Platinum catalysis rearrangements

Rearrangements of 1,3-diynes (43) to dienynes (45a) and (45b) have been carried out catalytically a metal-carbene intermediate (44) is likely to be involved.41 Interestingly, gold catalysis provides mainly (45a) whereas platinum catalysis under a CO atmosphere yields predominantly (45b). 

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

Except for naphthene dehydrogenation, which only requires a Pt site for catalysis, all the other major reactions require an interaction between sites. Ring and paraffin isomerization require the platinum function for dehydrogenation to olefin, the acid function for carbon skeletal rearrangement, and the metal function again for hydrogenation of the olefin. 

The first examples of skeletal rearrangements on metals were reported by the Soviet school of catalysis. A major step in hydrocarbon chemistry was the finding that platinum, unlike palladium and nickel, selectively catalyzes the hydrogenolysis of cyclopentane hydrocarbons. At about 300°C, on the classical Zelinskii platinum-charcoal catalyst, cyclopentane yields -pentane as sole reaction product (3, 4), while palladized charcoal is completely inactive (J) and nickel-alumina produces all the possible acyclic hydrocarbons, from methane to pentane (5-7). 

The mechanism of catalytic hydrosilylation is not well understood. Study of these reactions is hampered by their complexity induction periods are often involved, reaction conditions such as the nature of the catalyst and reacting groups are critical factors, and side-reactions, such as alkene rearrangements, are common. A widely accepted mechanism for homogeneous catalysis by platinum complexes is based on the work of Chalk and Harrod (Scheme 10)5,8,262. With Speier s catalyst, it appears that initially, and probably during the induction period, silane reduces the platinum to a Pt(0) or Pt(II) complex that is the active catalytic species265. 

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