We have been studying ketene formation by laser flash photolysis of aromatic α-diazoalkyl ketones. Among them 2-azaaryl sustituted compounds give species which have longer wave length absorption than do the corresponding ketenes. This absorption does not decay upon addition of amine nucleophiles. Wentrup found the ylide structure (IIa) coming from photolysis of 2-pyridyl diazomethyl ketone (Ia) by matrix isolation method [1]. We will report here the detection of the ylides by time-resolved IR technique.

The solution of Ia in MeCN (0.02 mM) was irradiated with 266 nm laser light (2 mJ) and the change at different wave numbers was recorded. The results are shown in the left figure. The large depletion at 2107 cm^-1 comes from the decrease of Ia by laser irradiation. The absorption at 2127 cm^-1 shows the formation of ketene IIIa and the one at 1725 cm^-1 derives from IIa. Similarly, Ic shows the depletion of Ic at 2112 cm^-1, the formation of ketene IIIc at 2132 cm^-1, and the ylide IIc at 1758 cm^-1. For Ib depletion of Ib and the ketene IIIb were observed at 2076 and 2104 cm^-1, respectively, but no ylide absorption is detected. In the presence of butylamine the ylide IIa and IIc had longer life times, while the decay rates of ketenes III were accelerated linearly with the concentration of the amine and gave second order rate constants of 2.15 x 10⁸ M^-1s^-1 for IIIa, 1.92 x 10⁶ M^-1s^-1 for IIIb, and 5.10 x 10⁸ M^-1s^-1 for IIIc. (see right Figure).

[1] A. Kuhn, C. Plug, C. Wentrup, J. Am. Chem. Soc. 2000, 122, 1945-1948.

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