Ring-Chain Tautomerism in 1,3-Diaza and 1,3-Oxaza Heterocycles

Abstract

The open-chain immonium tautomers (10-20%) are in equilibrium with the dominant ring form in CF3CO2H but are not present in CCI4 for the five- and six-membered 1,3-diaza and 1,3-oxaza compounds 1-4, which lack all carbon substituents. The alternative open-chain oxonium tautomer is suspected as a minor contributor for the five-membered oxaza heterocycle 3. The proportion of open-chain form increases with temperature in all cases, usually achieving about the 50% level by 100 °C. The ring and chain forms of the five-membered diaza heterocycle 1 undergo rapid interconversion on the NMR time scale above 60 °C. This observation comprises the first DNMR coalescence for a nucleophilic attack on an sp2 center (eq 1). The more rapid ring ↔ chain interconversion for the diaza system than for the oxaza system results from the relative O/N nucleophilicities and C-O/C-N bond energies. The more rapid ring ↔ chain interconversion for the five-membered ring (5-endo-trigonal) than for the six-membered ring (6-endo-trigonal) is an exception to the vectorial rules for ring closure. Either immonium ions or the ring-chain process itself may be a systematic exception to these rules. © 1980, American Chemical Society. All rights reserved.