An industrially viable catalyst system for palladium-catalyzed telomerizations of 1,3-butadiene with alcohols

Article abstract of DOI:10.1002/chem.200400182

The telomerization reaction of 1,3-butadiene with alcohols to give alkyl octadienyl ethers in the presence of palladium-carbene catalysts has been studied in detail. Unprecedented catalyst efficiency with turnover numbers (TON) up to 1500000 and turnover frequencies (TOF) up to 100000h^-1 have been obtained after optimization for the reaction of methanol in the presence of an excess of in situ generated carbene ligands. High yields (75-97%) and catalyst productivities (TON 15000-100000) are observed for other aliphatic alcohols and phenols. For comparison five carbenepalladium(0) complexes have been synthesized and characterized by X-ray crystallography. Both electronic and steric effects on the stability and reactivity of the catalysts have been discussed on the basis of density functional theory calculations.

Full text of DOI:10.1002/chem.200400182

FULL PAPER
An Industrially Viable Catalyst System for Palladium-Catalyzed
Telomerizations of 1,3-Butadiene with Alcohols
Ralf Jackstell,[a] Surendra Harkal,[a] Haijun Jiao,[a] Anke Spannenberg,[a]
Cornelia Borgmann,[b] Dirk Rˆttger,[b] Franz Nierlich,[b] Mark Elliot,[c] Stuart Niven,[c]
Kingsley Cavell,[c] Oscar Navarro,[d] Mihai S. Viciu,[d] Steven P. Nolan,[d] and
Matthias Beller*[a]
Abstract: The telomerization reaction
of 1,3-butadiene with alcohols to give
alkyl octadienyl ethers in the presence
of palladium carbene catalysts has
been studied in detail. Unprecedented
catalyst efficiency with turnover num-
bers (TON) up to 1500000 and turn-
over frequencies (TOF) up to
100000 hÀ1 have been obtained after
optimization for the reaction of metha-
nol in the presence of an excess of in
situ generated carbene ligands. High
yields (75 97%) and catalyst produc-
tivities (TON 15000 100000) are ob-
served for other aliphatic alcohols and
phenols. For comparison five carbene
palladium(0) complexes have been syn-
thesized and characterized by X-ray
crystallography. Both electronic and
steric effects on the stability and reac-
tivity of the catalysts have been dis-
cussed on the basis of density function-
al theory calculations.
Keywords: butadiene ¥ carbene ¥
homogeneous catalysis ¥ palladium ¥
telomerization
Introduction
tion characteristics. In this regard the palladium-catalyzed
telomerization of 1,3-dienes with nucleophiles is an interest-
ing methodology, that combines simple starting materials in
a 100% atom efficient manner to give functionalized octa-
2,7-dienes.[1,2] Due to their ready availability and low price,[3]
1,3-butadiene and alcohols, especially methanol, are attrac-
tive starting materials for this reaction (Scheme 1).
An important goal in organic chemistry is the development
of green processes that use fewer raw materials and less
energy, maximize the use of renewable resources, and mini-
mize or eliminate the use of dangerous chemicals. Clearly,
none of this is possible without catalysis. In general as the
science of accelerating chemical reactions, catalysis is about
using value-added transformations to convert simple raw
materials to more complex molecules with versatile applica-
Scheme 1. Telomerization of 1,3-butadiene with methanol.
[a] Dr. R. Jackstell, S. Harkal, Dr. H. Jiao, Dr. A. Spannenberg, Prof.
Dr. M. Beller
Leibniz-Institut f¸r Organische Katalyse
an der Universit‰t Rostock e.V.
