Conjugate Lewis acids

Evidently, however, the use of strong acids or stable dual-acid anions is only one of the necessary requirements to initiate carbonium ion reactions in general and cationic polymerizations in particular. A specific substrate (Lewis base) which is able to accept the proton (or carbonium ion) and can be converted into a new conjugated Lewis acid is equally important. This newly formed electrophile in conjunction with the deprotonated dual-acid will be the reactive species. 

Ishihara and coworkers also reported mixed zinc reagents, namely PhjZn/EtjZn, as the phenyl source for enantioselective phenyl additions to ketones (Scheme 7.51). The cat2ilyst was a chiral phosphoramides-Zn(II) complex prepared in situ [81]. These chiral Zn(II) cat2ilysts serve as conjugate Lewis acid-Lewis base catalysts (Figure 7.10). From a variety of aromatic and aliphatic ketones, optically active tertiary alcohols were obtained in high yields with high enantioselectivities (up to 98% ee) under mild reaction conditions (Scheme 7.51). 

Figure 7.10 Working model for the action of the chiral phosphoramide-Zn(ll) complex as a conjugate Lewis acid-Lewis base catalyst in ketone phenylations, as described by Ishihara and coworkers [81].

Ishihara et al. reported that conjugate Lewis acid-Lewis base catalysis was highly effective in enantioselective dialkylzinc addition to a series of aromatic, aliphatic, and heteroaromatic aldehydes (Scheme 4.48) [42]. Bifunctional BINOL ligands bearing phosphine oxides [P(=0)R2] (136), phosphonates [P(=0)(0R)2] (137) or phosphoramides [P(=0)(NMe2)2] (138) at the 3,3 positions were developed. The coordination of a NaphO-Zn(II)-R center as a Lewis acid to a carbonyl group... 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

The inactivity of pure anhydrous Lewis acid haUdes in Friedel-Crafts polymerisation of olefins was first demonstrated in 1936 (203) it was found that pure, dry aluminum chloride does not react with ethylene. Subsequentiy it was shown (204) that boron ttifluoride alone does not catalyse the polymerisation of isobutylene when kept absolutely dry in a vacuum system. However, polymers form upon admission of traces of water. The active catalyst is boron ttifluoride hydrate, BF H20, ie, a conjugate protic acid H" (BF20H) . 

Brmnsted-Lewis Superacids. Conjugate Friedel-Crafts acids prepared from ptotic and Lewis acids, such as HCl—AlCl and HCl—GaCl ate, indeed, supetacids with an estimated value of —15 to —16 and ate effective catalysts in hydrocarbon transformation (217). 

Electrophilic Addition. In the following example, an a-olefin reacts with a Lewis acid to form the most stable intermediate carbocation. This species, in turn, reacts with the conjugate base to produce the final product. Thus electrophilic addition follows Markovnikov s rule. 

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

Gothelf presents in Chapter 6 a comprehensive review of metal-catalyzed 1,3-di-polar cycloaddition reactions, with the focus on the properties of different chiral Lewis-acid complexes. The general properties of a chiral aqua complex are presented in the next chapter by Kanamasa, who focuses on 1,3-dipolar cycloaddition reactions of nitrones, nitronates, and diazo compounds. The use of this complex as a highly efficient catalyst for carbo-Diels-Alder reactions and conjugate additions is also described. 

The catalytic enantioselective cycloaddition reaction of carbonyl compounds with conjugated dienes has been in intensive development in recent years with the main focus on synthetic aspects the number of mechanistic studies has been limited. This chapter will focus on the development and understanding of cycloaddition reactions of carbonyl compounds with chiral Lewis acid catalysts for the preparation of optically active six-membered ring systems. 

For the reaction of carbonyl compounds with conjugated dienes two mechanistic pathways have generally been taken into account when Lewis acid-catalyzed reactions are considered ... 

