Efficient bulky phosphines for the selective telomerization of 1,3-butadiene with methanol

Article abstract of DOI:10.1021/ja100521m

A series of bulky phosphines containing substituted biphenyl, 2-methylnaphthyl, or 2,7-di-tert-butyl-9,9-dimethylxanthene moiety were prepared. They were used in the preparation of new monophosphine-palladium(0)- dvds complexes, which were employed as catalysts for the selective telomerization of 1,3-butadiene with methanol to obtain 1-methoxyocta-2,7-diene (1-MOD), the key intermediate in the Dow 1-octene process. Several ligands showed improved selectivity and yield compared to that of the benchmark ligand PPh₃. Especially 2,7-di-tert-butyl-9,9-dimethylxanthen-4-yl- diphenylphosphine (4, mono-xantphos ) stands out as an excellent ligand in terms of yield, selectivity, and stability.

Full text of DOI:10.1021/ja100521m

Published on Web 04/20/2010
Efficient Bulky Phosphines for the Selective Telomerization of
1,3-Butadiene with Methanol
Mathieu J.-L. Tschan,† Eduardo J. Garc´ıa-Sua´rez,† Zoraida Freixa,†
He´le`ne Launay,‡ Henk Hagen,‡ Jordi Benet-Buchholz,† and Piet W. N. M. van
Leeuwen*,†
Institute of Chemical Research of Catalonia (ICIQ), AVinguda Pa¨ısos Catalans, 16, 43007
Tarragona, Spain, and Dow Benelux B.V., Herbert H. Dowweg 5, P.O. Box 48,
4530 AA Terneuzen, The Netherlands
Received January 20, 2010; E-mail: pvanleeuwen@iciq.es
Abstract: A series of bulky phosphines containing substituted biphenyl, 2-methylnaphthyl, or 2,7-di-tert-
butyl-9,9-dimethylxanthene moiety were prepared. They were used in the preparation of new monophos-
phine-palladium(0)-dvds complexes, which were employed as catalysts for the selective telomerization
of 1,3-butadiene with methanol to obtain 1-methoxyocta-2,7-diene (1-MOD), the key intermediate in the
Dow 1-octene process. Several ligands showed improved selectivity and yield compared to that of the
benchmark ligand PPh3. Especially 2,7-di-tert-butyl-9,9-dimethylxanthen-4-yl-diphenylphosphine (4, “mono-
xantphos”) stands out as an excellent ligand in terms of yield, selectivity, and stability.
Introduction
being considered for on-purpose 1-octene production: ethene
tetramerization, the Sasol process based on hydroformylation
Selective synthesis of linear R-olefins [C6-C20] gained
considerable interest in both industry and academia due to the
high demand in diverse markets such as polyolefins, synthetic
lubricants, plasticizers, detergent intermediates, etc. Linear
R-olefins, especially 1-hexene and 1-octene, are key components
for the production of LLDPE and the demand for 1-hexene and
1-octene has increased enormously in recent years.1 Thus,
several processes have been developed in the past decades to
produce 1-hexene and 1-octene selectively.
In the late 1960s, Keim at Shell discovered a homogeneous
phosphine [P,O] nickel catalyst that oligomerizes ethene selec-
tively to higher homologues. This process is known as SHOP
(Shell higher olefin process), but as a result of the Schulz-Flory
distribution of the product formed, 1-hexene and 1-octene are
produced in limited amounts.2 Later on, other type of ligands
such as diimine [N,N]3 or [N,O]4 and [P,N]5 were also studied
for this reaction.
of 1-heptene, and the Dow process based on the telomerization
of 1,3-butadiene.
Ethene Trimerization and Tetramerization. 1-Hexene can be
obtained by ethene trimerization, and to this end, industrial
groups such as Phillips7 or academic ones developed a family
of catalysts that trimerize ethene to 1-hexene with high
selectivity, based mainly on Cr8 but also on Ti9 and Ta.10 To
achieve such high selectivity to 1-hexene, a fundamentally
different chemical pathway must be at work compared to “linear
growth” oligomerization reactions.11 The key difference between
this catalyst system and conventional oligomerization catalyst
is the propensity of the chromium-based catalyst to form seven-
membered ring metallacycles.12
Until recently, it was thought unlikely that further ethene
insertion to form metallacyclononane, which should eliminate
selectively 1-octene, would take place.13 However, researchers
Sasol’s coal-based high-temperature Fischer-Tropsch tech-
nology produces an Anderson-Schulz-Flory distribution of
hydrocarbons with high R-olefin content, and the desired olefins
are separated by distillation.6 Nowadays other technologies are
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W.; Mason, R. F. U.S. patent 3,647,914 (to Shell), 1972. (e) Bauer,
R.; Glockner, P. W.; Keim, W.; van Zwet, H.; Chung, H. U.S. patent
3,644,563 (to Shell), 1972. (f) Bauer, R.; Glockner, P. W.; Keim, W.;
Mason, R. F. U.S. patent 3,647,915 (to Shell), 1972. (g) Mason, R. F.
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† Institute of Chemical Research of Catalonia.
‡ Dow Benelux B.V.
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10.1021/ja100521m  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 6463–6473 6463

A R T I C L E S
Tschan et al.
2,7-octadienes from simple starting materials is a clear target.18
1,3-Butadiene and MeOH are attractive feedstocks because of
their ample availability and low price. The reaction leads to
Scheme 1. Sasol Process for the Production of 1-Octene from
1-Heptene
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that do indeed allow the formation of nine-membered metalla-
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Octanol Process by Sasol.
15
The chemistry of converting
1-heptene to 1-octene is shown in Scheme 1. To transform
1-heptene to 1-octene, three steps are required: (1) hydroformy-
lation of 1-heptene to octanal, (2) hydrogenation of octanal to
1-octanol, and (3) dehydration of 1-octanol to 1-octene. This
process is commercial.
Dow Process. The commercial route to produce 1-octene
based on butadiene as developed by Dow Chemical is presented
in Scheme 2.16 It came on stream in Tarragona in 2008. The
telomerization of butadiene with methanol in the presence of a
palladium catalyst yields 1-methoxy-2,7-octadiene,17 which is
fully hydrogenated to 1-methoxyoctane in the next step.
Subsequent cracking of 1-methoxyoctane gives 1-octene and
methanol for recycle.
The control of the selectivity and optimization of the reaction
is crucial in terms of the development of greener processes, and
efforts on catalyst design and/or tuning of the reaction param-
eters aim at these targets.
Thus, the palladium-catalyzed telomerization of 1,3-diene
with nucleophiles giving with 100% atom-efficiency substituted
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6464 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Phospines for Selective Telomerization of 1,3-Butadiene
A R T I C L E S
Scheme 2. Simplified View of the Dow Process
Scheme 3. Telomerization of 1,3-Butadiene with Methanol to Produce 1-Methoxy-2,7-octadiene (1-MOD)
Scheme 4. Extended Mechanism for the Telomerization of 1,3-Butadiene
the formation of 1-methoxy-2,7-octadiene (1-MOD) (Scheme
3), which is for instance a useful precursor for 1-octene.19 Main
byproduct of the reaction are the 3-methoxyocta-1,7-diene (3-
MOD), 1,3,7-octatriene (OCT), and less importantly 4-vinyl-
cyclohexene (VCH) formed by a Diels-Alder reaction of two
molecules of 1,3-butadiene.20
As a consequence of the increasing industrial use of 1-octene
as a co-monomer for polyethylene,21 the telomerization process
has been intensively studied by many industrial and academic
laboratories.22 The important contribution of Beller’s group in
understanding and optimizing the reaction has been reviewed
recently18h and has led to the development of the most active
and productive catalyst, a (NHC)Pd(0) complex, achieving
TONs up to 1,500,000.23d,e
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9
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A R T I C L E S
Tschan et al.
Figure 1. Series of bulky phosphines.
