Iminium ions mechanistic studies

This reaction is believed to proceed via nucleophilic combination of in situ generated Cu-acetylide and iminium ion. Mechanistic studies indicate a strong positive non-linear effect based on which a catalytic cycle is proposed that involves a dimeric Cu/quinap complex as the active catalytic species. 

A variety of double bonds give reactions corresponding to the pattern of the ene reaction. Those that have been studied from a mechanistic and synthetic perspective include alkenes, aldehydes and ketones, imines and iminium ions, triazoline-2,5-diones, nitroso compounds, and singlet oxygen, 10=0. After a mechanistic overview of the reaction, we concentrate on the carbon-carbon bond-forming reactions. The important and well-studied reaction with 10=0 is discussed in Section 12.3.2. 

Diphenylpyrrolidine (77) catalyses the enantioselective cy-chlorination of aldehydes.299 Mechanistic and computational studies suggest that - in contrast to pre- viously proposed mechanisms involving direct formation of the carbon-electrophile bond - iV-chlorination occurs first, followed by a 1,3-sigmatropic shift of chlorine to the enamine carbon. The product iminium ion is then hydrolysed in the ratedetermining step. 

Mechanistic details involved in imine and carbonyP - reductions are undoubtedly similar, although thorough investigations of the former are lacking. Certainly, hydride transfer to the electrophilic carbon, with or without prior activation by protonation or complexation is essential for both types of ir-systems (Scheme 1). Whether or not alcohol solvents participate in imine reductions by borohydride (in the absence of added acid) to furnish the amine proton (as is the case with carbonyls) is not known and must await detailed kinetic study and analysis of the initial intermediates formed before hydrolysis. Direct, in situ, reductive amination with NaBHsCN has been attributed to initial, reversible formation (via an intermediate hydroxyamine, (1) of an iminium ion (2) from carbonyls and amines followed by rapid attack by hydride (Scheme 2). However, the inermess of an imine (partial structure 3) to the usual reductive... 

Although this review has focused on muscle aldolase intensively because it is the most well studied enzyme of this kind, it is only one of a variety of aldolases catalyzing similar reactions using an iminium ion pathway. Many of the mechanistic features of these enzymes, including retention mechanism, borohydride reaction etc., are similar [60-63]. 

Based on previous studies where the imines were reduced with Hantzsch dihydropyridines in the presence of achiral Lewis [43] or Brpnsted acid catalysts, [44] joined to the capacity of phosphoric acids to activate imines (for reviews about chiral phosphoric acid catalysis, see [45-58]), the authors proposed a reasonable catalytic cycle to explain the course of the reaction (Scheme 3) [41]. A first protonation of the ketimine with the chiral Brpnsted acid catalyst would initiate the cycle. The resulting chiral iminium ion pair A would react with the Hantzsch ester lb giving an enantiomerically enriched amine product and the protonated pyridine salt B (Scheme 3). The catalyst is finally recovered and the byproduct 11 is obtained in the last step. Later, other research groups also supported this mechanism (for mechanistic studies of this reaction, see [59-61]). 

FeClj catalyses the carbonylation-arylation of A -arylacrylamides with aldehydes using TBHP oxidant in CsHsCl to afford oxindoles in 90% yield. Similar catalytic activity is shown by FeCl2 and Fe(OAc)2. Oxindoles can be further transformed to give diverse indole alkaloids. The FeCl2-catalysed oxidative amidation of tertiary amines with aldehydes using TBHP gives amides in moderate to excellent yields. Aromatic aldehydes are more reactive than aliphatic aldehydes. The mechanistic studies indicate that a peroxide and an iminium ion are the reactive intermediates. " ... 

The aim of this study was to gain insight into the feasibility of electron transfer (ET) in vivo by examining the electrochemical characteristics of two categories of anticancer agents quinones (diospyrin and 2,3-bis(bromomethyl)l,4 naphthoquinone) and conjugated quinolinium (iminium) ions from Dup 785 and camptothecin. Mechanistic ramifications are discussed. 

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