Catalysts magnesium bromide

The terms in Equation 1.3 (Malkin s autocatalytic model) are described in Nomenclature. In Malkin s autocatalytic model, the concentration of the activator, [A], is defined as the concentration of the initiator times the functionality of the initiator. For a difimctional initiator [e.g., isophthaloyl-bis-caprolactam, the concentration of the activator (acyllactam) is twice the concentration of the initiator]. The term [C] is defined as the concentration of the metal ion that catalyzes the anionic polymerization of caprolactam. In a magnesium-bromide catalyzed system, the concentration of the metal ion is the same as the concentration of the caprolactam-magnesium-bromide (catalyst) because the latter is monofunctional. 

The transmetallation of lithio derivatives with either magnesium bromide or zinc chloride has been employed to increase further their range of synthetic application. While the reaction of l-methyl-2-pyrrolyllithium with iodobenzene in the presence of a palladium catalyst gives only a poor yield (29%) of coupled product, the yield can be dramatically improved (to 96%) by first converting the lithium compound into a magnesium or zinc derivative (Scheme 83) (81TL5319). 

To a mixture of vinyl bromide (40 mmol) and the catalyst dichloro-[(R)-Af,N-dimethyl-l-[(.S)-2-(diphenylphosphino)ferrocenyl]ethylamine]-palladium(n) (0.2 mmol) was added an ethereal solution of [a-(trimethyl-silyl)benzyl]magnesium bromide (0.6-1 m, 80 mmol) at —78 °C. The mixture was stirred at 30 °C for 4 days, and then cooled to 0 °C and hydrolysed with dilute aqueous HC1 (3 m). The organic layer was separated, and the aqueous layer was re-extracted with ether. The combined organic extracts were washed with saturated sodium hydrogen carbonate solution and water, and dried. Concentration and distillation gave the chiral allylsilane (79%, 66% ee), b.p. 55°C/0.4mmHg. 

In our studies, the catalyst and initiator system was comprised of caprolactam-magnesium-bromide and isophthaloyl-bis-caprolactam, respectively. We determined the optimum values of the kinetic parameters in Malkin s autocatalytic model (Eq. 1.3), which consist of k, U, and b, by regression analysis. 

Figure 1.13 Isothermal complex viscosity rise during anionic polymerization of caprolactam using caprolactam-magnesium-bromide/isophthaloyl-bis-caprolactam as the catalyst/initiator system. Run numbers and polymerization temperatures are shown in the legend...

For sodium/hexamethylene-l,6-bis-carbamidocaprolactam system, Sibal et al. [64] found the value of the constant k in Equation 1.4 to be 17.5. Note that the values of the constant k in Equation 1.4 that defines the relative complex viscosity rise during anionic ring opening polymerization of caprolactam are comparable for both caprolactam-magnesium-bromide/isophthaloyl-bis-caprolactam and sodium/hexamethylene-l,6-bis-carbamidocapro-lactam as the catalyst/initiator systems even though the kinetic constants for anionic polymerization for these systems are extremely different (see Table 1.2). 

The kinetics of anionic ring opening polymerization of caprolactam initiated by iso-phthaloyl-bis-caprolactam and catalyzed by caprolactam-magnesium-bromide satisfactorily fit Malkin s autocatalytic model below 50 percent conversion. The calculated value of the overall apparent activation energy for this system is 30.2kJ/mol versus about 70kJ/mol for Na/hexamethylene-l,6,-bis-carbamidocaprolactam as the initiator/catalyst system. 

Thermal isomerization at 220° in the absence of catalyst,17 or isomerization at W in the presence of magnesium bromide,1 1 gives a mixture of S-methylcyclohexanone and noetylcyolopentane (Eq. H i... 

A large number of alkyl-substituted styrene and p-methoxy-styrene oxidra have been ieomerizod in various laboratories, notably those of Tiffeneau, ElieL and Cope. Among the catalysts found to be effective in this connexion are magnesium bromide, aluminum chloride, and boron trifluoride. Only a few illustrative examples are cited in tbe text, although a larger compilation may be found in Table 10. 

A more detailed picture of the influence of the ionicity of the catalyst have been given in other studies. Coleman and Fox (18) showed that the production of isotactic sequences decreased as the alkylmetal catalysts became more anionic. Phenyl magnesium bromide had a high probability for isotactic sequences. Butyl lithium and octylpotassium were progressively less able to give steric control (Table 2). 

Aluminum bromide AlBr3 is used as a catalyst and parallels AICI3 in this role. Strontium and magnesium bromides are used to a limited extent m phamiacentical applications. Ammonium bromide is nsed as a flame retardant in some paper and textile applications potassium bromide is used in photography. Phosphorus tribromidc PBr3 and silicon tetrabromide SiBi4 are nsed as intermediates and catalysts, notably in the production of phosphite esters. 

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