Pyridine ylide probe method

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. 

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. 

One of the most interesting products obtained from the capture of singlet nitrene 16e is ylide 22e produced by photolysis of 15e in pyridine (Scheme 4). Ylide 22e has a very intense absorption band with maximum absorption at 390 nm. The pyridine-ylide method was successfully used by our group to probe the dynamics of the fluoro-substituted singlet arylnitrenes - " -" ... 

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