As nucleophiles, simple alkenes are typically so unreactive that only highly active electrophiles, such as carbocations, peroxides, and halogens will react with them. For the generation of ...carbon-carbon bonds, milder methods will often be required. Fortunately, it is possible to increase the reactivity of alkene-type p-nucleophiles by introducing electron-donating substituents. Substitution of one H with an OH or OR gives an enol or a vinyl ether, which are already much better nucleophiles. Using nitrogen instead of oxygen, one obtains even better nucleophiles, enamines. Enamines are among the most reactive neutral carbon nucleophiles, exhibiting rates that are even comparable to some charged nucleophiles, such as enolates 1, 2. Most enamines, unfortunately, are sensitive to hydrolysis. The parent enamine, N,N-dimethylvinylamine, has in fact been prepared 3, but appears to be uns- ble. Enamines of cyclic ketones and many aldehydes can readily be isolated, however 4-7. The instability of enamines might at first appear to diminish the utility of enamines as nucleophiles, but actually this property can be viewed as an added benefit: enamines can be readily and rapidly generated catalytically by using a suitable amine and a carbonyl compound. The condensation of aldehydes or ketones with amines initially affords an imine or iminium ion, which then rapidly loses a proton to afford the corresponding enamine (Scheme 1).
This book follows the 2002 edition of Catalysis by Ceria and Related Materials, which was the first book entirely devoted to ceria and its catalytic properties. In the ten years since the first ...edition a massive amount of work has been carried out in the field, and ceria has gained a prominent position in catalysis as one of the most valuable material for several applications. This second edition covers fundamental and applied aspects of the latest advances in ceria-based materials with a special focus on structural, redox and catalytic features. Special emphasis is given to nano-engineered and nano-shaped systems which are a key factor in the predictive and rational design of ceria with novel properties.In addition, the book presents recent advances in emerging and traditional large-scale applications of ceria in catalysis, such as the treatment of emissions from mobile sources (including diesel and gasoline engines). The primary readership includes catalysis and material science researchers from academy and industry and postdoctorate and graduate students in chemistry, chemical engineering and physics.
S ciljem poboljšanja karakteristika katalizatora na bazi kroma, u ovom su se istraživanju kombinirano primijenile poluempirijske računalne metode i metode temeljene na teoriji funkcionala gustoće. ...Nastojao se dobiti uvid u katalitički potencijal izmjene kroma molibdenom i volframom u katalizatoru CrXO3 (gdje X označava krom, molibden i volfram) za dehidrogeniranje propana pri proizvodnji propilena. Rezultati su potvrdili da površina modificirana molibdenom pokazuje bolju selektivnosti u usporedbi s površinom modificiranom volframom, usporavajući dehidrogenaciju, pucanje i koksiranje propilena. Utvrđeno je da upotreba molibdena olakšava proizvodnju propilena zbog njegova nižeg afiniteta prema adsorbatima koji potiču pucanje i koksiranje, uključujući i lakšu desorpciju propilena te višu barijeru potpune dehidrogenacije što sprječava nastanak neželjenih produkata; to nije bio slučaj kod volframa. Ovo istraživanje stoga preporučuje upotrebu molibdena za poboljšanje svojstava katalizatora, što bi moglo rezultirati većim iskorištenjem u proizvodnji propilena te pomoći zadovoljavanju rastuće tržišne potražnje za propilenom.
