Amino-1,2,4-triazolium salts

Table 2 Density and thermal characteristics of amino-1,2,4-triazolium salts...

Drake, G., Hawkins, T., Tollison, K., Hall, L., Vi], A. and Sobaski, S., (lR)-4-Amino-1,2,4-triazolium Salts New Families of Ionic Liquids, in Ionic Liquids IIIB Fundamentals, Progress, Challenges, and Opportunities - Transformations and Processes, Eds. R.D. Rogers and K.R. Seddon, ACS Symposium Series, bl. 902 (American Chemical Society, Washington, 2005), pp. 259-302. 

H., Bonn, G. and Schottenberger, H., Quaternary 4-amino-1,2,4-triazolium salts Crystal structures of ionic liquids and iV-heterocyclic carbene (NHC) complexes, Z. Naturforsch. B 64 (6), 603-616 (2009). 

The partial inhibition of the hydrolysis observed at pH < 4, and the negative salt effect, may be attributed to a specific cation-anion interaction. The proposed mechanism receives further support from the observed effects on the hydrolysis rates of increased viscosity of the medium and of added inorganic nucleophiles. Hydrazine or alkylhydrazines cleave and recyclize 1,3,4-thiadiazolium salts (114) to 1,2- or l,4-dihydro-l,2,4,5-tetrazines (115) in high yield. The action of arylhydrazine results in the alternative recyclization of the probable intermediate ArNHN=CR2NR N=CR SH, to 4-amino-1,2,4-triazolium salts (116). 5-Amino-2-imino-3-phenacyl-l,3,4-thiadiazolines (117) isomerize in boiling ethanol to 5-amino-3-mercapto-l-phenacyl-l,2,4-triazoles (118). This example... 

This homoenolate methodology has been extended to the use of nitrones 170 as electrophiles [72]. Scheldt and co-workers have shown that enantiomerically enriched y-amino esters 172 can be prepared with excellent levels of stereocontrol from an enal 27 and a nitrone 170 using the NHC derived from triazolium salt 164 (Scheme 12.37). The oxazinone product 171, formally a result of a [3-1-3] cycloaddition, is cleaved to afford the y-amino ester product 172. The reaction shows broad substrate scope, as a range of substituted aryl nitrones containing electron donating and withdrawing substituents are tolerated, while the enal component is tolerant of both alkyl and aryl substituents. 

Arylhydrazones of A -acylbenzimidazoles (441) react with perchloric acid to rearrange into 1,2,4-triazolium salts 443, which can be isolated when R = Ar = Ph (Scheme 70). The protonated cycloadduct 442 represents a key intermediate. A reverse process was also pointed out neutralization of the triazolium salt 443 (R = Ar =Ph) with aqueous sodium carbonate gives back the corresponding 441, likely via a preliminary heter-ocyclization into a neutral cycloadduct. When R is a COMe or COOEt the unisolated triazolium salts 443 transform into final products by a condensation between the amino group and COMe or COOEt (89H339). 

The thiazolium-catalyzed addition of an aldehyde-derived acyl anion with a receptor is a valuable synthetic tool leading to the synthesis of highly funtionalized products. Acyl anion receptors include Michael acceptor (Stetter reaction), aromatic aldehyde (benzoin reaction), ketone, nitroalkene, aziridine, activated imine. Recently, nucleophilic addition of acyl anions to unactivated imines has been explored <07CC852>. Treatment of aryl aldehydes with imines 146 in the presence of triazolium salt 147 (20 mol%) and triethylamine (20 mol%) provides the a-amino ketones 148 in good yields. However, this methodology does not work for 4-pyridylaldehyde and tert-buty laldehyde. 

Acylation of 4-amino-1,2,4-triazole occurs on the 4-NH2 group <63JOC543) the result is not trivial as alternatives such as triazolium salt formation are conceptually possible. 

The /V-nitration of 4-amino-1,2,4-triazole, 1-aminobenzimidazole, N-aminobenzotriazole [73JCS(P 1)2624] and l,3-diamino-l,2,3-triazolium salts (891ZV2654) have been reported. The nitramine products exist in the... 

