Aluminum isopropoxide complexes

Coates and Ovitt reported ROP of wso-lactide catalyzed by a chiral aluminum isopropoxide complex 161b (Fig. 26), which exhibited a syndiotactic stereoselection (Pi = 96%) with alternating arrangement of stereocenters [160]. 

The milder metal hydnde reagents are also used in stereoselective reductions Inclusion complexes of amine-borane reagent with cyclodexnins reduce ketones to opucally active alcohols, sometimes in modest enantiomeric excess [59] (equation 48). Diisobutylaluminum hydride modified by zmc bromide-MMA. A -tetra-methylethylenediamme (TMEDA) reduces a,a-difluoro-[i-hydroxy ketones to give predominantly erythro-2,2-difluoro-l,3-diols [60] (equation 49). The three isomers are formed on reduction with aluminum isopropoxide... 

In contrast to the situation with copper-based catalysis, most studies on ruthenium-based catalysts have made use of preformed metal complexes. The first reports of ruthenium-mediated polymerization by Sawamoto and coworkers appeared in I995.26 In the early work, the square pyramidal ruthenium (II) halide 146 was used in combination with a cocatalyst (usually aluminum isopropoxide). 

The most common hydrox y oxime ligands are salicylaldoximes, complexes of which with copper(II), nickel(II) amd palladium(II), cobalt(II), iron(II), iron(HI) and manganese(II) have been investigated extensively including crystal and molecular structures13 of the first three. In an interesting study,71 the reactions of cobalt bis chelates of this type have been studied with aluminum isopropoxide. 

Allyl phosphates, 506 Allylsilanes, 43, 71-72, 529, 575 Allyl sulfones, 512 Allyltitaniiun ate complexes, 376-377 Allyltri-n-butyltin, 15-16 Allyltrimethylsilane, 16-20, 532 Allyltrimethyltin, 20-21 Almusone, 51 Alpine Boranc, 429 Alumina, 22-24 Aluminum amalgam, 24-25 Aluminum ate complexes, 17 Aluminum chloride, 25-28, 100 Aluminum chloride-ethanethiol, 28-29 Aluminum isopropoxide, 29, 296 Amidoalkylation, 16-17 Amidomercuration, 317 Animation, 221 Amino acid esters, 360... 

Aluminum chloride-phosphorus oxychloride complex, 31, 88 Aluminum isopropoxide, 3S, 40 Amberlite IR-4B resin, 32, 13 Amidation, of isocyanic add with bro-moaniline and other aromatic amines, 31, 8... 

Aluminum is used as the anode and nichrome as the cathode. There is also available in the cell a catalytic amount of titanium for fixation purposes, in addition to naphthalene, which serves a purpose again in the reduction stage. The electrolyte is tetrabutylammonium chloride. Aluminum isopropoxide increases the over-all efliciency and turns this process into a catalytic one. This system starts with titanium tetraiso-propoxide. Reduction takes place, presumably again to the titanium (II) level. We have evidence from electrochemical experiments that titanium (II) is produced and involved in the fixation. Titanium (II), once formed, picks up N2 from the atmosphere and forms the complex, which then is available for reduction by sodium naphthalenide. Naphthalene is present in a catalytic amount and is reduced at the cathode to the radical anion. In this experiment, one can actually see it as a greenish color at the cathode. Naphthalenide reduces the No compound, producing the nitride. Normally, the reaction would stop at this point. We believe that in the electrochemical process, aluminum (III) abstracts nitride from titanium and forms aluminum nitride. This nitride transfer also can be observed in nonelectrolytic reactions. Thus, aluminum nitride is stored and ammonia is available at any time, merely by protonation. Both titanium and naphthalene are catalytic and permit operation of an over-all catalytic process. 

Cram s model does not always predict the stereochemical result of kinetically controlled reductions with aluminum isopropoxide (Cram and Greene, 1953). For example, i -(—)-3-cyclohexyl-2-butanone is reduced to predominantly ZR,2R-erythro carbinol (erythro/threo = 1.9). Apparently special steric forces are important in this reduction. Recent work (Shiner and Whittaker, 1963) has shown that aluminum isopropoxide is trimeric or tet-rameric. It is therefore conceivable that some hydride transfers will involve Al(OR)3 units that are not coordinated to the carbonyl groups they reduce. These transfers may occur preferentially from the side opposite that exposed to a coordinated Al(OR).3 unit. Such competitive mechanistic pathways might well yield an isomer ratio not in agreement with that produced by less complex reducing agents. 

Figure 5.8 Coordination complex of aluminum isopropoxide consisting of three molecules...

The formation of the sol can be a critical part of the process. While boehmite and the amorphous hydroxide prepared by cold water hydrolysis can be peptized to a clear sol, bayerite will not form a sol and its formation during hydrolysis should therefore be avoided. In addition, the nature of the acid has a significant effect on the peptization step. Table 5.8 shows the peptizing effect of various acids on the precipitate formed by hydrolysis of aluminum s ec-butoxide. The results are similar when aluminum isopropoxide is used. It appears that only strong or fairly strong acids, which do not form chemical complexes (or form only very weak complexes) with aluminum ions, are effective for achieving peptization. For these acids, the concentration of the acid also has an effect. Peptization... 

Schubert, U., et al. (1999). Controlled polymerization of methyl methacrylate and ethyl acrylate using tris(4,4 -dimethyl-2,2 -bipyridine)copper(II) hexafluorophosphate complexes and aluminum isopropoxide. Polym. Bull., 43(4-5) 319-326. 

Reactions with Oxiranes, Oxetanes, and Aziridines. Lewis acids, lanthanide salts, and titanium tetraisopropoxide or aluminum isopropoxide catalyze the reactions of cyanotri-methylsilane with oxiranes, oxetanes, and aziridines, yielding ring-opened products. The nature of the products and the regio-selectivity of the reaction are primarily dependent on the nature of the Lewis acid, the substitution pattern in the substrate, and the reaction conditions. Monosubstituted oxiranes undergo regiospe-cific cleavage to form 3-(trimethylsiloxy)nitriles when refluxed with a slight excess of cyanotrimethylsilane in the presence of a catalytic amount of potassium cyanide-18-crown-6 complex (eqs 8-10). The addition of cyanide occurs exclusively at the least-substituted carbon. 

Stereochemistry of Reduction of Cyclohexanones by Organotin Hydrides, Complex Metal Hydrides, and Aluminum Isopropoxide... 

Tin(II) octanoate [Sn(Oct)2], aluminum isopropoxide a-Al(Oi-Pr)3] and, to a lesser extent, zinc(ll) lactate [Zn(Iact)2] are the most widely used complexes (Figure 10.1). Sn(Oct)2 is inherently more active than Al(Oi-Pr)3, with typical reaction times in bulk of a few hours versus a few days. Sn(Oct)2 is usually used in the presence... 

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