Iminium ions trapping

A thiophene unit can also be used as the iminium ion trap. 

Photolysis with visible light, DAP " TMSCN. The photochemical reaction generates an iminium ion that is trapped with cyanide."... 

Perhydrooxazolo[3,2- ]pyridines 338 are excellent precursors of iminium ions 339 obtained after treatment of the oxazolidine with either a Bronsted or Lewis acid. Trapping of these intermediate iminium ions with nucleophiles then allows for substitution at the C-8a position together with ring opening, yielding functionalized piperidines 340 (Scheme 93). 

In the presence of alcohols, the corresponding ethers are formed and added nucleophiles such as chloride ion40 or azide ion41 lead to the chloro- and azido-amine products, respectively. Rate constants are independent of the concentration of added nucleophile. Labelled 180 from the solvent is incorporated in the product42. All the evidence points to a reaction mechanism where water is lost from the O-protonated reactant to give a nitrenium ion-iminium ion intermediate which is rapidly trapped by a nucleophile (H2O in this case) to give the final product. This is shown in Scheme 7. Protonation at N- is likely to be more extensive, but there is no pathway to products from the N-protonated intermediate. 

Incorporation of CO into an organic substrate usually occurs by insertion of CO into a C-metal bond. The requisite Cl-metal bond is formed by oxidative addition of a Pd(0) species into the Cl-Br bond, the normal first step upon combining a Pd(0) compound and an aryl halide. Coordination and insertion of CO follows. Addition of N to the carbonyl and loss of Pd(0) gives an iminium ion, which is trapped by EtOH to give the product. 

In each of the tandem iminium ion/enamine cascade processes described above, the enamine is trapped in an intramolecular fashion. The ability to perform the trapping seQuence in an intermolecular manner would allow for the one—pot introduction of three points of diversity. IVIacNlillan has realised this goal and described a series of secondary amine catalysed conjugate addition—enamine trapping sequences with oc P Unsaturated aldehydes using tryptophan derived imidazolidinone 115 to give the products in near perfect enantiomeric excess (Scheme 47) [178]. 

Argoti, D. et al. (2005) Cyanide trapping of iminium ion reactive intermediates followed by detection and structure... 

Fragmentation of the initially formed chromate ester gives formaldehyde and an iminium ion that is trapped by dichromate. The resulting chromate ester evolves to a formamide. 

Other less common but highly effective trapping techniques include cyanide trapping via iminium ion formation and methoxylamine or semicarbazide for reactive... 

Although the anodic generation of a cation in a-position to nigrogen in aliphatic amines is not difficult, this type of reaction is not always useful to the synthesis of alkaloidal compounds, since the cation is not stable and a simple dealkylation is the usual follow-up reaction. N-monomethyl and N,N-dimethylanilines are, however, useful starting materials for the synthesis of the skeleton of tetrahydroquinoline. The anodic methoxylation of N,N-dimethylaniline 20 takes place at the methyl group, and an iminium ion intermediate 22 is easily generated by treatment of the methoxylat-ed product 21 with Lewis add. This intermediate can be trapped in situ with a variety of nucleophiles such as electron-rich olefins yielding tetrahydroquinolines 23 16). 

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. 

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... 

Interestingly, when the aforementioned AT-lithiopyrrolidine was trapped with water rather than methyl iodide, a mixture of 15 and 16 was obtained in 74% yield (Scheme 16.4). This gave Pearson the opportunity to make (—)-amabiline. For this, 16 was reacted with Eschenmoser s salt to produce the reactive iminium ion 17 which then underwent electrophilic aromatic substitution to give (-)-amabiline in excellent yield after acid-mediated transketalisation of the O-isopropylidene group. 

The Ru(II)/ROOH system can also be used to oxidize tertiary amines. The intermediate iminium ion is formed, as described earlier for secondary amines, and can be trapped by nucleophiles. Thus, the ruthenium-catalysed oxidation of tertiary amines with hydrogen peroxide in methanol can be performed to give the corresponding a-methoxyamines with high efficiency as illustrated in Fig. 24 [ 137]. Another example is the selective demethylation of tertiary amines in methanol with a combination of Ru(II) and H202, followed by hydrolysis of the intermediate a-methoxylated amines. For example, the methoxylation of N,N-dimethyl-p-toluidine followed by treatment with 2 N HC1 solution gave N-methyl-p-toluidine in 75% yield (Eq. 35) [137]. 

An important application of oxidation of a C-H bond adjacent to a nitrogen atom is the selective oxidation of amides. This reaction proceeds in the presence of ferf-BuOOH as the oxidant and Ru(II) salts. Thus in the example of Eq. (36), the a-tert-butylperoxy amide of the isoquinoline was obtained, which is an important synthetic intermediate for natural products [138]. This product can be conveniently reacted with a nucleophile in the presence of a Lewis add. Direct trapping of the iminium ion complex by a nudeophile was achieved in the presence of trimethylsilyl cyanide, giving a-cyanated amines as shown in Eq. (37) [45]. This ruthenium/peracid oxidation reaction provides an alternative to the Strecker reaction for the synthesis of a-amino acid derivatives since they involve the same a-cyano amine intermediates. In this way N-methyl-N-(p-methoxyphenyl) glycine could be prepared from N,N-dimethyl-p-methoxyaniline in 82% yield. 

Tertiary amines form complexes with mercury(II) ion, which then give iminium ions by loss of a piotoa Addition of perchloric acid permits isolation of the iminium ion as the perchlorate salt and generally the more-substituted ion is favored (equation 14). Intramolecular trapping by a hydroxyalkyl group is also possible to form aminals (equation 15). The lactam products result from over-oxidation, which is promoted by heat. Basification on the other hand usually allows the isolation of enamines although hydroxyenamines have been obtained by reaction of enamines with mercury(II) acetate (equation 16), while dihydroaromatic systems undergo aromatization (equation 17). ... 

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