Quite a number of asymmetric thiol conjugate addition reactions are known [84], but previous examples of enantioselective thiol conjugate additions were based on the activation of thiol nucleophiles by use of chiral base catalysts such as amino alcohols [85], the lithium thiolate complex of amino bisether [86], and a lanthanide tris(binaphthoxide) [87]. No examples have been reported for the enantioselective thiol conjugate additions through the activation of acceptors by the aid of chiral Lewis acid catalysts. We therefore focussed on the potential of J ,J -DBFOX/ Ph aqua complex catalysts as highly tolerant chiral Lewis acid catalyst in thiol conjugate addition reactions. 

With the success in Lewis acid-catalyzed thiol conjugate addition reactions mentioned above, we further tried to apply the J ,J -DBFOX/Ph-nickel(II) aqua complex catalyst to the catalyzed asymmetric conjugate addition reactions of hydroxyl-amines [88, 89]. However, after some preliminary examinations, we found that... 

The l ,J -DBFOX/Ph-transition metal aqua complex catalysts should be suitable for the further applications to conjugate addition reactions of carbon nucleophiles [90-92]. What we challenged is the double activation method as a new methodology of catalyzed asymmetric reactions. Therein donor and acceptor molecules are both activated by achiral Lewis amines and chiral Lewis acids, respectively the chiral Lewis acid catalysts used in this reaction are J ,J -DBFOX/Ph-transition metal aqua complexes. 

The coordination of the dienophile to a Lewis acid (in the calculations a proton was used as the Lewis acid) leads also to an increase in regioselectivity. The re-gioselectivity of reactions of electron-rich, or conjugated dienes, with electron-deficient dienophiles is also controlled hy the diene HOMO-dienophile LUMO interaction. From Fig. 8.2 it appears that the difference in magnitudes of the LUMO coefficients at carhon atoms 1 and 2 of acrolein (Ci -C2 = 0.20) is smaller than the same difference for protonated acrolein (Ci -C2 = 0.30-0.43) so that the reaction of the latter should he considerable more regioselective than the former in accordance with the experimental results [3]. 

Sdieire 3.3S. Tetrahydrofcifar cyrthecic by mearc of MOM a-allco 7allcylciipfate conjugate additionc followed by Lewie acid-promoted oyclization (MOM = metho rymethyl) [129]. 

According to Scheme 5-4 the diazonium ion (as a Lewis acid) is in equilibrium with its conjugate base, the diazohydroxide, and according to Scheme 5-5 the diazohydroxide (this time as acid) is in equilibrium with its conjugate base, the diazoate. This treatment applies only to those reaction steps that are moderately or very rapid and are reversible. 

In that reaction does boric acid act as a Lewis acid, a I ewis base, or neither Justify your answer by using Lewis structures of boric acid and its conjugate base. 

If an excess of Lewis acid is used, then the formation of conjugated anions built from the Lewis acid and the anion originating from it must be considered ... 

The interaction of a complex anion SbY5Z with excessive Lewis acid SbX5 leads to the formation of a ligand-bridged (conjugated 1)) anion (see part 4.2.2) according to... 

Table 18 summarizes the interaction energies between the anions SbYsZ and the Lewis acids SbX5, In general, the formation of a conjugated anion proceeds exothermically (—0.7. .. —1.9 eV). This energy is clearly smaller than those for the interaction between a Lewis acid SbX5 and a halide ion 7T (—4.1. .. —6.3 eV). The interaction of Lewis acids with complex anions decreases in the rows respectively ... 

Thus, the conjugated anion represents an intermediate for the halide transfer from a complex anion to a Lewis acid. The quantum chemical reaction energies for the halide transfer AE(r can be calculated using the values of the interaction energies from Table 18 in the equation AEtr = AE(I) — AE(II). The results are presented in Table 20 and allow the following generalization ... 

Lewis-acid catalyzed inverse electron-demand Diels-Alder reactions between conjugated carbonyl compounds and simple alkenes and enolethers also allow dihydropyranes to be prepared. SnCU-Catalyzed cycloaddition of... 

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

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