Buchbinderstrasse 5 6
18055 Rostock (Germany)
Fax : (+49)381-46693-24
The reaction usually leads to a mixture of cis/trans iso-
mers where 1-methoxyocta-2,7-diene 1 (n-products) is in
general the major product, which is a useful precursor for
plasticizer alcohols (octanols), solvents, corrosion inhibitors,
and monomers for polymers.[4] In addition, the by-products
of this reaction, mainly the 3-substituted methoxyocta-1,7-
dienes 2 (iso-products), 1,3,7-octatriene and 4-vinylcyclohex-
ene (VCH) also are of some commercial interest. Hence,
this telomerization process has been the subject of intensive
research in both academic and industrial laboratories.[5]
Important mechanistic studies of the telomerization of
methanol and 1,3-butadiene in the presence of palladium/
phosphine catalysts have been performed by Jolly and co-
E-mail: matthias.beller@ifok.uni-rostock.de
[b] Dr. C. Borgmann, Dr. D. Rˆttger, Dr. F. Nierlich
Degussa AG, OXENO C4-Chemie
Paul-Baumann-Strasse 1, 45764 Marl (Germany)
[c] Dr. M. Elliot, S. Niven, Prof. Dr. K. Cavell
Cardiff University
PO Box 912, Cardiff CF 103TB, Wales (UK)
[d] O. Navarro, M. S. Viciu, Prof. Dr. S. P. Nolan
University of New Orleans
Department of Chemistry
New Orleans, Louisiana 70148 (USA)
Chem. Eur. J. 2004, 10, 3891 3900
DOI: 10.1002/chem.200400182
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPER
workers.[6] More recently, we also investigated this reaction,
which led to an extended mechanistic proposal.[7] As shown
in Scheme 2, it is proposed that in the presence of palladi-
um(0) species, two molecules of 1,3-butadiene couple to
form the [PdPPh3(h1,h3-octadiendiyl)] complex 3. Protona-
tion of 3 by methanol at the C6 atom of the C8 chain leads
to the [PdPPh3(h2,h3-C8H13)] species 4.
Despite the economic attractiveness of the starting mate-
rials, a prerequisite for an industrial use of this telomeriza-
tion reaction is the required high catalyst efficiency due to
the relatively high price of palladium.[9] From the catalytic
cycle of the reaction it is apparent that only one external
ligand (L) on the palladium center is sufficient for a produc-
tive and highly selective catalyst system. Importantly, L
should be sterically demanding in order to prevent palladi-
um agglomeration and simple coordination of a second
ligand L, which leads to a lower n/iso-selectivity.
Inspired by the aforementioned catalytic cycle we thought
that stable palladium(0) 1,6-diene complexes resembling the
catalytic intermediate 5 should be ideal catalyst systems.
Unfortunately, the corresponding (phosphine)palladium(0)
1,6-diene[10] complexes did not prove to be superior for telo-
merizations compared to the standard phosphine/palladi-
um(ii) pre-catalysts. More recently, some of us discovered
that (carbene)palladium(0) diolefin complexes are extreme-
ly efficient catalysts for the reaction of 1,3-butadiene and
methanol. With these complexes were obtained the best cat-
alyst turnover numbers known so far for telomerizations
(TON up to 300000).[11]
In this paper we describe a full account of our work on te-
lomerization of 1,3-butadiene with alcohols in the presence
of different palladium carbene catalysts. Here, the synthesis
and detailed characterization of four new (carbene)palladi-
um(0) diolefin complexes and a comparison with in situ
generated catalysts in the palladium-catalyzed telomeri-
zation of 1,3-butadiene with methanol is presented. A sig-
nificant improvement in catalyst productivity (TON
> 1500000) is observed by adding an excess of imidazolium
salts to the reaction mixture. The optimized catalyst system
is useful for telomerizations of a variety of aliphatic alcohols
and substituted phenols.
Scheme 2. Proposed mechanism for the telomerization of 1,3-butadiene
with methanol.
In the following step of the reaction (path A), the meth-
oxide ion adds to either the allylic terminus C1 or C3 of the
C8 chain resulting in the formation of the telomers 1 or 2,
respectively. The formation of 1,3,7-octatriene 6 can occur
as a side reaction by b-hydrogen elimination from C4 in 4.
