Allenes, catalytic hydrogenation

Conjugated dienes can add hydrogen by 1,2 or 1,4 addition. Selective 1,4 addition can be achieved by hydrogenation in the presence of carbon monoxide, with bis(cyclopentadienyl)chromium as catalyst. With allenes catalytic hydrogenation usually reduces both double bonds. 

Interestingly, in the case of enantiomerically pure internal allenes, a diastereose-lective IH is observed (e.g.. Scheme 4-17), as shown by the catalytic hydrogenation of the Z/E mixture to give the trans-2,5-disubstituted pyrrolidine as a single compound. 

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. 

Most of the chlorinated hydrocarbon derivatives used as insecticides are practically ineffective against mites. At the same time, one example of chlorinated hydrocarbons, ineffective as insecticide, bis(pentachloro-2,4-cyc opentadien-l-yl) (1), attained importance in the chemical control of mites. It was introduced in 1960 under the name dienochlor. It is prepared by the catalytic hydrogenation, or Wurtz reaction, of hexachlorocyclopentadiene (Ladd, 1952 Rucker, 1955a, b Allen et al.,... 

Henze, H.R., Allen, B.B. Wyatt, W. (1942). Catalytic Hydrogenation of Cotton Hull Fibre. Journal of Organic Chemistry, Vol. 7, pp. 48 - 50... 

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... 

As shown in Scheme 12.1, reaction of the phosphine with an allenic ester gives all-carbon 1,3-dipole 262. This dipolar intermediate reacts at the a-position to form the cyclic intermediate 263, which is in equilibrium with 264 via hydrogen shift. Finally, the reaction affords the cycloadduct along with the regeneration of PPh3 as the catalytically active species. 

Irving, P. M., Lloyd Allen, W., Healey, T., Thomson, W. J., Catalytic micro reactor systems for hydrogen generation, in Matlosz, M., Ehrfeld, W., Baselt,... 

Whereas alkene insertion is followed by facile dehydropalladation whenever there is a /S-hydrogen to afford alkenes and Pd(0) catalytic species, the alkyne insertion produces the thermally stable alkenylpalladium species 2 and 7, which can not be terminated by themselves and further transformations are required in order to terminate the reactions and to regenerate Pd(0) species for catalytic recycling. In other words, it is generally believed that the reaction of generated alkenylpalladium species 2 and 7 can not be terminated, because the f-R elimination (formation of alkynes or allenes) even in the presence of a /1-hydrogen is not possible. Therefore the carbopalladation of alkynes is a living process, in which alkynes play a role of relay to pass the ability of carbon-carbon bond formation to other reactants. 

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