Platinum methyl group determination

Although the rate-determining step for C-0 bond formation involves nucleophilic attack of OR- at the platinum bound methyl group (k2),... 

In an attempt to prepare polycyclopentadiene which would be stable in toluene solution, the polymer was hydrogenated over a platinum oxide catalyst in a Parr bomb immediately after the completion of the polymerization reaction. Infrared analysis indicated the presence of residual unsaturation and the polymer became insolubilized on standing. An attempted copolymerization of cyclo-pentadiene with propylene gave a product whose infrared spectrum indicated the presence of C-methyl groups but which was still insoluble in toluene. No attempt was made to determine whether copolymerization had occurred. 

Rundle and Sturdivant carried out the determination by x-ray diffraction of the crystal structure of the substance to which the name platinum tetramethyl and the formula Pt(CH3)4 had been assigned. They found, however, that each molecule contains four platinum atoms and sixteen methyl groups, as shown in the adjacent drawing. The molecular formula is Pt4(CH3)i6. 

The selectivity for VA may be explained, in part, by the relative energies of the orbitals on the olefins and platinum. Meester et al. have reported orbital energies for platinum, VA and methyl acrylate in trans-PtClo(olefin)(pyridine) conplexes based on spectroscopic data (2i). Their results indicate a closer match for methyl acrylate in the energies of the unfilled olefin and platinum orbitals, but a closer match for VA in the energies of the filled orbitals. Apparently, the o-bond interaction predominates in determining the relative reaction rates (and product stabilities) for these olefin substitutions. The fact that methylmethacrylate does not react at all is probably due to steric hindrance of the additional methyl group on the double bond. A similar argument can... 

In contrast, comparable rates were determined over platinum of low dispersion suggesting that isomerization occurs without alkene formation.161 The carbene-alkyl species (21) formed with the involvement of terminal carbon atoms is a probable surface intermediate in this selective mechanism. Highly dispersed platinum catalysts are active in nonselective isomerization in which the precursor species is the 22 dicarbene allowing ring closure between methyl and methylene groups. On iridium a pure selective mechanism is operative,162 which requires a dicarbyne surface species (23). 

Valuable information can be derived from the comparison of reaction rates of various hydrocarbons, measured by the decrease of their concentration due to oxidation [18], It is essential to note the important role of steiic factors which manifest themselves in a very similar way to the case of H-D exchange. Thus methyl and methylene groups adjacent to the quaternary carbon atom practically do not participate in the oxidation. 2,2-Dimethylpropane and 2,2,3,3-tetramethylbutane were shown to be oxidized by platinum(lV) much more slowly than alkanes with the same number of carbon atoms but with no quaternary carbon. According to [18], the number of carbon atoms ( o) accessible to attack by the platinum compounds can be approximately determined by the formula... 

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