The Manganese‐Catalyzed Cross‐Coupling Reaction and the Influence of Trace Metals

Abstract

The substrate scope of the MnCl2‐catalyzed cross‐coupling between aryl halides and Grignard reagents has been extended to several methyl‐substituted aryl iodides by performing the reaction at elevated temperature in a microwave oven. A radical clock experiment revealed the presence of an aryl radical as an intermediate leading to the proposal of an SRN1 pathway for the coupling. The mechanistic information gave rise to suspicion about two previously published cross‐coupling reactions catalyzed by manganese(II) salts. As a result, the coupling between aryl halides and organostannanes as well as between aryl halides and amines were revisited. Both reactions were found impossible to reproduce without the addition of small amounts of palladium or copper and are therefore not believed to be catalyzed by manganese.

Development and mechanistic investigation of the manganese(iii) salen-catalyzed dehydrogenation of alcohols

“…Essentially no imine was formed when the reaction was conducted in the absence of complex 2 . The possibility for trace metal impurities is a serious concern in the development of reactions with new metal catalysts18 and studies have shown that some coupling reactions with manganese catalysts are most likely mediated by traces of other elements 19. For that reason, complex 2 was analyzed by inductively coupled plasma mass spectrometry (ICP-MS) for traces of other metals known to perform alcohol dehydrogenations.…”

Eco-friendly methoximation of aromatic aldehydes and ketones using MnCl 2 .4H 2 O as an easily accessible and efficient catalyst

“…This mild and moisture compatible Lewis acid has been found to promote a handful of relevant transformations, such as mesylate to chloride conversion [44], esterification and transesterification reactions [45,46], Michael addition [47], glucose to fructose isomerization [48], condensation of carbonyls with amine derivatives [49], carbonyl reduction [50] and the reductive amination of ketones [51]. The salt has also been employed as a catalyst for cross couplings leading to C-X (X=N, S) [52,53] or C-C bond formation [54,55], and in C-H activation reactions resulting in further C-C bond formation [56][57][58][59]. MnCl 2 .4H 2 O was also supported on montmorillonite K-10, and used in the catalytic synthesis of benzaldehyde derivatives, by the H 2 O 2 -mediated oxidation of the corresponding alcohols [60].…”

Zinc Oxide‐Catalyzed Dehydrogenation of Primary Alcohols into Carboxylic Acids

“…Zinc oxide was submitted to trace metal analysis by inductively coupled plasma mass spectrometry (ICP‐MS) to exclude other elements known to perform alcohol dehydrogenation . Fortunately, none of these metals were observed beyond their detection limits of typically 1 ppm and the transformation is therefore not believed to be catalyzed by trace metal residues in zinc oxide.…”