Reduction iminium ions

For Petasis borono-Mannich reactions of formaldehyde, methylation of the amine reportedly occurs as a side-reaction, presumably by iminium ion reduction [19]. The hydride source under these conditions may be formic acid, as in the Eschweiler-Clarke methylation reaction. This problem was overcome by the use of potassium trifluoroborate salts and Lewis acids in toluene at 90 °C (the reaction in highly polar solvents such as acetonitrile, DMF and DM SO gives the reduction product). 

This two-step methylation sequence, involving 2-azanorbornene synthesis and retro Diels-Alder initiated iminium-ion reduction, is applicable to a variety of amino acid derivatives. The protocol is compatible with unprotected phenols and hydroxyl groups. The 2-azanorbornenes derived from the methyl esters of unprotected L-tyrosine and L-serine both undergo the N-... 

Presumably the species that undergoes reduction here is a carbinolamine an iminium ion derived from it or an enamme... 

The reduction of the double bond of an enamine is normally carried out either by catalytic hydrogenation (MS) or by reduction with formic acid (see Section V.H) or sodium borohydride 146,147), both of which involve initial protonation to form the iminium ion followed by hydride addition. Lithium aluminum hydride reduces iminium salts (see Chapter 5), but it does not react with free enamines except when unusual enamines are involved 148). 

Amide reduction occurs by nucleophilic addition of hydride ion to the amicle carbonyl group, followed by expulsion of the oxygen atom as an alumi-nate anion leaving group to give an iminium ion intermediate. The intermediate iminium ion is then further reduced by JL1AIH4 to yield the amine. 

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... 

Rather than preforming the a-amino ketimines to be reduced, it is often advantageous to form in situ the more reactive iminium ions from a-aminoketones and primary amines or ammonium salts in the presence of the reducing agent, e.g., sodium cyanoborohydride. Use of this procedure (reductive amination) with the enantiopure a-aminoketone 214 and benzylamine allowed the preparation of the syn diamines 215 with high yields and (almost) complete diastereoselectivities [100] (Scheme 32). Then, the primary diamines 216 were obtained by routine N-debenzylation. Similarly, the diamine 217 was prepared using ammonium acetate. In... 

The one-step Polonovski rearrangement by Kobayashi, which contains the reduction of a similar iminium ion, also supports this prediction. See [16]. 

Laboratory transformations took advantage of fragmentation of dregamine (30), tabemaemontanine (31), and vobasine (32) as N4 oxide induced by treatment with trifluoroacetic anhydride (Polonovski-Potier reaction) (192) and reductive capture of the intermediate iminium ion (Scheme 1). 

The catalytic Pd complex and the aryl bromide together suggest the first step is oxidative addition of Pd(0) to the C5-Br bond. (The reduction of Pd(II) to Pd(0) can occur by coordination to the amine, p-hydride elimination to give a Pd(II)-H complex and an iminium ion, and deprotonation of Pd(IE)-H to give Pd(0).) The C10-C11 k bond can then insert into the C5-Pd bond to give the C5-C10 bond. P-Hydride elimination then gives the Cl 1-C12 n bond and a Pd(II)-H, which is deprotonated by the base to regenerate Pd(0). The overall reaction is a Heck reaction. 

Reductive cathodic amination of 2,5-hexanedione with ammonia and 1-pheny-lethylamine at the Hg cathode gave 2,5-dimethylpyrrolidines with satisfactory yields and excellent cis selectivity (90-98%). Other amines reacted less selectively to afford mixtures of pyrroles and diastereoisomeric pyrrolidines. A mechanism is proposed that involves the reduction of iminium ions, in which the stereoselectivity is controlled after the two le reductions of the cyclic iminium ion by the final protonation [350]. 

In the LAH reduction sequence, the C=N double bond in the iminium ion now behaves just as the C=0 bond of a carbonyl (see Section 7.7.1) and is also reduced by transfer of hydride from a further equivalent of LAH. The final product is thus an amine. 

The asymmetric alkynylation of isoquinoline iminium ion was reported in the presence of CuBr/QUINAP system (Scheme 5.7). ° Various alkynyl tetrahydroiso-quinolines were obtained in excellent yields and enantiomeric excesses. A natural product, homolaudanosine, was synthesized by the reduction of the obtained propargyl tetrahydroisoquinoline. 

However, formation of this bond through the conjugate addition of a soft sulfur nucleophile to a,P-unsaturated aldehydes is efficiently catalysed using iminium ion catalysis [116], Using diarylprolinol silyl ether 55 the addition of a series of sulfur based nucleophiles to a variety of a,P-unsaturated aldehydes was shown to be effective (73-87% yield 89-97% ee). The products were isolated as their p-hydroxy sulfide derivatives 73 after in situ reduction of the products (Scheme 33). 

In 2004, List reported that several ammonium salts including dibenzylammonium trifluoroacetate catalyzed the chemoselective 1,4 reduction of a, 5-unsaturated aldehydes with Hantszch esters as hydride sources [40]. It is assumed that substrate activation via iminium ion formation results in selective 1,4 addition of hydride. Subsequently, List [41] and MacMillan [42] reported asymmetric versions of this reaction promoted by chiral imidazoUdinone salts. In this context, several reports of this metal-free reductive process catalyzed by chiral phosphoric acids have appeared in the recent literature. 

Recently, List has described a cascade reaction promoted by phosphoric acid 1 in combination with stoichiometric amounts of achiral amine, which transforms various 2,6-diketones to the corresponding ds-cyclohexylamines (Scheme 5.28) [50]. This three-step process involves initial aldolization via enamine catalysis to give conjugate iminium ion intermediate A. Next, asymmetric conjugate reduction followed by a diastereoselective 1,2 hydride addition completes the catalytic cycle. 

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