Telomerisation of buta-1,3-diene and methanol: Superiority of chromanyl-type phosphines in the dow process for the industrial production of 1-MOD

Article abstract of DOI:10.1002/chem.201100684

Butadiene and methanol were telomerised in the presence of palladium catalysts with ligands containing 8-diphenylphosphinochromane-like substituents at phosphorus. MonoXantphos and monoSPANphos afforded the most active, stable and selective catalysts known to date under commercially relevant production conditions for 1-methoxyocta-2,7-diene, the precursor to oct-1-ene.

Full text of DOI:10.1002/chem.201100684

DOI: 10.1002/chem.201100684
Telomerisation of Buta-1,3-diene and Methanol: Superiority of Chromanyl-
Type Phosphines in the Dow Process for the Industrial
Production of 1-MOD**
Mathieu J.-L. Tschan,[a] Hꢀlꢁne Launay,[b] Henk Hagen,[b] Jordi Benet-Buchholz,[a] and
Piet W. N. M. van Leeuwen*[a]
Abstract: Butadiene and methanol were telomerised in the presence of palladium
catalysts with ligands containing 8-diphenylphosphinochromane-like substituents
at phosphorus. MonoXantphos and monoSPANphos afforded the most active,
stable and selective catalysts known to date under commercially relevant produc-
tion conditions for 1-methoxyocta-2,7-diene, the precursor to oct-1-ene.
Keywords: carbenes · ligand design ·
palladium phosphane ligands
telomerisation
·
·
Introduction
The telomerisation of 1,3-
dienes with nucleophiles has
been studied extensively in
both academic and industrial
laboratories.[1] The telomerisa-
Scheme 2. Telomerisation of buta-1,3-diene.
tion of buta-1,3-diene (Bd)
with methanol is the key step
of the commercial route devel-
oped by the Dow Chemical Company (Dow) to produce
oct-1-ene (Scheme 1).[2] The process came on stream in Tar-
ragona in 2007 and is based on the use of a crude C4 cut
(containing 50 wt% of buta-1,3-diene). The telomerisation
product, 1-methoxyocta-2,7-diene (1-MOD),[3] is fully hydro-
genated to 1-methoxyoctane in the next step. Subsequent
thermal cracking of 1-methoxyoctane gives oct-1-ene, the
desired product, together with methanol that can be recy-
cled.
Apart from 1-MOD, two main byproducts—3-methoxyoc-
ta-1,7-diene (3-MOD) and octa-1,3,7-triene (OCT,
Scheme 2) are formed. At high temperatures the thermal
Diels–Alder adduct vinylcyclohexene is also obtained. The
catalytic system currently used in the commercial process is
based on Pd/PPh3 and gives moderate results under typical
conditions in terms of activity, selectivity and stability:[4] the
best-performing catalyst under laboratory conditions is an
N-heterocyclic carbene (NHC)Pd complex reported by
Beller and co-workers.[5a,b]
Scheme 1. Simplified view of the Dow process.
Results and Discussion
[a] Dr. M. J.-L. Tschan, Dr. J. Benet-Buchholz,
Dr. P. W. N. M. van Leeuwen
We recently reported bulky phosphines that efficiently cata-
lysed the telomerisation of butadiene with methanol.[6] Phos-
phine 1 is a new, efficient ligand for the telomerisation of
buta-1,3-diene with methanol under commercial production
conditions.[6] This excellent result prompted us to investigate
the electronic and steric effects of bulky phosphine ligands
on the selectivity, productivity, activity and stability of the
catalyst for the telomerisation of buta-1,3-diene with metha-
nol. A new range of bulky phosphines (2–11) was thus de-
Institute of Chemical Research of Catalonia (ICIQ)
Avinguda Paꢀsos Catalans, 16, 43007 Tarragona (Spain)
Fax : +34 977 920 221
E-mail: pvanleeuwen@iciq.es
[b] Dr. H. Launay, Dr. H. Hagen
Dow Benelux B.V. Herbert H. Dowweg 5
P.O. box 48, 4530 AA, Terneuzen (Netherlands)
[**] 1-MOD=1-methoxyocta-2,7-diene
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100684.
8922
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8922 – 8928

FULL PAPER
formed by the ligand, in which
the palladium catalyst nicely
fits (Figure 1).[8]
Catalytic
experiments:
All
phosphines were tested in the
telomerisation of buta-1,3-diene
with MeOH under commercial
(Dow) production conditions
(908C, NaOMe/Pd (molar)=5,
Pd/Bd=0.0025 mol%,
crude
C4). Different temperatures
(60–1008C) and different meth-
anol/butadiene ratios (2, 2.6 wt/
wt, 3.4, 4.4 mol/mol, respective-
ly) were used in particular
cases, because higher MeOH/
butadiene ratios increase 1-
MOD selectivity.[5c]
veloped, with a focus on the chromane substituent and relat-
ed fragments.
Ligand synthesis: Phosphines 2–6 were prepared by the
same method as reported for 1,[6] from 2,7-di-tert-butyl-9,9-
dimethyl-9H-xanthene by selective bromination, lithiation
and phosphorylation. Phosphines 10 and 11 were prepared
by the same strategy. The synthesis thus proceeds through
the formation of two new monobrominated compounds: the
2-bromo-di-p-tolyl ether (12) and 8-bromo-4,4,4’,4’,6,6’-hex-
amethylspiro-2,2’-bichroman (13).
The bromo ether (12) is obtained as a di-p-tolyl ether/12/
2,2’-dibromo-di-p-tolyl ether mixture (5:80:15) by treatment
of di-p-tolyl ether with N-bromosuccinimide (NBS). In the
same manner, the 8-bromo-4,4,4’,4’,6,6’-hexamethylspiro-
2,2’-bichroman (13) is obtained as a mixture of the starting
material, compound 13 and the dibromo compound
(20:65:15) by treatment of the 4,4,4’,4ꢂ,6,6’-hexamethylspiro-
2,2’-bichroman (SPAN) backbone with NBS.
