Cobalt, hydrogenation conjugated dienes

The cobalt(I) complex CoBr(PPh3)3 as a boron trifluoride etherate selectively hydrogenates conjugated dienes to monoenes via an unusual 1,2-hydrogen addition at the more-substituted double bond (186). 

Cyanide-containing cobalt catalysts, particularly potassium pentacyanocobalta-te(II) K3[Co(CN)5], are used in the reduction of activated alkenes (conjugated dienes).26,31 [Co(CO)4]2 is best known as a hydroformylation catalyst, but hydrogenation is also possible under specific conditions. Phosphine-substituted analogs are more successful. 

The addition of hydrogen cyanide (HCN) to carbon-carbon double bonds activated by electron-withdrawing groups in the presence of a base as a catalyst (a variation of the Michael Reaction) has been known for a long time. Nitriles were also obtained by hydrocyanation of branched olefins, such as isobutylene and trimethylethylene, in vapor phase reactions in particular the reactions over alumina (3) and cobalt-on-alumina (4) were reported in the late 1940s and early 1950s. Addition of HCN to conjugated dienes in the presence of cuprous salts (vapor and liquid phase) was reported as early as 1947 (5). 

Conjugated dienes can be reduced to monoolefins by treatment with hydrogen, hydrated cobalt chloride, potassium cyanide, potassium chloride, sodium hydroxide, and tetramethylammonium chloride or benzyltri-ethylammonium chloride as the phase-transfer catalyst. The hydridopen-tacyanocobaltate anion, HCo(CN)s3, is the probable metal catalyst (47-... 

The cobalt(II) cyanide complex Co(CN)5 " in aqueous solution acts as a homogeneous catalyst for the selective hydrogenation of conjugated dienes to monoenes (81). The initial step in this catalysis is the reaction... 

HCN also adds to alkenes in the presence of an appropriate catalyst (Arthur et al., 1954 Jackson and Level, 1982). Thus, cobalt carbonyl leads to Markownikov addition, for example, 1-propene yields isopropyl cyanide in approximately 75% yield. HCN adds to alkynes in the presence of metal complexes, and the use of a nickel complex may lead to syn addition (Jackson and Level, 1983 Jackson et al., 1988). Hydrogen cyanide reacts with conjugated dienes, the mechanism involving a TT-allyl intermediate. The course of addition is complex and may lead to more than one product (Keim et al., 1982). 

For hydrogenation in water with an inexpensive catalyst, solutions containing cobalt salts and excess cyanide are useful10,11. The catalysts are selective for conjugated C=C bonds and are relatively unreactive with unconjugated dienes such as 1,5-cyclooctadiene. 

A wide variety of unsaturated substance can be hydroformylated by cobalt or rhodium catalysts but conjugated alkenes (e.g., butadiene) may give a number of products including hydrogenated monoaldehydes. The mechanism is different, since addition of M H to dienes leads to alicyclic species which may be present as a-bonded intermediate or as h3allyls. 

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