2-Pyridyl-2-phospholenes: New P,N ligands for the palladium-catalyzed isoprene telomerization

Article abstract of DOI:10.1016/j.jcat.2006.01.010

2-(2-Pyridyl)-2-phospholenes are efficient ligands for the palladium-catalyzed telomerization of isoprene with diethylamine. Good selectivities for the tail-to-head or tail-to-tail aminoterpenes are achieved on optimization of the ligand substituents and

Full text of DOI:10.1016/j.jcat.2006.01.010

Journal of Catalysis 238 (2006) 425–429
www.elsevier.com/locate/jcat
2-Pyridyl-2-phospholenes: New P,N ligands for the palladium-catalyzed
isoprene telomerization
François Leca, Régis Réau ∗
UMR 6509 CNRS-Université de Rennes 1, Institut de Chimie, Campus de Beaulieu, 35042 Rennes cedex, France
Received 25 October 2005; revised 5 January 2006; accepted 6 January 2006
Available online 30 January 2006
Abstract
2-(2-Pyridyl)-2-phospholenes are efficient ligands for the palladium-catalyzed telomerization of isoprene with diethylamine. Good selectivities
for the tail-to-head or tail-to-tail aminoterpenes are achieved on optimization of the ligand substituents and of the reaction conditions, such as the
nature of the catalyst precursor and of the solvent. Moreover, excellent catalytic activities are achieved on photochemical activation or addition of
acidic co-catalysts.
© 2006 Elsevier Inc. All rights reserved.
Keywords: Telomerization; Isoprene; Phospholene; P,N ligands; Palladium; Homogeneous catalysis; Photochemical reactions
1. Introduction
We have recently reported the synthesis of the first family
of P,N-chelates bearing a 2-phospholene moiety [12,13]. The
fact that the electronic properties of this P-ring are different
from those of the well-known phosphole or phospholane het-
erocycles prompted us to evaluate these novel P,N-donors for
palladium-catalyzed isoprene telomerization. In this paper we
present a detailed study including the optimization of the cat-
alytic parameters and variation of the ligand structure. We also
describe an unprecedented activation of the catalytic system on
photochemical activation.
Telomerization allows the synthesis of functionalized oligo-
mers from dienes and a nucleophile in a 100% atom efficient
manner. The first publication describing the synthesis of func-
tionalized linear octadienes via telomerization of 1,3-butadiene
with nucleophiles appeared in 1967 [1,2]. Since then, many
catalytic systems have been studied; the most efficient are pal-
ladium complexes bearing phosphine or carbene ligands [3–7].
The telomerization of other dienes has been poorly investigated
despite the great interest in the resulting telomers. For example,
with isoprene, natural products, such as functionalized C10 ter-
penes, could be synthesized. However, due to the nonsymmetri-
cal structure of the diene, four isomers can be formed, of which
only the head-to-tail telomer 4 is a natural terpene (Fig. 1).
The nature of the Pd ligand (e.g., mono- or di-phosphines, P,N-
donors) can influence the product distribution; however, none
of them allows good selectivities for the target compound 4
[8–11]. For instance, in most cases palladium/phosphine com-
plexes give the tail-to-tail 1 or tail-to-head 3 products (Fig. 1).
Hence the search for new ligands able to induce a good selectiv-
ity in one of the telomers 1–4 remains a challenge in this field.
2. Results and discussion
We have recently reported that (2-pyridyl)-2-phospholenes
8a–c can be obtained from the corresponding (2-pyridyl)phos-
pholes 5a–c via a three-step process involving: (i) coordina-
tion of 5a–c to a Pd(II)-center, (ii) addition of a base such
as pyridine, and (iii) release of 8a–c on addition of 1,2-
diphenylphosphinoethane (dppe) (Scheme 1) [12,13]. The role
of the base is to catalyze the [1,3]-H shift via the formation of
an allylic anion. Note that this step is very slow, requiring a
large excess of pyridine and long reaction times (ca. 2–3 days)
in refluxing CH2Cl2 [12,13].