Clearly, the regioselectivity determining step of the process
is the nucleophilic attack of methanol or methoxide ion on
the p-allylpalladium(phosphine) complex 4. The nucleophil-
ic attack at the C1 atom is favored for steric reasons, while
the attack at C3 atom is electronically favored.[8] Moreover,
the linear telomer is the thermodynamically more stable
product due to the presence of an internal double bond,
whereas the branched isomer with the terminal double bond
is less thermodynamically stable.
The selective formation of the linear telomer 1-methoxy-
octa-2,7-diene 1 is influenced to a large extent by the
ligand to metal ratio and the stoichiometry of the starting
materials. Good n/iso-selectivities (up to 97%) are realized
at low P and Pd ratio (1:1) and high methanol to 1,3-buta-
diene ratios in the absence of other coordinating species. A
second ligand, for example, phosphine bound to the metal
center, reduces the regioselectivity dramatically. This ex-
plains the formation of 7, which leads to lower n/iso-selectiv-
ity compared with 4 (reaction path B). Here, the regioselec-
tivity of the nucleophilic attack of methoxide anion on the
allyl palladium intermediate is no longer determined by the
formation of the favorable complex 5 with a chelating 1,6-
diene ligand. Hence, the internal coordination of the olefinic
side chain is one of the main driving forces governing the
n/iso-selectivity.
Results and Discussion
As a starting point for our investigation, we synthesized dif-
ferent (carbene)palladium(0) diolefin complexes (Figure 1).
In order to mimic intermediates of the catalytic cycle we de-
cided to use 1,3-dimethyldivinyl siloxane (dvds) as 1,6-diene
ligand. Due to the increased acceptor strength of this diole-
fin compared with other 1,6-dienes[12] the corresponding pal-
ladium(0) complexes are stable for months and can be easily
handled even under air.
First, the known complex (1,3-dimesitylimidazol-2-yli-
dene)-palladium(0)-h2,h2-1,1,3,3-tetramethyl-1,3-divinyl-disil-
oxane (8, [IMesPd(dvds)]) was synthesized by reacting the
palladium(0) diallylether complex [Pd2(dae)3][13] with 1,3-di-
mesitylimidazol-2-ylidene carbene (IMes)[14] in 1,1,3,3-tetra-
methyl-1,3-divinyl-disiloxane (dvds) at À308C. The new
complexes
(1,3-dimesityl-4,5-dimethylimidazol-2-ylidene)-
palladium(0)-h2,h2-1,1,3,3-tetramethyl-1,3-divinyl-disiloxane
(9), (1,3-dimesityl-4,5-dichloroimidazol-2-ylidene)-palladi-
um(0)-h2,h2-1,1,3,3-tetramethyl-1,3-divinyl-disiloxane
(10),
and {1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidine}-palla-
dium(0)-h2,h2-1,1,3,3-tetramethyl-1,3-divinyl-disiloxane (11,
[IPrPd(dvds)]), {1,3-bis-(2,6-diisopropylphenyl)-4,5-dimethyl-
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Palladium-Catalyzed Telomerizations
3891 3900
imidazol-2-ylidine}-palladium(0)-h2,h2-1,1,3,3-tetramethyl-1,3-
divinyl-disiloxane (12, [MeIPrPd(dvds)]) were obtained by
reacting stoichiometric amounts of the corresponding free
carbene with a Pd0/dvds solution (8%) in THF and subse-
quent crystallization from n-pentane at À308C.
tal structures of such complexes.[16] Nevertheless, suitable
crystals for X-ray crystallography were obtained in all cases
by crystallization from pentane or hexane at low tempera-
ture (<08C). The crystallographic data of 8 12 are given in
Table 1 and selected distances and angles are given in
Table 2.
