Detection of an ylide intermediate in the electrochemically-induced Stevens rearrangement of an ammonium salt by in situ UV-vis spectroelectrochemistry

Article abstract of DOI:10.1016/j.electacta.2013.01.079

The electrochemically-induced Stevens rearrangement of 2-(benzyldimethyl) ammonium acetophenone has been investigated by in situ UV-vis spectroelectrochemistry. Voltammetric analysis and absorption spectra recorded during the potentiostatic reduction indi

Full text of DOI:10.1016/j.electacta.2013.01.079

Electrochimica Acta 92 (2013) 446–451
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Detection of an ylide intermediate in the electrochemically-induced Stevens
rearrangement of an ammonium salt by in situ UV–vis spectroelectrochemistry
Amedeo Capobianco, Tonino Caruso, Laura Palombi∗, Andrea Peluso∗,1
Dipartimento di Chimica e Biologia, Università di Salerno, via Ponte Don Melillo, 84081 Fisciano, Sa, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemically-induced Stevens rearrangement of 2-(benzyldimethyl)ammonium acetophenone
has been investigated by in situ UV–vis spectroelectrochemistry. Voltammetric analysis and absorp-
tion spectra recorded during the potentiostatic reduction indicate that the reaction proceeds via a
one-electron transfer with a Platinum cathode and generation of an ammonium ylide intermediate.
© 2013 Elsevier Ltd. All rights reserved.
Received 26 September 2012
Received in revised form 10 January 2013
Accepted 12 January 2013
Available online 24 January 2013
Keywords:
Spectroelectrochemistry
Electrosynthesis
Ammonium ylides
Stevens rearrangement
1. Introduction
initiated by the enolate A arising from the two-electrons reductive
cleavage (Scheme 1).
The reactivity of -onium salts [1] is a subject of considerable
interest for organic chemists both from a synthetic [2–7] and mech-
anistic point of view [8–11]. An important class of such reactions
regards electrocyclic rearrangements (i.e. [1,2]- and [2,3]-shift)
which proceed via an ylide intermediate, by means of basic treat-
ment [12].
On the other hand, since many years, several literature reports
documented an electrochemical approach to the issue which avoid
the use of base reagents, the -onium salt being directly activated
on suitable electrodes [13–16].
The in situ spectroscopic characterization of species generated
by controlled-potential electrolysis could offer a valid help allow-
ing the detection of short-life intermediates and unstable oxidation
states [20–27].
Herein we report the results of a study in which the com-
bined use of spectroelectrochemistry, quantum computations and
standard chemical method, demonstrates the direct formation of
an ylide intermediate in the electrochemically-induced Stevens
rearrangement of 2-(benzyldimethyl) ammonium acetophenone
(1) [28] via one-electron transfer process at a platinum electrode.
Despite of the significant advantages of the electrosynthetic
process compared to the chemical one, its usefulness is some-
times reduced because of the formation of by-products resulting
from unselective C-heteroatom cleavage, especially when ammo-
nium salts are involved [17,18]. Further mechanistic insights are
therefore necessary for a better understanding of such interesting
electrochemically-induced rearrangements where the formation of
an ylide intermediate has been only indirectly deduced by the sub-
sequent chemical reactivity. In particular, in a pioneering study by
Iversen [19], the formation of the ammonium ylide B of the title
compound during the electroreductive process on a Hg cathode has
been postulated as a consequence of a chemical acid–base process
2. Experimental
2.1. General remarks
Starting material 1 was prepared accordingly to literature
[29] by using reagents and solvents as received from Aldrich
or Fluka, without further purification. Thin layer chromatogra-
phy (TLC) was performed using Merck Silica Gel 60 F254 plates
and visualized by fluorescence quenching at 254 nm. Column
chromatographic purification of the product 2 was carried out
using silica gel 60 (70–230 mesh, Merck). The NMR spectra were
recorded on Bruker DRX spectrometer (400 MHz, 1H; 100 MHz
13C). Spectra were referenced to residual CHCl3 (7.26 ppm, 1H;
77.00 ppm, 13C) or CD3CN (1.94 ppm, 1H; 118.0 and 1.4 ppm, 13C).
All voltammetric measurements were performed with an Autolab
PGSTAT302N potentiostat–galvanostat controlled by NOVA 1.8
∗
Corresponding authors. Tel.: +39 089 969575; fax: +39 089 969603.
E-mail addresses: lpalombi@unisa.it (L. Palombi), apeluso@unisa.it (A. Peluso).
1
Tel.: +39 089969577; fax: +39 089969603.
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.01.079