The mechanism of the reaction has been carefully studied
by Jolly,24 but factors explaining the changes in chemo- and
regioselectivity remain unraveled. Recent studies from Beller
et al. performed on the PPh3-based system showed that
increasing the L/Pd ratio led to systems with high productivity
(high TON) due to stabilization of the Pd(0) species, although
it had a detrimental effect on selectivity. Mechanistic investiga-
tions indicated that a palladium complex containing two
phosphines is responsible for the formation of the (undesired)
branched telomers. The formation of this palladium-bisphos-
phine species is enhanced by higher L/metal ratios.23h The
extended reaction scheme as proposed by Beller, based on
Jolly’s original one, is depicted in Scheme 4.
As pointed out by Beller et al., monoligated palladium
complexes are highly selective23d,f,h compared to catalysts
containing two ligands. Additionally, high L/Pd ratios are
required to increase TONs to guarantee metal stabilization.
Phosphine is lost via catalytic oxidation during the course of
the reaction and has to be replenished to avoid palladium
aggregation and metal precipitation.
To fulfill both (apparently contradictory) demands, we
considered the use of sterically demanding phosphine ligands.
They should be able to stabilize the complex toward palladium
aggregation and avoid or diminish coordination of a second
ligand due to repulsive steric interactions.
There are some examples in the literature of sterically
hindered phosphines that already serve this purpose, for instance,
in cross-coupling catalysis, also using palladium.25 Together
with newly designed ligands, we tested some of the known bulky
ligands for comparative purposes.
Concerning the metal precursor, palladium(0)-1,6-diolefin
complexes, containing only one phosphine ligand, resembling
the catalytic intermediate A (Scheme 2) seem ideal candi-
dates.23d,f,h The Pd(0)diolefin complexes of all of the ligands
used in this study have been prepared, and their catalytic
behavior has been tested in the telomerization of 1,3-butadiene
with methanol.
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Chem. 2002, 185, 105–112. (f) Jackstel, R.; Andreu, G. A.; Frisch,
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D.; Briel, O.; Karch, R.; Beller, M. Angew. Chem. 2002, 114, 1028-
1030; Angew. Chem., Int. Ed. 2002, 41, 986-989. (g) Vollmu¨ller, F.;
Ma¨gerlein, W.; Klein, S.; Krause, J.; Beller, M. AdV. Synth. Catal.
2001, 343, 29–33. (h) Vollmu¨ller, F.; Krause, J.; Klein, S.; Ma¨gerlein,
W.; Beller, M. Eur. J. Inorg. Chem. 2000, 1825–1832.
(24) (a) Benn, R.; Jolly, P. W.; Mynott, R.; Raspel, B.; Schenker, G.; Schick,
K.-P.; Schroth, G. Organometallics 1985, 4, 1945–1953. (b) Jolly,
P. W.; Mynott, R.; Raspel, B.; Schick, K.-P. Organometallics 1986,
5, 473–481. (c) Jolly, P. W. Angew. Chem. 1985, 97, 279-291; Angew.
Chem., Int. Ed. Engl. 1985, 24, 283-295. (d) Benn, R.; Jolly, P. W.;
Jowig, T.; Mynott, R.; Schick, K.-P. Z. Naturforsch. 1986, 41b, 680–
691. (e) Behr, A.; Ilsemann, G. V.; Keim, W.; Kru¨ger, C.; Tsay,
Y.-H. Organometallics 1986, 5, 514–518. (f) Do¨ring, A.; Jolly, P. W.;
Mynott, R.; Schick, K. P.; Wilke, G. Z. Naturforsch. 1981, 36b, 1198–
1199.
Results and Discussion
Ligand Syntheses and Properties. From the series of ligands
synthesized (Figure 1), phosphines 1, 2, 3, 7, and 8 were
(25) Buchwald, S. L.; Mauger, C.; Mignani, G.; Scholz, U. AdV. Synth.
Catal 2006, 348, 23–39.
9
6466 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Phospines for Selective Telomerization of 1,3-Butadiene
A R T I C L E S
Scheme 5. Synthesis of the Chlorophenoxaphosphine
Table 1. Phosphine Basicity and Coordination Induced Shift ∆ in
Rh Complex
Rh(CO)(phosphine)2Cl
31P{1H} NMR δ (ppm);
J (Hz)
ν(CO) cone angle
∆δ 31P{1H}
(complex-free ligand) (ppm)
phosphine
(cm-1
)
θ (deg)
1
2
3
4
5
6
9
10
11
32.2; JRh-P ) 129.5
31.8; JRh-P ) 129.2
25.2; JRh-P ) 124.1
28.4; JRh-P ) 128.7
1961
1966
1960
1969
19126
42.5
41.3
42.6
38.5
49.2
34.8
nd
nd
60.9
34.3
Scheme 6. Syntheses of Ligands 4, 5, 6: (i) n-BuLi, -78 °C, (ii)
Chlorophosphine, -78 °C
22326
-4.8; JRh-P ) 132.0 1972
-24.2; JRh-P ) 122.2 1972
nd
nd
1948
1930
1954
1967
20041
18442
52.3; JRh-P ) 96.7
PPh3 28.4; JRh-P ) 128
14543
prepared by previously described methods,26-28 whereas ligands
4-6 have been specifically designed for this work.
and used in homogeneous catalysis.35 The availability of the
xanthene skeleton in our group, combined with the fact that it
forms ortho-substituted aryl phosphines, prompted us to prepare
the new ligand 4 (“mono-xantphos”).
New phosphines 4, 5, and 6 were prepared by reacting the
corresponding bromo reagent, after lithiation, with the desired
chlorophosphine, namely, chlorodiphenylphosphine or 2,8-
dimethyl-10-chlorophenoxaphosphine (POP). Chlorophenox-
aphosphine was prepared by a Friedel-Crafts reaction between
di-p-tolylether and phosphorus trichloride in the presence of
aluminum trichloride (Scheme 5).36
Bromides such as 2′-bromo-2,6-dimethoxybiphenyl or 1-bromo-
2-methylnaphtyl were commercially available. For the synthesis
of phosphine 4, a new brominated compound 12, 5-bromo-2,7-
di-tert-butyl-9,9-dimethylxanthene, was prepared by selective
bromination of the ortho-position of the desired backbone with
N-bromosuccinimide in a DMF/THF mixture. Compound 12
was used to synthesize the phosphine without further purifica-
tion. The mixture contains, apart from the desired monobromi-
nated derivative, the starting material and the dibrominated
derivative in a molar ratio of 90/7/3, respectively, and can be
used without further purification to prepare the desired phos-
phine 4 (Figure 1).
Phosphines 1 and 3 were ortho-substituted aryl phosphines
initially developed to enhance the branched/linear ratio in
hydroformylation reactions.26 The related ligand 2, published
by Straub,27 is the diphenyl analogue of the commercially
available (dicyclohexyl) SPhos 11 (also tested in this study),
which was developed by Buchwald and used in a wide range
of coupling reactions (C-C and C-N bond formation).29 To
vary the electronic and steric properties of these ligands, the
2,7-phenoxaphosphine moiety was introduced instead of a
diphenylphosphino group on ligands 2 and 3 to give the new
phosphines 5 and 6, respectively. These phosphines should be
bulkier and less basic (see Table 1) than their diphenyl
counterparts, because of the higher rigidity and extended
π-system of the phenoxaphosphine moiety.30
The very bulky bowl-shaped phosphines (BSP) such as 7 and
8 developed by Goto28b,c and Tsuji,28a respectively, and suc-
cessfully used for hydrosilylation of unsaturated substrates28a,31
or Suzuki cross-coupling32 were prepared and used in this study.
Also the highly basic and bulky methoxy-substituted triph-
enylphosphines 9 and 10 developed by Wada were tested.33
We also designed a new bulky monophosphine based on the
backbone used for the construction of the wide bite angle
diphosphine Xantphos,34 which has been extensively studied
Typically, the phosphines were prepared by lithiation of the
brominated backbone at -78 °C with n-BuLi followed by the
reaction with the respective chlorophosphine, and they were
obtained in moderate yields (Scheme 6).
