Diazoketones photo

There are several transformations that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. An important example of this type of reaction is the thermal and photochemical reactions of x-diazoketones. When z-diazoketones are decomposed thermally or photo-chemically, they usually rearrange to ketenes. This reaction is known as the Wolff rearrangement. 

The singlet form of carbenes substituted with a carbonyl group is much lower in energy than the triplet, so the photolysis of a-diazocarbonyl compounds gives singlet carbenes. The photo-Wolff rearrangement (Chapter 2) of a-diazoketones occurs upon photolysis of these compounds. 

Intramolecular addition of carbenes to pendant alkenes also provides a cyclization path as e.g., the irradiation of the diazoketone 402 in benzene. No photo-Wolff reaction, rearrangements of the type described in the next section, occurs and the two products... 

There is no doubt that matrix isolation experiments have provided many examples of great interest in this and other areas where reactive intermediates are produced, a-Diazoketones were discussed in an earlier section since the singlet state reaction undergoes the well-known photo-Wolff process. Other studies on these compounds in matrices at 77 K have shown that the diazoketones 571 and 572 undergo equilibration via an oxirene intermediate. Again this is an example of intramolecular trapping . Others have also observed the formation of oxirenes and the existence of a keto-carbene/oxirene equilibrium in a variety of systems, such as the diazoquinone 573. ... 

Microwave irradiation has been used to modify selectivity in rearrangement and coupling reactions. Indeed, the photo-Fries rearrangement described above could also be classified in this section [56]. Sudrik described the Wolff rearrangement of a-diazoketones under conventional heating and microwave irradiation [76]. The... 

Diazoketones give rearranged products when decomposed thermally or photo-chemically. The reaction is known as the Wolff rearrangement and is of synthetic importance, since it constitutes a convenient method for one-carbon homologation... 

The photo Wolff rearrangement of 3- and 4-substituted pyridyl a-diazoketones 27 and 29 to yield pyridylketenes 28 and 30 was effected by irradiation at wavelengths of 300-400 nm. ... 

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