Haloarenes

Nitration of aromatic rings by nitronium tetrafluoroborate is a general method. Fifty-seven arenes, haloarenes, nitroarenes,... 

Last, the nickel catalyzed halogen exchange for haloarenes, actually the arylation of halide anions, has been investigated. The equilibrated exchange between the phenyl bromide (60 %) and the phenyliodide (40 %) allows with some... 

It is the metal catalyzed halogen exchange for haloarenes, that is to say the arylation of halides anions (chloride, bromide, iodide), acting as nucleophiles (Fig. 21). 

The protocol offers a direct and efficient method for the synthesis of the boronic ester in the solid phase, which hitherto met with little success using classical methodology (Scheme 1-42). A solid-phase boronate (113 [155], 114 [156]) is quantitatively obtained by treating a polymer-bound iodoarene with the diboron (82). The subsequent coupling with haloarenes furnishes various biaryls. The robot synthesis or the parallel synthesis on the surface of resin is the topic of further accounts of the research. 

Haloarenes routinely form XB. The same holds for haloheteroarenes and when the heteroaromatic ring is positively charged, the halogen becomes a particularly good XB donor [89-94]. This strategy has been applied with particular success in order to control the packing of bromo- and iodo-... 

The complete elimination of functional groups is often an undesirable side reaction in organic synthesis, but on the other hand it is a possibility for the recycling of environmentally harmful compounds, for example phenols and haloarenes such as polychlorinated dibenzodioxins (PCDDs or dioxins ). For example, aryl chlorides can be effectively dechlorinated with Pd(0) NPs in tetra-butylammonium salts with almost quantitative conversions also after 19 runs (entry H, Table 1.4) [96]. On the other hand, a C-0 bond cleavage reaction also seems suitable for the fragmentation of sugar-based biomass such as cellulose or cello-biose in that way, sugar monomers and bioalcohol can be derived from renewable resources (entry F, Table 1.4) [164]. 

The palladium-catalyzed arylation of alkenes by haloarenes, and applications in furan synthesis have been described earlier (see Eq. 14 in Section IV,B, 1). By employing o-aminoiodoarenes and appropriately substituted (Z)-alkenes it has proved possible to develop an efficient synthetic route to quinolin-2-ones (Scheme 140).213... 

Palladium-catalyzed cyclization reactions with aryl halides have been used to synthesize pyrazole derivatives. V-Aryl-lV-(c>-bromobenzyl)hydrazines 26 participated in a palladium-catalyzed intramolecular amination reaction to give 2-aryl-2W-indazoles 27 . Palladium-catalyzed cascade intermolecular queuing-cyclocondensation reaction of o-iodophenol (28) with dimethylallene and aryl hydrazines provided pyrazolyl chromanones 29 <00TL7129>. A novel one-pot synthesis of 3,5-disubstituted-2-pyrazolines 32 has been achieved with an unexpected coupling-isomerization sequence of haloarene 30, propargyl alcohol 31, and methylhydrazine <00ACIE1253>. 

Haloarenes have been found to undergo nucleopilic substitution when irradiated with the triphenyl stannyl anion46, reacting via a radical S l mechanism. In many cases the reaction will only occur under photochemical conditions. The reaction is found to proceed with chloro- and bromo-substituted arenes, but not iodo-compounds. The anion is produced either by treatment of triphenyltin chloride or hexaphenylditin with sodium metal in liquid ammonia, and will react with a wide variety of arenes (reaction 30). 

Abstract. This article describes recent achievements of my research group in the Leibniz-Institut fiir Katalyse e.V. in the area of applied homogeneous catalysis for the synthesis of biologically active compounds. Special focus is given on the development of novel and practical palladium and copper catalysts for the functionalization of haloarenes and haloheteroarenes. 

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... 

Several Czech research groups have studied the dehalogenation with NaAl-H2(OCH2CH2OCH3)2 (5). Hydrogenolysis of haloarenes with this hydride was accelerated by transition-metal species formed in situ from [(RCN)2PdCl2] (R=Ph, PhCH2) [53] or from 2,4-pentanedionates [54—56]. In the latter case, the efficiency decreased in the order Co x Ni x Pd>CuS>Mn>Fe [54]. Complete... 

Where reactivity is low, as for example with non-activatcd haloarenes, soliddiquid conditions are to be preferred over liquid liquid conditions and better yields are frequently obtained when TDA-1 is used as the catalyst instead of a quaternary ammonium salt [28]. Although not activated to nucleophilic attack, 2-chlorophenol reacts with ethanol under solid liquid conditions with microwave irradiation in the presence of benzyltrimethylammonium chloride to produce the ether in 70% yield [44], It is possible that the procedure has potential for further exploitation. 

Typical synthesis of aikoxy and aryloxyarenes from haloarenes... 

Haloarene Alcohol/phenol Reaction conditions % yield... 

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

There are relatively few reports of phase-transfer catalysed syntheses of phenols from activated haloarenes using quaternary ammonium salts, presumably because of the instability of the ammonium salts under the reaction conditions. A patented procedure for the conversion of, for example, 2,6-dichloropyridine into 6-chloropyrid-2-one (98%) using aqueous sodium hydroxide in the presence of benzyl-triethylammonium chloride at 120-150°C has been filed [32], A possible route to the phenols, however, comes from the observed reaction of phenols with potassium carbonateipotassium hydrogen carbonate to yield the aryl carbonates (80-85%) using the procedure described for the preparation of dialkyl carbonates (3.3.13) [50]. 

Selected examples of thioethers obtained from activated haloarenes... 

The thiophenol (60 mmol) and aqueous NaOH (10%, 100 ml) containing TBA-Br (0.96 g, 3 mmol) are stirred at room temperature and the haloarene (60 mmol) in PhMe (100 ml) is then added dropwise over 10 min. On completion of the reaction (1-4 h), as indicated by HPLC, the organic phase is separated, washed well with H20, dried (MgS04), and evaporated to yield the thioether. 

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]. 

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