Reductive couphng

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

The reductive couphng of the chiral planar di(imine) 42, prepared from the corresponding chiral (il)-dialdehyde, in turn available as a single enantiomer by standard reactions, can lead to four stereoisomers. The use of... 

In the future, further studies should be addressed to improve the chemose-lectivity and diastereoselectivity of the reductive coupling process, especially searching for novel reagents and milder experimental conditions. As a matter of fact, a few novel reductive couphng procedures which showed improved efficiency and/or stereoselectivity have not been further apphed to optically active imines. For example, a new electrochemical procedure which makes use of the spatially addressable electrolysis platform with a stainless steel cathode and a sacrificial aluminum anode has been developed for imines derived from aromatic aldehydes, and the use of the N-benzhydryl substituent allowed 1,2-diamines to be obtained with good yields and dl-to-meso ratios... 

The [Ir(cod)2]BARF complex also showed high catalytic activity in the hydrogena-tive coupling of alkyne with aldimines to lead to reductive couphng products, aUyl amines [69]. 

Procter [23] has reported the intermolecular, reductive couphng of aldehydes and ketones with o ,/ -imsaturated esters, immobihzed using an ephedrine chiral resin, to give enantiomerically enriched y-butyrolactones. 

Reductive cycHzations have been shown to be diastereoselective in the synthesis of bicyclic products as well as various macrocycles. There are also instances of chirahty transfer in intermolecular reductive couphngs. The synthesis of anti-1,2-diols has been demonstrated using a-alkoxyaldehydes with a methoxymethyl ether (MOM) protecting group and mono-aryl internal alkynes (Scheme 8.25) [49]. Dias-tereoselectivities are high for the formation of anfi-l,2-diols in cases where the aldehyde has a branched sp -P-carbon. 

The Cp2VCl2/R3SiCl/Zn catalytic system can be used for the reductive couphng of the imines 29 (Scheme 18) [55]. These components of the ternary catalyst are essential and, interestingly, meso-diastereoselectivity is observed in contrast to the coupling with cat. Cp2TiCl2/Sm system. The selectivity de-... 

Reductive elimination reactions are responsible for the final C-C/C-X bond formation step in catalytic processes. In principle, C-bound or any other anionic pincer ligands could engage in reductive couphng reactions, bringing about the decomposition of the pincer complex. In order to enable catalysis by such complexes, it is mandatory to avoid reductive elimination reactions involving the pincer ligand itself Fortunately, this seems to be usually the case. For example, reductive elimination of ethane from the Pd(IV) pincer complex 18 (Scheme 2.3) is preferred... 

Geary LM, Leung JC, Krische MJ (2012) Ruthenium-catalyzed reductive couphng of 1,3-enynes and aldehydes via transfer hydrogenation onrf-diastereoselective carbonyl propargylation. Chem Eur J 18 16823-16827... 

Scheme 2.39 Sm metal-induced reductive couphng of acetophenone in the presence of McsSiQ...

A related direction concerns the elaboration of coupled systems, where one substrate is oxidized into an added-value product, while another one is reduced, also giving a product with added value. As an example, coupled systems for selective oxidation of aromatic alcohols to aldehydes and reduction of nitrobenzene into anihne can be mentioned. This oxidation-reduction couphng was realized using a cadmium-based composite material as a photocatalyst under visible-hght illumination, giving, in one-pot, 45% and 26% yields of benzaldehyde and anihne, respectively. 

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