For almost 70 years, there have been attempts to advance the Williamson ether synthesis process to allow the use of low-cost, noncarcinogenic, weak alkylating agents and avoid salt production. These attempts to produce a "green" version of Williamson ether synthesis have been based on the use of weak alkylating agents such as carboxylic acid esters at relatively high temperatures (approximately 200 °C) and pressures. However, none of the processes considered was suitable for industrial application because of the high concentration of the alkali metal carboxylates required. By increasing the temperature to above 300 °C, it has now proved possible to carry out Williamson ether synthesis as a homogeneous catalytic process. The large temperature increase significantly boosts the alkylating power of weak alkylating agents such as alcohols, carboxylic acid esters, and ethers derived from weak Bro 1nsted acids, which are only weak alkylating agents at room temperature. At such temperatures, carboxylic acid esters such as benzoic acid methyl ester or acetic acid methyl ester demonstrate the alkylating power usually expected of alkylating agents derived from strong acids. In the catalytic cycle of this new process, for example, the low-cost alcohol methanol and phenol were converted into anisole and water at 320 °C via the intermediate methyl benzoate in the presence of catalytic quantities of alkali metal benzoate and phenolate. The catalytic Williamson ether synthesis (CWES) at high temperatures is especially well-suited for the production of alkyl aryl ethers such as anisole, neroline, and 4-methyl anisole which are of industrial importance. Selectivity values of up to 99% have been reached.
The preparation of 14‐membered peptide alkaloids was first investigated on model educts such as the pentafluorophenyl esters (1), Z = benzyloxycarbonyl. On the basis of these investigations it was subsequently possible to develop a method for the synthesis of dihydrozizyphin G (2), AA = CH2CH2, as the first compound of this type.
Our observations demonstrate that the Co-C bond of enzyme-bound ( I ) is not alternately closed and broken during the catalytic reaction with the substrate. If this were so, irradiation of the enzyme complex should not have had any significant effect on the rate of the subsequent enzymatic reaction in the dark.The present results do not necessarily support the currently popular idea that coenzyme B, dependent enzymatic reactions occur cia organic radicals and that the coenzyme is activated enzymatically by the homolysis of the C 4 bond. On the contrary, our findings are fully consistent with the previously postulatedr7-heterolytic mechanism of enzymatic coenzyme B,,-activation"
rate with high vapor pressure at T>2500"C, thus accounting for their low content; the high melting impurities Ta and W, on the other hand, remain in the niobium. The samples R-Nb-1 were electrolyzed; the major impurities Ta and W are thus removed, but metals such as Cr, Fe, and Co are apparently involved by contamination. The samples R-Nb-2 were additionally zone refined and annealed, which leads to a marked decrease in the Cr, Fe, and Co content.The niobium thus obtained can be additionally characterized by its specific electrical resistivity of @ (4.2 K)=6 x 0 cm in the normal state; the total content of non-metals is about 100 ppb. These findings show that niobium of particularly high purity and structural perfection can be prepared by the above method. The analytical method developed here for the determination of Cr, Fe, and Co is characterized by its extremely high sensitivity and freedom from error regarding contamination and losses.
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