Platinum on charcoal

Although palladium or platinum on charcoal are widely used, there is a preference for homogeneous reactions on both the laboratory and the industrial scale. Complexes of ruthenium (II) and rhodium (I), particularly with phosphine ligands, do have some importance in special applications [4], but... 

The first explicit information appeared in 1953 in two U.S. patents (9) which showed that platinum black as well as platinized asbestos or silica were effective for addition of trichlorosilane to olefins. Platinum on charcoal was unusually active with trichlorosilane and acetylene, ethylene, butadiene, vinyl chloride, or vinylidene fluoride. Temperatures as low as 130°C were sometimes employed. 

In fact, there are only two heterogeneous catalysts that reliably give high enantioselectivities (e.s. s) (90% e.e. or above). These are Raney nickel (or Ni/Si02) system modified with tartaric acid (TA) or alanine for hydrogenation of /(-kctocstcrs [12-30], and platinum-on-charcoal or platinum-on-alumina modified with cinchona alkaloids for the hydrogenation of a-ketoesters [31-73],... 

Platinum on charcoal provided a further means for directing the selectivity of the multiphasic hydrodehalogenation reactions. For example, the same reaction in Figure 6.20 conducted using Pt/C instead of Pd/C, yielded selectively the dehalo-genated benzylic alcohol (Figure 6.22)." The same reaction was conducted using... 

A third possible pathway could yield indan through cyclononane intermediate. We know that cyclononane undergoes transannular dehydrocycli-zation over platinum-on-charcoal catalyst at 300°C, to perhydroindan and then to indan (38) but so far there is no evidence for the direct cyclization of n-nonane to cyclononane. Unfortunately, Il in and Usov used an acidic catalyst and we cannot separate the contributions of acid and metal catalysis to the two mechanisms. Experiments over nonacidic platinum catalysts could show the relative importances of the platinum metal in the two cyclization pathways. 

Kazanskii and co-workers have described an interesting special case, double cyclization of -octane at 310°C over platinum-on-charcoal catalyst at 0.2 liquid hourly space velocity. The reaction product contains about 0.25% m-octahydropentalane and 2.2-4.5% alkylcyclopentanes (an approximately l lmixture of trans-l-methyl-2-ethylcyclopentane and n-pro-pylcyclopentane) (39, 40). Indirect evidence suggests that most of the octahydropentalane is formed from l-methyl-2-ethylcyclopentane, which cyclizes significantly faster than w-propylcyclopentane. 

This contrasts with the man-made chemistry of the laboratory and the industrial plant, which often employs the more reactive, but less readily accessible, second- and third-row transition elements. For example, nature chose nickel for the active site of many hydrogenases. Catalytic hydrogenations in the laboratory, on the other hand, are usually performed with palladium or platinum on charcoal. 

N-(3-Nitro-2-pyridinyl)-2-furanylmethanamine Platinum on charcoal Mercury (II) oxide... 

The conversion of primary alcohols into carboxylic acids is not a difficult task. Often, the same oxidant that has been used to oxidize alcohols to aldehydes is applicable to the oxidation to acids when used in appropriate amounts, in different solvents, at higher temperatures, or at longer reaction times. An example is the oxidation of primary alcohols with air or oxygen with platinum-on-charcoal or, better still, platinum dioxide as catalyst (equation 228) [56]. 

Alcohols in ethylene carbonate containing sodium acetate and palladium chloride are oxidized by oxygen at room temperature in 62-98% yields [70]. Oxygen passed at room temperature under irradiation through a solution of catalytic amounts of chloroplatinic acid and cuprous chloride in alcohols produces ketones in yields of up to 98% [57], Other catalysts used for this purpose are platinum [55], platinum-on-charcoal [56], and, better still, platinum oxide [56]. Such oxidations are carried out usually at room temperature and give fair to high yields. 

Increasing metal loading almost always leads to a decrease in dispersion. Hence with a series of platinum on charcoal catalysts ranging from a loading of 0.5% to 10% Pt the dispersion decreased from 47% for the 0.5% Pt catalyst to 23% for the 10% Pt catalyst. In other words, the total platinum surface area increased by ten times for a platinum loading increase of twenty times. 

D2O-DT exchange can be used for transferring tritium from heavy water to deuterium. Further enrichment is achieved by cryogenic distillation. Because of the similarity between deuterium and tritium, platinum on charcoal is the catalyst for vapor phase exchange, whereas hydrophobic catalyst is used for liquid-gas exchange. 

Vapor phase catalytic exchange (VPCE), followed by cryogenic distillation Platinum on charcoal catalyst is used for the exchange. The heavy water has to be converted into steam. The exchange is, therefore, carried out at 200° C. 

Selective Oxidation of Hydroxymethyl Groups. Platinum on charcoal in the presence of air is the conventional method for selectively oxidising hydroxymethyl groups to carboxylic acids, and thus hexoses to uronic acids anomeric protection is required, however, and the mechanism of this heterogeneous reaction is unknown. The reaction fails with polymers, probably for... 

Another elegant demonstration of the cis addition processes using platinum on charcoal as catalyst has been reported by Brook and coworkers (191) and is described in Scheme 45. Cis addition to diphenylacetylene affords the silylstil-benes 117. Their geometry was established by photochemical cyclization to the 9-silylphenanthrenes 118 as well as by comparison with the coupling product of chlorosilanes 116 and cis-stilbenyllithium. 

The same stereochemistry has been demonstrated in the platinum on charcoal catalyzed hydrosilylation of diphenylacetylene (191) (eq. [43]). 

Dry hydrogenation catalysts such as Raney nickel and palladium or platinum on charcoal are pyrophoric. One safety measure is the use of wet catalysts this is usually no problem because the hydrogenation reaction produces water. 

Addition occurs between diolefins, e.g., butadiene, and SiHCl3 in the presence of platinum on charcoal 361,363... 

Chromatographic separation of this mixture provides the two diastereomers 401 and 402 in 46% yield and in gram quantities. The highly selective catalytic hydrogenation of 401 in the presence of platinum on charcoal followed by reductive debenzylation in the presence of palladium on charcoal provides a mixture of L-nZ o-403 (3,5-trans) and L-xy/o-404 (3,5-cw) in a diastereomeric ratio of 93 7. Similarly, 402 in two steps affords L-lyxo-405 (3,5-trans) and L-arabino-406 (3,5-cis) in a diastereomeric ratio of 80 20. With the ready availability of (R)-390, the corresponding D-series compounds can be prepared with similar diastereochemical results [136] (Scheme 90). 

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