Pyrazole mercuration

It was found, during a study of synthetic routes leading to quinine analogues, that oxidation of a specific bicydic pyrazoline derivative with mercuric acetate gives an enamine-like pyrazole (87b). 

Cyclodesulfurization of thiosemicarbazides, containing pyrazole <2002PS67> or benzofuran <2002PS863, 2004PS1577> units, by yellow mercuric oxide or by l,3-dibromo-5,5-dimethylhydantoin in the presence of potassium iodide <2006TL4889> afforded the respectively substituted oxadiazoles. 

While there appears to have been no general study of the mercuration of azoles, the reaction seems to proceed readily in several systems. Thus pyrazoles are 4-chloromercurated by HgCl2. Mercuration of 1-phenylpyrazole and l-phenyl-3,4,5-trimethylpyrazole affords organomercury derivatives (152) and (153) (93AJC1323), the structures have been established using 199Hg NMR spectroscopy. 

Grandberg, Kost, and Zheltikova108 have shown recently that mercuric salts mercurate pyrazoles in the 4-position and also give other complexes with variable quantities of the mercuric salts. The proportion of mercuric salt bound as a complex depends on the basicity of the starting pyrazole. The mercuric group in mercurated pyrazoles behaves in a variety of reactions just as phenyl mercuric chloride.108,219,350... 

Phenylsydnone is the prototype of that class of mesoionic compounds called sydnones. On acidic hydrolysis it produces phenylhydrazine, whereas basic hydrolysis regenerates N-nitroso-N-phenylglycine. This sydnone undergoes a variety of electrophilic substitutions,3,4 8-19 including mercuration n-13,16 and formylation,19 with an ease comparable to thiophene, and a number of 1,3-dipolar cycloadditions with numerous alkenes,18 18 alkynes,17 and quinones8 to form, with loss of carbon dioxide, a variety of pyrazole derivatives. 

Cycloaddition approaches to pyrazole derivatives were employed extensively in 1997. Thus, aldhydrazones combine with mercuric acetate to form nitrileimines which react in situ with olefins to give 1,3,5-trisubstituted 2-pyrazolines <97SC3737>. Oxidation of the latter to pyrazoles with iodobenzene diacetate was reported <97SC2683>. The most common cycloaddition pathway, however, is from the reaction of alkenes or alkynes with diazo compounds. The facial selectivity of the regiospecific reactions of diazomethane (and other... 

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