Lewis acids aluminium-based

The best results before organic catalysis were with amides 125 and Lewis acid catalysts based on Al, Ti, and Cu(II) with C2-symmetric ligands. Corey s aluminium complex 127 derived from the diamine whose resolution was described in chapter 22 works well with substituted cyclo-pentadienes 124 and the product 126 was used in prostaglandin synthesis.28 There are three aspects of stereoselectivity in this reaction which diastereotopic face of 124 is attacked (that anti to the CH2OBn group), is the product exo or endo (endo) and which endo product is formed, 126 or its enantiomer Only for the last question is asymmetric catalysis necessary, though Lewis acid catalysis of any kind enhances endo/exo selectivity. 

Other fates are possible for the enolate formed in the initial conjugate addition and an obvious possibility is an aldol reaction. With an asymmetric catalyst, the combination of three simple molecules leads to one enantiomer of one diastereoisomer of the tandem Michael-aldol product14 83. The catalyst 84 is based on a BINOL A1 complex (see chapters 25, 26). It can be drawn either as a lithium salt with an aluminium cation or, better, as a lithium aryloxide with a Lewis-acidic aluminium atom. This is better because both basic ArCT and Lewis acidity are necessary for catalysis. 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

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%. 

In general terms, an acidic site may be defined as a site on which a base is chemically adsorbed, and basic site as a site on which an acid is chemically adsorbed. A Bronoted acid shows a tendency to donate a proton and the Bronsted base to accept a proton. A Lewis acid is able to accept an electron pair and a Lewis base to donate an electron pair. The Lewis acid can be, for instance, a coordinatively unsaturated aluminium ion in alumina or silicoaluminates, a cation such as Na(I) or Ca(II) in zeolites, etc. 

Based on the catalytic results, in combination with the information obtained by 27A1 Mas-NMR and FT-IR, the following reaction mechanism (32) has been proposed. The initial step is chemisorption of the sec-alcohol on a Lewis-acid site, consisting of coordinatively unsaturated Al. This results in the formation of surface alkoxide. The coordinative interaction of the carbonyl of the ketone with the same aluminium center allows the formation of a six-membered transition state in which hydride transfer can occur (Fig. 15.5). 

The tendency of the aluminium atom to complete its electron octet is manifested as the Lewis acidity of organoaluminium compounds. This is demonstrated by the formation of 1 1 complexes of trialkylaluminium with anions or bases such as ethers and amines. Organoaluminium compounds are generally more acid than the organoderivatives of boron, indium and gallium. For steric reasons, complex formation is more difficult in compounds of... 

Plesch summarised in his lecture at the Rouen meeting all the data concerning the selfionisation of aluminium-based Lewis acids. Most of the figures he gave have not yet been published and that table is therefore particularly important, but at the same time difficult to analyse in view of the different, and perhaps not equally stringent, experimental conditions. The most relevant deductions are qualitative and concern the fact that (i) the equilibrium concentration of ions derived from these selfionisation reactions is always very small, and (ii) the rate of ionisation (forward reaction) can be quite small compared with that of other important steps in the process, such as the subsequent attack of the positive moiety on the monomer and the propagation reaction. These points are particularly important to the discussion of the initiation mechanism developed below. 

Detailed studies of systems involving aluminium-based Lewis acids and hydrogen halides are scarce. Fontana and Kidder investigated the polymerisation of propene initiated by the pair aluminium bromideTiydrt n bromide. The cocatalytic role of the latter acid was clearly proved since no polymerisation could be detected in its absence. The dependence of the rate of polymerisation upon the cocatalyst concentration and the induction periods observed make this system similar to those in which stannic chloride induces the polymerisation of olefins in the presence of variable quantities of water (see Sect. IV-C-3-b). With relatively large quantities of added hydrogen bromide, addition of this acid to the mcmomer to give fso-propyl bromide must have constituted an important side reaction. 

We fully recognise the highly speculative character of these last remarks and offer them to the reado- as a (perhaps premature) new way of looking at the role of organic halides in initiation of cationic pol3mierisations promoted by aluminium-based Lewis acids. 

The idea of the proton as the acidic entity was extended by G. N. Lewis. Protons can accept electron pairs from bases Lewis acids are generalized electron-pair acceptors. For example, aluminium trichloride (1.60) behaves as a Lewis acid and reacts with the chloride ion (a Lewis base). Boron trifluoride may react with the lone pair of the oxygen of diethyl ether to form boron trifluoride etherate (1.61). A Lewis base is an electron pair donor, in this case the ether oxygen. 

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