A novel neutral organic electron donor with record half-wave potential

Article abstract of DOI:10.1039/c3ob41701h

Tricyclic donor 26 has been prepared and is the most reducing neutral ground-state organic molecule known, with an oxidation potential 260 mV more negative than the previous record. Cyclic voltammetry shows that a 2-electron reversible redox process occurs in DMF as solvent at -1.46 V vs. Ag/AgCl.

Full text of DOI:10.1039/c3ob41701h

Organic &
Biomolecular Chemistry
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A novel neutral organic electron donor with record
half-wave potential†
Cite this: Org. Biomol. Chem., 2013, 11,
8073
Hardeep S. Farwaha,a Götz Bucherb and John A. Murphy*a
Received 20th August 2013,
Accepted 10th October 2013
Tricyclic donor 26 has been prepared and is the most reducing neutral ground-state organic molecule
known, with an oxidation potential 260 mV more negative than the previous record. Cyclic voltammetry
shows that a 2-electron reversible redox process occurs in DMF as solvent at −1.46 V vs. Ag/AgCl.
DOI: 10.1039/c3ob41701h
www.rsc.org/obc
Table 1 Oxidation potentials of neutral organic electron donors
E11/2 vs. SCE E12/2 vs. SCE
Introduction
The most challenging electron transfer reductions, such as
Birch reductions,1 acyloin couplings2,3 and reduction of di-
nitrogen in nitrogenase enzymes,4 are achieved by reactive
metals and their complexes.5 Recently, a number of organic
electron donors have been synthesized that are very strong
reducing agents (1–8). All of these compounds critically
contain nitrogen atoms that are capable of stabilizing both
positive charge and radical character, as the donors undergo
oxidation. The aromaticity of their oxidation products (radical
cations and dications) following loss of one and two electrons
respectively, also plays an important role in determining the
strength of these electron donors. Table 1 compares the oxi-
dation potentials of these compounds with the widely used
sulfur-containing electron donor, tetrathiafulvalene (TTF).6
Among the nitrogen-containing electron-donors, TDAE (1)
is the parent compound in the series and the standard by
which the others can be judged.7 Neither 1 nor its oxidized
products is aromatic. Compound 2 could be considered anti-
aromatic8 if planar and so its oxidation through loss of two
electrons might expect to be strongly driven; however it is
quite deformed from planarity and it contains two aromatic
pyrrole rings – as a result it is not a strong reducing agent.
Compound 39 is already aromatic and hence its oxidation does
not benefit from aromatization as a driving force, and so it
also is not a strong donor of electrons. In contrast, donors 4–8
are all converted into aromatic products upon oxidation10–19
and this adds to their strength as reducing agents. To illustrate
the aromaticity that arises, the oxidation products of com-
pound 8 are also shown in Fig. 1. Loss of one electron leads to
Compound E11/2
E21/2
Solvent (converted) (converted)
TTF6
17
+0.37 V +0.67 V DCM
(SCE) (SCE)
−0.78 V −0.61 V MeCN
(SCE) (SCE)
+0.37 V
−0.78 V
−0.14 V
−0.32 V
−0.88 V
−1.03 V
−0.82 V
−1.20 V
−1.24 V
+0.67 V
−0.61 V
+0.19 V
—
28
−0.59 V −0.26 V THF
(Fc/Fc+) (Fc/Fc+)
39
−0.32 V
—
MeCN
(SCE)
413
511
610
710
814,19
−1.33 V −1.14 V DMF
−0.69 V
−1.03 V
−0.76 V
−1.20 V
−1.24 V
(Fc/Fc+) (Fc/Fc+)
−1.48 V −1.48 V THF
(Fc/Fc+) (Fc/Fc+)
−0.82 V −0.76 V DMF
(SCE)
(SCE)
−1.20 V −1.20 V DMF
(SCE) (SCE)
−1.69 V −1.69 V DMF
(Fc/Fc+) (Fc/Fc+)
radical cation 13 featuring one pyridinium ring, while loss of a
second electron affords the aromatic disalt 14. In terms of the
applications of these stronger electron donors, benzimidazole-
derived 6 converts iodoarenes into aryl radicals,15 while the
stronger donors 7 and 8 reduce the same substrates to aryl
anions.12,14 Donors 7 and 8 are also able to reduce arenesulfon-
amides,16 Weinreb amides17 and acyloin derivatives.18
Although these are highly reactive organic compounds,
their reducing power is significantly less than that of the stron-
gest metals (e.g. the oxidation potential of Li, E0 = −3.02 V)20
and questions arise about whether a limit is being approached
in the design of organic neutral donors.
Molecule 5a features a number of rings, all of which could
become aromatic (5b) on loss of two electrons.11 However, if
such a donor has a sufficient number of linked rings, the aro-
matic stabilization energy might ensure that the ground-state
of 5a will instead be diradical 5c and then the oxidation to 5b
by loss of two electrons would only convert the two terminal
aWestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde,
295 Cathedral Street, Glasgow, G1 1XL, UK. E-mail: John.murphy@strath.ac.uk
bWestCHEM, School of Chemistry, University of Glasgow, Joseph Black Building,
University Avenue, Glasgow, G12 8QQ, UK
†Electronic supplementary information (ESI) available: Computational coordi-
nates and NMR spectra are provided. See DOI: 10.1039/c3ob41701h
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Results and discussion
To probe whether more powerful donors could be prepared, it
was therefore important to avoid compounds like 7 and 9–12.
Compound 8 offered the best design lead. This compound was
a purple solid and was stable in the absence of air and moist-
ure. Unlike imidazole-derived 7, which required both trimethyl-
ene bridges, it had been possible to synthesise some
analogues of the pyridine-derived 8, including the compound
15 that features no trimethylene bridges. These compounds
were, together with 7, the strongest neutral organic ground-
state donors known. To enhance the donor strength, two possi-
bilities were considered: (i) introduction of appropriately
placed electron-releasing substituents on the pyridine-derived
rings or (ii) extension of the polycyclic system by inclusion of
more rings. We recently reported that our initial efforts to
prepare analogues derived from 2-(dialkylamino)pyridines had
led in an unexpected direction25,26 but we now address both
points in extending the polycyclic system.
The strategy for development of extended donors and
more powerful donors involved using pyrrole-derived units.27–30
Interpolation of a pyrroldiylidene between the two pyridine-
derived rings of donor 15 would result in 16. Here, five nitrogen
atoms would stabilize the transition states and products of oxi-
dation, and three rings would develop aromaticity in the two-
electron conversion to pyrrole-dipyridinium salt 18 (Scheme 1).
