Selective diene hydrogenation

The selective diene hydrogenation of monoterpenes such as myrcene, which contain both isolated monoene and diene moieties, forms a particular challenge [84]. The catalyst [RhH(CO)(PPh3)3] (60) has been reported to perform remarkably well for such hydrogenation reactions, and the diene moiety was shown to be selectively reduced to the monoene, while the isolated double bond remained unaffected under the reaction conditions used (Scheme 14.20). The rates of reaction expressed as average TOF (determined at ca. 80% conversion) ranged from ca. 640 (in benzene, 20 atm H2 at 100 °C) to 7600 mol mol 1 h 1 (in cyclohexane, 20 atm H2 at 80 °C). The hydrogenation in benzene solution resulted in... 

Finally, other rhodium catalysts for the selective diene hydrogenation worth mentioning include [RhCl(PPh3)3] (64), [RhCl(nbd)]2 (65), and the catalytic sys-... 

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

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. 

The selectivity for hydrogenation of dienes in the presence of monoolefins arises from the exceptional stability of jr-ally 1 complexes. In the case of Pt catalysts the reactions shown can compete with one another (equation 6)14. The second pathway is favored, especially when the olefin or diene must compete with excess ligands (phosphine, CO, SnCp ) for a coordination site. This is why the diene is almost completely hydrogenated before the concentration of olefin increases to the point that the olefin gains access to the catalyst. A similar phenomenon can be responsible for selectivity in hydrogenation of dienes with heterogeneous catalysts. 

Catalysts differ in their ability to promote double-bond migration and cis-trans isomerization, in their thermodynamic and mechanistic selectivities in diene hydrogenation and in their tendencies to catalyze 1,2-, 3,4-, or 1,4-addition34. 

In order to increase the selectivity in diene hydrogenation, low-temperature basic additives and the use of less polar solvents may help. In special cases, treatment of the catalysts with the salts of heavy metals (Zn, Cd, Pb) can be the method used to modify the activity and selectivity53. Rh and Ir catalysts could be selectively poisoned with CO-containing hydrogen, in order to saturate 1,3-butadiene to 1-butene without isomerization54. 

The selectivity of hydrogenation of dimethyl tetramethylbicyclo[2.2.0]hexa-2,5-diene-5,6-dicarboxylate depends on the catalytically active metal and on the bulk of the ester substituent (equations 59 and 60)147. 

Ethoxypenta-l, 3-diene underwent the Diels-Alder reaction withp-benzoquinone to give the cis-fused diketone I. Selective catalytic hydrogenation (Ni) of I, followed by reduction with lithium aluminum... 

A more challenging task is the selective partial hydrogenation (semihydrogenation) of alkynes to yield alkenes. This is a selectivity problem similar to the hydrogenation of dienes in that that the alkyne is hydrogenated preferentially in the presence of an alkene. The possibility of the formation of geometric isomers from nonterminal acetylenes raises the problem of stereoselective semihydrogenation. 

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... 

The site-selective reduction of a,co-dienes, in which the two double bonds are differentiated only by the presence of an allylic substituent, is a challenging task. A pentamethylyttrocene complex that is extremely selective in hydrogenating terminal alkenes exhibits good selectivity in this reaction 142... 

Another interesting reaction of importance is the selective partial hydrogenation of acetylene or of dienes on heterogeneous catalysts. The selective partial hydrogenation reaction will be established by one of the following three mechanisms ... 

Selective partial hydrogenation of acetylene or dienes caused by adsorption induced activation and deactivation of the surface. 

Selective partial hydrogenation of conjugated diene via alkylallyl anion intermediates. 

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). 

The Fe(CO)4 intermediates of types III and VIII in Scheme IV explain the direct reduction paths evidenced in the hydrogenation of mono- and diunsaturated fatty esters. Competition between monoene and diene hydrogenation can be related to the stability of the Fe(CO)3-and Fe(CO)4-complexes. At a low concentration of Fe(CO)5, the formation of Fe(CO)a complexes is favored because they are more stable. At a high concentration of Fe(CO)s, formation of mono- and di-Fe(CO)4 complexes becomes important, and selectivity for diene hydrogenation is decreased. Although the occurrence of olefin-Fe(CO)4 complexes has precedence in the literature (i9), no such species has yet been identified with either methyl oleate or linoleate. 

The selective hydrogenation of non-isomerizable dienes takes place over a variety of catalysts. The saturation of m-vinyl isopropenylbenzene (57) in the presence of m-diisopropenylbenzene (58) takes place over 5% Rh/C at room temperature and 4 atmospheres with about 75% selectivity. The hydrogenation of the more accessible double bond in 59 (Eqn. 15.36) takes place with 40 1 selectivity in 90% yield at 0°C and one atmosphere over 5% Pd/BaS04 in a 1 1 mixture of pyridine and ethanol. ... 

Selective catalytic hydrogenation of dienes and polyenes to monoenes over nickel sulphide supported on alumina or silica finds application in the partial hydrogenation of soybean oil and isoprene in the presence of methylbutenes. A sulphided C0-W/AI2O3 catalyst has been proposed for the selective hydrogenation of dienes in pyrolysis gasoline. 

This method of selective tropone cyclopropanation and partial diene hydrogenation, via formation of 28, was applied to the total synthesis of ( + )-cyclocolorenone. ... 

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 . ... 

---