Aryl aldehydes, reductive coupling reaction

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]. 

Nevertheless, it must be pointed out that the formation of such transient species has never been spectroscopically observed. Native CDs are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols, epoxydation,or oxidation " of olefins, reduction of a,/ -unsaturated acids,a-keto ester,conjugated dienes,or aryl alkyl ketones.Interestingly, chemically modified CDs like the partially 0-methylated CDs show a better catalytic activity than native CDs in numerous reactions such as the Wacker oxidation,hydrogenation of aldehydes,Suzuki cross-coupling reaction, hydroformylation, " or hydrocarboxylation of olefins. Methylated /3-CDs were also used successfully to perform substrate-selective reactions in a two-phase system. 

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]. 

The radicals R" generated by reduction of the halides RX (R = alkyl or aryl X = Cl, Br, I, etc.) by Sml2 (see Chap. 12.4.2) can be reduced (formation of RH by trapping an H atom from the solvent) or couple onto a carbonyl (aldehydes, ketones Barbier reaction), an olefin or an alkyne. For coupling reactions, the competitive reduction to RH indicated above must be avoided. The intramolecular coupling with formation of 5- or 6-atom rings, of synthetic interest, are promoted by the entropic factor. ... 

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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