Singlet carbene reaction structure

The ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

The intermediate produced by loss of nitrogen from a l.ff-triazole can be written as an imino-carbene, as a zwitterion, or as a diradical. Subsequent reactions of the intermediate which have been observed (Scheme 57) include (a) photochemical WoUf rearrangement (suggesting that the intermediate may have a singlet carbene structure) ... 

A deeper understanding of carbenic philicity requires a more detailed representation of the addition reaction transition state than that afforded by structure 4. Early MO calculations furnished structure 6 as representative of the transition state for addition of a singlet carbene to an alkene (Fig. 7.6). " ... 

A number of minima corresponding to oxonium ylides and H-bonded structures were found on the potential-energy surface for reaction of singlet carbenes with water and alcohols." Laser flash photolysis revealed that the rates of reaction between cyclopentadienylidene or fluorenylidene and alcohols increased with alcohol acidity and had linear Bronsted plots with slopes of 0.061 and 0.082, respectively.100 These results point to protonation with a very early transition state or to concerted OH insertion. For tetrachlorocyclopentadienylidene, the results showed that ylide formation (100) is predominant. 

Since data for the mechanistic details, such as the spin states and initial attacking site of the insertion of dichlorosilylene is lacking, inferences about the nature of the insertion reactions are at best a straightforward assumption based on the molecular structure of the final products. However, the insertion reactions of SiCl2 into C—X bonds, which proceed through elimination-recombination steps, do resemble the chemistry of singlet carbenes (59). It seems reasonably safe... 

Gloss and Moss have suggested the term carhenoid to describe intermediates that exhibit reactions quahtatively similar to those of carbenes without necessarily being free divalent carbon species. When carbenes are coordinated with metals they are known as carbenoids that have modified reactivities. lodomethylzinc iodide is often referred to as a carhenoid, because it resembles a carbene in its chemical reactions (Simmons-Smith reaction) (Scheme 5.14). Rhodium and copper carbenoids are unstable whereas some transition metals such as tungsten and chromium form stable and isolable carbenoids called metallocarhenes or Fischer carbenes. Carbenoids are structurally related to singlet carbenes and possess similar reactivity. 

---