The synthesis of 16 was achieved as shown in Scheme 2.31
Initially, the synthesis of diketone 22 directly from Weinreb
amide 19 and 4-DMAP 20 was attempted using Fort’s direct
deprotonation protocol,32 however this was unsuccessful.
Kessar33 used N-trifluoroboration of pyridine to acidify the
2-position of the ring, and the resulting pyridinium ylide was
used for C–C bond-formation. The same BF3 adduct has also
been utilized by Sammakia34 and Vedejs.35 The trifluoroborate
salt was easily prepared (76% yield) from reaction of 20 with
BF3·Et2O followed by filtration of the hygroscopic white solid.
Formation of diketone 22 from lithiation of the BF3 adduct
was unsuccessful using LDA, n-BuLi or t-BuLi and so a
lithium–halogen exchange using 2-bromo-4-DMAP 21 followed
by addition of Weinreb amide 19 was attempted. The synthesis
Fig. 1 Known neutral organic electron donors 1–8 and related compounds.
rings to aromatic rings. Whether 5 exists as 5a or 5c is not
entirely settled. The compound did not afford a well-defined
NMR spectrum that would characterize a closed-shell struc-
ture,11 indicating that it might be a diradical, although no EPR
evidence for radical character was seen. The second indication
of a reactivity limit related to the imidazole-derived com-
pounds 7, 9–12. Although the doubly trimethylene-tethered
compound 7 has been fully characterized, efforts to make
simpler analogues of this compound, for example 9–11, had
proved impossible.21,22 Attempted preparations of compound
11 had instead led to dicarbene 12, possibly through spon-
taneous rupture of the central alkene to the dicarbene, but
more likely through activation by a proton source or by a metal
cation23 – the dicarbene 12 is not an electron donor. The
instability of the tetraazafulvalene central double-bond was
evident also from calculations that showed bond strength for
molecule 10 of only 4 kJ mole−1 21
In fact, compound 9 and 10
.
have recently been prepared in our laboratories,24 but have
been shown to be very short-lived.
Scheme 1 Proposed new electron donors 16.
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Fig. 2 Comparison of cyclic voltammograms of 25 (purple) and 14 (green) vs.
Ag/AgCl in DMF at 50 mV s−1 scan rate.
Scheme 2 Synthesis of new electron donor 26.
cation 27 or may be another radical derived from 26. Quantitat-
ive ESR measurements undertaken using diphenylpicryl-
of 21 was successful and optimized by forming the trifluoro- hydrazyl (DPPH) as a calibrant indicated that the radical
borate adduct in situ and using tetrabromomethane as the concentration accounted for only 0.012% of the concentration
halogenation source, as opposed to bromine. A minor side- of 26, and so this cannot be the cause of the broadness in the
product 23 arose from nucleophilic attack by t-BuLi. From 22, NMR spectrum. The alternative possibility of rotation about
formation of the central pyrrole ring to give compound 24 was the inter-ring CvC bonds through a suitable low-energy triplet
efficient, as was the methylation of the pyridine rings to give was also explored. A low energy triplet (M05-2X/cc-pVTZ: ΔUT,calc
dication 25 (79% and 96% yields respectively, Scheme 2).
=
11.0 (12.9) kcal mol−1, gas phase (DMF)) exists (see ESI†
Cyclic voltammetry of compound 25 (Fig. 2 shows the vol- file); however since this is not observed in ESR, it could only
tammogram together with that of 14) revealed a reversible two- be a conduit between configurational isomers about the inter-
electron wave at E1/2 = −1.46 V vs. Ag/AgCl in DMF (equating to ring CvC bonds. However, the energies of the configurational
−1.50 V vs. SCE). This is 260 mV more negative than the half- isomers are very high relative to 26, (density functional calcu-
wave potential for 8, and so compound 26 is now by far the most lations using the B3LYP 6-31G* options in a DMF conti-
powerful neutral ground-state organic electron donor yet syn- nuum (Spartan’10 V1.1.0, Wavefunction Inc.) show that
thesized. Donor 26 was prepared by reduction of disalt 25 with changing one of the inter-ring alkenes from E to Z affords the
sodium amalgam in DMF. The compound was then removed next most favourable isomer, but that is 23 kcal mol−1 higher
from the amalgam for analysis and to explore its reactivity.
in energy than 26) and accordingly, populations of minor
Characterisation of 26 proved interesting. As previously isomers arising in this way are unlikely as the cause of the
seen for compound 5, this compound did not give a well broad signals in the NMR spectrum.
1
resolved H NMR spectrum in DMF-d7. Attempts to achieve a
The product of the oxidation of 26 by molecular iodine was
sharper spectrum by cooling, or by heating to 90 °C, were not characterized as the diiodide salt 25. Further characterization
successful, and so we sought further information at room of donor 26 was achieved through performing the experiment
temperature. To do this, ESR spectra were recorded and a weak quantitatively by using titration. The compound 26 was treated
signal detected that was consistent with an organic radical, with excess iodine to afford 25; following this reaction, the
but not with a triplet diradical. This species may be the radical unreacted residual iodine was then back-titrated with sodium
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Conclusions
A new powerful neutral organic electron donor 26 has been
synthesized and is able to reduce appropriate tosylamides with
greater efficiency than any previously synthesized neutral
organic donor. With a half-wave potential of −1.46 V vs. Ag/
AgCl in DMF (equating to of −1.5 V vs. SCE), it is the most
reducing neutral organic species known.
Experimental section
General
Proton NMR (1H) spectra were recorded at 500 MHz (on a
Bruker® AV500™ spectromer) or at 400.13 MHz (on a Bruker®
DPX 400™ or Bruker® AV400™ spectrometer). Carbon NMR
(13C) spectra were recorded at 125 MHz or 100 MHz using a
J-mod pulse program to determine carbon assignments.
Experiments were carried out using deuterated chloroform (CDCl3)
unless otherwise stated and chemical shifts are reported in
parts per million (ppm), calibrated on the solvent residual
peak and referenced to tetramethylsilane. Coupling constants J
are reported in hertz (Hz). The following abbreviations are
used for the multiplicities: s, singlet; d, doublet; t, triplet; q,
quartet; m, multiplet; bs, broad singlet.
