Catalysts conformation-switch

Scheme 14 Yamada s chiral conformation-switch catalyst applied to the KR of aryl alkyl sec-alcohols [136, 144]...

Poly[N-methyl-N -(2-methyl-6-isopropylphenyl)carbodiimide] maintains its chiral conformation even at elevated temperatures, and poly[-N-dodecyl-N -l-naphthylcarbodiimide] displays birefringent cholesteric mesophases. Stable helical polyguanidines are obtained from the same carbodiimide using chiral titanium catalysts in the polymerization. Also, using a chiral titanium catalyst chirooptical switching polyguanidines are obtained from N-(l-anthranyl)-N -octadecylcarbodiimide. ... 

At approximately 30 °C a break was found in the Eyring plot (see to the right). This break was interpreted as indicating a conformational change to an inactive form of the enzyme below 30 °C, thereby requiring isomerization to the active form for catalysis. The required isomerization imposes the thermodynamics of the isomerization on the observed rate constant. This creates a mechanism change, where the new mechanism pos.sesses a conformational switch in the catalyst prior to the catalytic steps. 

Water may play other roles for instance, in the conformational switch from one turn conformation to another with the opposite twist in the peptide catalyst 3, causing a reversal in enantioselectivity upon addition of water (up to 50%) to DMSO (Scheme 24.3) [21]. Such a switch of enantioselectivity of a given catalyst by changes of solvent is unusual, but a few reports have been described [22]. 

In 2006, Stephenson s group reported the involvement of a DKR process during an intramolecular asymmetric Heck reaction carried out in the presence of a BINAP-based Pd catalyst. It was shown that chiral helical conformations of the starting 2-iodoanilide interconverted through internal bond rotations, leading to the proposal of a DKR mechanism to account for the switch of the enantioselectivity (Scheme 2.61). 

In Special cases, this type of catalyst may oscillate, switching between the two stereoselective C2-symmetric racemic-like conformations and a nonstereoselective meso-Yikt conformation during the propagation of a single polymer chain. The result is a stereoblock polymer microstructure (see Chapter 8). The most-studied catalyst precursor of this type is the unbridged complex bis(2-phenylindenyl)zirconium dichloride (vide The oscillating stereocontrol mech-... 

Therefore, assuming that conformational equilibria similar to those observed for the precursor complexes occur for the active cations as well, Erker et al. proposed that (1) C2-symmetric con-formers can produce enantiomorphic-sites-controlled isotactic polypropylene with rr stereodefects, (2) Cl-symmetric conformers are (much) less stereoselective, and (3) reversible catalyst switches between the different accessible conformations can lead to stereoblock polypropylene microstructures, including the stereoblock-isotactic one (Figure 8.1c). 

Next, asymmetric NH3 on/off switching (on/off = 60/120 s) wifli variation of the temperature (150-400 °C) were conducted. Figure 8.14a-c shows the average NOx conversions for the Fe/ZSM-5, Cu/ZSM-5, and V-W/Ti02 catalysts. In case of the Fe/ZSM-5 catalyst (Fig. 8.14a), the NOx conversions under the periodic condition were higher than those under the steady-state condition below 300 °C, which is in conformity with the result depicted in Fig. 8.13b. Similar enhancement in the NOx reduction in the low temperature region was observed over H-ZSM-5 by Wallin et al. [28]. They have reported that NOx reduction was enhanced by up to five times as compared to a continuous supply of NH3 by changing the NH3 pulse condition at 200-300 °C [28]. 

The complex (13) reacts with H2 affording a dihydridoboratocobalt dihydride (14) in a reversible manner because of the boron ability to switch between a boryl and a dihydroborate conformation. The complex (14) reacts with HMe2N-BH3 to give a hydridoborane cobalt tetrahydridoborate complex (15), which is examined as the catalyst for olefin hydrogenation as well as amine-borane dehydrogenation/transfer hydrogenation. " " ... 

As in these last examples, the descriptions of diastereoselective epoxidations were accompanied already by details on their subsequent use in ring fission reactions, we shall now switch to the various options of controlled regioselective and diastereoselective epoxide transformations. The nucleophilic opening of epoxides shows a strong dependence on substrate structure, nature of the nucleophile, and the catalyst involved, similarly to nucleophilic substitutions. Lastly, one has to consider the solvent and other details of the reaction conditions including the transition state conformation. 

Conformational rigidity and flexibility are two key features for the development of an efficient chiral catalyst. Ma and coworkers developed a new generation of chiral dinuclear phase-transfer catalysts 47 by connecting two structurally rigid BINOL-derived chiral N-spiro quaternary ammonium salts with a flexible linker [80]. These catalysts were proven to act via dual activation of both nucleophiles and electrophiles and to be very efficient catalysts for the conjugate addition of hindered nitroalkanes to enones. Notably, a completely reversed enantioselectivity could be easily switched with catalysts (S,S)-47a and (S,S)-47b, which differ in the length of the methylene chain of the linker but not in the chiral element of backbone. 

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