Carbenes rings

Complexes of the nickel subgroup include C-coordinated, carbene, ring-opened, and cyclopalladated cases, while N-coordination is scarce. 

FPhosphoylide-aminocarbenes their protonated forms (possibly at the /S-carbon of the carbene ring) have the following Pi values —0.51 (PR3 = PPhs, PPh2(CH2Ph)), —0.60 (PR3 = PMes) V. iCyanoguanidine, at trr3f)s- Re(CNMe)(dppe)2. ... 

Irradiation of diazo quinoline 153 in alcohol provided the corresponding indole-3-carboxylic esters in 8-61% yield. Mechanistically, the transformation is a Wolff rearrangement and involves the formation of an intermediate carbene, ring contraction to ketene, and its solvolysis (76S754). 

Cp 2Sm(THF)2 reacts with l,3-diisopropyl-4,5-dimethylimidazoline-2-ylidene, C3N2Me2Pr 2 (= Pr -carben), forming Cp 2Sm(Pr -carben). In the carbene ring the values for the C-N distances alternate between 1.395(4) and 1.348(4) A. The Sm-C, C-C, and the exocyclic N-C bond lengths are in the normal range.585 Several related samarium(n) and ytterbium(ll) carbene complexes have been synthesized analogously. 

Several transformations of pyrazoles into pyridazines have been reported. In a carbene ring expansion, 3,4,5-trimethylpyrazole was treated with dichlorocarbene under neutral conditions. A very small amount of a pyridazine (51) together with a pyrimidine and two pyrazole derivatives were obtained. Under basic conditions no pyridazine was formed and the major product was the pyrazole 52, which could be rearranged with sodium ethoxide at 125° into the pyridazine 53. Pyridazines are formed also from certain pyrazoles upon heating, for example 54, - or from some pyrazolines. ... 

Another interesting route to tri-t-butylcyclobutadienes started from a tri-t-butylcyclopropene derivative and involved a carbenic ring expansion [38,54] ... 

Another most recent and successful example concerns the ring-expansion metathesis polymerization (REMP) of cycloolefins using cyclic carbene-ruthenium complex. " In this reaction, the cyclic olefin coordinates onto the ruthenium active center before insertion into the cyclic carbene ring. 

Diverse N-heterocyclic carbenes containing Lewis-acidic boron atoms within the carbene ring have been described in the literature. Carbene 128 derived from a four-membered heterocycle (Figure 1.11) showed several similarities with the previously discussed carbene 77b (see Scheme 1.11). Substitution of the... 

Heterocycles of type 145 are isoelectronic with the stable borazine. The carbene ring in 145c is nearly planar and the cyclohexyl groups are oriented... 

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