Iminium ions stability

It was concluded that enantioselectivity is controlled by different degrees of iminium ion stabilization via H-bonding interactions between the iminium ion NH proton and both the amide carbonyl group and the thiourea-bound cyanide ion. However, distinct structural features responsible for that more effective stabilization are not easy to specify. The 3D structures of TSIU, TS2U, TSlS, and TS2S can be found in the CD. 

Different rate-determining steps are observed for the acid-catalyzed hydration of vinyl ethers (alkene protonation, ks kp) and hydration of enamines (addition of solvent to an iminium ion intermediate, ks increasing stabilization of a-CH substituted carbocations by 71-electron donation from an adjacent electronegative atom results in a larger decrease in ks for nucleophile addition of solvent than in kp for deprotonation of the carbocation by solvent. 

Pyrrolidine itself is not sufficiently activated to allow direct C—C bond formation at positions 2 or 3. However, under certain conditions the nitrogen atom may stabilize a reactive carbanion or carbocation (iminium ion). Both approaches have been fruitful. 

TV-Ethyl substitution had very little effect on the measured rate constant, whereas a 4-methyl substituent increased the rate constant by a factor of ca 100. In this case the initial product (identified by Bamberger) is the iminocyclohexadienol 39, which slowly hydrolyses to the quinone 40. These substituent effects suggest that in the transition state the developing positive change is located mostly at the 4-position (stabilized by the 4-Me substituent) and very little on the nitrogen atom (no stabilization by a TV-Et substituent), so that the intermediate is more properly described by the iminium ion. This is supported by an earlier observation46 that whilst full incorporation of 180 from the solvent H2180 occurs in the product, there is no detectable 180 incorporation into the reactant phenylhydroxylamine. 

The bromination of Az-piperideine (131) has been reported to give iminium ion (132). Treatment of this ion with triethylamine gave the bromoenamine (133), while treatment with hydroxide ion resulted in a rearrangement to give the pyrrolidine (134 Scheme 15) (76TL2437). The presence of the 2-aryl substituent undoubtedly stabilized the iminium ion facilitating these reactions. 

Overman15 discusses two conceivable mechanisms for the cyclization. One possibility assumes a direct cyclization of iminium ion 9 via p-silyl cation intermediate 24 to the indolizidinone 10. Cation 24 is stabilized by a p-effect of the silicon atom. Alternatively, iminium ion 9 might first undergo a charge-accelerated cationic aza-Co/fe rearrangement to allylsilaniminium ion 25, which would then cyclize to 10 with loss of a silyl cation. 

Scheme 8 The role of formaldehyde in the formation of carbenium-iminium ion 17 and resonance stabilization of the latter...

From the observation that N-(mcLhylcnc)morpholinium cations (17) induced the autocatalytic decomposition of tertiary amine W-oxides in combination with the proof that these intermediates were actually present in Lyocell dopes, the question arose why Lyocell solutions were stable at all. The answer is found in the fact that carbenium-iminium ions generated are immediately consumed by reaction with water and carbohydrate structures as the most abundant and most reactive nucleophiles present. Only when the local concentration of 17 increases to such a high level that the consumption by reaction with water or cellulose cannot compensate for its production, does the reaction with NMMO become uncontrollable and lead to an exothermic event. The pulp used in the Lyocell process acts as a quasi-stabilizer for the solvent NMMO, albeit with the drawback of increased chromophore generation. 

A mechanistic rational for the ionization of pyrrole-3-triflates 72 was proposed as shown below (Eq. 8). Heterolytic cleavage of the sulfonate ester bond, assisted by donation of the nitrogen lone pair, results in loss of the triflinate ion59 and gives rise to the C-acyl iminium ion intermediate 73. This process is further facilitated by the stabilizing effect of a... 

When the hemiaminals A (Figure 9.12, Nu = NR3R4) are formed in a neutral or weakly basic solution, they also have the possibility to react further by an SN1 reaction albeit in a different manner than just described for the corresponding hemiacetals (Nu = OR3) and hemithioacetals (Nu = SR3). The OH group of hemiaminals A is then ejected without prior protonation (i.e., simply as an OH ion). This is possible because an especially well-stabilized carbocation is produced at the same time, that is, the iminium ion C (Nu = NR3R4). It reacts with the second equivalent of the N nucleophile. Proton loss affords the jVW-acetal B (Nu = NR3R4). 

