EPR and pulsed ENDOR study of intermediates from reactions of aromatic azides with group 13 metal trichlorides

Article abstract of DOI:10.3762/bjoc.6.84

The reactions of group 13 metal trichlorides with aromatic azides were examined by CW EPR and pulsed ENDOR spectroscopies. Complex EPR spectra were obtained from reactions of aluminium, gallium and indium trichlorides with phenyl azides containing a variety of substituents. Analysis of the spectra showed that 4-methoxy-, 3-methoxy- and 2-methoxyphenyl azides all gave 'dimer' radical cations [ArNHC₆H₄NH₂]⁺ and trimers [ArNHC₆H₄NHC₆H₄NH ₂]⁺ followed by polymers. 4-Azidobenzonitrile, with its electron-withdrawing substituent, did not react. In general the aromatic azides appeared to react most rapidly with AlCl₃ but this reagent tended to generate much polymer. InCl₃ was the least reactive group 13 halide. DFT computations of the radical cations provided corroborating evidence and suggested that the unpaired electrons were accommodated in extensive π-delocalised orbitals. A mechanism to account for the reductive conversion of aromatic azides to the corresponding anilines and thence to the dimers and trimers is proposed.

Full text of DOI:10.3762/bjoc.6.84

EPR and pulsed ENDOR study of intermediates
from reactions of aromatic azides with
group 13 metal trichlorides
Giorgio Bencivenni1, Riccardo Cesari1, Daniele Nanni1, Hassane El Mkami2
and John C. Walton*3
Full Research Paper
Open Access
Address:
Dipartimento di Chimica Organica “A. Mangini”, Università di
Beilstein J. Org. Chem. 2010, 6, 713–725.
1
doi:10.3762/bjoc.6.84
Bologna, Viale del Risorgimento 4, Bologna I-40136, Italy, 2School of
Physics and Astronomy, University of St. Andrews, St. Andrews, Fife
KY16 9SS, UK and 3School of Chemistry, University of St. Andrews,
EaStChem, St. Andrews, Fife KY16 9ST, UK
Received: 17 May 2010
Accepted: 23 July 2010
Published: 09 August 2010
Email:
Guest Editor: J. Murphy
John C. Walton* - jcw@st-andrews.ac.uk
©
2010 Bencivenni et al; licensee Beilstein-Institut.
*
Corresponding author
License and terms: see end of document.
Keywords:
aluminium; aromatic azides; ENDOR; EPR; gallium; indium
Abstract
The reactions of group 13 metal trichlorides with aromatic azides were examined by CW EPR and pulsed ENDOR spectroscopies.
Complex EPR spectra were obtained from reactions of aluminium, gallium and indium trichlorides with phenyl azides containing a
variety of substituents. Analysis of the spectra showed that 4-methoxy-, 3-methoxy- and 2-methoxyphenyl azides all gave ‘dimer’
radical cations [ArNHC6H4NH2]+• and trimers [ArNHC6H4NHC6H4NH2]+• followed by polymers. 4-Azidobenzonitrile, with its
electron-withdrawing substituent, did not react. In general the aromatic azides appeared to react most rapidly with AlCl3 but this
reagent tended to generate much polymer. InCl3 was the least reactive group 13 halide. DFT computations of the radical cations
provided corroborating evidence and suggested that the unpaired electrons were accommodated in extensive π-delocalised orbitals.
A mechanism to account for the reductive conversion of aromatic azides to the corresponding anilines and thence to the dimers and
trimers is proposed.
Introduction
The number of applications of indium [1-6], gallium [7-11] and N-centred radicals, although most such methods also require
other group 13 metal derivatives, as promoters of radical reac- organotin hydrides [17-23]. In seeking cleaner, less toxic and
tions, has been increasing ever since the original work of Baba more efficient synthetic methodology – not reliant on organotin
and co-workers with dichloroindium hydride [12-16]. Parallel to compounds [24-28] – some of us began investigating the reac-
that, organic azides are increasingly used as sources of tions of organic azides with dichloroindium hydride [29],
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Beilstein J. Org. Chem. 2010, 6, 713–725.
allylindium dichloride [30], and other group 13 metal deriva- deep blue or violet colour usually developed immediately or
tives. These reagents smoothly convert aromatic and aliphatic within a few minutes. In the case of AlCl3 the reactions were
azides into the corresponding amines, γ-azidonitriles into pyrro- very vigorous. Previously, we showed that the main product
lidin-2-imines [29], and δ-azidoesters and chlorides into ally- from the reaction of 4-methoxyphenyl azide 2 with GaCl3 was
lated nitrogen heterocycles [30].
