Amination reactions catalysis

Regarding the amine-catalyzed rearrangement of the p-nitrophenylhydrazone [81JHC723 83JCS(P2)1203], all the amines catalyze the reaction nevertheless, depending on the nature of the amine, different catalysis laws are observed. Tertiary amines and secondary amines with great... 

The formation of covalent substrate-catalyst adducts might occur, e.g., by single-step Lewis-acid-Lewis-base interaction or by multi-step reactions such as the formation of enamines from aldehydes and secondary amines. The catalysis of aldol reactions by formation of the donor enamine is a striking example of common mechanisms in enzymatic catalysis and organocatalysis - in class-I aldolases lysine provides the catalytically active amine group whereas typical organocatalysts for this purpose are secondary amines, the most simple being proline (Scheme 2.2). 

For amination with aliphatic amines, copper catalysis is normally superior to palladium catalysis, e.g., <2006T4435>. Thus, reactions of primary or secondary amines, including pyrrolidine and morpholine, with 5-bromopyrimidine using copper bromide and a phosphite ligand give 5-aminopyrimidines in excellent yields (Scheme 116) <2006T4435, 2005OL3965>. 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

The action of the tin catalyst was found to be quite different from the action of the tertiary amine catalyst. In the presence of the amine catalyst the reactivity of the phenol and benzyl alcohol was approximately equal (see Table IV). In the case of DBTDL, the reactivity ratio was similar to that of the non-catalyzed reaction, which indicates that the polarization of the isocyanate by the tin catalyst due to complex formation presumably played an important role in the reaction catalysis (see Table VI). 

The complex, C, is converted subsequently to the urea by transfer of an amine hydrogen to the 0 or N of the isocyanate. Similar complexes have been postulated to account for amine catalysis of urethane formation. Hydrogen transfer appears to be a mechanistically diflacult step, as judged by the complex kinetics of the isocyanate-amine reaction. The reaction of phenylisocyanate with aniline, for example, is self-catalyzed by the amine and autocatalyzed by the product urea. This complicated kinetic behavior may result from formation of a six-membered cyclic complex which facilitates hydrogen transfer. 

This overview about developments in the field of proline-catalysis unfortunately cannot take into full account the vast field of proline-derived catalysts, such as diarylprolinols, 4-silo)yprolines or proline-silyl-ether, to name only a few. These are covered in subsequent chapters of this volume. Furthermore, other great improvements have been made by using immobilised proline catalysts, such as PEG-supported proline or polyelectrolyte-bound pro-line. Going one step further, supported proline catalysts are then applicable in the striving field of continuous-flow reactions. Recent examples include aldol, a-amination reactions and Michael reactions under such conditions. ... 

Electrophilic a-amination reactions were also included in organocascade sequences. Jorgensen developed a formation of hydro) - and amino-esters in combination by aminocatalysis using diaiylprolinol silyl ether and a redox reaction followed by acyl transfer to the corresponding esters using NHC catalysis. This useful method did not require an inert atmosphere or anhydrous conditions to form the corresponding products in excellent enantioselectivities and high yield. 

Although the active participation of Pd(IV) intermediates in catalysis is currently well established [21, 22], it has been shown that the catalytic activity of paUadacycles is usually due to Pd nanoparticles or soluble Pd(0) species arising from partial or complete decomposition processes. Hartwig and Louie [23] showed that paUadacycles of type 1 are reduced by amines (in aryl amination reactions) or stan-nanes (in the Stille coupling) to afford Pd(0) phosphine complexes. Furthermore,... 

Another intramolecular synthesis of carbazoles 270 employed amino biaryls 269 as the starting materials with 2-picolinic acid as a directing group to facilitate a net C—H amination reaction under copper catalysis in the presence of Mn02 and acetic acid. The directing group is removed spontaneously after the initial amination (140L2892). 

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