Triazolo[1,5-a]pyridyl phosphines reflecting the influence of phosphorus lone pair orientation on spectroscopic properties

Article abstract of DOI:10.1039/c0dt01183e

A series of new triazolopyridine-based phosphines has been prepared. These compounds revealed unexpected spectroscopic patterns. In particular, the NMR spectra are highly dependent on the relative conformational preference of the phosphine substituent at

Full text of DOI:10.1039/c0dt01183e

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PAPER
[1,2,3]Triazolo[1,5-a]pyridyl phosphines reflecting the influence of phosphorus
lone pair orientation on spectroscopic properties†
Rafael Ballesteros-Garrido,a Laurence Bonnafoux,a Fernando Blanco,*c Rafael Ballesteros,*b
Fre´de´ric R. Leroux,*a Bele´n Abarca,*b Franc¸oise Colobert,*a Ibon Alkortad and Jose´ Elguerod
Received 7th September 2010, Accepted 12th November 2010
DOI: 10.1039/c0dt01183e
A series of new triazolopyridine-based phosphines has been prepared. These compounds revealed
unexpected spectroscopic patterns. In particular, the NMR spectra are highly dependent on the relative
conformational preference of the phosphine substituent at C7. Here, we report on their complete NMR
analysis, X-ray structures and DFT calculations that confirm the particular arrangement of the
phosphorus lone pair orbital related to the substituent pattern of the chosen phosphine.
Introduction
Nowadays, an immense number of tertiary phosphines and
their transition-metal complexes are known which reflects their
importance in homogeneous catalysis. The steric and electronic
properties at phosphorus have been often modified in order to fine-
Fig. 1 Influence of the lone pair orientation on the 2J coupling constant
in tertiary phosphines.
tune the efficiency of the transition-metal catalyst.1 The ligand-
binding properties of the phosphines are based on an available lone
pair orbital, whereas the steric environment as well as the electronic
effect of the phosphorus substituents clearly influences the binding
situation. The NMR coupling pattern2 of phosphine-derivatives
is conditioned by the relative orientation of the phosphorus lone
pair and the substituents. This phenomenon has been previously
studied. For instance Quin et al.3,4 reported on the influence of
2
the lone pair orientation on the J(P,C) coupling constant in 7-
(phosphamethyl)-norbornene (1a and 1b) derivatives (Fig. 1).
Some of us have recently reported on the preparation of the first
3-(2¢-pyridyl)triazolopyridine-phosphine derivatives 2 (Fig. 2).5
We could show that these compounds are in equilibrium6 between
two isomer forms A and B (i.e., the isomerization barrier is
generally low) and that the A–B ratio depends primarily on the
electronic properties of the substituent at C7. The triazolopyridine
ring system was revealed to be a sensitive sensor for the electronic
Fig. 2 First triazolopyridine-phosphine ligands.
situation at phosphorus. In a continuation of our ongoing studies
on triazolopyridines7 and phosphines derived thereof, we now
wanted to study the ability of the triazolopyridine ring to
reflect the conformational preferences of the attached phosphorus
substituents. However, the ring-chain equilibrium rendered an in-
depth NMR structural analysis dealing with rotational effects too
complicated.
Therefore, we decided to synthesize a new series of non-
isomerizable phosphines (PX2) of 3-methyl-[1,2,3]triazolo [1,5-
a]pyridine (3a), [1,2,3]triazolo[1,5-a]pyridine (3b), and 3-phenyl-
[1,2,3]triazolo[1,5-a]pyridine (3c) (Fig. 3), and to perform an
experimental and theoretical investigation on the influence of
phosphorus lone pair orientation on the spectroscopic properties.
We will show, how the structural and electronic properties of
the phosphine substituent (X) determines both the degree of
electron density of the phosphorus lone pair as well as its relative
orientation to the triazolopyridine ring, having a significant
influence on the NMR spectroscopic patterns.
aLaboratoire de ste´re´ochimie (UMR7509), CNRS-Universite´ de Strasbourg
(ECPM), 25 rue Becquerel, 67087, Strasbourg, France. E-mail: frederic.
leroux@unistra.fr, francoise.colobert@unistra.fr; Fax: +33 3.68.85.27.42
bDepartamento de Qu´ımica Orga´nica Facultad de Farmacia, Universidad
de Valencia Avda. Vicente Andre´s Estelle´s s/n, 46100, Burjassot, Valencia,
Spain. E-mail: belen.abarca@uv.es
cSchool of Chemistry, University of Dublin, Trinity College, Dublin 2, Ireland.
E-mail: blancof@tcd.ie
dInstituto de Qu´ımica Me´dica, CSIC, Juan de la Cierva 3, E-28006, Madrid,
Spain
† Electronic supplementary information (ESI) available: Experimental
details, characterization data, as well as X-ray crystallographic data for
4a, 4b, 4c, 5a and 5b. CCDC reference numbers 799866–799870. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01183e
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6.32 ppm in 4b and 6.45 ppm in 4c) compared to 6.9–7.0 ppm
in [1,2,3]triazolo[1,5-a]pyridine 3b (Fig. 5).
Fig. 3 Target compounds.
Results and discussion
1) Synthesis of triazolopyridine-phosphines
The new triazolopyridine-phosphines were synthesized analo-
gously to the method reported for 7-substituted triazolopy-
ridines. Regioselective metalation8 of the triazolopyridine ring
with butyllithium in toluene at -40 ◦C, followed by the trap-
ping of the 7-lithio-derivative with the corresponding diphenyl-,
dicyclohexyl-, or di-iso-propyl-phosphine chloride afforded the
desired phosphines 4–6 (Scheme 1). After purification by column
chromatography, compounds 4–6 were obtained in moderate
yields in almost all cases, except for 6c which was obtained in
a low 23% yield. All the phosphines were revealed to be insensitive
towards air and moisture.
Fig. 5 1H-NMR comparison (from up to down 3b, 4a, 4b and 4c), NMR
assignation for 3b: (from left to right: H7, H3, H4, H5 and H6).
In contrast, in dicyclohexyl- (5a–c) and di-iso-propyl-
phosphines (6a–c), the signal for H6 is shifted downfield and
overlaps with the signal corresponding to H5, forming a multiplet
at 7.10–7.20 ppm (Fig. 6).
Scheme 1 Preparation of the triazolopyridine-phosphine family.
