Aluminium Lewis acids

Note that the catalyst of Figure 10.6 contains the same titanium part as that of Figure 10.5, but that they differ in the aluminium Lewis acid and anions formed. The use of diethylaluminium chloride (DEAC, the common initiator for heterogeneous titanium catalysts) gave propene dimerisation only. This... 

The use of Lewis acids in controlling the stereoselective outcome of radical cyclization reactions has been explored, in particular the effect of aluminium-based Lewis acids using low temperature Et3B/Bu3SnH-initiated procedures.171,172 For example, cyclization of propargyl ether (78) or allyl ether (79) in the presence of Lewis acid (80) can completely reverse the normal selectivity (Scheme 34).171 The effect of aluminium Lewis acids on the diastereoselectivity of 6-exo cyclization of unsaturated chiral menthol esters has been studied.172 Cyclization at low temperature in the presence of the Lewis acid MAD modified the de of the reaction from 31 to 98%. 

A P- Al J-coupling constant for trimethylphosphine bound to the Lewis acid of Zeolite HY has been determined by Al to P INEPT methods,since coupling could not be resolved for this resonance (6= —49) in the P MAS NMR spectrum. The coupling was consistent with a five-coordinated aluminium Lewis acid/trimethyl phosphine complex, and thus a four-coordinated Lewis acid site. Bi- and tricyclic penta- and hexacoordinated-phosphoranes 25-28 have been... 

Diastereoselective complexation of a chiral [antagonist] ketone with a racemic aluminium Lewis acid catalyst effectively removes one enantiomer of the latter, leaving the uncomplexed antipode free to function as a chiral Lewis acid. E Asym. heterodiene synthesis. 0.1 eq. D-3-bromocamphor, 1.05 eqs. startg. siloxydiene, and benzaldehyde added sequentially to 0.1 eq. of the racemic aluminium complex in degassed methylene chloride at —78°, stirred for 3 h, then subjected to acidic work-up (2S,3S)-product. Y 78% (e.e. 82%, upgraded to > 98% by one recrystallization with ca. 60% recovery). F.e.s. K. Maruoka, H. Yamamoto, J. Am. Chem. Soc. Ill, 789-90 (1989). 

A chiral aluminium Lewis acid catalyst composed of Me3Al and 3,3 -bis-(trimethylsilyl)-BINOL has been reported to promote catalytic asymmetric ring expansion of cyclohexanone with a-substituted a-diazoacetates to give seven-membered rings with an all-carbon quaternary centre (Scheme 70). ... 

The tailored dinuclear aluminium Lewis acid displayed a high Lewis-acid catalytic activity due to a double electrophilic activation of a substrate s carbonyl group. This skilful ligand design turned a standard alkoxide into a real multitool for carbonyl chemistry. This type of catalyst was employed for instance in Mukaiyama aldol reactions, in MPV reductions and Oppenauer oxidations and related Tischchenko coupling reactions. 

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. 

BenZotrichloride Method. The central carbon atom of the dye is supphed by the trichloromethyl group from iJ-chlorobenzotrichloride. Both symmetrical and unsymmetrical triphenyhnethane dyes suitable for acryhc fibers are prepared by this method. 4-Chlorobenzotrichloride is condensed with excess chlorobenzene in the presence of a Lewis acid such as aluminium chloride to produce the intermediate aluminium chloride complex of 4,4, 4"-trichlorotriphenylmethyl chloride (18). Stepwise nucleophihc substitution of the chlorine atoms of this intermediate is achieved by successive reactions with different arylamines to give both symmetrical (51) and unsymmetrical dyes (52), eg, N-(2-chlorophenyl)-4-[(4-chlorophenyl) [4-[(3-methylphenyl)imino]-2,5-cyclohexadien-l-yhdene]methyl]benzenaminemonohydrochloride [85356-86-1J (19) from. w-toluidine and o-chloroaniline. 

Azaferrocene reacts with aromatic hydrocarbons in the presence of aluminium chloride, giving rise to the cationic complexes of the type (Ti -arene)(Ti -cyclopenta-dienyl)iron(l+) isolated as BF4 salts [87JOM(333)71]. The complex 28 is obtained by reaction of the sulfane compound [Cp(SMc2)3Fe]BF4 with pentamethyl-pyrrole [88AG(E)579 88AG(E)1468 90ICA(170)155]. The metallic site in this center reveals expressed Lewis acidity (89CB1891). 

