Pyridine catalyst deactivator

This heterogeneous process may constitute an interesting alternative to the classical synthetic route. Catalyst deactivation can be slowed down by deliberately poisoning the acidic surface site with added pyridine in the reactant feed. A feasible operation mode for a continuous heterogeneous process consists of reaction and subsequent reoxidation cycles of the catalyst. 

The deactivation of a lanthanum exchanged zeolite Y catalyst for isopropyl benzene (cumene) cracking was studied using a thermobalance. The kinetics of the main reaction and the coking reaction were determined. The effects of catalyst coke content and poisoning by nitrogen compounds, quinoline, pyridine, and aniline, were evaluated. The Froment-Bischoff approach to modeling catalyst deactivation was used. 

Thus, using Ni(0) and IMes, amines possessing a wide range of functionalities were obtained in yields ranging from 65 to 99%. No hydrogenation was observed with cyano- and pyridine-substituted imines and is likely due to ligand displacement and subsequent catalyst deactivation. 

The steric properties of the pyridine substrate are critical. Pyridines which lack ortho substituents form nonlabile, unreactive 18-electron pyri-dyl pyridine complexes 36 [see also Eq. (37)]. In fact, one of the principal catalyst deactivation processes in a-picoline coupling reactions is catalyst poisoning via formation of 36 by trace amounts of 3- and 4-methylpyridine impurities in the a-picoline feed. 

The next breakthrough was obtained when iridium was used instead of rhodium. This idea was inspired by results from Crabtree who had described an extraordinarily active Ir-tricyclohexylphosphine-pyridine catalyst that was able to hydrogenate even tetra-substituted C=C bonds. For the MEA imine hydrogenation very good ee values were obtained with an Ir-bdpp catalyst in the presence of iodide ions (ee 84% at 0°C) but the activity was disappointing. Turnover numbers (ton) of up to 10000 and tof numbers of 250/h (100 bar and 25 °C) but somewhat lower ee values were obtained with Ir-diop-iodide catalysts [10, 11], A major problem with these new Ir-diphosphine catalysts was an irreversible catalyst deactivation. 

During their studies on kinetic resolution (KR) of secondary alcohols, Connon et al. found that chiral pyridine catalyst 177 and its optimized analogue 178 promoted the synthetically useful KR of MBH adducts 179 derived from deactivated precursors (which were difficult to synthesize using catalytic asymmetric MBH reactions), allowing the convenient preparation of 179 in 62-90% ee and 82-97% ee, respectively (Scheme 2.87). This study also represents the first examples of effective non-enzymatic acylative KR of sec-sp -sp ... 

Another example of intramolecular Rh(I)-mediated biaryl coupling has been reported in the efficient cross-coupling between aryl cyanides and aryl chlorides in the presence of hexameth-yldisilane [52], Dibenzofuran, carbazole, and fluorene derivatives are readily obtained, albeit pyridine-containing substrates fail to react under the standard conditions, due to catalyst deactivation (Scheme 22.36). However, addition of catalytic amounts of InCl in place of phosphine ligand dramatically increases the product yield. 

The increased rate of epoxidation observed using pyridine as an additive has been studied by Espenson and Wang and was to a certain degree explained as an accelerated formation of peroxorhenium species in the presence of pyridine [62]. A stabilization of the rhenium-catalyst through pyridine coordination was also detected, although the excess of pyridine required in the protocol unfortunately led to increased catalyst deactivation. As can be seen above, MTO is stable under acidic conditions but at high pH an accelerated decomposition of the catalyst into perrhenate and methanol occurs. The Bronsted basicity of pyridine leads to increased amounts of HO2 which speeds up the formation of the peroxo-complexes and the decomposi-... 

Reaction of bisphenol with chloronitroaromatic compounds was generally performed in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) at reflux using K2C03 as a base.108 109 It is possible to achieve this condensation in Ullmann s conditions by using a cuprous chloride or iodide-pyridine system as a catalyst when this reaction is performed with deactivated aromatic compounds, it gives too poor yields110 ultrasounds can dramatically improve yields without solvent.111... 

The acetylation of carbazole by acetic anhydride in the presence of boron trifluoride produces the 9-acetyl derivative. Further acetylation requires more vigorous conditions, using aluminum trichloride as a catalyst, and yields 2,9-diacetylcarbazole, which, upon base-catalyzed hydrolysis, produces 2-acetylcarbazole (80T3017). Acetylation of 1-phenyl-isoindole under mild conditions in the presence of pyridine yields l-acetyl-3-phenylisoin-dole, whereas the presence of an ester group at the 1-position deactivates the ring sufficiently to prevent acylation (81AHC(29)34l). 

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