Lewis acid catalyst, in the

The most successful approach in this reaction category has been the use of chiral boron Lewis acid catalysts, in the addition of ethyl diazoacetate to imines reported by Wulff (Scheme 1.33) [59-60]. 

Other than energy considerations, on which there is little comparative data, the most important green role for photochemistry is in improving atom economy. Although only a preliminary research result, an excellent example of this is the avoidance of the need for stoichiometric amounts of Lewis acid catalysts in the synthesis of some acylated aromatic compounds. Benzoquinone can be reacted with an aldehyde under a sunlamp to yield an acylhydroquinone in up to 88% yield. The alternative procedure would involve reaction of an acyl chloride with hydroquinone and a... 

Cycloaddition of 2-cyanoalk-2-enones with several conjugated dienes proceeded under zinc chloride catalysis.636 Zinc halides have also shown reactivity with phenylacetylenes.637 Zinc chloride is an effective Lewis acid catalyst in the Diels Alder reactions of the keto esters and the effects on stereochemistry of catalysts used have been examined.638... 

Copper(II) complexes of amino acids have been explored as chiral Lewis acid catalysts in the Diels-Alder reaction of 3-phenyl-l-(2-pyridyl)-2-propen-l-one with cyclopentadiene. The best results were obtained using /V-methyl-/.-tryptophan, but more interestingly, the highest ee values for the major endo adduct were achieved in aqueous solution273. 

The neutral and cationic [Pd(II)NCN] pincer-containing dendritic assemblies 14 and 15 have been used as Lewis acid catalysts in the aldol condensation of benzaldehyde and methyl isocyanoacetate. These showed conversions, turnover frequencies and cis/trans ratios of the oxazoline products that were essentially identical to those of the monomeric model complex applied in the same reaction. 

Similarly, the aminated silane 9, obtained by alkylation of w-butylamine with the chlorosilane 7, can be transformed into a guanidine or a guanidinium sah.10 Here the corresponding grafted guanidinium salt was used as a Lewis-acid catalyst in the phosgenation of carboxylic acids (Figure 7). 

C2-Symmetric tridentate bis(oxazolinyl)pyri-dine-Cu(Il) complexes, introduced by Evans et al., can function as effective chiral Lewis acid catalysts in the Diels-Alder reaction [15a]. When applied to catalytic asymmetric aldol reactions [15], remarkable results were achieved (Scheme 8) [15a]. Only 0.5 mol % of catalyst 48 was needed for the reaction of 30 with the silylketene thioacetal 47 to yield after deprotection 49 in 99 % chemical yield. The ee values were determined to be 99 %. Today, catalysts... 

Lanthanide salts serve as efficient Lewis acid catalysts in the [3+2] cycloaddition of methylenecyclopropanes with activated aldehydes or ketones (Equation 100) <2003TL3839>. 

Diels-Alder reaction of aldehydes with activated dienes. This lanthanide shift reagent can function as a Lewis acid catalyst in the cyclocondensation of l-methoxy-3-trimethylsilyloxy-1,3-diene (2) with aromatic aldehydes, and permits isolation of the initial... 

You may note the inclusion of the Et2AICI Lewis acid catalyst in the second reaction. As we discussed in Chapter 35, the presence of a Lewis acid increases the rate of Diels-Alder reactions, and in this case is also vital for high stereoselectivity. 

On the other hand, thiocarbamoyl chlorides (15) may be used in thioamide synthesis, though via carbon-carbon bond formation in Friedel-Crafts-type reactions (c/. Volume 2, Chapter 3.1). The approach is limited to fairly electron-rich aromatics such as alkylbenzenes, anisole, and furan, but fails for benzene equation (7) gives some details. - Phenol does not require a Lewis acid catalyst in the reaction with H CSCl, but gives a mixture of 2-/4-hydroxythiobenzamides along with O-phenyl thiocarba-mate. ... 

In 1991, Mukaiyama et al. reported that high enantioselectivity can be achieved using the chiral tin (II) complex 6 derived from cinchonine (Cn) as a Lewis acid catalyst in the reactions of aldehydes and TMSCN [28]. However, the utilization of this protocol is limited to aliphatic aldehydes, where up to 96% ee was obtained (Table 4.4). However, when this protocol was applied to an aromatic aldehyde (benzaldehyde), no reaction was observed with TMSCN under the same conditions. 

Fig. 15.8 Selected Lewis acid catalysts in the screening experiments...

