N-Heterocyclic Carbenes

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

“…Consequently books [14–16] and reviews cover the range from NHC structures in the light of their early history [17], syntheses [6,18], coordination chemistry [19–20], and catalysis [21–22], to biological activities of NHC complexes [23–24]. In the last decade, attention has also been directed to anionic N-heterocyclic carbenes [25].…”

Purine-substituted imidazolium mesomeric betaines and their tautomeric N-heterocyclic carbenes. Formation of a cyclic borane adduct

“…The bond length between the boron atom and the former carbene carbon atom is 1.6643(16) Å, whereas the bond between the boron atom and the methylene carbon atoms of the ethyl groups was found to have a bond length of 1.6226(16) Å and 1.6315(17) Å. The bond distance between the boron atom and the nitrogen atom is 1.6003 (14) Å.…”

“…13 Thus, carbene 7 is an example of a type of anionic N-heterocyclic carbene which differs from all aforementioned molecules. The chemistry of N-heterocyclic carbenes in the light of history 14 as well as the intersection between the substance classes of mesomeric betaines and N-heterocyclic carbenes have been reviewed. 15 In continuation of our interest in mesomeric betaines 16 and their conversion into N-heterocyclic carbenes, 14,15 and as part of our studies directed toward the chemical and physical consequences of different types of conjugation, we describe here imidazol-2-ylidenes which are in π-conjugation with an anionic purinate which serves as π-electron donator via N1 of the imidazole.…”

Interaction of N‐Heterocyclic Carbenes and Simple Carbenes with Small Molecules (One to Three Atoms) Excluding Metals: Formation of Covalent C–X Bonds

“…Interaction of N-heterocyclic Carbenes and Simple Carbenes with Small Molecules (One to Three Atoms) Excluding Metals: Formation of Covalent C-X Bonds In general, the neutral forms are more stable; heating these compounds results in the loss of CO 2 forming neutral azoles, such as 28 from 27N and 31 from 30N [82]. However, when both N atoms are substituted (this is true even for triazoles and tetrazoles), then the loss of CO 2 leads to the corresponding carbenes 5 and 33 [83]. This is the reverse of the reaction of NHCs with CO 2 of Scheme 17 that has been reported in several books dealing with NHCs [84][85][86] and the mechanism discussed based on using the B3LYP, B97D, or M06-2X DFT functionals with the conjunction of 6-31+G(d), 6-311+G(d,p), and aug-cc-pvTZ basis sets [87].…”