Tolman introduced the cone angle θ (deg) and the ꢀ value
to describe the steric and electronic properties of phos-
phines.37 Both parameters have been used extensively as a
measure of ligand properties.38 To evaluate the relative
basicity of our ligands, we synthesized the corresponding
Rh(phosphine)2(CO)Cl complexes by reacting 4 equiv of
phosphine with the rhodium dimer [Rh(CO)2Cl]2. The multiplic-
ity of the signals (only one doublet in all cases) and the value
of the coupling constants JRh-P are consistent with the formation
of Rh complexes in which the two phosphine ligands are
(26) Riihima¨ki, H.; Suomalainen, P.; Reinius, H. K.; Suutari, J.; Ja¨a¨skel-
a¨inen, S.; Krause, A. O. I.; Pakkanen, T. A.; Pursiainen, J. T. J. Mol.
Catal. A: Chem. 2003, 200, 69–79.
(27) Keller, J.; Schlierf, C.; Nolte, C.; Mayer, P.; Straub, B. F. Synthesis
2006, 2, 354–365.
(28) (a) Niyomura, O.; Iwasawa, T.; Sawada, N.; Tokunaga, M.; Obora,
Y.; Tsuji, Y. Organometallics 2005, 24, 3468–3475. (b) Ohzu, Y.;
Goto, K.; Sato, H.; Kawashima, T. J. Organomet. Chem. 2005, 690,
4175–4183. (c) Ohzu, Y.; Goto, K.; Sato, H.; Kawashima, T. Angew.
Chem., Int. Ed. 2003, 42, 5714–5715.
(29) (a) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L.
J. Am. Chem. Soc. 2005, 127, 4685–4696. For reviews, see: (b) Surry,
D. S.; Buchwald, S. L. Angew. Chem., Int. Ed. 2008, 47, 2–26. (c)
Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461–1473.
(30) Bronger, R. P. J.; J.; Kamer, P. C.; van Leeuwen, P. W. N. M.
Organometallics 2003, 22, 5358–5369.
(35) van Leeuwen, P. W. N. M. Homogeneous Catalysis, Understanding
the Art; Kluwer Academic Publishers: Dordrecht, Netherlands, 2004
and references therein.
(36) Herwig, J.; Bohnen, H.; Skutta, P.; Sturm, S.; van Leeuwen,
P. W. N. M.; Bronger, R. (Celanese Chem. Europe Gmbh) Patent
WO02068434 A1, 2002.
(37) Tolman, C. A. Chem. ReV 1977, 77, 313–348, and references therein.
(38) (a) Moser, W. R.; Papile, C. J.; Brannon, D. A.; Duwell, R. A.;
Weininger, S. J. J. Mol. Catal. 1987, 41, 271–292. (b) van Leeuwen,
P. W. N. M.; Roobeek, C. F. J. Organomet. Chem. 1983, 258, 343–
350.
(31) Niyomura, O.; Tokunaga, M.; Obora, Y.; Iwasawa, T.; Tsuji, Y. Angew.
Chem., Int. Ed. 2003, 42, 1287–1289.
(32) Ohta, H.; Tokunaga, M.; Obora, Y.; Iwai, T.; Iwasawa, T.; Fujihara,
T.; Tsuji, Y. Org. Lett. 2007, 9, 89–92.
(33) Wada, M.; Higashizaki, S. J. Chem. Soc., Chem. Commun. 1984, 482–
483.
(34) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J. Organometallics 1995,
14, 3081–3089.
9
J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6467

A R T I C L E S
Tschan et al.
Figure 2. Phosphine-palladium-dvds complexes.
arranged in a trans fashion, as expected due to steric reasons
and the trans influences.39 The fact that along the series the
same type of complex is formed validates the construction of
the electronegativities series based on the comparison of their
carbonyl IR frequencies, ν(CO), Table 1. According to these
data, the series follows the following basicity order trend: 10 >
9 > 11 > 3 > 1 > 2 > PPh3 > 4 > 5 ) 6. As expected methoxy-
substituted triphenylphosphines are the most basic33,40 of the
series, and the POP-based ones are located at the opposite end.30
Sphos 11 is also rather basic, thanks to the presence of two
cyclohexyl moieties, and phosphines 2 and 4 are electronically
similar to triphenylphosphine.
Phosphine-Palladium-dvds Complexes. Phosphine-palladi-
um-diolefin complexes were synthesized and studied as they
represent a model of the catalytic intermediate A in the
telomerization reaction (Scheme 4). The synthesis of the
complexes was achieved using the elegant method developed
by Po¨rschke et al.44 by reacting [(TMEDA)Pd(CH3)2]45 (TME-
DA ) 1,2-bis(dimethylamino)ethane) with 1 equiv of the
phosphine in the presence of an excess of the diolefin.
Tetramethyldivinyldisiloxane (dvds) is used as the diolefin
ligand because it stabilizes the palladium(0) diolefin complex
thanks to its increased π-acceptor ability (Scheme 7).
Thus, we prepared a series of new palladium complexes,
obtained in moderate to good yield (30-100%) following the
procedure described in Scheme 7. The new complexes are
depicted in Figure 2.
Scheme 7. Synthesis of Phosphine-Pd-dvds Complexes
In the case of the BSP phosphine 8, the palladium(0) complex
does form but is not stable and decomposes with time and/or
during the workup. The main difference between the two BSP
phosphines 7 and 8 is the depth (d) of the bowl, which is 2.1
and 2.8 Å for 7 and 8, respectively,28 which could account for
the instability of 8a compared to 7a. In complex 8a, the steric
repulsion between the m-terphenyl moieties and dvds should
be larger than in 7a and could destabilize the corresponding
Pd-complex.
Single crystals suitable for X-ray diffraction have been
obtained for complexes 1a, 2a, and 3a by crystallization from
a concentrated solution of the complex in diethyl ether at -25
°C. The molecular structures are presented Figure 3.
The X-ray structure analysis of complexes 1a, 2a, and 3a
reveals that the palladium atom has a trigonal planar coordina-
tion environment, created by the phosphorus atom and the two
CdC bonds of the dvds ligand. As observed by Po¨rschke, the
Pd(1,6-diene) moiety adopts a stable chair like conformation.46
On the one hand, we can observe that in the case of 1a and 2a
the biphenyl moiety pointed over the Pd atom because of
favorable interactions between the aromatic π-system and the
palladium center as observed by Buchwald for SPhos.29b,c On
the other hand, because such interaction is not possible in 3a,
the naphthyl moiety is inside the “cone” of the phosphine ligand
and the methyl substituent points to the Pd atom.
(39) (a) Freixa, Z.; van Leeuwen, P. W. N. M. Coord. Chem. ReV. 2008,
252, 1755–1786. (b) Bessel, C. A.; Aggarwal, P.; Marschilok, A. C.;
Takeuchi, K. J. Chem. ReV. 2001, 101, 1031–1066.
(40) Pruchnik, F. P.; Smolenski, P.; Wajda-Hermanowicz, K. J. Organomet.
Chem. 1998, 570, 63–69.
(41) Grim, O. S.; Yankowsky, A. W. J. Org. Chem. 1977, 42, 1236.
(42) Dunbar, K. R.; Haefner, S. S. C. Polyhedron 1994, 13, 726.
(43) (a) Vallarino, L. J. Chem. Soc. 1957, 2287. (b) Evans, D.; Osborn,
J. A.; Wilkinson, G. Inorg. Synth. 1968, 11, 99.
The trigonal planar geometry around the palladium for the
three complexes is highly distorted as revealed by the value of
the three angles (deg) D1-Pd1-D2 (1a 132.72(4), 2a 130.48(7),
(44) Krause, J.; Cestaric, G.; Haack, K.-J.; Seevogel, K.; Storm, W.;
Po¨rschke, K.-R. J. Am. Chem. Soc. 1999, 121, 9807–9823.
(45) De Graaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten,
G. Organometallics 1989, 8, 2907–2917.
(46) Krause, J.; Haack, K.-J.; Cestaric, G.; Goddard, R.; Po¨rschke, K.-R.
Chem Commun. 1998, 1291–1292.
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6468 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Phospines for Selective Telomerization of 1,3-Butadiene
A R T I C L E S
Figure 3. Ortep plots (thermal ellipsoids shown at 50% probability level) of the complexes 1a, 2a, and 3a. Hydrogen atoms have been omitted for the sake
of clarity. D1: C27dC28 (left), C27dC28 (center), C26dC27 (right). D2: C25dC26 (left), C29dC30 (center), C24dC25 (right).
