Magnesium catalysts

The parent system 3 was reported fw the first time as a product obtained in low yield originating from the vapor-phase cyclodehydrogenaticMi of /V,/V -diphenyl-m-phenylenediamine (137) over a platinum-on-magnesium catalyst (Scheme 17) (61JOC1509). 

A chiral magnesium catalyst prepared from magnesium iodide and 1,2-diphenyl-ethylenediamine was also found to he effective in asymmetric aza Diels-Alder reaction of a-imino ester 21b with 7a (Scheme 5.12) [32]. The novel catalyst was discovered using parallel comhinatorial methods. 

A rather unexpected discovery was made in connection to these investigations [49]. When the 1,3-dipolar cycloaddition reaction of la with 19b mediated by catalyst 20 (X=I) was performed in the absence of MS 4 A a remarkable reversal of enantioselectivity was observed as the opposite enantiomer of ench-21 was obtained (Table 6.1, entries 1 and 2). This had not been observed for enantioselective catalytic reactions before and the role of molecular sieves cannot simply be ascribed to the removal of water by the MS, since the application of MS 4 A that were presaturated with water, also induced the reversal of enantioselectivity (Table 6.1, entries 3 and 4). Recently, Desimoni et al. also found that in addition to the presence of MS in the MgX2-Ph-BOX-catalyzed 1,3-dipolar addition shown in Scheme 6.17, the counter-ion for the magnesium catalyst also strongly affect the absolute stereoselectivity of the reac-... 

It should also be mentioned that in connection with the investigations on MgX2-BOX catalysts, Desimoni et al. also tested a Zn(C104)2-B0X catalyst for the 1,3-di-polar cycloaddition of a nitrone and acryloyloxazolidinone (see Scheme 6.17). Contrary to the magnesium catalysts, this zinc catalyst was exo selective as an 27 73 exojendo ratio was observed, and 84% ee of the exo isomer was obtained [51]. 

An extensive review of the use of chiral Lewis acid catalysts in Diels-Alder cycloadditions has been presented. Brpnsted acid-assisted chiral Lewis acids have been shown to be highly efficient catalysts for the enantioselective Diels-Alder reactions of a- and /3-substituted-Q, /i-enals with numerous dienes. The chiral Lewis acid-catalysed Diels-Alder reaction between cyclopentadiene and alkenoyloxazolidi-nones can be catalysed by bis(oxazolone)magnesium catalysts. ... 

The above examples clearly demonstrate unusual catalytic properties of the products synthesized in magnesium-containing organic films. However, binary magnesium organic compound systems do not exhaust the possibilities of such synthesis. The range of magnesium catalysts synthesized in films... 

Table 4. EPR data on Ti3+ ions formed in titanium-magnesium catalysts of various composition 80) By permission of Hiithig Wepf Verlag...

Whiting et al. [20] found the catalytic system for an aza Diels-Alder reaction by the use of a combinatorial approach to catalyst selection. When methyl glyoxylate-derived aldimine 25 was reacted with Danishefsky s diene 24 in the presence of the chiral magnesium catalyst (10 mol %), prepared in-situ from chiral diphenylethylenediamine 23, Mgl2, and 2,6-lutidine, the Diels-Alder product 26 was obtained in 64 % yield with 97 % ee (Sch. 9). 

The maximum number of AQ in propylene polymerization by supported titanium-magnesium catalysts may be estimated on the basis of the maximum activity of catalysts and kp values ( 10 l/(mol x s). The initial activity of this catalytic system was as high as 80 kg C3H6/(g Ti x h x atm) (in the presence of Al(i-Bu)3 and ethyl /7-methoxybenzoate), not less than 90% of isotactic polymer being formed. One can conclude that is 0.07 mol/mol Ti. Taking into account Cp, the total number of active centers for this catalyst is 10 % of the content of titanium in the catalyst. These data are close to those obtained in ref (see Table 2). 

Oxidative dehydrogenation of butanediol on the selected vanadium-magnesium catalysts allowed to reduce the reaction temperature of butadion synthesis by about 100°C. The reaction was studied in the temperature range of 160 - 350°C at LHSV-equal to 1 h" and butanediol oxygen molar ratio equal to 1 1 (Table 2). Already at 250°C more than 85% of butanediol was converted. 

Traditional ZN Catalysts and Supported Titanium-Magnesium Catalysts. 121... 

A new and more effective and reliable variant of the kinetic method is the stopped flow method (SF method), which has been offered by Keii and Terano [153] for determination of the number of active centers and the propagation rate constant in olefin polymerization on ZN catalysts. The main feature of this method is determination of Cp and k values in conditions of quasi-living polymerization, when transfer reactions of a polymer chain practically do not proceed and linear dependences of molecular weight of formed polymer and yield of polymer on polymerization time are observed. It has been shown that these conditions are obtained for propylene polymerization on supported titanium-magnesium catalysts (TMC) at low temperature (30°C) and at times of polymerization less than 0.2 s in these cases, values of Cp and can be calculated from Eqs. (14) and (15) ... 

Cycloaddition of cyclopropanes to aldehydes leads to the formation of tetrahydrofurans derivatives, whose enantiomeric form can be obtained either by using enantioenriched cyclopropane substrates or by a dynamic kinetic asymmetric transformation. In this regard, Johnson et al. reported a dynamic kinetic asymmetric [3 -I- 2] cycloaddition of racemic cyclopropanes 63 for the enantioselective synthesis of tetrahydrofurans 64. In this study, the magnesium catalyst can promote the ring opening of the racemic cyclopropane and catalyses the reaction of one of the ring-opened enantiomers with the aldehydes (Scheme 3.19). 

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