The bromo compound mixtures, which give backbone/
monophosphine/diphosphine mixtures after lithiation/phos-
phorylation, were used without further purification, because
in our hands it remains easier to separate the phosphine
product mixtures. In the cases of 7–9, one-pot, one-step syn-
theses—direct lithiation of the backbone at room tempera-
ture followed by phosphorylation as reported for the synthe-
sis of DPEphos and xantphos derivatives[7]—were investigat-
ed.
Non-optimised overall yields were 45% for phosphines 2
and 3, 52% for 6, around 20% for 4 and 5 and 37% for 11.
Phosphines 7–9 were obtained pure in 21–45% yields. The
yields are moderate for all ligands, due to the difficulty of
selective monofunctionalisation of a symmetrical backbone
(e.g., concomitant formation of the corresponding diphos-
phines).
Figure 1. ORTEP plot (thermal ellipsoids shown at 50% probability
level) of compound 11. Selected distances (ꢃ) and angles (8): P1–C2:
1.8390(8); P1–C24: 1.8376(9); P1–C30: 1.8337(9). C2-P1-C24: 101.94(4);
C2-P1-C30: 100.40(4); C24-P1-C30: 102.89(4).
No differences were observed with use of well-defined L-
Pd0-diolefin complexes or PdII/ligand precatalysts,[6,5a,b] so
catalytic reactions were carried out with a phosphine/PdII
precatalyst. All phosphines were tested and compared with
1 and PPh3 to study structure–reactivity relationships of the
phosphine for the telomerisation reaction.
The performances of 2–11 are presented in Table 1.
Almost all of the new ligands gave more productive and se-
lective catalysts than PPh3, up to 10 and 11% respectively.
It seems not to be a straightforward matter to find general
rules relating to the influence of steric/electronic properties
of the ligand, because the two factors are always subtly asso-
ciated. It appears that for a similar ligand type, a phosphine
Crystals of 11 suitable for X-ray diffraction analysis were
obtained. The molecular structure of 11 shows the niche
Chem. Eur. J. 2011, 17, 8922 – 8928
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8923

P. W. N. M. van Leeuwen et al.
Table 1. Catalytic results obtained for the ligands 1–11 at 908C over
(cf. 7, 8). Higher Pd loss, up to 41%, was observed with use
of a less bulky and less rigid backbone (cf. 9, 10), which also
gave a slightly lower selectivity. Catalyst selectivity and pro-
ductivity dropped when phosphine 8 was used, probably be-
cause of the presence of the sulfur atom.
MonoSPANphos 11 performed similarly to 1 in terms of
productivity, selectivity and stability at 908C (Table 2). Inter-
estingly, ligand 11 exhibited a remarkable activity at low
2.5 h.[a]
Ligand Conv. Bd 1-MOD 3-MOD OCT Chemo. Pd loss
[%]
[%]
[%]
[%]
[%][c]
[%][b]
1
2
3
4
5
6
7
8
PPh3
85
93
75
93
91
95
83
92
86
87
93
94
82
89
77
90
90
93
91
87
83
87
88
88
5
5
5
5
5
4
5
5
5
5
5
6
13
6
18
5
5
3
4
8
12
8
7
87
94
82
95
95
97
96
92
88
92
93
94
17
6
1
2
3
4
5
6
7
8
9
17
14
14
22
10
22
25
41
23
8
Table 2. Catalytic results obtained for 11.[a]
9
10
11 10
12 11
L (L/Pd)
Pd/Bd
[10À6 mol] [h] [8C] Pd
[mol]
t
T
Base/
Conv.
Bd
[%]
1-MOD Pd
G
sel.
loss
A
[%][b]
[%][c]
6
1
2
3
4
5
6
7
1 (2)
1 (2)
1 (2)
11 (2)
11 (2)
11 (2)
IMesHCl
(1)
25
25
25
25
25
25
25
2.5 60
2.5 75
2.5 90
2.5 60
2.5 75
2.5 90
2.5 90
5
5
5
5
5
5
5
36
67
93
71
82
94
32
94
93
89
94
91
88
86
2
–
6
0
–
[a] Reaction conditions: Pd (0.0025 mol% vs. Bd), NaOMe
(0.0125 mol% vs. butadiene), ligand/Pd 2:1, MeOH/Bd 2.6, wt/wt.
[b] Measurement error Æ5% absolute. [c] Chemoselectivity (%)=(1-
MOD+3-MOD)/(1-MOD+3-MOD+OCT)ꢄ100.
8
23
that is less basic than 1 gives a less productive catalyst and/
or is less active (cf. 1 with 2 (p-CF3) and 6 (POP); Table 1,
entries 2, 3, 7 and Figure 2) and also gives a less chemoselec-
8
9
IMesHCl
(1)
IMesHCl
(1)
25
25
2.5 90
2.5 90
100
87
93
97
96
45
77
1000
[a] Reaction conditions: MeOH/Bd: 2.6 wt/wt. [b] 1-MOD selectivity (%)
is 1-MOD/(1-MOD+3-MOD+OCT)ꢄ100. Measurement error Æ5%
absolute.
temperature and it was studied in detail (Table 2). Com-
pound 11 should thus permit higher selectivity and produc-
tivity to be obtained and energy to be saved relative to 1 or
PPh3. Moreover, to highlight the excellent performances of
1 and 11 for the telomerisation, they were compared with
IMes
(IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-yli-
dene, used as its IMesHCl salt), an N-heterocyclic carbene
reported by Beller and co-workers,[5a,b] under suitable pro-
duction conditions (crude C4, low amounts of promoter).
The results are presented in Table 2 (results obtained for 1
were reported previously).[6]
Surprisingly, at 908C, the ligands 1 and 11 form more pro-
ductive catalysts than IMes and the last compound shows a
lower selectivity than reported previously (Table 2, entries 3,
6, 7).[5a,c] Obviously, activity and productivity cannot be com-
pared with the reported values because [Bd] in the medium
is lower than that in our work (higher MeOH/Bd ratio,
crude C4, impurities). To achieve excellent performances
with an NHC ligand, a high NaOMe/Pd ratio is required—
from 100 to 1000 (Table 2, entries 8 and 9; Figure 3)—proba-
bly due to poor precatalyst formation at low base concentra-
tions,[10] but such conditions are not viable industrially.
Moreover, under such conditions the loss of palladium is
very important, up to 77% (Table 2, entry 9).
