Al Lewis acids

Shirakawa, E. et al., Nickel-catalyzed carbostannylation of alkynes with allyl-, acyl-, and aUcynylstan-nanes Stereoselective synthesis of trisubstituted vinylstannanes, J. Am. Chem. Soc., 121, 9907, 1999. Matsukawa, Y. et al., Lewis acid catalyzed allylstannylation of unactivated alkynes. Tetrahedron, 155, 3779, 1999. 

Figure 2.30 Difference FTIR spectra of CD3CN adsorbed at ambient temperature on HZSIVl-5 with increasing coverage. Spectrum of the pretreated catalyst is subtracted. Hydroxyis absorbing at 3610 cm are shifted and split to an ABC pattern with maxima at 2780, 2415, and 1695 cm The bands at 2329 and 2299 cm are assigned toC-N modes of CD3CN bound to Al Lewis acid sites and bridging hydroxyls, respectively. Reproduced from Ref. (375).

In place of Ti(O Pr)2-BINOLate, (147) Al-BINOLates (148) are easily prepared by changing the metal source (Scheme 19.29). The polymer-supported chiral Al Lewis acid (148) was used for the enantioselective 1,3-dipolar cycloaddition of diphenyl nitron to vinyl ethers [65]. High levels of exoselectivity (>95 5) and enantioselectivity (96% ee) were obtained. 

Chu J, Han X, Kefalidis CE, et al. Lewis acid tri ered reactivity of a Lewis base stabilized scandium-terminal imido complex C—H bond activation, cycloaddition, and dehydrofluorination. J m Chem Soc. 2014 136 10894-10897. 

MAPH and ATPH have also been known as more bulky Al Lewis acids compared to methylaluminum bis(2,6-di-tert-butyl-4-alkylphenoxide) derivatives such as MAD and MAT. MAPH can discriminate between structurally similar aldehydes such as pentanal and cyclohexanecarboxaldehyde, thereby MAPH acts as a carbonyl protector of less hindered aldehydes. Therefore, butyllithium reacts preferentially with uncomplexed, more hindered aldehydes (Scheme 6.39) [48]. 

There are few research for Al Lewis acid-catalyzed [3 + 2]cycloaddition reactions, although Kanemasa reported ATP H catalyzed 1,3-dipolar cycloaddition of nitrones to a,P-unsaturated aldehydes (Scheme 6.112). In this reaction, ATPH acts as a pinhole Lewis acid, therefore in the presence of ATPH the opposite regioselectivity... 

This approach has been extended to the conjugate addition of primary dialkylzincs to 2-aryl- and 2-heteroaryl-nitroolefins to obtain enantioenriched 2-arylamines. Dendritic styryl TADDOLs and polymer-bound Ti-TADDOLates have proved to be very practical chiral catalysts for the enantioselective addition of organozinc to aldehydes. Likewise, the immobilization of BfNOL by a cross-linking copolymerization of styryl derivatives has allowed several enantioselective Ti- and Al-Lewis acid-mediated additions to aldehydes. Dialkylzines obtained via an 1/Zn exchange or a B/Zn exchange have also been successfully used for the enantioselective additions to... 

Desimoni et al. initially advocated the Acceptor Number (AN) as the dominant solvent parameter" The AN describes the ease with which a solvent can act as an electron pair acceptor (Lewis acid) and... 

Analogous to the classification of Lewis acids and bases in hard and soft species, Ahrland et al. have su ested a division of donors and acceptors into classes a and 6. See Ahrland, S. Chatt, J. Davies, N.R. Quart. Rev. 1958, 77, 265... 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

The effect of ligands on the endo-exo selectivity of Lewis-acid catalysed Diels-Alder reactions has received little attention. Interestingly, Yamamoto et al." reported an aluminium catalyst that produces mainly exo Diels-Alder adduct. The endo-approach of the diene, which is normally preferred, is blocked by a bulky group in the ligand. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

Catalytic asymmetric Diels-Alder reactions are presented by Hayashi, who takes as the starting point the synthetically useful breakthrough in 1979 by Koga et al. The various chiral Lewis acids which can catalyze the reaction of different dieno-philes are presented. Closely related to the Diels-Alder reaction is the [3-1-2] carbo-cyclic cycloaddition of palladium trimethylenemethane with alkenes, discovered by Trost and Chan. In the second chapter Chan provides some brief background information about this class of cycloaddition reaction, but concentrates primarily on recent advances. The part of the book dealing with carbo-cycloaddition reactions is... 

In 1994 Yamamoto et al. developed a novel catalyst which they termed a "Brmsted acid-assisted chiral Lewis acid" (BLA) [10] (Scheme 1.14, Table 1.3). The catalyst 7 was prepared from (R)-3,3 -dihydroxyphenyl)-2,2 -dihydroxy-l,l -binaphthyl by reaction with B(OMe)3 and removal of methanol [10a, dj. The Brmsted acid is essential for both the high reactivity of the Lewis acid and the high enantioselectivity - the... 

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. 

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). 

Corey et al. synthesized a chiral bis(oxazoline)Fe(III) catalyst 30, the ligand of which was prepared from chiral phenylglycine. The catalyst was formed by the reaction of the ligand with Fel3 in the presence of I2.12 greatly enhances the Lewis acidity of the catalyst owing to the formation of a cationic species [39] (Scheme 1.49). 

Shibasald et al. reported that lithium-containing, multifunctional, heterobimetallic catalysts such as LaLi3tris((l )-6,6 -dibromobinaphthoxide) 35, with moderate Lewis acidity in non-polar solvents, promote the asymmetric Diels-Alder reaction to give cycloadducts in high optical purity (86% ee) [53] (Scheme 1.67). The lithium... 

Chiral boron(III) Lewis acid catalysts have also been used for enantioselective cycloaddition reactions of carbonyl compounds [17]. The chiral acyloxylborane catalysts 9a-9d, which are also efficient catalysts for asymmetric Diels-Alder reactions [17, 18], can also catalyze highly enantioselective cycloaddition reactions of aldehydes with activated dienes. The arylboron catalysts 9b-9c which are air- and moisture-stable have been shown by Yamamoto et al. to induce excellent chiral induction in the cycloaddition reaction between, e.g., benzaldehyde and Danishefsky s dienes such as 2b with up to 95% yield and 97% ee of the cycloaddition product CIS-3b (Scheme 4.9) [17]. 

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