*
Because we wanted to investigate the Pd-catalyzed telo-
merization of isoprene with diethylamine, it was of interest
Corresponding author. Fax: +33 (0)223236939.
E-mail address: regis.reau@univ-rennes1.fr (R. Réau).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.01.010

426
F. Leca, R. Réau / Journal of Catalysis 238 (2006) 425–429
Fig. 1. Products of telomerization of isoprene with nucleophiles.
Scheme 1. Synthesis of 2-pyridyl-2-phospholenes 8a–c.
Scheme 2. Reaction between diethylamine and Pd(II) complex 6a.
to know whether this base can also promote the isomeriza-
tion process depicted in Scheme 1. Indeed, in presence of one
equivalent of diethylamine, complex 6a gives quantitatively
the corresponding [2-pyridyl-2-phospholene]Pd(II) complex 7a
within 1 h at room temperature in dichloromethane (Scheme 2).
Hence diethylamine is by far the most efficient base for promot-
ing phosphole–phospholene isomerization. This result suggests
that phosphole-complexes 6 could be used as precursors of
the corresponding phospholene complexes 7 in the isoprene-
diethylamine telomerization. This is an appealing possibility
avoiding the stepwise preparation of 2-pyridylphospholene lig-
ands (Scheme 1). To check this hypothesis, the performance
of the 2-pyridylphospholes 5a,b and of their 2-pyridyl-2-
phospholene isomers 8a,b (Scheme 1) in the Pd-catalyzed
telomerization were compared. The reactions were conducted
at 40 ◦C over 48 h. The catalytic systems are formed in situ
by reacting (CH3CN)2PdCl2 with one equivalent of the P,N-
ligands.
Table 1
Activities and selectivities of complexes (5a,b)PdCl and (8a,b)PdCl . See
Scheme 1 for structure numbering
2
2
Entry Ligand Solvent Yield (%) 1 (%) 2 (%) 3 (%) 4 (%)
1
2
3
4
5a
8a
5b
8b
CH CN 54
40
43.5
41
21
26
10
7
26.5
23.5
27
12.5
7
22
17
3
CH CN 48
3
CH CN 40
3
CH CN 38
40
36
3
◦
General conditions: 0.025 mmol of ligand and metallic precursor, 40 C, 48 h.
Yields are determined by GC.
complex bearing a 2-pyridyl-2-phospholenes ligand. Hence it
is possible to conduct this study using 2-pyridylphospholes,
which are easily available on a gram scale, as precursors of
2-pyridylphospholenes.
The nature of the pyridylphospholene ligand influences the
catalytic activity and product distribution. The yields are higher
with di(2-pyridyl)phospholene 8a (48%) than with (2-pyridyl)-
(2-thienyl)phospholene 8b (38%). With this latter ligand, the
selectivity for the head-to-tail telomere 4 is higher (Table 1, en-
tries 2 and 4), although in both cases the major product is the
derivative 1. The next step was to optimize the reaction condi-
tions (i.e., solvent, metallic salts, temperature) to improve both
the catalytic activity and the selectivity. In a first round of ex-
The yields and selectivities obtained with the phosphole
ligands 5a and 5b are very similar to those recorded with
their phospholene analogues 8a and 8b (Table 1). These
data confirm that under the catalytic reaction conditions, iso-
merization is rapid and the catalytic species is a palladium

F. Leca, R. Réau / Journal of Catalysis 238 (2006) 425–429
427
Table 2
Table 3
Effect of solvent and temperature on yields and product distribution using cat-
Catalytic experiments conducted with complexes 9
alytic precursor 6a
−
Entry
X
Solvent
Yield (%) 1 (%) 2 (%) 3 (%) 4 (%)
Entry
Solvent
Yield (%)
1 (%)
2 (%)
3 (%)
4 (%)
−
9
10
11
12
13
14
Cl
Cl
CH CN
3
THF
4
20
3
48
1
57
2.5
38
32
64
0
5.0
32
3.5
0
27
43
92.5
19
37
36
0
0
12
27
0
−
5
6
7
8
CH CN
3
THF
54
8
15
16
40
5
46
58
20.5
3
12
0
27
92
34
42
12.5
−
−
0
8
0
BF
4
BF
4
CH CN
3
a
CH CN
THF
THF
THF
3
−
CH CN/THF
SbF
OTf
3
6
−
7
43.5
29.5
0
◦
General conditions: 0.025 mmol of ligand and metallic precursor, 40 C, 48 h.