High resolution mass spectra were recorded at the EPSRC
National Mass Spectrometry Service Centre, Swansea on a JLZX
102™, VGZAB-E™ or a VG™ micromass instrument. Low
resolution mass spectra were recorded at the University of
Strathclyde Mass Spectrometry Service on a ThermoFinnigan™
PolarisQ Ion Trap Spectrometer and trace GC instrument using
a ZB-5 column (30 metres).
Scheme 3 Reduction of organic substrates using donor 26.
Infrared spectra were recorded on a Perkin Elmer Spectrum
One FT-IR™ spectrometer as films applied on sodium chloride
thiosulfate. This titration showed that the donor had reacted plates or mixed and pressed into potassium bromide disks.
with exactly one equivalent of iodine, in line with expectation Melting points were recorded using either a Griffin or a Gallen-
for two-electron donor 26.
kamp melting point apparatus.
The reactivity of 26 towards organic substrates was now
Column chromatographies on silica gel were performed
tested. Donor 26 reduced Weinreb amide 28 (96% yield) using using Prolabo 35–75 µm particle sized silica gel 60
just 1.5 eq. of donor at room temperature (Scheme 3). (200–400 mesh). Crude mixtures were studied using thin layer
Reduction of tosylamides 30, 32 and 34 was then carried out. chromatography (TLC) carried out on Merck silica gel 60 F254
Substrate 30 was of interest as 4-DMAP-based donor 8 had precoated aluminium plates. Visualization was achieved under
reduced substrate 30 with difficulty in 22% yield at 100 °C, UVP mineralight UVG-11 lamp or by developing plates with
and six equivalents of imidazole-based donor 7 had been methanolic vanillin or potassium permanganate.
required to reduce 32 in 96% yield at 110 °C. However, donor
Concentration of solutions under reduced pressure
26 now reduced 30 and 32 in 68% and 87% yield respectively, (1–10 mbar) was achieved using a diaphragm pump vacuum.
both times requiring only 3 equivalents of donor at 100 °C Drying of solids under reduced pressure was performed at
indicating that it is more efficient at performing difficult room temperature firstly under 1–10 mbar using a diaphragm
reductions. It has been previously established that the depro- pump vacuum then under 0.001–0.01 mbar using a rotary oil
tection of these sulfonamides affords nitrogen anions and sul- pump.
finate anions. In this way, reaction of substrate 34 affords
All reagents were obtained from commercial suppliers and
dianion 36, leading to isolated sulfonamide 35, on workup. used without further purification unless stated otherwise.
Overall, pyrroldiylidene donor 26 represents a new generation Tetrahydrofuran, dichloromethane, hexane, diethyl ether and
of highly electron-rich and purely organic reducing agents with toluene were dried and deoxygenated with a Pure-Solv 400
a half-wave potential of −1.5 V vs. SCE and the ability to carry solvent purification system (by Innovative Technology Inc.,
out ever more challenging reductions.
USA). n-BuLi was obtained as a 2.5 M solution in hexane and
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t-BuLi as a 1.7 M solution in hexane. Titration of both reagents, Preparation of N,N′-dimethoxy-N,N′-dimethylsuccinamide 19
prior to use, was achieved by dropwise addition of either
reagent solution via syringe to a solution of diphenylacetic
Potassium hydroxide (50.95 g, 908 mmol, 6 eq.) was added to a
solution
of
N,O-dimethylhydroxylamine
hydrochloride
acid (1 mmol) in THF (10 mL) under argon. Addition was
stopped with the first appearance of a yellow colour (diphenyl-
acetate dianion) and the volume of lithiating reagent was
measured. The procedure was carried out in triplicate so that
an average concentration could be calculated for each reagent.
N,N-Dimethylformamide was obtained from commercial sup-
pliers as anhydrous (99.98%) and used directly. Sodium
hydride was supplied as a 60% suspension in mineral oil and
was washed with hexane to remove oil prior to use. Dry aceto-
nitrile was dried by distillation over phosphorus pentoxide. All
reactions were carried out under argon unless otherwise
stated.
(59.05 g, 605 mmol, 4 eq.) in water (150 mL) slowly at 0 °C
with vigorous stirring. The free amine was distilled from the
solution at 42 °C and added via dropping funnel to a solution
of succinyl chloride (16.6 mL, 151 mmol, 1 eq.) in dry DCM
(300 mL) under argon at −10 °C with vigorous stirring. The
reaction mixture was brought to room temperature and left
stirring under argon atmosphere for 16 h. The reaction
mixture was concentrated under reduced pressure to 100 mL,
washed with 0.5 M hydrochloric acid (5 × 50 mL), NaHCO3(aq)
(3 × 50 mL), brine (50 mL) and dried over Na2SO4. The crude
solution was concentrated under reduced pressure and puri-
fied by column chromatography (firstly with neat EtOAc to
remove less polar impurities then 20% MeOH–EtOAc) to afford
N,N′-dimethoxy-N,N′-dimethylsuccinamide 19 (20.1 g, 65%) as
a clear oil, which crystallised on standing to give white crystals;
m.p. 73–75 °C (lit.:40 73–75 °C); [Found: (ESI+) (M + H)+
205.1185, C8H17N2O4 (MH) requires 205.1183]; νmax(film)/cm−1
3493, 2963, 2942, 2830, 1731, 1660, 1460, 1422, 1390, 1194,
1097, 994, 934, 795, 745; 1H-NMR (500 MHz, CDCl3) δ 2.78
(4H, s, CH2), 3.12 (6H, s, NCH3), 3.74 (6H, s, OCH3); 13C-NMR
(125 MHz, CDCl3) δ 25.9 (CH2), 31.1 (NCH3), 60.7 (OCH3),
172.9 (C); m/z (ESI+) 205 ([M + H]+, 100%), 239 (21), 351 (7),
515 (2).
