Arylchlorides

SUZUKI CROSS-COUPLING OF ARYLCHLORIDES WITH ARYLBORONIC ACIDS... 

Normally, the oxidative addition of an aryl chloride to Pd(0) is reluctant to take place. But such a process is greatly accelerated in the presence of sterically hindered, electron-rich phosphine ligands [e.g., P(/-Bu)3 or tricyclohexylphosphine]. In late 1990s, Reetz [76] and Fu [77] successfully conducted intermolecular Heck reactions using arylchlorides as substrates, as exemplified by the conversion of p-chloroanisole to adduct 77 [77], The applications of this discovery will surely be reflected on future Heck reactions of non-activated heteroaryl chlorides. 

The Suzuki-Miyaura reactions with relatively inert arylchloride are known to require palladium complexes possessing highly electron-rich ligands which favor the oxidative addition of the arylchloride into Pd(0)-complex (Scheme 11) [67-69]. Herrmann et al. showed that the utilization of NHC ligands with bulky substituents... 

The main requirement to methylchloride, ethylchloride and chlorobenzene is the absence of impurities, by-products and especially moisture. With even the slightest amount of liquid entering the reaction zone, the products start to hydrolyse and condense, the activity of the contact mass or copper-silicon alloy decreases, and the process subsides. That is why the technology of direct synthesis usually provides for a device to dehydrate alkyl- and arylchlorides. For this purpose one can pass methyl- or ethylchloride through the tower sprayed with sulfuric acid, or use other dehydrating substances (burnt CaCF, AI203 and zeolites, e.g. burnt klinoptilo-lite). 

Gaseous alkylchloride (or arylchloride) is fed through a special distribution device into the lower zone of the reactor. The products formed are withdrawn from the reactor through a side tube and are sent to purification in filter 4, and then into cooler system 5 (the diagram shows one) for condensation. 

Two coupled synthesis reactors turned out to be an efficient means to increase the yield of target products and the conversion degree of alkyl-and arylchlorides. 

Assessing on the whole the method of the production of alkyl- and aryl-chlorosilanes based on the interaction of alkyl- and arylchlorides with free silicon (i.e. direct synthesis), we should say that this method in comparison with metalorganic synthesis is more efficient, especially for the production of methyl- and phenylchlorosilanes. As for unsaturated chlorosi-lanes (vinyl- and allylchlorosilanes) and organochlorosilanes with higher radicals (hexyl-, heptyl-, octyl- and nonylchlorosilanes), no direct synthesis technique has yet been developed. 

The synthesis of silicone monomers based on the interaction of hy-dridechlorosilanes with alkyl-, alkenyl- or arylchlorides, is a convenient technique, which has recently found a wide application. 

Very promising results have also been observed using phosphonium-based ionic liquids in both Heck and Suzuki coupling reactions.[1012] Notably, good activities were obtained with arylchlorides as substrate, which in molecular solvents are often found to react only sluggishly, if at all. 

C4)4N]Br Palladium(O) carbene complexes NaOAc 140-160 °C. Phosphine-free arylation of styrene with arylchlorides excellent conversions with both activated and non-activated arylchlorides product extracted with Et20. [67]... 

C4)4N]Br Supported Pd nanoparticles Bu3N 130 °C. Phosphine-free arylation of styrene with arylchlorides palladium immobilised on layered double hydroxide microwave irradiation leads to significant rate acceleration product isolated by distillation. [81]... 

C4)4N]Br Pd nanoparticles [(C4)4N][OAc] 130 °C. Nanoparticles immobilised on chitosan products extracted with cyclohexane high reaction rates for deactivated arylbromides and activated arylchlorides highly recyclable catalyst system. [82]... 

Comparable reaction conditions were applied in the coupling of activated and non-activated arylchlorides with styrene or 2-ethylhexyl acrylate, using the palladium carbene catalysts shown in Scheme 6.6. While compounds 22 and 23 were found to be highly active catalysts, complex 21 was thermally unstable and decomposed to palladium black during the catalysis.1671 The yield and selectivity were only moderate in DMA, but results improved markedly when the reaction was carried out in [(C4)4N]Br. 

Palladium nanoparticles were further immobilised on chitosan flakes and from TEM it was established that they have a core-shell structure in which the metallic palladium is surrounded by a stabilising shell composed of a [(C4)4N]+ monolayer that is surrounded by Br and [PdCl4]2- species.1821 High catalytic activity for arylhalides including activated arylchlorides was observed at 130°C in [(C4)4N]Br using tetrabutylammonium acetate as base. 

A charged support, double layered hydroxide, that can be described by the general formula, Mgi xAlx(OH)2Cl zH20, was used to immobilise [PdCl4]2-/811 While results with this catalyst were only poor in A-methyl pyrrolidinone, good conversion and selectivity was obtained in [(C4)4N]Br with arylchlorides as substrates. By employing microwave irradiation coupling between chlorobenzene and styrene afforded the desired product in 95% yield within 30 minutes. The products were isolated by distillation and the catalytic activity remained essentially stable for at least 5 runs. 

Molten tetrabutylammonium bromide was shown to be another suitable solvent for the coupling of arylhalides with arylboronic acids.1 031 With 1.2 mol% of Pd(PPh3)4 and two equivalents of aqueous K2CO3 high yields were observed after 10 minutes at 120°C. Both the presence of water and base was necessary for the reaction to proceed. Other palladium sources such as Pd(OAc)2 or palladium black were also active, albeit with lower reaction rates. Notably, arylchlorides also react and chlorobenzene reacted with phenylboronic acid to afford biphenyl in 64% yield after 30 minutes. Recycling of the catalyst was possible and a higher activity was observed in the second and third cycles. 

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