4-amino-4′-methoxydiphenylamine (11b, Variamine blue),
together with traces of anisole, oxidised derivatives (including
To help in elucidating the mechanisms of these reductions, we 4-(4-methoxyphenylamino)phenol, 4-((4-methoxyphenyl)-
used CW EPR spectroscopy and attempted to characterise the imino)-cyclohexa-2,5-dienone) and much dark-coloured
reactive intermediates in selected reactions involving gallium polymer [31]. The EPR spectrum showed the radical cation of
trichloride. Surprisingly, we found that treatment of phenyl Variamine blue (11b+•) plus broad signals which we attributed
azide and 4-methoxyphenyl azide with gallium trichloride to oligomer and/or polymer radical cations (Scheme 2).
resulted in strong EPR spectra of long-lived paramagnetic
species. By combining the results of product analyses with the
results of EPR spectroscopy, we were able to show that persist-
ent radical cations of ‘dimers’ (4-aminodiphenylamines) and
‘
trimers’ (4′-phenylamino-4-aminodiphenylamines) were being
formed [31]. We have now broadened the scope of this investi-
gation to aromatic azides with a range of functionality. We
report here our findings on the behaviour of aromatic azides
when treated with the group 13 trichlorides of gallium, indium
and aluminium.
Results and Discussion
Reaction of 4-methoxyphenyl azide (2) with
Scheme 2: Reaction of 4-substituted-phenyl azides with GaCl3.
group 13 metal chlorides
A set of aromatic azides, each containing an electron-releasing
or an electron-withdrawing substituent in the 4-position, was When anhydrous AlCl3 in DCM – instead of the gallium halide
chosen for this study. The position of the substituent was also – was added to a solution of azide 2, a vigorous reaction took
varied and several other azide types were included (Scheme 1). place. The resulting deep-coloured solution was transferred to
Each organic azide was reacted with the metal halide in the EPR spectrometer and initially the spectrum, Figure 1a, was
dichloromethane/pentane or acetonitrile solution at rt, and an obtained at 300 K. The broad, poorly resolved signal suggested
aliquot (~0.1 mL) was placed in a quartz capillary tube (diam 1 that the mixture was dominated by polymeric material.
mm), purged with nitrogen for 15 min and transferred to the However, when the solution was cooled down to 220 K, the
resonant cavity of an X-band EPR spectrometer. When either well-resolved spectrum, Figure 1b, was obtained. The reso-
AlCl3, or GaCl3 or InCl3 was used, the reaction was accom- lution improvement may be due to the fact that most of the
panied by copious evolution of gas (probably nitrogen) and a polymer separates from the solution at the lower temperature.
A good computer simulation was achieved by utilising the
hyperfine splitting constants (hfs) listed in Table 1. A well-
resolved EPR spectrum of 11b+•, generated from 2 with GaCl3,
is shown in Figure 1c for comparison. Figure 1d shows the
experimental spectrum obtained from treatment of 2 with InCl3,
together with the corresponding computer simulation. Although
the three EPR spectra appear different at first sight, the hfs
derived from the simulations (Table 1) are actually quite
similar. The contrasts in the spectral appearances are mainly the
result of different line widths with consequently different reso-
lutions. It is evident that the main species in each case is the
radical cation 11b+•. The acceptable agreement between the
DFT-computed isotropic hfs of 11b+• (Table 1) and the experi-
Scheme 1: Organic azides studied.
mental data provides additional support for this identification.
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Beilstein J. Org. Chem. 2010, 6, 713–725.
Figure 2: EPR spectrum after treatment of tetra-deuterated azide 3
with AlCl3. Top: 2nd derivative spectrum at 290 K in DCM. Bottom:
computer simulation using the hfs from Table 1.
Reactions of phenyl azide and 4-substituted-
phenyl azides with group 13 metal chlorides
Azides 1, 6 and 7 were chosen to vary the electronic properties
and leaving group abilities of 4-substituents. We showed
previously that treatment of phenyl azide 1 with GaCl3 gave
well-resolved spectra of 4-aminodiphenylamine radical cation
(
11a+•, the dimer) and of the trimer under different reaction
conditions [31]. On treatment with InCl3, 1 gave little sign of
reaction. No colour developed and no EPR spectra were
obtained. However, a vigorous reaction took place between 1
and AlCl3 with nitrogen evolution and development of a deep
blue colour. The EPR spectrum, Figure 3a, was dominated by a
broad component, probably due to polyaniline type material,
together with some fine structure. The second derivative
spectrum at low modulation amplitude discriminated against the
broad signal, and spectrum, Figure 3b, was obtained after
digitally removing the residual broad component.
Figure 1: EPR spectra after treatment of azide 2 with MCl3. (a) AlCl3
in DCM; 1st derivative spectrum at 300 K. (b) AlCl3 in DCM; 2nd
derivative spectrum at 220 K; below: computer simulation with the
parameters listed in Table 1. (c) GaCl3 in DCM/pentane at 300 K with
the computer simulation below. (d) InCl3 in DCM:CH3CN; 4:1 at 320 K;
computer simulation below.
The small differences in the hfs obtained for the different group
1
3 metal chlorides can probably be attributed to the different
counter ions, solvents and temperatures.
The 4-methoxy-tetradeuterio-azide 3 was also treated with
AlCl3 in DCM, and the resulting spectrum and simulation are
shown in Figure 2.
The unpaired electron interacts with two non-equivalent
N-atoms, a single comparatively large H-atom and a pair of
equivalent H-atoms. The spectrum obtained previously on treat-
ment of 3 with GaCl3 was better resolved (Table 1) [31].