Calculations
All calculations in the ESI file were performed using the
Gaussian09 suite of programs.36 The M05-2X hybrid functional37
was employed in combination with a cc-pVTZ basis set.38 All
stationary points were fully optimised and characterised via a
vibrational analysis. The influence of solvation was taken into
account via a polarisable continuum model (scrf=pcm).39
Cyclic voltammetry conditions
Cyclic voltamograms were carried out using a glassy carbon
working electrode, Ag/AgCl reference electrode and platinum
counter electrode. The electrolyte solution used was 0.1 M
tetrabutylammonium hexafluorophosphate in degassed anhy-
drous DMF and the concentrations of the ferrocene external
Preparation of 2-bromo-4-dimethylaminopyridine 21
(a) Preparation of 4-dimethylaminopyridinium trifluoro-
standard and analyte were also 0.1 M. All solutions were pre- borate. To a solution of 4-dimethylaminopyridine (4.89 g,
pared in the glovebox under an inert atmosphere. A three-elec- 40 mmol, 1 eq.) in a 1 : 1 mixture of dry THF (50 mL) and Et2O
trode set up was used to obtain the cyclic voltammograms. (50 mL) was added boron trifluoride diethyl etherate (6.01 mL,
Electrochemical measurements were carried out using the 48.9 mmol, 1.22 eq.) dropwise. The white suspension was
Autolab®/PGSTAT302N potentiostat.
stirred at room temperature for 3 h and the precipitate was fil-
tered via argon pressure onto a sinter funnel. The white solid
was dissolved in DCM (100 mL), washed with water (2 ×
50 mL), brine (50 mL), dried over Na2SO4 and concentrated
General procedure for reductions using pyrrole-based tricyclic
electron donor 26
Sodium amalgam was prepared by addition of freshly cut under reduced pressure to afford 4-dimethylaminopyridinium
sodium (50 mg) to a flame-dried 25 mL round bottom flask trifluoroborate (5.80 g, 76%) as a white solid; mp: 128–130 °C
containing mercury (5 g) under a strong flow of argon. To this (lit.:41 129.8–130.9 °C); [Found: (ESI+) (M + NH4)+ 208.1226;
was added anhydrous DMF (15 mL) followed by pyrrole-based C7H14(11B)F3N3, (M + NH4) requires 208.1227]; νmax(KBr)/cm−1
1
diiodide salt 25. The formation of a deep purple colour indi- 2939, 1646, 1568, 1404, 1113, 914; H-NMR (400 MHz, CDCl3)
cated donor formation and at this stage the reaction was left δ 3.18 (6H, s, NCH3), 6.64 (2H, s, J = 7.3 Hz, ArH), 8.12 (2H, d,
stirring for 3 h to ensure reaction had gone to completion. The J = 7.3 Hz, ArH); 13C-NMR (125 MHz, CDCl3) δ 40.0 (CH3),
purple solution was transferred by cannula to a flask contain- 106.3 (CH), 142.2 (CH), 156.9 (C); 19F-NMR (125 MHz, CDCl3)
ing the desired substrate and left to stir overnight at the stated δ 152.07–152.18 (BF3); m/z (ESI+) 207 ([M + NH4]+, 10B, 22%),
temperature. There was no transfer of sodium amalgam to the 208 ([M + NH4]+ 11B, 100%), 213 [(M + Na)+, 68], 231 (13).
flask containing the substrate. This was further verified by
(b) Preparation
of
2-bromo-4-dimethylaminopyridine
inspecting the flask for traces of mercury at the work-up stage. 21. To
a
solution of 4-dimethylaminopyridine (25.45 g,
The reaction solution was partitioned between EtOAc (100 mL) 208 mmol, 1 eq.) in dry THF (300 mL) at room temperature
and water (50 mL). The organic phase was washed with water was added freshly purchased boron trifluoride diethyl etherate
(2 × 50 mL) to remove traces of DMF and brine (50 mL). After (30.9 mL, 250 mmol, 1.2 eq.) and solution was stirred for
drying over Na2SO4 and concentrating under reduced pressure, 30 min before cooling to −78 °C under vigorous flow of argon.
the residue was purified using column chromatography with n-BuLi in hexane (100 mL, 250 mmol, 1.2 eq.) was added drop-
the stated eluent mixture.
wise under argon to the cream-coloured suspension, keeping
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1
the temperature below −70 °C. After 30 min, a solution of 1376, 1224, 1054, 985, 947, 818, H-NMR (500 MHz, CDCl3) δ
carbon tetrabromide (82.91 g, 250 mmol, 1.2 eq.) in dry THF 1.14 (9H, s, CH3), 2.86 (2H, t, J = 6 Hz, CH2), 2.95 (6H, s,
(100 mL) was added dropwise via cannula under argon (again NCH3), 3.39 (2H, t, J = 6 Hz, CH2), 6.53 (2H, dd, J = 2.5 Hz, 6
keeping temperature below −70 °C) and the dark brown reac- Hz, ArH), 7.17 (2H, d, J = 2.5 Hz, ArH), 8.20 (2H, d, J = 6 Hz,
tion mixture was left to warm to room temperature overnight. ArH); 13C-NMR (125 MHz, CDCl3) δ 26.6 (CH3), 30.8 (CH2), 32.0
THF was removed under reduced pressure and the residue was (CH2), 39.1 (NCH3), 43.9 (C), 104.6 (CH), 109.2 (CH), 149.0
partitioned between DCM (250 mL) and sat. NaHCO3(aq) (CH), 153.5 (C), 154.8 (C), 201.6 (C), 214.4 (C); m/z (ESI+) 263
(100 mL). The organic layer was washed with sat. NaHCO3(aq) ([M + H]+, 100%), 264 (16), 265 (5).