Figure 2. Formation of 1-MOD (% yield) over time:[a] influence of elec-
tronic properties. Reaction conditions: cf. Table 1.
tive catalyst (cf. 1 with 2). More basic ligands thus give
higher chemoselectivity (cf. 3, 4, 5; Table 1, entries 4, 5, 6,
respectively,). A positive influence of the o-MeO substitu-
ents in 5 on catalyst selectivity, activity and productivity is
highlighted.[6,9]
High activity (Figure 2), high conversion and high selec-
tivity (95 and 93%, respectively; Table 1, entry 6) for 1-
MOD was thus achieved with 5 as the ligand at 908C under
commercial production conditions. More basic ligands led to
increases in Pd loss because the phosphine is more easily
oxidised during the catalysis.
At 608C, the ligand 11 is highly superior to 1 and IMes,
showing a turn-over number (TON) of 26696 after 2.5 h and
a selectivity for 1-MOD of 94% (Table 2, entries 4 and 1).
As shown in Figure 3, the presence of more equivalents of
NaOMe significantly increased the rate of formation of 1-
Decreasing the steric bulk of the xanthene backbone in-
fluenced the stability of the catalyst, as expressed in Pd loss,
whereas selectivity and productivity decreased only slightly
8924
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8922 – 8928

Telomerisation of Buta-1,3-diene and Methanol
FULL PAPER
(5 min) ꢀTOF (30 min)) only at 908C (TOFs are calculated
with respect to butadiene).
Conclusion
In summary, we have studied the steric and electronic ef-
fects of phosphines on the telomerisation of butadiene and
methanol. It appears that the presence of chromanyl-like
moieties such as xanthene or bischromane backbones
(SPANphos) in phosphine ligands is exceptionally well
suited for the telomerisation, especially with regard to selec-
tivity, activity and productivity/stability of the catalyst. The
ligand 5 gives rise to 95% Bd conversion and 93% selectivi-
ty for 1-MOD at 908C and is the best ligand of the monoX-
antphos series. Under industrial production conditions, mon-
oXantphos 1 and monoSPANphos 11, especially, are better
ligands than IMes even at 908C. To the best of our knowl-
edge, 11 is the most active ligand at low temperature for this
reaction. The moderate steric bulk of the substituent and
the weakly coordinating nature of the chromanyl oxygen
atom are believed to be responsible for this performance.
Figure 3. Formation of 1-MOD (% yield) over time (Pd (0.0025 mol%),
MeOH/Bd=2.6, NaOMe (5 equiv vs. Pd)); comparison of IMes, 11 and
1.
MOD in the case of IMes ligand; a similar effect was ob-
served for PPh3 (see the Supporting Information), so the ki-
netic profiles (Figure 3) of IMes with NaOMe (1000 equiv
vs. Pd) and of 1 or 11 with NaOMe (5 equiv vs. Pd) are not
comparable.[11] IMes performed less well than 11 in the pres-
ence of 5 or even 100 equiv of NaOMe, giving rates inter-
mediate between those of 1 and 11. The attractive feature of
the NHC ligand is that, under suitable conditions, the 1-
MOD selectivity does not depend on reaction temperature
and remains constant at ꢀ96%.
Experimental Section
General procedures: All reactions were carried out under argon with use
of Schlenk techniques. Deuterated chloroform was distilled over calcium
hydride under argon prior to use. All chlorodiphenylphosphine deriva-
tives were purchased from Alfa Aesar and used as received. Di-p-tolyl
ether, 9,9-dimethylxanthene, and phenoxathiine were purchased from Al-
drich and used without further purifications. 4,4,4’,4’,6,6’-Hexamethyl-
spiro-2,2’-bichroman was prepared by a previously described method.[12]
4-Bromo-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene (impure) was pre-
pared by the reported method.[6a] NMR spectra were recorded with a
Bruker 400 MHz spectrometer. Chemical shifts are reported in ppm, and
The formation of 1-MOD versus time at different temper-
atures for 11 and 1 is shown in Figure 4 at a MeOH/Bd ratio
of 2 (2.6 is reported in the Supporting Information). In the
case of 11, the 1-MOD formation rate is significantly higher
than in that of 1 (Figure 4). The initial rate at 608C for 11 is
similar to that of 1 at 908C. With regard to the activity of
1
were calibrated with the aid of the residual H and 13C resonances of the
deuterated solvents. Coupling constants (J) are expressed in Hz. Mass
spectra and X-ray structure analyses were carried out at the Research
Support Unit of the ICIQ (Tarragona, Spain). Analyses of the catalytic
reactions were performed with a GC-FID and HP-5 (5% phenyl methyl
siloxane; 30 mꢄ320 mmꢄ0.25 mm) capillary column. Elemental analyses
were performed at the Unidade de Anꢅlise Elemental of the Universi-
dade de Santiago de Compostela (Spain).
the catalyst, an impressive turn-over frequency (TOFmax
;
5 min) of 170000 hÀ1 (30 min; 52000 hÀ1) is observed at
908C with 11 (conversion of Bd=35 and 65%, respective-
ly); compound 1 exhibits a TOFmax of 40000 hÀ1 (for 1, TOF
Synthesis of 4-(bis-p-trifluoromethylphenylphosphino)-2,7-di-tert-butyl-
9,9-dimethylxanthene (2): nBuLi (2.5m in hexane, 1.24 mL, 1.1 equiv)
was added slowly at À788C to a solution of 4-bromo-2,7-di-tert-butyl-9,9-
dimethyl-9H-xanthene (1,13 g, 2.8 mmol) in THF (30 mL), and the mix-
ture was allowed to stir at À788C for 2 h. A solution of chloro-bis-p-tri-
fluoromethylphenylphosphine (1 g, 2.8 mmol in 15 mL of THF) was
added dropwise to the reaction mixture at À788C, and the mixture was
allowed to stir for 2 h at À788C and overnight at RT. The solvent was
evaporated under vacuum and dichloromethane (or diethyl ether, 20 mL)
and degassed water (10 mL) were then added. The organic layer was re-
covered and dried with MgSO4. Evaporation of the solvent afforded a
white solid. The product was purified by column chromatography over
silica gel with use of a gradient elution from hexane to hexane/CH2Cl2
(8:1) and was obtained from the third fraction as a white solid (55%).