2) 1H-NMR study of compounds 4–6a–c
The typical 1H-NMR chemical shifts and coupling constants
for hydrogen at C4, C5, C6 and C7, which can be found in
[1,2,3]triazolo[1,5-a]pyridines 3a–c, are summarized in Fig. 4.9
Fig. 4 1H-NMR chemical shifts and J constants for compounds 3a, 3b
and 3c.
1
The analysis of the H-NMR spectra of the newly prepared
triazolopyridine-phosphine series 4–6 revealed some interesting
features. The most significant one is the influence of the substituent
at phosphorus on the 1H-NMR signal corresponding to H6.
Depending on their nature (phenyl phosphines 4(a–c) or alkyl
phosphines (X = Cy, iPr) 5(a–c) and 6(a–c)) and, thus, on
their electronic and/or steric properties, different trends can be
observed.
Fig. 6 1H-NMR comparison (from up to down 3b, 5b and 6b), NMR
assignation for 3b: (from left to right: H7, H3, H4, H5 and H6).
We assumed that the differences in the relative orientation of the
substituents X and the lone pair orbital of the phosphine groups
in 5 and 6 compared to 4 could explain our observations. In order
to confirm this hypothesis, single crystal X-ray analyses and DFT
calculations were performed.
In diaryl phosphines 4a–c, the signal corresponding to pro-
ton H6 is significantly shifted upfield (d = 6.37 ppm in 4a,
1388 | Dalton Trans., 2011, 40, 1387–1395
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3) Single crystal X-ray analysis
These results encouraged us to further investigate the role of the
phosphine orientation, i.e. the influence of the relative position of
the phosphorus lone pair and the substituents on the H-NMR
Suitable single crystals of triazolopyridine phosphines for X-ray
analysis were obtained by slow diffusion of iso-propanol into
chloroform.
1
chemical shift of H6. As it was confirmed by single crystal X-ray
analysis in diaryl phosphines (4a–c) H6 is located in the shielding
region of the anisotropic influence of the phenyl rings which
explains the observed upfield shift (Fig. 9). This phenomenon has
been reported in many cases, the most representative examples
being paracyclophanes.10,11 Contrary, in dialkyl phosphines (5a–c
and 6a–c), X-ray analysis shows the phosphorus lone pair pointing
towards H6 (Fig. 9), which could be related with the significant
H6 downfield shift observed in these compounds.
Thus, single crystal X-ray analysis of 4–6 confirmed that the
relative position of the phosphine group in the solid state is not the
same for dialkyl and diaryl phosphine derivatives. Opposite effects
on the 1H-NMR spectra suggest that for each kind of phosphorus
substituent a preferential conformation in solution results. In
order to confirm our assumption, a theoretical rotational study
was performed.
As predicted, triazolopyridine-phosphines adopt different con-
formations in the solid state. X-Ray analyses of diaryl phosphines
4a–c (Fig. 7) reveal a spatial arrangement where the phosphorus
lone pair is almost in the plane of the triazopyridine ring, oriented
towards the triazole system with one of the phenyl groups located
above and the other one below the ring (Fig. 9). On the other
hand, in alkyl systems, the lone pair remains in the plane of the
triazolopyridine ring but pointing to the opposite side near H6
(Fig. 8).
4) DFT conformational study12
Significant effects on the H6 chemical shift observed in the
1H-NMR spectra suggested a preferential conformation of the
phosphorus substituent PX2 depending on the nature of the
substituent X. Fig. 10 shows the general formula for the two
conformers used in the theoretical study based on the two different
geometries found in the X-ray analysis: the conformer A, detected
in dialkyl substituted phosphines (X = Cy: 5a–c and iPr: 6a–c) and
the conformer B, detected in the phenyl series (X = Ph: 4a–c).
Fig. 7 X-Ray structures for compounds 4a (up left), 4b (up right) and 4c
(down).
Fig. 10 Preferred conformations of the phosphorus substituent according
to the rotational study.
a) Rotational profile. As a first approach we analyzed the
rotation of the phosphine group over the triazolopyridine system
by IRC (Intrinsic Reaction Coordinate) calculations. In order to
simplify the model and minimize the steric factor, we screened
the rotation in three phosphines substituted by small groups with
different electronic properties: e-donor (X = H, CH3) and e-
acceptor (X = F). As shown in Fig. 11, starting from the minimum
corresponding to the conformer A, through the transition states
TS AB1 or TS AB2, the rotation in either direction lead to two
equivalent minima B1 and B2 (with substituents located on the
opposite side of the ring). The transition between B1 and B2 goes
through the transition state TS B1B2 that presents the lone pair
orbital of N1 and the lone pair at phosphorus in the same plane
and in a close proximity. Thus, the repulsive effects between both
lone pairs could play a determining role in the stability of this
structure. This particular role of the phosphorus lone pair in the
Fig. 8 Molecular structures for compounds 5a (left) and 5b (right) in the
solid state (X-ray diffraction).
Fig. 9 Different 1HNMR effects depending on the phosphine orientation.
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Table 1 Theoretical and experimental ratio of the conformers A/B (see
Fig. 10)
X
Compound
DE (kJ mol-1)
Theoretical
Experimental
Cy
iPr
CH3
H
Ph
F
5b
6b
-7.5
-7.4
-0.8
0.9
8.1
9.9
A
A
A/B
B/B
B
A
A
4b
B
B
Br
11.1
B
Fig. 11 Two views of rotational mechanism from A to B (R = H; X = F).
Arrows show the orientation of the phosphorus lone pair for each case.
A more detailed study including the analysis of the transition
states suggests interesting conclusions concerning the influence of
the phosphine lone pair in the rotational equilibrium:
molecular geometry, has already been the subject of numerous
studies in organometallics and in organic chemistry in the past,
being named the gauche effect.13
– TS AB1/TS AB2, correspond to the geometry where the
phosphine substituents (-F, -H or -CH3) are close to N1. For X = F
and CH3 the relative energy of this transition state is significantly
high (18.0–20.0 kJ mol-1), as a result of the repulsion between
N1 and the X substituent. In contrast, this barrier decreases for
X = H (7.50 kJ mol-1) as a result of the decrease of repulsion
and the possible extra stabilizing interaction by hydrogen bonding
(N1–H).