Amino-l-alkyl-s-triazolium salts, on condensation with acetoacetic ester and its derivatives, give mesoionic triazolopyridazines 364 (73JPR97). The reaction needs a base for the generation of ylid 363. 

However, iV-ammonium salts as well as the IV-imines possess further acidic hydrogen atoms. In 0.2 N NaOD, (V-aminopyridinium salts (14) exchanged their hydrogen atoms at the 2-, and 4-, and 6-positions for a D atom within the time the NMR spectra could be measured.17 For the 4-amino-s-triazolium salts (93) base catalysis is not needed for the hydrogen exchange.134 The ylid or nucleophilic carbene structures 94 and 95 are intermediates (see also Wanzlick135 and Quast and Hunig136) and can be intercepted by sulfur or ethyl acetoacetate.134... 

In addition to traditional organocatalysis, ionic liquids as new catalytic systems have been explored. The first examples used nonimmobilised OTBDPS-L-Ser, protonated arginine or lysine in the presence of ionic liquids based on l-alkyl-3-methyl imidazolium ([bmim], [hmim], [omim]) or JV-butyl-N-methyl pyrrolidinium ([bmpy]) ions. The systems, in addition to giving the aldol adducts with high yields and ee, are efficient for catalyst recovery and reuse. Since 2010 new structures containing a primary amino acid coupled with a 1,2,3-triazolium salt, an acyl group or a polystyrene have been developed. The more effective ones for the aldol reaction depicted Scheme 12.3 (13, 14 and 15, respectively) are represented in Figure 12.3. 

In 2002 and 2003, the Rovis group developed ehiral triazolium salts with amino indanoi and pyrrolidine frameworks whieh were found to be excellent eatalysts for intramolecular Stetter reaetions (up to 98% yield, 99% ee). The amino indanoi derived eatalysts and pyrrolidine derived catalysts, developed by Rovis and co-workers, turn out to be the most efficient catalysts so far for the intramolecular Stetter reaetions, providing the desired products in high yields and excellent enantioseleetivity. 

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). 

The amino-indanol-derived triazolium salt F5 was also found to be an efficient precatalyst for the intramolecular Stetter reaction of a-substituted cyclo-hexadienones (Scheme 20.17), furnishing tricyclic products 35 bearing multiple stereocentres in up to 96% yield and with >99% enantiomeric excess. ... 

The syntheses of 1,2,3-triazolium salts consists of two major steps, which are construction of the 1,2,3-triazole ring system and then its N-alkylation (Scheme 1). While l-amino-1,2,3-... 

In 1966, Sheehan reported a remarkable asymmetric benzoin reaction catalyzed by chiral thiazolium salts with moderate levels of enantioselectivity [56-59]. In 2002, Enders and coworkers made an important breakthrough when they reported the first highly enantioselective intermolecular benzoin reaction catalyzed by a triazolium salt derived from ferf-leucine [Eq. (1)] [60]. Since then, catalyst development for NHC catalysis has seen exponential growth for new triazolium salts derived from chiral amino acids and amino alcohols. 

For this field to continue to develop at this breakneck pace, several existing roadblocks must be overcome. First, the synthesis of the catalysts remains challenging. Although the amino alcohol-derived triazolium salts affect a... 

For this field to continue to develop at this breakneck pace, several existing roadblocks must be overcome. The synthesis of the catalysts remains challenging. Although the amino alcohol-derived triazolium salts effect a remarkably broad range of transformations with exceptional enantiocon-trol, it is difficult to rapidly modify the NHC pre-catalysts to improve those reactions that are less selective. Also, major gaps remain in the mechanistic understanding of these reactions and the elegant dance of proton transfer and redox reactions that characterize the novel cascades that have become the hallmark of NHC-catalyzed processes. 

Certain AMmines formed by deprotonation of N-aminoazolium salts also failed to undergo cycloaddition to give azapentalenes. TV-Amino-thiazolium mesylate on treatment with potassium carbonate and DMAD added 2 moles of dipolarophile to give the fused diazepine 219 2°5 4-Amino-l-methyl-s-triazolium iodide and DMAD in the presence of base gave the pyrazole 221, probably by ring opening of the first-formed adduct 220. Dehydrogenation to 222 did not occur.297... 

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