Iminium ions generation

Other methods for the synthesis of a-oxygenated A-alkylamides (or carbamates) include addition of oxygen nucleophiles to A-acyl (or A-alkoxycarbonyl) iminium ions, generated via either... 

Iminium ions, generated in aqueous solution from secondary amines and formaldehyde, undergo a Barbier-type allylation mediated by tin, aluminum, and zinc. The reaction is catalyzed by copper and produces tertiary homoallylamines in up to 85% yield.67 The imines generated in situ from 2-pyridinecarboxaldehyde/2-quinolinecarboxaldehyde and aryl amines undergo indium-mediated Barbier allylation in aqueous media to provide homoallylic amines.68 Crotyl and cinnamyl bromides... 

Petasis reported an efficient addition of vinyl boronic acid to iminium salts.92 While no reaction was observed when acetonitrile was used as solvent, the reaction went smoothly in water to give allyl amines (Eq. 11.54). The reaction of the boron reagent with iminium ions generated from glyoxylic acid and amines affords novel a-amino acids (Eq. 11.55). Carboalumination of alkynes in the presence of catalytic Cp2ZrCl2 and H2O affords vinylalane intermediates, which serve as nucleophiles in the subsequent addition to enantiomerically enriched... 

The starting material was prepared with the modification on the epibatidine bicychc ring system by repositioning the nitrogen atom to a methylene group. We carried out the Hetero-Diels Alder reaction of cyclopentadiene and iminium ion generated from ammonium chloride and formaldehyde in the aqueous medium and protected resulted unstable secondary amine with benzoyl chloride to provide (3) in good yields [5] (Scheme 38.1). 

Using this information in conjunction with a study into the preferred conformations of iminium ions generated from catalysts 12 and 21, Houk suggests that the additional steric bulk of the ferf-butyl group causes the benzyl arm of the catalyst to shield better the Si face of the C=C double bond - a requirement for high ees in an open transition state. For both the Diels-Alder and pyrrole/indole alkylation... 

The iminium ion generated in situ from the oxazolidine in equation 66 undergoes cyclization with (Z)-vinylsilane satisfactorily. Attempts to cyclize the corresponding E-isomer are unsuccessful136. 

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. 

This Strecker synthesis is of phenylglycine, an amino acid not found in proteins. Cyanide reacts more rapidly with the iminium ion generated in the first step than it does with the starting benzalde-... 

The a-aminoalkyl radicals as well as iminium ions generated as intermediates in electron-transfer reactions of amines can be used for bringing about synthetically useful transformations of amines. The synthetic applications of amine oxidation reactions brought about by thermal, electrochemical and photochemical methods as discussed below. 

Enamines can be obtained as the products of the Polonovski reaction of amine oxides and, in particular, by reaction of piperidine A -oxides with acetic anhydride. This is primarily due to the fact that when acetate is the counterion the intermediate iminium ions are labile and readily tautomerize. The formation of enamines during the Polonovski reaction is also favored by the presence of a base. In fact, enamines are often obtained in high yield from the reaction of an IV-oxide with trifluoroacetic anhydride in the presence of triethylamine or pyridine. Conversion of intermediate iminium ions, generated under modi-fred Polonovski conditions, to enamine products can also occur during hydrolytic work-up. 

In this section, reactions are featured that involve the ortho trapping of an iminium ion generated oxidatively without the abstraction of H by the adjacent group. (In principle, this process could also involve an a-amino radical). Some years ago, we demonstrated that Mn02 oxidation of o-substituted /-anilines can lead to intermediates that can be trapped by appropriate ortho nucleophiles [68JCS(C)1722 69CI(L)443]. Thus o-amino- and o-carboxyphenyldialkylamines were cyclized to give... 

The aza-Diels-Alder reaction combines three components (an aldehyde, an amine salt and a diene) to produce heterocyclic products which are useful synthetic intermediates. Grieco first reported that such a reaction could occur in water. Preliminary studies focused on the reaction of dienes with iminium ions generated by an amine hydrochloride and 37% aqueous formaldehyde solution [59]. When dienylamine hydrochlorides are treated with aqueous formaldehyde at 50 °C, bicyclic ring systems are formed (Scheme 9). 

While these various intramolecular cyclization modes make accessible a myriad of azacycles, the practitioner is faced with the delicate art of orchestrating an efficient cyclization. Cyclization conditions must be such that the reactivity of the nucleophile is preserved, while unleashing intramolecularly the electro-philicity of an iminium ion. A listing of the most important routes to iminium ions is found in Scheme It is important to note that many of the methods for iminium ion generation are not conducive to the preservation of certain nucleophiles. The judicious choice of reaction conditions is the key to an effective intramolecular Mannich or Mannich-related reaction. 

DiaJkylamino-l-(trimethylsUyiyi,2-butadienes Iminium ions generated by the... 

Cyanide will react with iminium ions to form a amino nitriles. Although these compounds are relatively unimportant in their own right, a simple hydrolysis step produces a amino adds. This route to amino acids is known as the Strecker synthesis. Of course, it s not usually necessary to make the amino acids that Nature produces for us in living systems they can be extracted from hydrolysed proteins. This Strecker synthesis is of phenylglycine, an amino acid not found in proteins. Cyanide reacts more rapidly with the iminium ion generated in the first step than it does with the starting benzaldehyde. 

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