Alkenations selective

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. 

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

Tin radical-induced elimination from P-nitro sulfones or P-nitro sulfides proceeds in a stereoselective way to give anti elimination products.153 When diastereomers of P-nitro sulfones can be separated, each diastereomer gives ( )- and (Z)-alkenes selectively (Eq. 7.111). Such... 

Stereoselective preparation of ( )-allyl alcohols via radical elimination from anti-j-phenylthio-P-nitro alcohols has been reported.154 The requisite anti-P-nitro sulfides are prepared by protonation of nitronates at low temperature (see Chapter 4), and subsequent treatment with Bu3SnH induces anti elimination to give (E)-alkenes selectively (see Eq. 7.112). Unfortunately, it is difficult to get the pure yyw-P-nitro sulfides. Treatment of a mixture of syn- and anti-P-nitrosulfides with Bu3SnH results in formation of a mixture of (E)- and (Z)-alkenes. 

Alkalization of iron catalysts causes two different effects. The selectivities of 1-alkenes are raised and both the growth probability a2 and the fraction f2 are markedly increased, as already shown in Figure 11.2. Detailed studies on the promoter effect of alkali have revealed the effect on 1-alkene selectivity to saturates at 1 mass% of K2C03, while the effect on f2 already begins at 0.2 mass% of K2C03.1213 This difference points to specific active sites in Fischer-Tropsch syn-... 

Table 11.7. Substituted cyciopropanols by intramolecular hydroxycyclopropanation of a terminally alkoxycarbonyl-substituted alkene (selected examples).

Light alkene selectivities from MTO over SAPO-34 at 400-450 °C (ethylene > propylene butylenes > pentenes) are quite different than those predicted from thermodynamic equilibrium (butylenes > propylene > pentenes > ethylene). Over... 

ZSM-5 propylene and higher alkene selectivities are typically higher and ethylene selectivity is typically lower compared to SAPO-34 [97]. Over Beta light alkene breakdown is closer to thermodynamic equilibrium compared with ZSM-5 and SAPO-34 [97]. While the SAPO-34 pore opening is not large enough to let aromatics and branched alkanes out, di- and trimethylbenzenes and even some tetra-methylbenzenes are observed over ZSM-5 as products in the effluent and hexamethylbenzenes are observed over Beta as products in the effluent [98, 99]. 

The conditions that lead to the formation of the first C-C bond during MTO, the mechanism for making ethylene and propylene from methanol, the critical catalyst parameters that are responsible for the wide variation in light alkene selectivities observed among different framework types and between fresh versus aged catalysts are some of the most intriguing questions of the catalysis field today [100-105]. 

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.

The alkene selectivity as a function of conversion is shown in Figure 2. 

The 1,4-addition of sp3-geminated organodimetal compounds, prepared by the Gaude-mar/Normant coupling reaction, to alkylidenemalonate gives Z-alkene selectively through elimination of the malonate moiety (equation 33)56,57. 

The methanol transformations discussed precedingly can be modified to produce high amounts of light alkenes.437 454 474 475 The key to achieve this change is to prevent C2-C4 olefinic intermediates to participate in further transformations. Such decoupling of alkene formation and aromatization can be done by the use of small-pore zeolites or zeolites with reduced acidity. Reduced contact time and increased operating temperature, and dilution of methanol with water to decrease methanol partial pressure, are also necessary to achieve high alkene selectivities. This approach has led to the development of the MTO (methanol-to-olefin) process, which yields C2-C5 alkenes with about 80% selectivity. 

Reaction with ally lie sulfoxides (1) or sulfones (2).4 The allylic sulfoxides or sulfones undergo y-substitution on reaction with lithium dialkylcuprates in ether to provide trisubstituted (E)-alkenes selectively. 

A typical example is the dehalogenation of 1,2-dihalides. The elimination of 1,2-dibromides occurs stereospecifically in the form of an anh -elimination. Thus, the /neso-dibromide yields the (E)- and the d, /-dibromide yields the (Z)-alkene selectively [37] ... 

To explain the observed selectivities, a similar model was proposed as for the epoxidation reactions (see Section 10.2.1.10) [96]. In this model, the ylide conformation is controlled as before, and the alkene selectively attacks one face in an analogous manner to the aldehyde in the epoxidation reactions. It was noted that in this case betaine formation was non-reversible so the diastereoselectivity is controlled by non-bonding interactions during the betaine-formation step [79]. 

In addition to epoxides, three-membered nitrogen heterocycles, aziridines, can be obtained by means of catalytic asymmetric aziridinations (Eq. 30). To this aim, chiral ruthenium(salen) complexes 67 [56] and 68 [57] were useful (Fig. 1). The former phosphine complexes 67 gave the aziridine from two cy-cloalkenes with 19-83% ee [56]. On the other hand, terminal alkenes selectively underwent aziridination in the presence of the latter carbonyl complex 68 with 87-95% ee [57]. In these examples, N-tosyliminophenyliodinane or N-tosyl azide were used as nitrene sources. Quite recently, catalytic intramolecular ami-dation of saturated C-H bonds was achieved by the use of a ruthenium(por-phyrin) complex (Eq. 31) [58]. In the presence of the ruthenium catalyst and 2 equiv iodosobenzene diacetate, sulfamate esters 69 were converted into cyclic sulfamidates 70 in moderate-to-good yields. 

As shown in Equation (30), furan reacted with ethyl acetylenecarboxylate under gold-catalyzed conditions to form the hydroarylation product that contained a (Z)-alkene selectively <2003EJO3485>. 

Reaction with allylic nitro compounds. All. lie nitro compounds such as 1 undergo Sn2 substitution with lithium dialkyl cuprates to form (E)-trisubstitutcd alkenes selectively. 

Since the rate of oxidation of terminal alkenes is much higher than that of internal alkenes, selective oxidation of terminal alkenic bonds is possible without attacking the internal alkenic bonds in various dienes (equation 7)... 

Pascual P, lingerer P, Tavitian B, and Boutin A. Development of a transferable guest-host force field for adsorption of hydrocarbons in zeolites. II. Prediction of alkenes adsorption and alkane/alkene selectivity in silicalite. J Phys Chem B 2004 108 393-398. 

Palladium-catalyzed reaction of conjugated dienes in the presence of a halide anion can be controlled to give a l-acyloxy-4-halo-2-alkene selectively under appropriate reaction conditions. The catalyst for this system is a palladium(II) salt, usually Pd(OAc)2 or Li2PdCl4. The reaction may be intermolecular or intramolecular. In most cases, it is stereoselective and results in a 1,4-cis addition to the diene. The products obtained from such reactions are useful synthetic intermediates since they have two allylic leaving groups with a large difference in reactivity (see below, under Synthetic applications ). 

Both Wittig reagents have carbanions that are further stabilized by carbonyl groups and should -alkenes selectively. The first example reacts with the aldehyde rather than with the ester to give trans enal in good yield. 

All of the common hydrogenation catalysts can effect the complete saturation of the alkyne to alkane, but all catalysts are not equally effective in the selective hydrogenation to produce alkenes. Selectivities for cis 2-pentene formation from 2-pentyne decreased in the order Pd > Rh > Pt > Ru > Ir. Palladium is the most selective of the noble metal catalysts for alkyne semihydrogenation with respect... 

---