1,3-Pentadiene selective reduction

Certain monohydride catalysts exhibit high selectivity toward hydrogenation of conjugated dienes and polyenes to give monoenes. Potassium pentacyanocobaltate prepared from CoCl2 and KCN is one of the most active catalysts, but reduces only conjugated double bonds. The reduction of dienes with nonequivalent double bonds follows the usual substitution trend noted for monoenes. 1,3-Pentadiene, for example, is converted selectively to trans-2-pentene via 1,2 addition 134... 

Mixed cyclopentadienyl-diene titanium complexes, Cp TiX(diene)(X = Cl, Br, I), have been prepared in 30-60% yield by the stoichiometric reaction of CpTiXs with (2-butene-l,4-diyl)magnesium derivatives or by the reduction of CpTiXs with RMgX (R = i-Pr, f-Bu, Et X = Cl, Br, I) in the presence of conjugated dienes, as shown in Scheme 4. The butadiene, 1,3-pentadiene, and 1,4-diphenylbutadiene complexes of Cp TiX exhibit a unique prone (endo) conformation (13), while the isoprene, 2,3-dimethylbutadiene, and 2,3-diphenylbutadiene complexes prefer the supine (exo) conformation (14). Reduction of Cp TiX(diene) with RMgX or Mg gives a low-valent species, which catalyzes a highly selective (>99%) tail-to-head linear dimerization of isoprene and 2,3-dunethylbutadiene. " ... 

Reduction of dienes and polyenes In the hydrogenation of 1,3-pentadiene the selectivity sequence is the following... 

PEI-RhCl3 and PEI-RUCI3 complexes exhibit different catalytic activity with regard to a mixture of the cis- and trans isomers of pentadiene [61]. The former complex is more selective for pentene (0.94) than the latter. A quantitative aniline yield results from the reduction of nitrobenzene in the presence of PEI complexes with Ni(II), Co(ll), Sn(II), Pd(II) and Rh(III) [61]. The reduction proceeds rapidly at 20- 70°C and at 1-25 atm pressure of H2 both in a solvent and without. Besides aniline, cyclohex-ylamine is produced by further hydrogenation of the aromatic ring in the presence of the PEI-Rh(III) complex. Polymer-metal catalysts do not lose their catalytic stability after repeated application. For example, when eight hydrogenation reactions were catalyzed with PEI-Pd(II), in each case a 100% product yield of aniline was attained. 

Examples of this kind of enantiomorphic or chiral selectivity are now being found in organic synthesis. Asymmetric synthesis, for example, has been demonstrated with stereo-controlled Michael addition in the synthesis of beta-lactams using chiral catalysts, where an acyl ligand such as acetyl is bound to cyclo-pentadiene carbonyl triphenylphosphine. Essentially complete enantiomorphic selectivity has been achieved in this Michael addition synthesis. Another case is enantio-morhic ketone reduction in ethylbenzene reduction in the ethylation of benzaldehyde. Using chiral catalysts, 97% selectivity has been achieved. Closely related research involves the making of catalytic antibodies and hybrid enzymes. ... 

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