5 behave as pseudohalides after decarbonylation under certain conditions. Sulfonyl chlorides 6 react with evolution of SO2. Allylic halides are reactive, but their reactions via 7t-allyl complexes are treated in Chapter 4. Based on the reactions of those pseudohalides, several benzene derivatives such as aniline, phenol, benzoic acid and benzenesulfonic acid can be used for the reaction, in addition to phenyl halides. In Scheme 3.1, reactions of benzene as a parent ring compound are summarized. Needless to say, the reactions can be extended to various aromatic compounds including heteroaromatic compounds whenever their halides and pseudohalides are available. 

Ni(0) complexes react with halides and pseudohalides. Their reactions are somewhat different from those of Pd(0). Chlorides add to Ni(0) much more easily than to Pd(0). Even C—O bonds such as aryl alkyl ether bonds are cleaved with Ni(0) under certain conditions. Not only triflates, but also mesylates react with Ni(0). Oxidative addition to Ni(0) and subsequent transformations are summarized in Scheme 3.6. 

Oxidative addition of aryl and alkenyl halides, and pseudohalides, followed by transmetallation with various metal hydrides generates Ar—M—FI species, reductive elimination of which results in hydrogenolysis of halides. In the main, Pd is used as an efficient catalyst for the hydrogenolysis. 

Palladium-catalyzed synthesis of aryl amines. Starting materials are aryl halides or pseudohalides (for example triflates) and primary or secondary amines. 

The Hiyama Coupling is the palladium-catalyzed C-C bond formation between aryl, alkenyl, or alkyl halides or pseudohalides and organosilanes. This reaction is comparable to the Suzuki Coupling and also requires an activating agent such as fluoride ion or a base. 

The Suzuki Coupling, which is the palladium-catalysed cross coupling between organoboronic acid and halides. Recent catalyst and methods developments have broadened the possible applications enormously, so that the scope of the reaction partners is not restricted to aryls, but includes alkyls, alkenyls and alkynyls. Potassium trifluoroborates and organoboranes or boronate esters may be used in place of boronic acids. Some pseudohalides (for example triflates) may also be used as coupling partners. 

Aryl chlorides are more reluctant to participate in amination than most other aryl halides/pseudohalides. To tackle this problem, Caddick et al. examined the effect of palladium-N-heterocyclic carbenes as catalysts in rapid microwave-promoted reactions [87]. Para-tolyl and -anisyl chloride were reacted with aromatic and aliphatic amines in mostly good yields within 6 minutes of heating at 160 °C. Reactions using anisyl, tolyl or phenyl chlorides and aliphatic amines have also been reported by Maes et al. using a phosphine ligand and a strong base, which creates the desired products after 10 minutes of heating at 110-200 °C [88]. 

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