Solvents aryl chlorides

To allow more efficient stirring, it was beneficial to run this reaction at half of the concentration (2 ml. solvent per mmol of aryl chloride) of the original published procedure (1 mL solvent per mmol of aryl chloride). 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

As an example, consider the use of PVPy as a solid poison in the study of poly(noibomene)-supported Pd-NHC complexes in Suzuki reactions of aryl chlorides and phenylboroiuc acid in DMF (23). This polymeric piecatalyst is soluble under some of the reaction conditions employed and thus it presents a different situation from the work using porous, insoluble oxide catalysts (12-13). Like past studies, addition of PVPy resulted in a reduction in reaction yield. However, the reaction solution was observed to become noticeably more viscous, and the cause of the reduced yield - catalyst poisoning vs. transport limitations on reaction kinetics - was not immediately obvious. The authors thus added a non-functionalized poly(styrene), which should only affect the reaction via non-specific physical means (e.g., increase in solution viscosity, etc.), and also observed a decrease in reaction yield. They thus demonstrated a drawback in the use of the potentially swellable PVPy with soluble (23) or swellable (20) catalysts in certain solvents. 

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

Long-lived homogeneous Pd(0)f4 [L = P(Ph)3] catalysts for the cyanation of aryl chlorides (295) (X = Cl) have been developed, which are activated by cathodic reduction (Scheme 113) [433]. A high temperature, 130 °C, and a high-boihng solvent, dimethylformamide, are required because Pd does not insert into the aryl C—Cl bond at lower temperatures. Cat-alytically inactive metal species formed in undesirable side reactions with cyanide are electrochemically restored to a catalytically... 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

Heterogeneous Pd catalysts can activate the C-Cl bond in aryl chlorides for the Suzuki-Miyaura reaction, presumably due to a synergistic anchimeric and electronic effect that occurs between the Pd surface and the aryl chlorides. Pd on carbon has been found to be a very effective pre-catalyst for a variety of substrates even under very mild reaction conditions and aqueous solvent mixtures.In 2001, Kabalka and co-workers described that Pd powder and KF as base were useful to couple aryl iodides with arylboronic acids in methanol." At the conclusion of the reaction, Pd metal could be recovered by simple decantation. The use of microwave irradiation accelerates the reaction by... 

Formation of aldehydes. Aldehydes can be prepared by the carbonylation of halides in the presence of various hydride sources. The carbonylation of aryl and alkenyl iodides and bromides with CO and H (1 1) in aprotic solvents in the presence of tertiary amines affords aldehydes[373,374]. Aryl chlorides, as tricarbonylchromium derivatives, are converted into aldehydes at 130 C[366], Sodium formate can be used as a hydride source to afford aldehydes. Chlorobenzene (514) was carbonylated at 150 °C to give benzaldehyde with CO and sodium formate by using dippp as a ligand[375,376]. 

Another one-pot, large-scale procedure for the preparation of thiophenols from unactivated aryl chlorides is available [7]. The reaction of an aryl chloride with a sodium alkanethiolate in NMP as a solvent affords, after acidification, the thiol in a high yield. 

In addition, aryl chlorides, bromides, and iodides can be converted to arene-amines ArNH2 by the conjugate bases of amines. In fact, the reaction of potassium amide with bromobenzene is extremely rapid, even at temperatures as low as —33°, with liquid ammonia as solvent ... 

The use of formamide as solvent in the ammonolysis of activated aryl chlorides has been recommended when the substrate carries cyano ring substituents which can undergo hydrolysis in water.126... 

The problem with aryl chlorides seems to be that they do not compete effectively with reactant alkenes or even many solvents as ligands or reactants at palladium(0). Even if the arylpalladium chloride is formed its equilibrium concentration under the usual reaction conditions is probably very low. 

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