First isolation of a Wheland intermediate in the azo-coupling reaction, its X-ray crystal structure determination and products from its evolution

Article abstract of DOI:10.3998/ark.5550190.p008.311

The reaction between tris(N-piperidinyl)benzene and 4- methoxybenzenediazonium tetrafluoroborate afforded a Wheland intermediate (W) that was isolated and crystallized at low temperature. This permitted to obtain, for the first time, the X-ray structure analysis of a σ- complex in the diazo coupling reaction. In solution, W slowly evolves to the rearomatized product PH⁺ (as salt). The single crystal X-ray diffraction determination of the neutral azo compound P is also reported. ARKAT-USA, Inc.

Full text of DOI:10.3998/ark.5550190.p008.311

General Papers
ARKIVOC 2014 (iv) 51-66
First isolation of a Wheland intermediate in the azo-coupling
reaction, its X-ray crystal structure determination and products
from its evolution
Carla Boga,a* Erminia Del Vecchio,a Silvia Tozzi,a Luciano Forlani,a Magda Monari,b
Gabriele Micheletti a and Nicola Zanna a
a. Department of Industrial Chemistry ‘Toso Montanari’, Alma Mater Studiorum – Università di
Bologna, Viale del Risorgimento, 4 - 40136 Bologna, Italy
b. Department of Chemistry ‘G. Ciamician’, Alma Mater Studiorum – Università di Bologna, Via
Selmi, 2 - 40126 Bologna, Italy
E-mail: carla.boga@unibo.it
DOI: http://dx.doi.org/10.3998/ark.5550190.p008.311
Abstract
The reaction between tris(N-piperidinyl)benzene and 4-methoxybenzenediazonium
tetrafluoroborate afforded a Wheland intermediate (W) that was isolated and crystallized at low
temperature. This permitted to obtain, for the first time, the X-ray structure analysis of a σ-
complex in the diazo coupling reaction. In solution, W slowly evolves to the rearomatized
product PH+ (as salt). The single crystal X-ray diffraction determination of the neutral azo
compound P is also reported.
Keywords: Aromatic substitution, Wheland intermediate, X-ray crystal structure analysis, azo-
coupling, triaminobenzenes, diazonium salts
Introduction
The protonation of the benzene ring1–4 (unsubstituted or holding several electron-donating
groups) has, in recent times, allowed to explore the structure of possible complexes (both, π and
σ) on the pathway of the electrophilic aromatic substitution reaction (SEAr). However, these σ-
complexes are usually very short-lived intermediates and, even if the first observations of
persistent arenium ions in strong acids date back about half a century ago,5–8 the simplest among
+
them, the benzenium ion (C6H7 ), remained an elusive specie until 1971, when it was first
characterized at dry-ice temperature.9 Later, its characterization by X-ray diffraction analysis
used carborane superacid anion.10 In general, good candidates for the observation and isolation
of σ-cationic complexes are arenes bearing electron-donor substituents such as methylated
benzenes (e.g. hexamethylbenzene)11 or triaminobenzene derivatives, such as compounds 1–3
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showed in Scheme 1, first used by Effenberger.12 These latter form σ cationic intermediates
(Wheland intermediates)13 by protonation14–20 and tris(N-pyrrolidinyl)benzene (3) gave
moderately stable intermediates also in alkylation with alkyl halides15,21–23 and in halogenation
reactions.24,25 When compounds 1–3 were coupled with a superelectrophilic reagent such as 4,6-
dinitrobenzofuroxan (DNBF) zwitterionic σ-intermediates which are contemporaneously
Wheland and Meisenheimer intermediates (WM in Scheme 1) were detected and characterized
for the first time.26 Later, WM complexes similar to those of Scheme 1 were obtained by using
2-aminothiazole derivatives27,28 and 4,6-dinitrotetrazolopyridine.29
Scheme 1. Reactions between triaminobenzene derivatives 1–3 and DNBF forming WM
complexes.
The difficulties in the isolation of Wheland intermediates (W) in simple electrophilic
aromatic substitution reactions1–4,30 probably arise from the unsuitable experimental conditions
used. Consequently, the idea that proton elimination is a fast step in the pathway of SEAr
reactions has grown. The use of aromatic rings activated by the presence of several strongly
electron-donating substituents, such as compounds 1–3 allowed us to obtain important
information on the steps of the electrophilic aromatic substitution reaction.31 Actually, using 1
and 2 in azo-coupling reactions, we were able to obtain the first W intermediates in this kind of
reaction.32 Their stability allowed us to carry out a kinetic investigation33 (by UV/Vis
spectroscopy) on the hydrogen elimination from the W intermediate to obtain the final products
by rearomatization. In this study we found that the spontaneous proton release is a slow step, in
spite of the presence of several amino nitrogen atoms on the molecule which may exert an
internal base catalysis.
