Aryl aldehydes, reductive coupling

Under the conditions of iridium-catalyzed transfer hydrogenation employing isopropanol as reductant, 1,3-cyclohexadiene couples to aryl aldehydes to provide... 

To date, no report has appeared using manganese metal. We wish to report a mild method for reductive coupling reactions of aryl aldehydes, aryl ketones, andaldimines [100]. 

As in all C-C cross-coupling reactions, the Negishi reaction mechanism consists in three steps (Fig.4.1) oxidative addition, transmetalation, and reductive elimination. The former and the latter are common to all the other cross-coupling reactions, whereas the transmetalation step is particular of this reaction. Unfortunately, this transmetalation has been less studied compared to the ones in the Stille [12-17] or Suzuki reactions, [18-21] in spite of the fact that the transmetalation between organozinc and palladium complexes is also involved in other relevant processes, such as the hydroalkylation of styrenes, [22] the asymmetric allylation of aryl aldehydes, [23] the coupling propargylic benzoates and aldehydes, [24] or the double-transmetalation oxidative cross-coupling reaction [25, 26]. 

Indium chloride is known to have little water sensitivity and this has led to the discovery of a novel multicomponent synthesis of monocyclic 1,4-diazepines from aldehyde, amine, and a,p-xmsaturated ketone [89]. The initial step was the bimolecular reductive coupling of the aldimine formed in situ leading to the formation of N, N -diphenyl-l,2-diaryl-l,2-diamino ethane, which underwent aza-Michael addition to the a,p-xmsaturated ketone. The product 129 (Scheme 26) was isolated by simple recrystallization and obtained in very good yield witir excellent diastereose-lectivity favoring the trans-isomer. In the dimerization of tire radical Zn anion of aldimine, the repulsion of the lone pair on nitrogen and steric hindrance between the aryl groups seemed to have contributed toward frans-selectivity. 

This reaction has been extended to the synthesis of vicinal arylamines, and a one-pot reductive coupling of aryl aldehydes and amines has also been reported ... 

For reductive cross-coupling between a,P-unsaturated ketones and aldehydes, Kim et al. made use of In-InCl3 in aqueous media to prepare p,y-unsaturated ketones from methyl vinyl ketone and aryl aldehydes [282] (Figure 8.128). A radical mechanism was proposed, with InCb activating the carbonyl group for cyclopropanation and addition. Uemura also studied the same reaction, with InCl3 (10 mol%) as catalyst, Al as reductant, and requiring the presence of TMSCl [283]. 

Aromatic nitro compounds are often strongly colored. They frequently produce characteristic, colored, quinoid derivatives on reaction with alkali or compounds with reactive methylene groups. Reduction to primary aryl amines followed by diazotization and coupling with phenols yields azo dyestuffs. Aryl amines can also react with aldehydes with formation of Schiff s bases to yield azomethines. 

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