As shown in Figure 1 the central palladium atom is coor-
dinated by the diolefin unit H2C=CHSiMe2OSiMe2HC=CH2
and the corresponding carbene ligand in a trigonal planar
coordination in 8 12. The plane (g1) of the carbene hetero-
In order to rationalize catalytic effects as a function of
structural features of the complexes we were interested in
the detailed structural information of 8 12. It is noteworthy
that despite the catalytic potential of palladium(0) carbene
complexes[15] so far there exist only a small number of crys-
Figure 1. Monocarbenepalladium(0) dvds complexes 8 12. Hydrogen atoms are omitted for clarity; the thermal ellipsoids correspond to 30% probabili-
ty.
Chem. Eur. J. 2004, 10, 3891 3900
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M. Beller et al.
FULL PAPER
Table 1. Crystallographic data.
at the B3LYP/LANL2DZp
level are used for comparison
and discussion.
As given in Table 3, the com-
puted bond lengths and angles
are very close to X-ray data in
9
10
11
12
crystal system
space group
a [ä]
b [ä]
c [ä]
monoclinic
P21/n
12.865(3)
17.157(3)
15.903(3)
monoclinic
P21/n
12.869(3)
17.138(3)
15.884(3)
orthorhombic
P212121
12.893(3)
13.899(3)
20.645(4)
triclinic
≈
P1
11.079(2)
17.721(4)
19.684(4)
84.50(3)
89.92(3)
84.96(3)
3831.8(14)
4
À
Table 2. The computed Pd
a [8]
À
Ccarbene and Pd CEt bond lengths
b [8]
112.56(3)
112.79(3)
g [8]
are longer than the X-ray data
by an average value of 0.052
and 0.046 ä, respectively. The
V [ä3]
Z
3241.6(11)
4
1.281
0.671
200
3229.7(11)
4
1.3701.223
0.838
200
7886
4161
3533
358
3699.6(14)
4
1.230
0.593
200
16781
4833
4364
1calcd [gcmÀ3
]
À
m(MoKa) [mmÀ1
T [K]
]
0.576
200
11514
11514
9274
difference for the C=C and N
Ccarbene bond lengths within the
five-membered ring is only
0.026 and 0.010 ä, respectively.
The angle differences are less
than 18. However, large differ-
ences are found for the torsion
angle between the two planes
in 9’ (7.68), 10’ (9.38) and 11’
(À17.48), while those in 8’ and
12’ are much smaller (À1.6 and
1.2/0.28), respectively.
no. rflns (measd)
no. rflns (indep)
no. rflns (obsd)
no. params
9596
5198
4322
335
370823
0.027
0.056
R1 (I>2s(I))
wR2 (all data)
0.039
0.112
0.033
0.089
0.033
0.106
Table 2. Selected bond lengths [ä] and angles [8] in complexes 8 12.
8[11]
9
10
11
12
À
Pd C(carbene)
2.076(5)
2.028
2.051
2.093(3)
2.048
2.045
2.080(3)
2.054
2.050
2.084(3)
2.032
2.034
2.114(3)/2.114(3)
2.056/2.059
Next, we tested palladi-
um(0) monocarbene complexes
8 12 in the telomerization of
1,3-butadiene with methanol. In
addition, two (allyl)palladi-
um(ii) (carbene) complexes 13
[a]
À
Pd CE
2.057/2.052
C=C(carbene)
C=C(olefin)
1.320(6)
1.408(6)
1.388(6)
1.369(6)
1.373(5)
102.0(4)
128.7(4)
129.3(3)
130.3
1.341(5)
1.409(5)
1.402(5)
1.365(4)
1.369(4)
102.7(3)
128.4(2)
128.9(2)
129.6
1.330(5)
1.394(5)
1.398(6)
1.368(4)
1.376(4)
103.1(3)
127.9(2)
129.0(2)
129.5
1.335(5)
1.409(6)
1.377(5)
1.370(4)
1.376(4)
102.2(3)
125.1(2)
131.9(2)
129.1
1.352(4)/1.349(4)
1.391(5)/1.395(5)
1.398(5)/1.396(5)
1.362(4)/1.362(4)
1.369(3)/1.366(4)
103.3(2)/103.1(2)
125.0(2)/124.4(2)
131.3(2)/132.0(2)
129.9/130.3
À
C(carbene) N
{[(IMes)Pd(allyl)Cl],
IMes=
N-C(carbene)-N
N-C(carbene)-Pd
1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene} and 14[18]
CE1-Pd-CE2
{[(IPr)Pd(allyl)Cl],
IPr=1,3-
C(carbene)-Pd-CE
115.3
115.3
115.3
112.3
112.3/111.0
