Activation, -Isomer selective

A series of 2-(4-chlorophenoxy)-3-oxoalkanoate were reduced by baker s yeast, and Kluyveromyces marxianus. Yeast reduction of ethyl 2-(4-chlorophenoxy)-3-oxo-3-phenylpropanoate (R=Ph) afforded enantiomerically pure ethyl 2R, 3S)-2-(4-chlorophenoxy)-3-hydroxy-3-phenylpropanoate out of the four possible stereoisomers in >99% de [29h]. Although baker s yeast reduction of butanoate (R = CFl3) was not selective (92% de), the use of K. marxianus afforded (2R, 3S)-isomer selectively [29i]. The products are intermediates for potential peroxisome prolifera-tor-activated receptor isoform a-agonists (Figure 8.39a). 

The aza-bis(oxazoline) 14, bearing sterically hindering groups, led to very good results in terms of activity and selectivity, comparable to those obtained from corresponding aza-semicorins or bis(oxazolines). For the enantioselec-tive cyclopropanation of styrene, the trans isomer was obtained in 92% ee... 

The percentage of linear product increases greatly by replacing CO with the much bulkier phosphine ligands. Due to the increased steric hindrance the catalyst shows a distinct preference for the n- over the tw-isomer. Tkatchenko (1991) has reported a detailed analysis of this system in terms of the correlation between catalyst performance (activity and selectivity) and detailed structure. 

However, (Ph3P)2Rh(CO)Cl on alumina or activated carbon were effective hydroformylation catalysts under more severe conditions 108). At 148°C and a pressure of 49 atm (CO 37.5 mol%, H2 37.5, propylene 25), good activity was found. The propylene conversion was 30% at a contact time of 0.92 cm3 of reactor void space/cm3 of feed per minute. Isomer ratios of 1.3 to 1.9 1 n iso were realized. By-product formation was low, with <1% conversion to alcohols plus alkanes and 2.2% high-boiling materials. This system was stable for a 300 hour operating time, with no detectable loss of activity or selectivity. 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

Octadecane hydroprocessing behavior of Pt-containing bifunctional catalysts with TON and MTT framework types was compared, as illustrated in Figure 13.31 [28]. While the two zeolitic catalysts showed similar activities, the selectivity vs conversion performances were different. At any given conversion, the selectivity to dibranched isomers was lower and the selectivity to mono-branched isomers... 

Another class of ligands for ATH is represented by multidentate Schiff bases and their derivatives. Zassinovich and Mestroni reported on the effective reduction of alkyl aryl ketones catalyzed by a series of lr(l) complexes with chiral bidentate pyridylaldimines, of the form [lr(cod)(NNR )]C104 (76a-f see Scheme 4.31). It was observed that both the activity and selectivity depended heavily on the nature of the subshtuents at the chiral center of the ligand, and also at the prochiral center of the substrate. Optical yields of up to 50% (R-isomer) at 100% conversion were obtained in the ATH of BuC(0)Ph and PhCH2C(0)Ph using [lr(cod)(PPEl)]C104 as the precatalyst (0.1% mol, 83 °C, PrOH, KOH) [66]. 

The molar ratio of the phosphorus ligand to rhodium has pronounced effects on catalyst activity and selectivity. It is well established that increasing the molar ratio of the ligand to the rhodium leads to a higher linear to branched isomer ratio at the... 

Different isomers may have different rates of degradation in the environment which would change the overall risk. For example, in the case of cypermethrin, the trans-isomers were preferentially degraded over the cis-isomers. Of the two active isomers of this insecticide, the (lR)-ds-(aS)-isomer was degraded more slowly than (1 R)-tmns-(u. S)-i sorn er by bacteria and in whole sediments [150]. By selecting for the isomer (or process that produces a majority of that isomer) with the more rapid degradation profile, the ecological risk should be reduced. 

Bell et al. (81) presented forced diffusion calculations of butene isomers in the zeolite DAF-1. DAF-1 (82) is a MeALPO comprising two different channel systems, both bounded by 12-rings. The first of these is unidimensional with periodic supercages, while the other is three-dimensional and linked by double 10-rings. The two channel systems are linked together by small 8-ring pores. It is a particularly useful catalyst for the isomerization of but-l-ene to isobutylene (S3) its activity and selectivity are greater than those of ferrierite, theta-1, or ZSM-5. 