1H NMR (400 MHz, CDCl3, 258C): d=7.61 (m, 4H; H arom.), 7.50 (m,
5H; H arom.), 7.40 (d, J=2.3 Hz, 1H; H arom.), 7.10 (dd, J=2.3, 8.5 Hz,
1H; H arom.), 6.75 (dd, J=2.3, 7.0 Hz, 1H; H arom.), 6.46 (d, J=8.5 Hz,
Figure 4. Formation of 1-MOD (% yield) over time (Pd (0.0025 mol%),
1H; H arom.), 1.67 (s, 6H; CH3), 1.32 (s, 9H; CACHTUNGTREN(NGNU CH3)), 1.20 ppm (s, 9H;
MeOH/Bd=2); comparison of the ligands 11 and 1.
C
N
=
Chem. Eur. J. 2011, 17, 8922 – 8928
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8925

P. W. N. M. van Leeuwen et al.
11.6 Hz), 148.0, 146.0, 145.7 (d, JP, C =3.6 Hz), 141.4 (d, JP,C =15.0 Hz),
133.9 (d, JP, C =20.3 Hz), 131.0, 130.0 (d, JP, C =1.4 Hz), 129.3, 129.2 (d,
1.16 ppm (s, 9H; C
154.4, 149.8 (d, JP, C =9.5 Hz), 148.1, 145.5, 144.7 (d, JP, C =3.0 Hz), 135.8,
(CH3)); 13C{1H} NMR (100 MHz, CDCl3, 258C): d=
ACHTUNGTRENNUNG
J
P, C =10.2 Hz), 125.1 (m), 124.5, 124.3, 124.0 (d, JF,C =270.0 Hz) 122.0,
135.5, 132.3 (d, JP, C =9.3 Hz), 131.5, 128.9 (d, JP, C =9.3 Hz), 128.1 (d, JP, C
9.0 Hz), 125.3 (d, JP, C =21 Hz), 124.2, 123.9, 122.5, 117.2, 117.1 (d, JP,C
=
121.2 (d, JP,C =13.4 Hz), 115.5, 34.8, 34.6, 34.34, 31.8, 31.5 31.4 ppm;
19F{1H} NMR (376 MHz, CDCl3, 258C): d=À62.87 ppm; 31P{1H}NMR
(160 MHz, CDCl3, 258C): d=À9.03 ppm; elemental analysis calcd (%)
for C37H37F6OP: C 69.15, H 5.80; found: C 69.35, H 5.89.
=
2.1 Hz), 115.6, 34.4, 34.4, 34.3, 32.5, 31.59, 31.3, 20.6 ppm; 31P{1H} NMR
(160 MHz, CDCl3, 258C): d=À60.50 ppm; elemental analysis calcd (%)
for C37H41O2P: C 80.99, H 7.53; found: C 80.95, H 7.50.
Synthesis of 4-(bis-p-tolylphosphino)-2,7-di-tert-butyl-9,9-dimethylxan-
thene (3): The same procedure as described for the synthesis of 2 was fol-
lowed (chloro-bis-p-tolylphosphine, 0.696 g, 2.8 mmol in 15 mL of THF).
The product was purified by column chromatography over silica gel with
use of a gradient elution from hexane to hexane/CH2Cl2 (10:3) and ob-
Synthesis of compounds 7–9: nBuLi (1.6m in hexane, 1.2 equiv, 6 mmol,
3.75 mL) was added dropwise at room temperature to a stirred solution
of the backbone (5 mmol; 9,9-dimethylxanthene (1.05 g), phenoxathiin
(1.013 g), di-p-tolyl ether (0.991 g)) and TMEDA (1.2 equiv, 6 mmol,
0.697 g, 0.899 mL) in THF (50 mL), and the mixture was stirred for 16 h.
Chlorodiphenylphosphine (1 equiv, 5 mmol, 1.103 g, 0.897 mL) was then
added at room temperature and the mixture was stirred for 16 h more.
After 16 h, solvent was evaporated to dryness and diethyl ether or
CH2Cl2 (ꢀ30 mL) was added, followed by water (10 mL). The organic
layer was recovered and dried over MgSO4 and the solvent was evaporat-
ed to dryness. The pure products 7, 8 and 9 were obtained in 21, 41 and
45% yields, respectively, after purification by column chromatography
over silica gel (eluent: CH2Cl2/hexane 2:10 v/v).
tained as
a
white solid from the third fraction (56%). 1H NMR
(400 MHz, CDCl3, 258C): d=7.40 (dd, J=2.3, 12.9 Hz, 2H; H arom.),
7.29 (m, 4H; H arom.), 7.15 (m, 4H; H arom.), 7.09 (dd, J=2.3, 8.5 Hz,
1H; H arom.), 6.72 (dd, J=2.3, 5.8 Hz, 1H; H arom.), 6.58 (d, J=8.5 Hz,
1H), 2.32 (s, 6H; CH3 tolyl), 1.65 (s, 6H; CH3), 1.31 (s, 9H; C
ACHTUNGTNER(NUGN CH3)),
1.17 ppm (s, 9H; C
ACHTUNGTRENNUNG
150.1 (d, JP,C =13.0 Hz), 148.3, 145.4, 145.1 (d, JP, C =2.3 Hz), 138.5, 133.9
(d, JP, C =19.7 Hz), 131.8 (d, JP, C =10.9 Hz), 129.4, 129.2 (d, JP,C =2.2 Hz),
129.1, 129.1, 128.9 (d, JP,C =4.9 Hz), 124.0, 123.3, 121.9, 115.9, 34.5, 34.4,
34.4, 31.8, 31.5, 31.4, 31.3 ppm; 31P{1H} NMR (160 MHz, CDCl3, 258C):
d=À12.06 ppm; elemental analysis calcd (%) for C37H43OP: C 83.11, H
8.11; found: C 82.95, H 8.16.