Table 2. Selected Bond Length and Bond Angles for Complexes
1a, 2a, and 3a
Table 3. Coordination Induced Shift ∆ for the
Phosphine-Pd-dvds Complexes
1a
2a
3a
free
ligand
δ
31P{1H} chemical
shift (ppm)
δ
31P{1H} chemical
shift (ppm)
∆δ 31P{1H}
(complex-free ligand) (ppm)
complex
Pd1-P1 (Å)
2.2975(6)
2.0621(11)
2.0620(11)
1.3922(15)
1.3910(15)
132.72(4)
114.09(3)
113.03(3)
3.38
2.3192(5)
2.068(2)
2.088(2)
1.401(3)
1.402(2)
130.48(7)
119.91(5)
109.60(5)
3.27
2.3255(3)
2.0733(13)
2.0622(13)
1.399(2)
1.3987(19)
132.78(5)
112.36(4)
114.72(4)
3.30
1
2
3
4
5
6
7
9
-10.3
-9.5
-17.4
-10.1
-54.0
-59.0
-4.4
1a
2a
3a
4a
5a
6a
7a
9a
10a
11a
27.1
24.1
15.5
37.4
33.6
32.9
37.0
43.9
31.2
34.2
54.1
45.1
44.4
36.144
Pd1-D1 (Å)
Pd1-D2 (Å)
D1 (Å)
D2 (Å)
D1-Pd1-D2 (deg)
D1-Pd1-P1 (deg)
D2-Pd1-P1 (deg)
Pd1· · ·C7 or C11 (Å)
Pd1· · ·C8 (Å)
Pd1· · ·C12 (Å)
26.9
-10.1
-27.8
29.8
17.7
-21.6
35.8
-36.4
-66.7
-8.6
10
11
PPh3
3.09
4.01
3.56
3.50
-5.9
PPh3-Pd-vds
30.2
3a 132.78(5)); D1-Pd1-P1 (1a 114.09(3), 2a 119.91(5), 3a
112.36(4); D2-Pd1-P1 (1a 113.03(3), 2a 109.60(5), 3a
114.72(4)) (Table 2), which are far from 120° as observed by
Beller in the case of (NHC)-Pd-dvds complexes (D1-Pd-D2
≈ 130°, D1-Pd1-P1 ≈ D2-Pd1-P1 ≈ 115°).23d Moreover,
the CdC bond lies exactly in the coordination plane and the
CdC bonds length (1.40 Å) is lengthened as compared with a
typical CdC bond distance (1.34 Å), indicating a weak
backbonding of the Pd. By contrast to 2a, the phenyl ring
covering the Pd in 1a is tilted, as reflected by comparison of
the Pd1-C8 and Pd1-C12 bond lengths in 1a and 2a.
In 1a and 2a, as observed by Buchwald in (SPhos)2Pd
complexes,29a,c no Pd-Cipso interaction is observed as shown
by the large distance of Pd1-C7 (3.27 Å); this is in contrast
with (SPhos)Pd(dba) in which the Pd-Cipso bond length is
around 2.37 Å.
Table 4. Telomerization of 1,3-Butadiene in the Presence of
Phosphine-Palladium-dvds Complexa
entry
Pd complex
conv Bd (%) TON 1-MOD (%) 3-MOD (%) OCT (%)
1
2
3
4
5
6
7
8
9
1a
2a
3a
4a
5a
6a
7a
9a
10a
58
99
24
99
99
14
96
99
10
90
90
99
11600 89
19800 90
4800 94
19800 93
19800 87
2800 90
19200 76.5
19800 96
2000 72.5
18000 83.5
18000 87.5
19800 93
4
8
4
5
10.5
4
12.5
3
1.5
9.5
7.5
5
7
2
2
2
2.5
6
11
1
26
7
10 11a
11 Pd(OAc)2/PPh3 (1/2)
12 Pd(OAc)2/4 (1/2)
5
2
a Reaction conditions: 60 °C, 16 h, MeOH/1,3-butadiene 2/1 (molar
ratio), Pd complex (0.005 mol % vs 1,3-Butadiene), NaOMe (1 mol %).
In the case of phosphine rhodium complexes, Shaw observed
a linear correlation between the chemical shift of the free
phosphine and the coordination induced shift ∆, which was
attributed to the widening of the cone angle of the ligand upon
coordination.47 The magnitude of ∆ tends to be less for larger
ligands than for smaller ligands, as ligands with large substit-
uents generally open less on coordination, as the C-P-C angles
are already large in the free ligand.37 In this work, neither in
the case of rhodium nor in the case of palladium complexes
was such a correlation found (cf. PPh3 is the least bulky
phosphine and gives one of the lowest ∆ in the case of rhodium
(Table 1)).
bond strength, meaning the stronger the metal phosphine bond,
the larger ∆.40 Unfortunately, no such relation can be found
here either (cf. Table 3), pointing out the unusual steric
properties of “Buchwald-type” phosphines, containing two
substituents such as phenyl or cyclohexyl and one very bulky
moiety such as substituted biphenyl, xanthene, or 2-methyl-
naphtyl.
Catalytic Studies. The complexes prepared were tested in the
telomerization of 1,3-butadiene with MeOH. The conditions
used are 60 °C, 16 h, MeOH/1,3-butadiene (2 mol/mol), NaOMe
(1 mol % vs butadiene), catalyst (0.005 mol % vs butadiene) to
evaluate their overall productivity and selectivity to the linear
product. The results obtained are presented in Table 4. The
standard triphenylphosphine system was tested as a reference,
and the results found are similar to those found by Beller.23g,h
We observed that complexes 2a, 4a, and 9a are highly selective
A linear relation has also been found between the basicity of
the phosphines and ∆, suggesting that ∆ is related to the P-M
(47) Mann, B. E.; Masters, C.; Shaw, B. L. J. Chem. Soc. A 1971, 1104–
1106.
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J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6469

A R T I C L E S
Tschan et al.
Table 5. Telomerization of 1,3-Butadienea
entry ligand T (°C) MeOH/Bd w/w ratio conv Bd (%) 1-MOD (%) Pd lossb (%)
toward linear 1-MOD and more productive and selective toward
the linear product than the triphenylphosphine system under
these conditions.
1
2
3
4
5
6
7
8
2
2
4
4
4
4
4
5
9
9
9
9
60
90
60
60
90
90
100
90
70
70
90
90
60
60
90
60
60
90
90
2
2
2
2.6
2
38
50
40
36
89
93
91
87
57
38
82
53
10
10
30
32
29
82
85
85
70
94
94
88
89
84
82
95
92
92
89
75
61
76
91
92
80
83
0
35
0
2
2
6
2
30
Concerning the productivity, ligands 3 and 6 render the less
productive catalysts, exhibiting a TON of only 4560 and 2700,
respectively (Table 4, entries 3, 6). Both ligands containing the
2-methylnaphthyl moiety are probably among the bulkier
phosphine of the series (the cone angle of 3 is θ ) 223°). Thus,
it seems that, as observed by Beller in the case of NHC ligands,
although bulky ligands could activate the catalyst by destabiliz-
ing the diolefin complex, it does not always translate into an
increased catalytic activity, maybe because decomposition takes
place.23d By contrast, complexes 2a, 4a, and 5a are the most
productive catalysts of the series, showing nearly complete
conversions (TON of 20,000) under such conditions. The
commercially available tris(o-methoxyphenyl)phosphine 9 is the
more selective catalyst, yielding 96% of 1-MOD (with 99%
chemoselectivity and 97% regioselectivity). Unfortunately, the
productivity of the catalyst with 9a is strongly dependent on
the quality of the methanol, because 9 is more sensitive to
oxidation than 4 or PPh3.48 It seems also difficult to find a trend
concerning the properties of ligand and the productivity/
selectivity of the catalyst as ligands 2, 4, and 9, which are
electronically and sterically different, give the most promising
results. We also observed that in situ preparation of the catalyst
has no detrimental effect on the productivity and on the
selectivity of the catalyst (Table 4, entry 12).
