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

Two different C-13-labeled 7-trifluoromethyl-N-1,N-10-ethyleneisoalloxazinium chlorides were utilized to examine the mechanism of amine dehydrogenation. H-1 NMR studies in CD3CN (C-13 NMR in DMSO-d(6)), as confirmed using N-15-labeled benzylamine, indicate that primary and secondary amines add to give tetrahedral C-10a adducts that persist for hours at 25 degreesC. Upon heating, the C-10a amine adducts partition between rearrangement to C-4a spirohydantoin amidines, and, in the case of benzylic amines, beta -elimination to give reduced flavin and imine dehydrogenation product. A C-10a tetrahedral hydroxy adduct, generated under basic conditions when water was present, was also confirmed by H-1/C-13 NMR. (C) 2001 Elsevier Science Ltd. All rights reserved.