Yields are determined by GC, T = 60 C, 48 h.
◦
◦
General conditions: 0.025 mmol of 9, 40 C, 48 h. Yields are determined
by GC.
−
or BF4 as the counter-anion, the allylic palladium complex 9
gave a very low yield in CH3CN (3–4%; Table 3) compared
with the PdCl2-complex 6a (54%). However, the catalytic ac-
tivity increased when using THF (Table 3, entries 9–12). In this
solvent, the nature of the counter-anion strongly influences the
yields (Table 3, entries 10–14), as has been observed for allylic
palladium complexes bearing carbene ligands [7].
+
−
complexes 9.
Fig. 2. [(5a)Pd(allyl)]
X
The counter-anion has a dramatic influence on the product
distribution. With Cl−, the tail-to-tail telomer 3 is the major
product (Table 3, entry 10), as observed using the PdCl2 pre-
cursor 6a. Very interestingly, with BF4−, a rather high amount
of the natural isomer 4 was obtained (Table 3, entry 12). This
result is promising, because it can be expected that further vari-
ations of the ligand structure will allow an increase in this se-
lectivity.
periments, ligand 8a, which is the most efficient, was used. Its
phosphole precursor 5a was reacted with (CH3CN)2PdCl2 in
dichloromethane for 1 h, affording complex 6a (Scheme 1).
Then dichloromethane was replaced by the desired solvent. The
influence of several solvents was evaluated for a temperature
of 40 ◦C for a reaction time of 48 h. No reaction occurs using
methanol, toluene, or dichloromethane as solvent. The forma-
tion of telomers is observed only in a coordinating solvent, such
as THF and CH3CN. The yield decreases considerably when
using THF instead of CH3CN (Table 2, entries 5 and 6); how-
ever, the catalytic system is more selective in THF. It gives the
tail-to-tail telomer 3 as the major product, whereas in CH3CN
a mixture of the four telomers is isolated (Table 2, entry 5). The
nature of the coordinated solvent plays thus a crucial role in ac-
tivity and selectivity. This is nicely illustrated by the fact that
when an equivolume of THF and CH3CN is used, the yield is
still low, and the main products are the isomers 1 and 3 (Ta-
ble 2, entry 8). Note that increasing the temperature results in
lowering the yield, with the main products remaining almost
similar. A catalytic run was conducted in ionic liquid medium
using a cationic complex and butyl-methyl-imidazolium salts.
The yield was about 15% after 48 h without profound varia-
tion of the selectivity. These experiments clearly show that only
coordinated solvents allow good activity, and that the best tem-
perature for this reaction is 40 ◦C.
In this preliminary study with ligand 5a, we have shown the
importance of the solvent and of the counter-anion depending
on the metallic precursor used. With PdCl2(CH3CN)2, the use
of a coordinated solvent, such as THF or acetonitrile, is re-
quired, whereas with the allylic palladium complex 9, Cl− and
−
BF4 counter-anions are needed.
In these optimized conditions, we evaluated the behavior
of different ligands in which electronic and steric properties
were been varied by modifying the substitution pattern of the
phosphole ring. Ligands 5b, 5aꢀ, and 10 (Fig. 3) were se-
lected as phospholene precursors. The phosphorus substituent
(phenyl vs. cyclohexyl) will directly influence both the steric
and electronic properties of the P-donor atom, whereas the sec-
ond substituent at the C5-atom of the phosphole ring (2-pyridyl
versus 2-thienyl) is expected to influence the electronic prop-
erties of the N-donor atom via the extended π-conjugated sys-
tem [14,15]. Finally, the size of the fused saturated carbocycle
(six-membered vs. five-membered) is expected to influence the
steric bulk of the ligands. We have previously reported that
these variations can modify the behavior of Pd complexes dur-
ing catalytic processes [16].
