The first calibration of an aminiumyl radical ion clock: Why N-cyclopropylanilines may be poor mechanistic probes for single electron transfer

Article abstract of DOI:10.1039/b702157g

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 × 10⁴ s^-1. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702157g

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The first calibration of an aminiumyl radical ion clock: why
N-cyclopropylanilines may be poor mechanistic probes for single
electron transfer{
Xiangzhong Li, Michelle L. Grimm, Kazuo Igarashi, Neal Castagnoli, Jr. and J. M. Tanko*
Received (in Cambridge, UK) 12th February 2007, Accepted 19th March 2007
First published as an Advance Article on the web 3rd April 2007
DOI: 10.1039/b702157g
magnitude to that of the neutral radical (e.g., 3), whose rate
constant for ring opening is estimated to be .107 s21 (eqn (1)).5
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 6 104 s21
.
(1)
N-Cyclopropylamines have been used in attempts to detect single
electron transfer (SET) mechanisms for amine oxidations in both
chemical and biochemical systems. The supposition is that if
Indirect evidence and MO calculations imply that N-cyclopropyl
radical cations generated from aliphatic amines undergo rapid ring
opening.6–9 Less is known about the behavior of aromatic
aminiumyl radical ions such as those derived from
N-cyclopropyl-N-methylaniline, and derivatives, which have been
used as SET probes in both enzymatic10–13 and chemical
systems.14,15 In this paper we report results pertaining to the
mechanism and kinetics of radical cations generated from
N-cyclopropylanilines which directly address these issues.
+
?
electron transfer occurs, the resulting aminiumyl radical ions (1 )
will undergo rapid ring opening (Scheme 1, path a) to generate
+
?
distonic radical cations 2 , in direct analogy to the cyclopropyl-
carbinyl A homoallyl neutral free radical rearrangement.1 The
isolation or detection of cyclopropane ring-opened products, or a
loss of catalytic activity for an enzyme-catalyzed process, provides
evidence that a radical cation was formed, and thus an SET event
had occurred. The most compelling evidence for radical inter-
mediates in oxidations catalyzed by cyctochrome P450s,2 horse-
radish peroxidase,3 and monoamine oxidase A and B4 is derived
from the behavior of N-cyclopropylamines (1).
Direct photoionization (266 nm, 5% CH3OH in CH3CN) of
N,N-dimethylaniline (4), N-ethyl-N-isopropylaniline (5), N-methyl-
N-cyclopropylaniline (6) and N-methyl-N-(1-methylcyclopropyl)a-
niline (7) generated transients with absorption maxima in the
region 460–490 nm, consistent with the reported spectroscopic
Despite their widespread use, there remain several unresolved
issues with N-cyclopropyl compounds as probes for SET in amine
oxidations, the most important of which is that no rate constants
for ring opening of the corresponding radical cations (ko) have ever
been directly measured. It is often assumed that the rate constant
for ring opening of these radical cations will be of comparable
features of 4 +.16 The lifetimes of each of these radical cations were
?
nearly identical (ca. 0.2–0.7 ms) and their decay is attributable to
their consumption in bimolecular reactions with other paramag-
netic species produced by photoionization. The introduction of an
+
?
+
?
N-cyclopropyl group (e.g., 6 , 7 ) did not affect the lifetime,
suggesting the rate of cyclopropane ring opening is not competitive
with these other processes, and that, ko must be ¡106 s21 (i.e., ring
opening occurs slowly on the LFP time scale).
Next, the oxidation of 4 and 6 was studied by cyclic and linear
sweep voltammetry (Pt electrode, CH3CN solvent). In these
experiments, the rate law for radical cation decay can be
determined by the characteristics of the voltammogram.17
Consistent with previous electrochemical studies,18,19 the radical
Scheme 1 Possible SET and HAT pathways for the reaction of an
N-cyclopropylamine.
+
?
cation derived from N,N-dimethylaniline (4 ) undergoes bimole-
cular decay.19 The product of the electrochemical oxidation is the
expected dimeric product N,N,N9,N9-tetramethylbenzidine (8).18 In
contrast, the radical cation generated from N-methyl-
Department of Chemistry, Virginia Polytechnic Institute and State
University, Blacksburg, VA, 24061, USA. E-mail: jtanko@vt.edu;
Fax: +01 540 2313255; Tel: +01 540 2316687
{ Electronic supplementary information (ESI) available: Details and data
pertaining to the direct and indirect electrochemistry, and EC/ESI-MS of 4
and 6. See DOI: 10.1039/b702157g
+
?
N-cyclopropylaniline (6 ) undergoes clean, first-order decay,
consistent with a unimolecular rearrangement attributable to
2648 | Chem. Commun., 2007, 2648–2650
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cyclopropane ring opening (Scheme 1, path a). The fact that the
decay is zero order in methanol effectively rules out the
nucleophile-assisted pathway for ring opening, analogous to that
documented for cyclopropylarene radical cations20,21 (Scheme 1,
path c), at least for methanol as a nucleophile.
Finally, homogeneous redox catalysis17 was used to study
further the electrochemical oxidation of 6. Three ferrocene-based
