Lewis-acids, CUS

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... 

In organic solvents Lewis-acid catalysis also leads to large accelerations of the Diels-Alder reaction. Table 2.2 shows the rate constants for the Cu -catalysed Diels-Alder reaction between 2.4a and 2.5 in different solvents. 

Surprisingly, the highest catalytic activity is observed in TFE. One mi t envisage this to be a result of the poor interaction between TFE and the copper(II) cation, so that the cation will retain most of its Lewis-acidity. In the other solvents the interaction between their electron-rich hetero atoms and the cation is likely to be stronger, thus diminishing the efficiency of the Lewis-acid catalysis. The observation that Cu(N03)2 is only poorly soluble in TFE and much better in the other solvents used, is in line with this reasoning. 

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

In contrast to the situation in the absence of catalytically active Lewis acids, micelles of Cu(DS)2 induce rate enhancements up to a factor 1.8710 compared to the uncatalysed reaction in acetonitrile. These enzyme-like accelerations result from a very efficient complexation of the dienophile to the catalytically active copper ions, both species being concentrated at the micellar surface. Moreover, the higher affinity of 5.2 for Cu(DS)2 compared to SDS and CTAB (Psj = 96 versus 61 and 68, respectively) will diminish the inhibitory effect due to spatial separation of 5.1 and 5.2 as observed for SDS and CTAB. 

Chapter 5 also demonstrates that a combination of Lewis-acid catalysis and micellar catalysis can lead to accelerations of enzyme-like magnitudes. Most likely, these accelerations are a consequence of an efficient interaction between the Lewis-acid catalyst and the dienophile, both of which have a high affinity for the Stem region of the micelle. Hence, hydrophobic interactions and Lewis-acid catalysis act cooperatively. Unfortunately, the strength of the hydrophobic interaction, as offered by the Cu(DS)2 micellar system, was not sufficient for extension of Lewis-acid catalysis to monodentate dienophiles. 

Chapter 2 describes the results of the first detailed study of Lewis-acid catalysis of a Diels-Alder reaction in water. Substituted 3-phenyl-l-(2-pyridyl)-2-propen-l-one dienophiles (la-gin Scheme 1) were found to coordinate to Co, Cu" and Zn ions in aqueous solution. This process forms... 

Lewis acids, such as the haUde salts of the alkaline-earth metals, Cu(I), Cu(II), 2inc, Fe(III), aluminum, etc, are effective catalysts for this reaction (63). The ammonolysis of polyamides obtained from post-consumer waste has been used to cleave the polymer chain as the first step in a recycle process in which mixtures of nylon-6,6 and nylon-6 can be reconverted to diamine (64). The advantage of this approach Hes in the fact that both the adipamide [628-94-4] and 6-aminohexanoamide can be converted to hexarnethylenediarnine via their respective nitriles in a conventional two-step process in the presence of the diamine formed in the original ammonolysis reaction, thus avoiding a difficult and cosdy separation process. In addition, the mixture of nylon-6,6 and nylon-6 appears to react faster than does either polyamide alone. 

Chiral Cu(ll)-complexes ofbis-oxazolines as Lewis acids for catalyzed cycloaddition, carbonyl addition, and conjugate addition reactions 99PAC1407. 

With few exceptions chiral Lewis acids are usually moisture-sensitive and require anhydrous conditions, but bench-stable aquo complexes such as [Cu(S,S)-t-Bu-box)(H20)2](SbF6)2 were found to promote the Diels-Alder reaction as effectively as the anhydrous copper reagent. 

Evans et al. reported that the bis(imine)-copper (II) complex 25, prepared from chiral bis(imine) ligand and Cu(OTf)2, is also an effective chiral Lewis acid catalyst [34] (Scheme 1.44, Table 1.18). By tuning the aryl imine moiety, the bis(2,6-dichlor-ophenylimine) derivative was found to be suitable. Although the endojexo selectivity for 3-alkenoyloxazolidinones is low, significant improvement is achieved with the thiazolidine-2-thione analogs, for which both dienophile reactivity and endojexo selectivity are enhanced. 

Thermolysis rates are enhanced substantially by the presence of certain Lewis acids (e.g. boron and aluminum halides), and transition metal salts (e.g. Cu ", Ag1).46 There is also evidence that complexes formed between azo-compounds and Lewis acids (e.g. ethyl aluminum scsquichloridc) undergo thermolysis or photolysis to give complexed radicals which have different specificity to uncomplexed radicals.81 83... 

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 ... 

FIGURE 15.4 When aqueous ammonia is added to a copper(ll) sulfate solution, first a light-blue precipitate ot Cu(OH)2 forms (the cloudy region at the top, which appears dark because it is backlit). The precipitate disappears when more ammonia is added to form the dark blue complex ru(NH))4 T by a Lewis acid-base reaction. 

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. 

Cationic bis(oxazoline) and pyridil-bis(oxazoline) Cu(ll) and Zn(ll) Lewis-acid catalysts. A comparative study in catalysis of Diels-Alder and aldol reactions [101]... 

The complex obtained from commercially available chiral a-amino acids (AA) with Cu + ion induces asymmetry in the Diels-Alder reaction of 31 (R = H) with 32. By using 10% Cu(II)-AA (AA = L-abrine) the cycloaddition occurs e/iJo-stereoselectively in 48 h at 0°C with high yield and with acceptable enantioselectivity ee = 1A%). This is the first example of enantioselective Lewis-acid catalysis of an organic reaction in water [9b]. 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

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