However, it is clear that the same ‘dimer’ species was formed
with AlCl3, probably having picked up the NH and NH2
hydrogen atoms from the solvent. The line width of the
spectrum with AlCl3 was ca. 0.7 G. Therefore, it is not
surprising that hfs from aromatic ring D-atoms were not
resolved. Again, differences in the hfs of the spectra from AlCl3
and GaCl3 can be attributed to the different counter ions.
Figure 3: EPR spectra after treatment of azide 1 with AlCl3. (a) 1st
derivative spectrum in DCM at 280 K. (b) 2nd derivative spectrum after
digital removal of residual broad central component. (c) computer
simulation.
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Beilstein J. Org. Chem. 2010, 6, 713–725.
Table 1: EPR parameters of ‘dimers’ [ArNHC6H4NH2]+• from treatment of aryl azides with group 13 metal chlorides.a
Radical
Precursor
MCl3/solvent
cation 1N 1N (N)H
species
(N)H2
2H
2H
2.0
2H
1.0
2H
0.6
Other
PhN3
1H,
1.0
GaCl3/DCM
11a+• 4.9 4.9
1a+• 4.5 4.5
6.8
6.3
5.6
5.0
3.1
2.2
1b
PhN3
1
AlCl3/DCM,
ACN
1
PhN3
1
1H,
−2.3
DFTc
11a+• 4.3 2.2 −8.4
−4.8
3.7
−2.0
1.2
−1.8
1.2
−1.1
1.0
0.4
4
4
4
4
4
4
-MeOC6H4N3
AlCl3/DCM
GaCl3/DCM
11b+• 4.9 4.3
11b+• 5.2 4.4
7.3
7.3
7.8
2
-MeOC6H4N3
3.8
3.1
2.2
0.8
1.8
−0.8
–
2d
-MeOC6H4N3 InCl3/DCM 4,
11b+• 5.2 4.0
3.2
3.2
3.2
2
ACN 1
-MeOC6H4N3
MeO,
1.2
DFTc
11b+• 4.2 2.0 −8.1
−4.4
4.9
−2.0, −1.6 −1.6, −1.4
0.2
–
2
-MeOC6D4N3
AlCl3/DCM
GaCl3/DCM
GaCl3/ACN
DFTc
5.9 4.8
5.2 4.4
6.7
7.1
5.7
–
–
3
-MeOC6D4N3
3.6
0.66, 2D
5.7, 5.7
0.55, 2D
5.2, 5.7
3d
3
-MeOPhN3
17a+• 4.3
–
5.2, 5.2
–
–
4b
3
-MeOPhN3
MeO,
−0.1
17a+• 3.8 1.1 −7.6 −2.3, −2.0 −7.2, −7.3 2.6, −3.4 −2.3, 1.3 −0.9, −0.2
4
2
-MeOC6H4N3
GaCl3/DCM
DFTc
17b+• 4.1 4.1
17b+• 4.0 1.5 −8.1
1c+• 4.4 3.9 4.0
11c+• 4.0 2.5 −7.9
.4 3.4 5.6
5.5
5.5
−3.9
4.0
5
2-MeOC6H4N3
MeO,
0.8
−3.1, −2.3 −1.5, −1.6 0.8, 0.7
−0.3, 0.1
0.5
5
4-FC6H4N3
HGaCl2/ACN,
TESe
1F,
6.6
1
2.0
−1.7
2.0
1.0
6
4-FC6H4N3
DFTc
−5.5
2.8
−1.4
−1.4
0.5
4.3f
6
2-NapN3
InCl3/DCM 4,
ACN 1
3
2.8 (1H)
8
aAll g-factors were 2.0032 ± 0.0005. Assignments of hfs to specific atoms are tentative and are based on the DFT computations. Note that only the
magnitudes and not the signs of hfs can be derived from the EPR spectra.
bTreatment of PhN3 and 3-MeOC6H4N3 with InCl3 gave only very weak and broad unresolved spectra.
cDFT computations: geometries optimised to UB3LYP/6-31+G(d,p) then single point calculations with 6-311++G(d,p) basis.
dData from ref [31].
eHGaCl2 prepared from GaCl3 and Et3SiH (TES). fThe computed a(F) varied from 4.3 G, with the 6-311++G(d,p) basis set, to 9.5 G with the DGDZVP
basis set.
The hfs were similar to those of 11a+• (Table 1, entry for 1 with colour developed and no paramagnetic species could be
GaCl3) except that the smaller hfs were not resolved. Minor detected.
differences in the magnitudes of the hfs can be attributed to the
different counter ions. The trimer radical cation was not Very interesting results were obtained from reactions of
observed, but clearly a contribution from this species could be 1-azido-4-fluorobenzene (6). When 6 was treated with GaCl3 in
hidden under the broad component.
DCM, a deep blue-violet colour developed and the spectrum
was dominated by a broad feature, Figure 4a, although under-
No reaction of 4-azidobenzonitrile 7 with InCl3, GaCl3 or AlCl3 lying fine structure was evident. When dichlorogallium hydride,
was observed and no paramagnetic species were detected by prepared from GaCl3 and Et3SiH in CH3CN, was used to
EPR spectroscopy. It appears the electron-accepting property of promote the reaction, a beautifully resolved spectrum resulted,
the CN group inhibited the coupling process at some stage. It is Figure 4b.
also worth mentioning that, as expected, aliphatic azides such as
ethyl 5-azidopentanoate 9 did not react in the same way either. The good simulation of this spectrum, Figure 4c, enabled the
Treatment of 9 with InCl3 or GaCl3 led to gas evolution but no hfs shown in Table 1 to be determined. Comparison of these hfs
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Beilstein J. Org. Chem. 2010, 6, 713–725.