A. Capobianco et al. / Electrochimica Acta 92 (2013) 446–451
447
max. theoretical yield: 50%
Scheme 1. Reaction pathway proposed for the formation of the ammonium ylide B during the electrolysis of 1 on Hg cathode [19].
software version (by Metrohm-Autolab), using a positive feed-
back to compensate for ohmic drop. A standard three-electrode
configuration was used, consisting of a Pt disk electrode as work-
ing (by Metrohm), a Pt bar as counter (by Metrohm) and a
double junction Ag/AgCl (KCl 3 M) electrode (by Metrohm) in
which the outer compartment is filled with LiClO4 0.10 M in
acetonitrile.
Constant current electrolysis was performed using an Hewlett
Packard DC Power Supply Mod. E3612A. Potentiostatic electrolyses
for spectroelectrochemical measurements were performed using
an 2051 Amel Potentiostat.
2.3. Procedure for electroinduced Stevens rearrangement of
benzyldimethylphenacylammonium bromide 1 under
galvanostatic conditions
The experiment was carried out in the cathodic compart-
ment of a U-divided glass cell separated through a porous G-4
glass frit. Platinum spirals (apparent area 2.5 cm2) were used
as anode and cathode. Catholyte was constituted by 2 mL of
a solution of 1 (0.05 M) in CH3CN. Anolyte was constituted
by 2 mL of a solution of (Et)4NClO4 0.1 M in CH3CN. After 1.2
electron/molecule (1) were passed (current intensity 20 mA)
and TLC analysis indicated the complete consumption of com-
pound 1, the reaction was prolonged at 50 ◦C for 2 h and then
the catholyte was concentrated under vacuum. The crude mix-
ture was purified on silica gel (gradient hexane:ethylacetate)
yielding 2-(dimethylamino)-1,3-diphenyl-1-propanone 2 in 56%
yield. Product 2 gave spectral and analytical data as reported in
literature [32].
2.2. Determination of the electrons number transferred in the
electrochemical process
The relationship produced during cyclic voltammetry between
peak current (ip) and scan rate (v1/2
) is described by the
Randles–Sevcik equation, modified for irreversible systems [30]:
2.4. Spectroelectrochemical measurements (Fig. 4)
ip = (2.99 × 105)n(˛ na)1/2AD1/2Cv1/2
The experiments were performed on a single beam Varian
Cary-50 spectrophotometer (scan rate 600 nm/min, resolution
1.0 nm) by using a ALS thin layer quartz glass spectroelectro-
chemical cell with a 0.5 mm optical path length, equipped with
a Pt as counter electrode, a Pt gauze as working electrode and
a double junction Ag/AgCl (KCl 3 M) electrode in which the
outer compartment is filled with LiClO4 0.10 M in acetonitrile as
reference.
The voltammetric experiments was performed with C = 0.5 mM on
a Pt electrode. The diffusion coefficient D = 2.325 × 10−9 m2/s was
determined by NMR experiments (DOSY) [31]. The derived plot of
ip vs. v1/2 is reported below (Fig. 1).
In such a conditions, assuming na = 1 and 0.3 < ˛ < 0.7 as reported
in literature [30], it has been derived n = 1.0 0.2.
Fig. 1. (a) Cyclic voltammograms of 1 [C = 0.5 mM in CH3CN 100 mM LiClO4. Working electrode: Pt. Internal reference electrode: double junction Ag/AgCl (3 M KCl)] at different
scan rates. (b) Dependence of the “normalized” peak current (Ip) on scan rate (v) observed in CV measurements.