(2 × 100 mL), brine (100 mL) and dried over Na2SO4. The crude
solution was concentrated under reduced pressure and puri-
fied by column chromatography (10–30% EtOAc–hexane) to
Preparation of N-ethyl-2,5-bis-(4-dimethylamino-2-pyridyl)-
pyrrole 24
afford 2-bromo-4-dimethylaminopyridine 21 (30.59 g, 75%) as To a solution of 1,4-bis-(4-dimethylamino-2-pyridyl)butane-1,4-
an orange semi-solid; [Found: (ESI+) (M + H)+ 202.0022; dione 22 (1.56 g, 4.79 mmol, 1 eq.) in methanol (10 mL) was
C7H9(79Br)N2 requires MH, 202.0022]; νmax(film)/cm−1 2932, added freshly distilled ethylamine(aq) (20 mL, excess). The reac-
2820, 1594, 1520, 1441, 1222, 1131, 1073, 978; 1H-NMR tion was stirred at room temperature overnight and concen-
(500 MHz, CDCl3) δ 2.99 (6H, s, NCH3); 6.43 (1H, dd, J = 2.5 trated under reduced pressure. The residue was purified by
Hz, 6 Hz, ArH); 6.63 (1H, d, J = 2.5 Hz, ArH); 7.93 (1H, d, J = 6 column chromatography (EtOAc to 2% Et3N–EtOAc) and
Hz, ArH); 13C-NMR (125 MHz, CDCl3) δ 39.5 (NCH3), 106.4 washed in a sinter funnel with diethyl ether to remove further
(CH), 109.4 (CH), 143.2 (C), 149.5 (CH), 156.0 (C); m/z (CI+) 202 impurities to afford N-ethyl-2,5-bis-(4-dimethylamino-2-pyridyl)-
([M + H]+, 79Br, 63%), 204 ([M + H]+, 81Br, 4%), 123 (100). Data pyrrole 24 (1.26 g, 79%) as a light brown semi-solid; [Found:
were consistent with precedent.42
(ESI+) (M + H)+ 336.2185, C20H25N5 requires MH, 336.2183];
νmax(KBr)/cm−1 3091, 2978, 2927, 2814, 1593, 1542, 1501, 1443,
1370, 1325, 1278, 1224, 989, 804, 770; 1H-NMR (500 MHz,
CDCl3) δ 1.04 (3H, t, J = 7 Hz, CH3), 3.04 (12H, s, NCH3), 4.80
(2H, q, J = 7 Hz, CH2), 6.42 (2H, dd, J = 2.5 Hz, 6 Hz, ArH), 6.76
Preparation of 1,4-bis-(4-dimethylamino-2-pyridyl)butane-1,4-
dione 22, with 1-(4-dimethylamino-2-pyridyl)-5,5-dimethyl-
hexane-1,4-dione 23 as by-product
To
a
solution of 2-bromo-4-dimethylaminopyridine 21 (2H, d, J = 2.5 Hz, ArH), 8.27 (2H, d, J = 6 Hz, ArH); 13C-NMR
(300 mg, 1.49 mmol, 2.05 eq.) in dry THF (20 mL) at −78 °C (125 MHz, CDCl3) δ 16.6 (CH3), 39.2 (NCH3), 40.7 (CH2), 104.7
under argon was added t-BuLi (2.01 mL, 3.02 mmol, 4.15 eq.) (CH), 105.9 (CH), 110.6 (CH), 136.0 (C), 144.0 (CH), 153.0 (C),
dropwise using argon pressure. The reaction was stirred for 154.8 (C); m/z (ESI+) 336 ([M + H]+, 100%), 337 (20), 338 (3).
60 min and a solution of N,N′-dimethoxy-N,N′-dimethylsuccin-
EPR measurements
amide (149 mg, 0.73 mmol, 1 eq.) in THF (10 mL) was added
dropwise using argon pressure. The reaction was then left to Deoxygenated dry DMF (15 mL) was added under a flow of
warm to room temperature overnight. After quenching the argon to an oven-dried 25 mL round-bottomed flask contain-
reaction by dropwise addition of water (5 mL), THF was ing sodium amalgam (prepared from 50 mg of sodium in 5 g
removed under reduced pressure. The red residue was dis- of mercury) and the donor precursor diiodide salt (69.7 mg,
solved in DCM (40 mL) and washed with water (2 × 20 mL), 0.113 mmol, 1 eq.). The deep purple reaction mixture was
sat. NaHCO3(aq) (1 × 20 mL), brine (1 × 20 mL) and dried over stirred at room temperature under fast argon flow for 3 h.
Na2SO4. The crude solution was concentrated under reduced Three test aliquots of this 0.0075 M solution were transferred
pressure and purified by column chromatography (1% Et3N– into an oven dried syringe needle and the centre of a freshly
EtOAc). Since the product is insoluble in EtOAc, the crude created stream of this air-sensitive solution was immediately
material was washed with EtOAc in a sinter funnel to remove drawn from inside the tip of the syringe needle into a 1 mm
unreacted 4-dimethylaminopyridine. The yellow solid was then diameter glass capillary (with a sample height of around 5 cm)
dried under reduced pressure to afford 1,4-bis-(4-dimethyl- before being sealed at the open end by flame-gun. This was
amino-2-pyridyl)butane-1,4-dione 22 (5.3 g, 49%) as a yellow repeated a fourth time until there was great confidence that no
powder; m.p. 165–167 °C; [Found: (EI+) (M+) 326.2; oxygen was present in the sample solution (which maintained
C18H22N4O2 requires M+, 326.2]; νmax(film)/cm−1 2924, 1692, a deep purple colour). ESR data were then obtained for this
1600, 1509, 1432, 1377, 1226, 985, 810; 1H-NMR (400 MHz, sample, which showed a signal with g-factor 2.0035).
CDCl3) δ 3.04 (12H, s, NCH3), 3.64 (4H, s, CH2), 6.63 (2H, dd, J
To an oven-dried 25 mL round bottom flask containing
= 2.5 Hz, 6 Hz, ArH), 7.31 (2H, d, J = 2.5 Hz, ArH), 8.31 (2H, d, diphenylpicrylhydrazyl (DPPH, 44.5 mg, 0.113 mmol, 1 eq.)
J = 6 Hz, ArH); 13C-NMR (125 MHz, CDCl3) δ 32.5 (CH2), 39.2 was added deoxygenated dry DMF (15 mL) and a sample of
(NCH3), 104.9 (CH), 109.2 (CH), 149.1 (CH), 153.7 (C), 155.1 this 0.0075 M solution was transferred into a 1 mm capillary
(C), 201.6 (C); m/z (EI+) 326 ([M+], 100%), 327 (21), 328 (5). in an identical manner to that mentioned above so that a refer-
1-(4-Dimethylamino-2-pyridyl)-5,5-dimethylhexane-1,4-dione 23 ence ESR signal could be obtained (g-factor 2.0035). Compari-
(623 mg, 6.5%) was also separately isolated as a yellow oil son of the signal intensities indicated a concentration of
[Found: (ESI+) (M + H)+ 263.1757; C15H22N2O2 requires MH, 9 × 107 M, corresponding to 0.012 M conversion of the donor
263.1754]; νmax(film)/cm−1 2966, 1698, 1601, 1540, 1509, 1432, to the radical cation within that solution.