XC6H4NHC6H4NH2, to the trimer XC6H4NHC6H4NH-
C6H4NH2, thence to oligomers and eventually polyaniline type
polymers X[C6H4NH]nC6H4NH2. Some polymer radical cation
was always observed by EPR spectroscopy, but whether dimer
or trimer or oligomer dominated the spectrum depended on a
delicate balance between solvent, metal halide and other factors.
Reactions of 2-methoxy- and
3-methoxyphenyl azides with group 13 metal
chlorides
Aromatic azides 4 and 5 were chosen to investigate how the
position of the MeO substituent influenced the reaction. Treat-
ment of the 3-methoxy precursor 4 with InCl3 or HInCl2 gave
only very weak and broad EPR spectra. However, reaction of 4
with GaCl3 in CH3CN gave a strong EPR spectrum and the hfs
derived from the computer simulation are presented in Table 1.
The comparatively large line width (~0.9 G) did not permit the
resolution of small hfs from aromatic ring H-atoms. For the
same reason, the hfs from the second N-atom were not resolved.
However, it is clear that this species is probably a ‘dimer’
although the connectivity of the angular structure 17a+• is
somewhat different from that of the 4-aminodiphenylamines
derived from the 4-substituted phenyl azides (Scheme 3).
Figure 4: EPR spectra after GaCl3 and InCl3 reactions of azide 6. (a)
st derivative spectrum from 6 and GaCl3 in DCM at 300 K. (b) 1st
derivative spectrum of dimer (11c+•) from 6 and HGaCl2 in CH3CN at
00 K. (c) computer simulation of (b) with hfs of Table 1. (d) 1st deriva-
1
3
tive spectrum of trimer (19+•) from 6 and InCl3 in DCM at 300 K. (e)
computer simulation of (d) with hfs of Table 2.
with those of the other species in Table 1 supports the identifi-
cation of this intermediate as the corresponding dimer cation
1
1c+•, containing a single F-atom. The DFT computation on the
dimer hfs gave satisfactory agreement (Table 1), with the
possible exception of the para-F hfs. However, DFT-computed
a(F) values varied from 7.2, to 7.1, to 9.5 and 4.3 G with
6
-31G(d), 6-31+G(d,p), DGDZVP and 6-311++G(d,p) basis
sets, respectively. This spread indicates the comparative unreli-
ability of the DFT spin density computations for F-atoms in
these cations.
Scheme 3: Dimer and trimer radical cations.
Treatment of 6 with InCl3 in DCM led to the usual broad signal
from oligomeric and polymeric species superimposed on a Treatment of the 2-methoxy precursor 5 with GaCl3 in DCM, or
spectrum with much narrower lines. On recording the spectrum with HGaCl2 in CH3CN, gave essentially the same strong
with a smaller modulation amplitude, and digitally removing spectrum, see Figure 5a and Supporting Information. The hfs
the residual broad feature, the spectrum shown in Figure 4d derived from the simulations (Table 1) suggest that this is also a
resulted. This is obviously a different species from that of dimer type radical cation 17b+•. However, with the passage of
Figure 4b and, after many trials, a satisfactory simulation was time a central peak began to appear in this spectrum. When 5
obtained, Figure 4e. The derived hfs are presented in Table 2 was treated with HGaCl2, prepared from GaCl3 and Et3SiH, the
and they clearly correspond to a trimer, also probably species with a central peak dominated the spectrum, Figure 5b.
containing a single F-atom, i.e. 19b+•.
Treatment of 5 with InCl3 in DCM or with HInCl2 in THF
prepared from InCl3 and DIBAL-H) also gave rise to a
(
It seems clear that the MCl3 reactions with aromatic azides spectrum of this same species, Figure 5c. A well-resolved
entail a progression from the aniline XC6H4NH2, to the dimer spectrum of this species was obtained by treatment of 5 with
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Beilstein J. Org. Chem. 2010, 6, 713–725.
