Conjugated diene complexes hydrogenation

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

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). 

Hydrogenation of butadiene with K3Co(CN)5, which is known to hydrogenate selectively conjugated dienes [75], was possible with 100% conversion and selectivity and a TOF up to 72 h-1 in the ionic liquid [BMIM][BF4], but the catalyst was deactivated after the first run and the inactive complex (BMIM)3Co(CN)5 was formed [69]. 

A selective hydrogenation of conjugated dienes was carried out with a Pd complex which was preactivated with oxygen. Besides the conversion of dienes with good selectivity (98%), diene esters, ketones and nitro compounds were also hydrogenated with fairly good selectivities (equation 9)29. 

In Grignard reactions, Mg(0) metal reacts with organic halides of sp carbons (alkyl halides) more easily than halides of sp2 carbons (aryl and alkenyl halides). On the other hand, Pd(0) complexes react more easily with halides of sp2 carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C rr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes, conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /(-hydrogen. At the same time, the PdfO) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg. Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Certain chromium complexes also exhibit similar selectivities in the hydrogenation of conjugated dienes.139 140 Coordination of the diene in the cisoid conformation and 1,4 addition can account for the selectivity. 

The conjugation process 1,4-diene — 1,3-diene goes by interconversion of 7r-Pd complexes I and III via the intermediate 7r-allyl-Pd hydride complex II and involves 1,3-hydrogen shifts. Conversion of cis,trans to the thermodynamically favored trans,trans-conjugated dienes occurs, but both of these isomers would be carboxylated the same way. CO insertion via a Pd carbonyl hydride (allyl or olefin) intermediate gives unsatu-... 

The hydrogenation of trans-l-phenyl-1,3-butadiene has been discussed49 in terms of the mechanism originally proposed,45 and 773-allyl species were invoked. NMR studies on the hydrogenation of butadiene have shown that Tj -but-2-enyl and r 3-l-methylallyl complexes are present in the reaction mixture. With isoprene, an q1 -methylbut-2-enyl complex was observed.50 By using a two-phase system and a phase-transfer catalyst, the turnover numbers attainable with conjugated dienes can be increased to about 30.51... 

The nitrosyl complex [Rh(NO)(PPh3)3] (36) catalyzes the hydrogenation of both 1-hexene and cyclohexene in dichloromethane as solvent and was also found to add deuterium to cyclohexene without H/D scrambling.138 A further study extended the range of substrates hydrogenated to internal alkenes, to conjugated and non-conjugated dienes, activated alkenes and terminal and internal alkynes.139... 

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