Hypervalent Iodine Oxidation of 5-Substituted and 4,5-Disubstituted Pyrazol-3(2H)-ones: A Facile Synthesis of Methyl-2-alkynoates and Methyl 2,3-alkadienoates

Article abstract of DOI:10.1016/S0040-4020(01)80023-5

Hypervalent iodine oxidation of various 5-substituted pyrazol-3(2H)-ones (1a-h) with iodobenzene diacetate or iodosobenzene in methanol results in fragmentative loss of molecular dinitrogen to yield methyl 2-alkynoates (2a-h).However, 5-methyl-4-substituted pyrazol-3(2H)-ones (3a-d) under similar conditions yield methyl 2,3-allenic esters (4a-d).Methyl 2,3-cycloalkadienoates (6a-e) are obtained by the oxidative cleavage of 4,5-polymethylene pyrazol-3(2H)-ones (5c-e) using iodobenzene diacetate or iodosobenzene in methanol. 5-Methyl-4,4-disubstituted pyrazol-3(2H)-ones (7a-b) were found not to undergo raction under similar conditions.The scope and mechanism of these reactions are discussed.