Carbenes, generation solvent effects

In particular cases the presence of cosolvents seems to be somewhat useful. In the generation of dihalocarbenes, the addition of a small amount of ethanol to chloroform in some cases remarkably improves the yield of the corresponding carbene This is likely due, however, to the formation of a more lipophilic alkoxy anion, which participates in the reaction, rather than to a solvent effect. As a further example, the extraction of tetraalkylammonium hydroxides into aromatic hydrocarbons increases by 1 to 3 orders of magnitude when a small amoimt of alcohol is added however, the extracted species is mainly RO than OH ... 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Halide ions, especially F , are weak bases in aqueous solution. However, in dry aprotic solvents Et4N F effectively induces elimination of HBr from 2-bromoethylbenzene (7). The ammonium chloride and bromide only promote halogen exchange. At 80°, the fluoride decomposes into EtjN, HF, and ethylene, thus demonstrating the high protophilicity of the hard fluoride ion. This same property is also shown by its usefulness for generating dihalo-carbenes. Naked fluoride ion (cation sequestered by a crown ether) promotes facile elimination of haloethylenes to form acetylenes and allenes (8). 

The development of transition metal-free hydroelementation reactions has been a goal for the synthetic community for many years. As mentioned in the previous sections on Michael addition reactions, A-heterocyclic carbenes have been shown to be efficient catalysts for these addition reactions. One of the challenges that must be overcome when using these catalysts is the sensitivity of the catalysts to electrophiles. To circumvent this problem, carbon dioxide adducts have been synthesized and used as precatalysts for the phospha-Michael addition of secondary phosphine oxides to activated alkenes (Scheme 4.45) [96]. The success of this approach hinged on the observation that these adducts underwent cleavage in solution to generate the free carbene and carbon dioxide. The concentration of the free carbene was found to be dependent upon the solvent, and THF was found to be an effective solvent for the... 

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