8078 | Org. Biomol. Chem., 2013, 11, 8073–8081
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Paper
Preparation of N-ethyl-2,5-bis-(N′-methyl-4-dimethylamino-2-
pyridinium iodide)pyrrole 25
7.20–7.23 (8H, m, ArH), 7.27–7.29 (2H, m, ArH), 7.55–7.56 (2H,
m, ArH); 13C-NMR (125 MHz, CDCl3) δ 21.2 (CH3), 54.7 (CH2),
127.6 (CH), 127.8 (CH), 128.4 (CH), 128.5 (CH), 128.8 (CH),
129.0 (CH), 129.5 (CH), 135.7 (C), 136.0 (C), 139.0 (C), 143.5
(C). Data were consistent with those previously published.16
Methyl iodide (12 mL, 192 mmol, 15 eq.) was added dropwise
to
a solution of N-ethyl-2,5-bis-(4-dimethylamino-2-pyridyl)-
pyrrole 24 (4.30 g, 12.8 mmol, 1 eq.) in acetonitrile (80 mL)
was under argon. The solution was refluxed and the suspen-
sion formed was left stirring overnight. The suspension was
Preparation of N-benzylaniline 31
cooled to room temperature, diethyl ether (100 mL) was added The general procedure for electron transfer reactions was
and the solid was filtered via suction and washed with diethyl applied to N-phenyl-N-benzyl-4-methylbenzenesulfonamide 30
ether several times to afford N-ethyl-2,5-bis-(N′-methyl-4-di- (90.8 mg, 0.27 mmol, 1 eq.) using pyrrole salt 25 (500 mg,
methylamino-2-pyridinium iodide)pyrrole 25 (7.50 g, 95%) as 0.81 mmol, 3 eq.). The reaction was heated to 100 °C and the
a light brown powder; m.p. (dec.) T > 250 °C; [Found: (NSI+) crude material was purified by column chromatography (20%
(M − 2I)2+ 182.6281 C22H31N5 (M − 2I)2+ requires 182.6284]; EtOAc–Pet. Ether) to give N-benzylaniline 31 (33 mg, 68%) as a
1
νmax(KBr)/cm−1 2924, 1674, 1567, 1382, 1314, 1059; 1H-NMR white solid; m.p. 35–37 °C (lit.7 35–38 °C); H-NMR (500 MHz,
(500 MHz, DMSO-d6) δ 0.91 (3H, t, J = 7 Hz, CH3), 3.23 (12H, s, CDCl3) δ 4.08 (1H, bs, NH), 4.35 (2H, s, CH2), 6.65–6.67 (2H,
NCH3), 3.74 (6H, s, NCH3), 3.82 (2H, bs, CH2), 6.69 (2H, s, m, ArH), 6.74 (1H, t, J = 7 Hz, ArH), 7.12–7.21 (2H, m, ArH),
ArH), 7.11 (2H, d, J = 3 Hz, ArH), 7.15 (2H, dd, J = 3 Hz, 7.5 Hz, 7.29–7.30 (1H, m, ArH), 7.34–7.40 (4H, m, ArH); 13C-NMR
ArH), 8.42 (2H, d, J = 7.5 Hz, ArH); 13C-NMR (125 MHz, DMSO- (125 MHz, CDCl3) δ 48.7 (CH2), 112.0 (CH), 118.1 (CH), 128.5
d6) δ 16.7 (CH3), 40.5 (NCH3), 41.1 (CH2), 43.3 (NCH3), 107.9 (CH), 128.7 (CH), 129.1 (CH), 129.7 (CH), 139.8 (C), 148.4 (C);
(CH), 111.6 (CH), 113.7 (CH), 126.4 (C), 144.2 (C), 145.2 (CH), Data consistent with those previously published.35
156.4 (C), m/z (NSI+) 182 ([M − 2I]2+, 100%), 337 (42), 492 (29),
651 (13).
Preparation of 1-(toluene-4-sulfonyl)-1H-indole 32
An oven-dried flask containing indole (5 g, 42 mmol, 1 eq.)
and NaH (60% dispersed in mineral oil, 2.05 g, 51 mmol, 1.2
Preparation of N-methyl-1-naphthamide 29
The general procedure for electron transfer reactions was eq.) under flow of argon, was washed with dry hexane (3 ×
applied to N-methoxy-N-methyl-1-naphthamide (116 mg, 50 mL) prior to addition of dry THF (200 mL). To this was
0.54 mmol, 1 eq.) using pyrrole salt 25 (500 mg, 0.81 mmol, 1.5 added a solution of tosyl chloride (8.14 g, 42 mmol, 1 eq.) in
eq.). The reaction was stirred at room temperature and the THF (50 mL). The solution was stirred at room temperature
crude material was purified by column chromatography (5% overnight. THF was removed under reduced pressure and par-
EtOAc–DCM) to give N-methyl-1-naphthamide 29 (102 mg, titioned between EtOAc and 1 M HCl(aq). The organic phase
96%) as a white crystalline solid m.p. 159–161 °C (lit.:43 was washed with NaHCO3(aq) (1 × 100 mL), brine (1 × 100 mL)
1
159–160 °C); H-NMR (500 MHz, CDCl3) δ 3.08 (3H, d, J = 4.9 and dried over Na2SO4. The crude solution was concentrated
Hz, CH3), 6.05 (1H, bs, NH), 7.42–7.47 (1H, m, ArH), 7.51–7.60 under reduced pressure and recrystallised in DCM–hexane to
(3H, m, ArH), 7.85–7.92 (2H, m, ArH), 8.29–8.31 (1H, m, ArH); afford 1-(toluene-4-sulfonyl)-1H-indole 32 (10.9 g, 94%) as a
13C-NMR (125 MHz, CDCl3) δ 26.8 (CH3), 124.7 (CH), 124.9 white solid m.p. 82–84 °C (lit.:16 83–84 °C); 1H-NMR (500 MHz,
(CH), 125.5 (CH), 126.4 (CH), 127.1 (CH), 128.3 (CH), 130.1 (C), DMSO-d6) δ 2.29 (3H, s, CH3), 6.81–6.83 (1H, m, ArH),
130.5 (CH), 133.7 (C), 134.7 (C), 170.3 (C).
7.22–7.25 (1H, m, ArH), 7.31–7.37 (3H, m, ArH), 7.58–5.60 (1H,
m, ArH), 7.77–7.78 (1H, m, ArH), 7.83–7.85 (2H, m, ArH),
7.91–7.93 (1H, m, ArH); 13C-NMR (125 MHz, DMSO-d6) δ 20.5
(CH3), 108.7 (CH), 113.5 (CH), 120.2 (CH), 123.5 (CH), 125.2
(CH), 126.3 (CH), 130.6 (CH), 131.0 (C), 134.8 (C), 135.1 (C),
145.1 (C).
