Allenes, hydrogenation studies

Fluxionality studies have been carried out on 343 (Scheme 44).137 No coalescence of the two allene hydrogens (HI and H6) was observed in temperature ranges where the complex is stable (ca. 100°C). However, spin saturation transfer was observed between 60 and 100°C, which yielded activation parameters (AH = 26.8 1.3 kcal/mol A5J = 15.1 1.3 eu) for migration of the metal between the two double bonds. At 80°C, 343... 

J. F. Watson, J. L. Christian, and J. W. Allen, A Study of the Effects of Nuclear Radiation on High Strength Aerospace Vehicle Materials at the Boiling Point of Hydrogen, ERR-AN-085. 

Addition Reactions. — A detailed study of the free-radical chlorination of penta-2,3-diene with t-butyl hypochlorite shows that substitution products are obtained by both allylic and allenic hydrogen-abstraction addition... 

Even substituted allenes, RCH=C=CH2, are protonated at the 1 position, giving methyl ketones by hydration (60-63) and mostly 2-halo-2-alkenes, RCH=CXCH3, by the addition of hydrogen halides (62, 64, 65). Jacobs and Johnson (65), in a careful study, have shown that addition of HCl to... 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

Only a few reactions have been studied with N-allenylsulfenimides. Despite the presence of a sulfur function in allene 281, van Vranken and co-workers carried out its heterogeneous catalytic hydrogenation (Eq. 8.39) [148], The reaction occurs only at the terminal C=C bond, providing the Z-configuration enamide 296 in 70% yield. 

Evidence derived from a study of the stereochemistry of hydrogenation of 1,2-cyclononadiene and 1,2-cyclodecadiene led Moore (108) to conclude that allyl complexes like those postulated above must be intermediates. He noted that, of the four ways in which either allene could be adsorbed on a surface, two, a and b, would yield via cis addition of hydrogen the cis-cycloalkene and two, c and d, the tram isomer. Examination of... 

Because only a small number of stable selenoketones exist, studies of their decomposition are scarce. Irradiation of dwerf-alkylselones with UV light in hydrogen-donating solvents affords diselenides (80CJC6 87MI1), and the selone 37 behaves likewise. A second path of photolysis involves extrusion of molecular nitrogen from 37 to yield eventually tetramethyl-allene and elemental selenium, probably via the elusive... 

The efficiency of conjugated dehydrogenation of olefins with hydrogen peroxide was also confirmed in studies of propylene dehydrogenation to allene [60], The well-known process of propylene oxidative dehydrogenation to allene in the presence of iodine [61] at 625-700 °C gives low yields of the target product (3.6% allene) at low selectivity of the process (6%). 

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