– TS B1B2 shows similar energetic barriers for X = H and
CH3 (15.00–20.00 kJ mol-1), while it visibly decreases for X =
F. In this case, the most decisive factor is the interaction of the
phosphorus lone pair and N1. For X = F the effect of the electron-
attracting properties of fluorine reduces the electron density of the
phosphorus lone pair, decreasing the repulsion with the lone pair
at N1. Oppositely, for X = H and CH3, this repulsive interaction
would be increased due to the donor character of the substituents
that increase the electronic density of the phosphorus lone pair.
Fig. 12 shows the reaction path for the studied derivatives in
terms of relative energy with respect to the minimum A. For
each case the profile is characteristic. Transition barriers found
were smaller than 30 kJ mol-1, suggesting that in all cases a free
rotation exists. Consequently, the conformational preference can
be explained based on a thermodynamic approach. Concerning
the electronic nature of the substituents, there are clear differences
in the rotational profile when the phosphine is substituted by an
e-attracting group (X = F) or by an e-donor one (X = H, CH3).
b) Conformational A–B ratios. Subsequently to the rota-
tional study, we performed a conformational analysis of the
triazolopyridine-phosphine system as a function of the properties
of the phosphine substituent (X). As shown in Table 1, we selected
7 different groups, the experimentally studied (-PPh2: 4b, -PCy2:
5b, and PiPr2: 6b), and the new ones (X = H, CH3, F and Br), to
optimize their geometries and locate the corresponding minima.
In the case of substituents with secondary conformations as a
result of the internal movement (cyclohexyl or isopropyl groups),
some different geometries were minimized. Table 1 shows the
relative energies only between the most stable configurations in
the respective conformations A or B for the different substituents
considered. Negative values indicate that the conformer A is
energetically favoured while positive values indicate a better
stability of the isomer B.
The general trend suggests that the substituents with positive
inductive effect (+I) stabilize the conformer A (X = Cy, iPr, CH3),
whereas substituents with negative inductive effect (-I) stabilize
the conformer B (X = Br, F, Ph). This observation is in accordance
with the experimental results obtained in this work. Moreover, the
calculated energy differences between the respective conformers
for the compounds 4b, 5b and 6b (above 5 kJ mol-1) are high
enough to explain that these compounds are present only as the
most stable form in solution and not as an isomeric mixture.
Fig. 12 Reaction path for the studied derivatives (kJ mol-1).
As shown in Fig. 12, when X = F the absolute minimum
corresponds to the conformer B, with an energy difference to
A close to 10 kJ mol-1. The orientation of one of the fluorine
atoms toward H6 (possible stabilizing interactions) and the low
repulsion between the N1- and the phosphine lone pair orbitals
(as a consequence of the low electron density of the phosphorus
lone pair due to the electron-attracting effect exerted by fluorine),
could explain the better stability of this conformation. Moreover,
in the conformation A, two fluorine atoms are located very close
to the N1 lone pair creating a clearly destabilizing situation by
electronic repulsion.
For X = H and X = CH3, however, the energy differences between
the conformers A and B are very low, although in the case of X =
CH3 the conformer A is slightly favoured. In this case there are
no obvious electronic or steric factors favouring one of the two
conformations.
c) Comparison between predicted and experimental NMR chemi-
cal shifts. 1H-NMR of compounds 4b, 5b, and 6b were calculated
using the GIAO methodology. Table 2 presents the comparison
1390 | Dalton Trans., 2011, 40, 1387–1395
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Table 2 1H-NMR spectra comparison
and 6a–c the carbon signals C7, C6, C5, C4 and C3 can be easily
assigned (Table 3).
Compound
Data
Conf.
H3
H4
H5
H6
If we compare the 13C-NMR chemical shifts of the parent
compounds 3a and 3c with the corresponding phosphines, we can
highlightsomeobservations:(1)adownfieldshiftofalmost15ppm
for the carbon atom C7, (2) a downfield shift of 5–10 ppm for the
carbon C6 and (3) no significant influence on the carbons C4 and
C5. This pattern is the same for alkyl- and/or aryl-phosphines.
The value of C3, on the other hand, is strongly conditioned by the
R substituent.
5b (X = Cy)
5b (X = Cy)
5b (X = Cy)
Calc.
Calc.
Exp.
Calc.
Calc.
Exp.
Calc.
Calc.
Exp.
A
8.08
8.01
8.07
8.10
8.04
8.02
8.03
7.99
7.98
7.61
7.50
7.7–7.6
7.64
7.53
7.7–7.6
7.67
7.53
6.99
7.01
7.2–7.1
7.03
7.06
7.2–7.1
7.12
6.92
7.27
6.88
7.2–7.1
7.34
6.90
7.2–7.1
7.61
6.20
B
Aassig.
A
i
6b (X = Pr)
i
6b (X = Pr)
B
6b (X = iPr)
4b (X = Ph)
4b (X = Ph)
4b (X = Ph)
Aassig.
A
B
Bassig.
7.61
7.01
6.32
As it can be seen, alkyl phosphines (5a–c and 6a–c) show a J(C–
P) coupling for carbons C7, C6 and C5 of the triazolopyridyl ring.
In contrast, aryl phosphines (4a–c) show J(C–P) coupling only for
C7 and C3 (Fig. 13). Furthermore, for compounds 4a–c, coupling
constants (2J and 3J) can be found with the carbon of the phenyl
substituents of the phosphine group.
between experimental and calculated14 values. In all cases the most
relevant data concerns the chemical shift of the proton H6. As it
can be seen, for dicyclohexyl phosphine-substituted compound 5b,
and the iso-propyl derivative 6b, the experimental values for H6
(d = 7.2–7.1 ppm) are closer to the calculated one corresponding to
conformer A (d = 7.27 ppm), whereas for the phenyl derivative the
chemical shift of H6 (d = 6.32 ppm) is closer to that calculated for
conformer B (d = 6.20 ppm). That is consistent with our previous
conformational study. Moreover, considering the other significant
1H-NMR signals, the DFT calculated values can be used as a good
approximation.
Fig. 13 Selective coupling of different phosphines with the triazolopyri-
dine ring.
5) 13C-NMR study of compounds 4–6a–c
We deduce that the relative position of the phosphorus lone pair
orbital plays an essential role in the carbon-phosphorus coupling.
This phenomenon could be observed due to the relative simplicity
of compounds 4a–c and 6a–c.