Table 2: EPR hfs of ‘trimer’ species {[ArNH]2C6H4NH2}+• from treatment of aryl azides with group 13 metal chlorides.a
Trimer
radical
cation
MCl3/solvent or
DFT
Precursor
PhN3
N
N
N(H2)
(N)H2
(N)H (N)H
H-rings
Other
2.1 (3H)
1.0 (3H)
GaCl3/DCM
DFTc
19a+•
19a+•
5.0
5.4
4.9
3.4
3.0
2.0
6.5
4.9
2.1
1b
−
1.6 (4H)
−1.0 (3H)
±0.6 (6H)
.2 (1H)
1.5 (2H) 2.2 (1F)
.5 (6H)
1.3 (4H)
−0.6 (4H) 1.5 (1F)
PhN3
1
−3.0
−7.8 −4.8
<
2
4-FC6H4N3
InCl3/DCM
DFTd
19b+•
19b+•
5.5
3.7
5.5
2.1
2.6
1.5
6.2
7.2
4.5
6
0
−
4-FC6H4N3
−3.2
−7.2 −4.2
6
<0.6 (4H)
2
-MeOC6H4N3
InCl3/DCM
18+•
18+•
4.8
4.5
4.8
4.5
4.8
3.0
5.1
5.1
5.5
5.1
5.5
–
5
2-MeOC6H4N3
4.1
1.7
1.7 (1H)
0.5 (4H)
HGaCl2e/CN
5
−2.0 (1H)
2
-MeOC6H4N3
−2.0
−1.6
−1.7 (2H)
−1.1 (1H) (CH3O)
0.7 (8H)
0.4
DFTc
18+•
4.4
3.9
1.5
−6.6 −5.1
5
<
aAll g-factors 2.0032 ± 0.0005, assignments of hfs to specific atoms are tentative and are based on the DFT computations. Note that only the magni-
tudes and not the signs of hfs can be derived from the EPR spectra. bData from ref [31]. cDFT: UB3LYP/6-31G(d). dDFT computations: UB3LYP/6-
31+G(d,p) then single point calculation with 6-311++G(d,p) basis. eHGaCl2 prepared from GaCl3 and Et3SiH (TES).
HGaCl2 prepared with Et3SiH in CH3CN, Figure 5d. The hfs
derived from the computer simulation, Figure 5e and Table 2
show the presence of three N-atoms and of four H-atoms with
sizeable hfs that can probably be attributed to NH or NH2
groups. Thus, this species is almost certainly a ‘trimer’ although
this will necessarily have an angular structure 18+• rather than
the linear type structure of the trimers from 4-subsitituted azides
such as 19+• (Scheme 3).
The results from azides 4 and 5 showed that the position of the
MeO substituent in the phenyl azides was not critical. The reac-
tions with gallium and indium promoters proceeded along
similar lines to that of phenyl and 4-substituted phenyl azides to
give dimers, trimers and polymers.
The spectra obtained on treatment of 2-azidonaphthalene 8 with
InCl3, GaCl3 and AlCl3 are shown in Figure 6a, Figure 6b and
Figure 6c, respectively.
The broad signal in Figure 6c shows that polymerisation domin-
ated the reaction with AlCl3. Similarly, the main broad feature
in Figure 6b suggests that polymerisation was again dominant
in the reaction with GaCl3. The comparatively well-resolved
species observed in the InCl3-promoted reaction, Figure 6a, was
well simulated on using the parameters shown in Table 1. The
data show that the unpaired electron interacted with two
Figure 5: EPR spectra after GaCl3- and InCl3-promoted reactions of
-methoxyphenyl azide 5. (a) 1st derivative spectrum of 17b+• from 5
with GaCl3 in DCM. (b) 1st derivative spectrum of 17b+• from 5 with
HInCl2 in THF at 300 K. (c) 1st derivative spectrum of 18+• from 5 with
InCl3 in DCM at 300 K. (d) 1st derivative spectrum of 18+• from 5 with
HGaCl2 in CH3CN at 300 K. (e) computer simulation of (d).
2
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Beilstein J. Org. Chem. 2010, 6, 713–725.
An additional complication arises if the nuclear spin is >½,
which adds another term describing the nuclear quadrupole
interaction in the above equations [33]. In frozen solution all
orientations of the paramagnetic species are observed and there-
fore an anisotropic ENDOR spectrum is expected. The latter is
more complex and requires a detailed understanding of the
anisotropy of the system. The above equations are not suitable
for such a situation and a more complete resonance condition
that considers all the orientations is needed. In the case of a
system with I = ½ the parameter aiso in Equation 1 and Equa-
tion 2 is replaced by Ai (i.e. one of the principal components of
the hyperfine tensor).
The Davies ENDOR spectrum from the species derived from
the 4-fluoroazide 6 sample at 50 K is shown in Figure 7. The
inset shows the ESE-EPR spectrum, with an arrow indicating
the magnetic field position at which the ENDOR experiment
was performed. The ENDOR spectrum shows powder pattern
lineshapes, as expected for frozen solutions, due to the
anisotropic hyperfine interactions. Two main features cover the
Figure 6: EPR spectra after In-, Ga- and Al-promoted reactions of
azide 8. (a) intermediate from InCl3 treatment of 8 at 260 K in DCM
and CH3CN (4:1). (b) spectrum from GaCl3 treatment of 8 at 300 K in
DCM/pentane. (c) spectrum from AlCl3 treatment of 8 at 300 K in
DCM.
N-atoms, two (N)H2 atoms, one (N)H-atom and one other whole spectrum; a powder pattern centred about the 1H Larmor
H-atom; other splittings were not resolved. The magnitudes of frequency and a second broad signal located at lower frequency
the hfs are somewhat smaller than those of analogous atoms in and spread over 8 MHz width.
the dimer from 1. This is exactly as would be expected from the
greater extent of aromatic delocalisation in a dimer from 8.
Clearly, however, more than one isomer is possible.
