Using direct and indirect electrochemical methods, the rate constant for ring opening of the radical cation generated from N-cyclopropyl-N-methylaniline was found to be 4.1 x 10(4) s(-1).

N-Cyclopropyl-N-methylaniline (5) is a poor probe for single electron transfer (SET) because the corresponding radical cation undergoes cyclopropane ring opening with a rate constant of only 4.1 × 104 s–1, too slow to compete with other processes such as radical cation deprotonation. The sluggish rate of ring opening can be attributed to either (i) a resonance effect in which the spin and charge of the radical cation in the ring-closed form is delocalized into the phenyl ring, and/or (ii) the lowest energy conformation of the SET product (5 •+) does not meet the stereoelectronic requirements for cyclopropane ring opening. To resolve this issue, a new series of N-cyclopropylanilines were designed to lock the cyclopropyl group into the required bisected conformation for ring opening. The results reveal that the rate constant for ring opening of radical cations derived from 1′-methyl-3′,4′-dihydro-1′H-spiro­[cyclopropane-1,2′-quinoline] (6) and 6′-chloro-1′-methyl-3′,4′-dihydro-1′H-spiro­[cyclopropane-1,2′-quinoline] (7) are 3.5 × 102 s–1 and 4.1 × 102 s–1, effectively ruling out the stereoelectronic argument. In contrast, the radical cation derived from 4-chloro-N-methyl-N-(2-phenylcyclopropyl)­aniline (8) undergoes cyclopropane ring opening with a rate constant of 1.7 × 108 s–1, demonstrating that loss of the resonance energy associated with the ring-closed form of these N-cyclopropylanilines can be amply compensated by incorporation of a radical-stabilizing phenyl substituent on the cyclopropyl group. Product studies were performed, including a unique application of EC–ESI/MS (Electrochemistry/ElectroSpray Ionization Mass Spectrometry) in the presence of 18O2 and H2 18O to elucidate the mechanism of ring opening of 7 •+ and trapping of the resulting distonic radical cation.

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The electrochemical reduction of several α,β-epoxyketones was studied using cyclic (linear sweep) voltammetry, convolution voltammetry, and homogeneous redox catalysis. The results were reconciled to pertinent theories of electron transfer. α,β-Epoxyketones undergo dissociative electron-transfer reactions with CÀ O bond cleavage, via both stepwise and concerted mechanisms, depending on their structure. For aliphatic ketones, the preferred mechanism of reduction is consistent with the "sticky" concerted model for dissociative electron transfer. Bond cleavage occurs simultaneously with electron transfer, and there is a residual, electrostatic interaction in the ring-opened (distonic) radical anion. In contrast, for aromatic ketones, because the ring-closed radical anions are resonancestabilized and exist at energy minima, a stepwise mechanism operates (electron transfer and bond cleavage occur in discrete steps). The rate constants for ring opening are on the order of 10 8 s À 1 , and not significantly affected by substituents on the 3membered ring (consistent with CÀ O bond cleavage). These results and conclusions were fully supported and augmented by molecular orbital calculations.