Diazocarbonyl protonation

Synthesis of a-alkoxyketones from a-diazocarbonyl compounds and alcohols under the influence of copper or rhodium catalysts is well established as an alternative to the Lewis or proton acid catalyzed variant of this synthetic transformation. The sole recent contribution to the aspect of general reactivity deals with the competition between O/H insertion and cyclopropanation of unsaturated alcohols 162). The results... 

The reaction of diazomethane with acyl chlorides starts as a simple acylation to give a diazo-nium compound. If there is an excess of diazomethane, a second molecule acts as a base to remove a rather acidic proton between the carbonyl and the diazonium groups to give the diazocarbonyl compound. 

Attempts to protonate diazocarbonyl compounds in superacids at C(l) failed (Allard et al., 1969 Wentrup and Dahn, 1970). When diazoacetone (2.10) was treated at -60 to -80°C in FSO3H - SbFs - SO2 or in HF-SbF5-S02, the O-protonated enoldiazonium ions with (Z)- and (F )-configuration (2.11 and 2.12) were detected by NMR spectroscopy. It may be that there is a hydrogen bond formed between the OH and the diazonio group in 2.11. 

Proton, and NMR spectra probably provide less ambiguous results on rotamer equilibria of diazocarbonyl compounds in solution, but only at low temperature ( H Kaplan and Meloy, 1966 Wentrup and Dahn, 1970 Kessler and Rosenthal, 1973 Curd et al., 1974 Zellhofer, 1980 Lichter et aL, 1977). 

Heteroaromatic cyclic cations like pyrylium and thiopyrylium ions (9.22, X = O or S, respectively R = H or alkyl) react with diazocarbonyl and diazophosphoryl compounds in the 2-, 4- and 6-positions if these are not substituted (R = H). Triethylamine is necessary for removal of the proton in one of these positions, to form the 4/f-pyran or -thiopyran (9.23 9-16), as found by the group of Regitz (Regitz and Khbeis, 1984 Regitz et al, 1985). 

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