mediators were used, varying in oxidation potential. For each
mediator, the chemical step was rate-limiting, with the electron
transfer as a rapid pre-equilibrium step; the results are summarized
in Fig. 2. Under these conditions, the composite rate constant kobs
= kok1/k21 = koK1 can be determined by fitting of the ip/ipd vs.
log(1/n) data to theoretical working curves.17,23,24
Bulk electrolysis of 6 proved problematic. Reasonable mass
balances could only be obtained when the product mixture was
quenched with NaBH4. For example, electrolysis of 6 (2 equiv.
electrons) resulted, after NaBH4 treatment, in 21%
N-methylaniline, 10% N-methyl-1,2,3,4-tetrahydroquinoline (9),
and 70% unreacted starting material. Prolonged electrolysis
produced these same products in approximately the same ratio,
but with lower total mass balance. Formation of 9 is consistent
Reconciliation of the direct and indirect electrochemical results
leads to the rate constant for ring opening (ko = 4.1 6 104 s21).
+
?
The rate constant for ring opening of 6 is exceptionally low,
likely too low for 6 to be a dependable probe for single electron
transfer (vide infra). Two explanations are offered (Scheme 2): (1)
+
with ring opening of 6 + to the distonic radical cation followed by
?
?
the lowest energy conformation of 6 does not meet the
stereoelectronic requirements for cyclopropane ring opening,24,25
radical cyclization as was observed by Hanzlik et al. in horseradish
peroxidase incubation mixtures of 6.12,22 No dimeric products of
any type were detected, consistent with the results described above
+
?
and (2) 6 is stabilized by resonance in the cyclopropane ring-
closed form.
+
?
which revealed overall first order decay of 6 .
These results are of particular interest because N-cyclopropyl-
N-methylaniline and several structurally related substrates have
been used to probe for single electron transfer pathways in amine
oxidations mediated by cytochrome P450. Based upon the results
reported herein, if electron transfer were occurring in these
systems, the sluggish rate of ring opening would be unable to
compete radical cation deprotonation.
Because of these low mass balances, products arising from the
oxidation of 4 and 6 were studied further by electrochemical-
electrospray ionization/mass spectrometry (EC-ESI/MS).2 This
technique involves pumping a solution containing the electroactive
species through an electrochemical flow cell that is coupled directly
to an electrospray ionization mass spectrometer. The mass
voltammogram (plot of ion intensity vs. electrochemical cell
potential) is presented in Fig. 1. The observed ion intensities of the
product(s) relative to the starting material suggest good mass
balance under these conditions for the oxidation of 4 and 6.
Structural assignments for products arising from 4 and 6 are
discussed in detail in the ESI.{ To summarize: only dimeric
products were observed for the EC-ESI/MS oxidation of 4,
consistent with the dimerization as the only available pathway for
To amplify this statement: The probe approach requires that the
rearrangement is rapid relative to other competing processes, such
as deprotonation. If, in cases where a paramagnetic intermediate is
almost certainly involved (e.g., the cP450 system), the rate of ring
opening is uncompetitive with deprotonation (ko , kd[B2], paths a
and b, Scheme 1), then it is impossible to distinguish an SET
pathway from other pathways such as direct hydrogen atom
transfer (HAT). The rate constant for the deprotonation of
N,N-dimethylaniline radical cation by acetate anion has been
measured under very similar conditions (kd = 1.2 6 108 M21
s21).26,27 Assuming that the rate constant for deprotonation from
+
?
decay of 4 . In contrast, only cyclopropane ring-opened products
were observed for 6; product assignments appear in Fig. 1. No
products arising from radical cation deprotonation were detected
in these products studies. This is not surprising since there is no
base present in sufficiently high concentration for this pathway to
become competitive with dimerization of 4, or ring opening of 6.
+
the methyl group of 6 is the same (kd = 6 6 107 M21 s21
,
?
correcting for the number of hydrogens) and 0.1 M acetate, the
Fig. 2 Mediated oxidation of N-cyclopropyl-N-methylaniline by ferro-
cenecarbaldehyde (m), benzoylferrocene ($), and ferrocene carboxylic
acid (r) at various concentrations of substrate; constant [M]/[substrate].
Fig. 1 The mass voltammogram of N-cyclopropyl-N-methylaniline (6) in
70% methanol–30% water–0.7% formic acid at a flow rate of 50 mL min21
.
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150 : 1 ratio. In the context of an enzymatic study, a carboxylate
base present in an effective molar concentration of 0.1 or greater
would certainly overwhelm the ring opening in a similar manner,
and a bona fide SET process would likely go undetected by this
method.
This work was funded by the National Science Foundation
(CHE-0108907 and CHE-0548129) and the Harvey W. Peters
Research Center. M. L. G. was partially supported by NSF-
IGERT (DGE-0333378).
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2650 | Chem. Commun., 2007, 2648–2650
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