bis(2,6-diisopropylphenyl)imi-
dazol-2-ylidene} and several in
situ catalyst systems, for exam-
ple, Pd(OAc)2/PPh3 were exam-
ined for comparison. To be of
interest for practical application
and to distinguish catalyst pro-
114.3
64.7
76.6
77.7
114.9
59.2
77.4
81.3
115.0118.2
117.1/118.0
g1[b]
g2[c]
g3[c]
58.8
77.2
80.6
73.8
82.3
88.7
56.5/57.9
69.7/70.6
72.0/76.2
[a] CE: mid-points of the coordinated C=C bonds. [b] g1 angle between the planes defined by the carbene het-
erocycle and the coordination plane. [c] g2, g3 angle between the planes defined by the carbene heterocycle
and the aryl plane.
ductivity
a comparably small
cycle and the coordination plane form angles of 56.5 73.88
(Table 2). The distance between the palladium and the car-
bene carbon atom varies in between 2.076(5) and
2.114(3) ä, which is in the expected range.[17] Surprisingly,
the substituents on the carbene backbone in 9 and 10 do not
influence significantly the carbene palladium bond length.
In 12 an elongation of the carbene palladium bond length is
observed.
For a detailed comparison of the complexes we have also
carried out high level density functional theory calculations.
In our modeling, the whole carbene ligand composition was
used, while the diolefin ligand, 1,3-dimethyldivinyl siloxane
(H2C=CHSiMe2OSiMe2HC=CH2) was replaced by two eth-
ylene molecules. All complexes studied (8’ 12’) have C2
symmetry and are energy minimum structures on the poten-
tial energy surface according to the frequency calculations
at the B3LYP/LANL2DZ level of theory (see computational
part). The distances (ä) and angles (degree), and energies
amount of catalyst (1,3-butadiene/catalyst 100000:1) was
used in all experiments.
As shown in Table 4 (entries 1 4, 6, 7) the presence of
N,N-diarylcarbene ligands is crucial for obtaining good
yields and selectivity as well as high catalyst productivity.
Using standard reaction conditions (708C, methanol/buta-
diene 2:1, 1 mol% NaOMe) the palladium(0) monocarbene
complexes 8 and 10 give nearly quantitative yield (96%) of
the desired telomers 1 and 2, excellent chemoselectivity
(>99%) and an n/iso ratio of 98:2 (Table 4, entries 1 and 3),
while the ™classic∫ phosphine catalyst system (Pd(OAc)2/
3 equiv PPh3) gives only a product yield of 26% and a sig-
nificant lower chemoselectivity (Table 4, entry 8). Interest-
ingly, 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidine palla-
dium complexes 11 and 14 are less efficient and less selec-
tive compared with the 1,3-dimesitylimidazol-2-ylidine palla-
dium complexes 8 and 13 (Table 4, entries 4, 7 vs 1, 6).
Methyl substitution in the backbone of the carbene in com-
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Palladium-Catalyzed Telomerizations
3891 3900
Table 3. Computed bond lengths [ä] and angles [8] for complexes 8’ 12’.
mol% NaOMe, 1,3-butadiene/
MeOH 1:2). Using complex 8
at 908C, the reaction is very
8’
9’
10’
2.1278
2.0971
1.3629
1.4016
1.3789
11’
2.1415
2.0901
1.3609
1.4022
1.3797
12’
2.1625
À
Pd C(carbene)
2.1334
2.0910
1.3622
1.4032
1.3766
2.1396
2.0908
1.3674
1.4031
1.3736
[a]
À
Pd CE
2.0950 fast and nearly complete after
C=C(carbene)
C=C(olefin)
1.3663
1.4006
1.3788
102.73
only 2 3 h. At 708C the reac-
tion requires approximately 6 h
to reach completion, while at
508C 80% conversion is ob-
tained after 16 h.
