Singlet carbene reaction

These results are inconsistent with a radical rebound mechanism because this mechanism is a two-step process that requires the involvement of intermediates. Instead the results suggest that the hydroxylation is a concerted process, much like a singlet carbene reaction, which does not involve intermediates. However, this conclusion is in conflict with the properties of singlet carbene reactions discussed above. Subsequent studies on a number of substituted methylcyclopropanes and other stained hydrocarbon systems established that these findings were not anomalous. 

A valuable boost to the study of reactions of triplet species came from the observation by Moss that the chemistry of some carbenes is very sensitive to temperature. Photolysis of 9-diazofluorene (207) in isobutylene at 273 K produced the cyclopropane (208) characteristic of a singlet carbene reaction, but at 133 K addition took place to yield 209. The change in products was shown to occur at the temperature of change in phase of the solution. 

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). 

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. 

Given the zwitterionic natnre of single carbenes, the possibility exists for coordinating solvents such as ethers or aromatic compounds to associate weakly with the empty p-orbital of the carbene. Several experimental stndies have revealed dramatic effects of dioxane or aromatic solvents on prodnct distribntions of carbene reactions. Computational evidence has also been reported for carbene-benzene complexes. Indeed, picosecond optical grating calorimetry stndies have indicated that singlet methylene and benzene form a weak complex with a dissociation energy of 8.7kcal/mol. ... 

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... 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

Insertion reactions are processes in which a reactive intermediate, in this case a carbene, interposes itself into an existing bond. In terms of synthesis, this usually involves C—H bonds. Many singlet carbenes are sufficiently reactive that insertion can occur as a one-step process. 

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