Radical ions are calculated t o have large nonadditive captodative stabilisation, in contrast to their neutral analogues.The synergetic stabilising influence of a combination of an acceptor and a donor substituent at a radical centre, termed the captodative (cd) effect, has been suggested to play a crucial role in a number of reactions proceeding via radical and diradical intermediates.l.2 However, many neutral radicals with prototypical captodative substituents fail to show any extra stabilisation under critical experimental3 and theoretical scrutiny.4.5 We now show that cd stabilisation is unambiguously large in radical ions.The possibility of nonadditive substituent interactions in radical ions, e.g. (1)-(4), has been suggested ear1ier.lc.d However, the cd effect in radical ions has not yet been quantitatively established. We have chosen to quantify the effect in two representative radical anions, ( 5 ) and ( 6 ) , derived from the neutral captodative radicals, (7) and (8), respec-
N-Carbamylmaleamic acid (malur) undergoes cyclodehydration under favourable conditions, as expected, to give N-carbamyl maleimide. N-(Carboxymethyl) maleamic acid (malgly), however, does not undergo a similar cyclization reaction. Strong π bonding between the C and N of the amide group as well as two intramolecular hydrogen bonds makes malgly a planar molecule, as revealed by single-crystal X-ray studies.
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