Immonium structures

The ultraviolet absorption at 222-232 nm is comparable only with immonium structure (186a). No active hydrogen (Zerewitinov) was present in the immonium salts (1,186b) and no deformation vibrations of nitrogen-hydrogen linkage were detected (186a). 

In the formation of salts, addition of a proton occurs at the free electron pair of one of the mesomerie forms of the enamine. The salts are usually derived from the immonium structure. With tertiary enamines, there is a substantial difference between the free bases which possess a fixed vinylamine structure and their immonium salts. 

For an immonium structure of the enamine salt, protonation of the polarized mesomeric form on the j3-carbon atom is necessary. The behavior of dienamines shows59-61 that primary formation of an... 

Indole and alkyl-indoles (38) are protonated in position 3 by the action of strong mineral acids.198,197 The tendency of the enamine salts to assume the immonium structure is very general the salts of ethyl j3-aminoerotonate are derived from the imino form at the expense of the conjugation between the carbonyl group and the double bond.12,198... 

The UV spectral data for some 2,5-diaryl-l,3-oxathiolium perchlorates are shown in Table 6. Also shown are the UV and IR data of a series of 2-(disubstituted amino)-5-aryl-l,3-oxathiolium salts. The presence of a strong band at ca. 1652 cm-1 is explained by a strong contribution from the immonium structure. With one exception, no detailed IR studies have been reported in the series. A complete assignment of the bands in the spectrum of 1,3-dioxolane has been made (59JCS802). The non-planarity of the ring is shown by the broad nature of the bands. 

Little has been reported on tautomerism of dioxoles and oxathioles. The spectral data for a series of 2-dialkylamino-l,3-oxathiolium salts has been interpreted as favoring the immonium structure (43) (72CPB304). Similarly, the spectra of several oxathiol-2-ylidene derivatives (44) indicate that the dipolar contribution to the resonance hybrid structure is small (73CPB2224). 

The structure of this cyclic intermediate B, which contains an immonium structure, suggests the possibility of undergoing a facile reduction by hydride, if present during the course of photocyclization. This expectation was visualized as expected on the various enamides and therefore opened up a new phase of the application of enamide photocyclization (15) (Scheme 16). 

The reaction of the nitrone (23) with alkyl phosphites has been studied. With trimethyl phosphite in refluxing MeOH (23) afforded a mixture of the epimers (32), whereas in AcOH (33) was obtained. It was shown that (33) was not formed from (32) by loss of MeOH. With triethyl phosphite analogous products were formed, but in this case a large amount of the deoxygenated compound (34) was also obtained. With trimethyl phosphite the formation of an immonium structure, reducible to conanine (35), was suggested by the presence in the n.m.r. spectrum of the reaction mixture of a N—Me group. The structures of these compounds were deduced from their spectral data. 

Mild treatment of N-oxides with trifluoroacetic anhydride yields immonium trifluoroacetates the immonium structure may be hydrolyzed to a carbonyl group ... 

Unstable chlorides were converted to stable SnCU complexes. In their IR spectra there is an intense absorption band in the 1900 cm region, which is consistent with the band of allenic system (structure C). Unlike unstable chlorides A and B, the SnCl4 complexes are stable and, when kept in an inert atmosphere, remain intact for several days. The allenic structure of the immonium salt was confirmed by studying the mercuration of the same aminobutenynes (74DIS). 

The dimerization of skatole proceeds in an entirely analogous manner, cation (44) now being the electrophilic reagent. This is sufficiently reactive to effect substitution at the a-position of a neutral skatole molecule. Attack by the less hindered side of cation (44) will be favored, leading to the stereochemistry shown in structure (30). The failure of 2-methylindole to dimerize is paralleled by the failure of 2-methylpyrrole dimer to react with a further molecule of 2-methylpyrrole. The main reason is almost certainly again the reduction in the electrophilic character of the immonium carbon by... 

Figure 5.13 Structures of the immonium and acyl ions formed in the mass spectral fragmentation of peptides. From Chapman, J. R. (Ed.), Protein and Peptide Analysis by Mass Spectrometry, Methods in Molecular Biology, Vol. 61, 1996. Reproduced by permission of Humana Press, Inc.

The products from the cyclopropenium immonium cations 153 and primary and secondary amines88 vary with amine structure and basicity of the amino function... 

Figure 6.7. A general chemical structure of immonium ions.

Aliphatic onium ions such as immonium, oxonium, and sulfonium ions have been introduced as even-electron ionic products of the a-cleavage occurring from molecular ions of amines, alcohols, and ethers or thiols and thioethers, respectively (Chap. 6.2.5). All these and analogous onium ions are capable of further fragmentation reactions, the majority of which are alkene losses [141] yielding fragments of high relevance for structure elucidation. 

It was observed that a-amino oximes 511, when treated with sodium borohydride in boiling acetonitrile, produced the expected fragmentation products 513 in moderate to good yields (31-87%) (equation 225). The use of the hydride-induced fragmentation in cyclic oximes leads to amino nitrile compounds, as a result of the reduction of the immonium salt intermediates 512. Careful selected oxime structures showed that the reaction time increases when the stabihty of the immonium intermediate decreases, showing the importance of the mesomeric assistance. 

The first clues that compounds of structure I might be involved in nitrosamine-forming reactions came during the study of tertiary amine nitrosations. Smith and Loeppky had proposed ( ) in their detailed, classical investigation of the mechanism of this reaction that the first steps involve nitrosammonium ion formation followed by elimination of nitroxyl (HNO). The resulting immonium ion was postulated to hydrolyze to the secondary amine, which reacted with nitrosating agent to form the observed product. These mechanistic proposals are summarized in Fig. 2a. 

Nitrosamine formation is not the only conceivable fragmentation mechanism for compounds of structure I. By analogy to the nitrosative dealkylation reactions discussed above, one might predict that such compounds could also undergo cis elimination of nitroxyl in amide-forming reactions. Such a transformation has possibly been observed (14). During an attempt to synthesize the nitrosamino aldehyde VIII from immonium ion IX, Hecht coworkers were able to isolate only 5-10% of the desired product. The major product proved to be N-methyl-2-pyrrolidone, as shown in Fig. 10. We interpret this as evidence that an intermediate such as li can fragment not only by the Fig. 1... 

There is an interesting difference in the cydization of the closely related pyrrole derivatives 7 and 9. A reaction of N-vinylpyrrole 7 with hydrogen chloride in ether yielded a dark blue immonium salt, represented by resonance structures 8a and 8b. The N-ethyl analog (9), on the other hand, gave the 1-amino-3//-pyrrolizine (10) under the same conditions. Hydrolysis of 8 did not give the expected pyrrolizinone(12) but stopped half way to give adduct 11.19... 

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