Bimetallic Aluminum Catalyst

From the polymerization data it is evident that complexes 98 can initiate the ROP of racemic-LA under the conditions given in Table 6.15. Complete conversion of 500 equiv of LA occurs within 48 hrs at 70°C in 

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The use of group 4 metallocene alkyne complexes924 and bimetallic aluminum derivatives925 as CL polymerization initiators has also been described. These catalysts generally exhibit poor control with Mn values much lower than expected and Mw/Mn= 1.4-2.6. End groups have not... 

Very recently North s group revealed that the bimetallic aluminum(salen) complex 1 (Scheme 9) when used in conjunction with tetrabutylammonium bromide constitutes the only catalyst system capable of catalyzing the insertion of carbon dioxide into oxirane at 1 atm (760 mmHg) and at ambient temperature (25-30... 

Cyclodiphosphazanes(III) 27 shown in Scheme 16 undergo oxidation reactions to give the cyclodiphosphazanes(V) of type 28. These are prospective ligands in catalysis since these ligands due to lack of phosphorus lone-pairs are less susceptible to the destructive cycloreversion of the ligands. Hence they could prevent catalyst deactivation in the process. When treated with trimethyl aluminum the cyclodiphosphazanes form symmetrically substituted bimetallic species of type 29 [90]. Characterization by single-crystal X-ray studies show... 

This finding is also in agreement with another three-component Michael/aldol addition reaction reported by Shibasaki and coworkers [14]. Here, as a catalyst the chiral AlLibis[(S)-binaphthoxide] complex (ALB) (2-37) was used. Such hetero-bimetallic compounds show both Bronsted basicity and Lewis acidity, and can catalyze aldol [15] and Michael/aldol [14, 16] processes. Reaction of cyclopentenone 2-29b, aldehyde 2-35, and dibenzyl methylmalonate (2-36) at r.t. in the presence of 5 mol% of 2-37 led to 3-hydroxy ketones 2-38 as a mixture of diastereomers in 84% yield. Transformation of 2-38 by a mesylation/elimination sequence afforded 2-39 with 92 % ee recrystallization gave enantiopure 2-39, which was used in the synthesis of ll-deoxy-PGFla (2-40) (Scheme 2.8). The transition states 2-41 and 2-42 illustrate the stereochemical result (Scheme 2.9). The coordination of the enone to the aluminum not only results in its activation, but also fixes its position for the Michael addition, as demonstrated in TS-2-41. It is of importance that the following aldol reaction of 2-42 is faster than a protonation of the enolate moiety. 

A catalyst system with a zinc complex that also induces the chain-transfer reaction has been developed by Jerome et al When the polymerization of PO is conducted by using a mixture of zinc/aluminum bimetallic complex (Bu0)2A10Zn0Al(0Bu)2 and phenoxyethanol ([PO]/[Zn]/[phenoxyethanol] = 1000/1/20), the conversion reaches 97% to give PPO that contains low and high molecular weight fractions. An additive of lithium chloride or... 

Raney copper is prepared by intensive leaching of a commercial copper-aluminum alloy (50-50 wt%) washed with water until neutral. Bimetallic catalysts are obtained using an oxido-reduction method summarized as n Cu-s + 2 Mn+ -> n Cu2+ + 2 Ms... 

These complexes are the first examples of multifunctional catalysts and demonstrate impressively the opportunities that can reside with the as yet hardly investigated bimetallic catalysis. The concept described here is not limited to lanthanides but has been further extended to main group metals such as gallium [31] or aluminum [32]. In addition, this work should be an incentive for the investigation of other metal-binaphthyl complexes to find out whether polynuclear species play a role in catalytic processes there as well. For example, the preparation of ti-tanium-BINOL complexes takes place in the presence of alkali metals [molecular sieve ( )]. A leading contribution in this direction has been made by Kaufmann et al, as early as 1990 [33], It was proven that the reaction of (5)-la with monobromoborane dimethyl sulfide leads exclusively to a binuclear, propeller-like borate compound. This compound was found to catalyze the Diels-Alder reaction of cyclopentadiene and methacrolein with excellent exo-stereoselectivity and enantioselectivity in accordance with the empirical rule for carbonyl compounds which has been presented earlier. 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

Recently, Doi152) speculated on the presence of two types of bimetallic active centers, based on 13C NMR analysis of the structure and stereochemistry of polypropylene fractions obtained with different Ziegler-Natta catalyst systems (see Fig. 44). Site A produces highly isotactic polypropylene, site B atactic polypropylene consisting of isotactic and syndiotactic stereoblocks. The formation of the latter fraction would be due to the reversible migration of the aluminum alkyl, made... 

