Heavy Metal Salt Promoters

However, the hydrogenation catalysts can be promoted also by compounds which are ineffective for the iron ammonia catalysts, e.g., by silica and silicates, silicofluorides, borates, and phosphates. An interesting type of hydrogenation catalysts was found in the form of zeolites which by ion exchange were impregnated with heavy metal salts. 

It has been discovered that the performances of platinum and palladium catalysts may be improved by promotion with heavy metal salts. However, there is little information available about the role and chemical state of the promoter 8,9). We have recently found that a geometric blocking of active sites on a palladium-on-activated carbon catalyst, by lead or bismuth, suppresses the by-product formation in the oxidation of l-methoxy-2-propanol to methoxy-acetone 10). 

In the Koenigs-Knorr method, glycosyl halides are the glycosyl donors. The procedure consists of the treatment of glycosyl bromides or chlorides, easily obtained as pure a anomers [3], with alcoholic or phenolic acceptors in the presence of an excess of a heavy metal salt [92], Silver carbonate or silver oxide were originally used, but at present silver triflate or mercury (II) salts are the most frequently promoters employed [93]. 

Heavy-metal salts in this class may reduced directly since their anions also are reducing agents and thus promote the overall reaction [36]. In some cases (see p. 1665), simple thermal decomposition of the formate or oxalate will yield the metal in such cases the hydrogen acts only as a protective gas which prevents reoxidation. Metals obtained by this method are not always completely free from carbon [33, 37]. The nature of the starting material may influence the activity of the product metal catalyst to some extent this is especially true if the material Is reduced at a temperature just sufficient to effect the reaction [42]. 

The Koenigs-Knorr glycosylation method using glycosyl bromides or chlorides as donors dates back to 1901 (13), and it is still in wide use. The glycosylation reaction was originally achieved with silver carbonate as a promoter, but other heavy metal salts, especially silver triflate (21), have been introduced as more effective... 

Cleavage of the sulfur-C2 bond of penicillins has been achieved by reactions promoted by electrophilic attack of alkylating agents, carbenes, and heavy-metal salts as thiophiles. Use of reactive alkyl halides, in particular Mel, in conjunction with a strong anhydrous base (NaH, KOBu ), is known as the Nayler reaction [77]. The reaction is very limited in scope, but can be run on penams 117a, b and 119d f. 

Usually, 0-acetylated methyl ester methyl glycoside 2 is prepared by glycosida-tion, and conventional saponification of 2 gives methyl sialoside 3. Although several methods for the preparation of 0-acetylated methyl ester methyl glycoside 2 have been developed,most of them rely on the use of heavy metal salts or other toxic or expensive promoters. 

Penicillamine (D-(3,(3-dimethylcysteine) is an effective chelator of copper, mercury, zinc and lead which promotes the excretion of these metals in the urine. It is clinically used in patients with Wilson s disease, with rheumatoid arthritis and with heavy-metal intoxications. Ziram and ferbam are the zinc and the iron salts respectively of dimethyl-dithiocarbamic acid (Figure 20.44). They are widely used as selective fungicides in agriculture. Pyrithione (1 -hydroxy-2(l//)-pyridinethione), as its zinc salt is used in dermatology as antiseborrheic. 

The autoxidation reactions are promoted by catalysts which are organic salts of polyvalent heavy metals. By far the most powerful of these are cobalt salts, e.g. cobalt octanoate (2-ethylhexanoate, C4H9CH(C2H5)C00H) or cobalt naphthenate, and these are usually accompanied by other organic salts, e.g. of lead, zirconium or calcium. 

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