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Breslow intermediates (aminoenols) and their keto tautomers: first gasâphase characterization by IR ion spectroscopy
Breslow intermediates (BIs) are the crucial nucleophilic amino enol intermediates formed from electrophilic aldehydes in the course of Nâheterocyclic carbene (NHC) catalyzed Umpolung reactions. Both in organocatalytic and enzymatic Umpolung, the question whether the Breslow intermediate exists as the nucleophilic enol, or in the form of its electrophilic ketoâtautomer, is of utmost importance for its reactivity and function. We herein report the preparation of chargeâtagged Breslow intermediates/keto tautomers derived from three different types of NHCs (imidazolidinâ2âylidenes, 1,2,4âtriazolinâ5âylidenes, thiazolinâ2âylidenes) and aldehydes. An ammonium chargeâtag is introduced by either the aldehyde unit or the NHC. ESIâMS IRâIon spectroscopy allowed for the unambiguous conclusion that in the gasâphase, the imidazolidinâ2âylidene derived BI indeed exists as a diamino enol, while both 1,2,4âtriazolinâ5âylidenes and thiazolinâ2âylidenes give the ketoâtautomer. This result coincides with the tautomeric states observed for the BIs in solution (NMR) and in the crystalline state (XRD), and is in line with our earlier calculations on the energetics of BI ketoâenol equilibria