Stable carbenes

Keywords Carbene Stable carbene Non-NHC NHC Ligand Catalysis... 

The superficially similar carbene 4.116, another carbene stable enough to be isolated, has the best of all worlds. With six electrons for the n system coming from the double bond and the two nitrogen lone pairs, it has an aromatic sextet without having to fill the pz orbital. The pz orbital is empty and the px orbital is filled, making this a nucleophilic carbene, which reacts with the electrophile carbon disulfide to give the zwitterion 4.117. 

Most carbenes R-C-R, in which R and R are H, alkyl, vinyl, or aryl, are too reactive to be observable inside a hemicarcerand. " For example, incarcerated arylcarbenes rapidly insert into C-H or C-0 bonds of the hemi-carcerand or add to one of the cavitand s aryl units even at very low temperature. " However, stability and reactivity of carbenes can be tailored especially with heteroatom substituents that stabilize the carbene s singlet state through electron donation (push effect). " " In fact, many diaminocarbenes are stable and isolable at room temperature. " " In cases where intrinsic stabilization (e-donation) is not sufficient, extrinsic effects (incarceration) may render an otherwise fleeting singlet carbene stable under normal conditions. 

On the other hand, triplet carbenes stable enough for X-ray analysis have not been realized for a long time. This is partly because a triplet state is less readily stabilized thermodynamically than its singlet counterpart. Many aOonpts have been made to stabilize triplet carbenes kinetically by protecting the carbene center with a bulky group, but the voracious appetite for electrons has made it extranely difficult to realize a triplet carbene stable enough for isolation and X-ray analysis, although fairly stable triplet carbenes have been available [11,12]. 

Lavallo, V., Canac, Y, Donnadieu, B., Schoeller, W.W. and Bertrand, G. (2006) CO fixation to stable acydic and cyclic alkyl amino carbenes stable amino ketenes with a small HOMO-LUMO gap. Angewandte Chemie-Intemational Edition, 118, 3568-3571 Angewandte Chemie-Intemational Edition, 2006, 45, 3488-3491. 

Comments The diene A is symmetrical so it doesn t matter which double bond is attacked by the carbene. On the other hand, it may be difficult to stop carbene addition to the second double bond. The only control over the stereochemistry will be that the trans compound we want is more stable. Japanese chemists have recently synthesised optically active trans chrysanthemic acid by this route (Tetrahedron Letters. 1977, 2599). 

The remaining classes oF monohapto organic ligands listed in Table 19.2 are carbene (=CR2), carbyne (=CR), and carbido (C). Stable carbene complexes were first reported in 1964 by E. O. Fischer and A, Maasbol. Initially they OMe... 

It was observed occasionally that the DTDAF 51a can revert back to the stable carbene so that the dimerization appears to be reversible (97LAR365). Tills splitting reaction could not always be clearly reproduced and seems to be influenced by trace contaminants that are as yet unidentified. 

Imidazol-2-ylidene and its N,N -disubstituted derivatives as stable carbenes 99ACR913. 

Diisopropylphenyl)-4,5-dimethylthiazol-2-ylidene as a stable carbene 99ACR913. 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

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