Nucleophilic carbenes and the wanzlick equilibrium: A reinvestigation

Evidence for an Equilibrium between an N-heterocyclic Carbene and Its Dimer in Solution

“…[7] In contrast to this, Denk et al recently showed by NMR spectroscopy that metathesis products of type 2 a 2 b can be obtained from two different enetetramines at elevated temperatures. [8] Scheme 2. Metathesis experiment using two different enetetramines.…”

“…Denk et al have already pointed out that the formation of the metathesis product 2 a 2 b from (2 a) 2 and (2 b) 2 can also be explained by a [22] cycloaddition/cycloreversal. [8] Only the detection of 2 and (2) 2 at the same time in the absence of any electrophile can be regarded as unequivocal proof for an equilibrium between carbene and enetetramine. In this paper we describe dibenzotetraazafulvalene (3 d) 2 which at ambient temperature is already in equilibrium with the corresponding annelated N-heterocyclic carbene 3 d. In addition, we present the opening of the bridged compound 3 c to form a molybdenum dicarbene complex (Scheme 3).…”

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

“…According to Lemal the pure dimers do not dissociate. However, Denk's experiments 318 give reason to acknowledge the possibility of a cross-dissociation occurring in the dimer forming reaction. On the other hand, Taton and Chen have shown 319 that dimerization is observed for the cyclic bisimidazole-2-ylidenes with two linkers of three methylene units, while for the analogue with four methylene links only the biscarbene was detected.…”

Conformational analysis of N,N′-dinaphthyl heterocyclic carbenes: imidazol-2-ylidenes and imidazolin-2-ylidenes

“…Their stability depends on two factors: aromaticity, i.e., I versus II, and steric hindrance due to the N-substituents [24][25][26]. For a discussion on the dimerization of NHCs and the reversibility of this reaction, see [27,28].…”