Rearrangement on decomposition of diazo compounds

Schroter172 in 1909 found that azideoxybenzoin loses nitrogen when warmed, yielding diphenylketene  

According to Smith and Hoehn,173 diphenylketene is obtained in 64% yield when a benzene solution of azideoxybenzoin is dropped slowly into a Claisen flask placed in an oil-bath at 110° at this temperature the benzene distils off and the residual azideoxybenzoin loses nitrogen and rearranges. 

According to Horner, Spietschka, and Gross,174 the decomposition can be effected under very mild conditions, namely by UV-irradiation at 0°, yields then being around 90%. 

The light source used was an S81 lamp from Quarzlampengesellschaft Hanau and was placed in a quartz vessel cooled in ice-water. The reaction vessel (capacity 170 ml) was placed in a Dewar vessel with ice or a freezing mixture. 

Azideoxybenzoin (4.4 g) was dissolved in anhydrous ether (170 ml) and illuminated for 3 h while a slow stream of nitrogen was passed in. At the end of the reaction the ether was removed under nitrogen, and the diphenylketene was distilled in a vacuum (yield 3.5 g). 

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