Generally, in SEAr reactions the catalysis to proton abstraction cannot be observed because
the usual experimental conditions involve the presence of large amount of bases (as proton
acceptors, in neutral or anionic form) able to perform base catalysis on proton elimination from
the W complex. Contrary to the usual idea, reported in textbooks, that the driving force of the
overall reaction is the energy gain in the re-aromatization step, we found that the proton
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abstraction is a process showing a relevant base catalysis.17,33 Our findings support the relative
importance of the two main steps in which the attack of the electrophile is a fast step, while the
proton abstraction is a slow step or a catalyzed process, as shown in Scheme 2, in which the
formation of π complexes with H+ has been omitted. Further studies gave experimental evidence
that the whole reaction is a reversible process in both steps.34
Scheme 2. General proposed pathway for SEAr reactions.
Although there are a few literature examples of characterization of Wheland intermediates
1
mainly by H NMR and UV/Vis spectroscopy, the transient nature of these species has allowed
for only a few examples of characterization by single crystal X-ray structural analyses.14,35,36
This approach remains a challenge for chemists.
The X-ray analysis of a W intermediate from azo-coupling reactions has never been reported
so far. The slow, catalyzed last step demonstrated in earlier research33 led us to believe that it
should be feasible to isolate and crystallize this intermediate in order to gain information on its
structure in the solid state and also to compare the data with those of the final neutral substitution
product.
Results and Discussion
Since in the azo-coupling reaction between compounds 1 and 2 and a series of p-substituted
benzenediazonium salts the presence of an electron-donating substituent on the benzene ring of
the latter causes the deceleration of all the processes32 including the back reaction34 and the
proton abstraction, we selected 4-methoxybenzenediazonium tetrafluoroborate as a suitable
candidate to obtain single crystals of W.
The reaction (Scheme 3) shows NMR and UV/Vis spectroscopic evidence of the formation
of the Wheland intermediate W that evolves toward the salt PH+ that, in turn, can be converted
into the final neutral substitution compound P.
The moderate stability of W in common solvents suggested we should search for the optimal
medium suitable to grow crystals of X-ray diffraction quality. After several attempts, some
crystals of solid W were obtained by precipitation from a mixture of CH2Cl2/n-hexane at low
temperature (about –60 °C). The X-ray structure determination was carried out twice on two
different crystals at different temperatures. The first data collection was carried out at 223 °K,
whereas the second one at 100 °K. In both cases the nature of W was the same but the lower
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temperature data set allowed to understand the disorder involving the N1, N2 atoms and the
methoxyphenyl ring (see later) and therefore this discussion will comment the structural features
of W based on the X-ray structure determination at 100 °K.
Scheme 3. Wheland intermediate and products of the reaction between tris-(1-
piperidinyl)benzene and 4-methoxybenzenediazonium tetrafluoroborate.
The molecular structure of W (cation only) in the crystal is shown in Figure 1.
Figure 1. ORTEP drawing of W (cation only). Broken lines indicate the minor conformer arising
by the ‘pedal motion’.37,38
In Table 1 selected atomic distances of W and of the final product P (obtained from
compound PH+ by column chromatography on basic aluminum oxide)32 are reported.
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Table 1. Selected bond lengths (Å) for W and P
W
P
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C1-C6
C1-N1
N1-N2
N2-C7a
C2-N3
C4-N4
C6-N5
1.470(4)
1.372(4)
1.422(4)
1.435(4)
1.360(4)
1.461(4)
1.324(5)
1.326(6)
1.415(6)
1.362(4)
1.343(4)
1.380(4)
1.423(2)
1.383(2)
1.401(2)
1.395(2)
1.386(2)
1.418(2)
1.402(2)
1.262(2)
1.425(2)
1.407(2)
1.389(2)
1.414(2)
a Referred to the main image of the disordered 4-methoxyphenyl group.
The cation of W consists of a tris-(1-piperidinyl)substituted ring bearing a 4-
methoxyphenylazo group. The steric crowding of the substituents is alleviated by the orientation
of the two piperidinyl groups in ortho position to the 4-methoxyphenylazo substituent that lie on
the opposite side of the ring with respect to the azo group that, in turn, is in syn position with
respect to the para-piperidinyl substituent. The triaminosubstituted ring shows a significant loss
of planarity [dihedral angle between the C2-C3-C4-C5-C6 and C2-C1-C6 planes 23.3(2)°] and
the C1 carbon of W is 0.31 Å above the five-membered C2-C3-C4-C5-C6 plane. The C-C bond
distances in the ring fall in the range 1.360-1.470(4) Å. In particular the C2-C3 and C5-C6 bonds
are the shortest ones [1.372 and 1.360(4) Å, respectively], the C3-C4 and C4-C5 bonds are
longer [1.422 and 1.425(4) Å, respectively] and the C1-C2 and C1-C6 bonds are the longest ones
[1.470 and 1.461(4) Å, respectively]. This bond length distribution evidences the loss of
aromaticity and highlights the importance of a quinoid structure like 4 in delocalizing the
positive charge (Figure 6) and is similar to that found in the benzenium cation and in other
arenium ions.10 In fact the analogous triaminosubstituted ring in the neutral compound P (Figure
S2 in SI) is different since it is planar (maximum deviation from planarity 0.020(1) Å) with bond
lengths falling in the range 1.383-1.418(2) Å typical for aromatic rings.