The modified organoaluminum reagent, BINAL, can be used as a chiral Lewis acid catalyst in the asymmetric hetero-Diels-Alder reaction [41]. Reaction of various aldehydes with activated dienes under the influence of catalytic BINAL (5-10 mol%) at -20 °C gave, after exposure of the resulting hetero-Diels-Alder adducts to tri-fluroacetic acid, predominantly c/v-dihydropyrone in high yield with excellent enan-tioselectivity. 

With the ability to make use of fluoroglycosides as C-glycosidation substrates, chloroglycosides have been comparably useful. As shown in Scheme 2.5.10, Allevi, et al.,89 condensed silylenol ethers with 2,3,4,6-tetra-Obenzyl-a-D-glucopyranosyl chloride. Silver triflate was utilized as the Lewis acid catalyst. In the examples studied, the yields exhibited were consistently greater than 80%. Furthermore, complete a selectivity was observed. 

For designing enzyme-mimicking catalysts exploiting metal ions as catalytic centers, it is necessary to understand catalytic repertories (32, 33) of metal ions acting as Lewis acid catalysts in the hydrolysis of peptide bonds and related carboxyl derivatives (e.g., esters). Although esters are much easier to hydrolyze than peptides, important mechanistic information for catalysis in peptide hydrolysis can be obtained from that in ester hydrolysis. 

Polymer supported catalysts have advantages because of the ease of catalyst recovery and the opportunity for simultaneously using otherwise incompatible catalytic systems. Indeed, the immobilization of several catalysts onto a polymer matrix is a unique way of avoiding antagonistic reactions between them, and of lowing reagents to participate in a cascade of reactive processes. For example, polymer-supported catalysts have been used as the Lewis acid catalysts in the carbocationic polymerization of isobutylene. After the reaction, polyisobutylene is obtained by simply filtering the supported catalyst. The reaction cycle can be repeated many times. 

Another important main group metal based Lewis acid catalyst in the aldol reaction is bismuth(III) compounds because of low toxicity, low cost, and good stability. Numerous examples of aldol and aldol-type reactions have been studied using bismuth(III) salts, especially BiCL and Bi(OTf)3, in catalytic amounts (188). 

In 1993, a tris(pentafluorophenyl)boron was first recognized by Yamamoto and co-workers as a water-tolerant Lewis acid catalyst in the aldol reaction of silyl enol ethers (237). Subsequently, Kobayashi and co-workers developed the first strategy for catalytic generation of boron enolates, employing catalytic amount of diphenylborinic acid (Ph2BOH) to promote the Mukaiyama aldol reaction in the presence of sodium dodecyl sulfate (SDS) (Scheme 59). The authors presumed that the active species of the reactions are boron enolates. Perhaps it is the first example of catalytic use of a boron source in boron enolatemediated diastereoselective aldol reactions (238). 

The same researchers found that lanthanide triflates could catalyze aldol reactions and allylations in aqueous media [10]. However, Wang et al. exploited the potential of lanthanide triflates to act as stable Lewis-acid catalysts in the aqueous imino Diels-Alder reaction [11]. This variant of the aqueous heterocycloaddition protocol also expands the scope of such reactions. The use of higher aldehydes in such reactions generally met with limited success under the conventional protocol. For example, under the standard conditions, the reaction of hexanal and benzylamine hydrochloride with cyclopentadiene in water is sluggish and affords only 4% of the Diels-Alder adducts 26 and 27 in a 2.7/1 ratio. In sharp contrast, the addition of various lanthanide(lll) triflates (0.25 M) to this reaction results in substantial increases in both the rate and yield of Diels-Alder adduct formation. In particular, use of praseodymium(iii) triflate results in a 68% yield of adducts 26 and 27 ... 

This interesting and promising result prompted us to examine the use of Sc(OTf)3 as a Lewis-acid catalyst in the aldol reactions of silyl enolates with carbonyl compounds. Several examples of the Sc(OTf)3-catalyzed aldol reactions of silyl enolates with aldehydes were examined. Silyl enolates derived from ketones, thioesters and esters reacted smoothly with aldehydes in the presence of 5mol% of Sc(OTf)3 to afford the aldol adducts in high yields. Sc(OTf)3 was also found to be an effective catalyst in the aldol-type... 

The use of T1CI4 acting as Lewis acid catalyst in the Ugi reaction between (5)-a-amino acids 20, such as valine, serine, phenylalanine, tryptophan, and even pyroglutamic acid, aromatic aldehydes 9, and a isonitrile 17b (Scheme 11.8), provided mixtmes of products 21 with moderate to good diastereoselectivity [25]. 

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