Lewis Acids Links

PoIysuIfonyIa.tlon, The polysulfonylation route to aromatic sulfone polymers was developed independendy by Minnesota Mining and Manufacturing (3M) and by Imperial Chemical Industries (ICI) at about the same time (81). In the polymerisation step, sulfone links are formed by reaction of an aromatic sulfonyl chloride with a second aromatic ring. The reaction is similar to the Friedel-Crafts acylation reaction. The key to development of sulfonylation as a polymerisation process was the discovery that, unlike the acylation reaction which requires equimolar amounts of aluminum chloride or other strong Lewis acids, sulfonylation can be accompHshed with only catalytic amounts of certain haUdes, eg, FeCl, SbCl, and InCl. The reaction is a typical electrophilic substitution by an arylsulfonium cation (eq. 13). 

The mineral talc is extremely soft (Mohs hardness = 1), has good sHp, a density of 2.7 to 2.8 g/cm, and a refractive index of 1.58. It is relatively inert and nonreactive with conventional acids and bases. It is soluble in hydroduoric acid. Although it has a pH in water of 9.0 to 9.5, talc has Lewis acid sites on its surface and at elevated temperatures is a mild catalyst for oxidation, depolymerization, and cross-linking of polymers. 

Since Evans s initial report, several chiral Lewis acids with copper as the central metal have been reported. Davies et al. and Ghosh et al. independently developed a bis(oxazoline) ligand prepared from aminoindanol, and applied the copper complex of this ligand to the asymmetric Diels-Alder reaction. Davies varied the link between the two oxazolines and found that cyclopropyl is the best connector (see catalyst 26), giving the cycloadduct of acryloyloxazolidinone and cyclopentadiene in high optical purity (98.4% ee) [35] (Scheme 1.45). Ghosh et al., on the other hand, obtained the same cycloadduct in 99% ee by the use of unsubstituted ligand (see catalyst 27) [36] (Scheme 1.46, Table 1.19). 

The primary aim of most studies on Lewis acid controlled copolymerization has been the elucidation of mechanism and only low conversion polymerizations are reported. Sherrington et al.m studied the high conversion synthesis of alternating MMA-S copolymers in the presence of Lewis acids on a preparative scale. Many Lewis acids were found lo give poor control (i.e. deviation from 50 50 composition) and were further complicated by side reactions including cross-linking. They found that the use of catalytic BCI- as the Lewis acid and photoinitiation gave best results. 

Alkyl halides, interaction with Lewis acids 207 Amination of polymer 156 Anionic end-linking 164... 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

Complexes of other metals such as gallium, indium, lead, and antimony have also been used as Lewis acids. Catalytic enantioselective meso-epoxide ring-opening reactions using a chiral gallium(III) catalyst (Ga-Li-linked-BINOL) have been reported (Scheme 84).348 The chemical yields are much improved by linking two BINOL units. 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

Kusumoto and coworkers have found that the treatment of hemiacetal 1 with trifluoro- or trichloroacetic anhydride 94 (1 equiv) and trimethylsilyl perchlorate (0.2 equiv) selectively provides the corresponding anomeric ester intermediate 91 [152], Hemiacetal acylation occurs even in the presence of the alcohol acceptor. With Lewis acid assistance, the glycosyl ester intermediate is displaced to provide disaccharide products in good yields. This transformation allowed the synthesis of disaccharides 98 (81%) and 99 (91%). In some cases, acetic anhydride has been used as the electrophilic activator of hemiacetal donors and the reaction with thiol acceptors yields S-linked glycosides [153,154],... 

In this section, the literature about Diels-Alder reactions will be presented in a conceptual and illustrative way. After a profound introduction dealing with the development of mechanistic understanding of the Diels-Alder reaction, some interesting recent synthetic developments and applications will be presented. The reaction types and fields of interest are structured in such a way that they can be easily linked to ongoing research from the past ten years. Special attention will be paid to the application of chiral auxiliaries and chiral Lewis acids in asymmetric Diels-Alder reactions. 

The nature of the acidic sites is still subject of lively discussion. One school of thought, based on a proposition by Thomas (348), attributes the acidity to substitution of AP+ ions for Si + ions in a tetrahedrally linked silica network. Electroneutrality is obtained by addition of protons. Others think that Lewis acid sites, as proposed by Milliken et al. (349), are responsible for the catalytic activity, Gray (350) suggested that only the alumina content was responsible and that a spinel-like phase was formed on heating with protons on certain octahedral positions. 

The metal is initially covalently linked to silica and a methyl is extracted by a weakly coordinating Lewis acid such as B(C5F5)3 (BARF). 

E.4. Scandium Cross-Linked Dendrimers as Lewis Acid Catalysts... 

Fig. 13). The cross-linked scandium-modified dendrimer was tested in a number of Lewis acid-catalyzed reactions, including Mukaiyama aldol additions to aldehydes and aldimines, Diels-Alder reactions, and Friedel-Crafts acylations. The dendritic catalyst was recovered by a simple filtration. The Mukaiyama aldol... 

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