As far as the piperidinyl substituents in W are concerned, the N-C(phenyl) distances C2–N3,
C4–N4 and C6–N5 [1.362, 1.343 and 1. 380(4) Å, respectively] are slightly shorter than the
related N-C(phenyl) bonds in P that are 1.406, 1.389 and 1.414(2) Å, respectively, indicating the
participation of the amino nitrogen atoms in supporting the positive charge of W. A comparison
of the N-C(phenyl) distances, as well as the lengthening of the C1–C2 and C1–C6 bonds in W
with respect to the corresponding values in the final product P [1.423 and 1.418(2) Å] shows a
good agreement with the data previously reported for the tris(pyrrolidino)cyclohexadienylium
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cation.14 In addition in the recently reported crystal structure of the protonated derivative of
1,3,5-tris(piperidinyl) benzene39 it has been shown that addition of H+ takes place at the N atom
of one piperidinyl ring with lengthening of the N-C(aryl) bond distance from 1.413(2) to
1.476(2) Å without significant variation of the inner benzene ring. Finally, the 4-
methoxyphenylazo group in W was found to be disordered over two sites with 70 and 30%
occupancy factors, respectively. This disorder is a common feature in azobenzenes and stilbenes
known as ‘pedal motion’.37,38 The triamino substituted benzene ring is not affected by this
disorder because of the presence in the two ortho positions of the bulky piperidinyl substituents.
Conversely this type of disorder has not been observed in the 4-methoxyphenylazo group in P
presumably because of the very low population of the minor component. The two substituted
phenyl rings in P do not lie on the same plane [dihedral angles between the C1-C2-C3-C4-C5-C6
and C7-C8-C9-C10-C11-C12 rings 39.46(5)°] and therefore there is no π delocalization through
the azo group. The N1-N2 distance in P of 1.262(2) Å has the typical double bond character
whereas the equivalent N=N distance in W of 1.326(6) Å is longer than a normal double bond.
This lengthening of the N=N bond in W is associated to a shortening of the C1-N1 distance of
1.324(5) Å which is indicative of some double bond character. In fact the N2-C7 bond is
1.415(6) Å and is comparable to the equivalent distances N1-C1 and N2-C7 in the neutral P that
are 1.402 and 1.425(2) Å, respectively. It should be pointed out that the N=N double bond in the
cationic W is affected by dynamic disorder (see experimental section) and therefore it is not
possible to completely rule out the contemporary presence in the crystal in addition to the
Wheland compound of different protonated species that could be present in trace amounts and
not detectable by X-ray diffraction. The crystal packing of W (Figures 3 and 4) show that every
–
BF4 anion is surrounded by three cations and therefore, in addition to the electrostatic
interactions, each fluorine atom is engaged in weak C-H···F interactions. Also the O atom of the
methoxy group is involved in a O···H-C non classical H bonding.
Figure 2. View along the a axis of the crystal packing of W. The black dotted lines indicate the
C-H...F and C-H...O hydrogen bonds.
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Figure 3. View along the c axis of the crystal packing of W. The black dotted lines indicate the
C-H...F and C-H...O hydrogen bonds.
In the crystal packing of the neutral compound P (Figure 4) the molecules are arranged in
dimers arising by two O···H27A-C27(piperidinyl) hydrogen bondings.
Figure 4. Arbitrary view of the crystal packing of P showing a few dimers generated by C-H...O
hydrogen bonds (black dotted lines).
We also carried out a theoretical calculation in order to compare the experimental structures
with the calculated ones. The optimized structures of both compounds W and P were
superimposed to those obtained by X-ray diffraction analysis and reported in Figure 5.
For P, the overlap of the two structures is really good (overlay similarity 0.99). For W, as
showed in the picture, a good agreement in the tris(N-piperidinyl)benzene moiety was obtained
while the azobenzene moiety is twisted by about 90°. The explanation could be related to the
closed packing in the crystal structure which forces the azo moiety to be more sterically hindered
with respect to the gas phase theoretical structure.
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Figure 5. Superimposed structures of theoretical (red) and experimental (blue) structures of
compounds W (left) and P (right).
