Reaction of amines with N1,N10-ethylene-bridged flavinium salts: the first NMR spectroscopic evidence of C10a tetrahedral amine adducts

Two different 13C-labeled 7-trifluoromethyl-N1,N10-ethyleneisoalloxazinium chlorides were utilized to examine the mechanism of amine dehydrogenation. 1H NMR studies in CD3CN (13C NMR in DMSO-d6), as confirmed using 15N-labeled benzylamine, indicate that primary and secondary amines add to give tetrahedral C10a adducts that persist for hours at 25°C. Upon heating, the C10a amine adducts partition between rearrangement to C4a spirohydantoin amidines, and, in the case of benzylic amines, β-elimination to give reduced flavin and imine dehydrogenation product. A C10a tetrahedral hydroxy adduct, generated under basic conditions when water was present, was also confirmed by 1H/13C NMR.