Carbenium-iminium ions

Interconversion of NMMO-Derived Carbenium-iminium Ions. 170... 

To prove the presence of formaldehyde and NMMO-derived carbenium-iminium ions under Lyocell mixtures was a key issue here, as the occurrence of the latter is critical with regard to autocatalytic degradation and instabilities of Lyocell solutions. Again, trapping methodology was used for this purpose, it served also to investigate the general chemistry of the NMMO-derived carbenium-iminium ions. 

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

Scheme 11 Carbenium-iminium ion conversion of 22 into 17 schematic representation of the computed reactant and transition state geometries as well as reaction energies. The trigonal planar environments of the carbenoid carbons are shaded in gray. The water molecule participating in the transition state is circled by a dotted line...

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. 

The mechanism of the thermal decomposition of NMMO and Lyocell solutions is extremely complex since the reaction, initiated by the action of carbenium-iminium ions, quickly enters an uncontrollable course. A central question was whether the heterocyclic ring of NMMO was cleaved during the reaction and whether products of this cleavage, having either vinyl ether or enamine structures, could be detected. Employing the trapping agents... 

The mechanism and kinetics of a novel autocatalytic degradation of A-methylmor-pholine A-oxide (70) into morpholine and formaldehyde, induced by carbenium-iminium ions (Mannich intermediates) has been explored. " The study was prompted by the observation that NMMO as an oxidant is often consumed far beyond the stoichiometric ratio, with generation of morpholine and some formaldehyde, and at a rate faster than its reaction with reductant. Decomposition of NMMO was apparently promoted by the combination of products mentioned and could also be induced by dimethyl(methylene)iminium iodide. The proposed mechanism is in Scheme 8. Addition of small amounts of base favour the abstraction of the proton from the A-methyl group but do not prevent carbenium-iminium ion formation however, larger concentrations of base terminate the reaction. The presence of only trace amounts of water is prerequisite for the protonation-deprotonation steps to proceed. 

By analogy to carbenium ions, it can be expected that alkylnitrenium ions will experience 1,2-alkyl and hydride shifts. These would presumably be orbital symmetry allowed reactions that convert the nitrenium ion into a more stable iminium ion. This idea is supported by several theoretical studies suggesting that simple... 

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. 

Iminium ions with short chains fragment by forming an ion-neutral complex [12], The slow step of the process is the 1,2 proton transfer leading to a secondary carbenium ... 

Enol XV is certainly not prone to undergo rearrangement since the required vinyl amine would now be an a,j8-unsaturated iminium ion. As a consequence, the route open to this intermediate (XIV) is only the jS elimination of XV and the trapping of the resulting carbenium ion (XVI) with the vinyl-ogous amide function. 

The chemistry of isonitriles, now readily accessible by this very general method, has thus flourished in the last four decades, not only proving that these compounds are not merely a curiosity in the field of organic chemistry, but also indicating that they possess an outstanding versatility. Noteworthy synthetic procedures, all of them involving attack of electrophilic species (e.g., carbenium and iminium ions) on the... 

The mechanism was then reexamined 25 years later in 1997 by Kappe. Kappe used H and C spectroscopy to support the argument that the key intermediate in the Biginelli reaction was iminium species 16. In the event, 5 reacted with 3a to form an intermediate hemiaminal 17 which subsequently dehydrated to deliver 16. Iminium cation 16 then reacted with 6 to give 14, which underwent facile cyclodehydration to give 15. Kappe also noted that in the absence of 6, bisureide 8 was afforded as a consequence of nueleophilic attack of 16 by urea (3a). This discovery confirmed the conclusion of Folkers and Johnson in 1933. As far as the proposal from 25 years earlier by Sweet and Fissekis, Kappe saw no evidenee by H and NMR spectroscopy that a carbenium ion was a required species in the Biginelli reaetion. When benzaldehyde (5) and ethyl... 

In a - Amino-nitrilen wird die Cyan-Gruppe durch Lithiumalanat, Lithiumala-nat/Aluminiumchlorid und Natriumboranat reduktiv abgespalten, wenn die elektronen-liefemde Amino-Gruppe die Bildung eines Iminium-carbenium-Ions ermoglicht2 ... 

The Ritter reaction [6] proceeds by the electrooxidation of alkyl iodides (56) in an MeCN-(Pt) system to form Ai-alkyl acetamides (58) (Scheme 21). Attack of carbenium ion intermediate - from dissociation of the initially formed alkyl cation radical - to acetonitrile would give the iminium cation (57). However, a different mechanism is proposed, whereby the alkyl iodide reacts with the electrogenerated iodo cation [I]" " [73]. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Sufficiently stable electrogenerated carbenium ions can be trapped with nucleophiles or nucleophilic solvents. Acetoxylation and methoxylation proceed by electrolysis in either acetic acid or methanol. Acetamidation occurs in wet acetonitrile, in which the nucleophilic attack of the nitrile group on the carbenium ions R tends to give iminium cations subsequent hydrolysis with water in the medium completes the reaction ... 

Thioamides with bulky substituents bound to the nitrogen can be rearranged to iminium compounds (119 equation 75) by treatment with HCl, provided the bulky groups can form stable carbenium ions. - ... 

Two of the factors that determine the reactivity of tethered ir-nucleophiles in Mannich-type cycliza-tions have been emphasized stereoelectronic effects and reaction medium effects. The stereoelectronics of orbital overlaps between the ir-nucleophile and the iminium electrophile are best evaluated by considerations such as antiperiplanar addition trajectories and Baldwin s rules for ring formation. The critical importance of the reaction medium has received serious attention only recently. However, it already appears clear that Tr-nucleophiles that would lead, upon cyclization, to relatively unstable carbocations can have their reactivity markedly increased by carrying out the cyclization in the presence of a nucleophilic solvent or additive which, by nucleophilic participation, can obviate the formation of high energy cyclic carbenium ion intermediates. 

The discrimination of the face of the chiral carbenium ion is determined by the hindrance of the flanking groups. In the case of chiral Bronsted acids, the chiral counter ion, formed after effective protonation or partial donation of the proton, surrounds the created cationic intermediate. One face of the intermediate is effectively covered by the chiral counter ion and the nucleophile reacts with the less covered face. However, in many SNl-type transformations mediated by Lewis acids, a transition state in which a couple of protons interact with the phosphates is often invoked. This double interaction seems cmcial and only a partial success in the use of nucleophiles was achieved. Nevertheless, a series of successful S l-type reactions were discovered and are highUghted in this section. The chiral ion pairs between phosphate anions and iminiums [64] or metal ions were developed [65]. This powerful strategy is called asymmetric counteranion-directed catalysis (ACDC) and it has been applied successfully to several innovative transformations. The generation of carbenium ion in situ from alcohol, with the use of acids able to form a tight chiral ion pair with the phosphate anion, can be used in S il-type... 

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