2.6
2
2.6
0.6
2.6
0.6
2.6
2
2.6
2
2
9
10
11
12
13 11
14 11
0
4
32
0
0
16
17
15 11
16 PPh3
17 PPh3
18 PPh3
19 PPh3
2.6
2
2.6
a Reaction conditions Pd (0.0025 mol % vs butadiene), NaOMe
(0.0125 mol % vs butadiene), 2.5 h, ligand/Pd 2/1. b Measurement error
(5%.
Thus, we decided to study in detail the best phosphines (2,
4, 5, 9, and 11) compared to PPh3 under different reaction
conditions, such as scale-up (from 7 g of pure 1,3-butadiene to
150 g of C4 crude fraction containing 47.5 wt % of 1,3-
butadiene), MeOH/1,3-butadiene ratio, and temperature. We also
studied the activity of the catalyst by following the rate of
formation of 1-MOD and/or butadiene conversion with time.
Palladium loss at the end of the catalytic reaction was also
evaluated. For these experiments 0.0025 mol % of Pd and
0.0125 mol % of NaOMe (vs butadiene) were used, and the
catalysts were prepared in situ (TPP ) PPh3, Bd ) 1,3-
butadiene, w/w ) weight ratio, reaction time )2.5 h, ligand/
Pd ) 2/1; see Supporting Information). The results obtained
for 2, 4, 5, 9, 11, and PPh3 at different temperature and MeOH/
Bd weight ratio are summarized in Table 5.
Figure 4. Rate of formation of 1-MOD (MeOH/Bd (w/w) ) 2/1, Pd
(0.0025 mol % vs butadiene); comparison of ligand 2 and PPh3.
significant decrease of selectivity is observed for 2 (1-MOD )
70% vs 80% for PPh3; Table 5, entry 2).
In the case of TPP at 60 °C, when only 0.0025 mol % of Pd
loading is used, higher selectivity is obtained compared to
previous results (1-MOD selectivity up to 92.4%; Table 5,
entries 16 and 17). As observed by Beller et al., the concentra-
tion of 1,3-butadiene influences the yield of 1-MOD: decreased
concentration of butadiene increases the yield of the linear
telomer (cf. increase the regioselectivity) and increasing the
temperature decreases the yield to 1-MOD (cf. decrease of the
chemoselectivity) but increases butadiene conversion (Table 5,
entries 16-19).23h Palladium precipitation at 90 °C is about 17%
(Table 5, entries 18 and 19).
We observed that under the new reaction conditions, ligand
2 gives low selectivity compared to previous results (85% vs
89% 1-MOD selectivity at 60 °C) and compared to PPh3 under
the same conditions (85% vs 91% 1-MOD selectivity for ligand
2 and PPh3, respectively (Table 5, entries 1 and 16)). At 90 °C,
Concerning the reaction rate, at 60 °C 2 gives a more active
catalyst (see Figure 4) than PPh3. However at 90 °C, the reaction
rate decreases strongly after 10 min (initial reaction rates for
PPh3 and 2 seem equivalent at 90 °C), meaning that the catalyst
derived from 2 is unstable. High palladium loss was measured
in the case of 2 at 90 °C (up to 35%), which is in favor of this
hypothesis.
As observed previously (Table 4, entry 8) for ligand 9, high
selectivity to 1-MOD was obtained (up to 92% at 90 °C; see
Supporting Information), but in this case low MeOH/Bd ratio
(0.6 instead of 2.7) enhances the selectivity and the butadiene
conversion (53% with MeOH/Bd ) 2.6 and 82% with MeOH/
Bd ) 0.5). In this case, this could be explained by the fact that
degassed commercially available methanol was used for these
experiments, pointing out the high sensitivity of 9 to oxidation.48
The system based on 5 shows similar performance in terms
of productivity, selectivity, and activity as the one using PPh3,
except that the palladium loss is higher (up to 30%). Ligand 11
gives very bad results in term of selectivity, conversion, and a
very high Pd precipitation (see Supporting Information).
(48) For example, we observed that butadiene conversion drops to 50%
instead of 99% if non-degassed commercially available extra dry
methanol (ACROS) is used as received (amount of water in degassed
and distilled methanol at the bench being around 10 ppm).
9
6470 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Phospines for Selective Telomerization of 1,3-Butadiene
A R T I C L E S
catalyst, reaching TOF up to 140,000 h-1 at 100 °C with high
selectivity (up to 84%) for the production of 1-MOD. Surpris-
ingly, ligand 4 also highly stabilizes the catalytic active species
as the loss of palladium during the catalytic reaction is much
lower than that of the catalyst based on PPh3.
Further studies on the structure-reactivity relationship and
new derivatizations of monoxantphos 4 are currently carried
out to understand the steric and electronic influence of the
phosphine on the telomerization reaction and to optimize the
systems accordingly.
Experimental Section
General Procedures. All reactions were carried out under an
argon atmosphere using Schlenk techniques. Deuterated chloroform
was distilled over calcium hydride under argon prior to use. MeOH
was dried by distillation over Mg under Ar and degassed twice by
a freeze, pump, and thaw cycle. Tetramethyldivinyldisiloxane (dvds)
was purchased from Aldrich, dried over molecular sieves 4 Å, and
degassed with argon. 2′-Bromo-2,6-dimethoxybiphenyl, 1-bromo-
2-methylnaphthyl, phosphorus trichloride, n-BuLi solutions in
hexane, and tetramethylethylenediamine (TMEDA) were purchased
from Aldrich and used as received. Chlorodiphenylphosphine was
purchased from Fluka and distilled under inert atmosphere prior to
use. Tetracarbonyldi-µ-chlorodirhodium was purchased from Acros
and used as received. Palladium dichloride was purchased from
Strem and used as received.
Figure 5. Rate of formation of 1-MOD (MeOH/Bd (w/w) 2/1, Pd (0.0025
mol % vs butadiene); comparison of ligand 4 (monoxantphos) and PPh3.
Phosphine 4, which is the most promising ligand of the series,
has been studied in more detail. In all conditions tested, ligand
4 gives better results than PPh3 (Table 5, entries 3-7)
concerning productivity and selectivity to 1-MOD, which stays
constant during the course of the reaction, meaning that
formation of byproduct such as 3-MOD and OCT is directly
dependent on the catalyst features and is not a consequence of
catalyst decomposition. At 60 °C with 4, the selectivity to
1-MOD is 2-3% higher than the one observed with PPh3 and
butadiene conversion increases from ∼30% to ∼38% (Table 5,
entries 3, 4, 16, and 17), but the largest differences are observed
at 90 °C. Selectivity to 1-MOD and butadiene conversion are
6-8% and 8% higher, respectively, than PPh3. Moreover, Pd-
loss is significantly lower in the presence of ligand 4, decreasing
from 17% to 6% under the same conditions (Table 5, entries 6
and 18).
The high potential of monoxantphos 4 is also demonstrated
by the kinetics depicted in Figure 5. At 60 °C the rate of
formation of 1-MOD is slightly higher than with PPh3. At 90
°C after 30 min of reaction, a max TOF of ∼40,000 and of
25,500 h-1 (Beller reported a TOF 25,000 after 30 min for
PPh3)23g is reached in the case of 4 and PPh3, respectively. The
most impressive result is obtained at 100 °C: after 5 min of
reaction, a TOF of ∼140,000 h-1 is observed for 4; in the case
of PPh3, TOF (5 min) ≈ 41,600 h-1. Only in the case of an
NHC ligand has such a high TOF (up to 100,000 h-1 at 90 °C)
been observed, although this was not achieved under the
conditions of the commercial process.23d
Phosphines 1,26 2,27 3,26 7,28 and 828 were prepared by
previously described methods, and commercially available phos-
phines 9-11 were purchased from Aldrich and used without further
purifications. (TMEDA)Pd(Me)2 was prepared by following the
published procedure.45 NMR spectra were recorded on a Bruker
400 MHz spectrometer. Chemical shifts are reported in ppm and
were calibrated using residual 1H and 13C resonances of deuterated
solvents. Coupling constants (J) are expressed in Hertz. Mass
spectra and X-ray structure analysis were done at the Research
Support Unit of the ICIQ (Tarragona, Spain). Analyses of the
catalytic reactions have been performed on a GC-FID equipped
with a HP-5 (5% phenyl methyl siloxane; 30 m × 320 m × 0.25)
capillary column. Elemental analysis were performed at the Unidade
de Ana´lise Elemental of the Universidade de Santiago de Com-
postela (Spain).
