1.2.4- Triazolium salts reduction

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

The meso-ionic l,2,4-triazol-3-ones (200) are stable to acid, but alkaline hydrolysis gives 1,4-disubstituted semicarbazides. They do not normally participate in 1,3-dipolar cycloadditions, but the meso-ionic 1,4-diphenyl-l,2,4-triazol-3-one (200, R = R = Ph, R = H) and benzyne yielded 2-phenylindazole. 1,2,4-Triazolium salts (211) are formed with triethyloxonium tetrafluoroborate. Reduction of the meso-ionic compound 200, R = Me, R = R = Ph, with lithium aluminum hydride gives the triazolidinone 212. ... 

For the structural optimization of the tricyclic triazolium salt 119 the cw-tricyclic lactam 126 was chosen as the precursor for the synthesis of the tetracyclic triazolium salt 127. The diastereo- and enantiopure y-lactam 126 was synthesized following a procedure reported by Ennis et al. (Scheme 32) (Ennis et al. 1996 Nieman and Ennis 2000). a-Tetralone (124) was a-alkylated with ethyl bromoacetate and subsequently hydrolyzed to the corresponding carboxylic acid. Condensation with (R)-phenylglycinol yielded the lactam 125 as a single stereoisomer. Stereoselective reduction, dehydration of the alcohol, and acid-catalyzed enamine hydrolysis provided the cis-tricyclic lactam 126. The one-pot procedure that had previously been successful in the synthe-... 

The ring system of 1,2,3-triazoles (257) is stable to dissolving metals in acids," and 1,2,3-triazolium salts are resistant to reduction unless they are substituted at both positions N-1 and N-2. Thus whereas salts of the type (258) are not reduced, analogs of the form (259) are converted into triazolines (260) by reaction with sodium borohydride. ... 

Triazolium salts (335) are not readily reduced by NBH. Indeed, 4-phenyl- and 4,5-diphenyl-1,3-dimethyl-1,2,3-triazolium iodides do not undergo reduction, yet l-methyl-2-phenyl-l,2,3-triazolium triflate (336) is reduced to the 4-triazoline 337. This reactivity pattern is explained by the need for an immonium ion for successful reaction. The presence of a C-2 substituent in 336 makes this possible the lack of such a substituent localizes the double bond joining C-4 and C-5. 

In addition, this synthetic scheme offers a mild reaction route for the degradation of a-hydroxy acids to the corresponding aldehydes containing one less carbon atom or for their conversion to aldehydes with the same number of carbon atoms. For example, reaction of 1,4-diphenylthio-semicarbazide with a-acetoxyacyl chlorides yields 5-(a-hydroxyalkyl)-1,2,4-triazolium salts, which, upon sodium hydride reduction and acid treatment, give benzaldehyde along with 138 (R = H) (Scheme 58) (75TL1889). Moreover, the 5-(a-hydroxyalkyl)-l,2,4-triazolium salts can be easily converted to the 5-alkyl compounds (Scheme 58) and borohydride reduction of the latter followed by acid hydrolysis provides a method for the transformation of a-hydroxy acids to aldehydes with the... 

NHCs have become popular ligands in coordination chemistry owing to the facile access to this type of ligands and to metal-NHC complexes. Most NHC ligands are prepared from azohum compounds such as imidazolium, triazolium, benzimidazo-lium, imidazolidinium, or thi azolium salts [1]. Alternatively, the reductive desulfurization of imidazolin-, benzimidazolin-, and imidazolidin-2-thiones to yield a variety of NHCs has been described. The preparation of suitable azolium salts and imidazolin-2-thiones is presented in Sect. 2.1. This is followed by the description of methods to liberate the free NHCs from these compounds. Today, stable singlet... 

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