Alkenes and Hydrogen

This co-oxidation has been used by James to convert a mixture of hydrogen, oxygen, and cyclooctene into water and cyclooctanone. The catalytic intermediate is proposed to be [IrHCl2(C8Hi2)]2. The reaction is accompanied by the catalytic hydrogenation of cyclooctene to cyclooctane. No definitive mechanistic details have been given, and no data yet produced to confirm whether the two oxidations occur by a coupled pathway. 

A brief report on the reaction of CO with Ni02(f rr-BuNC)2 at 20° in chlorobenzene solvent shows that both CO2 and r-BuNCO are formed (34). The yield of isocyanate is substantial, but again no mechanistic details are given as to whether the oxygen transfer reactions occur by separate or integrated pathways. 

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Decomposition of Thiols. Thiols decompose by two principal paths (i43— i45). These are the carbon—sulfur bond homolysis and the unimolecular decomposition to alkene and hydrogen sulfide. For methanethiol, the only available route is homolysis, as in reaction 29. For ethanethiol, the favored route is formation of ethylene and hydrogen sulfide via the unimolecular process, as in reaction 30. 

The reaction of carbon atoms with A-unsubstituted aziridines leads to alkenes and hydrogen cyanide (72IA3455), probably via extrusion from the initially formed adduct (285). The fragmentation does not appear to be concerted, although this would be a symmetry-allowed process, since only about half the alkene formed retains the aziridine stereochemistry in the case of cM-2,3-dimethylaziridine. 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

The third-order process presumably involves reaction of a complex formed between the alkene and hydrogen halide with the second hydrogen halide molecule, since there is little likelihood of productive termolecular collisions. 

The actual spacings of the metal atoms in the surface will clearly be of importance in making one face of a metal crystal catalytically effective, and another not, depending on how closely the actual atom spacings approximate to the bond distances in alkene and hydrogen molecules. In practice only a relatively sma l proportion of the total metal surface is found to be catalytically effective—the so-called active points . These adsorb alkene strongly, and then desorb immediately the resultant alkane, thus becoming free for further alkene adsorption. 

The catalyst used is typically platinum, palladium, rhodium, or ruthenium, or sometimes an appropriate derivative. Precise details of the reaction remain vague, but we believe the catalyst surface binds to both the substrate, e.g. an alkene, and hydrogen, weakening or breaking the jr bond of the alkene and the a bond of hydrogen. Sequential addition of hydrogen atoms to the alkene carbons then occurs and generates the alkane, which is then released from the surface. 

Linford, M. R., Fenter, P., Eisenberger, P. M. and Chidsey, . E. D. Alkyl monolayers on silicon prepared from 1-alkenes and hydrogen-terminated silicon. Journal of the American Chemical Society 117, 3145 (1995). 

Linford MR, Fenter P, Eisenberger PM, Chidsey CED (1995) Alkyl Monolayers on Silicon Prepared from 1-Alkenes and Hydrogen-Terminated Silicon, J Am Chem Soc 117 3145—3155... 

The hydrogenation of alkenes (Section 7-7) is exothermic, with values of AH° around -80 to -120 kJ/mol (-20 to -30 kcal/mol). Therefore, dehydrogenation is endothermic and has an unfavorable (positive) value of A 77°. The entropy change for dehydrogenation is strongly favorable (AS0 = +120 J/kelvin-mol), however, because one alkane molecule is converted into two molecules (the alkene and hydrogen), and two molecules are more disordered than one. 

The mechanisms of the two key steps are worth discussion. Hydrometallation occurs by initial n-complex formation followed by addition of the metal to one end of the alkene and hydrogen to the other. Both of these regioisomers are formed. The carbonyl insertion reaction is another migration from the metal to the carbon atom of a CO ligand. 

Two acetatomthenium catalysts have been shown to form alkyl complexes when allowed to react with alkene and hydrogen. However, a second molecule of hydrogen is not responsible for converting the alkyl complex to alkene. Instead, the alkyl complexes are decomposed by a proton from the carboxylic acid solvents. ... 

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