Salts azolium

Reactivity of neutral azoles Azolium salts Azole anions... 

Condensation Reaction of Cyclopentadienides with Azolium Salts. 130... 

As described in Section II,A,1, the condensation of azolium salts with aza derivatives of cyclopentadienide (pyrroles, imidazoles, pyrazoles) also plays... 

Keywords Azolium salts AJ-heterocydic carbenes transition metal complexes asymmetric catalysis... 

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... 

In 2007, Fernandez et al. demonstrated that transition-metal complexes with heterobidentate S/C ligands based on imidazopyridin-3-ylidene and thioether functionalities could be readily prepared from the corresponding azolium salts by reaction with Ag20 and transmetalation of the resulting silver carbenes with appropriate metal sources. The cationic Pd(allyl)(carbene-S) complexes have proven to be active catalysts in the test reaction, reaching enantioselectivities of... 

Recent European patents,645,646 drawing on much earlier work647 649 describe an assay for NADH based on its ability to reduce naphthotri-azolium salts (236) to the azo dye (237) in close similarity to tetrazolium salts. 

Synthesis of [1,2,3]triazolo[1,5-c]pyrimidines and [1,2,4]triazolo[1,5-c]pyrimidines A novel approach to [l,2,3]triazolo[l,5-c]pyrimidines is shown in Scheme 55. Batori and Messmer - in the course of their investigations on fused azolium salts - described a synthetic pathway to l,3-disubstituted[l,2,3]triazolo[l,5-c]-pyrimidinium salts <1994JHC1041>. The cyclization was accomplished by transformation of the hydrazone 436. This compound was subjected to an oxidative ring closure by 2,4,4,6-tetrabromo-2,5-cyclohexadienone to give the bicyclic quaternary salt 437 in acceptable yield. 

The formation of azolium salt by intramolecular cyclization has been reported < 1993JAN1866> as well as a similar cyclization under desilylating conditions (Equations 8 and 9). 

The most practical approach is the direct treatment of azolium salts with metal complexes under neutral or basic conditions [39,154-159]. Alternatively, the free carbenes can be generated in the presence of a suitable metal complex by reduction of a carbene precursor, e.g. a thiourea [160]. Stable, uncomplexed imidazoline-2-ylidenes, isolated for the first time in 1991 by Arduengo [161] (for further examples see [162-166]), are also convenient starting materials for the preparation of carbene complexes [167,168]. The corresponding diaminocarbene complexes can be obtained by treatment of the stable diaminocarbenes with transition metal complexes. Finally, at high temperatures many transition metal complexes catalyze the carbon-carbon bond scission of tetraaminoethylenes, forming carbene complexes [169-171]. Examples of such preparations are given in Table 2.8. 

The following mechanism for condensation of a nucleoside phosphoroamidite and a nucleoside activated by an azolium salt AzH X has been proposed [13, 20j]. This Scheme 3 includes the additional pathway d related to recent stereochemical studies. 

In many cases the synthesis of NHC complexes starts from iV,A/ -disubstituted azolium salts. Imidazolium salts as precursors for imidazolin-2-ylidenes are generally accessible by two ways complementing each other (i) nucleophilic substitution at the imidazole heterocycle or (ii) a multicomponent reaction building up the heterocycle with the appropriate substituents in a one-pot reaction. 

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

Scheme 6. Synthesis of ruthenium-alkylidene complexes starting at the azolium salt without isolating the NHCs.

Up to now, NHCs or their azolium precursors, respectively, have been screened only in palladium- or nickel-catalyzed Heck-type reactions. This is probably based on the fact that the generation of catalytically relevant palladium and nickel complexes starting directly at the azolium salt had been known before (vide supra). 

The in situ preparation of a ruthenium-alkylidene catalyst for olefin metathesis is the first step for extending this high-throughput approach toward other catalytic transformations and opens up the way to the screening of azolium salt libraries for olefin metathesis reactions. ... 

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