Double bond migration during hydrogenation

Double-bond migrations during hydrogenation of olefins are common and have a number of consequences (93). The extent of migration may be the key to success or failure. It is influenced importantly by the catalyst, substrate, and reaction environment. A consideration of mechanisms of olefin hydrogenation will provide a rationale for the influence of these variables. 

Nevertheless, during hydrogenation of the dimethylcyclohexenes, the fact that the quantities of trans isomer increase with the double bond migration ability of the various catalysts90 suggests that double bond migration sites (or the sites nearby) are involved in apparent trans addition. 

An alternative mechanism for double bond migration has recently been proposed by Smith (Fig. 2.12).113 It is based in part on theoretical calculations,114 in part on the recent surface science work suggesting that hydrogen occupies threefold hollows,115 and in part on the experimental observation that during hydrogenation an allylic deuterium moves 1-3 across the bottom of an adsorbed allylic system without being exchanged.116... 

Takaya and co-workers46 found that BINAP-based Ru(II) dicarboxylate complexes 31 can serve as efficient catalyst precursors for enantioselective hydrogenation of geraniol (2E)-32 and nerol (2Z)-32. (R)- or (iS )-citroncllal 33 is obtained in nearly quantitative yield with 96-99% ee. The nonallylic double bonds in geraniol and nerol were intact. Neither double bond migration nor (fi)-/(Z)-isomerization occurred during the catalytic process. Furthermore, the S/C ratio was extremely high, and the catalyst could easily be recovered (Scheme 6-18). This process can be applied to the asymmetric synthesis of a key intermediate for vitamin E. 

Isomerization during hydrogenation may alter the functional groups present in the molecule. For instance, reduction of cyclohexen-2-ol over 5% platinum-on-carbon occurred with rapid absorption of 1 mole of hydrogen at substantially constant rate and quantitative formation of cyclohexanol. On the other hand, reduction over palladium ceased abruptly at about two-thirds of a mole, and the product was a mixture of cyclohexanol and cyclohexanone, the latter arising through double bond migration to the enol of cyclohexanone (29). 

Disproportionation is a special form of double bond migration in which the double bond is transferred from one molecule to another. Reactions of this type are especially liable to occur over palladium, and for this reason palladium sometimes is best avoided in olefin hydrogenation when the double bond is contained in an incipient aromatic system. Disproportionation activity in the hydrogenation of cyclohexene (and presumably other incipient aromatic systems will follow the same order) decreases with the metal in the order palladium >> platinum > rhodium (16). An example of the complication that can be caused by disproportionation during hydrogenation is found in the attempted reduction of... 

Palladium is the most active transition metal for double-bond isomerization relative to saturation. It can also saturate olefins in the presence of aromatics, and exchange large amounts of hydrogen for deuterium during saturation. The mechanistic picture of saturation on a palladium surface is of a molecule in a state of rather violent agitation with many double-bond migrations and flipovers and much... 

Quite recently in the asymmetric hydrogenation of ( )-2-methylpent-2-enoic acid in hexane over Engelhard Pd-alumina catalysts (E 40692 and E 5220), modified with Cnd, were used. Deuteriumation experiments showed that double bond migration and E-Z isomerization occurs during deuteriumation, but these processes are less important at the higher pressure of hydrogen (40 bar) and in the presence of modifier Cnd (Salladie-Cavallo et al. ). 

In addition to the stereoisomerism during catalytic hydrogenation, double bond migration is also observed. Show how esters of c/s-9-octadecenoic acid are converted to cis- and frans-S-octadecenoic esters under these conditions. 

Further investigation on the metathesis of 1-decene (an expected olefin during the alkane metathesis of -decane) and the hydrogenation of the products formed clearly demonstrated that the distribution resulting from alkane and olefin metathesis completely differs with the same catalyst. If there is no double-bond migration, 9-octadecene and ethylene are expected to be the major primary products. Indeed, these primary products are observed, as the temperature reaches 150°C. However, after just 15 min, Cj to C12 and C13 to C20 olefins are also observed, clearly indicating that some isomerization of double bond occurs leading to several competitive metatheses (Fig. 5). 

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