Platinum hydrogenolysis, cyclopropanes

The catalyst exerts some influence on the bonds broken in hydrogenolysis of saturated cyclopropanes (775), but in vinyl and alkylidene cyclopropanes the effect is pronounced. Platinum or palladium are used frequently. In one case, Nishimura s [124a) catalyst, rhodium-platinum oxide (7 3), worked well where platinum oxide failed (.75). An impressive example of the marked influence of catalyst is the hydrogenation of the spirooctane 42, which,... 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

One of the most extensively used addition reactions of cyclobutanes is hydrogenolysis.36 With regard to the mechanistic aspect, evidence has been provided that hydrogenolysis of cyclobutane is structure sensitive to the particle size of the platinum on alumina catalysts.37 Moreover, a kinetic study has also revealed that the mechanism for the hydrogenolysis of cyclobutanes is likely to be different from that for cyclopropanes.37... 

Comparison of Initial Specific Rate Data for the Cyclopropane Hydrogenolysis on Platinum Catalysts... 

In addition, the same studies that were carried out on the Pt(lll) crystal face result in reaction rates identical to those found on stepped crystal surfaces of platinum. These observations support the contention that well-defined crystal surfaces can be excellent models for polycrystalline supported metal catalysts. It also tends to verify Boudart s hypothesis that cyclopropane hydrogenolysis is an example of a structure-insensitive reaction. The initial specific reaction rates, which were reproducible.within 10%, are within a factor of two identical to published values for this reaction on highly dispersed platinum catalysts. The activation energies that were observed for this reaction, in addition to the turnover number, are similar enough on the various platinum surfaces so that we may call the agreement excellent. 

As illustrated in Fig. 20.1, the C-C bond that is broken in the hydrogenolysis of alkyl cyclopropanes is that one between the two least substituted carbon atoms of the ring. With unsymmetrical 1,2-dialkyl-cyclopropanes (1, R iR ) the bond adjacent to the smaller alkyl group is preferentially cleaved. When coupled with the facile cyclopropanation of alkenes, the hydrogenolysis of alkyl cyclopropanes over platinum oxide has served as a useful means of introducing a methyl group onto a tertiary carbon atom as illustrated by the preparation of tert-butyladamantane (3) (Eqn. 20.1) and the 9-methyldecalins, 5 and 6 (Eqn. 20.2). > ... 

Halogens on a cyclopropane ring can be removed without hydrogenolysis of the ring itself. The selective removal of one bromine from 1,1-dibromo-cyclopropanes occurred over palladium on charcoal or platinum oxide in methanolic potassium hydroxide at room temperature and atmospheric pressure. Complete debromination resulted from the use of Raney nickel under the same conditions (Eqn. 20.56). 31... 

D.R. Kahn, E.E. Petersen, and G.A. Somoijai. The Hydrogenolysis of Cyclopropane on a Platinum Stepped Single Crystal at Atmospheric Pressure. J. Catal. 34 294... 

Tn recent years ultrahigh vacuum methods have been applied to cata-lytic studies on initially clean metal surfaces having low surface area. In several instances (the hydrogenolysis of cyclopropane over platinum (I) and the catalytic methanation reaction over rhodium (2) and nickel (3)) a link between ultrahigh vacuum methods and conventional catalytic measurements was established. That is, specific reaction rates over low area cm ) catalyst samples agreed with specific reaction... 

One of the significant features of the reaction of cyclopropane with hydrogen is that on metals active for hydrogenolysis, i.e. all the metals of Groups 8 to 10 excepting palladium and platinum, fragmentation to methane and ethane takes place at much lower temperatures than would occur with propane. Ethene and propene are also observed under certain conditions. 

Cyclopropane and Cyclopropene Adds.— The undesirable physiological properties of cyclopropene acids such as malvalic acid (20 R = H) and sterculic acid (21 R = H) and their presence, albeit in small amount, in cottonseed oil has aroused interest in these compounds. The content of cyclopropene acids is reduced by heating with palladium catalysts, though not with nickel or platinum catalysts. Palladium treatment cleaves the cyclopropene ring and produces methyl- and methylene-substituted acids. Hydrogenolysis of cyclopropane acids is reported to give methyl-substituted fatty acids which can be identified by their g.l.c. behaviour on capillary columns. The chemical reactions of cyclopropene acids (malvalic and sterculic) shown in Scheme 5 have been reported. ... 

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