Science of Synthesis

Kubik, S., Science of Synthesis, (2010) 41, 514.

Protonation of diazoalkanes is another way to generate alkanediazonium ions (‌Scheme 14‌). This reaction constitutes the final step in the generation of diazonium ions by photolysis of N-sulfonylhydrazones or their anions in protic solvents.[‌85‌–‌88‌] The degree to which protonation of diazo compounds occurs depends on substrate structure and pH. While diazoalkanes are protonated to a significant extent even in alkaline solution, protonation of diazo ketones, diazo esters, and diazo sulfones requires acidic media. Interestingly, not only the thermodynamics but also the kinetics of this reaction is controlled by substrate structure. Thus, protonation of diazomethane,[‌18‌,‌89‌,‌90‌] primary diazo ketones [R1C(=O)CHN2],[‌91‌,‌92‌] diazo esters,[‌93‌] and diazo sulfones[‌94‌–‌96‌] is fast whereas secondary diazo ketones [R1C(=O)CR2N2][‌97‌–‌100‌] and diazo esters[‌101‌,‌102‌] display rate-determining proton transfer that is slower than the subsequent reactions of the diazonium ion. 1-Diazobutane exhibits irreversible protonation in acidic solution, but reversible protonation in alkaline solution.[‌103‌] Reaction of diazoalkanes with acids in nonpolar solvents leads to ion pairs.[‌55‌,‌56‌,‌104‌–‌106‌]

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References