Catalysis via

Lewis-Base Catalysis via Intermediate Formation of a Chiral Zwitterionic Enolate... 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

The use of mesitoic acid esters has again been successfully employed by Burrows and Topping (1975) in the elucidation of intramolecular carbon acid participation. Under basic aqueous conditions, 2-acetylphenyl mesitoate [41] hydrolyses to yield mesitoic acid and 2-hydroxyacetophenone, reacting with intramolecular catalysis via the monoanion of the ketonic hydrate (see p. 192). However, in 47.5% aqueous ethanol containing potassium hydroxide, the reaction products from l-acetyl-2-naphthyl mesitoate [45] were found... 

Practical systems based on Eq. (66), but using Ni(COD)2/RNC mixtures, were reported. Catalysis via Ni(CNR)3 or Ni(CNR)2 intermediates could not be completely ruled out in these cluster systems. The nickel isonitrile and acetylene clusters did not effect hydrogenation of the triple bond in nitrogen (394). 

Probably the most interesting aspect of catalysis via surface organometallic chemistry is the fact that if the system is well defined it may be possible to follow the various steps of the catalytic cycle, understand deactivation, increase activity and/or selectivity by changing the ligand environment of the active site. We review here some of our recent catalytic results obtained on oxides and on metals. 

The majority of the Michael-type conjugate additions are promoted by amine-based catalysts and proceed via an enamine or iminium intermediate species. Subsequently, Jprgensen et al. [43] explored the aza-Michael addition of hydra-zones to cyclic enones catalyzed by Cinchona alkaloids. Although the reaction proceeds under pyrrolidine catalysis via iminium activation of the enone, and also with NEtj via hydrazone activation, both methods do not confer enantioselectivity to the reaction. Under a Cinchona alkaloid screen, quinine 3 was identified as an effective aza-Michael catalyst to give 92% yield and 1 3.5 er (Scheme 4). 

Like cAMP, 3 -5 -cGMP is widespread as an intracellular messenger substance. Analogous to cAMP, cGMP is formed by catalysis via guanylyl cyclase from GTP (review Lohmaim, 1997). 

With 5-33.3 vol.% water/acetone mixtures, it is found136 that common-ion salts have no effect on the rate of hydrolysis of benzoyl chloride whereas the rate in 15% (but not 33.3%) water is increased by the addition of neutral salts such as lithium bromide or potassium nitrate. The increase in ionic strength on the addition of neutral salts is not the major reason for the increase in rate and nucleophilic catalysis via the more easily hydrolysed benzoyl bromide was postulated. 

ASYMMETRIC CATALYSIS VIA CHIRAL METAL COMPLEXES SELECTED EXAMPLES... 

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