Haloarene complexes

Following an initial report [2] including carbon nucleophiles for the cycliza-tion, a series of papers have defined useful possibilities with heteroatom nucleophiles, especially N and O derivatives [2]. The ortho-lithio haloarene complexes (e.g., 16) are highly basic, and the electrophilic trapping species is restricted to those with low kinetic acidity and high electrophilicity. Good results are obtained with isocyanates, ketenes, and acyl derivatives with a-protons of low acidity. 

Nucleophilic aromatic substitution reactions of haloarenes complexed to transition metal moieties with oxygen-, sulfin-, and nitrogen-containing nucleophiles allows for the synthesis of a wide variety of aryl ethers, thioethers, and amines. These metal-mediated reactions proceed under very mild conditions and allow for the incorporation of a number of different functional groups. Nucleophilic substitution reactions of chloroarenes complexed to the cyclopentadienyliron moiety have been the focus of many studies directed toward the design of functionalized organic monomers. ... 

The ortho-C-H activation of haloarenes was achieved using an Ir(i) complex, with the halogen acting as a directing group (Equation (61)).62... 

Aryl ethers from haloarene chromium tricarbonyl complexes (Table 2.4)... 

Haloarene chromium tricarbonyl complexes are activated to nucleophilic attack by thiolate anions [58, 59]. High yields of the thioethers are obtained under liquiddiquid two-phase conditions, but optimum yields are achieved under soliddiquid conditions. In many cases the thioether is produced directly but, where the reaction mixture contains thioether and its chromium complex, the thioether can be isolated by degradation of the complex with iodine or an excess of the thiol. Both 1,2- and 1,4-dichlorobenzenes yield only monothioethers, even when an excess of thiolate anion is used. In contrast, 1,3-dichlorobenzenes produce a mixture of the mono- and dithioethers [59]. Aryl allyl thioethers have been produced under catalysed Heck reaction conditions from S-allyl thiocarbamates and iodobenzene [60]. 

Selected examples of the reaction of haloarene chromium tricarbonyl complexes [Cr(CO)jArX] with thiolate anions... 

Activated aryl halides react with thiols [e.g. 4] to produce aryl thioethers and thioethers, derived from non-activated aryl halides, can be synthesized via the Cr(CO), complexes of the haloarenes [29] (Scheme 4.2, see also Chapter 2). 

Aryl methyl ketones have been obtained [4, 5] by a modification of the cobalt-catalysed procedure for the synthesis of aryl carboxylic acids (8.3.1). The cobalt tetracarbonyl anion is converted initially by iodomethane into the methyltetra-carbonyl cobalt complex, which reacts with the haloarene (Scheme 8.13). Carboxylic acids are generally obtained as by-products of the reaction and, in several cases, it is the carboxylic acid which predominates. Unlike the carbonylation of haloarenes to produce exclusively the carboxylic acids [6, 7], the reaction does not need photoinitiation. Replacement of the iodomethane with benzyl bromide leads to aryl benzyl ketones in low yield, e.g. 1-bromonaphthalene produces the benzyl ketone (15%), together with the 1-naphthoic acid (5%), phenylacetic acid (15%), 1,2-diphenylethane (15%), dibenzyl ketone (1%), and 56% unchanged starting material [4,5]. a-Bromomethyl ketones dimerize in the presence of cobalt octacarbonyl and... 

Vinylic halides are virtually unreactive and a high selectivity is to be found in the preferential cleavage of aliphatic carbon-halogen bonds of haloalkanoic amides and esters, and of nitro- and cyanoaryl derivatives. Activated haloarenes, e.g. 1-chloro-2,4-dinitrobenzene, however, give a complex mixture of products [7]. 

In contrast to these results, C y —H bond activation of related Ir—PNP pincer complexes appeared to be a heteroatom-directed process. The iridium(I) complex 14 induces ortho C—H bond achvation of a variety of haloarenes and anisole to afford the corresponding iridium(III) complexes 15 (Equation 12.7) [26]. 

Manganese, iron, cobalt, and nickel vapors do not give arene complexes with haloarenes. Interactions with hexafluorobenzene have been reported, but the explosive products are unlikely to be complexes containing planar C8F8 rings. The Ni-C8F8 cocondensate is a source of... 

The formation of phenantridone analogues from o-halobenzamides and haloarenes, reported by Catellani, could be considered a vaiation of this approach, where the acetylene bond is formally replaced by a bcnzync moiety. In reality the reaction follows a more complex and mechanisticly distinct pathway 44... 

Copper complexes of substituted haloarenes have been shown to be particularly prone to displacement of halide by nucleophiles. Most commonly, copper(n) species are involved, but in a few cases copper(i) has also been shown to be effective. This is not surprising in view of the facile inter-conversion of copper(i) and copper(n) in aerobic condi-... 

Catalysts for coordination polycondensation involving haloarene derivatives are usually based on Pd complexes, although Ni complexes can be employed in some instances. 

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... 

The reactivity of complexed haloarenes toward thiolates has been studied, and it has been reported that o-, m-, and p-dichlorobenzenetricarbonylchromium complexes 18a-c react with thiolates (RS R = Me, nBu, tBu Scheme 17, path i) under phase-transfer conditions or in DMSO to give 39 and 40a-c. The ortho- and para-complexes 18a and 18c undergo stepwise substitution of the two Cl atoms in a reaction sequence that can be easily controlled by the amount of added thiolate. The meta complex 18b shows a lower selectivity and gives a mixture of mono- and disubstituted products even in the presence of substoichiometric amounts of thiolate (Scheme 17) [22]. Similarly, LiCH(C02Et)CN and BuSH react with the o-dichlorobenzene complex 18a to give complex 39d and then disubstituted arene 40d, showing that this substitution can be performed with two different nucleophiles (Scheme 17) [23]. Phase-transfer catalysis has also been applied to fluoroarene-Cr(CO)3 complexes, which are more reactive toward thiolates than are the corresponding chloro derivatives [22]. 

The proposed mechanism for a standard Heck reaction is depicted in Scheme 6.5. Generally, a haloalkene or haloarene undergoes oxidative addition to an in situ generated, coordinatively unsaturated 14-electron palladium(O) complex, but other substrates such as tosylates, triflates or diazonium salts can also be applied. Subsequent, sy -insertion into the C=C double bond of a complexed olefin yields a t7-(j -alkenyl) or (j- aryl)alkylpalladium complex. If no hydrogen atom in a pseudo cis-position relative to the palladium is present, an internal rotation step is required prior to syw-elimination of the olefin to afford the traws-olefin product and a palladium(II) hydride complex. The latter is restored to the initial Pd(0) species by base-induced reductive elimination.137"401... 

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

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