W spontaneously and slowly produces PH+ (Scheme 3), the salt of the usual product of
substitution derived by the shift of the proton from the carbon atom of W to a nitrogen atom. The
1H NMR spectrum of W at –40 °C in CD3CN shows the signal related to the sp3 carbon atom at δ
= 6.41 ppm which disappears and a signal, which may be attributed to N-H bond of the salt PH+,
at δ =12.52 ppm grows. The fact that the chemical shifts of protons H-4 and H-6 of the salt PH+
are different32 (δ = 5.81 ppm and 6.13 ppm in CDCl3 at 25 °C) indicates a not-symmetric
situation caused by the presence of the proton bound to the heterocyclic nitrogen in 1 (or 3)
position. The presence of the proton in other positions (on the nitrogen atom of the heterocyclic
moiety in 4 position or on the nitrogen atoms of azo group) should cause an equivalence of the
C-4 and C-6 positions. Probably, the nitrogen atoms in positions 1 or 3 are preferred to the
nitrogen atom in 4 position because of the sharing of the proton between the piperidinyl moiety
and the two nitrogen atoms of the azo group, as depicted in 5 (Figure 6).
N
H
N
N
N
N
H
3
1
N
N
+
5
+
N
5
4
Figure 6. Left: representation of a quinoid resonance structure of complex W. Right: schematic
representation of the interaction between proton and nitrogen atoms in PH+.
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This interaction between the proton and the two nitrogen atoms may be considered an
extremely favourable situation. Actually, this is supported by the fact that the back transfer of the
proton on the carbon atom was not observed even when the final product P (obtained from PH+
using a base32 such as NaOH, tertiary amines, etc.) was dissolved in the presence of large amount
of HCl. In the same way, the decrease of the temperature of a solution of PH+ (obtained directly
in the NMR probe) did not cause a return to the W intermediate.
Conclusions
In conclusion, the isolation, crystallization and the first X-ray investigation on the Wheland
intermediate in azo-coupling provided new insights on the structure of “sigma complexes” of the
electrophilic aromatic substitution (SEAr). The isolation of W further supports our previous
findings on this reaction that, when carried out in the absence of bases (different from the
starting triaminobenzene derivative) revealed that the two steps pathway, as reported in Scheme
2, involves first the attack of the electrophilic reagent as a fast step, while the spontaneous proton
releasing process is a slow step with the possibility of a relevant base catalysis leading to the re-
aromatized substrate. Crystal data of W were compared to those of the neutral azo compound P
obtained from the salt PH+, in turn derived from the spontaneous evolution of W in solution.
Even if we reported evidence on the reversibility of the whole substitution process, in the case
studied herein there was some evidence pointing to a hydrogen shift in solution from a carbon
atom to a nitrogen atom within the same molecule without evidence of a back reaction.
Experimental Section
13
General. 1H and C NMR spectra were recorded at 400 and 100 MHz respectively on a Varian
Mercury 400 instrument and the chemical shifts were measured in δ (ppm) using the solvent as
1
13
reference (δ = 7.27 and 2.0 ppm and δ = 77.0 and 0.3 ppm for H and C NMR in CDCl3 and
CD3CN, respectively). J values are given in Hz. Signal multiplicities were established by DEPT
experiments. HRMS were recorded in E.I. mode (70 eV) on a high resolution magnetic
instrument. Electron spray ionization mass spectra (ESI-MS) were recorded with a WATERS 2Q
4000 instrument. Melting points were measured on a Büchi 535 apparatus and are uncorrected.
1,3,5-Trichlorobenzene, phenyllithium, and p-methoxy-benzenediazonium tetrafluoroborate
were purchased from Sigma Aldrich (Milan, Italy). Compound 1 was prepared from 1,3,5-
trichlorobenzene using a modification of the method reported in the literature.40 Compounds PH+
and P were prepared as previously described.32
1,3,5-Tris(1-piperidinyl)benzene (1). In a flame-dried apparatus, under argon atmosphere, 18
mL of piperidine (182 mmol) were added to a magnetically stirred solution of 1,3,5-
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trichlorobenzene (2.72 g, 15 mmol) in anhydrous diethyl ether (100 mL). To this solution
phenyllithium (50 mL 1.8 M in dibutyl ether, 90 mmol) was added dropwise (about 90 min).
After 16 h water was added to the reaction mixture until complete dissolution of the salt and the
organic layer was separated. After further extraction with water (2 x 10 mL), the organic layer
was dried over anhydrous MgSO4 and filtered. The solution was concentrated under reduced
pressure. The residue (3.25 g, 66% yield) was washed with acetone and compound 1 was
obtained in 55% yield as a pale yellow solid, mp 178–181 °C (from chloroform), Lit.:41 oil,
1
Lit.:42 183–184 °C (from acetone). H NMR (300 MHz, CDCl3), δ (ppm): 6.11 (s, 3H), 3.15–
3.00 (m, 12 H), 1.77–1.60 (m, 12 H), 1.60–1.47 (m, 6 H); 13C NMR (75.46 MHz, CDCl3), δ
(ppm): 153.7, 99,0, 51.3, 25.9, 24.3; ESI-MS (ES+): m/z = 328.
