Lewis acids copper® chloride

Lewis acids (dicthylaluminum chloride, ethyl aluminum scsquichloridc) have been used in conjunction with ATRP to provide greater alternating tendency in S-MMA copolytnerization.519 However, poor control was obtained because of interaction between the catalyst (CuCI/dNbpy) and the Lewis acid. Better results were obtained by RAFT polymerization/10 Copper catalysts, in particular Cu(lI)Br/PMDETA, have been shown to coordinate monomer but this has negligible influence on the outcome of copolymerization/6 ... 

Hussey et al. carried out an aluminum bulk deposition on copper foil using a Lewis acidic aluminum chloride 1 -ethyl-3-methyl-imidazolium chloride-based ionic liquid [9]. The thickness of the observed deposits were in the range 24—30 pm. Without additives the deposits were not shiny and only poorly adherent. The addition of benzene enhanced the quality of the deposit. XRD measurements confirmed that the composition of the deposits was 100% aluminum metal. 

Continuous chlorination of benzene at 30—50°C in the presence of a Lewis acid typically yields 85% monochlorobenzene. Temperatures in the range of 150—190°C favor production of the dichlorobenzene products. The para isomer is produced in a ratio of 2—3 to 1 of the ortho isomer. Other methods of aromatic ring chlorination include use of a mixture of hydrogen chloride and air in the presence of a copper—salt catalyst, or sulfuryl chloride in the presence of aluminum chloride at ambient temperatures. Free-radical chlorination of toluene successively yields benzyl chloride, benzal chloride, and benzotrichloride. Related chlorination agents include sulfuryl chloride, tert-huty hypochlorite, and /V-ch1orosuccinimide which yield benzyl chloride under the influence of light, heat, or radical initiators. 

Many of the d-block elements form characteristically colored solutions in water. For example, although solid copper(II) chloride is brown and copper(II) bromide is black, their aqueous solutions are both light blue. The blue color is due to the hydrated copper(II) ions, [Cu(H20)fJ2+, that form when the solids dissolve. As the formula suggests, these hydrated ions have a specific composition they also have definite shapes and properties. They can be regarded as the outcome of a reaction in which the water molecules act as Lewis bases (electron pair donors, Section 10.2) and the Cu2+ ion acts as a Lewis acid (an electron pair acceptor). This type of Lewis acid-base reaction is characteristic of many cations of d-block elements. 

Many years ago, geochemists recognized that whereas some metallic elements are found as sulfides in the Earth s crust, others are usually encountered as oxides, chlorides, or carbonates. Copper, lead, and mercury are most often found as sulfide ores Na and K are found as their chloride salts Mg and Ca exist as carbonates and Al, Ti, and Fe are all found as oxides. Today chemists understand the causes of this differentiation among metal compounds. The underlying principle is how tightly an atom binds its valence electrons. The strength with which an atom holds its valence electrons also determines the ability of that atom to act as a Lewis base, so we can use the Lewis acid-base model to describe many affinities that exist among elements. This notion not only explains the natural distribution of minerals, but also can be used to predict patterns of chemical reactivity. 

Diels-Alder reactions are one of the most famous examples which are accderated by a Lewis acid. Various water-stable Lewis adds such as Ln(OTf)3,1371 methylrhenium trioxide,1381 copper nitrate,1391 copper bis(dodecyl sulfate) (4b),1401 indium chloride,1411 and bismuth triflate1421 have been used for Diels-Alder and aza-Diels-Alder reactions in water. Furthermore, a catalytic asymmetric Dids-Alder reaction in water using a copper complex of an amino... 

The reverse reaction is catalysed by copper sulphate in an ethanol/water (50 50) mixture297 298. Indium(III) chloride catalysis of Diels-Alder reactions was also reported, but the effects were poor and comparison to uncatalysed reactions was made only in a few cases299,300. A very versatile Lewis acid catalyst for such reactions is methylrhenium trioxide (MTO)300. This catalyst can be used without a solvent, in pure organic solvents like chloroform and even in pure water. While the catalyst is active in the latter two solvents (Table 22), it gives the best results in water (Table 23). 

Figure 8-8. The copper-mediated conversion of an anisole derivative to a co-ordinated phenoxide. This provides a useful alternative to conventional methods of ether hydrolysis, which involve treatment with Lewis acids such as boron tribromide, hydriodic acid or pyridinium chloride.

Copper(l) chloride in combination with tributyltin hydride shows unique character as an initiator of certain radical reactions. Hydrostannation of a,/3-unsaturated ketones with Bu3SnH is initiated by CuCl and the resulting tin enolates react with aldehydes under the influence of CuCl as a Lewis acid catalyst (Equation (81))/... 

When R-C O+ is used as the electrophile a ketone is produced If an aldehyde were wanted, H-C=0+ would have to be used but it cannot be made from HCOC1 because that is unstable. Instead, it can be generated by passing carbon monoxide and hydrogen chloride through a mixture of the aromatic hydrocarbon, a Lewis acid, and a co-catalyst, usually copper (I) chloride. Copper (I) chloride is known to form a complex with carbon monoxide and this probably speeds up the proto-nation step. 

Lewis acids can also be exploited for the cleavage of isopropylidene derivatives. One of the mildest examples comes from a synthesis of Lankacidin in which cleavage of an isopropylidene acetal without harm to a p-methoxybenzyl ether was effected with copper(Il) chloride dihydrate in methanol at reflux [Scheme 3.11].13 Alternatively, zinc(II) nitrate hexahydrate in acetonitrile at 50 DC can be used in which case even a primary tert-butyldimethylsilyl ether survives [Scheme 3.12].14 During a synthesis of Quinocarin, Katoh and co-workerscleaved an isopropylidene group using iron(IIT) chloride adsorbed onto silica gel [Scheme 3.13]. 

A diazonium salt reacts with copper(I) chloride or copper(I) bromide to form an aryl chloride or aryl bromide, respectively. This is called the Sandmeyer reaction. It provides an alternative to direct chlorination and bromination of an aromatic ring using CI2 or Br2 and a Lewis acid catalyst. 

Another advantage over other Lewis acids is the thiophilic nature of copper this has led to its utility in the hydrolysis of thioacetals. Copper(II) chloride in conjunction with copper(II) oxide was introduced by Mukaiyama [16] for the deprotection of 1,3-dithianes and this method has found utility in a variety of synthetic protocols (Sch. 6) [17]. This combination, in which copper oxide plays the role of a buffer to prevent the medium from becoming too acidic, has also found application in the hydrolysis of a-heteroatom substituted and vinyl sulfides [18]. Acetals, which are prone to epimerization under acid-catalyzed hydrolysis conditions (21), can be con-... 

Copper(I) triflate was used as a co-catalyst in a palladium-catalyzed carbonylation reaction (Sch. 27). The copper Lewis acid was required for the transformation of homoallylic alcohol 118 to lactone 119. It was suggested that the CuOTf removes chloride from the organopalladium intermediate to effect olefin complexation and subsequent migratory insertion [60]. Copper(I) and copper(II) chlorides activate ruthenium alkylidene complexes for olefin metathesis by facilitating decomplexation of phosphines from the transition metal [61]. 

The rearrangement of propargyl chlorides to chloroallenes occurs stereoselectively in the presence of BU4NCUCI2 (Sch. 28) [62]. Copper(I) dichloride performs as a reagent to deliver chloride, and CuCl might be acting as a Lewis acid to activate the pro-pargylic chloride for elimination. 

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