A bimetallic catalyst prepared from BINOL and lithium aluminum hydride has been found to result in useful asymmetric induction in the Pudovik reaction [17]. The (f )-ALB catalyst 64 (10 mol %) facilitates the addition of dimethyl phosphite to a variety of electron-rich and electron-poor aryl aldehydes in high yield with induction in the range 71-90 % ee. The nature of the solvent is important in this reaction—the induction for addition to benzaldehyde dropped from 85 % ee to 65 % ee when the solvent was changed from toluene to dichloromethane. Aluminum seems to be a key to the success of this reaction, because reaction with benzaldehyde was not as successful with other bimetallic catalysts. BINOL catalysts with lanthanum and potassium gave only 2 % ee, a catalyst with lanthanum and sodium gave a low 32 % ee, and a catalyst with lanthanum and lithium gave only a 28 % ee [18]. Aliphatic aldehydes were not successfully hydrophosphonylated with dimethyl phosphite by catalyst 64 (Sch. 9). Induction was low (3-24 % ee) for unbranched and branched substrates. a,/3-Unsaturated aldehydes were, however, reported to work nearly as well as aryl aldehydes with four examples in the range 55-89 % ee. The failure of aliphatic aldehydes with this catalyst can be overcome by reduction of the product obtained from reactions with a,)3-unsaturated aldehydes. As illustrated by the reduction of 67 with palladium on carbon, this can be done without epimerization of the a-hydroxy phos-phonate. 

Inspired by the bimetallic catalyst developed by Shibasaki and coworkers with 2 1 complexes of BINOL with aluminum, Manickam and Sundararajan prepared 2 1 complexes of the aminodiol 420 with aluminum [87,88]. Reaction of malonate esters with cyclopentenone or cyclohexenone results in asymmetric induction of at least 90 % ee with dibutyl malonate, as detailed in Sch. 57. A catalyst prepared by the reaction of 2 equiv. diol 419 with lithium aluminum hydride was found to result in asymmetric induction for the reaction of cyclohexenone with malonate 390d similar to that observed with the catalyst derived from 420 and from BINOL, although the rate was slightly slower. 

Indeed, the H2(ext/obs) ratio which is a measure of the reduction degree and therefore indicates if there is an interaction with the zeolite or between cerium and vanadium, exhibited different values, depending on the way of introduction and species formation. The catalyst treated with steam, EXV, presented a low H2 (ext/obs) ratio, which indicates a better reduction. On the other hand, the impregnated catalyst (IMPV) presented a high H2 (ext/obs) ratio, and thus low reduction. This could explain the indication that an interaction occurred during the treatment, with the formation of bimetallic or alloys or even the formation of aluminum silicate-metal interaction. 

While a limited amount of experimental evidence does lend support to the bimetallic concept, major objections were voiced by Ziegler, who was of the opinion that like dimeric aluminum alkyls the Ti-Al complex is not likely to be the effective catalyst agent. Other more recent work also favors the second and simpler alternative, the monometallic mechanism. 

The coordination catalysts for these reactions are diverse. They can be compounds of alkaline earth metals, like calcium amide, or calcium amide-alkoxide. They can also be Ziegler-Natta-type catalysts. These can be alkoxides of aluminum, magnesium, or zinc combined with ferric chloride. Others are reaction products of dialkylzinc with water or alcohol. They can also be bimetallic //-oxoalkoxides, such as [(RO)2A102]Zn. Other catalysts are aluminum or zinc metalloporphyrin derivatives (see Fig. 4.1). 

Figure 19-1. Structures of Ziegler catalysts based on titanium/aluminum. X, Anion O, vacant ligand site. As shown, the complexes responsible for the monometallic mechanism may be monometallic or bimetallic, (a) Known structure in a soluble Ziegler catalyst. Also shown are proposed structures in heterogeneous Ziegler catalysts for (b) a bimetallic mechanism (c) or (d) a monometallic mechanism.

Various residues, such as X = C2H5 or Cl, or even aluminum alkyls as in bimetallic complexes, can act as ligands. Of course, every complex with a vacant coordination site or an uneven electron distribution is a potential Ziegler catalyst. Consequently, complexes between two compounds of different metals (Ti/Al), between two forms of the same metal differing in... 

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