Pulse ENDOR spectrum of the intermediate
from 4-fluorophenyl azide 6
Pulsed ENDOR experiments, based on the ESE effect, were
carried out on the frozen solution from azide 6 at 50 K. The
echo signal was created by the microwave pulse sequence, and
an rf pulse was applied during the ‘mixing period’, which
corresponded to the time T in the Davies ENDOR sequence
[
32]. The rf pulse drove the nuclear spin transitions, which led
to a change in the ESE intensity. The ENDOR signal was there-
fore measured by monitoring the ESE intensity while the rf
frequency was varied. In the case of an S = ½ system coupled
with a nucleus with nuclear spin I = ½, the Davies ENDOR
spectrum consists of two lines at the nuclear resonance fre-
quencies να and νβ, which correspond to the transitions asso-
ciated with the electron spin manifolds Ms = +½ and Ms= −½,
respectively. If the Larmor frequency (νn) of the nucleus in
question is larger than the hyperfine interaction, then the reso-
nance frequencies are given by:
Figure 7: Experimental and simulated Davies ENDOR spectrum after
the Ga-promoted reaction of azide 6 recorded at 50 K. The inset
shows the field-swept EPR spectrum with an arrow indicating the
magnetic field position of the ENDOR experiment.
The lack of resolution encountered in the ENDOR spectrum
makes an unequivocal analysis difficult. Therefore, our
ENDOR simulation was based mainly on the CW EPR results.
A simulated spectrum is displayed in Figure 7, and a deconvo-
luted version is in the Supporting Information together with one
chosen set of ENDOR hyperfine tensor parameters. Almost all
the 1H hyperfine splittings fit well within the ENDOR spectra,
(1)
If νn is less than ½aiso, the frequencies are then given by:
(2)
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Beilstein J. Org. Chem. 2010, 6, 713–725.
but it is worth noting, as mentioned above, that a satisfactory character. The CNC angles in 17a+• and 17b+• were 128.9 and
simulation was only achieved by assuming an anisotropic line- 131.6°, showing significant widening from trigonal. The
shape of the hyperfine couplings. Extra weak hyperfine aromatic rings in all the structures were twisted significantly out
couplings, unresolved in the CW EPR, were also included in the of co-planarity. As might be expected on steric grounds, this
simulation. These probably correspond to hyperfine coupling in increased in the dimers as the substitution site moved from the
polymer which was undoubtedly present. The broad feature at 4- to the 3- to 2-position. For example, the computed dihedral
low frequency is related to a mixture of fluorine (19F) and angles between the rings increased from 26.8 to 30.7 to 39.0 in
nitrogen (14N) contributions. The anisotropy and the unre- the series 4-MeO-11b+•, 3-MeO-17a+• and 2-MeO-17b+•,
solved nuclear quadrupole of the nitrogen couplings make the respectively. It seems that a compromise was reached in which
spectra difficult to interpret. Each 1H contributes three sets of the repulsive steric interaction between substituents of neigh-
peaks to the spectrum times the number of 1H’s present. This bouring rings was balanced against the stabilising effect from
represents an enormous number of lines in one spectrum. Obvi- conjugation of the π-systems. The SOMOs depicted in Figure 8
ously, they cannot all be assigned from this broad unresolved show that there was still sufficient orbital overlap in the linear
powder pattern. Almost axial tensors were assumed (see and angular dimer and trimer radical cations to support lengthy
Supporting Information). However, it should be noted that it π-systems extending over all the rings and N-atoms. This is in
may well be possible to simulate these spectra with other para- accordance with the EPR spectroscopic data, that show exten-
meter sets. The experimental Davies ENDOR data support the sive delocalisation of the unpaired electron in dimer and trimer
CW EPR data in confirming the magnitudes of the hyperfine radical cations. The computed hfs in Table 1 and Table 2 show
couplings and the nitrogen interactions. Further pulse tech- reasonable correspondence with the experimentally observed
niques such as electron spin echo envelope modulation values.
(
ESEEM) and its multidimensional extension Hyscore would be
required to get more insight into the nitrogen contribution. Conclusion
Regarding the 19F contribution; only the low-frequency part of Literature reports show that anilines can easily be oxidised
the 19F coupling fits well with the experimental data. The 19F to the corresponding resonance-stabilised radical cations,
high-frequency line in our simulation is not consistent with the which can couple with more aniline to afford very persistent
experimental spectrum, which suggests that the latter might be radical cation dimers [37,38]. The generation of these
highly asymmetric. Such situations have been previously radical cations depends critically on the reaction conditions, in
reported in other studies where it was shown that this could be particular on the degree of protonation, which can facilitate
related to the relaxation time. Sometimes relaxation processes electron transfer (ET) [39,40]. It has also been reported that
can lead to a partial saturation in the nuclear transitions such electrochemical oxidation of aromatic amines can generate the
that the observed signal is the result of a transition in one mani- same radical cations which can polymerise giving oligo- and
fold only [34]. Partial saturation may explain the absence of the poly-anilines [41]. In view of the fact that product analyses [31]
1
9F high-frequency line in our spectrum.
identified aniline amongst the products from 1 and anisole
amongst the products from 2, it seems probable that the
DFT computations of radical cation properties aromatic amines are the precursors of the dimer and trimer
Quantum chemical calculations were carried out with the species.