4-(Diphenylphosphino)-9,9-dimethylxanthene (7): 1H NMR (400 MHz,
CDCl3, 258C): d=7.46–7.34 (m, 12H), 7.09–6.98 (m, 3H), 6.63 (m, 2H),
1.64 ppm (s, 6H); 13C{1H} NMR (100 MHz, CDCl3, 258C): d=152.3 (d,
J
P, C =14.3 Hz), 150.5, 136.5 (d, JP, C =10 Hz), 134.0 (d, JP, C =20.1 Hz), 131.4
Synthesis of 4-(bis-p-methoxyphenylphosphino)-2,7-di-tert-butyl-9,9-di-
methylxanthene (4): The same procedure as described for the synthesis
(d, JP, C =2.7 Hz), 130.4, 130.3, 128.8, 128.5 (d, JP, C =7.0 Hz), 127.2, 126.5,
125.3, 125.1 (d,
JP, C =14.4 Hz), 123.3, 123.2, 116.6, 34.4, 31.5 ppm;
31P NMR (160 MHz, CDCl3, 258C): d=À11.72 ppm; elemental analysis
of
2 was followed (chloro-bis-p-methoxyphenylphosphine, 0.785 g,
2.8 mmol in 15 mL of THF). The product was purified by column chro-
matography over silica gel with use of a gradient elution from hexane to
hexane/CH2Cl2 (1:1) and obtained as a white solid (28%). 1H NMR
(400 MHz, CDCl3, 258C): d=7.40–7.30 (m, 6H; H arom.), 7.08 (dd, J=
2.3, 8.5 Hz, 1H; H arom.), 6.89 (dd, J=0.8, 8.7 Hz, 4H; H arom.), 6.68
(dd, J=2.2, 5.6 Hz, 1H; H arom.), 6.57 (d, J=8.5 Hz, 1H), 3.82 (s, 6H;
calcd (%) for C27H23OP: C 82.21, H 5.88; found: C 82.56, H 5.98.
4-(Diphenylphosphino)phenoxathiin (8): 1H NMR (400 MHz, CDCl3,
258C): d=7.42–7.32 (m, 10H), 7.14 (m, 1H), 7.08 (m, 1H), 6.94 (m, 3H),
6.54 (ddd, J=1.5, 4.2, 7.6 Hz, 1H), 6.27 ppm (m, 1H); 13C{1H} NMR
(100 MHz, CDCl3, 258C): d=153.5 (d, JP, C =14.7 Hz), 151.9, 135.9 (d,
J
P, C =10.4 Hz), 134.1 (d, JP, C =20.4 Hz), 131.5 (d, JP,C =3.0 Hz), 129.0,
OCH3), 1.65 (s, 6H; CH3), 1.31 (s, 9H; C
(CH3)); 13C{1H} NMR (100 MHz, CDCl3, 258C): d=160.1, 150.0 (d, JP,C
13.7 Hz), 148.3, 145.4, 145.1 (d, JP, C =1.4 Hz), 135.4 (d, JP,C =21.7 Hz),
ACHTUNGTREN(NUGN CH3)), 1.17 ppm (s, 9H; C-
128.6 (d, JP, C =7.4 Hz), 127.8 (d, JP, C =16.7 Hz), 127.5, 127.4, 126.5, 124.6,
124.5, 118.0 ppm; 31P NMR (160 MHz, CDCl3, 258C): d=À11.38 ppm; el-
emental analysis calcd (%) for C37H43O3P: C 74.98, H 4.46; found: C
74.76, H 4.35.
A
=
129.5, 129.3 (d, JP, C =1.4 Hz), 128.6 (d, JP, C =4.2 Hz), 127.8 (d, JP,C
5.9 Hz), 124.5 (d, JP, C =13.0 Hz), 124.1, 123.0, 121.8, 115.9, 114.0 (d, JP,C
=
=
2-(Diphenylphosphino)-di-p-tolyl ether (9): 1H NMR (400 MHz, CDCl3,
258C): d=7.45–7.36 (m, 10H; H arom.), 7.10 (d, J=8.3 Hz; H arom.),
7.06 (d, J=8.4 Hz, 2H; H arom.), 6.78–6.73 (m, 3H; H arom.), 6.65 (dd,
J=2.3, 4.7 Hz, 1H), 2.32 (s, 3H; CH3), 2.23 ppm (s, 3H; CH3);
8.4 Hz), 55.2, 34.7, 34.5, 34.4, 31.8, 31.5, 31.4 ppm; 31P{1H} NMR
(160 MHz, CDCl3, 258C): d=À13.62 ppm; elemental analysis calcd (%)
for C37H43O3P: C 78.42, H 7.65; found: C 78.67, H 7.72.
Synthesis of 4-(bis-o-methoxyphenylphosphino)-2,7-di-tert-butyl-9,9-di-
methylxanthene (5): The same procedure as described for the synthesis
13C{1H} NMR (100 MHz, CDCl3, 258C): d=157.6 (d,
JP,C =16.5 Hz),
154.7, 136.5 (d, JP, C =10.8 Hz), 134.2, 134.0 (d, JP, C =19.8 Hz), 132.6, 132.5,
130.8, 129.9, 128.4, 128.6, 128.3, 118.9, 117.38, 20.8, 20.7 ppm;
31P{1H} NMR (160 MHz, CDCl3, 258C): d =À12.99 ppm; elemental anal-
ysis calcd (%) for C26H23OP: C 81.66, H 6.06; found: C 81.72, H 6.14.
