Expanding the Scope of Chelating Triazolylidenes: Mesoionic Carbenes from the 1,5‐“Click”‐Regioisomer and Catalytic Synthesis of Secondary Amines from Nitroarenes

Abstract

Chelating 1,2,3-triazolylidenes have been established as privileged ligands in homogeneous catalysis. We present herein a new approach towards chelating 1,2,3-triazolylidene ligands based on the 1,5-regioisomer of the corresponding triazole, which can be obtained through simple click chemistry. The new ligands are compared to their 1,4-regioisomeric counterparts through coordination to the ruthenium p-cymene fragment. The complexes are characterized structurally and spectroscopically and are employed as (pre)catalysts in the reductive condensation of nitroarenes and primary alcohols to yield secondary amines. The activity of chelating mesoionic carbene ligands obtained from the two different regioisomers of the triazoles are compared and contrasted in catalysis. The performance of the ruthenium complexes with mesoionic carbenes could be improved through the choice of the employed base and reaction conditions, giving rise to the most effective systems thus far. The results presented here prove the utility of chelating mesoionic carbenes as an extremely potent class of ligands for the synthesis of secondary amines from nitroarenes.

Mono‐ and Di‐Mesoionic Carbene‐Boranes: Synthesis, Structures and Utility as Reducing Agents

“…The mono triazolium salts ( L1 – L3 , Scheme 1) were synthesized by reaction of the corresponding triazolium with CH 3 I, [18] and the bitriazolium salt L4 by the reaction of the bitriazole with Meerwein salt [19] . For the methylene‐bridged bitriazolium salt L5 a procedure, recently reported by us, which is based on the preactivation of diiodomethane with AgOTf followed by a reaction with the 1,5‐disubstituted triazole T5 was used (see experimental section) [20] …”

“…[19] For the methylene-bridged bitriazolium salt L5 a procedure, recently reported by us, which is based on the preactivation of diiodomethane with AgOTf followed by a reaction with the 1,5disubstituted triazole T5 was used (see experimental section). [20] The mono MIC-borane adducts L B 1-L B 3 as well as the diborane aduct L B 4 were all synthesized using the same procedure (Scheme 2 and experimental section). The corresponding triazolium salt was suspended in THF and cooled to 0°C, whereupon a base was immediately added, and the mixture was stirred for 30 minutes.…”

“…Synthetic route for the synthesis of L1-L5. Method A [a] : CuSO 4 • 5H 2 O, sodium ascorbate, K 2 CO 3 , pyridine, water/t-BuOH, r.t., 3d; [13][14][15][16] Method B [b] : tetramethylazanium hydroxide, DMSO, r.t., 12 h; [17] Method C [c] : CH 3 I, MeCN, 70°C, 2d; [18] Method D [d] : Me 3 OBF 4 , DCM, r.t., 3d; [19] Method E [e] : Me 2 I 2 , AgOTf, hexane/toluene, refluxe, 12 h. [20] Scheme 2. Synthesis of the Mesoionic Carbene-Boranes.…”

Aerobic dehydrogenation of amines to nitriles catalyzed by triazolylidene ruthenium complexes with O2 as terminal oxidant

“…This frequency is in agreement with previously reported chelating triazolylidene ruthenium complexes. 40,41 The absence of the triazolylidene proton as well as the splitting of the aromatic p-cymene protons into four distinct doublets indicate chelation and therefore N,Cbidentate coordination of the pyridyl-triazolylidene ligand in complexes 4-6. Furthermore, the pyridyl α proton is considerably deshielded, e.g., a doublet at δ H = 8.81 for complex 6b, which is characteristic [41][42][43] for N-coordination of the pyridyl unit to the ruthenium center (cf.…”

A Palladium-Free Sonogashira Coupling Protocol Employing an In Situ Prepared Copper/Chelating 1,2,3-Triazolylidene System

“…This protocol requires a simple experimental setup and can be easily followed in any synthetic laboratory. After screening a number of low-cost, widely available copper sources, in combination with the in situ prepared chelating 1,2,3-triazolylidene (Trz* Ph,Ph ) 2 CH 2 (L, Figure 1), 25 the most efficient catalytic system was found to be the one employing Cu(OAc) 2 . A number of electron-poor or electron-rich aryl iodides along with a variety of terminal alkynes were coupled with satisfactory results.…”