Catalysis with rhenium-platinum

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

Other Catalysts. Other catalysts with metals of rhenium and platinum have shown catalytic reactivities for cyclopropanation. Methylrhenium trioxide (MTO) was the first rhenium catalyst for catalytic cyclopropanation, with yields of 57-87% obtained for the cyclopropanation of alkyl or aryl alkenes with EDA (45). As for platinum, a number of complexes have been screened for cyclopropanation catalytic activity (46). PtCl4 was the most active, giving good yield (79%) of cyclopropane from styrene and EDA. However, all reactions had to proceed at elevated temperature. Nonmetal catalysts such as tris(4-bromophenyl)-aminium hexachloroantimonate have been utilized as catalysts for mechanistic studies of cyclopropanation of a series of raras-stilbenes with EDA (47). A cation radical mechanism for this catalysis has been proposed. 

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