The final parameter studied was the nature of the metallic
salts. The (5a)NiBr2, (5a)PtCl2, and (5a)RuCpCl complexes
are totally inactive in the conditions described below. With Cl−
Fig. 3. 2-Pyridylphosphole precursors of phospholene ligands tested in telomerization.

428
F. Leca, R. Réau / Journal of Catalysis 238 (2006) 425–429
Table 4
Table 6
Reaction in presence of ligand precursors 5b and 10
Influence of addition of an acidic co-catalyst
Entry Ligand Metal
Solvent Yield
(%)
1
2
3
4
Entry
Co-catalyst
CH CO H
Yield (%)
1 (%)
2 (%)
3 (%)
4 (%)
(%) (%) (%) (%)
26
27
90
35
52
100
16
0
32
0
0
0
3
2
16
17
18
19
20
21
22
23
5b
5b
5b
10
10
10
10
10
PdCl (CH CN)
CH CN 40
3
41 10 27 22
12
0
43 20 22 15
25
5
BF ·OEt
2
3
2
2
2
2
2
3
2
PdCl (CH CN)
THF
THF
4
7
3
7
85
93
0
0
2
3
2+
−
General conditions: 0.025 mmol of ligand 5a and [Pd(CH CN) ] 2BF
20 eq. of acidic co-catalyst. T = 40 C, 48 h.
,
3
4
4
PdCl (CH CN)
2
3
◦
PdCl (CH CN)
CH CN 65
2
3
3
PdCl (CH CN)
THF
THF
THF
THF
5
8
11
6
3
6
7
0
72
89
88
90
0
0
0
0
2
3
Pd(allyl)Cl
Pd(allyl)Cl
Table 7
Catalysis under UV with 5a
a
5
10
a
PdCl (CH CN)
2
3
2
Entry Metal
Solvent Yield (%) 1 (%) 2 (%) 3 (%) 4 (%)
◦
General conditions: 0.025 mmol of ligand and metallic precursor, 40 C, 48 h.
Yields are determined by GC.
2+
−
−
28
29
30
31
Pd(CH CN)
2BF
THF
DCM
91
8
46 20
18
0
16
17
0
0
3
4
4
4
PdCl (CH CN)
100
0
40 43
38.5 21
a
◦
2
3
2
T = 60 C, 48 h.
2+
2+
Pd(CH CN)
Pd(CH CN)
2BF
2SbF
CH CN 30
3
4
4
3
−
THF
75
22.5 18
3
6
Table 5
Catalytic reaction using [(5a)Pd(CH CN)] 2BF
2+
−
2+
−
4
General condition: 0.025 mmol of ligand 5a and [Pd(CH CN) ] 2BF , 48 h
under UV lamp.
3
4
4
3
Entry Complex
Yield (%) 1 (%) 2 (%) 3 (%) 4 (%)
2+
−
−
24
25
[(5a)Pd(CH CN)] 2BF
30
82
51
31
0
13
49
23
0
33
3
4
4
(20 eq.) in the reaction mixture was evaluated (Table 6). Adding
acetic acid, the yield reached 90%, and the main product was
telomer 1 (Table 6, entry 26). But the most interesting result was
obtained with BF3·OEt2. With this Lewis acid, only one telomer
was formed (Table 6, entry 27). These results clearly show a
considerably enhanced yield and selectivity using an acidic co-
catalyst, in agreement with results reported by Keim [17].
Finally, we were able to develop a catalytic system for
isoprene telomerization under photochemical activation. This
work was motivated by the fact that the 2-pyridylphosphole lig-
ands are chromophores exhibiting broad absorption in the UV–
vis range [15]. Photoinduced electron transfer could drive some
catalytic reaction in the solid state [18,19] or could be used
for the synthesis of complex molecules [20,21]. Recently, Bach
et al. described efficient organocatalytic enantioselective reac-
tions performed under photochemical conditions, suggesting
that photochemical routes also could be used in catalysis [22].