Synthesis of 4-Diphenylphosphino-2,7-di-tert-butyl-9,9-dim-
ethylxanthene (4). To a solution of 4-bromo-2,7-di-tert-butyl-9,9-
dimethylxanthene (90%) (12) (1.53 g, containing 3.44 mmol of 12)
in THF (30 mL) at -78 °C was added slowly n-BuLi 2.5 M in
hexane (1.52 mL), and the mixture was stirred for 1 h at -78 °C.
Then, chlorodiphenylphosphine (3.8 mmol, 0.82 g, 0.67 mL) was
added at -78 °C, and the mixture was stirred 1 h at -78 °C and
1 h at rt. Then, the solvent was evaporated under vacuum,
dichloromethane (20 mL) was added, the solution was filtered off
in order to eliminate the inorganic salts formed during the reaction,
and the solvent was removed under reduced pressure. The product
was purified by column chromatography over silica gel using as
eluent hexane/dichloromethane (4:1). Compound 4 is obtained as
Conclusions and Outlook
In addition to several published sterically hindered phos-
phines, new ones possessing different electronic and steric
properties were prepared in moderate to good yield. The
xanthene backbone, well-known for its diphosphine Xantphos,
as well as the phenoxaphosphine moiety, have been used to
prepare new phosphines.
A series of phosphine-palladium-dvds complexes, contain-
ing those ligands, were synthesized and tested as catalyst for
the selective telomerization of 1,3-butadiene with methanol. In
comparison with PPh3, monoxantphos 4 emerged as a very
promising alternative giving a more active and selective catalyst
than PPh3 in the standard system optimized for PPh3.
The testing of phosphines under “Dow production conditions”
(scale-up, crude C4 fraction, high temperature) reveals the high
potential of 4. Remarkably, ligand 4 gives a highly active
1
white solid (0.95 g, 55%). H NMR (400 MHz, 25 °C, CDCl3) δ
7.41-7.31 (m, 12H, H arom.), 7.05 (dd, 1H, J ) 2.3 Hz, J ) 8.5
Hz, H arom.), 6.65 (dd, 1H, J ) 2.3 Hz, J ) 5.8 Hz, H arom.),
6.52 (d, 1H, J ) 8.5 Hz) 1.64 (s, 6H, CH3), 1.29 (s, 9H, C(CH3)),
1.14 (s, 9H, C(CH3)); 13C{1H} NMR (100 MHz, 25 °C, CDCl3) δ
150.4 (d, J(P,C) ) 13.5 Hz), 148.5, 145.6, 145.3, 137.0 (d, J(P,C)
) 10.2 Hz), 134.1 (d, J(P,C) ) 20.0 Hz), 129.6, 129.5, 129.1, 129.0,
128.69, 128.4 (d, J(P,C) ) 7.0 Hz), 124.2, 123.7 (d, J(P,C) ) 14.0
Hz), 123.41, 121.9, 115.9, 34.9, 34.6, 34.5, 31.9, 31.7, 31.5; 31P{1H}
(160 MHz, 25 °C, CDCl3) δ -10.18. Anal. Calcd for C35H39OP:
C, 82.97; H, 7.76. Found: C, 82.77; H, 7.74.
Synthesisof2-(2.8-Dimethylphenoxaphosphino)-2′,6′-dimethoxy-
biphenyl (5). To a solution of 2-bromo-2′,6′-dimethoxybiphenyl
(0.88 g, 3 mmol) in 20 mL of THF cooled at -78 °C was added
9
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A R T I C L E S
Tschan et al.
n-BuLi (1.1 equiv, 3.3 mmol, 1.32 mL, 25 M in hexane) dropwise
via syringe. The resulting mixture was stirred at -78 °C for 0.5 h.
Then 10-chloro-2,8-phenoxaphosphine (1 equiv, 3 mmol, 0.80 g)
dissolved in 10 mL of THF was added via syringe. The mixture
was stirred at -78 °C for 1 h and then allowed to slowly warm to
rt. The solvent was removed under vacuum. The resulting solid
was dissolved in dichloromethane (20 mL) and filtered off through
Celite in order to eliminate the inorganic salts formed during the
reaction. Then the solvent was removed under reduced pressure to
afford a yellow solid. Recrystallization from ethanol (20 mL)
equiv, 1.03 × 10-4 mol) were dissolved in dichloromethane (3 mL).
In the case of 10, the reaction was carried out in THF (because 10
reacts with CH2Cl2 to give the phosphonium salt).33 The solution
was stirred for 1 h, and then solvent was evaporated to dryness.
The yellow solid obtained was analyzed by IR (solid phase) and
31P{1H}NMR (160 MHz, 25 °C, CDCl3).
General Procedure for the Synthesis of the Phosphine-
Palladium-dvds Complexes. A suspension of (TMEDA)Pd(CH3)2
(0.1 g, 0.40 mmol) and of the desired phosphine (1 equiv, 0.40
mmol) in degassed and dried tetramethyldivinyldisiloxane (dvds,
4 mL) was stirred at room temperature overnight. Then, the solvent
was removed under vacuum, and the white solid obtained was
washed with a small portion of diethylether (or pentane) at -50
°C to remove small quantities of unreacted reagents. Because of
theirlowstabilityatroomtemperature,thephosphine-palladium-dvds
complexes have been characterized only by 1H and 31P NMR
spectroscopy and MS spectrometry and should be stored under inert
atmosphere at -20 °C.
1
provided the phosphine as a white solid (0.68 g, 51%). H NMR
(400 MHz, 25 °C, CDCl3) δ 7.42 (t, J ) 8.3 Hz, 1H, H arom.),
7.28 (brt, J ) 7.9 Hz, 1H, H arom.), 7.15-7.10 (m, 2H, H arom.),
7.07-6.98 (m, 5H, H arom.), 6.82 (brd, J ) 9.8 Hz, 2H, H arom.),
6.72 (d, 2H, J ) 8.3 Hz, H arom.) 3.77 (s, 6H, OCH3), 2.22 (s,
6H, CH3 arom.); 13C{1H} NMR (100 MHz, 25 °C, CDCl3) δ 158.0,
153.4, 140.5 (d, J(P,C) ) 21.9 Hz), 139.8 (d, J(P,C) ) 32.8 Hz),
134.8 (d, J(P,C) ) 32 Hz), 133.48 (d, J(P,C) ) 1.5 Hz), 132.3 (d,
J(P,C) ) 13.5 Hz), 130.9, 130.7 (d, J(P,C) ) 5.8 Hz), 129.4, 128.7,
127.83, 127.78, 119.67, 119.61, 119.3 (d, J(P,C) ) 6.5 Hz), 117.25,
103.9, 55.7, 20.8; 31P{1H} (160 MHz, 25 °C, CDCl3) δ -54.50.
Anal. Calcd for C28H35O3P: C, 74.64; H, 7.83. Found: C, 74.82;
H, 7.75.
Synthesis of 1a. Yield of 1a: 61%, 0.152 g (1: 0.40 mmol, 0.136
g). 1H NMR (400 MHz, 25 °C, CDCl3) δ 7.50-7.30 (brm, 14H, H
arom.), 7.15-6.95 (brm, 5H, H arom.), 3.20 (m, 2H, dvds), 2.85
(m, 2H, dvds), 2.66 (m, 2H, dvds), 0.27 (s, 6H, dvds), -0.20 (s,
6H, dvds). 31P{1H} (160 MHz, 25 °C, CDCl3) δ 27.1. ESI-MS:
m/z ) 630.2 [M]+, 653.1 [M + Na]+ and decomposition peaks.