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A. Capobianco et al. / Electrochimica Acta 92 (2013) 446–451
2.5. Computational details
3. Results and discussion
A systematic conformer search was carried out for B and
C by using the Spartan 04 program [33] Geometry optimiza-
tions and time dependent DFT (TDDFT) computations were
carried out at the PBE0 and M052X levels of computations
in conjunction with the 6-31+G** basis set. The unrestricted
formalism was adopted for radical C. Data reported in the
text and figures refer to TD-DFT calculations performed using
the PBE0 functional, since M052X yields qualitatively similar
results, but in slightly poorer agreement with experimental
data. DFT and TDDFT computations were carried out by using
the Gaussian 09 package [34]. Effects due to solvent polariza-
tion were included through the polarizable continuum model
[35].
Accordingly with a recent study of one of the authors con-
cerning the electrochemically-induced Stevens rearrangement of
proline-derived ammonium salts [36], a preliminary galvanostatic
electrolysis of the title compound (1) was performed on Pt cath-
ode as detailed in Section 2. After the electrolysis, the product of
Stevens’ rearrangement 2 was isolated in 56% yield. Noteworthy,
although a poorly selective electrolytic process should be expected
due to the galvanostatic condition, the monitoring of this elec-
trolytic process by means of 1H NMR analyses of the catholyte,
showed the formation of 2 as the most abundant product, while N-
N-dimethyl benzylamine (3) and acetophenone (4) were detected
in significant lower extent (Fig. 2). The analysis of the reaction prod-
ucts suggests that on Pt cathode the ylide formation could not be
a
b
c
Fig. 2. (a) 1H NMR spectrum of starting material 1. (b) 1H NMR spectrum of Stevens rearrangement product 2. (c) Portion of the 1H NMR spectrum of the reaction mixture
after 0.7 electron/molecule 1 passed.

A. Capobianco et al. / Electrochimica Acta 92 (2013) 446–451
449
a
b
Fig. 3. (a) DPV curve of 1 [0.5 mM] in CH3CN; sweep rate 100 mV/s; (b) CV curve of 1 [0.5 mM] in CH3CN 100 mM LiClO4; sweep rate 100 mV/s. Working electrode: Pt. Internal
reference electrode: double junction Ag/AgCl (3 M KCl).
a
b
c
d
Fig. 4. (a) UV–vis spectra of 1 and 2 in CH3CN. (b) UV–vis spectra recorded at 600 nm/min during the electroreduction of 1 (0.55 mM in CH3CN) at −1.4 V vs. Ag/AgCl (3 M
KCl), at different times. (c) Difference absorption spectrum recorded after 180 s. (d) Predicted (PCM/PBE0/6-31+G**) electronic transitions of C (red lines) and B (blue lines).
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Scheme 2. Proposed reaction pathway for the electro-induced Stevens rearrangement of 1.