In our case, we expected that on irradiation, activation of the Pd-
complexes could occur. Indeed, under UV irradiation, the yield
obtained with complex [(5a)Pd(CH3CN)2]2+2BF4− (91%) was
far superior to that recorded without irradiation (8%). Note that
under irradiation with [Pd(CH3CN)4]2+2BF4−, the yield was
very low (ca. 3%). The results reported in Table 7 show that
the catalytic system was always more active under UV–vis acti-
vation than under thermal activation. The best catalytic system
was obtained in THF at 40 ◦C in 48 h (91% yield, compared
with a 30% yield under thermal activation) with a selectivity in
favor of 1 (46%, entry 28). Activities dropped when acetonitrile
or another counter-anion was used in the same conditions (en-
tries 30 and 31). To the best of our knowledge, this is the first
report of a photochemical activation of a catalytic system for
telomerization.
a
2+
[(5a)Pd(CH CN)] 2BF
3
◦
General conditions: 0.025 mmol of ligand and metallic precursor, 40 C, 48 h.
Yields are determined by GC.
a
◦
T = 40 C, 7 days.
In the same reaction conditions, the yields obtained with 5b
(Table 4, entries 16–18) are lower than those observed with
di(2-pyridyl)phosphole 5a (ca. 54%). The selectivity is similar
for both systems. Moreover, no catalytic activity has been de-
tected with the complex [(5b)Pd(allyl)]+Cl−. Experiments with
5aꢀ showed yields down to 3%. These results can be explained
by the presence of a bulky substituent on the phosphorus atom,
which disfavors the coordination of isoprene on metallic center
or the diethylamine nucleophilic attack.
Ligand 5a, bearing two 2-pyridyl moieties and one phenyl
fragment on a P atom, is the most efficient ligand in the pres-
ence of palladium salts for telomerization of isoprene. Inter-
estingly, the yield is further increased using ligand 10 (65 vs.
54% for 5a) in acetonitrile. Unfortunately, the four telomers are
formed, with 1 being the major product (43%). Variations of
solvent or metallic precursors do not allow an increase in selec-
tivity for one of these telomers.
To avoid the abstraction step of the chlorine on the PdCl2
complex, which can be a difficult step, we decided to use an-
other palladium salt, namely [Pd(CH3CN)4]2+2[BF4−], which
contains four labile acetonitrile ligands. In each experiment,
only one equivalent of ligand 5a was added to [Pd(CH3CN)4]2+
2[BF4−]. In THF, the yield reached 30% after 48 h, and only
two telomers, 1 and 3, are formed in equal amounts. If the
reaction mixture were stirred during 7 days, then the yield
would increase to 82%, but the product distribution would be
completely different. The four isomers 1–4 are produced; the
head-to-tail telomer 4 is the major product (Table 5, entry 25).
In conclusion, this new palladium precursor affords active but
nonselective catalysts.
3. Conclusion
We have described new and efficient catalysts bearing
2-pyridyl-2-phospholenes ligands for the telomerization of
isoprene and diethylamine. We have demonstrated that di-
ethylamine can induce a rapid quantitative isomerization of
The addition of acidic co-catalysts to the reaction media
could strongly drive variations in activity and selectivity [17].
Thus, the influence of acetic acid (20 eq.) and BF3·OEt2

F. Leca, R. Réau / Journal of Catalysis 238 (2006) 425–429
429
2-pyridylphospholes in 2-pyridyl-2-phospholenes under mild
conditions in the coordination sphere of palladium. Catalytic
studies show the crucial role played by the solvent and metallic
salts. Coordinating solvents were needed to obtain good yields,
generally directed toward telomer 1 (tail-to-head coupling) and
3 (tail-to-tail coupling). Unfortunately, telomer 4 (the natural
product) was obtained at only 32% in the presence of complex
[(5a)Pd(CH3CN)2]2+2BF4−. High activity could be reached in
the presence of acidic cocatalyst, such as acetic acid. In the
same way, the use of BF3·OEt2 enables achievement of high
selectivity in 1. We have shown the first UV-activation of a cat-
alytic system for this reaction. This latter feature holds much
promise, because the ligand will influence the catalytic cen-
ter not only by its steric and electronic properties, but also by
the nature of the ligand–metal charge transfer. Complementary
research is in progress to further explore this unique catalytic
system based on ligands that are also chromophores.