Synthesis of 2a. Yield of 2a: 59%, 0.162 g (2: 0.40 mmol, 0.160
Synthesis of 1-(2,8-Dimethylphenoxaphosphino)-2-methyl-
naphtalene (6). To a solution of 1-bromo-2-methylnaphtalene (0.72
g, 3.2 mmol) in THF (20 mL) cooled at -78 °C was added dropwise
via syringe n-BuLi (3.6 mmol, 1.43 mL, 2.5 M in hexane). The
resulting mixture was stirred at -78 °C for 0.5 h. Then 10-chloro-
2,7-phenoxaphosphine (3.2 mmol, 0.85 g) dissolved in 10 mL of
THF was added via syringe. The mixture was allowed to stir at
-78 °C for 1 h and then allowed to slowly warm to rt. The solvent
was evaporated under vacuum. The resulting solid was dissolved
in dichloromethane (20 mL) and filtered off through Celite in order
to eliminate the inorganic salts formed during the reaction. Then
the solution was concentrated under reduced pressure, and a yellow
solid was obtained. Recrystallization from ethanol (20 mL) rendered
the phosphine as a white solid (0.58 g, 48%). 1H NMR (400 MHz,
25 °C, CDCl3) δ 8.85 (m, 1H, H arom.), 7.86 (d, 1H, J ) 8.3 Hz,
H arom.), 7.83 (m, 1H, H arom.), 7.41 (m, 2H, H arom.), 7.32 (dd,
1H, J ) 2.0 Hz, 8.3 Hz, H arom.), 6.99 (s, 4H, H arom.), 6.65 (d,
J ) 9.2 Hz, 2H, H arom.) 2.48 (s, 3H, CH3 napht.), 2.08 (s, 6H,
CH3); 13C{1H} NMR (100 MHz, 25 °C, CDCl3) δ 151.0 (d, J(P,C)
) 5.5 Hz), 145.7 (d, J(P,C) ) 12.7 Hz), 137.6 (d, J(P,C) ) 17.3
Hz), 133.0 (d, J(P,C) ) 5.9 Hz), 132.8 (d, J(P,C) ) 4.0 Hz), 131.49,
131.48 (d, J(P,C) ) 20.9 Hz), 130.3, 130.2 (d, J(P,C) ) 3.8 Hz),
128.6, 127.4, 127.2, 126.66 (d, J(P,C) ) 2.1 Hz), 125.06 (d, J(P,C)
) 1.3 Hz), 119.1 (d, J(P,C) ) 8.0 Hz), 117.1, 23.2 (d, J(P,C) )
17.5 Hz), 20.6; 31P{1H} (160 MHz, 25 °C, CDCl3) δ -59.08. Anal.
Calcd for C25H21OP: C, 81.50; H, 5.75. Found: C, 81.17; H, 5.84.
Synthesis of 4-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthene
(12). 2,7-Di-tert-butyl-9,9-dimethylxanthene (3 g, 9.3 mmol) and
NBS (14.9 g, 84 mmol) were dissolved in DMF/THF 1/1 in volume
(200 mL) and stirred at rt for 3 days. The conversion was monitored
by GC/MS. If necessary, more NBS was added. The solvent was
evaporated to dryness, and the solid was washed with water and
extracted with dichloromethane. The organic layer was separated
and dried over MgSO4 and evaporated to dryness. The solid
obtained was washed with ethanol to give 2.8 g of a white solid
(containing a molar mixture of monobromo/dibromo/starting ma-
terial 90/7/3). The mixture was used without further purification to
prepare the phosphine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 7.46
(d, J ) 2.2 Hz, 1H, H arom.), 7.41 (d, J ) 2.3 Hz, 1H, H arom.),
7.37 (d, J ) 2.2 Hz, 1H, H arom.), 7.46 (d, J ) 2.2 Hz, 1H, H
arom.), 7.26 (dd, J ) 2.2, 8.5 Hz, 1H, H arom.), 7.10 (d, J ) 8.5
Hz, 1H, H arom.), 1.66 (s, 6H, CH3), 1.36 (s, 9H, C(CH3)3), 1.34
(s, 9H, C(CH3)3).
1
g). H NMR (400 MHz, 25 °C, CDCl3) δ 7.53-7.41 (m, 6H, H
arom.), 7.37-7.28(m, 7H, H arom.), 7.20-7.17 (m, 1H, H arom.),
7.11 (t, J ) 8.4 Hz, 1H, H arom.), 6.30 (d, J ) 8.4 Hz, 2H, H
arom.), 3.45 (s, 6H, OCH3), 3.29 (dd, J ) 2.2, 11.8 Hz, 1H, dvds),
3.29 (dd, J ) 2.7, 11.4 Hz, 1H, dvds), 2.60-2.41 (m, 4H, dvds),
0.20 (s, 3H, dvds), -0.39 (s, 3H, dvds). 31P{1H} (160 MHz, 25
°C, CDCl3) δ 24.1. ESI-MS: m/z ) 713.2 [M + Na]+.
Synthesis of 3a. Yield of 3a: 68%, 0.168 g (3: 0.40 mmol, 0.131
g). 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.27 (d, J ) 8.4 Hz, 1H,
H arom.), 7.87 (d, J ) 8.4 Hz, 1H, H arom.), 7.81 (d, J ) 8 Hz,
1H, H arom.), 7.52 (m, 4H, H arom.), 7.36-7.28 (m, 8H, H arom.),
7.08 (t, J ) 8 Hz, 1H, H arom.), 3.35-3.20 (m, 4H, dvds), 2.84
(dd, J ) 4.8, 15.8 Hz, 2H, dvds), 2.30 (s, 3H, CH3 arom.), 0.24 (s,
3H, dvds), -0.30 (s, 3H, dvds). 31P{1H} (160 MHz, 25 °C, CDCl3)
δ 26.9. ESI-MS: m/z ) 641.2 [M + Na]+.
Synthesis of 4a. Yield of 4a: 71%, 0.226 g (4: 0.40 mmol, 0.203
1
g). H NMR (400 MHz, 25 °C, CDCl3) δ 7.55 (m, 4H, H arom.),
7.45 (d, J ) 2.1 Hz, 1H, H arom.), 7.34 (m, 7H, H arom.), 6.97
(dd, d, J ) 2.1, 8.5 Hz, 1H, H arom.), 6.86 (dd, d, J ) 2.0, 10.6
Hz, 1H, H arom.), 6.06 (d, J ) 8.5 Hz, 1H, H arom.), 3.35 (d, J )
5.8 Hz, 1H, dvds), 3.32 (d, J ) 5.2 Hz, 1H, dvds), 3.13 (m, 4H,
dvds), 1.64 (s, 6H, CH3), 1.29 (s, 9H, C(CH3)3), 1.18 (s, 9H,
C(CH3)3), 0.25 (s, 3H, dvds), -0.25 (s, 3H, dvds). 31P{1H} (160
MHz, 25 °C, CDCl3) δ 15.3. ESI-MS: m/z ) 821.4 [M + Na]+.
Synthesis of 5a. Yield of 5a: 57%, 0.166 g (5: 0.40 mmol,
1
0.176.2 g). H NMR (400 MHz, 25 °C, CDCl3) δ 7.45-7.35 (m,
2H, H arom.), 7.28 (m, 1H, H arom.), 7.13 (t, J ) 8.3 Hz, 1H, H
arom.), 7.04 (brm, 5H, H arom.), 6.94 (m, 2H, H arom.), 6.29 (d,
J ) 8.3 Hz, 2H, H arom.), 3.47 (s, 6H, O(CH3)), 3.33 (d, J ) 5.8
Hz, 1H, dvds), 3.29 (d, J ) 5.6 Hz, 1H, dvds), 2.91 (m, 2H, dvds),
2.70 (d, J ) 6.0 Hz, 1H, dvds), 2.66 (d, J ) 6.2 Hz, 1H, dvds),
2.25 (s, 6H, CH3(POP)), 0.22 (s, 6H, dvds), -0.33 (s, 6H, dvds).
31P{1H} (160 MHz, 25 °C, CDCl3) δ -10.1. ESI-MS: m/z ) 755.2
[M + Na]+.
Synthesis of 6a. Yield of 6a: 85%, 0.234 g (6: 0.40 mmol, 0.148
g). 1H NMR (400 MHz, 25 °C, CDCl3) δ 9.08 (brs, 1H, H arom.),
7.95 (d, J ) 7.9 Hz, 1H, H arom.), 7.86 (d, J ) 7.3 Hz, 1H, H
arom.), 7.43-7.25 (m, 3H, H arom.), 7.02 (brs, 4H, H arom.), 6.81
(d, J ) 10.5 Hz, 2H, H arom.), 3.27 (brs, 4H, dvds), 2.85 (brm,
2H, dvds), 2.35 (s, 3H, CH3(napht.)), 2.13 (s, 6H, CH3(POP)), 0.21
(s, 6H, dvds), -0.39 (s, 6H, dvds). 31P{1H} (160 MHz, 25 °C,
CDCl3) δ -27.8. ESI-MS: m/z ) 683.2 [M + Na]+.