The chemical behavior of Franklin acidic chloroaluminate(III) ionic liquids (where X(A1C13) > 0.50) [6] is that of a powerful Lewis acid. As might be expected, it catalyzes reactions that are conventionally catalyzed by aluminium(III) chloride, without suffering the disadvantage of the low solubility of aluminium(III) chloride in many solvents. 

The role of Lewis acids in the formation of oxazoles from diazocarbonyl compounds and nitriles has primarily been studied independently by two groups. Doyle et al. first reported the use of aluminium(III) chloride as a catalyst for the decomposition of diazoketones.<78TL2247> In a more detailed study, a range of Lewis acids was screened for catalytic activity, using diazoacetophenone la and acetonitrile as the test reaction.<80JOC3657> Of the catalysts employed, boron trifluoride etherate was found to be the catalyst of choice, due to the low yield of the 1-halogenated side-product 17 (X = Cl or F) compared to 2-methyI-5-phenyloxazole 18. Unfortunately, it was found that in the case of boron trifluoride etherate, the nitrile had to be used in a ten-fold excess, however the use of antimony(V) fluoride allowed the use of the nitrile in only a three fold excess (Table 1). 

As mentioned several times Lewis acids are highly valuable catalysts but the most commonly used ones such as aluminium chloride and boron trifluoride are highly water sensitive and are not usually recovered at the end of a reaction, leading to a significant source of waste. In recent years there has been much research interest in lanthanide triflates (trifluoro-methanesulfonates) as water stable, recyclable Lewis acid catalysts. This unusual water stability opens up the possibility for either carrying out reactions in water or using water to extract and recover the catalyst from the reaction medium. 

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

All acids but especially Lewis acids (particularly aluminium chloride), give rise to dangerous interactions with nitrated derivatives and nitrates (there is not much information about nitrates). Aluminium chloride causes a large number of accidents due to nitrobenzene and sometimes nitromethane when used as a solvent in Friedel-Crafts reactions for which aluminium chloride is the common catalyst. 

An alternative route to anthraquinone, which involves Friedel-Crafts acylation, is illustrated in Scheme 4.3. This route uses benzene and phthalic anhydride as starting materials. In the presence of aluminium(m) chloride, a Lewis acid catalyst, these compounds react to form 2-benzoyl-benzene-1-carboxylic acid, 74. The intermediate 74 is then heated with concentrated sulfuric acid under which conditions cyclisation to anthraquinone 52 takes place. Both stages of this reaction sequence involve Friedel-Crafts acylation reactions. In the first stage the reaction is inter-molecular, while the second step in which cyclisation takes place, involves an intramolecular reaction. In contrast to the oxidation route, the Friedel-Crafts route offers considerable versatility. A range of substituted... 

Mixtures of C4 alkene isomers (largely isobutene) are polymerised commercially in contact with low levels of aluminium chloride (or other Lewis acid) catalysts. The highly exothermic runaway reactions occasionally experienced in practice are caused by events leading to the production of high local levels of catalyst. Rapid increases in temperature and pressure of 160°C and 18 bar, respectively, have been observed experimentally when alkenes are brought into contact with excess solid aluminium chloride. The runaway reaction appears to be more severe in the vapour phase, and a considerable amount of catalytic degradation contributes to the overall large exotherm. 

The ready decomposition of most aryl azides with sulfuric acid and Lewis acids may be vigorous or violent, depending on structure and conditions. In absence of a diluent (carbon disulfide), phenyl azide and aluminium choride exploded violently. 

Yoshino reports a novel and general method for the C-3 acylation of indoles with acyl chlorides in the presence of dialkylaluminium chloride which obviates the need for prior N-protection . Interestingly, as described in this preliminary communication, the unprotected indoles 147 are first treated with the Lewis acids prior to addition of the acid chlorides, yielding the desired 3-acyl derivatives 148. In reactions more typical of indoles under acidic conditions, Nakatsuka determined the structures of the dimers and trimers of 1-trimethylacetylindole produced in the presence of aluminium chloride . 

Trimethyl-d9-chlorogermane was prepared for an infrared study by reacting tetramethyl-di2-germane with acetyl chloride in the presence of a Lewis acid such as aluminium trichloride12 (equation 11). 

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