Formation of N‐Heterocyclic Carbenes by Tautomerization of Mesomeric Betaines: Cyclic Boron Adducts and Palladium Complexes From 2‐(Imidazolium‐1‐yl)phenolates

Abstract

2-(Imidazolium-1-yl)phenolates are conjugated heterocyclic mesomeric betaines in tautomeric equilibrium with the corresponding N-heterocyclic carbenes (NHCs), 3-(2-hydroxyphenyl)-imidazol-2-ylidenes. The carbene tautomers can be trapped as thiones (X-ray analysis). Moreover, bis(triphenylphosphine)palladium(II) dichloride in THF trapped the carbene tautomer as a palladium complex without participation of the phenolate group (X-ray analysis). The corresponding anionic NHCs, 2-phenolate-substituted imidazol-2-ylidenes, can be trapped by triethylborane or triphenylborane to form 4,4-diethyl- or 4,4-diphenyl-4H-benzo[e]imidazo[2,1-c][1,4,2]oxaza-borininium-4-ides, respectively (two X-ray analyses). These tricyclic systems are the first representatives of a new heterocyclic ring system. The results of DFT calculations concerning the HOMO/LUMO profiles and partial charges are also presented.

From betaines to anionic N-heterocyclic carbenes. Borane, gold, rhodium, and nickel complexes starting from an imidazoliumphenolate and its carbene tautomer

“…Salt 5 is available via a three-step procedure [35]. Deprotonation with potassium carbonate resulted in the formation of the tautomeric equilibrium of the mesomeric betaine 6A and its NHC 6B (Scheme 1).…”

“…Preparation of lithium 2-(3-butyl-1 H -imidazol-2-ylidene-1-yl)phenolate (7): A solution of 0.02 g (0.09 mmol) of 2-(3-butyl-1 H -imidazolium-1-yl)phenolate [35] and 0.10 mL of lithium bis(trimethylsilyl)amide (1.0 M solution in THF) in 0.7 mL of pyridine was stirred for 30 minutes at rt. The product was characterized in solution, as traces of moisture reconstituted the starting material.…”

Anionic N-heterocyclic carbenes derived from sydnone imines such as molsidomine. Trapping reactions with selenium, palladium, and gold

“…Other examples of conjugated systems with related structures have been described as well. [24][25][26][27][28][29][30][31][32] Scheme 1.…”

Coinage Metal Complexes of the Carbenic Tautomer of a Conjugated Mesomeric Betaine Akin to Nitron

“…Nitron belongs to the class of conjugated mesomeric betaines (CMBs) [ 34 ]. Related work on other five-membered CMBs in equilibrium with their tautomeric NHCs was published by the groups of César and Lavigne [ 35 , 36 , 37 ], Braunstein and Danopoulos [ 38 , 39 , 40 ], Ganter [ 41 ] and Schmidt [ 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ], who also provided the first review of this burgeoning field [ 50 ]. We demonstrated that the electronic properties of 2 ′ are very similar to those of the “Enders carbene” ( 1 ) [ 31 , 32 , 33 ], as is reflected by their essentially identical Tolman Electronic Parameter (TEP) values [ 51 ] as well as by the very similar 77 Se NMR chemical shift values (∆δ = 4 ppm) determined for their corresponding selenone derivatives.…”