Alkenes selective hydrogenation

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

The used Pd/ACF catalyst shows a higher selectivity than the fresh Lindlar catalyst, for example, 94 1% versus 89 + 2%, respectively, at 90% conversion. The higher yield of 1-hexene is 87 + 2% with the used catalyst versus 82 + 3% of the Lindlar in a 1.3-fold shorter reaction time. Higher catalyst activity and selectivity is attributed to Pd size and monodispersity. Alkynes hydrogenation is structure-sensitive. The highest catalytic activity and alkene selectivity are observed with Pd dispersions <20% [26]. This indicates the importance of the Pd size control during the catalyst preparation. This can be achieved via the modified ME technique. 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Partial reduction of alkynes to Z-alkenes is an important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that... 

TABLE 2.1 Order of ris-Alkene Selectivity from Hydrogenation of Alkynes25... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

Three new methods for the conversion of alkynes to (Z)-alkenes were reported, although Lindlar semi-hydrogenation still remains as the most convenient method. Copper (I) hydride reagent could reduce alkynes to (Z)-alkenes as shown in Scheme 3 [12]. Yoon employed nickel boride prepared on borohy-dride exchange resin for selective hydrogenation of alkynes to (Z)-alkenes (Scheme 4) [13]. 

The complex [Rh(COD)L L2]+, where L1 = PPh3 and L2 = pyridine, and a neutral benzoate complex, Rh(COD)(PPh3)(OCOPh), also effect highly selective hydrogenation of 1-alkynes to 1-alkenes as well as reduction of 1-alkenes and ketones to alcohols (139) the one equivalent of base required may be related to monohydride formation [Eq. (25)]. The bisphosphine complexes also catalyze reduction of styrene oxide to 2-phenylethanol and phenylacetaldehyde (140) ... 

Similarly, in some cases, dienes can be selectively hydrogenated into the corresponding alkenes, but they usually provide the corresponding alkane or a mixture of alkanes and alkenes [6, 45, 49, 59]. For example, the hydrogenation of 50 equiv. of 1,3- or 1,4-cyclohexadiene in the presence of [Cp2TiCl2]-iPrMgBr... 

The selective hydrogenation of hex-2-yne into ds-hex-2-ene with Pd colloids stabilized by 1,10-phenanthroline and derivatives has been reported by Schmid. Selectivity in alkenes up to 99% was obtained [25]. The use of PVP-stabilized Pt colloids with an average particle size of 1.4 nm dispersed in a propanol mixture prepared from Pt2(dba)3 provided 81% and 62% selectivity to ds-hexene at 50% and 90% hex-2-yne conversion, respectively. Bradley has shown that selectivity up to 89% in ds-hex-2-ene could be obtained with colloids supported in an... 

Conjugated dienes (such as 1,3-cyclohexadiene, cyclopentadiene, 2,4-hexadienoic-sorbic-acid) and polyenes can be selectively hydrogenated to monoenes unactivated alkenes are totally unreactive [20]. Unfortunately, the possibilities for modification of the catalyst by ligand alteration or by the use of additives are very limited [50, 51]. 

Figure 3.11 Propane metathesis catalyzed by (=SiO)2TaH (3) in a continuous flow reactor. Contact time effect (520mg of 5.3% Ta/SiOj 1 atm, 150 C, 1 to lOOmLmin or VHSV = 38 to 3800 h ) (a) alkane selectivity (b) alkene selectivity (c) hydrogen selectivity (d) alkane alkene ratio.

Shape- and Size-Selective Hydrogenation of Alkenes on the Imprinted Rh Dimer Catalyst... 

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