As it was shown for 1H-NMR, the relative orientation of the phos-
phine lone pair and substituents also has a significant influence on
the 13C-NMR spectra.2 It is known that the angle and proximity
between the phosphorus lone pair orbital and a corresponding
coupled atom provide an enhancement of the coupling constant.
Recently, Contreras and Peralta15 reported on the evolution of
the 2J(P,C) value for ethyl phosphine, observing a dependence on
the C–C–P angle. Gil and Philipsborn also showed the influence
of the angle on the coupling constants.16 However, although
many phosphine-based compounds are published every year, the
assignment of carbon signals and the phosphorus lone pair effect
are usually not reported. Furthermore, the already commented
free rotation makes the study of these phenomena difficult. As it
was mentioned above, in the case of the systems studied in the
present work these effects are clearly observed due to the existence
in solution of only the most favourable conformer. Contrary to
previous studies with phosphines where the carbon assignation
was found to be extremely difficult, in the phosphines 4a–c, 5a–c
6) Controlling the rotation of aryl phosphines
As outlined before, the dependence of the angle between the
phosphorus lone pair and the corresponding carbon atoms must
be taken into account in order to analyze the NMR spectra.
As we have shown, the same triazolopyridine ring system can
have different NMR coupling (P–C) patterns depending on the
phosphine type, due to the existence of preferential rotamers.
We were also interested in the possibility to control this
angle, by the introduction of large substituents at the 6-position
of the triazolopyridine ring. They should increase the steric
hindrance and force the aromatic ring of the diphenylphos-
phino group to turn along the triazolopyridine-phosphine bond.
Therefore, we synthesized 6-substituted phosphines starting from
Table 3 13C-NMR chemical shifts for triazolopyridines 4a–c, 5a–c and 6a–c. C7, C6, C5, C4 and C3 assignation, multiplicity, chemical shift (ppm) and
J(P–C) coupling constant (Hz)
Entry
Compound
C7
C6
C5
C4
C3
1J(C7–P)
2J(C6–P)
3J(C5–P)
4J(C3–P)
1
2
3
4
5
6
7
8
3a
3b9
3c
4a
5a
6a
4b
5b
6b
4c
5c
6c
124.9
124.7
126.0
115.1
115.1
115.7
120.9
124.1 d
124.2 d
120.9
122.5 d
122.7 d
120.9
125.1
125.0
126.1
124.5
124.6 d
124.2 d
124.5
124.5 d
124.4 d
124.9
117.7
117.6
118.8
117.4
118.2
118.1
117.2
117.8
117.9
117.9
118.8
118.8
133.5
133.3
131.9
134.2 d
134.2
134.4
125.6 d
125.7
125.6
137.8 d
137.8
137.9
0
0
0
0
0
0
0
25.9
24.2
0
26.7
26.1
0
0
0
0
0
8.2
7.6
0
8.5
8.3
0
0
0
0
1.2
0
0
1.6
0
0
1.9
0
138.4 d
137.1 d
137.5 d
138.5 d
136.7 d
137.4 d
139.3 d
137.6 d
138.2 d
22.4
41.8
42.2
22
41.8
41.8
23.3
44.2
43.6
9
10
11
12
125.1 d
124.6 d
124.6 d
124.7 d
27.6
26.2
8.7
8.3
0
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3-methyl-6-bromo-[1,2,3]triazolo[1,5-a]pyridine (7),17 by regios-
elective metalation with LDA followed by trapping with
diphenylphosphine chloride. In this way, compound 8 was ob-
tained in 57% yield (Scheme 2).
Scheme 2 Synthesis of compound 8.
Analysis of the 13C-NMR spectra now revealed P–C coupling
constants with C7, C6, C5 and C3a. This kind of coupling has
previously not been observed with diaryl phosphines and can
therefore be taken as confirmation of our hypothesis (Fig. 14).
Fig. 15 Lone pair relative position upon different substrates.
Experimental section
General methods
Starting materials, if commercial, were purchased and used as
such, provided that adequate checks (melting ranges, refractive
indices, and gas chromatography) had confirmed the claimed
purity. When known compounds had to be prepared according
to literature procedures, pertinent references are given. Air-
and moisture-sensitive materials were stored in Schlenk tubes.
They were protected by and handled under an atmosphere of
argon, using appropriate glassware. Tetrahydrofuran was dried by
distillation from sodium after the characteristic blue colour of
sodium diphenyl ketyl (benzophenone-sodium “radical-anion”)
had been found to persist. Ethereal or other organic extracts
were dried by washing with brine and then by storage over
sodium sulfate. Melting ranges (M.p.) given were determined
on a Kofler heated stage and found to be reproducible after
recrystallization, unless stated otherwise (“decomp.”), and are
uncorrected. If melting points are missing, it means all attempts
to crystallize the liquid at temperatures down to -75 ◦C failed.
Column chromatography was carried out on a column packed
Fig. 14 13C-NMR coupling pattern of compound 8.
According to the 13C-NMR spectrum, the phosphorus lone pair
orbital should be perpendicular to the triazolopyridine ring to
allow the coupling with C6, C5 and C3a. This orientation seems to
be reasonable if one considers the interaction between the bromine
atom and the phenyl ring. The bond rotation (triazolopyridine-
PPh2) is the only way to minimize steric interaction moving the
lone pair from the right side (close to the nitrogen atoms) towards
the P/C7/C4 based molecular plane perpendicular to that of the
pyridine ring. It is interesting to note that the presence of the
bromine atom induces a significant shift in the 31P-NMR of 8 (d =
-2.0 ppm) compared to the parent compounds 4a–c (d = -15.4,
-15.0 and -14.6 ppm, respectively) probably due to both electronic
and steric effects.
1
with silica-gel 60 N spherical neutral size 63–210 mm. H and
(1H decoupled) 13C nuclear magnetic resonance (NMR) spectra
were recorded at 400 or 300 and 101 or 75 MHz, respectively.
Chemical shifts are reported in d units, parts per million (ppm)
and were measured relative to the signals for residual chloroform
(7.27 ppm). Coupling constants J are given in Hz. Coupling
patterns are abbreviated as, for example, s (singlet), d (doublet), t
(triplet), q (quartet), quint (quintet), sp (septuplet), td (triplet of
doublets), m (multiplet), app. s (apparent singlet) and br (broad).
