Catalyzed Hydrogenation of Alkynes and Conjugated Dienes

Chromium-Catalyzed Hydrogenation of Alkynes and Conjugated Dienes 

The best-studied precatalysts for this reaction are of the type L3Cr(CO)3, in which L3 is an arene, (CH3CN)3, or (CO)3. - The arene catalysts are thought to generate Cr(CO)3(S)3 (in which S = solvent) after thermal or photochemical activation of the coordinatively saturated precatalyst. The most active catalyst of this type is the naphthalene complex (Ti -naphthalene)Cr(CO)3.  

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Sea sponge Negombata magnifica, [Mo]-VII/CH2Cl2 system catalyzed the chemoselective alkyne RCM and the subsequent semi-hydrogenation of the cyclic conjugated enyne brought about the formation of cyclic Z,E-1,3-dienes. 

The nitrosyl complex [Rh(NO)(PPh3)3] (36) catalyzes the hydrogenation of both 1-hexene and cyclohexene in dichloromethane as solvent and was also found to add deuterium to cyclohexene without H/D scrambling.138 A further study extended the range of substrates hydrogenated to internal alkenes, to conjugated and non-conjugated dienes, activated alkenes and terminal and internal alkynes.139... 

In DMF as solvent, PdCl2 or [PdCl2(DMF)2] will catalyze the hydrogenation of conjugated dienes and alkynes to alkenes.194 The above systems quite generally give selective reduction of dienes to monoenes. 

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