Isolation and characterization of W. p-Methoxy-benzenediazonium tetrafluoroborate (16.35
mg, 0.05 mmol) was dissolved in acetone/dichloromethane (8:2) at ~ –80 °C. To this solution
compound 1 (11.1 mg, 0.05 mmol) was added. After about 30 min n-hexane was added to the
solution until a precipitate was formed. The solvent was quickly removed from the mixture and
the semi-solid residue was quickly dissolved in CD3CN at –40 °C and analyzed at this
1
temperature by 1H NMR spectroscopy. Since the H NMR spectrum showed mainly presence of
13
1
the W, also C NMR spectrum was recorded. After the DEPT experiment, a further H NMR
analysis of the solution showed presence of W and of compound PH+ (see spectra in Supporting
1
Information). NMR data of compound W are: H NMR (300 MHz, CD3CN, –40 °C), δ (ppm):
7.70 (d, J 8.8 Hz, 2 H), 7.10 (d, J 8.8 Hz, 2 H), 6.41 (s, 1H), 5.50 (s, 2 H), 3.92 (s, 3 H), 3.97–
13
3.23 (m, 12 H), 2.07–1.46 (m, 18 H); C NMR (75.46 MHz, CD3CN, –40 °C), δ (ppm): 163.2,
158.3, 157.4, 135.7, 124.5, 114.6, 85.1, 68.3, 55.5, 49.4, 48.9, 26.2, 25.8, 23.84, 23.75. Crystals
of solid W were obtained by precipitation from a mixture of CH2Cl2/n-hexane at low temperature
(about –60°C).
1-[2-(4-Methoxyphenylazo)-3,5-dipiperidin-1-ylphenyl]piperidinium
tetrafluoroborate
(PH+)32: Dark red solid, yield: 90%, mp 199–201 °C (CH2Cl2/n-hexane); H NMR (300 MHz,
CDCl3, 25 °C), δ (ppm): 12.52 (s, 1H, NH+), 7.25 (d, J 9.0 Hz, 2 H), 6.94 (d, J 9.0 Hz, 2 H), 6.13
(d, J 2.1 Hz, 1 H), 5.81 (d, J 2.1 Hz, 1 H) 3.81 (s, 3H), 3.75–3.65 (m, 4 H), 3.65–3.52 (m, 4 H),
3.40–3.27 (m, 2 H), 2.96–2.80 (m, 2 H), 2.08–1.60 (m, 18 H, overl. with solvent); 13C NMR
(75.46 MHz, CDCl3, 40 °C), δ (ppm): 160.0, 158.2, 157.7, 151.58, 135.56, 124.2, 117.0, 115.2,
98.5, 92.4, 55.7, 52.5, 51.9, 49.6, 26.3, 26.2, 25.9, 24.1, 23.9, 23.3; ESI-MS (ES+): m/z = 462.
1-(4-Methoxyphenyl)-2-(2,4,6-tripiperidin-1-ylphenyl)diazene (P)32: mp 155–157 °C
1
1
(CH2Cl2/n-hexane); H NMR (300 MHz, CDCl3, 25 °C), δ (ppm): 7.90 (d, J 8.9 Hz, 2 H), 6.99
(d, J 8.9 Hz, 2 H), 6.27 (s, 2 H), 3.87 (s, 3 H), 3.43–3.16 (m, 4 H), 3.16–2.86 (m, 8 H), 1.84–
1.43 (m, 18 H); 13C NMR (75.46 MHz, CDCl3, 40 °C), δ (ppm): 160.2, 153.1, 149.5, 131.5,
124.0, 123.2, 113.9, 100.0, 55.4, 54.4, 50.1, 26.4, 25.7, 24.6, 24.5. HRMS: m/z = 461.3142,
calcd. for C28H39N5O. 461.3155.
X-ray crystallography. The data collections for W and P were carried out on flash-cooled
crystals at 100 K. For W another data collection was carried out at 223 K but since the crystal
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was of poorer quality this dataset was abandoned. Crystal data and other experimental details for
W and P are reported in Table 2. Cell dimensions and the orientation matrix were initially
determined from a least-squares refinement on reflections measured in three sets of 20
exposures, collected in three different ω regions, and eventually refined against all data. A full
sphere of reciprocal space was scanned by 0.3° ω steps. The collected frames were processed for
integration by the SAINT program,43 and an empirical absorption correction was applied using
SADABS.44 The structures were solved by direct methods (SIR 97)45 and subsequent Fourier
syntheses and refined by full-matrix least-squares on F2 (SHELXTL),46 using anisotropic thermal
parameters for all non-hydrogen atoms. The H atoms were refined using pertinent riding models.
The displacement parameters for the H atoms were fixed at 1.2 U eq of their parent carbon atoms
for aromatic groups and 1.5 U of their parent atoms for methyl and methylene groups. In
eq
compound W a residual electron density ( 0.32 e/Å3) at a distance of 0.98 Å from C1
approximately indicated the position of the proton attached to C1 but the hydrogen was added in
calculated position. No conformational disorder was observed in P whereas in the cation of W
the azo nitrogen atoms N1 and N2, the substituted phenyl ring C7-C12 and the methoxy O1 and
C13 atoms were completely disordered over two positions, showing the existence of two
different conformers. After the refinement the occupation factors of the conformers were 0.70
–
and 0.30, respectively. Some disorder was observed also in the BF4 anion of W. In the final
difference Fourier map of W a residual electron density of 0.36 e/Å3 was found close to one of
the nitrogen of the azo group (N2). Crystal data and other experimental details for W and P are
reported in Table 2.
