Pentadiene selective hydrogenation

The third member of the tandem inter [4+2]/intra [3+2] cycloaddition family is classified as the bridge mode, in which the dipolarophile is attached to the dienophile. Simple, 1,4-pentadi-enes as well as 2-alkoxy-l,4-pentadienes can function effectively as dienophiles and dipolarophile combinations with excellent chemical selectivity and regio- and diastereoselectivity. Hydrogenation of the bridged nitroso acetals produces hydroxymethylated derivatives in high diastereo- and enantioselectivity (Eq. 8.116).185... 

Attempted reaction of 1,3-pentadiene with the optically active diboron derived from dialkyl tartrate in the presence of a phosphine-free platinum catalyst gave poor diastereoselectivity (20% de).63 Better selectivity has been attained with a modified platinum catalyst bearing a PCy3 ligand (Scheme 6).64 The reaction of allylborane thus obtained with an aldehyde followed by oxidation with basic hydrogen peroxide affords the corresponding diol derivative with moderate ee. 

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... 

This step has been shown to be stereospecific in the case of hydrogenation (deuteration) catalyzed by dicyclopentadienylmolybdenumdihydride (dideuteride) (62) with cis addition of the metal hydride to the double bond (70). In these systems there is spectral evidence for the initial formation of cr- and 7r-electron donor-acceptor complexes between the catalyst and substrate prior to 7r-complex formation and n-a- rearrangement (70, 71). This catalyst (62) has also been used for the selective homogeneous hydrogenation of 1,4- or 1,3-dienes to monoenes, for example, cyclo-pentadiene (63). The reactions are run at elevated temperature (72). 

Several complexes having arenes, [Cr(CO)3(arene)] (arene = benzene, mesitylene, methyl benzoate, etc.) and [Cr(CO)3(cycloheptatriene)], are excellent catalysts for selective 1,4-hydrogen addition to 1,3-dienes. " 1,4-Hydrogenation of methyl sorbate (34), isoprene (36) and 1,3-pentadiene (38) in cyclohexane affords methyl cw-3-hexenoate (35), 2-methyl-2-butene (37) and cw-2-pentene (39), respectively. [Cr(CO)3(MeCN)3] is also an active catalyst under mild conditions (40 C, H2 1.4 atm), as are [Cr(CO)e] or [Cr(NBD)(CO)4] under UV irradiation. ... 

A deuterium kinetic isotope effect of 2 has been found in the hydrogenation of 1,3-pentadiene using a Ziegler-Natta catalyst, cobalt(ll,lll) /r -oxostearate-AlEts, Co O(Ci7H35C02)6(H20)3 5H2O-AICI3. The reaction was found to be of a kinetic order of 0.3 in the diene, and first order in the hydrogen and the catalyst. The kinetics and the selectivity of the reaction has been studied at 253-293 K. 

Not only metals but some oxide catalysts are active in diene hydrogenation ZnO modified by Sn(CH3)4 afforded 1-butene in hydrogenation of butadiene at room temperature. Reduced and sulfided moUbdena on alumina catalyst hydrogenated butadiene and cyclohexadiene selectively5 . When the transition metal complex Mo(CO)6 was encapsulated in NaY zeolite cages, it converted //wnv-l, 3-pcnladicnc to cA-2-pentene and 1,4-pentadiene to c -l,3-pentadiene at 150°C °. Cr(CO)3 encaged in LiX or NaX zeolite was efficient and selective in butadiene hydrogenation to cw-2-butene . ... 

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

The partial hydrogenation of dienes was successfully carried out by Michalska et al. [21] using palladium supported on heterocyclic polyamides. Under the reaction conditions employed (MeOH, atmospheric pressure, 25 °C) the resin-sup-ported catalyst was able to selectively hydrogenate one of the two double bonds present. Recycling experiments proved the high stability of the used catalysts. For example, in 11 hydrogenation runs with 2-methyl-l,3-pentadiene, which is equivalent to 4300 catalytic cycles per palladium atom, neither loss of activity nor changes in selectivity were observed. 

Table 2. Activation Parameters and Relative Rate Constants for [1,5] Sigmatropic Hydrogen Shifts in Selected Acyclic 1,3-Pentadienes...

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