Preparation of N-phenyl-N-benzyl-4-methylbenzenesulfon-
amide 30
An oven-dried flask containing N-benzylaniline (2.01 g,
11.0 mmol, 1 eq.) and NaH (60% dispersed in mineral oil,
526 mg, 13.2 mmol, 1.2 eq.) under flow of argon, was washed
with dry hexane (3 × 20 mL) prior to addition of dry THF
Preparation of 1H-indole 33
(100 mL). To this was added a solution of tosyl chloride The general procedure for electron transfer reactions was
(2.10 g, 11.0 mmol, 1 eq.) in THF (30 mL). Solution was stirred applied to 1-(toluene-4-sulfonyl)-1H-indole 32 (73 mg,
at room temperature overnight. THF was removed under 0.27 mmol, 1 eq.) using pyrrole salt 25 (500 mg, 0.81 mmol,
reduced pressure and partitioned between EtOAc and 1 M 3 eq.). The reaction was heated to 100 °C and the crude material
HCl(aq). The organic phase was washed with NaHCO3(aq) (1 × was purified by column chromatography (20% EtOAc–Pet.
100 mL), brine (1 × 100 mL) and dried over Na2SO4. The crude ether) to give 1H-indole 33 (27 mg, 87%) as a white solid;
solution was concentrated under reduced pressure and recrys- m.p. 52–53 °C (lit.16 51–54 °C); 1H-NMR (500 MHz, CDCl3)
tallised in DCM–hexane to afford N-phenyl-N-benzyl-4-methyl- δ 6.64–6.65 (1H, m, ArH), 7.19–7.31 (3H, m, ArH), 7.41–7.43 (1H,
benzenesulfonamide 30 (3.38 g, 91%) as a white solid; m. m, ArH), 7.74–7.76 (1H, m, ArH), 8.01 (1H, bs, NH); 13C-NMR
1
p. 138–140 °C (lit.16 139–140 °C); H-NMR (500 MHz, CDCl3) δ (125 MHz, CDCl3) δ 102.6 (CH), 111.3 (CH), 119.9 (CH), 120.8
2.45 (3H, s, CH3), 4.73 (2H, s, CH2), 6.98–7.00 (2H, m, ArH), (CH), 122.1 (CH), 124.3 (CH), 127.9 (C), 135.8 (C).
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Organic & Biomolecular Chemistry
Preparation of N,N′,N′-tris(p-toluenesulfonyl)tryptamine 34
Notes and references
To a solution of tryptamine (1.00 g, 6.24 mmol, 1 eq.) in dry
THF (100 mL) was added NaH (60% mineral oil, 1.25 g,
31.2 mmol, 5 eq.) and light brown suspension was stirred for
10 min. To this was added a solution of tosyl chloride (4.76 g,
25.0 mmol, 4 eq.) in dry THF (50 mL) via cannula and suspen-
sion was stirred for 18 h under argon. Reaction was concen-
trated under reduced pressure and partitioned between EtOAc
(100 mL) and 2 M NaOH(aq) (50 mL). Organic layer was washed
with brine (50 mL) and dried over Na2SO4. The crude solution
was concentrated under reduced pressure and purified by
column chromatography (10% EtOAc–hexane) to give N,N′,N′-
tris(toluenesulfonyl)tryptamine 34 (1.53 g, 37%) as a pink foam;
[Found: (NSI)+ (M + NH4)+ 640.1600, C31H34N3O6S3 (M + NH4)+
requires 640.1604]; νmax(KBr)/cm−1 3387, 2941, 2074, 1635,
1566, 1526, 1448, 1390, 1310, 1252, 1206, 1163, 1118, 1050,
1 A. J. Birch, Q. Rev. Chem. Soc., 1950, 4, 69–93.
2 K. Rühlmann, Synthesis, 1971, 236–253.
3 K. T. Finley, Chem. Rev., 1964, 64, 573–589.
4 D. V. Yandulov and R. R. Schrock, Science, 2003, 301, 76–
78.
5 N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877–
910.
6 J. L. Segura and N. Martin, Angew. Chem., Int. Ed., 2001, 40,
1372–1409.
7 C. Burkholder, W. R. Dolbier and M. Medebielle, J. Org.
Chem., 1998, 63, 5385–5394.
8 T. P. Vaid, J. Am. Chem. Soc., 2011, 133, 15838–15841.
9 D. Emeljanenko, A. Peters, V. Vitske, E. Kaifer and
H.-J. Himmel, Eur. J. Inorg. Chem., 2010, 4783–4789.
10 J. R. Ames, M. A. Houghtaling, D. L. Terrian and
T. P. Mitchell, Can. J. Chem., 1997, 75, 28.
11 W. W. Porter, T. P. Vaid and A. L. Rheingold, J. Am. Chem.
Soc., 2005, 127, 16559–16566.
12 J. Murphy, S. Z. Zhou, D. Thomson, F. Schoenebeck,
M. Mahesh, S. Park, T. Tuttle and L. Berlouis, Angew.
Chem., Int. Ed., 2007, 46, 5178–5183.
1
924, 811, 624; H-NMR (500 MHz, CDCl3) δ 2.36 (3H, s, CH3),
2.47 (6H, s, CH3), 3.09–3.13 (2H, m, CH2), 3.88–3.92 (2H, m,
NCH2), 7.23–7.35 (9H, m, ArH), 7.60 (1H, d, J = 8.2 Hz, ArH),
7.77 (4H, d, J = 8.4 Hz, ArH), 7.91 (2H, d, J = 8.4 Hz, ArH), 7.98
(1H, d, J = 8.2 Hz, ArH); 13C-NMR (125 MHz, CDCl3) δ 21.1
(CH3), 21.2 (CH3), 26.1 (CH2), 48.6 (NCH2), 113.7 (CH), 118.2
(C), 119.5 (CH), 123.3 (CH), 123.8 (CH), 124.9 (CH), 126.8 (CH),
128.2 (CH), 129.8 (CH), 129.9 (CH), 130.4 (C), 135.1 (C), 135.4
(C), 136.9 (C), 144.9 (C), 145.1 (C). m/z (ESI+) 640 [(M + NH4)+,
100%].
13 Z. Han, T. P. Vaid and A. L. Rheingold, J. Org. Chem., 2008,
73, 445–450.
14 J. A. Murphy, J. Garnier, S. R. Park, F. Schoenebeck,
S. Z. Zhou and A. T. Turner, Org. Lett., 2008, 10, 1227–1230.
15 J. A. Murphy, T. A. Khan, S. Z. Zhou, D. W. Thomson and
M. Mahesh, Angew. Chem., Int. Ed., 2005, 44, 1356–1360.
16 F. Schoenebeck, J. A. Murphy, S. Z. Zhou, Y. Uenoyama,
Y. Miclo and T. Tuttle, J. Am. Chem. Soc., 2007, 129, 13368–
13369.