CCDC-894028 (for W), and -894029 (for P), contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif
Table 2. Crystal data and structure refinement for W and P
Compound
W
C28H40BF4N5O
549.46
P
Formula
C28H39N5O
461.64
M
T, K
100(2)
153(2)
Crystal symmetry
Orthorhombic
P212121
10.683(3)
29.325(8)
8.808(3)
90
Monoclinic
P21/c
Space group
a, Å
13.5426(4)
8.3251(3)
22.2471(7)
90
b, Å
c, Å
α, °
β, °
90
96.118(1)
90
γ, °
V, Å3
90
2759(1)
2493.9(1)
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Table 2. Continued
Compound
W
P
Z
4
1.323
4
1.230
Dc, Mg m-3
µ(Mo-Kα), mm-1
F(000)
0.100
0.076
1168
1000
Crystal size, mm
0.30 x 0.30 x 0.15
0.25 x 0.25 x 0.10
θ limits, °
1.39 – 27.00
28457
1.51 – 30.16
31360
Reflections collected
Unique obs. reflections [Fo > 4σ(Fo)]
3417
7368
Goodness-of-fit-on F2
1.246
0.0518, 0.1275
0.359 and –0.550
2 2 1/2
0.962
0.0546, 0.1360
0.324 and -0.315
2
R1 (F)a, wR2 (F2)b [Fo > 4σ(Fo)]
Largest diff. peak and hole, e. Å-3
R1 = Σ||Fo|-|Fc||/Σ|Fo|.b wR2 = [Σw(Fo -Fc2)2/Σw(Fo ) ] where w = 1/[σ2(Fo ) + (aP)2 + bP]
a
2
2
where P = (Fo + Fc2)/3.
Computational details. All DFT calculations were carried out with the Gaussian 09 program
package (Rev. A.02).47 The initial guess geometry of the compounds W and P were taken from
the X-Ray coordinates. The geometries were fully optimized using ultrafine grid at the
B3LYP/6-31G(d) level of theory,48–51 which is known to produce reliable geometries and
-
frequencies of the stationary points. The BF4 counterion was not taken into account in the
optimization process because its interactions could bring to deformed structures of both W and P
compounds. Frequency were calculated at the same level of theory as the geometry optimization
to verify the nature of the stationary points.
Supporting Information. ORTEP drawings of W and P, cartesian coordinates of all structures
optimized at B3LYP/6-31G(d), and 1H and 13C NMR spectra of compounds 1, PH+ and P.
Acknowledgements
Work supported by Alma Mater Studiorum – Università di Bologna (RFO funds).
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Supplementary Material
First isolation of a Wheland intermediate in the azo-coupling
reaction, its X-Ray crystal structure determination and products
from its evolution
Carla Boga,a* Erminia Del Vecchio,a Silvia Tozzi,a Luciano Forlani,a Magda Monari,b
Gabriele Micheletti a and Nicola Zannaa
a Department of Industrial Chemistry ‘Toso Montanari’, Alma Mater Studiorum – Università di
Bologna, Viale del Risorgimento, 4 - 40136 Bologna, Italy
b Department of Chemistry ‘G. Ciamician’, Alma Mater Studiorum – Università di Bologna, Via
Selmi, 2 - 40125 Bologna, Italy
E-mail: carla.boga@unibo.it
Table of Contents
1. Figure S1. ORTEP drawing of W
S2
2. Figure S2. ORTEP drawing of compound P
3. Cartesian coordinates of W and P structures optimized at B3LYP/6-31G(d)
4. Figure S3. 1H NMR of compound 1
5. Figure S4. 13C NMR spectrum of compound 1
6. Figure S5. 1H NMR spectrum of W
S2
S3
S6
S6
S7
7. Figure S6. 13C NMR spectrum and DEPT of W
8. Figure S7. 1H NMR spectrum of compound PH+
9. Figure S8. 13C NMR spectrum of compound PH+
10. Figure S9. 13C NMR spectrum of compound P
11. Figure S10. 1H NMR spectrum of compound P
S8
S9
S10
S11
S12
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Figure S1. ORTEP drawing of W (cation only). Broken lines indicate the minor conformer
arising by the ‘pedal motion’.
Figure S2. ORTEP drawing of compound P.
