Carbenes singlet

Relative rates of some prototypical carbenes, obtained by Stem-Volmer methods, are listed in Table 2. Although many of these carbenes have triplet ground states, reaction with nucleophiles Y occurs prior to spin equilibration. Most often, ylide formation with solvent molecules was analysed in terms of Eq. 3. The pyridine-ylide served as the probe for 154. 

The results for methylene confirm the indiscriminate nature of the parent carbene. A small deuterium isotope effect is apparent for MeOH vs. MeOD, 

Linear dependencies of k0bS on [ROH] are also rare for the aryl(chloro)carb-enes 200 and 201 91 hence the ranking of these carbenes is somewhat uncertain. The aryl(oxy)carbenes 203, 154a, and 154c are clearly superior to their alkyl(oxy) counterparts (198, 199) as proton acceptors.76,77 The 4-OMe 

The reactivities of carbenes toward alkenes have been correlated with the inductive and resonance effects of the carbene substituents, log k — a Eat + fcEaR+ + c.m Analogous correlations cannot be obtained for the reaction rates of carbenes with alcohols, neither with the substituent parameters used by Moss,109 nor with related sets.110 In particular, the substituent parameters do not describe the strong, rate-enhancing effect of aryl groups. For a detailed analysis, see the discussion of proton affinities (Section V.A). 

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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. 

In the photolysis of difiuorodiazirine (218) a singlet carbene was also observed (65JA758). Reactions of the difiuorocarbene were especially studied with partners which are too reactive to be used in the presence of conventional carbene precursors, such as molecular chlorine and iodine, dinitrogen tetroxide, nitryl chloride, carboxylic acids and sulfonic acids. Thus chlorine, trifiuoroacetic acid and trifiuoromethanesulfonic acid reacted with difiuorodiazirine under the conditions of its photolysis to form compounds (237)-(239) (64JHC233). 

A novel route to 2-fluoropyridines involved the base-induced decomposition of substituted N-fluoropyridinium salts. Abstraction of the 2-H produces a singlet carbene (11) that removes F from a counterion. This is in contrast to the reaction with C nucleophiles, which are fluorinated, and is attributed to the high stability of C—F compared to O—F and N—F (89JOC1726). 

Wanzlick showed that the stability of carbenes is increased by a special substitution pattern of the disubstituted carbon atom [12-16]. Substituents in the vicinal position, which provide n-donor/a-acceptor character (Scheme 2, X), stabilize the lone pair by filling the p-orbital of the carbene carbon. The negative inductive effect reduces the electrophilicity and therefore also the reactivity of the singlet carbene. 

This is not surprising, since triplet carbenes are free radicals. But singlet carbenes can also give this reaction, though in this case only halogen atoms are abstracted, not hydrogen. 

For evidence that at least some singlet carbenes add by a two-step mechanism, see Giese, B. Lee, W, Neumann, C. Angew. Chem. Int. Ed. Engl., 1982, 21, 310. 

Figure 7.11. Singlet carbene stabilizing substituents D = electron donating groups, W electron withdrawing groups.

Figure 7.12. Generalized chemical structures of selected singlet carbenes with known X-ray structures.

Singlet Carbene C-H Insertions Although [1,2]-H shifts are formally carbene C-H insertions, these rearrangements have different orbital symmetry aspects than those of intramolecular insertions. As described above, overwhelming evidence exists that triplet carbenes undergo abstraction-recombination reactions to... 

Carbon Tunneling in Singlet Carbenes As described earlier in... 

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