Conclusion
In summary, we have reported the preparation of non-isomerizable
triazolopyridine-phosphines and we have highlighted their spec-
troscopic (NMR) properties. By means of DFT calculations, X-ray
and NMR analysis we have been able to show and predict how
the phosphine orientation can influence the NMR pattern. We
have also pointed out how its relative position is dependent on the
phosphine substituent, but also on the steric hindrance (Fig. 15).
The orientation of the phosphorus lone pair induces an
uncommon 13C-NMR coupling pattern. This phenomenon was
clearly identified due to the simplicity of compounds 4–6(a–c) and
8. The lone pair has been found to favour P–C NMR coupling
constants when the carbon atoms are near to, coplanar to, or on
the same side as it.
General protocol: synthesis of [1,2,3]triazolo[1,5-a]pyridine based
monophosphines
At -40 ◦C, butyllithium (1 eq) in hexane (1.54 M) was
added dropwise to
a solution of the corresponding 3-
substituted[1,2,3]triazolo[1,5-a]pyridine (1 eq) in toluene. After
0.5 h, a solution of chlorophosphine (1 eq) in toluene was
added dropwise. After 1 h, the solution was allowed to reach
room temperature. Water was added, followed by extraction with
1392 | Dalton Trans., 2011, 40, 1387–1395
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dichloromethane. The combined organic layers were dried over
sodium sulfate, filtered, and evaporated.
3-methyl-[1,2,3]triazolo[1,5-a] pyridine (5a; 1.0 g 45%) as a
◦
1
colorless solid. mp 93–95 C. – H NMR (300 MHz, CDCl3):
d = 7.56 (d, J = 8.2 Hz, 1 H, H4), 7.15–7.00 (m, 2 H, H6 + H5),
2.73–2.57 (m, 2 H, PCy2), 1.94 (dd, J = 16.5, 8.6 Hz, 2 H, PCy2),
1.72 (d, J = 7.5 Hz, 2 H, PCy2), 1.53 (m, 4H, PCy2), 1.14 (m, 12 H,
PCy2). – 13C NMR (75 MHz, CDCl3): d = 137.1 (d, J = 41.2 Hz,
1C, C7) 134.2 (s, 1 C, C3), 131.5 (s, 1 C, C3a), 124.1 (d, J = 25.9 Hz,
1 CH, C6), 122.5 (d, J = 9 Hz, 1 CH. C5), 117.8 (s, 1 CH, C4),
32.4 (d, J = 11 Hz, 2 CH, (CH)-PCy2), 30.9 (d, J = 19 Hz, 2 CH2,
CH2), 29.9 (d, J = 9 Hz, 2 CH2, CH2), 26.7 (d, J = 3.2, Hz, 2 CH2,
CH2), 26.6 (bs, 2 CH2, CH2), 26.2–25.9 (bs, 2 CH2, CH2), 10.4 (s,
CH3). – 31P (161 MHz, CDCl3): d = 6.5. – MS (EI): m/z(%) = 329.2
(35) [M+], 301.2 (33) [M+ - N2], 246. (81) [M+ - Cy], 218.2 (100)
[M+ - N2 - Cy], 137 (66) [M+ - N2 - 2Cy]. – HRMS for C19H28N3P
[M+Li]: calcd. 336.2176; found 336.2215.
7-(Diphenylphosphino)-3-methyl-[1,2,3]triazolo[1,5-a]pyridine
(4a). Starting from 3-methyl-[1,2,3]triazolo[1,5-a] pyridine (3a,
1.0 g, 7.5 mmol) in toluene (50 mL). The crude product was pu-
rified by filtration on silicagel to afford 7-(diphenylphosphino)-3-
methyl-[1,2,3]triazo◦lo[1,5-a]pyridine (4a; 1.0 g, 42%) as a colorless
1
solid. mp 142–144 C. – H NMR (300 MHz, CDCl3): d = 7.60
(d, J = 8.8 Hz, 1 H, H4), 7.45–7.30 (m, 10 H, PPh2), 7.05 (dd, J =
8.8, 6.9 Hz, 1 H, H5), 6.37 (d, J = 6.8 Hz, 1 H, H6), 2.62 (s, 3 H,
CH3). – 13C NMR (75 MHz, CDCl3): d = 138.4 (d, J = 22 Hz, C7)
134.5 (d, J = 1.2 Hz, 1 C, C3), 134.2 (d, J = 21 Hz, 4 CH, o-Ph),
132.6 (d, J = 8,3 Hz, 2 C, ipso-Ph), 131.3 (s, 1 C, C3a), 129.7 (s, 2
CH, p-Ph), 128.84 (d, J = 7.7 Hz, 4 CH, m-Ph), 123.1 (s, CH, C5),
120.8 (s, CH, C6), 117.2 (s, CH, C4), 10.5 (s, CH3). – 31P NMR
(161 MHz, CDCl3): d = -15.4. – MS (EI): m/z(%) = 317.1 (63)
[M+], 288 (100) [M+ - N2], 212.1 (39) [M+ - N2 - Ph], 183.1 (93).
– HRMS for C19H17N3P: calcd. 318.1155; found 318.1145.
7-(Dicyclohexylphosphino)-[1,2,3]triazolo[1,5-a]pyridine (5b).
Starting from [1,2,3]triazolo[1,5-a]pyridine (3b, 0.4 g, 3.4 mmol)
in toluene (17 mL). The crude product was purified by filtration on
silicagel to afford 7-(dicyclohexylphosphino)-[1,2,3]triazolo[1,5-
a]pyridine (5b; 0.6 g, 58%) as a colorless solid. mp 124–126 ◦C.
7-(Diphenylphosphine)-[1,2,3]triazolo[1,5-a]pyridine
(4b).
