Ketenes, TRIR spectroscopy

The photoinduced Wolff rearrangement of 5-diazo-2,2-dimethyl-l,3-dioxan-4,6-dione has also been examined by TRIR methods [116], These ultrafast measurements, conducted in a PMMA matrix, revealed that the formation of the ketene rearrangement product was complete within 20 ps a carbonyl carbene was not detected in this study. Other applications of TRIR spectroscopy to the study of carbene-related chemistry include investigations of diazirine to diazo rearrangements [117] and of oxygen and sulfur atom transfer reactions [118]. 

As discussed in the previous section, diazocarbonyl compounds are another common photochemical precursor to ketenes. A number of substituted ketenes have been prepared in this way and studied by TRIR spectroscopy. [123]. The diagnostic ketene asymmetric stretching frequency, typically observed between 2100 and 2200 cm has been correlated with the inductive (field) effect of... 

Spectroscopic methodologies have provided a wealth of information concerning the amination of ketenes. Scaiano and co-workers have measured rate constants for ketene reactions with various classes of amines in acetonitrile. The reaction rate is influenced by the basicity of the amine as well as by steric factors in both the ketene and the amine. Ketene amination has also been studied by time-resolved infrared (TRIR) spectroscopy. The strong ketene IR band near 2100 cm" makes it an excellent candidate for study by this spectroscopic technique. Scaiano, Wagner, Lusztyk, and co-workers provided evidence for the first nucleophilic attack being rate determining and that the transition state involves an enol amide. They further found that the asymmetric stretching IR band of substituted ketenes... 

The lack of excited state involvement for 21 is in contrast to the behavior in systems that have substantial equilibrium concentrations of the syn conformer. To examine the effect that conformation has on ketene growth kinetics, 4-diazo-3-isochromanone (22), ° a cyclic analogue (phenyl version) of 21 that is locked in the syn conformation, was also recently studied by TRIR spectroscopy. In this case, a carbene IR band is observed at 1686 cm that decays with a lifetime of 526 50 ns in Freon-113. The ketene IR band at 2116 cm, however, in dramatic contrast to the data observed with acyclic diazocarbonyl 21, is produced faster than the experimental time resolution. Ketene 24, therefore, is formed entirely from a non-carbene source, presumably the excited state of 22 (Scheme 8). In agreement with this hypothesis, oxygen and methanol quench carbene 23, but they leave the initial intensity of the ketene IR band unaffected. These... 

Their kinetics of ring closure (Eqn (4.55)) were measured by UV or by time-resolved infrared spectroscopy (TRIR), and gave the rate constants reported in Table 4.4. The measured reactivities vary by a factor of 10 °, showing the powerful and opposite influence of substituents on the ketenes and on the cyclobutenediones, and this may be extended to 10 " if the estimated rate constant for the bis(Me3Si) substrate is included. Computations of the ring-closure barriers are consistent with the measured reactivities. 

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