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Cartesian coordinates of W and P structures optimized at B3LYP/6-31G(d)
Compound W
N
N
C
C
H
C
C
C
H
C
H
O
C
H
H
H
N
N
N
C
H
C
C
H
C
C
H
C
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
-1.00924500 0.09378200 -1.00466300
-1.05083700 -0.11692100 0.23193200
-2.33203500 -0.17663100 0.81019800
-3.54447800 -0.04020400 0.11053800
-3.52267500 0.12228300 -0.96157800
-4.75588200 -0.11443700 0.78175800
-4.77354300 -0.32755200 2.17593900
-3.56246800 -0.46578400 2.87941500
-3.60266900 -0.62969100 3.95096500
-2.35979100 -0.39192900 2.20057300
-1.41525600 -0.49648000 2.72524800
-5.89382400 -0.41482900 2.92045900
-7.16710200 -0.28606200 2.28805500
-7.27712100 0.69681900 1.81502000
-7.90209000 -0.39009300 3.08638400
-7.32132800 -1.07566400 1.54331000
0.39959600 2.60197300 -1.33359500
4.09832100 0.20174100 0.61403000
0.58425000 -2.29700300 -1.55224100
0.41060300 0.15080100 -1.50583700
0.32889600 0.20526300 -2.59721600
1.05852800 1.44771800 -1.03777200
2.24446000 1.40793900 -0.33871800
2.75380700 2.32976900 -0.11427000
2.91504000 0.18162000 -0.04972000
2.36806400 -1.05730000 -0.49732900
2.89752800 -1.96402900 -0.26162100
1.17105400 -1.12786300 -1.17487700
-0.58455700 2.72251000 -2.43173600
-0.95912300 1.73824900 -2.69950000
-0.06585000 3.14533000 -3.30692100
-1.76702100 3.61943800 -2.05267300
-2.33981600 3.12998000 -1.25473100
-2.42679900 3.70202000 -2.92423700
-1.28914400 4.99818700 -1.59042500
-0.81721600 5.52677200 -2.43087800
-2.13594700 5.61262900 -1.26588200
-0.28227200 4.83169000 -0.45036900
-0.78745700 4.41517600 0.43056400
0.14071300 5.79788500 -0.15179500
0.87844700 3.90291500 -0.83298700
1.51232600 4.37452900 -1.59999500
1.49239200 3.73736600 0.05117500
4.56932000 1.33084700 1.43688000
3.84512300 2.14149300 1.41218700
5.51112800 1.70468200 1.01158200
4.77910200 0.88056300 2.89400800
3.80031400 0.63182300 3.32447500
5.17481400 1.72925400 3.46416300
5.71794400 -0.32837700 2.99671200
5.77342100 -0.67783900 4.03334200
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H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
H
6.73557600 -0.02675900 2.71118200
5.24946500 -1.45851700 2.06967000
5.98342200 -2.27238800 2.04585700
4.30382700 -1.88264600 2.43220800
5.05259400 -0.92164300 0.64703900
6.00875100 -0.52850100 0.27245500
4.74510600 -1.69832000 -0.04971600
-0.45201300 -2.37691800 -2.60558700
0.04574200 -2.64013000 -3.55277500
-0.92640900 -1.40619000 -2.72882100
-1.52939300 -3.41705700 -2.28456400
-2.22879100 -3.45486000 -3.12783500
-2.09603400 -3.08166200 -1.40643400
-0.91112300 -4.79144100 -2.01831400
-1.68305400 -5.51520700 -1.73526500
-0.44155400 -5.17040200 -2.93722400
0.13774700 -4.66804100 -0.91140900
-0.35351000 -4.40339700 0.03382000
0.65679500 -5.61980600 -0.74929200
1.19340100 -3.60042200 -1.23046700
1.83788400 -3.48791500 -0.36024200
1.82263400 -3.92133700 -2.07538600
-5.68117700 -0.00870400 0.22776200
Compound P
N
N
C
N
C
C
H
C
H
H
N
C
N
C
C
H
C
O
C
H
C
H
H
C
C
H
C
1.02991700 0.32295300 0.05760200
1.87224100 -0.57925400 0.35985300
-1.12436300 1.30025400 0.21194500
-4.54420300 -0.05433700 -0.34170900
-3.15558700 -0.01704000 -0.11355900
-2.50498900 1.22203700 0.03779100
-3.08624000 2.13300200 -0.01081300
0.34389900 2.82273700 1.48051300
0.82768200 1.90168400 1.80117700
-0.31182000 3.16283800 2.30648100
-0.47385600 2.54342600 0.28927100
-0.32911100 0.10219600 0.24377700
-0.28500700 -2.37047800 0.25448000
3.19197700 -0.26350600 -0.05206400
4.22625000 -1.02805000 0.49787200
3.97376900 -1.80264700 1.21584200
-1.00212500 -1.16082500 0.18625900
7.13273500 0.51859900 -1.18352500
-2.38962100 -1.18802700 -0.02170300
-2.88464600 -2.14876800 -0.03875700
