Chiral azolium salts

The azolium salts being the main precursors of the chiral diaminocar-benes, the next section will concern their preparations. 

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

The efficiency of synthesized chiral azolium salts (260)-(262), derived from (5)-pyroglutamic acid, as carbene precursors was evaluated in the [Rh(cod)Cl]2-catalysed asymmetric transfer hydrogenation of aromatic ketones in isopropanol, acting as the hydrogen donor, and KOH as promoter to the corresponding alcohol. It was reported that the use of (262) displayed the highest activity and asymmetric induction for the transfer hydrogenation. The yield was up to 94% and enantioselectivities up to 90% ee were observed. ... 

Chiral azolium salts 52 and 53 have been used in the intramolecular vinylogous Stetter reaction of oxygen substrates 54 to provide 3-substituted chromanones (Scheme 77) (13CEJ15852). Other examples are obtained from the free-radical cascade reaction of O-allyl acylphosphonate with various functionalized P-ketoxanthates in the presence of dilauroyl peroxide, with moderate to good yields (130L4818). A series of 2,3-disubstituted chromanones are synthesized from the reaction of acrylic acids with arynes in the presence of CsF (13T2789). 

Scheme 61, yielded thiazole 200 as the major product, along with minor amounts of carbinol 201 [152]. On the other hand, treatment of the imine formed from 199 and p-methoxyphenylamine with catalytic tetrabutylammonium cyanide, produced suc-cinimide derivative 202. In both cases, the process is initiated by nucleophilic attack to the carbaldehyde C=0 (or azomethine s C=N) group, which is followed up by an anionic rearrangement. A variation of the above process using as catalysts /V-heterocyclic carbenes (NHC) derived from base treatment of azolium, imidazo-lium, or triazolium salts, has also been developed to access gem-disubstituted succinimides [153, 154]. Unfortunately, an attempt of kinetic resolution of racemic 4-formyl (3-lactams by using chiral NHC resulted in moderate selectivities only [154]. 

In 2013, the Chi group realized an NHC-catalyzed asymmetric p-functional-ization reaction of aldehydes via the transformation of saturated aldehydes to formal Michael acceptors via double oxidation. By using the catalyst derived from the chiral amino indanol triazolium salt in combination with quinone as the oxidant, the p-aryl substituted saturated aldehydes were converted to the o,p-unsaturated acyl azolium intermediates which further reacted with 1,3-dicarbonyl compounds or p-keto esters to generate the corresponding 5-lactones. It was found the use of LiCl and 4 A MS as additives was beneficial to improve the ee s of the products. Notably, the p-alkyl substituted saturated aldehydes were not viable substrates, probably due to the reduced acidity of the p-C—H bonds (Scheme 7.118). 

In order to separate structural effects from the electronic differences of these two catalyst classes. Bode synthesized chiral imidazolium salt 57 (Scheme 14.28). This allowed direct comparison of imidazolium versus triazolium precatalysts across a number of different reaction manifolds including those involving the catalytic generation of homoenolate equivalents, ester enolate equivalents, and acyl anions. These studies conclusively demonstrated that imidazolium-derived catalysts are superior for homoenolate reactions with less reactive electrophiles, while the triazolium-derived pre-catalysts are preferred for all other reactions. Interestingly, from the currently published body of the work, it does not appear to be any effects from the counterion of the azolium pre-catalysts in the presence of bases. 

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