The study of activity of com-
plexes 8 11 at 708C demon-
strates the influence of different
substitutions on the 4,5-position
at the mesitylcarbene backbone
(Figure 3). The dichlorosubsti-
tuted dimesitylcarbene complex
10 is faster than the unsubstitut-
À
C(carbene) N
N-C(carbene)-N
N-C(carbene)-Pd
CE1-Pd-CE2
C(carbene)-Pd-CE
g1[b]
g2[c]
g3[c]
102.79
128.60128.61
124.62
117.69
63.05
102.79
103.69
102.59
128.64
128.16
128.71
123.96
118.02
66.81
123.705124.70122.0
118.02
68.05
117.65
56.36
104.37
118.97
57.66
105.77
100.11
81.5085.1084.51
96.50
96.56
78.54
75.22
[a] CE: mid-points of the coordinated C=C bonds. [b] g1 angle between the planes defined by the carbene het-
erocycle and the coordination plane. [c] At the B3LYP/LANL2DZp level. g2 and g3 are the two torsion angles
of the phenyl ring to the carbene center.
plex 12 decreased dramatically the catalyst performance.
Apparently, substitution in the backbone of the mesitylsub-
stituted carbene ligand (position 4 and 5) and the co-ligands
(allyl or 1,6-diene) have only a minor influence on the yield
and selectivities after 16 h. Unfortunately, all N,N-dialkyl-
substituted imidazol-2-ylidine ligands showed basically no
catalyst activity under our conditions (Table 4, entries 10
12). An in situ generated system from Pd(OAc)2 and IM-
esHCl forms apparently the same catalytic species as com-
plex 8 and also leads to good catalyst performance (Table 4,
entries 1, 9).
While all experiments shown in Table 4 were run for 16 h,
we also performed catalyst activity studies in which conver-
sion and product yield were continuously measured. In Fig-
ures 2 and 3 selected runs are shown at different tempera-
tures (general conditions: 0.001 mol% Pd, 15 g 1,3-buta-
diene, 25 mL THF, 5 mL isooctane as internal standard, 1
ed complex 8, while the dimethyl-substituted dimesitylcar-
bene complex 9 is slower than 8. Diisopropylphenyl substi-
tution in complex 11 instead of mesityl substitution in com-
plex 8 leads to a significantly lower reaction rate and prod-
uct yield.
In order to gain more insight into the relative stability
and reactivity of these complexes upon carbene ligand coor-
dination, we considered the ligand exchange reaction of
complex 8’ shown in Scheme 3 as a model system. For a
given complex, a positive reaction enthalpy (endothermic)
indicates its reduced relative stability, while a negative reac-
tion enthalpy (DH, exothermic) reveals the enhanced rela-
tive stability with regard to 8’.
On the basis of these calculations, that the introduction of
two methyl groups at the five-membered ring stabilizes com-
plex (9’) by 2.1 kcalmolÀ1, and this indicates that complex
(9’) should be more stable (less active) in the catalytic reac-
Table 4. Telomerization of 1,3-butadiene with methanol in the presence of different catalysts.[a]
Entry
Catalyst
Yield [%][b]
n:iso [%]
Chemosel. [%][c]
TON (1+2)[d]
TOF (1+2)[d]
1
2
3
4
5
6
7
8
8
9
10
11
12
13
14
96
93
96
90
2
94
46
26
94
5
98:2
98:2
98:2
92:8
91:9
98:2
92:8
96:4
98:2
98:2
>99
99
>99
97
96000
93000
96000
90000
2000
94000
46000
26000
94000
7000
6000
5813
6000
5625
125
5875
2875
1625
5875
438
99
96
87
> 99
71
Pd(OAc)2/3 equiv PPh3
Pd(OAc)2/4 equiv IMesHCl
2/4 equiv
9
10Pd(OAc)
11
12
Pd(OAc)2/4 equiv
0
0
Pd(OAc)2/4 equiv[e]
[a] General conditions: 16 h, 708C, 1 mol% NaOMe, MeOH/1,3-butadiene 2:1. [b] Yield of 1+2. [c] Chemoselectivity=(1+2)/(1+2+6+VCH) î 100;
VCH=4-vinylcyclohexene. [d] Calculated with respect to 1,3-butadiene. [e] Ligand was prepared by Dr. A. Dervisi, synthetic details will be provided
elsewhere.
