Iminium salts, and

The first correlation for the determination of enamines and iminium salts was that of Leonard and Gash (7), who prepared a series of enamines and the corresponding iminium salts and compared the infrared spectra. There was observed a shift of 20-50 cm" toward higher frequencies whenever an... 

The reaction has been examined more recently by several groups with simple iminium salts and the results are analogous. Treatment of J -dehydro-sparteinium perchlorate (79) gave (—)-6-cyanosparteine (80) (46). Other examples of this reaction are listed in Table 11. 

As a result, one might expect that there is only a slight difference between the iminium salts and the parent ketone (4-rm-butylcyclohcxanone) concerning the stereochemical course of the addition reaction and. actually, this assumption proves to be true. In each case the diastereomer resulting from the less hindered equatorial attack of the nucleophile clearly predominates10. 

Conversion of Carbonyl Croups into Schiff Bases, Iminium Salts, and Enamines... 

V-Phenylbenzylimines undergo stilbene-type photocyclizations (equation 83) to yield heterocyclic compounds143. The reactions usually take place via the iminium salt and need oxidants like oxygen or iodine. Six-electron electrocyclic reactions have been observed for 1-aza-l,3-dienes144 (equation 84). 

Synthesis of antx-a has been done through a ring expansion of cocaine (20), from 1, 5-cyclooctadiene (21) and by intramolecular cyclization between an iminium salt and a nucleophilic carbon atom (22). The structure of antx-a is given in Fig. 1. 

The epoxidation of olefins catalyzed by iminium salts and amines (or ammonium salts) is emerging as a new technique for the functionalization of simple aUcenes. These catalysts have relatively simple structures and hence are easily produced at scale they offer potential as green oxidation catalysts. These organic salts are effective oxygen transfer reagents towards electron-rich unfunctionalized olefins. For the iminium salt systems oxone oxidizes an iminium salt to the oxaziridi-nium intermediate, which then transfers oxygen to the olefin and as oxone reacts readily with iminium ions to regenerate the oxaziridinium species catalyti-cally, efficient oxidation is possible. 

P. S. Mariano, in W. M. Horspool (ed.), Synthetic Organic Photochemistry, Plenum (1984. This is a comprehensive account of the photochemical reactions of iminium salts and related species. 

The cyclization reaction developed by Scheffold, and shown in Scheme 65, makes use of the addition of an amidine to an iminium salt, and produces a product of reduced porphinoid character.196 197... 

Aliphatic amines can be readily oxidized by Pd(II) to imines or iminium salts and hydrido complexes. The latter can transfer hydrogen to alkenes, leading to the formation of alkanes as byproducts of the Heck reaction (last example, Scheme 8.18). Such reactions can be avoided by using alkali carbonates as base instead of aliphatic amines [148]. Treatment of stannanes or organoboron derivatives with electron-deficient alkenes under acidic reaction conditions can also lead to formal products of Michael addition instead of the products of a Heck-type reaction [149, 150] (Scheme8.19). 

The reduction of the iminium salt to an anionic species has been suggested to be the initiation step, since the best yield is obtained when the working potential of the cathode has a value inbetween the reduction peak potential of the iminium salt and that of the alkylating agent. 

Other C-electrophiles besides iminium salts and carbonyl compounds have not yet extensively been tested for C-C-forming cyclopropane ring cleavage. However, whereas acyl or tertiary alkyl halides do not give addition products, benzaldimine at least provides the expected secondary amine 194 with good yield and diasteroselectivity 61). 

Figure 5. 1H NMR spectra of (a, b) two types of iminium salt and (c) the iminium salt prepared as described in Figure 7 (4)...

Allyltriorganosilanes react with activated C-N double bonds such as iminium salts and Lewis acid-coordinated imines at the y-position to give homoallylamines.14,118 For example, in the presence of BF3, iV-acylimines generated in situ by the reaction of aldehydes or acetals with carbamates are efficiently allylated with allyltrimethylsilanes (Equation (26)).119,119a,12° The use of homochiral crotylsilanes such as 20 leads to highly diastereo- and enantioselective synthesis of homoallylamines (Equation (27)). a Allenylation of the iV-acylimines can be performed with propargylsi lanes. 

ADDITION TO IMINES, IMINIUM SALTS AND RELATED COMPOUNDS... 

This remains a developing area for the synthesis of chiral oxiranes and has attracted interest from several research groups. As with the use of dioxiranes (above), it is not necessary to form the reactive oxaziridinium salt rather, the epoxidation reaction can be mediated by the corresponding iminium salt and Oxone. 

Numerous reviews have appeared which are relevant to aromatic photochemistry and are also of general interest and application these include accounts of the chemistry of excited-state complexes by Davidson, the photochemical electron-transfer reactions of ethylenes and related compounds by Mattes and Farid, photosolvolyses and photoreactions involving carbo-cations by Cristol and Bindel, conformational flexibility and photochemistry by Wagner, asymmetric photochemistry in solution by Rau, and the photochemistry of iminium salts and related heteroaromatic systems by Mariano." The photochemistry of fragrance materials has been considered by Shibamoto and Mihara, and in their second article they deal with aromatic compounds and phototoxicity. ... 

Among the systems which have played the role of cyclopropanone equivalents have been cyclopropane derivatives containing both a secondary amino group and a hydroxyl substituent on the same carbon atom. These 1,1-disubstituted carbinol amines appear to be in equilibrium with the corresponding iminium salts and thereby act as reactive electrophilic sites for attack by a variety of nucleophiles. 

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