of
2 was followed (chloro-bis-o-methoxyphenylphosphine, 0.785 g,
2.8 mmol in 15 mL of THF). The product was purified by column chro-
matography over silica gel with use of a hexane/CH2Cl2 gradient elution
(8:2 to 1:1) and was obtained as
a
white solid (20%). 1H NMR
Synthesis of 2-(2,7-dimethylphenoxaphosphino)-di-p-tolyl ether (10):
nBuLi (1.6m in hexane, 1.2 equiv, 2.2 mL) was added dropwise at À788C
to a solution of 2-bromo-di-p-tolyl ether 15 (80%, 1 g, 2.9 mmol) in THF
(30 mL) and the mixture was stirred at À788C for 2 h. Chloro-2,7-dime-
thylphenoxaphosphine (3.48 mmol, 0.913 g) in THF (15 mL) was then
added dropwise at À788C and the mixture was stirred for 2 h at À788C
and for 2 h at RT. Solvent was evaporated to dryness and diethyl ether or
CH2Cl2 (ꢀ30 mL) was added, followed by water (10 mL). The organic
layer was recovered and dried over MgSO4 and the solvent was evaporat-
ed to dryness. The pure product was obtained in 70% yield after purifica-
tion by column chromatography over silica gel (eluent: from hexane to
CH2Cl2/hexane 2:10 v/v). 1H NMR (400 MHz, CDCl3, 258C): d=7.39
(brd, J=10.5 Hz, 2H; H arom.), 7.19–7.07 (m, 6H; H arom.), 6.95 (brd,
J=8.3 Hz; H arom.), 6.76 (m, 2H; H arom.), 6.64 (ddd, J=8.2, 4.1,
1.8 Hz, 1H; H arom.), 6.50 (m, 1H; H arom.), 2.37 (s, 3H; CH3), 2.27 (s,
6H; CH3 POP), 2.15 ppm (s, 3H; CH3); 13C{1H} NMR (100 MHz, CDCl3,
258C): d=156.8 (d, JP, C =15.5 Hz), 155.2, 154.6, 136.1, 135.7, 132.6 (m),
132.5, 132.3, 131.5, 130.6, 130.5 (d, JP, C =25 Hz), 130.0, 118.5, 118.0 (d,
(400 MHz, CDCl3, 258C): d=7.38–7.30 (m, 4H), 7.07 (dd, J=2.3, 8.5 Hz,
1H), 6.93–6.90 (m, 2H), 6.83 (m, 4H), 6.73 (dd, J=2.3, 5.8 Hz, 1H), 6.61
(d, J=8.5 Hz, 1H), 3.77 (s, 6H), 1.65 (s, 6H), 1.31 (s, 9H), 1.16 ppm (s,
9H); 13C{1H} NMR (100 MHz, CDCl3, 258C): d=161.5 (d, JP,C =16.8 Hz),
150.6 (d, JP, C =13.8 Hz), 148.6, 145.1, 144.7 (d, JP, C =2.2 Hz), 134.1 (d,
J
P, C =1.4 Hz), 129.9, 129.6, 129.2 (d, JP, C =6.3 Hz), 128.8 (d, JP,C =1.3 Hz),
124.7 (d, JP,C =12.7 Hz), 123.9, 122.7, 122.3 (d, JP, C =15.0 Hz), 121.7, 120.9,
116.1, 110.0 (d, JP, C =1.5 Hz), 55.7, 34.7, 34.5, 34.4, 31.7, 31.6, 31.4 ppm;
31P NMR (160 MHz, CDCl3, 258C): d=À32.38 ppm; elemental analysis
calcd (%) for C37H43O3P: C 78.42, H 7.65; found: C 78.14, H 7.59.
Synthesis of 4-(2,7-dimethylphenoxaphosphino)-2,7-di-tert-butyl-9,9-di-
methylxanthene (6): The same procedure as described for the synthesis
of 2 was used (chloro-2,7-dimethylphenoxaphosphine, 0.735 g, 2.8 mmol
in 15 mL of THF). The product was purified by column chromatography
over silica gel with use of a gradient elution from hexane to hexane/
CH2Cl2 (5:1) and obtained as a white solid from the third fraction
(58%). 1H NMR (400 MHz, CDCl3, 258C): d=7.51 (brdd, J=1.9,
10.7 Hz, 2H; H arom.), 7.38 (d, J=2.3 Hz, 1H; H arom.), 7.29 (m, 2H;
H arom.), 7.20 (d, J=8.5 Hz, 1H; H arom.), 7.16 (brdd, J=2, 8.5 Hz,
2H; H arom.), 7.09 (d, J=8.3 Hz, 2H; H arom.) 6.65 (dd, J=2.3, 7.4 Hz,
J
P, C =1.4 Hz), 117.3, 117.1 (d, JP, C =3.0 Hz), 20.8, 20.7 20.5 ppm;
31P{1H} NMR (160 MHz, CDCl3, 258C): d =À62.51 ppm; elemental anal-
1H; H arom.), 2.33 (CH3 POP) 1.59 (s, 6H; CH3), 1.36 (s, 9H; C
N
ysis calcd (%) for C28H25O2P: C 79.23, H 5.94; found: C 79.10, H 5.92.
8926
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8922 – 8928

Telomerisation of Buta-1,3-diene and Methanol
FULL PAPER
Synthesis of 8-diphenylphosphino-4,4,4’,4’,6,6’-hexamethylspiro-2,2’-bi-
chroman (11) (mono-SPANphos): A solution of 8-bromo-4,4,4’,4’,6,6’-
hexamethylspiro-2,2’-bichroman (16, 65%, 2.2 g, containing 3.44 mmol of
16) in THF (40 mL) was cooled to À788C. nBuLi (2.5m in hexane,
1.9 mL) was then added slowly. The mixture was stirred at À788C for 1 h
and ClPPh2 (0.9 mL) was added. The mixture was stirred at À788C for
1 h and at RT for 1 h. The solvent was then evaporated and degassed
water was added. The product was extracted with CH2Cl2. The organic
layer was separated and dried over MgSO4. Evaporation of the solvent
gave crude product (2.2 g), which was purified by column chromatogra-
phy on silica gel (eluent gradient from hexane to CH2Cl2/hexane 2:5).
The product was isolated as a white solid from the third fraction (1 g,
1.92 mmol, 55%). 1H NMR (400 MHz, CDCl3, 258C): d=7.33–7.36 (m,
3H; H arom.), 7.21–7.15 (m, 3H; H arom.), 7.13–7.09 (m, 1H; H arom.),
7.05–6.95 (m, 4H; H arom), 6.83 (brs, 1H; H arom.), 6.71 (brd, J=
8.2 Hz, 1H), 6.44 (d, J=8.2 Hz, 1H), 6.25 (dd, J=1.9, 4.0 Hz, 1H), 2.24
(s, 3H; CH3 arom.), 2.15 (s, 3H; CH3 arom.), 2.10–2.00 (m, 3H; CH2),
1.89 (d, J=14.1 Hz, 1H), 1.69 (s, 3H; CH3), 1.48 (s, 3H; CH3), 1.38 (s,
3H; CH3), 1.25 ppm (s, 3H; CH3); 13C{1H} NMR (100 MHz, CDCl3,
258C): d=150.9 (d, JP,C =16.7 Hz), 147.9, 137.7 (d, JP, C =11.4 Hz), 136.3
(d, JP,C =10.7 Hz), 134.2 (d, JP, C =19.8 Hz), 133.2 (d, JP, C =19.0 Hz), 132.4,
130.9 (d, JP, C =2.3 Hz), 130.3, 130.3, 130.0, 128.1, 128.0 (d, JP,C =6.6 Hz),
127.8 (d, JP, C =6.6 Hz), 127.6, 127.4, 126.5, 124.5 (d, JP,C =13.1 Hz), 116.8,
containing approximately 50 weight percent of buta-1,3-diene, 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 tem-
perature in the autoclave was raised to the desired work temperature
(908C unless otherwise indicated).
