Hydrogenation conjugated alkenes

Drivers for Performing Conjugated Alkene Hydrogenation in Micro Reactors... 

Beneficial Micro Reactor Properties for Conjugated Alkene Hydrogenation... 

Conjugated Alkene Hydrogenation Investigated in Micro Reactors Cas/liquid reaction 19 [CL 19) Hydrogenation of Z-(a)-acetamidocinnamic methyl ester... 

Copper-catalyzed monoaddition of hydrogen cyanide to conjugated alkenes proceeded very conveniently with 1,3-butadiene, but not with its methyl-substituted derivatives. The most efficient catalytic system consisted of cupric bromide associated to trichloroacetic acid, in acetonitrile at 79 °C. Under these conditions, 1,3-butadiene was converted mainly to (Z )-l-cyano-2-butene, in 68% yield. A few percents of (Z)-l-cyano-2-butene and 3-cyano-1-butene (3% and 4%, respectively) were also observed. Polymerization of the olefinic products was almost absent. The very high regioselectivity in favor of 1,4-addition of hydrogen cyanide contrasted markedly with the very low regioselectivity of acetic acid addition (vide supra). Methyl substituents on 1,3-butadiene decreased significantly the efficiency of the reaction. With isoprene and piperylene, the mononitrile yields were reduced... 

Isolated double and triple bonds are reduced readily, whereas conjugated alkenes and aromatic systems are difficult to hydrogenate. Carbonyl double bonds react only very slowly, if at all, so it is possible to achieve selective reduction of C=C double bonds in the presence of aromatic and carbonyl functions. 

Enones are selectively hydrogenated electrochemically at the conjugated alkene bond. A high degree of selectivity is achieved at an electrodeposited nickel surface... 

As a rule, alkynes react in bromo- and iodofluorinations by monoaddition because vinylic a./f-halofluorides as electron-deficient alkenes are generally less reactive. However, some aryl-conjugated alkenes, such as cinnamate or stilbene derivatives 3 or 5, respectively,184 may also be halofluorinated by hydrogen fluoride/pyridine/A-halosuccinimide reagent.185... 

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. 

VIV propene to equimolar mixtures of propane + oxygen reduced the cool-flame induction period by ca. 18 % at 300 °C. Like the previous work, these results showed the importance of the conjugate alkene in the autocatalytic oxidation of propane. However, at 247 °C, the yield of cyclohexene just prior to the cool flame was < 1 % of the total products. In contrast, at temperatures above 300 °C, it becomes the major product and is formed in roughly equal amounts with hydrogen peroxide prior to a stabilized cool flame [52]. Tipper concluded that above 300 °C reaction (2) occurs to an appreciable extent until well after the initial stage of oxidation since the differential yield of the alkene (d[C H2 ]/ d [C Hj 2 ]) was > 25 % over at least a quarter of the reaction. 

Above ca. 400—450 °C abstraction of a hydrogen atom from alkyl radicals by oxygen to yield the conjugate alkene and hydroperoxy radical... 

While most catalysts reduce the least-substituted double bond preferentially, the novel catalyst system CoBr(PPh3)3 BF30Et2 can selectively hydrogenate conjugated dienes to monoenes via 1,2-hydrogen addition to the more substituted double bond (Scheme 92). Unfortunately, hydrogenation of functionalized alkenes, such as methyl vinyl ketone, methyl acrylate and -butyl vinyl ether, does not occur even under forcing conditions. 

The hydrogenation of alkenes conjugated to aromatic systems is usually much less difficult than partial reduction of polyenes, as the aromatic ring is less susceptible to reduction. Many catalysts of Group 10 metals have been used for this transformation, including Raney nickel, Pd adsorbed on carbon, Pd adsorbed on calcium carbonate, Pd on barium sulfate, metallic palladium, PtOa, chloroplatinic acid or platinum metal. Normally, the stereochemistry of the reduced product arises from syn addition of a hydrogen molecule to the less-hindered face of the double bond, as exemplified in the catalytic hydrogenation of a steroidal styrene over metallic palladium (Scheme 93). ... 

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