Aldimines, reductive coupling

Reductive coupling of aldimines obtained from aromatic aldehydes and aromatic amines to generate vicinal diamines mediated by indium was carried out in aqueous ethanol (Eq. 11.59)." Small indium rods were used in this study. No side-product was observed due to unimolecular reduction. The presence of NH4CI was found to accelerate the reaction. The reaction fails completely in CH3CN, DMF, or wet DMF. The use of nonaromatic substrates also resulted in the failure of the reaction. 

More recently, Tamaru and co-workers reported nickel-catalyzed reductive coupling of dienes with in situ-generated aldimines from aldehydes and amines (Equation (88)).446,446a The reaction exhibits high regio- and diastereoselectivity. 

For use of conjugated enones as nucleophilic partners in catalytic intermolecular reductive couplings to aldimines, see ... 

Reductive coupling of aromatic aldimines leading to vicinal diamines proceeds with SmU, and is strongly accelerated by addition of a catalytic amount of or a stoichiometric amount of Yb(OTf)3. In the former... 

Reductive coupling of imines.2 Aromatic ketimines are reduced to the corresponding secondary amines by Sml2, but aromatic aldimines couple to 1,2-diamines (equation II). 

Coupling of aldimines.1 The Ti(II) reagent obtained from TiCl4 and Mg effects reductive coupling of aryl aldimines to 1,2-diarylethylenediamines as a 1 1 mixture of meso- and /-isomers. 

In the presence of metallic zinc and a catalytic amount of BiCl3, the reductive coupling between a-diketones and aldehydes takes place to yield a,/3-dihydroxyketones with moderate //-selectivity (Equation (48)).83 The reaction proceeds even in aqueous media. Phenylglyoxal and methylglyoxal can be converted to the corresponding coupling products. The reductive coupling of aldimines to vicinal diamines can also be accomplished by the action of Bi-KOH in methanol at room temperature.84... 

Isomerization of alkenes is catalyzed by bis(j7 -allyl)(jj -cyclopentadienyl)vanadium(IV). Cyclohexene nndergoes catalytic stereoselective aerobic oxidation with CpV(CO)4. Reductive coupling (see Reductive Coupling) of aldehydes and aldimines is catalyzed by a Cp-substituted vanadium compound in the presence of a chlorosilane and zinc or aluminum. Vanadocene or bis( ] -cyclopentadienyl)dimethylvanadium(IV) catalyzes the synthesis of 1,2-diphenyldisilane and 1,2,3-triphenyltrisilane from phenylsilane. ... 

The addition of organometallic agents to aldimines and ketimines provides a useful route to substituted amines, although this reaction is sensitive to imine/organometallic substitution. Along with addition, competitive enolization, reduction and bimolecular reduction (coupling) reactions are also possible. 

Reductive couplings. Aldehydes and aldimines undergo reductive coupling in the presence of a chlorotriorganosilane to give vicinal diols and diamines, respectively. However, quite different stereoselectivities are observed. Cyclization is also observed for unsaturated aldehydes which fulfill certain structural requirements. ... 

Reductive couplings. Claims for simple reduction and reductive coupling of aldimines by Zn in aqueous NaOH under rather similar conditions have been recorded. Using MejSiCl in MeCN to trap the ionic site of radical anion intermediates the coupling process is thereby favored. Although nearly equal amounts of dl and meso-dimers are generated, the method is practical owing to the ease of their separation (very different solubility in certain solvents). 

Reductive Coupling Reaction of Aldimines (13 and 15) into Vicinal Diamines (14 and 16)... 

Another example of reductive coupling is the reaction of aldimines promoted by indium in aqueous ethanol [82]. The reaction fails in MeCN and DMF but is accelerated by NH4CI ... 

The Cp2VCl2/R3SiCl/Zn catalytic system can be also applicable to the reductive coupling of aldimine (Scheme 2.24) [72], mew-Diamine 33 is obtained as a major product. The diastereoselectivity depends on the substituents on both the nitrogen and silane atoms. The allyl or benzyl group on the nitrogen atom is advantageous for me so selection. 

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. 

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]. 

To avoid secondary reductive steps, low concentrations of the active low-valent metal are advantageous, i.e. the use of catalytic one-electron reduction cycles [31]. Low-valent vanadium species are efficient catalysts for pinacolization. After the primary electron transfer, the oxidized vanadium catalyst can be reduced by Zn(0) or by aluminum [32], TrialkyIsilanes have to be added to decomplex the oxidized vanadium catalyst from the pinacol [33]. The same catalytic system can also be used for the synthesis of 1,2-diamines via coupling of aldimines [34]. Alternatively, a titanium (Il)-samarium system gives the 1,2-diamines with moderate D,L-selectivity [35]. 

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