Preparation of 3-(2-(p-toluenesulfonylamido)ethyl)indole 35
The general procedure for electron transfer reactions was
applied to N,N,N-tris(toluenesulfonyl)tryptamine 34 (200 mg,
0.30 mmol, 1 eq.) using pyrrole salt 25 (748 mg, 1.20 mmol, 4
eq.). The reaction was heated to 100 °C and the crude material
was purified by column chromatography (30% EtOAc–hexane)
to give 3-(2-(p-toluenesulfonylamido)ethyl)indole 35 (89 mg,
17 S. P. Y. Cutulic, J. A. Murphy, H. Farwaha, S.-Z. Zhou and
E. Chrystal, Synlett, 2008, 2132–2136.
18 S. P. Y. Cutulic, N. J. Findlay, S.-Z. Zhou, E. J. T. Chrystal
and J. A. Murphy, J. Org. Chem., 2009, 74, 8713–8718.
19 J. Garnier, A. R. Kennedy, L. E. A. Berlouis, A. T. Turner and
J. A. Murphy, Beilstein J. Org. Chem., 2010, 6, 73.
20 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chem-
istry, J. Wiley & Sons, Oxford, 4th edn, 1980.
21 Z. Shi, V. Goulle and R. P. Thummel, Tetrahedron Lett.,
1996, 37, 2357–2360.
22 T. A. Taton and P. Chen, Angew. Chem., Int. Ed. Engl., 1996,
35, 1011–1013.
23 R. W. Alder, M. E. Blake, L. Chaker, J. N. Harvey, F. Paolini
and J. Schutz, Angew. Chem., Int. Ed., 2004, 43, 5896–5911.
24 P. I. Jolly, S. Zhou, D. W. Thomson, J. Garnier,
J. A. Parkinson, T. Tuttle and J. A. Murphy, Chem. Sci.,
2012, 3, 1675–1679.
1
94%) as a clear oil; H-NMR (500 MHz, CDCl3) δ 2.41 (3H, s,
CH3), 2.94 (2H, t, J = 6.4 Hz, CH2), 3.28 (2H, q, J = 6.4 Hz,
CH2), 4.50 (1H, t, J = 6.4 Hz, NH), 6.97 (1H, ArH), 7.06 (1H, t, J
= 8 Hz, ArH), 7.18–7.23 (3H, m, ArH), 7.38 (1H, d, J = 8 Hz,
ArH), 7.43 (1H, d, J = 8 Hz, ArH), 7.64 (2H, d, J = 8 Hz, ArH),
8.09 (1H, bs, ArNH); 13C-NMR (125 MHz, CDCl3) δ 21.5 (CH3),
25.5 (CH2), 43.1 (NCH2), 111.3 (CH), 111.6 (C), 118.5 (CH),
119.5 (CH), 122.3 (CH), 122.4 (CH), 126.9 (C), 127.0 (CH), 129.6
(CH), 136.4 (C), 136.8 (C), 143.3 (C). Data were consistent with
those published in literature.44
Acknowledgements
25 M. J. Corr, K. F. Gibson, A. R. Kennedy and J. A. Murphy,
J. Am. Chem. Soc., 2009, 131, 9174–9175.
We thank Professor John C. Walton and Dr Michael J. Corr for
EPR measurements (St. Andrews University), and we thank 26 M. J. Corr, M. D. Roydhouse, K. F. Gibson, S.-Z. Zhou,
University of Strathclyde, Scottish Funding Council and BAE
Systems for a SPIRIT studentship to HSF. Mass spectrometry
A. R. Kennedy and J. A. Murphy, J. Am. Chem. Soc., 2009,
131, 17980–17985.
data were acquired at the EPSRC UK National Mass Spectro- 27 B. Bolto, R. McNeill and D. Weiss, Aust. J. Chem., 1963, 16,
metry Facility at Swansea University.
1090–1103.
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28 A. Watanabe, S. Murakami, K. Mori and Y. Kashiwaba,
Macromolecules, 1989, 22, 4231–4235.
29 C. L. Gaupp, K. Zong, P. Schottland, B. C. Thompson,
C. A. Thomas and J. R. Reynolds, Macromolecules, 2000, 33,
1132–1133.
30 C. C. Bof Bufon, T. Heinzel, P. Espindola and J. Heinze,
J. Phys. Chem. B., 2009, 114, 714–718.
31 J. W. Ciszek, Z. K. Keane, L. Cheng, M. P. Stewart, L. H. Yu,
D. Natelson and J. M. Tour, J. Am. Chem. Soc., 2006, 128,
3179–3189.
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaus-
sian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009.
32 P. Gros, S. Choppin, J. Mathieu and Y. Fort, J. Org. Chem., 37 Y. Zhao, N. E. Schultz and D. G. Truhlar, J. Chem. Theory
2001, 67, 234–237. Comput., 2006, 2, 364–382.
33 S. V. Kessar, P. Singh, K. N. Singh and M. Dutt, J. Chem. 38 R. A. Kendall, T. H. Dunning and R. J. Harrison, J. Chem.
Soc., Chem. Commun., 1991, 570–571. Phys., 1992, 96, 6796–6806.
34 T. Sammakia and T. B. Hurley, J. Org. Chem., 1999, 64, 39 S. Miertuš, E. Scrocco and J. Tomasi, J. Chem. Phys., 1981,
4652–4664. 55, 117–129.
35 E. Vedejs and X. Chen, J. Am. Chem. Soc., 1996, 118, 1809– 40 K. Uchiyama, Y. Hayashi and K. Narasaka, Tetrahedron,
1810. 1999, 55, 8915–8930.
36 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, 41 S. V. Kessar, P. Singh, K. N. Singh and M. Dutt, J. Chem.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, Soc., Chem. Commun., 1991, 570–571.
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, 42 D. Cuperly, P. Gros and Y. Fort, J. Org. Chem., 2001, 67,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, 238–241.
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, 43 Y. Jo, J. Ju, J. Choe, K. H. Song and S. Lee, J. Org. Chem.,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, 2009, 74, 6358–6361.
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, 44 D. L. Priebbenow, L. C. Henderson, F. M. Pfeffer and
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
S. G. Stewart, J. Org. Chem., 2010, 75, 1787–1790.
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