Undesirable side reactions of diazo compounds

Unfortunately not only carbenes are formed when diazoacyl and related compounds are photolysed. A major product is that formed by rearrangement of the carbene (or perhaps the excited diazo compound) to a ketene the Wolff rearrangement (Fig. 3.9). 

Ketenes are highly reactive electrophiles but not nearly so indiscriminate as carbenes. When the properties of diazoacetyl photoaffinity reagents were evaluated the Wolff rearrangement was found to be a major problem accounting for 30 to 60% of the products arising from O-esters and 100% of the products fromS-esters. For instance, diazoacetyl-chymotrypsin gave rise to O-carboxymethyl serine formed by the attack of water on the ketene (Shafer et al., 1966) (Fig. 3.10). 

Diazo compounds with electron withdrawing substituents were introduced both to improve the stability of the reagent in the dark (Table 3.1) and to 

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