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 on total weight of raw materials, into a dry box, and the
catalyst solution was then placed in a stainless steel sample cylinder. The
catalyst solution was added to the autoclave with use of high-pressure ni-
trogen (19 bar to 20 bar, or 1900 kPa to 2000 kPa). After catalyst addi-
tion, the reaction began, producing the final product. Samples were
taken from the autoclave at set times (five minutes after catalyst addition
and at 30-minute intervals thereafter), and gas and liquid phases of the
samples were analysed by GC (internal standard m-xylene).
Palladium precipitation measurement: Palladium precipitation in the re-
actor was determined by measurement of palladium concentration in the
liquid phase after the reaction and comparison of that with 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 concen-
tration in the liquid was measured by inductively coupled plasma atomic
emission spectroscopy (ICP-AES).
97.8, 46.9, 46.8, 33.0 32.8, 32.3, 31.8 (d, JP, C =8.9 Hz), 31.6, 30.8 (d, JP,C
=
1.7 Hz), 30.1, 20.9, 20.8 ppm; 31P{1H} NMR (160 MHz, CDCl3, 258C): d=
À13.05 ppm; elemental analysis calcd (%) for C35H37O2P: C 80.74, H
7.16; found: C 80.53, H 7.09.
Acknowledgements
Synthesis of 2-bromo-di-p-tolyl ether (12): Di-p-tolyl ether (5.0 g,
25.2 mmol) and NBS (5.5 g, 31 mmol) were dissolved in DMF (150 mL)
and the mixture was stirred at RT for 2 days. The conversion was moni-
tored by GC/MS. If necessary, more NBS can be added. After one day, a
mixture containing the monobromo compound, the dibromo compound
and the starting material, in an 80:15:5 ratio, had been formed. The sol-
vent was evaporated to dryness and the solid was washed with water and
extracted with diethyl ether. The organic layer was dried over MgSO4
and concentrated to dryness. The obtained solid was washed with ethanol
to give a white solid (4.5 g, containing a mixture of the monobromo com-
pound, the dibromo compound and the starting material 80:15:5). The
mixture was used without further purification to prepare the phosphine.
This work was supported by Dow Chemicals (Benelux). We thank the
European Union for
a Marie Curie Chair of Excellence Grant
(P.W.N.M.v.L.) (MEXC-CT-2005-0023600, http://europa.eu.int/mariecur-
ieactions).
[1] For reviews, see: a) N. Yoshimura, Applied Homogeneous Catalysis
with Organometallic Compounds, 2nd ed. (Eds.: B. Cornils, W. A.
Herrmann), Wiley-VCH, Weinheim, 2002, pp. 361–367; b) J. Tsuji,
Acc. Chem. Res. 1973, 6, 8–15; c) J. Tsuji, Palladium Reagents and
Catalysts: Innovations in Organic Synthesis Wiley, New York, 1995,
p. 422; d) J. M. Takacs, Comprehensive Organometallic Chemistry II,
Vol. 12 (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson), Pergamon
Press, Oxford, 1995, p. 785; e) R. F. Heck, Palladium Reagents in Or-
ganic Syntheses, Academic Press, London, 1985; f) A. Behr, Aspects
of Homogeneous Catalysis, Vol 5 (Ed.: R. Ugo), Reidel, Dordrecht,
1984, p. 3; g) D. J. Nielsen, K. J. Cavell, N-Heterocyclic Carbenes in
Synthesis (Ed.: S. P. Nolan), Wiley-VCH, Weinheim, 2006, pp. 73–
102; h) N. D. Clement, L. Routaboul, A. Grotevendt, R. Jackstell,
M. Beller, Chem. Eur. J. 2008, 14, 7408–7420; i) A. Behr, M.
Becker, T. Beckmann, L. Johnen, J. Leschinski, S. Reyer, Angew.
Chem. 2009, 121, 3652–3669; Angew. Chem. Int. Ed. 2009, 48, 3598–
3614.
Synthesis of 8-bromo-4,4,4’,4’,6,6’-hexamethylspiro-2,2’-bichroman (13):
4,4,4’,4’,6,6’-hexamethylspiro-2,2’-bichroman (2 g, 5.9 mmol) and NBS
(1.05 g, 5.9 mmol) were dissolved in DMF (100 mL) and stirred at RT for
1 day. The conversion was monitored by GC/MS. If necessary, more NBS
can be added. After two days, a mixture of the monobromo compound,
the dibromo compound and the starting material in a 65:15:20 ratio had
been formed. The solvent was evaporated to dryness and the solid was
washed with water and extracted with diethyl ether. The diethyl ether so-
lution was dried over MgSO4 and evaporated to dryness. The obtained
solid was washed with ethanol to give a white solid (2 g, containing a
mixture of the monobromo compound, the dibromo compound and the
starting material 65:15:20). The mixture was used without further purifi-
cation to prepare the phosphine.
[2] R. C. Bohley, G. B. Jacobsen, H. L. Pelt, B. J. Schaart, M. Schenk,
D. A. G. van Oeffelen (Dow Benelux N.V.), WO 92/10450, 1992.
[3] G. B. Jacobsen, H. L. Pelt, B. J. Schaart, WO 91/09822 (Dow Bene-
lux N.V) 1991. J. Briggs, J. Patton, S. Vermaire-Louw, P. Margl, H.