Starting from [1,2,3]triazolo[1,5-a]pyridine (3b, 0.4 g, 3.4 mmol)
in toluene (17 mL). The crude product was purified by filtration
on silicagel to afford 7-(diphenylphosphine)-[1,2,3]triazolo[1,5-
a]pyridine (4b, 0.5 g, 44%) as a colorless solid. mp 160–162 ◦C. –
1H NMR (300 MHz, CDCl3): d = 7.98 (d, J = 2.4 Hz, 1 H, H3),
7.61 (d, J = 8.8 Hz, 1 H, H4), 7.30–7.21 (m, 10 H, PPh2), 7.01
1
– H NMR (300 MHz, CDCl3): d = 8.07 (s, 1 H, H3), 7.73–7.65
(m, 1 H, H4), 7.21–7.10 (m, 2 H, H5 + H6), 2.72–2.55 (m, 2 H,
CH-PCy2), 2.02–1.90 (m, 2 H, CH2), 1.79–1.70 (m, 2 H, CH2),
1.63–1.51 (m, 4 H, CH2), 1.35, 1.16 (m 10 H, CH2). – 13C NMR
(75 MHz, CDCl3): d = 137.1 (d, J = 41.8 Hz, 1 C, C7), 133.7 (s,
1 C, C3a), 125.6 (s, 1 C, C3), 124.6 (d, J = 25.9 Hz, 1 CH, C6),
124.1(d, J = 8.2 Hz, 1 CH, C5), 118.2 (s, 1 CH, C4), 32.6 (d, J =
11.3 Hz, 2 CH, (CH)-PCy2), 30.9 (d, J = 19.1 Hz, 2 CH2, CH2),
30.0 (d, J = 9.0 Hz, 2 CH2, CH2), 26.7 (d, J = 3.6 Hz, 2 CH2,
CH2), 26.6 (s, 2 CH2, CH2), 26.8 (d, J = 1.1 Hz, 2 CH2, CH2). –
31P NMR (161 MHz, CDCl3): d = 7.0. – MS (EI): m/z(%) = 315.2
(42) [M+], 287.2 (33) [M+ - N2], 232.1 (57) [M+ - Cy], 204.2 (100)
[M+ - N2 - Cy], 151.1 (45) [M+ - 2Cy + 2H], 123 (25) [M+ - N2
- 2Cy + 2H]. – HRMS for C18H26N3P [M + Li]: calcd. 322.2019;
found 322.1975.
(dd, J = 8.8, 6.9 Hz, 1 H, H5), 6.32 (d, J = 6.8 Hz, 1 H, H6). – 13
C
NMR (75 MHz, CDCl3): d = 138.5 (d, J = 22.4 Hz, 1 C, C7) 133.9
(d, J = 21.1 Hz, 4 CH, o-Ph), 133.2 (s, 1 C, C3a), 132.3 (d, J =
8.2 Hz, 2 CH, ipso-Ph), 129.6 (s, 2 CH, p-Ph) 128.7 (d, J = 7.8 Hz,
4 CH, m-Ph), 125.6 (d, J = 1.6 Hz, 1 C, C3), 124.5 (s, 1 CH, C5),
120.8 (s, 1 CH, C6), 117.2 (s, 1 CH, C4). – 31P NMR (161 MHz,
CDCl3): d = -15.0. – MS (EI): m/z(%) = 303.1 (78) [M+], 275.1
(10) [M+ - N2], 274.1 (55) [M+ - N2 - H], 198.1 (23) [M+ - N2 -
Ph], 183.1 (93). – HRMS for C18H14N3P: [M+H+] calcd. 304.0998;
found 304.0969.
7-(Diphenylphosphino)-3-phenyl-[1,2,3]triazolo[1,5-a] pyridine
(4c). Starting from 3-phenyl-[1,2,3]triazolo[1,5-a] pyridine (3c,
0.4 g, 2.1 mmol) in toluene (14 mL). The crude product was
purified by filtration on silicagel to afford 7-(diphenylphosphine)-
[1,2,3]triazolo[1,5-a]pyridine (4c; 1.1 g, 52%) as a yellow oil. – 1H
NMR (300 MHz, CDCl3): 7.98 (m, 3 H, H4 + 3-o-Ph), 7.52–7.33
(m, 13 H, PPh2 + 3-m-Ph + 3-p-Ph), 7.17 (dd, J = 8.8, 6.9 Hz, 1 H,
H5), 6.45 (d, J = 6.9 Hz, 1 H, H6). – 13C NMR (75 MHz, CDCl3):
d = 139.3 (d, J = 23.3 Hz, 1 C, C7), 137.8 (d, J = 1.9 Hz, 1 C, C3),
134.2 (d, J = 21.2 Hz, 4 CH, P(o-Ph)2), 132.4 (d, J = 8.3 Hz, 2 C,
ipso-PPh2), 131.6 (s, 1 C, C3a), 130.2 (s, C, 3-ipso-Ph), 129.8 (s,
2 C, P(p-Ph)2), 128.9 (d, J = 7.8 Hz, 4 CH, P(m-Ph)2), 128.8 (s,
2 CH. 3-o-Ph), 127.7 (s, 1 CH, 3-p-Ph), 126.6 (s, 2 CH, 3-m-Ph),
7-(Dicyclohexylphosphino)-3-phenyl-[1,2,3]triazolo[1,5-a] pyri-
dine (5c). Starting from 3-phenyl-[1,2,3]triazolo[1,5-a] pyridine
(3c, 0.3 g, 1.3 mmol) in toluene (25 mL). The crude product was pu-
rified by filtration on silicagel to afford 7-(dicyclohexylphosphino)-
3-methyl-[1,2,3]triazolo[1,5-a] pyridine (5c; 0.3 g, 52%) as a
◦
1
colorless solid. mp 93–95 C. – H NMR (300 MHz, CDCl3):
d 8.00 (m, 3 H, H4 + 3-o-Ph), 7.51 (J = 7.6, 7.6 Hz, 2 H, 3-m-Ph),
7.37 (m, 1 H, 3-p-Ph), 7.25–7.20 (m, 2 H, H5 + H6), 2.82–2.60 (m,
2 H, CH-PCy2), 2.02–1.91 (m, 2 H, CH2), 1.81–1.70 (m, 3 H, CH2),
1.63–1.51 (m, 5 H, CH2), 1.39–1.04 (m 10 H, CH2). – 13C NMR
(75 MHz, CDCl3): d = 137.8 (s, 1 C, C3), 137.6 (d, J = 44.5 Hz,
1 C, C7), 131.7 (s, 1 C, C3a), 130.7 (s, 1 C, 3-ipso-Ph), 128.9 (s, 2
CH, 3-o-Ph) 127.7 (s, CH, 3-p-Ph), 126.6 (s, 2 CH, 3-m-Ph), 125.1
(d, J = 27.6 Hz, 1 CH, C6), 24.6 (d, J = 8.7 Hz, 1 CH, C5), 118.8
(s, 1 CH, C4), 32.7 (d, J = 27.6 Hz, 2 CH, (CH)-PCy2), 31.1 (d,
J = 19.3 Hz, 2 CH2, CH2), 30.2 (d, J = 9.0 Hz, 2 CH2, CH2), 26.9
(s, 2 CH2, CH2), 26.8 (d, J = 3.5 Hz, 2 CH2, CH2), 26.7 (s, 2 CH2,
CH2), 26.2 (s, 2 CH2, CH2). – 31P NMR (161 MHz, CDCl3): d =
6.5. – MS (EI): m/z(%) = 391.2 (25) [M+], 363.2 (80) [M+ - N2],
280. (100) [M+ - N2 -Cy], 199.1 (90), 146.1 (90). – HRMS for
C24H30N3P [M + O + Li]: calcd. 414.2280; found. 414.2270.