Gaussian 03 programme package [35,36]. Density functional
theory with the UB3LYP functional was employed. The equi- A possible mechanism for production of anilines from the
librium geometries were fully optimised with respect to all aromatic azides is set out in Scheme 4. Coordination of the
geometric variables, no symmetry being assumed either with metal halide to the starting azide should produce the Lewis
the 6-31+G(d,p) basis set (dimers) or with the 6-31G(d) basis base–acid adduct 12 that could undergo reduction by ET from
set (trimers). Isotropic EPR hfs were derived from computed more azide to afford, after nitrogen loss, the metal-coordinated
Fermi contact integrals evaluated at the H- and N-nuclei. The aminyl radical 13 together with the ArN3+• radical cation.
hfs were taken directly from the Gaussian output files and are Aminyl radical 13 could then abstract an H-atom from solvent
shown in Table 1 and Table 2.
RH (or from HMCl2 when the metal hydrides were used) with
the production of metal-coordinated amine 14. The latter can
The optimum structures of the radical cations 17a+• and 17b+• then pick up a proton to produce an aromatic amine and
their associated SOMOs are shown in Figure 8.
regenerate the metal halide. The reason 4-azidobenzonitrile 7
did not react with any of the group 13 metal chlorides may well
The C–NH bond lengths in radical cation 17a+• (1.40 and 1.38 be that the ET step 12 → 13 was inhibited by the presence of
Å) and 17b+• (1.38, 1.39 Å) indicated significant double bond the electron acceptor CN group.
720

Beilstein J. Org. Chem. 2010, 6, 713–725.
Figure 8: DFT structures and SOMOs for dimer and trimer radical cations.
Several mechanisms have been proposed in the literature for the and trimer radical cations we observed is shown in Scheme 5
formation of ‘dimers’ from anilines. These include [42-44]: (i) for the case of 2-methoxyaniline.
initial formation of the radical cation ArNH2+• which then
couples with more aniline and forms the 4-aminodiaryl amine Ipso attack by radical 13a on the aniline would lead to the
radical cation after loss of HX and (ii) formation of the aniline production of delocalised radical 15. Elimination of MeOMCl3−
radical ArNH•, which couples with ArNH3+, ArNH2 or would then yield radical 16, which, on protonation, would
ArNH2+•. A plausible mechanism for formation of the dimer afford the observed long-lived dimer radical cations 17+•. Of
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Beilstein J. Org. Chem. 2010, 6, 713–725.
tion was observed with PhN3 and very little reaction occurred
with the 3-methoxyazide 4. GaCl3 and HGaCl2 were the best
promoters. The dimers were the main products from reactions
of excess 1 and 2 with GaCl3 such that the process could have
synthetic potential. In general, the GaCl3- and HGaCl2-
promoted reactions were also best for spectroscopic studies
because they gave the most intense and well-resolved spectra of
[
ArNHArNH2]+• and/or {[ArNH]2ArNH2}+• with the narrowest
line widths.
Experimental
Scheme 4: Possible mechanism of formation of aromatic amines.
EPR and ENDOR spectroscopy. EPR spectra were obtained
with a Bruker EMX X-Band 10/12 spectrometer fitted with a
course, proton transfer could occur earlier in the reaction, such rectangular ER4122 SP resonant cavity and operating at 9.4
that coupling takes place with the anilinium cation instead. GHz with 100 kHz modulation. An aliquot (~0.1 mL) of the
Trimer 18+• could be produced by coupling of 17+• with more reaction mixture from each aromatic azide and the metal chlo-
1
3a followed by a similar sequence of steps. The trimer could ride in CH2Cl2/pentane or CH3CN solution was placed in a 1
then grow into oligomer and polymer by a succession of such mm o.d. quartz capillary tube, de-aerated by bubbling nitrogen
coupling reactions.
for 20 min and transferred to the resonant cavity. Spectra were
examined at several temperatures but generally best resolution
In general, the aromatic azides appeared to react most rapidly and signal intensity were obtained at around 300 K. Most of the
with AlCl3 but this reagent tended to generate much polymer. EPR spectra were recorded with 2.0 mW power, 1.0–0.2 Gpp
InCl3 was the least reactive group 13 halide such that no reac- modulation intensity and a gain of ~106. In all cases where
spectra were obtained, hfs were assigned with the aid of
computer simulations using the Bruker SimFonia and NIEHS
Winsim2002 software packages.
Pulsed EPR and ENDOR were performed using a pulsed EPR
X-band spectrometer (Bruker Elexsys E580) equipped with a
Dice-ENDOR accessory, a radio frequency (rf) amplifier and a
dielectric-ring ENDOR resonator (Bruker EN4118X-MD-4-
W1). Samples were maintained at 50 K using liquid helium in
an Oxford CF-935 cryostat. Field-swept electron spin echo
(
ESE) spectra were recorded using a two-pulse ESE sequence
while ESE-ENDOR experiments were carried out using Davies
three-pulse sequence π-T-π/2-τ-π-echo with a selective rf pulse
of variable frequency applied during time T. The pulse lengths
used were 128 and 256 ns for π/2 and π respectively, and 10 μs
for the π-rf pulse. ENDOR data were processed and simulated
using the EasySpin package (freeware from http://
www.easyspin.org/).
