Invisible carbenes

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. 

Observing Invisible Carbenes By Trapping Them with Pyridine... 

The transient spectra of neither singlet nor triplet 1-naphthylcarbene have ever been observed upon LFP of 10. The kinetics of these invisible carbenes were obtained from the growth of their ylide reaction products and not by direct observation of either singlet or triplet 1 -naphthylcarbene. 

In summary, the pyridine ylide method has told us quite a lot about 1,2-hydrogen migration reactions of invisible carbenes. We learned that simple aUcyl and dialkyl carbenes are true intermediates with nanosecond lifetimes. The pyridine ylide method revealed that the rate of rearrangement of alkylcarbenes are influenced by the cationic stabilizing power of X k. increases as X = FI < CH ... 

TRUV-VIS spectroscopy is an established method that has been routinely applied to the study of carbene chemistry. 5- 0" Obviously, to study an intermediate of interest by absorption spectroscopy, it must possess a useful chromophore in the UV-VIS region. To study spectroscopically invisible carbenes, Platz and co-workers introduced the pyridine-yhde probe methodology. " " Here, carbenes without useful absorptions react with probes such as pyridine to form strongly absorbing yhdes from which kinetic data can be derived. "" ""... 

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