Hydrogen-deuterium exchange has been examined under phase-transfer conditions for 1 5 thiazoles and a number of other heteroaromatic compounds (pyridines, thiophens, and imidazoles). The effects of 1 1 quaternary (ammonium, phosphonium, and arsonium) salts and 17 solvents have been studied. The effect of chain-length of the salt [Me(CH,),],NBr-( n = 1-5) has been examined. First-order kinetics (in thiazole) are reported for seven thiazoles. The present method offers a practical route to specifically and higher deuteriated (D L 90%) thiazoles and other aromatics.Some of the results (such as the effects of various catalysts and solvent polarity) are at variance with existing data in the field of phase-transfer catalysis and it is suggested that interfacial and micellar effects may arise. PHASE-TKANSFER catalysis (and its variants) has been very widely used and examined in organic chemistry and there are numerous, recent reviews of the subject .2-8 However, there are only a few reports of the use of phase-transfer catalysts to achieve isotopic (H-D) exchange. These involved replacement of the active hydrogens of octan-2-one (at C(l) and C(3)] and of indene [at C(l) and C(3)] and fluorene [at C(9)l lo by deuterium in a sodium deuterioxide-deuterium oxide medium. I t has been predicted that in addition to ketones, other compounds, e.g. nitro-compounds and nitriles, etc., should be amenable to ready H-D exchange under phase-transfer conditi0ns.G We report herein several examples where the introduction of a nitro-group promoted H-D exchange and in earlier work l b we have shown that a cyano-group has a similar effect. In the present study we have examined H-D exchange in thiazole, benzothiazole, and a number of 2-, 4-, and 5substituted thiazoles and also in several pyridines and thiophens, and in imidazole and N-methylimidazole. Many of the compounds examined underwent extensive isotopic exchange under the phase-transfer conditions used and this method offers a practical route to specifically labelled heteroaromatic compounds having a deuterium content >,90%.
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