Iminium salts, addition

The first reaction is a Lewis-acid-catalysed reaction between the amine and the aldehyde ar.i addition of the allyl silane to the resulting iminium salt. Addition occurs at the other end of th allylic system and on the opposite face to the silicon. The most obvious way to put the reagent, together looks excellent until we realize, from the Newman projection, that the two Hs in tht product would then be anti whereas they are actually syn. 

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. 

A hypsochromic shift of 20-50 cm is observed in the double-bond stretching region, when the enamines are converted to the corresponding iminium salts by the electrophilic addition of a proton at the /3-carbon atom. The shift is accompanied by an enhancement in the intensity of the band. Leonard and co-workers (68,71-74) have used this absorption shift as a diagnostic tool for the determination of the position of the double bond... 

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). 

In most reviews of enamine chemistry the reactions of iminium salts are scattered throughout the review and are consequently not covered in a comprehensive manner. This chapter will be an attempt to look at reactions that, at one stage or another, proceed by nucleophilic addition to the iminium intermediate. The subject of enamines has been reviewed 1-4) and certain aspects of iminium salt chemistry such as reduction of aromatic quaternary salts have been treated in detail (5). Consequently, the reduction of aromatic quaternary salts with complex hydrides will be presented here only briefly. Although the literature (especially 1950-1967) has been checked with care, the author can make no claim to completeness. The... 

It should be noted that the products from addition to conjugated iminium salts occur primarily by 1,2-and not 1,4-addition (14). 

Another class of nucleophilic addition to iminium salts can be found in the addition of diazoalkanes. These are of great interest since they were known to add and then reaet further to form three-membered rings as in the case of C=S, C=0, and C=C functions 103). Leonard and Jann 104-106) found that treatment of iminium perchlorates with diazomethane and other diazoalkanes yielded aziridinium salts. Treatment of an iminium salt such as N-cyclohexylidinepyrrolidinium perchlorate (66) with diazomethane yielded a new product whose structure was established by spectral and chemical means to be 5-azoniadispiro[4.0.5.1]dodecane perchlorate (67). The UV spectrum was devoid of any absorption above... 

Azides have been shown to react with itniniutn salts to give addition products. The same product is obtained if the iminium salt is treated with azide ion or if the enamine is treated with hydrazoic acid 14). The yields of the products were all very high (85-95 %). The interest in this reaction centers on the fact that the azides react with isonitriles to give substituted tetrazoles (83) 44). 

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,... 

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. 

An interesting example for the modification of steroids by the addition of 5a-cholestanes to an Af(V-dimethyl(methylene)iminium salt with good stereoselectivity has been reported7. 

Auxiliary-Induced Diastereoselection I.4.3.3.I. Auxiliary Croup in the Enolate Addition to Imines or Iminium Salts... 

The addition of electron-rich enamines2-5 to preformed imines and imine derivatives so far has scarcely been investigated6 -15 compared to the addition of enolatcs (see Section 1.4.3., refs 16 and 17 and literature cited therein). Most of the known examples refer to iminium salts because they are more reactive than the corresponding iinines due to their highly polarized C-N double bond (see Section 1.4.1.6., ref 16 and literature cited therein). 

The stereoselectivity of the addition of an enamine to an iminium salt is not significantly influenced by its anion. Therefore, iminium chlorides 2a can also be used. Indeed, they are the reagents of choice on large scale syntheses. Compared to iminium chlorides 2a the corresponding tctrachloroaluminates 2b are clearly less hygroscopic and for that reason particularly easy to handle 1. 

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