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A. Capobianco et al. / Electrochimica Acta 92 (2013) 446–451
ascribed to the acid–base reaction between the two-electron reduc-
tion product A and the starting material, since the reaction pathway
depicted in Scheme 1 implies the formation of 3 and 4 in equimolar
amount (with respect to the ylide B).
As indicated in Fig. 3a, the differential pulse voltammogram
(DPV) exhibits a single signal peaked at −1.4 V vs. Ag/AgCl (KCl, 3 M)
due to an irreversible reduction process of 1 as showed by the cyclic
voltammetry (Fig. 3b). Since, as demonstrated by the voltammetric
experiments (Fig. 1a), the reduction at the electrode is free of spe-
cific substrate and surface/product interactions and thus represents
pure diffusion-controlled electroreduction of a solution species,
this peak could be reasonably ascribed to a single electron transfer
by using the equation of Randles–Sevcik, modified for irreversible
systems.
With the aim of identifying the electrochemically-generated
intermediates, UV–vis spectra were recorded in situ during the
potentiostatic electrolysis of 1, and compared with the ones of
the compounds 1 (ꢀmax 250 nm) and 2 (ꢀmax 242 nm), reported in
Fig. 4a.
No significant changes in the absorption spectrum have been
observed during potentiostatic electrolysis for applied potentials
in the range (0 → −1.4 V). Conversely, transient absorption spectra,
recorded in the time interval 0–180 s during the potentiostatic elec-
trolysis of 1 at E = −1.4 V, Fig. 4b, show a clear bleaching of the band
at 250 nm, where 1 exhibits an absorption peak, and two increasing
absorptions, a broad one growing at about 300 nm, and another one
peaked at ca. 225 nm, as better evidenced in the difference spec-
trum, drawn in Fig. 2c for t = 180 s. No further signals are observed
in the visible region up to 700 nm.
Furthermore, the transient absorption spectra of Fig. 4b show
the existence of two quasi-isosbestic points, falling at ca. 235 and
264 nm, suggesting a simple reaction mechanism in which only
two species, the reagent and a reaction intermediate, are present
in solution. The hypothesis that the second species is simply the
reaction product 2 must be disregarded both because of the num-
ber of the observed quasi-isosbestic points and of their wavelength
positions and because the increasing absorption at 300 nm is not
compatible with the formation of 2.
Assuming that the reduction proceeds via one-electron trans-
fer, the intermediate species responsible of the observed transient
spectra could be both the neutral radical C, which is continuously
generated at the electrode surface, or the ylide B, formed from C by
fast H atom extraction on Pt cathode [37–44] (Scheme 2).
In order to discriminate between the two most probable inter-
mediates, the electronic absorption spectra of C and B have been
computed by using time dependent density functional theory
Fig. 5. UV spectra at 600 nm/min of compound 1 (black line), 1 treated with
0.5 equiv. of NaOH (red line) and 1 equiv. of NaOH (blue line). (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of the article.)
(TDDFT). The results are summarized in Fig. 4d, where the tran-
sition strengths, averaged over Boltzmann conformer populations,
are reported as a function of the absorption wavelength. The most
intense transitions of C are predicted at wavelengths higher than
400 nm, where no signal was detected by spectro-electrochemical
measurements, whereas intense transitions of B are predicted at ca.
305 nm, well compatible with the increasing absorption observed
in the region 270–320 nm (Fig. 4b). Thus TDDFT computations rule
out the possibility that the transient absorption spectrum can be
assigned to the radical species C, and strongly suggest that the
observed intermediate species is indeed the ylide B.
The latter hypothesis has been confirmed by collecting the
UV–vis spectra during the reaction of the ammonium salt 1 with
NaOH in AN, a reaction which is well known to chemically generate
the long lived ammonium ylide B, via proton extraction [12]. The
UV absorption spectrum recorded in the same conditions, is shown
in Fig. 5. Remarkably, it strongly resembles those of Fig. 4b, giving
further support to the assignment of the broad band observed in
the electro-activated reaction to the ylide B [45].
The ylide formation via one-electron process at the platinum
cathode rather than by acid–base reaction as indicated in Scheme 1,
is further confirmed by comparing the UV-spectra of Fig. 4b with
the absorption spectrum of acetophenone 4 reported in Fig. 6a. In
a
b
Fig. 6. (a) UV–vis spectrum of acetophenone 4 in CH3CN; calculated (PCM/TDPBE0/6-31+G**) electronic transitions are superimposed as red vertical lines. (b) Predicted
(PCM/TDPBE0/6-31+G**) electronic transitions of enolate A.

A. Capobianco et al. / Electrochimica Acta 92 (2013) 446–451
451
fact, if the attainment of the ylide B were due to the reaction path-
way depicted in Scheme 1, an isosbestic point between the max
absorbances of 1 and 4 should be expected.
On the other hand, as shown by the computed spectrum
reported in Fig. 6b, the increasing absorption at 300 nm observed
during the electrolysis is also not compatible with the formation
of the transient enolate A (predicted ꢀmax ca. 350 nm), the pri-
mary reduction product expected if the two-electron cleavage was
operating.
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