Mitsubishi Kasei introduced a process to manufacture isononyl alcohol, an important PVC (polyvinyl chloride) plasticizer, via the hydroformylation of octenes (a mixture of isomers produced by dimerization of the C4 cut of naphtha cracker or FCC processes).95 First a nonmodified rhodium complex exhibiting high activity and selectivity in the formation of the branched aldehyde is used. After the oxo reaction, before separation of the catalyst, triphenylphosphine is added to the reaction mixture and the recovered rhodium-triphenylphosphine is oxidized under controlled conditions. The resulting rhodium-triphenylphosphine oxide with an activity and selectivity similar to those of the original complex, is recycled and used again to produce isononanal. 

Traditionally, solid acidic catalysts are applied in industry for the oligomerization of butenes and are still studied. MTS-type aluminosilicates,522 a NiCsNaY zeolite,523 and a silica-alumina containing 13% alumina524 proved to be active and selective catalysts. Moreover, deactivation rates of these catalysts are also favorable. Sulfated zirconia promoted with Fe and Mn was active and selective to yield primarily dimethylbutene isomers under supercritical conditions.525 A small amount of water improved productivity and decreased deactivation. A study showed that the blending octane number of Cg hydrocarbons is directly linked to the number of allylic hydrogens in the molecules.526... 

Similar correlations between the acid-base properties of catalysts and activ-ity/selectivity were earlier observed in the rearrangement of simple oxiranes (refs. 5-8). In our case it seems reasonable to suppose that the observed changes are due to the different competing mechanisms discussed above. WO, with strong acidic sites in high concentration, is able to form the carbenium ion. Since the density and the strength of the basic sites on WO are low, formation of the double-bonded surface species depicted in Fig. 3 has only a low probability. The single-bonded open carbenium ion is then mainly transformed to ketone 3. In harmony with this, the isomers exhibit identical selectivity, a... 

The anticancer activity of our racemic p-lactams has prompted us to devise method for the preparation of the optically active analogues. It is known that an optically active isomer of a racemic compound has better and much selective biological... 

Considering the available technologies for isomerization, it would be interesting to develop a new generation of environmentally friendly catalysts with high activity and selectivity to branched isomers that can work at lower reaction temperatures than zeolites and show an increased resistance to sulfur and water. 

In either the presence or absence of GTP, half-maximal stimulation of enzyme activity is achieved with 3 uM dopamine. Both 6,7-ADTM and epinine (K-methyl dopamine) stimulate adenylate cyclase activity to the same degree as does dopamine (Figure 8). In contrast, apomorphine is a partial agonist eliciting only 30 of the maximal effect of dopamine. The dopamine-stimulated adenylate cyclase activity is selectively blocked by cis-flupenthixol rather than the trans-isomer of this antagonist (JJL). Among the antagonists tested, the order of potency is cis-flupenthixol = fluphenazine > chlorpromazine > haloperidol > trans-flupenthixol (Table I). 

Davies and co-workers have explored the role of ligand conformation in the ruthenium(II)-catalyzed cyclopropanation of styrene.10 This study was based on results reported by Nishiyama in which the catalyst prepared in situ from pyridine-bis(oxazoline) 62 and RuCI2(/>cymene) 2 was found to be highly active and selective in the reaction of ethyl diazoacetate with styrene (66% yield, 84% de, and 89% ee of major trans-isomer).52 Several ligands hindered on the oxazoline ring, including 3, were tested and poorer yields and selectivities were obtained (for 3, 50% yield, 81% de, and 59.5% ee of major trans-isomer), which indicated unfavorable steric interactions between styrene and the Ru(in-pybox) carbene complex (Scheme 17.22).10... 

HMordenite, HFaujasite-780, HFaujasite 720 and Na-Faujasite zeolites. Among the different catalysts, HFaujasite-720 was the most active and selective catalyst towards 2,4-dinitrotoluene, achieving a yield of dinitrotoluenes of 92 % with a ratio of 2,4- to 2,6- isomers of 4.3 1 in 3 min reaction time. Using this zeolite, l-chloro-2-nitrobenzene and pyrazole were also nitrated regioselectively to obtain l-chloro-2,4-dinitrobenzene in a l-chloro-2,4-dinitro l-chloro-2,6-dinitro ratio of 30 1, and 1,4-dinitropyrazole in 80% yield, respectively. The authors proposed a nitration mechanism in which the protons in the zeolite are replaced by nitronium ions derived from N2Os in a fast pre-equilibrium process. This produces active sites for transfer of nitronium ion from faujasite to aromatic in the rate-controlling step. 

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