Iminium salt, formation

Another interesting example was reported by Yamaguchi et al. by using a simple L-proline rubidium salt 27 (Scheme 8D. 15) [32], A reversible iminium salt formation, involving the amine moiety of the catalyst and the carbonyl group of an enone, was proposed as the key intermediate. 

The NaBD4 reduction of sulfoxides of thiohemiaminals performed on 6-hydroxythiobinupharidine syn-sulfoxide (24), 6 -hydroxythiobinupharidine syn-sulfoxide (25), and 6,6 -dihydroxythiobinupharidine syn-sulfoxide (26) follows a single steric mode (90% of deuterium incorporation) and introduces axial deuterium at both C-6 and C-6. This reduction may not follow a mechanism with intermediate iminium salt formation (36). The presence of strong hydrogen bonding and the absence of a-iminium salts in the reacting mixture support this conclusion. 

Figure 2.7. The materials used for the BOM process. Exposure at 365 nm produces acid derived from CBr4. This photogenerated acid protonates the imine dye, which causes a dramatic hathochromic shift in the absorbance. As exposure proceeds, the absorbance at 365 nm is bleached and the absorbance at 436 nm increases because of iminium salt formation.

This general synthesis of a-aminonitriles is explained by the iminium salt formation, via the formal hydride abstraction with the ruthenium catalyst, and addition of cyanide. The oxidant has the role of reoxidizing intermediate ruthenium species formed during the catalytic cycle. Slightly different catalytic steps have been proposed for the reaction using oxygen or hydrogen peroxide as oxidant [25, 26]. 

The formation of 88 is postulated to be occurring by the nucleophilic attack of a hydride ion (47), abstracted from the secondary amine, on the a-carbon atom of the iminium salt (89). The resulting carbonium ion (90) then loses a proton to give the imine (91), which could not be separated because of its instability (4H). In the case of 2-methyIhexamethylenimine, however, the corresponding dehydro compound /l -2-methylazacyclo-heptene (92) was isolated. The hydride addition to the iminium ion occurs from the less hindered exo side. 

The halogenation of enamines is formally analogous to protonation with salt formation. Thus the steroidal enamine (164) undergoes bromination (7/5) to give the j8-bromo iminium bromide (165), which is readily hydrolyzed to the /5-bromo aldehyde (166). 

The reaction of iminium salts such as 66 with salts of trichloroacetic acid has been shown to yield amides such as 84 on hydrolysis 126). It was suggested that the reaction proceeds by addition of dichlorocarbene to give an aziridinium intermediate (85), which was opened by trichloroacetate followed by hydrolysis to give the observed products 126). The observed products from the reaction can be accounted for by formation of CCI3,... 

The intermediate formation of iminium salts is postulated in the reduction of (x-amino ketones by the Clemmensen method, occurring with concomitant ring enlargement or contraction (244-246). Reduction of l,2,2-trimethyl-3-piperidone (154) in this manner gave l-methyl-2-iso-propylpyrrolidine (155). 

Some covalent compounds, such as a-aminonitriles D (formation of an iminium ion by solvolysis) or TV-substituted 1,3-oxazolidincs E can be regarded as masked iminium salts because there is evidence that in reactions of these species with organometallic reagents iminium intermediates are involved101214-17. 

Iminium ions bearing an electron-withdrawing group bonded to the sp carbon of the iminium function are very reactive dienophiles. Thus, iminium ions 26 generated from phenylglyoxal (Scheme 6.15, R = Ph) or pyruvic aldehyde (R = Me) with methylamine hydrochloride, react with cyclopenta-diene in water at room temperature with good diastereoselectivity [25] (Scheme 6.15). If glyoxylic acid is used, the formation of iminium salt requires the free amine rather than the amine hydrochloride. 

Although some examples of C-substitutions of silylated Schiff bases and iminium salts, in particular the formation of / -lactams, have already been mentioned in Sections 5.1.3 and 5.1.5 (cf. also C-substitutions of lactones and amides in Section 4.8) in this section several additional and typical C-substitutions of 0,0- and 0,N-acetals and of iminium salts derived from carbonyl groups are discussed. 

Besides the allylation reactions, imines can also undergo enol silyl ether addition as with carbonyl compounds. Carbon-carbon bond formation involving the addition of resonance-stabilized nucleophiles such as enols and enolates or enol ethers to iminium salt or imine can be referred to as a Mannich reaction, and this is one of the most important classes of reactions in organic synthesis.104... 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

The combination of a TiCl4-catalyzed formation of an iminium salt from 193, a subsequent [3,3]-sigmatropic rearrangement followed by a return of the cyanaide provided 194 (Scheme 1.87) [611]. 

The key intermediate 21 is in principle accessible in any of several ways. Thus reaction of thiophenecarbox-aldehyde with amninoacetal would lead to the Schiff base treatment with acid would result in formation of the fused thiophene-pyridine ring (21). Alkylation of that intermediate with benzyl chloride gives the corresponding ternary iminium salt. Treatment with sodium borohydride leads to reduction of the quinolinium ring and thus formation of ticlopidine (24). ... 

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. 

The formation of isomeric products can be reconciled by consideration of the mechanistic pathway. Deprotonation of the more acidic methyl group leads to formation of iminium salt 88 that can then form two ylides, 89 and 90, furnishing a mixture of products after subsequent cycloaddition (Scheme 3.24). 

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