(0.08 g, 0.45 mmol) in dichloromethane (3 mL). Then one
equivalent of silver tetrafluoroborate AgBF4 was added to the
reaction mixture. The solution was filtered on Celite, and the
solvent was removed. The product was washed with diethyl
ether (2×5 mL), affording 9 as an air-stable yellow solid (yield,
1
0.51 g, 0.86 mmol, 96%). H NMR (200 MHz, CDCl3): δ =
1.83 (m, 6H; CH2 and syn-H allyl), 2.92 (d, 3JH–H = 18.8 Hz,
2H; anti-H allyl), 3.36 (d, 3JH–H = 12.5 Hz, 2H; anti-H allyl),
4.24 (m, 4H; CH2), 5.9 (broad s, 1H; H allyl), 7.26–7.34 (m,
6H; arom-H Ph and H5 Py), 7.41–7.58 (m, 3H; H3 Py), 7.88
(t, 3JH–H = 7.4 Hz, 2H; H4 Py), 8,67 (d, 3JH–H = 4.8 Hz, 2H;
H6 Py). 13C{1H} NMR (50.323 MHz, CDCl3): δ = 22.5 (s,
C=CCH2CH2), 22.3 (d, JP–C = 2.4 Hz, C=CCH2), 52.4 (s,
CH=CH2), 123.4 (s, C5 Py), 124.3 (d, JP–C = 6.73 Hz, C3 Py),
126.9 (d, JP–C = 43.0 Hz, ipso-C Ph), 129.5 (d, JP–C = 11.4 Hz,
m-C Ph), 131.7 (s, CH2CH allyl), 131.8 (s, p-C Ph), 133.7
(d, JP–C = 13.5 Hz, o-C Ph), 138.6 (s, C4 Py), 140.0 (d,
JP–C = 47.6 Hz, PCα=C), 151.1 (d, JP–C = 11.0 Hz, C2 Py),
152.7 (s, C6 Py), 153.1 (d, JP–C = 21.8 Hz, PC=Cβ). 31P{1H}
NMR (81.014 MHz, CDCl3): δ = +49.7; HR–MS (ESI) m/z:
515.0879 [M+]. Calcd. for C27H26N2PPdBF4: 515.0877; el-
emental analysis (%) calcd. for C27H26N2PPdBF4(515.0879):
C 53.81, H 4.35, N 4.65; found: C 53.85, H 4.32, N 4.62.
4. Experimental
4.1. General remarks
All experiments were performed under an atmosphere of
dry argon using standard Schlenk techniques. Commercially
available reagents were used as received without further pu-
rification. Solvents were freshly distilled under argon from
sodium/benzophenone (tetrahydrofuran, diethylether, toluene)
or from phosphorus pentoxide (dichloromethane, acetonitrile).
1H, 13C, and 31P NMR spectra were recorded on Bruker model
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4.2. Standard procedure for isoprene telomerization
A solution of 0.025 mmol of the ligand in dichloromethane
was added to a solution of the metallic precursor (0.025 mmol)
in dichloromethane. The mixture was stirred for 2 h, after which
the solvent was removed. Then the solvent used for the catalytic
test was added. After 30 min of stirring, an excess of isoprene
(0.5 mL) and diethylamine (0.5 mL) were injected into the re-
action mixture. The reaction was stirred for 48 h at 40 ◦C, after
which flash chromatography was done to stop the reaction and
the liquid filtrate was injected in GC–MS.
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