Butadiene hydrogenation, palladium catalyzed

It deserves mention that related palladium-catalyzed C-C coupling cascades have been combined with a carbonylation terminating step [41]. In such cases vinyl-, alkyl- or allylpalladium(II) intermediates were generated in situ and trapped by carbonylation reactions, mainly carboxylations. As an example pelar-gonic (nonanoic) acid, an industrially interesting synthetic fatty acid, has been prepared via butadiene telomerization in the presence of methanol, subsequent carbonylation of the resulting allylic ethers and hydrogenation (eqs. (13) and (14)) [42]. 

It is not intended that the literature concerning the hydrogenation of alkenylalkynes and dialkynes shall be reviewed in detail. However, the hydrogenation of molecules as unsaturated as these provides further examples of the operation of the thermodynamic factor which are of interest. The palladium-, platinum-, and nickel-catalyzed hydrogenations of vinylacetylene (H2C=CH—C=CH) provides 1,3-butadiene as the major initial product butenes and butane are also produced (57). The product distributions are constant in liquid phase reactions until the parent hydrocarbon has been removed, showing that vinylacetylene is more strongly adsorbed than 1,3-butadiene and the butenes. The relative yields of butenes and butane resemble those obtained in 1,3-butadiene hydrogenation over these metals (see Section III, F, 6). 

The palladium-catalyzed hydrogenation of this molecule at room temperature has been investigated both in the gas phase, using a flow-system (90), and in the liquid phase using a solvent (57). The products of the gas phase reaction were cis-2-butene, 52% trans-2- mtene, 7% 1-butene, 40% and w-butane, 1%. Isomerization of the reactant was also observed, 2-butyne being produced to the extend of about 10% of the total olefin yield, together with traces of 1-butyne and 1,3-butadiene. 

To summarize, it may be said that the addition of hydrogen to 1,3-butadiene in gas phase reactions occurs partly by a 1-2-inechanism over all metal catalysts giving 1-butene in the gas phase. 2-Butene is either produced directly by a simultaneous 1-4-addition process as in the cobalt- and palladium-catalyzed reactions, or it is produced indirectly by the isomerization of 1-butene after its initial formation on the surface as is the case with the remaining metals of Group VIII and copper. The fraction of adsorbed olefin which is hydrogenated to w-butane depends upon the manner in which the thermodynamic and mechanistic factors, discussed previously, operate in each particular reaction. 

If ILs are to be used in metal-catalyzed reactions, imidazoHum-based salts may be critical due to the possible formation and involvement of heterocyclic imidazo-lylidene carbenes [Eqs. (2)-(4)]. The direct formation of carbene-metal complexes from imidazolium ILs has already been demonstrated for palladium-catalyzed C-C reactions [40, 41]. Different pathways for the formation of metal carbenes from imidazolium salts are possible either by direct oxidative addition of imidazolium to the metal center in a low oxidative state [Eq. (2)] or by deprotonation of the imidazolium cation in presence of a base [Eq. (3)]. It is worth mentioning here that deprotonation can also occur on the 4-position of the imidazolium [Eq. (4)]. The in-situ formation of a metal carbene can have a beneficial effect on catalytic performances in stabilizing the metal-catalyst complex (it can avoid formation of palladium black, for example). However, given the remarkable stability of this imidazolylidene-metal bond with respect to dissociation, the formation of such a complex may also lead to deactivation of the catalyst This is probably what happens in the telomerization of butadiene with methanol catalyzed by palladium-phosphine complexes in [BMIMj-based ILs [42]. The substitution of the acidic hydrogen in the 2-position of the imidazolium by a methyl group or the use of pyridinium-based salts makes it possible to overcome this problem. Phosphonium-based ILs can also bring advantages in this case. 

It will be recalled that cobalt and palladium may be the only two metals to catalyze the l 4-addition of hydrogen to 1,3-butadiene, and that an adsorbed 7r-allylic species proposed to feature in the mechanism only time will show whether this is related to the above-mentioned propensity of these two metals to from 77-allylic complexes or whether this correlation is merely a coincidence. 

In the dimerization reaction of butadiene catalyzed by palladium complexes, nucleophiles (YH), such as amines, alcohols, phenols, carboxylic acids 41 4S>, and active methylene compounds 46) are introduced. This reaction can be explained by the attack of these nucleophiles on the jr-allylic complexes formed as intermediates-in the reactions. Takahashi, Shibano, and Hagihara confirmed by using deuterium that the hydrogen of YH migrates to C6 of the dimeric product, probably via the oxidative addition reactions of YH to the palladium species 42). 

Nickel and palladium complexes also catalyze the reaction of 1,3-butadiene with compounds containing active hydrogen such as alcohols, amines, carboxylic acids, and active methylene and methyne compounds [equations (13.90)-(13.94)]. The mechanism... 

The palladium(O)-catalyzed reaction of 1,3-dienes with active methylene compounds to give 1,4-addition of a hydrogen atom and a stabilized carbanion is complicated by the formation of 2 1 telomerization products [27]. It was found by Hata et al. [21a] that bidentate phosphines such as l,2-(diphenylphosphino)ethane favor formation of the 1 1 adduct. More recent studies by Jolly have shown that the use of more a-donating bidentate phosphines on palladium gave a high selectivity for 1 1 adducts [23]. For example, 1,3-butadiene reacted with 11 to give the 1,4-addition product 12 in 82% yield, along with 18% of the 1,2-addition product 13 (Eq. (7)). 

Removal of hydrogen liom catalytic dehydrogenation of 1-butene to butadiene (surface catalyzed palladium membrane)... 

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