124.9 (s, 1 CH, C5), 120.9 (s, 1 CH, C6), 117.9 (s, 1 CH, C4). – 31
P
NMR (161 MHz, CDCl3): d = -14.6. – MS (EI): m/z(%) = 379.1
(8) [M+], 351.1.1 (37) [M+ - N2], 350.1 (20) [M+ - N2 - H], 185.1
(72) [P(Ph)2], 183.1 (100). – HRMS for C24H18N3P: [M+O+Li+]
calcd. 402.1343; found 402.1316.
7-(Dicyclohexylphosphino)-3-methyl-[1,2,3]triazolo[1,5-a] pyri-
dine (5a). Starting from 3-methyl-[1,2,3]triazolo[1,5-a] pyridine
(3a, 1.0 g, 7.5 mmol) in toluene (50 mL). The crude product was pu-
rified by filtration on silicagel to afford 7-(dicyclohexylphosphino)-
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7-(Di-iso-propylphosphino)-3-methyl-[1,2,3]triazolo[1,5-a] pyri-
dine (6a). Starting from 3-methyl-[1,2,3]triazolo[1,5-a] pyridine
(3a, 1 g, 7.5 mmol) in toluene (17 mL). The crude product was puri-
fied by filtration on silicagel to afford 7-(di-iso-propylphosphino)-
3-methyl-[1,2,3]triazolo[1,5-a]pyridine (6a, 0.9 g, 48%) as a brown
7-(Diphenylphosphino)-6-bromo-3-methyl-[1,2,3]triazolo [1,5-
a]pyridine (8). At 0 ◦C butyllithium (18.2 mmol, 11.6 mL, 1.3 eq)
in hexanes (1.56 M) was added to a solution of di-iso-propylamine
(18.2 mmol, 2.6 mL, 1.3 eq) in tetrahydrofuran (150 mL). Then it
was cooled to -40 ◦C and added to a stirred solution of 6-bromo-
3-methyl-[1,2,3]triazolo[1,5-a]pyridine (7; 3.0 g, 14.1 mmol,
1.0 eq) in tetrahydrofuran (150 mL) at -40 ◦C. The mixture
was kept at -40 ◦C for one hour before diphenylphosphine
chloride (0.9 g, 42 mmol, 1.3 eq) in tetrahydrofuran (5 mL)
was added. After 1 h, the solution was allowed to reach room
temperature. Water (20.0 mL) was added, followed by extraction
with dichloromethane (3 ¥ 20.0 mL). The combined organic
layers were dried over sodium sulfate, filtered, and evaporated.
The crude product was purified by chromatography on silicagel
(ethyl acetate/cyclohexane 1 : 4) to afford 7-(diphenylphosphino)-
6-bromo-3-methyl-[1,2,3]triazolo[1,5-a]pyridine (8; 3.2 g. 57%) as
1
oil. – H NMR (300 MHz, CDCl3): d = 7.60 (d, J = 8.6 Hz,
1 H, H4), 7.21–7.06 (m, 2 H, H5 + H6), 2.97–2.78 (m, 2 H,
P(CH(CH3)2)2), 2.61 (s, 3 H, 3-CH3), 1.21 (dd, J = 16.1, 7.0 Hz, 6
H, P(CH(CH3)2)2), 0.88 (dd, J = 13.2, 7.0 Hz, 6 H, P(CH(CH3)2)2).
–
13C NMR (75 MHz, CDCl3): d = 137.4 (d, J = 41.8 Hz, C7), 134.4
(s, 1 C, C3a), 131.6 (s, 1 C, C3), 124.4 (d, J = 26.1 Hz, 1 CH, C6),
122.7 (d, J = 8.3 Hz, 1 CH, C6), 117.9 (s, 1 CH, C4), 22.6 (d, J =
10.4 Hz, 2 CH, P(CH(CH3)2)2), 20.7 5 (d, J = 14.5 Hz, 2 CH3,
P(CH(CH3)2)2), 20,5 (d, J = 4.6 Hz, 2 CH3, P(CH(CH3)2)2), 10.4
(s, 3-CH3). – 31P NMR (161 MHz, CDCl3): d = 16.1. – MS (EI):
m/z(%) = 249.2 (56) [M+], 221.2 (11) [M+ - N2], 206.1 (26) [M+ -
i
1
iProp], 178.1 (100) [M+ - N2 - Prop], 136.1 (64) [M+ - N2 - 2Cy].
a yellow oil. – H NMR (300 MHz, CDCl3): 7.49–7.40 (m, 4H),
– HRMS for C13H20N3P [M+Li]: calcd. 256.1550; found 256.1519.
7.24 (d, J = 9.22 Hz, 1H, H4), 7.18–7.08 (m, 7H), 2.32 (s, 3H,
CH3). – 13C NMR (75 MHz, CDCl3): d = 135.4 (s, C, C3), 133.9
(d, J = 47.30 Hz, 1 C, C7), 133.3 (d, J = 21.17 Hz, 4 CH, o-Ph),
132.3 (d, J = 10.17 Hz, 2 C, ipso-Ph), 130.62 (d, J = 1.5 Hz, C,
C3a), 129.25 (s, 2 CH, p-Ph), 128.64 (d, J = 3.00 Hz, CH, C5),
128.50 (d, J = 6.96 Hz, 4 CH, m-Ph), 123.71 (d, J = 34.04 Hz,
C, C6), 119.18 (s, CH, C4), 26.99 (s, CH3). 31P NMR (161 MHz,
CDCl3): d = -2.0. – MS (EI): m/z(%) = 397.1 (25) [M+], 368.0
(60) [M+ - N2], 288.1 (80) [M+ - N2 - Br], 183.1 (100). – HRMS
for C19H1579BrN3P: [M + H] calcd. 396.0265 found 396.0261.
