Kinetic Evidence for Carbene Protonation

Proton transfer to carbenes is indicated by the following kinetic data (i) the rates of X-H insertion reactions increase with increasing acidity of the proton donor HX (ii) normal deuterium kinetic isotope effects are observed, kHx DX (hi) alcohols react faster than ethers. However, mechanistic conclusions cannot be drawn from rates that are close to the diffusion limit. Thus, 

Rate measurements are straightforward if the carbenes can be monitored directly. As a rule, the decay of carbene absorption is (pseudo) first-order, due to rearrangement and/or reaction with the solvent. In the presence of a quencher, the decay is accelerated (Eq. 1), and the rate constant kq is obtained from a plot of k0bs versus [Q], Curved plots were often observed with proton donors (HX) as quenchers, particularly for high concentrations of weakly acidic alcohols. Although these effects have been attributed to oligomerization of the alcohols,91 the interpretation of curved plots remains a matter of dispute.76 Therefore, the rate constants reported in Tables 2-4 are taken from linear (regions of) obs-HX plots, or refer to a specified concentration of HX. 

Spectroscopically invisible carbenes can be monitored by the ylide method .92 Here, the carbene reacts with a nucleophile Y to form a strongly absorbing and long-lived ylide, competitively with all other routes of decay. Although pyridine (Py) stands out as the most popular probe, nitriles and thiones have also been used. In the presence of an additional quencher, the observed pseudo-first-order rate constant for ylide formation is given by Eq. 2.92,93 A plot of obs vs. [Q] at constant [Y ] will provide kq. With Q = HX, complications can arise from protonation of Y and/or the derived ylides. The available data indicate that alcohols are compatible with the pyridine-ylide probe technique. 

However, care should be exercised with more acidic proton donors. 

If the growth of ylide absorption is too fast to be monitored, relative rates can still be obtained by a Stem-Volmer approach, Eq. 3. The yield of ylide, measured as the change in optical density (AOD), decreases in the presence of 

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