0.27517800 -2.72291800 1.56965700
-0.53593500 -3.09056700 2.22837000
0.69338400 -1.82183400 2.01621600
5.87043800 0.20861500 -0.76491500
3.51799300 0.74303700 -0.98402700
2.71359700 1.32375800 -1.42187700
1.39322500 3.89807500 1.19249800
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H
H
C
H
H
C
H
C
H
H
C
H
C
H
H
C
H
H
C
H
H
C
H
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
2.10149300 3.50337300 0.45348600
1.95762900 4.10634100 2.11042700
-1.17133800 3.73099100 -0.19697600
-1.90060300 4.10972000 0.54725900
-1.73292900 3.46053200 -1.09560300
4.83707400 0.97483400 -1.33370300
5.10304400 1.74034100 -2.05675200
-0.85075900 -3.53177400 -0.43550100
-1.25127400 -3.19855000 -1.39756200
-1.68885300 -3.98031100 0.13440500
5.55990500 -0.79762100 0.15732600
6.33789600 -1.40247200 0.60900400
0.84590300 -5.02712900 0.68773900
0.08477900 -5.53856000 1.29632400
1.65648900 -5.74861100 0.52559600
-5.15520000 -1.32064900 -0.74119300
-5.30990600 -1.98655400 0.13037800
-4.47325600 -1.83218400 -1.42530100
-0.16950300 4.84417600 -0.53506700
0.44775700 4.51025000 -1.37905100
-0.72170700 5.73336500 -0.86555500
8.21600500 -0.23276300 -0.66182500
8.29418800 -0.12688600 0.42853400
9.11542500 0.17383200 -1.12820700
8.12712200 -1.29809800 -0.91345200
0.22326300 -4.60458700 -0.64846200
-0.22636900 -5.46557600 -1.15972100
1.00004000 -4.19841200 -1.30857900
0.73115900 5.17506100 0.66094300
0.12405000 5.62905900 1.45867000
1.48699800 5.91845700 0.37883300
1.36031800 -3.79295800 1.43899900
1.71709900 -4.06815200 2.44016300
2.20258900 -3.35342000 0.89324100
-6.49667400 -1.08170000 -1.44709400
-6.30410100 -0.55960200 -2.39336800
-6.94585000 -2.05138100 -1.69592900
-5.41574500 0.74095500 0.53471400
-4.90783300 1.66691900 0.80553000
-5.60017300 0.19035100 1.47750100
-6.75576000 1.04797500 -0.13949600
-7.38966900 1.60987500 0.55807600
-6.57628400 1.69165900 -1.01071300
-7.44864400 -0.24609500 -0.58374300
-8.37240800 -0.02417500 -1.13186900
-7.73971000 -0.82537300 0.30509200
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N
N
N
Figure S3. 1H NMR (300 MHz, CDCl3, 25 °C) spectrum of compound of 1,3,5-tris(1-
piperidinyl)benzene (1).
N
N
N
Figure S4. 13C NMR (75.46 MHz, CDCl3, 25 °C) spectrum of compound of 1,3,5-Tris(1-
piperidinyl)benzene (1).
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OCH3
N
N
H
R2N
NR2
–
BF4
NR2
NR2 = N-piperidinyl
W complex
Figure S5. 1H NMR (300 MHz, CD3CN, –40 °C) spectrum (and expanded view) of W [with
presence of PH+ (spectrum recorded after time used to record 13C NMR spectrum) and presence
of acetone, dichloromethane, and n-hexane used as solvents].
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OCH3
N
N
H
R2N
NR2
–
BF4
NR2
NR2 = N-piperidinyl
W complex
Figure S6. 13C NMR (75.46 MHz,, CD3CN, –40 °C) (and DEPT) of W [with traces of PH+
(spectrum recorded immediately after precipitation of W) and of
acetone/dichloromethane/hexane used as solvents].
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General Papers
ARKIVOC 2014 (iv) S1-S12
OCH3
N
+
.
H
N
R2N
NHR2
–
BF4
NR2
NR2 = N-piperidinyl
PH+
Figure S7. 1H NMR (300 MHz, CDCl3, 25 °C) spectrum of compound PH+.
Page S9
©ARKAT-USA, Inc

General Papers
ARKIVOC 2014 (iv) S1-S12
Figure S8. 13C NMR (75.46 MHz,, CDCl3, +40 °C) of compound PH+.
Page S10
©ARKAT-USA, Inc

General Papers
ARKIVOC 2014 (iv) S1-S12
OCH3
N
N
R2N
NR2
NR2
NR2 = N-piperidinyl
P
Figure S9. 13C NMR (75.46 MHz,, CDCl3, +40 °C) of compound P.
Page S11
©ARKAT-USA, Inc

General Papers
ARKIVOC 2014 (iv) S1-S12
OCH3
N
N
R2N
NR2
NR2
NR2 = N-piperidinyl
P
Figure S10. 1H NMR (300 MHz, CDCl3, 25 °C) spectrum of compound P.
Page S12
©ARKAT-USA, Inc

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