C19H1581BrN3P: [M + H] calcd. 398.0245 found 398.0241.
7-(Di-iso-propylphosphino)-[1,2,3]triazolo[1,5-a]pyridine (6b).
Starting from [1,2,3]triazolo[1,5-a]pyridine (3b, 0.4 g, 3.4 mmol)
in toluene (17 mL). The crude product was purified by filtration on
silicagel to afford 7-(di-iso-propylphosphino)-[1,2,3]triazolo[1,5-
a]pyridine (6b; 0.4 g, 50%) as a brown oil. – 1H NMR (300 MHz,
CDCl3): d = 8.02 (d, J = 8.9 Hz, 1 H, H3), 7.71–7.62 (m, 1 H,
H4), 7.17–7.09 (m, 2 H, H5 + H6), 2.77 (m, 2 H, P(CH(CH3)2)2),
1.13 (dd, J = 15.9, 7.0 Hz, 6 H, P(CH(CH3)2)2), 0.81 (dd, J =
13.4, 7.0 Hz, 6 H, P(CH(CH3)2)2). – 13C NMR (75 MHz, CDCl3):
d = 137.5 (d, J = 42.2 Hz, 1 C, C7) 133.6 (s, 1 C, C3a), 125.6 (s, 1
C, C3), 124.3 (d, J = 24.2 Hz, 1 CH, C6), 124.2 (d, J = 7,6.7 Hz,
1 CH, C5), 118.1 (s, 1 CH, C4), 22.5 (d, J = 10.9 Hz, 2 CH,
P(CH(CH3)2)2), 20.4 (d, J = 10.0 Hz, 2 CH3, P(CH(CH3)2)2), 20.2
(s, 2 CH3, P(CH(CH3)2)2). – 31P NMR (161 MHz, CDCl3): d =
15.4. – MS (EI): m/z(%) = 235.2 (49) [M+], 207.2 (21) [M+ - N2],
DFT conformational study on isomerism
Geometries of the minima were fully optimised at the B3LYP
theoretical level with the 6-31G** basis set as implemented in
the Gaussian 03 program.12 Harmonic frequency calculations
verified the nature of the stationary points as minima (all real
frequencies). The scanning of the rotation was performed using
the IRC type calculation implemented in the Gaussian 03 program
at the same level. 1H shieldings of compounds have been calculated
over the fully optimized geometries within the GIAO14 (Gauge-
Independent Atomic Orbital) approximation.
i
i
192.1 (100) [M+ - Prop], 164.1 (39) [M+ - N2 - Prop], 149.1 (85)
[M+- 2iProp], 122.1 (42) [M+ - N2 - 2iProp + H]. – HRMS for
C12H18N3P [M + Li]: calcd. 242.1393; found 242.1371.
7-(Di-iso-propylphosphino)-3-phenyl-[1,2,3]triazolo[1,5-a] pyri-
dine (6c). Starting from 3-phenyl-[1,2,3]triazolo[1,5-a] pyridine
(3c, 0.4 g, 2.1 mmol) in toluene (14 mL). The crude prod-
uct was purified by filtration on silicagel to afford 7-(di-
iso-propylphosphino)-3-phenyl-[1,2,3]triazolo[1,5-a]pyridine (6c;
X-ray structure determination
CCDC 799866 (4a), 799867 (4b), 799868 (4c), 799869 (5a) and
799870 (5b) contain the supplementary crystallographic data for
this paper.†
1
0.3 g, 23%) as a brown oil. – H NMR (300 MHz, CDCl3): d =
7.95–7.85 (m, 3 H, H4 + 3-o-Ph), 7.46–7.39 (m, 2 H, 3-m-Ph), 7.34–
7.26 (m, 1 H, 3-p-Ph), 7.19–7.13 (m, 2 H, H5 + H6), 2.85 (m, 2 H,
P(CH(CH3)2)2), 1.17 (dd, J = 16.1, 7.0 Hz, 6 H, P(CH(CH3)2)2),
0.84 (dd, J = 13.5, 7.0 Hz, 6 H, P(CH(CH3)2)2). – 13C NMR
(75 MHz, CDCl3): d = 138.2 (d, J = 43.6 Hz, 1 C, C7), 137.9 (s,
1 C, 3-ipso-Ph), 131.6 (s, 1 C, C3a), 130.6 (s, 1 C, C3), 128.9 (s,
2 CH, 3-o-Ph), 127.8 (s, 1 CH, 3-p-Ph), 126.7 (s, 2 CH, 3-m-Ph),
124.7 (d, J = 26.2 Hz, 1 CH, C6), 124.6 (d, J = 8.3 Hz, 1 CH, C5),
118.8 (s, 1 CH, C4), 22.8 (d, J = 10.7 Hz, 2 CH, P(CH(CH3)2)2),
20.7 (d, J = 7. 9 Hz, 2 CH3, P(CH(CH3)2)2), 20.5 (d, J = 1.7 Hz,
2 CH3, P(CH(CH3)2)2). – 31P NMR (161 MHz, CDCl3): d = 17.3.
– MS (EI): m/z(%) = 311.2 (28) [M+], 283.2 (58) [M+ - N2], 240.1
Acknowledgements
We are indebted to Dr Lydia Brelot (Service de Radiocristal-
lographie, University of Strasbourg, France) for her assistance
with the single crystal structure elucidation. This work was
carried out with financial support from the Ministe`re de la
Recherche (France), from the CNRS (France), from the Ministerio
de Educacio´n y Ciencia (Project No. CTQ2007-61901/BQU
and CTQ2006-15672-C05-03), and the Comunidad Auto´noma
de Madrid (Project MADRISOLAR, ref. S-0505/PPQ/0225).
Thanks are given to the CTI (CSIC) and CESGA for allocation
of computer time. RBG is much indebted to the Ministe`re de
i
(55) [M+ - N2 - Prop], 198.1 (100) [HM+-N2 - 2iProp], 167.1 (51)
[M+ - N2 - 2iProp + H]. – HRMS for C18H22N3P [M+Li]: calcd.
´
318.1716; found 318.1721.
l’Education Nationale, de la Recherche et de la Technologie
1394 | Dalton Trans., 2011, 40, 1387–1395
This journal is The Royal Society of Chemistry 2011
©

View Article Online
(France) for a doctoral fellowship. LB thanks LONZA AG
(Switzerland) for a PhD grant.
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