Hydrogenation of 1,3-dienes

The hydrogenation of dienes to monoenes introduces several problems of selectivity. Regioselective saturation of one of the double bonds is governed basically by the same effects that determine the relative reactivities of monoalkenes in a binary mixture that is, a terminal double bond is reduced preferentially to other, more substituted double bonds. During the reduction of a diene, a new competition also emerges since the newly formed monoene and the unreacted diene compete for 

Nonconjugated dienes, namely, allenes and isolated dienes, react preferentially on the terminal double bond.10 Hydrogenation of 1,2-butadiene over palladium yields 1-butene and d.s-2-butene as the main products with moderate discrimination of the two double bonds.68 Deuteration experiments indicated that the dominant syn addition to either the 1,2- or the 2,3-olefinic bond occurs. Different vinyl and Jt-allyl intermediates were invoked to interpret the results.69 70 

The monohydrogenation of conjugated dienes can occur by either 1,2 or 1,4 addition. 1-Butene (53%) and trans-2-butene (42%) are the main products in the hydrogenation of 1,3-butadiene on palladium with only a small amount of cis-2-butene.68 Deuterium distribution reveals that the trans isomer is produced by 1,4 addition. 

Cyclopentene is produced in excellent yield by the selective hydrogenation of cyclopentadiene. Both Raney nickel in cold ethanol solution74 and Cu/A1203 aerogel in the gas phase75 are selective catalysts. The cyclopentene is converted to cyclopentane at a rate much lower than that of diene transformation to cyclopentene. Other catalytic processes for the hydrogenation of cyclopentadiene yield cyclopentane as the principal product. 

Selective hydrogenation of diolefins and alkenylaromatics in steam-cracked gasoline is of industrial importance. Specific refining by selective hydrogenation of these polymerizable hydrocarbons without hydrogenating other unsaturated compounds (alkenes, aromatics) is required to increase the stability of gasoline (see Section 11.6.1). 

For small-scale synthetic organic chemistry, the half-hydrogenation of dienes is sometimes useful. For example, at —10°C the half-hdrogenation by 

4- addition of (3a,2aa,5p,9aP)-2,3,3a,4,5,9a-hexachydro-9a-hydroxy-3,5, 8-trimethyl-lH-cycl open ta-cyclooctene-5-methanol was accomplished (Fig. 2.6).51 Possibly, this is a hydrogenation of one double bond followed by isomerization of the other to the more stable position however, in other experiments conducted by the authors, a position different from the one in the product appears to be more stable, so this may be an unambiguous example of 

After half-hydrogenation, migration of the remaining double bond (sometimes mistaken for 1,4-addition) can be inhibited by poisoning with CO.53 

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Improved feedstock pretreatment is important to minimize catalyst consumption and reduce subsequent spent-catalyst handling requirements. Selective hydrogenation of dienes can be used to reduce acid consumption, both in HF and H2SO4 alkylation (29). More effective adsorptive treating systems have been appHed to remove oxygen-containing contaminants that are frequently introduced in upstream processing steps. 

Hydrogenation of acetylenes to olefins, 134 Hydrogenation of dienes, 124 Hydrogenation of dienones, 130 Hydrogenation of enones, 130... 

A. Andreetta, F. Conti, and G. F. Ferrari Selective homogeneous hydrogenation of dienes and polyenes to monoenes, pp. 204-295. 

The condensation of methanol with isobutene and the highly chemoselective hydrogenation of dienes and alkynes are independently promoted by the active acidic sites (-SO3H) and by the active metal. 

Toshima N, Yonezawa T, Harada M, Asakuara K, Iwasawa Y (1990) The polymer-protected Pd-Pt bimetallic clusters having catalytic activity for selective hydrogenation of diene. Preparation and EXAFS investigation on the structure. Chem Lett 19 815-818... 

Another subject of dispute was the mechanism of the photochemical, chromium carbonyl catalyzed hydrogenation of dienes /42/. The question here was whether the catalytic reaction is started by the dissociation of CO (Equation 42) or by the dissociation of the coordinated diene (Equation 43) /42, 43/. 

A m-RuCl2(CO)2(PPh3)2 complex, which catalyzes hydrogenation of dienes and monoenes, becomes useful in the presence of added phosphine for selective hydrogenation of 1,5,9-cyclododecatriene to cyclododecene the catalyst is a HRuCl(CO)2(PPh3)2 hydride that operates via steps analogous to those of reactions (9)-(11) (144, 145). 

Hydrogenation of Dienes and Alkynes with Croup III and Lanthanide Complexes... 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

Scheme 14.15 Proposed hydrogenation mechanism for the 1,4-hydrogenation of dienes by [Cr(CO)3 (arene)] (45).

Apart from Cr(0) carbonyl complexes, similar Mo, W and Co complexes also catalyze 1,4-cis-hydrogenation of dienes, though the selectivity of these catalysts is relatively low [63]. 

Scheme 14.18 Proposed 1,4-hydrogenation mechanism for the hydrogenation of dienes by cationic rhodium complexes [P2Rh(diene)]+A (54).

This chapter provides a review of the progress in reaction art, reactor techniques and process technology with respect to homogeneous catalytic hydrogenation of diene-based polymers, in accordance with the homogeneous hydrogenation theme of this handbook. 

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