Hagen, D. Beigzadeh (Dow Global Technologies, Inc.), WO 2010/
019360A2, 2010.
[4] Under Dow production conditions at 908C, typical results for PPh3-
based systems are: Bd conversion=85% (TON=34000, TOF
(30 min)=25500 hÀ1), 1-MOD=82%, Pd loss=17%.
[5] a) R. Jackstell, S. Harkal, H. Jiao, A. Spannenberg, C. Borgmann, D.
Roettger, F. Nierlich, M. Elliot, S. Niven, C. Kingsley, O. Navarro,
M. S. Viciu, S. P. Nolan, M Beller, Chem. Eur. J. 2004, 10, 3891–
3900; b) R. Jackstell, G. A. Andreu, A. Frisch, K. Selvakumar, A.
Zapf, H. Klein, A. Spannenberg, D. Rottger, O. Briel, R. Karch, M.
Beller, Angew. Chem. 2002, 114, 1028–1031; Angew. Chem. Int. Ed.
2002, 41, 986–989; c) F. Vollmꢆller, J. Krause, S. Klein, W. Mꢇger-
lein, M. Beller, Eur. J. Inorg. Chem. 2000, 1825–1832.
Pre-catalyst solution: The catalyst was prepared with palladium acetyl
acetonate (PdACHTUNGTRENNUNG(acac)2) plus two molar equivalents of the phosphine. One
molar equivalent of acetic acid may be added to increase storage stabili-
ty. The catalyst was prepared in methanol by dissolving all three compo-
nents such that the palladium concentration in methanol equalled about
500 ppm.
Catalytic reaction: Each reaction was carried in a 1 L Parr reactor made
from electropolished stainless steel. For each reaction, the autoclave of
the Parr reactor was filled with specified amounts of methanol (MeOH/
Bd ratio 2 or 2.6 by weight, 3.4 or 4.4 by mol), promoter (sodium meth-
oxide, at a promoter to palladium molar ratio of 5:1) and inhibitor (di-
ethyl hydroxyl amine, approximately 20 parts by weight per million parts
by weight (ppm) based on total weight of methanol plus crude C4 load).
The autoclave was closed and purged twice with low-pressure nitrogen
(6 bar or 600 kPa) to substantially remove oxygen contained in the auto-
clave. A stainless steel sample cylinder was filled with a crude C4 stream
Chem. Eur. J. 2011, 17, 8922 – 8928
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8927

P. W. N. M. van Leeuwen et al.
[6] a) M. J.-L. Tschan, E. J. Garcia-Suꢈrez, Z. Freixa, H. Launay, H.
Hagen, J. Benet-Buchholz, P. W. N. M. van Leeuwen, J. Am. Chem.
Soc. 2010, 132, 6463–6473; b) M. J.-L. Tschan, J.-M Lopez-Valbue-
na, Z. Freixa, H. Launay, H. Hagen, J. Benet-Buchholz, P. W. N. M.
van Leeuwen, Organometallics 2011, 30, 792–799.
[7] M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer, P. W. N. M.
Van Leeuwen, K. Goubitz, J. Fraanje, Organometallics 1995, 14,
3081–3089.
Eur. J. 2008, 14, 8995–9005; c) P. J. C. Hausoul, P. C. A. Bruijincx,
R. J. M. Klein Gebbink, B. M. Weckhuysen, ChemSusChem 2009, 2,
855–858; d) R. Palkovits, A. N. Parvulescu, P. J. C. Hausoul, C. A.
Kruithof, R. J. M. Klein Gebbink, B. M. Weckhuysen, Green Chem.
2009, 11, 1155–1160; e) A. N. Parvulescu, P. J. C. Hausoul, P. C. A.
Bruijnincx, R. J. M. Klein Gebbink, Catal. Today 2010, 158, 130–
138; f) P. C. J. Hausoul, A. N. Parvulescu, M. Lutz, A. L. Spek,
P. C. A. Bruijnincx, B. M. Weckhuysen, R. J. M. Klein Gebbink,
Angew. Chem. 2010, 122, 8144–8147; Angew. Chem. Int. Ed. 2010,
49, 7972–7975.
[8] Crystals of 11 suitable for X-ray analysis were obtained by slow
evaporation of a concentrated solution of 11 in CH2Cl2 at low tem-
perature (À258C). Crystal data for 11 at 100 K: C36H39Cl2O2P1;
[10] A ratio of 1:1 for Pd/NHC or Pd/ACTHNGUETRNNU(G NHC)HCl is optimal for this cata-
605.54 gmolÀ1
;
triclinic; P1; a=11.4134(4), b=12.0574(4), c=
lytic system when a low amount of base is used; an excess of
(NHC)HCl is even detrimental.
¯
13.3831(5) ꢃ; a=65.6340(10), b=76.4820(10), g=67.9480(10)8; V=
1548.15(9) ꢃ3; Z=2, 1calcd =1.299 MgmÀ3
;
R1 =0.0420 (0.0541);
[11] The amount of NaOMe also increased the initial rate of the reaction
in the case of PPh3 (by more than two times, see the Supporting In-
formation). The influence of the amount of NaOMe on the systems
of 1 and 11, was not investigated, but it should be similar to that on
PPh3.
[12] This product was obtained by an improved procedure with respect
to that already described: a) A. Ryavov, Neftekhimiya 1987, 27,
215–218; b) A. J. Caruso, J. L. Lee, J. Org. Chem. 1997, 62, 1058–
1063.
wR2=0.1156 (0.1257); for 3164 reflections with I>2s(I) (for 11819
reflections (Rint: 0.0314) with a total measured of 43206 reflections);
goodness-of-fit on F2 =1.055; largest diff. peak (hole)=0.742
(À0.612) eꢃÀ3. CCDC-794329 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[9] a) J. Briggs, J. Patton, S. Vermaire-Louw, P. Margl, H. Hagen, D.
Beigzadeh (Dow Global Technologies Inc., USA), WO 2010/019360,
2010; [Chem. Abstr. 2010, 152, 212563]; b) R. Palkovits, I. Nieddu,
C. A. Kruithof, R. J. M. Klein Gebbink, B. M. Weckhuysen, Chem.
Received: March 4, 2011
Published online: June 21, 2011
8928
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8922 – 8928