Iminium group

Sodium borohydride is sometimes used in conjunction with CeCl3 (Luche s reagent).70 The active reductants under these conditions are thought to be alkoxyborohydrides. Sodium cyanoborohydride is a useful derivative of sodium borohydride.71 The electron-attracting cyano substituent reduces reactivity and only iminium groups are rapidly reduced by this reagent. 

Bisquinolizidine alkaloids have also been widely studied by this technique. For instance, the crystal structure of (—)-A16(17 ,-dehydrolupaninium perchlorate 23 was obtained from sealed-tube and synchroton X-ray diffraction data, and showed that the A, B, C, and D rings assume distorted half-chair, chair, distorted sofa, and chair conformations, respectively it was also used to determine the most precise dimensions so far known for the iminium group <1999JST245>. The crystal structure of quinolizinium hexafluorophosphate has also been studied <2001CSC174>. 

C-NMR chemical shifts of the heterocyclic rings of selected bicyclic compounds and of homoberbine-type isoquinobenzazepines (including the alkaloid saulatine, 11) are listed in Tables 9 and 10, respectively. Shifts of a-olefinic and (at higher field) )S-olefinic carbon atoms of enamines 77a, 288, and 292-294 and of the iminium group carbon atom in bicyclic salt 60 are found in the respective regions reported in the literature to be typical of such compounds (82T1975, p. 1986). 

Polyethyleneimine is also extensively cyclized as a result of intramolecular nucleophilic attack of primary and secondary amines on the iminium group. This results in cyclic oligomer as well as polymer molecules containing large-sized rings as part of their structure. 

Irradiation of chloranil with />-xylene [224] or with hexamethylbenzene [225] gives the corresponding 4-hydroxyphenyl benzyl ethers. Another good trap for benzyl radicals is the iminium group. Thus, irradiation of 1-methyl-2-phenylpyrrolinium perchlorate with toluene, benzyltrimethylsilane, or... 

Although coupling of aryl diazonium salts at the -position of dienamines yields indazoles on acid-catalysed cyclization, the presence of an electron-donor substituent may render the aromatic ring sufficiently nucleophilic to cyclize onto the iminium group to give cinnolines or 6,7-diazasteroids2,67 (Scheme 40). 

Internal Sn2 displacement of sulfide results in formation of a 5-membered ring containing an iminium group. 

The product from this first step is an iminium cation that reacts with pyrrole to give a more stable iminium salt. The extra stability comes from the conjugation between the pyrrole nitrogen and the iminium group. 

Anhydrides may also be employed instead of an acid chloride. The mixed anhydride of acetic and formic acid reacts with 1-iV-morpholinocyclohexene to give 2-hydro-xymethylenecyclohexanone . Acylation of isobutyraldehyde enamines with trichloroacetic anhydride in tetrahydrofuran at room temperature gives the a-trichloromethyl-j5-trichloroacetyl adduct . This occurs by initial ) -acylation to give the acyl iminium trichloroacetate, followed by decarboxylation and nucleophilic addition of the tri-chloromethyl anion to the iminium group. Trifluoroacetic anhydride under the same conditions just gave the -trifluoroacetyl derivative . 

When X is an iminium group, the reaction is spontaneous even at —60°C, whereas with the trichloromethyl group it occurs only upon heating to 60-100 °C. When X is not electron-withdrawing, the reaction does not occur. 

The aromatic <5-sultam, 9,10,11,1 la-tetrahydronaphtho[l,8-d,e] pyrrolo[l,2-b]-thiazine-7,7-dioxide (116) has been synthesized by Adesogan and Alo by cyclization of the iminium salt 115, which is itself prepared from N-arylsulphonylpropyl chloride by the action of silver triflate and triflic acid (equation 196)252. The cyclization step presumably proceeds via C—C bond formation involving electrophilic attack of the activated iminium group carbon atom on the 8-position of the naphthalene ring (equation 197). 

A final equilibrium of synkinetic interest concerns the reversible hydration of carbonyl and iminium groups. In both cases electron-withdrawing groups strongly favor the hydration of C=0 and C=N bonds. Scheme 1.4.6 shows the approximate hydration upon dissolution of the carbonyl compound in water. Acid catalysis is usually required to establish equilibria. 

Sodium in liquid ammonia, in the presence of ethanol, affords the 1,4-dihydropyridine and 4-pyridones are reduced to 2,3-dihydro-derivatives. " Metal/acid combinations, which in other contexts do bring about reduction of iminium groups, are without effect on pyridines. Samarium(II) iodide in the presence of water smoothly reduces pyridine to piperidine. ... 

Iminium groups are more easily reduced than carbonyl groups in acid solution, and this has been exploited in a method for reductive amination of aldehydes and ketones by way of the iminium salts formed from the carbonyl compounds and... 

The product from this first step is an iminium cation that reacts with pyrrole to give a more stable iminium salt. The extra stability comes from the conjugation between the pyrrole nitrogen and the iminium group. The work-up with aqueous Na2C03 hydrolyses the imine salt and removes any acid formed. This method is particularly useful because it works well with Me2NCHO (DMF) to add a formyl (CHO) group. This is difficult to do with a conventional Friedel-Crafts reaction. 

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