General Procedure for the Synthesis of the Rh(CO)-
(phosphine)2Cl. Tetracarbonyldi-µ-chlorodirhodium complex (10
mg, 2.57 × 10-5 mol) and the desired amount of the phosphine (4
Synthesis of 7a. Yield of 7a: 30%, 0.145 g (7: 0.40 mmol, 0.621
1
g). H NMR (400 MHz, 25 °C, CDCl3) δ 7.27 (dd, J ) 1.4, 9.9
9
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Phospines for Selective Telomerization of 1,3-Butadiene
A R T I C L E S
Hz, 6H, H arom.), 7.14-7.10 (m, 6H, H arom.), 7.04 (brs, 8H, H
arom.), 7.03 (brs, 4H, H arom.), 6.94 (m, 3H, H arom.), 3.31-3.15
(m, 6H, dvds), 1.88 (s, 36H, CH3 arom.), 0.21 (s, 3H, dvds), -0.47
(s, 3H, dvds).31P{1H} (160 MHz, 25 °C, CDCl3) δ 29.8. ESI-MS:
m/z ) 1201.6 [M + Na]+ and intense decomposition peaks.
Synthesis of 9a. Yield of 9a: 60%, 0.155 g (9: 0.40 mmol, 0.141
load). The autoclave was closed, purged twice with low pressure
nitrogen (6 bar or 600 kPa) to substantially remove oxygen
contained in the autoclave. A stainless steel sample cylinder was
filled with a crude C4 stream that contained approximately 50 wt
% of 1,3-butadiene, based upon total crude C4 stream weight and
pressure. The content was added to the autoclave with low pressure
nitrogen (6 bar or 600 kPa). The temperature in the autoclave was
raised to the desired work temperature (60, 75, 90, or 100 °C, as
shown in Table 5). The catalyst was prepared with palladium acetyl
acetonate (Pd(acac)2) plus 2 molar equiv of the phosphine. One
molar equivalent of acetic acid may be added to increase storage
stability. The catalyst was prepared in methanol by dissolving all
three components such that the palladium concentration in methanol
equals about 500 ppm. An amount of the catalyst solution was
weighed, such that the palladium concentration in the reactor after
addition of all raw materials was 10 ppm based upon total weight
of raw materials, into a drybox, and then the catalyst solution was
placed into a stainless steel sample cylinder. The catalyst solution
was added to the autoclave using high pressure nitrogen (19-20
bar, or 1900-2000 kPa). Following catalyst addition, the reaction
began, producing the final product. Samples were taken from the
autoclave at set times (5 min after catalyst addition and at 30-min
intervals thereafter), and gas and liquid phases of the samples were
analyzed via GC.
Palladium Precipitation. In the reactor, precipitation is deter-
mined by measuring palladium concentration in the liquid phase
after the reaction and comparing that to a theoretical number based
on total amount of palladium added and total liquid volume,which
includes liquids added at the beginning of the reaction and liquids
formed due to the butadiene conversion. Palladium concentration
in the liquid is measured using inductively coupled plasma atomic
emission spectroscopy (ICP-AES). Tests for conversion, selectivity
to MOD-1, and palladium precipitation were made at methanol to
butadiene ratios of 2, 2.6, and 5, respectively.
1
g). H NMR (400 MHz, 25 °C, CDCl3) δ 7.43-7.31 (m, 6H, H
arom.), 6.91 (brt, J ) 7.5 Hz, 3H, H arom.), 6.82 (dd, J ) 3.9, 8.1
Hz, 3H, H arom.), 3.40 (s, 9H, OCH3), 3.06-2.87 (m, 6H, dvds),
0.24 (s, 3H, dvds), -0.21 (s, 3H, dvds).31P{1H} (160 MHz, 25 °C,
CDCl3) δ -17.7. ESI-MS: m/z ) 667.2 [M + Na]+.
Synthesis of 10a. The complex is obtained after evaporation of
dvds without washing. Yield of 10a: quant, 0.325 g (10: 0.40 mmol,
1
0.213 g). H NMR (400 MHz, 25 °C, CDCl3) δ 5.98 (d, J ) 2.9
Hz, 6H, H arom.), 3.80 (s, 9H, p-OCH3), 3.46 (s, 18H, o-OCH3),
3.22 (dd, J ) 6.6, 13.4 Hz, 2H, dvds), 2.79 (m, 2H, dvds), 2.64
(m, 2H, dvds), 0.21 (s, 3H dvds), -0.21 (s, 3H, dvds). 31P{1H}
(160 MHz, 25 °C, CDCl3) δ -21.6.
Synthesis of 11a. Yield of 11a: 65%, 0.191 g (11: 0.40 mmol,
1
0.164 g). H NMR (400 MHz, 25 °C, CDCl3) δ 7.59 (m, 1H, H
3
arom.), 7.39-7.31 (m, 2H, H arom.), 7.21 (t, J ) 8.3 Hz, 1H, H
arom.), 6.99 (m, 1H, H arom.), 6.43 (d, J ) 8.3 Hz, 2H, H arom.),
3.64 (s, 6H, OCH3), 3.15 (dd, J ) 5.5, 13.4 Hz, 2H, dvds), 2.92
(dd, J ) 4.7, 15.7 Hz, 2H, dvds), 2.42 (m, 2H, dvds), 2.15-2.05
(m, 2H, P-Cy), 1.82-1.60 (m, 10H, P-Cy), 1.35-1.10 (m, 10H,
P-Cy), 0.26 (s, 3H, dvds), -0.23 (s, 3H, dvds). 31P{1H} (160 MHz,
25 °C, CDCl3) δ 35.8. ESI-MS: m/z ) 725.2 [M + Na]+.
Catalysis. Catalytic reaction carried out at 60 °C with
phosphine-palladium-dvds complexes (cf. Table 4). In a glovebox,
the desired amounts of NaOMe (1 mol %, 0.07 g), MeOH (0.266
mol, 8.52 g, 10.8 mL), and palladium complex (0.005 mol %) were
mixed in a stainless steel autoclave equipped with a Teflon inset
(25 mL). At the bench, the desired amount of 1,3-butadiene (0.133
mol, 7.2 g) was condensed from the gas bottle into a cooled (N2
liquid/acetone bath) metallic cylinder (Swagelok, 30 mL). Then,
the 1,3-butadiene was condensed from the cylinder into the cooled
autoclave, and the vessel was heated to the reaction temperature
for the desired time. After 16 h, the autoclave was cooled down
with an ice bath, and the remaining butadiene was recondensed in
a cylinder. Then, the autoclave was opened, and decane (0.5 mL,
0.365 g) was added to the reaction mixture. The conversion and
yield of the telomerization product were determined by GC-FID
analysis.
Acknowledgment. This work was supported by Dow Chemicals
(Benelux). We thank the Spanish Ministerio de Educacio´n y Ciencia
for a “Ramon y Cajal” contract (Z.F.), Project CTQ2008-00683,
Consolider Ingenio 2010 (Grant no. CSD2006_0003) and the
European Union for a Marie Curie Chair of Excellence Grant
(P.W.N.M.v.L.) (MEXC-CT-2005-0023600).
Scale-Up Catalytic Reaction (cf. Table 5). The reaction was
carried out in a 1 L Parr reactor made from electropolished stainless
steel. For each reaction, the Parr reactor’s autoclave was filled with
specified amounts of methanol, promoter (sodium methoxide, at a
promoter to palladium molar ratio of 5:1), and inhibitor (diethyl
hydroxyl amine, approximately 20 parts by weight per million parts
by weight (ppm) based on total weight of methanol plus crude C4
Supporting Information Available: Representative experi-
mental procedures, kinetics, data for the catalytic experiments
and X-ray crystallography data in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA100521M
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J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6473