The esterification of carboxylic acids with DMF acetal (Formula A in Figure 9.16) proceeds in line with the esterification of carboxylic acids with ortho formates (Formula A in Figure 9.15). The reaction conditions are even milder since no acid needs to be added. This is due to the fact that all cationic intermediates of an esterification according to Figure 9.16 are iminium ions in which the positive charge is better stabilized than by the cationic intermediates of esterifications shown in Figure 9.15, because they are carboxonium ions. 

This iminium ion C can he stabilized by combining with a nucleophile. As Figure 9.12 shows, when Nu = R R2N, this step completes an SN1 reaction in the initially formed addition... 

Figure 15.20 shows the multistep mechanism of the [4+2]-cycloaddition between 1-(dimethylamino)-l,3-butadiene and cis-dicyanoethenedicarboxylic acid diester. The reaction proceeds via an intermediate, which must be zwitterion conformer B. The anionic moiety of this zwitterion is well stabilized because it represents the conjugate base of a carbon-acidic compound (Section 13.1.2). The cationic moiety of zwitterion B also is well stabilized. It is an iminium ion (i.e., a species with valence electron octet) rather than a carbenium ion (which is a species with valence electron sextet). Moreover, the iminium ion is stabilized by conjugation to a C=C double bond. 

This iminium ion C can be stabilized by combining with a nucleophile. As Figure 7.15 shows, when Nu = R1R2N, this step completes an SN1 reaction in the initially formed addition product B. Thereby, an A,lV-acetal (formula A in Figure 7.23) or a derivative thereof (see Figure 7.23) is produced. We will treat this kind of reaction in more detail in Section 7.4. The other important reaction mode of the iminium ions C in Figure 7.23 is the elimination of a proton. This step would complete an El elimination of H20 from the primary adduct B. This kind of reaction also occurs frequently and will be discussed in Section 7.4, as well. 

The deprotonation of an iminium ion (formula A in Figure 7.27) to give an enam-ine is reversible under the usual reaction conditions. Therefore, the most stable enam-ine possible is produced preferentially. Figure 7.28 emphasizes this using the example of an enamine formation from a-methylcyclohexanone (i.e., from an asymmetrical ketone). The enamine with the trisubstituted double bond is produced regioselectively and not the enamine with the tetrasubstituted double bond. Since the stability of olefins usually increases with an increasing degree of alkylation, this result is at first... 

Electrophilic substitutions of alkenyl-, aryl-, and alkynylsilanes with heteroatom-stabilized cationic carbon species generated by the action of a Lewis or Brpnsted acid (acyl cation, oxocarbenium ion, etc.) provide powerful methods for carbon-carbon bond formation. Particularly, intramolecular reactions of alkenylsilanes with oxocarbenium and iminium ions are very valuable for stereoselective construction of cyclic ether and amine units.21-23 For example, the BFj OEt -promoted reaction of (E)- and (Z)-alkenylsilanes bearing an acetal moiety in the alkenyl ligand gives 2,6-disubstituted dihydropyrans in a stereospecific manner (Scheme l).23 Arylsilanes also can be utilized for a similar cyclization.24... 

The most common fragmentation of amines is a cleavage to give a resonance-stabilized cation an iminium ion. This ion is simply a protonated version of an imine (Section 18-15). 

Figure 19-9 shows the mass spectrum of butyl propyl amine. The base peak (m/z 72) corresponds to a cleavage with loss of a propyl radical to give a resonance-stabilized iminium ion. A similar a cleavage, with loss of an ethyl radical, gives the peak at m/z 86. 

Mass spectrum of butyl propyl amine (N-propylbutan- 1-amine). Note the odd mass number of the molecular ion and the even mass numbers of most fragments. The base peak corresponds to a cleavage in the butyl group, giving a propyl radical and a resonance-stabilized iminium ion. 

The nucleophilic carbon atom attacks an electrophile to give a resonance-stabilized cationic intermediate (an iminium ion). 

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