DFT calculations. All computations were done with the
Gaussian 03W programme package (Version 6.1.0.0) [35].
Geometries were optimised at the UB3LYP/6-31+G(d,p) level
[
45] (dimers) and the UB3LYP/6-31G(d) level (trimers) and
single point calculations at these geometries with a triple zeta
quality basis set (6-311++G(d,p)) were used to predict isotropic
EPR hfs. The DGTZVP basis set, similar to that recommended
by Schäfer et al. [46], was also employed for some compu-
tations.
Scheme 5: Possible mechanism for dimer and trimer formation.
722

Beilstein J. Org. Chem. 2010, 6, 713–725.
General procedure for the reaction of aryl azides with different temperatures. Products analysis was performed as
indium trichloride. The starting azide (1 mmol) was added at above.
0
°C to an acetonitrile solution of indium trichloride (1.1 mmol)
in DCM (4 mL) and stirred for 5 min at 0 °C. Gas was Ethyl 5-azidopentanoate (9) [48] was prepared by treatment of
evolved and the solutions took on a dark blue or violet colour. the corresponding alkyl bromide with sodium azide in
The resulting solutions were rapidly transferred into a DMSO [49]; IR (νmax, CHCl3), 1718 (CO) and 2092 (N3) cm−1;
quartz capillary tube and purged with nitrogen for few minutes. 1H NMR (400 MHz) δ 1.24 (t, J = 7.2 Hz, 3H), 1.54–1.77 (m,
The tube was sealed and placed in the EPR resonant 4H), 2.32 (t, J = 6.9 Hz, 2H), 3.28 (t, J = 6.6 Hz, 2H), 4.13 (q, J
cavity. Spectra were recorded at several different temperatures. = 7.2 Hz, 2H).
Some samples were photolysed with a 500 W super pressure
Hg arc.
Aromatic azides 1–8 were prepared by standard diazotisation of
the corresponding anilines followed by treatment with sodium
General procedure for the reaction of aryl azides with azide, and were identified by comparison with literature data:
dichloroindium hydride. The starting azide (1 mmol) was phenyl azide (1) [50], 1-azido-4-methoxybenzene (2) [48],
added at 0 °C to an acetonitrile solution of dichloroindium 1-azido-3-methoxybenzene (4) [51], 1-azido-2-methoxy-
hydride (1.1 mmol), generated in situ by stirring under an argon benzene (5) [48], 1-azido-4-fluorobenzene (6) [48], 4-azido-
atmosphere anhydrous indium trichloride (243 mg, 1.1 mmol, benzonitrile (7) [52] and 2-azidonaphthalene (8) [53].
previously dried by heating at 130 °C under argon for 1 h) and 2,3,5,6-Tetradeuterio-4-methoxyphenyl azide (3) was prepared
triethylsilane (177 μL, 1.1 mmol) in ACN (4 mL) for 5 min at by diazotisation of 2,3,5,6-tetradeuterio-4-methoxyaniline,
0
°C [47]. The resulting solution was rapidly transferred into a derived in turn from the reaction of 3,5-dideuterio-4-
quartz capillary tube and nitrogen was bubbled inside for few methoxyaniline hydrochloride with boiling D2O for 4 days in a
minutes. The tube was sealed and placed in the EPR sealed tube [31].
cavity. Spectra were recorded at several different temperatures.
Some samples were photolysed with a 500 W super pressure
Supporting Information
Hg arc. Selected samples were given an aqueous work-up with
NaHCO3 followed by extraction with diethylether. In each case
Supporting information features general procedures, EPR
the corresponding aromatic amine was identified by compari-
spectra from azides 4 and 5, deconvolution of ENDOR
son with literature data.
spectrum from azide 6, Cartesian coordinates for
DFT-computed structures of dimer and trimer radical
General procedure for the reaction of aryl azides with
cations.
AlCl3. Aluminium trichloride (1.1 mmol) was dried under
reduced pressure at 25 °C for 1 h. Then DCM (3 mL) was added
and a DCM solution of the azide (1 mmol in 1 mL) was intro-
duced at rt. Gas was evolved, sometimes violently, and dark
blue or violet colours developed. The resulting solution was
then transferred to a capillary quartz tube and purged with
nitrogen. The capillary was sealed and several EPR spectra
were run at different temperatures. Product analysis was
performed by quenching the reaction with an aqueous solution
Supporting Information File 1
EPR and pulsed ENDOR study of intermediates
from reactions of aromatic azides with
group 13 metal trichlorides
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-84-S1.pdf]
of NaOH and extracting with DCM. The mixtures were Acknowledgements
analysed by GC–MS and, when possible, by 1H NMR and 13C We thank EaStChem and the EPSRC (UK Basic Technology
NMR spectroscopy.
Programme grant GR/S85726/01) for financial assistance. We
also acknowledge financial support from MIUR, Italy (2008
General procedure for the reaction of aryl azides with PRIN funds for ‘Properties and reactivity of free radicals in
GaCl3. A pentane solution of gallium trichloride (0.55 mL of complex environments and their role in oxidative processes and
0
.5 M; 0.28 mmol) was added under a nitrogen atmosphere to a in organic synthesis’).
DCM solution of the azide (0.25 mmol in 4 mL) at rt. Gas was
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