Silver Carbonate Promoted Reaction of Hydrazonyl Chlorides with Allylic Alcohols

The title reaction affords two main kinds of products: (i) pyrazole derivatives due to cycloaddition of a nimle imine intermediate to the ethylenic bond; (ii) open-chain carbonyl compounds due to the electrophilic attack of a nitrilium-hke carbocation to the ethylenic bond.Mechanistic possibilitm are discussed.Nitrile iminesl are familiar to a large number of chemists interested in heterocyclic synthesis.The intermediacy of these labile species was first envisaged by Fusco and ~omani' and later established by Huisgen's monumental w ~r k .~~~ D~rect evidence of the existence of nitrile imines has been achleved by uvand ir spectroscopies at low temperature6~y and by photoelectron spectroscopy in gas ~hase.''Among the various procedures for the in situ generation of nitrile imines, the most common method is the base treatment of hydrazonyl halides, which involves deprotonation of the NH group and subsequent loss of the halide anion.'A few paperslt.tZhave described that silver carbonate behaves as an efficient enhancer of the redctlvity of hydrazonyl chlorides.Although such a fact is far from being strange, doubts arise about the formation mechanism of nitrile imines under the latter conditions and even about the real intervention of nitrile imtne species in silver carbonate promoted reactions of hydrazonyl chlorides.To gain information on these points, we have undertaken the present investigation, dealing with the reactions between chlorohydrazone (1) and a series of allylic alcohols(2) in the presence of silver carbonate.