1,3-dipolar cycloaddition reactions salts

In 1997 the application of two different chiral ytterbium catalysts, 55 and 56 for the 1,3-dipolar cycloaddition reaction was reported almost simultaneously by two independent research groups [82, 83], In both works it was observed that the achiral Yb(OTf)3 and Sc(OTf)3 salts catalyze the 1,3-dipolar cycloaddition between nitrones 1 and alkenoyloxazolidinones 19 with endo selectivity. In the first study 20 mol% of the Yb(OTf)2-pyridine-bisoxazoline complex 55 was applied as the catalyst for reactions of a number of derivatives of 1 and 19. The reactions led to endo-selective 1,3-dipolar cycloadditions giving products with enantioselectivities of up to 73% ee (Scheme 6.38) [82]. In the other report Kobayashi et al. described a... 

The first report on metal-catalyzed asymmetric azomethine ylide cycloaddition reactions appeared some years before this topic was described for other 1,3-dipolar cycloaddition reactions [86]. However, since then the activity in this area has been very limited in spite of the fact that azomethine ylides are often stabilized by metal salts as shown in Scheme 6.40. 

Pyranopyrroloimidazoles have been prepared stereospecifically by an intramolecular 1,3-dipolar cycloaddition reaction. Either enantiomer of the imidazoline derivative 176 (the -enantiomer is shown) may react with the bromoacetyl-containing acrylate dipolarophile 177, in the presence of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU), to give the diastereomerically pure tricyclic product 178 in moderate yield (Equation 15). This reaction involves quaternization of the imidazole N, reaction of the quaternary salt with base to give the 1,3-dipole, which can then react, intramolecularly and stereospecifically, with the tethered dipolarophile <1997TL1647>. 

In 1981 we published the first paper [22] on the synthesis of s-triazolo[4,3-a]pyridinium salts, 4, by the anodic oxidation of hydrazones 3 in the presence of pyridine (Scheme 5). In our working mechanistic scheme we proposed nitrilimine as the possible intermediate and pointed out that this reaction opens the door to a wide range of heterocyclic systems via anodic oxidation of aldehyde hydrazones through 1,3-dipolar cycloaddition reactions of the nitrilimine involved. 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

Treatment of meso-ionic l,2,3,4-oxatriazole-5-thiones (286) (Section VII, I, 3) with boiling ethanoUc ammonia yields the isomers 297. These belong to a new class of meso-ionic heterocycle, which by O-alkylation with triethyloxonium tetrafluoroborate 3rield the salts 298, These are useful intermediates for the sjmthesis of a number of novel types of meso-ionic 1,2,3,4-thiatriazoles (299, 300, and 301). The l,2,3,4-thiatriazol-5-ones (297) have dipole moments in accord with their meso-ionic formulation. They are remarkably stable to acidic hydrolysis, and 1,3-dipolar cycloaddition reactions have not been observed alkaline hydrolysis yields aryl azides. 

Aminofuro[3,2- ]pyridinium tosylates can be prepared from furopyridines by treatment with 0-(4-methylbenzene-sulfonyl)hydroxylamine. Treatment of the tosylate salt with base, such as potassium carbonate, leads to a pyridinium A -imide, 50. Compound 50 readily undergoes 1,3-dipolar cycloaddition reactions <1999CCC539>. 

Salts of the linear [S=N=S]" cation (isoelectronic with CS2) are readily prepared. This cation undergoes quantitative, 1,3-dipolar cycloaddition reactions with unsaturated molecules such as alkenes, alkynes and nitriles, and also with NS" , to give a variety of ring compounds (Scheme 2.8). The... 

The intermediate l-aza-2-azoniallene cations can undergo 1,3-dipolar cycloaddition reactions with inverse electron demand [119]. Thus, oxidation of a A -phenyl hydrazone in the presence of pyridine leads to the formation of a. y-triazolo[4,3-a]-pyridinium salt by attack of pyridine as a nucleophile on the intermediate nitrilimine [Eq. (17)] [120]. Other examples are reported by Jugelt [121]. 

Dipolar cycloaddition reaction of propargyl ammonium salt and 2-azidoethylammonium salt is greatly enhanced in the presence of cucurbituril because the hydrophobic cavity of cucurbituril is an excellent reaction field for the cyclocondensation [175]. Mock et al. obtained [2]rotaxane consisting... 

The quaternary salts of 3-hydroxy-pyri(jines are converted by mild base into zwitterionic, organic-solvent-soluble species, for which no neutral resonance form can be drawn. These pyridinium-3-olates undergo a number of dipolar cycloaddition reactions, especially across the 2,6-positions. ... 

TABLE 4.14. 1,3-DIPOLAR CYCLOADDITION REACTIONS OF IN SITU-GENERATED MUNCHNONES AND VINYL PHOSPHONIUM SALTS"... 

TABLE 4.15 1,3-DIPOLAR CYCLOADDITION REACTIONS OF MUNCHNONES (AZLACTONES) AND VINYL PHOSPHONIUM SALTS, 515... 

Rate accelerations in 1,3-dipolar cycloaddition reactions between alkenes and nitrones as well as improvements in endo/exo selectivity were achieved with certain Lewis acids, such as ZnCl2, TiCl(OiPr)3, TiCl2(OiPr)2, Mg(II) salts or BF3 0Et2, ... 

Miscellaneous Mn02-mediated Reactions. Treatment of a quinoxalinium salt with various alkenes in the presence of Mn02 has been found to effect a novel 1,3-dipolar cycloaddition reaction to give the corresponding pyrrolo[l,2-a]quinoxalines (eq 65). ... 

---