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M. Beller et al.
FULL PAPER
Both the stabilizing effect in 9’ and the destabilizing effect
in 10’ are electronic in origin; that is, an electron donating
methyl group stabilizes the different palladium carbene
complexes and most likely reduces the reactivity of the in-
termediate 4 (see catalytic cycle, Scheme 2), while an elec-
tron withdrawing chlorine substitution destabilizes the palla-
dium carbene complexes and promotes the reactivity of 4 in
the rate determining reaction step. Apart from the electron-
ic effect, we were also interested in the contribution of the
steric effect to catalyst stability. Instead of a mesityl group
at the nitrogen centers, we used 2,6-diisopropylphenyl with
the hope that the four isopropyl groups can influence coor-
dination of substrates. Apparently, the calculated positive
reaction enthalpy of 2.4 kcalmolÀ1 indicates the destabiliza-
tion of complex 11’. An even stronger destabilizing effect
(+6.5 kcalmolÀ1) is found for complex 12’, despite of the
electron donating substitution of methyl group at the back-
bone. Structural analysis reveals that the origin of destabili-
zation in 12’ is due to the repulsive interaction between the
methyl groups of the five-membered ring and the isopropyl
group (the shortest H¥¥¥H distance in 12’ is 2.086 ä, while it
is 2.742 ä in the free carbene ligand, respectively). The con-
sequence is the rotation of the benzene rings (g2 and g3) and
the longer Pd C(carbene) distance (Table 3), which reduces
the catalyst activity.
^
Figure 2. Catalyst performance of complex 8 at 50, 70 and 908C; : 908C,
~
&
: 708C, : 508C.
However, this enhanced reactivity does not lead to the
corresponding activity, as found in Figure 3. For example,
the reaction with catalyst 11 is much slower and the corre-
sponding yield is lower, as compared to catalyst 8. The total
yield with catalyst 12 is only 2% (Table 4, entry 5), and we
did not consider it further. Therefore, the steric effect of the
bulky isopropyl substituents at the benzene rings destabilize
the catalyst on one hand, and on the other hand reduces its
catalytic reactivity.
Economic calculations clearly show that the productivity
shown above is still not sufficient in order to allow industrial
bulk chemical applications using 1,3-butadiene. Here, cata-
lyst turnover numbers in the range of 1000000 have to be
realized at high product yield. Therefore, we were interested
in further optimizing this catalyst system. Selected results
from more than 300 experiments are shown in Table 5.
Most experiments were performed in the presence of cat-
alysts 8 and 13 with a 1,3-butadiene to Pd ratio of
1000000:1 at 908C. At this temperature the reaction is
somewhat faster, but both catalysts exhibit comparable sta-
bility. With or without ligand in the absence of palladium no
product formation is observed at low conversion (<5% for-
mation of vinylcyclohexene, which results from the Diels
Alder reaction of 1,3-butadiene) (Table 5, entry 1). Simply
using catalysts 8 and 13 under the conditions described in
Table 4 telomers 1 and 2 are obtained in low yield (ca.
20%), which corresponds to a turnover number of 200000
(Table 5, entries 2 and 3). Again both catalysts do not differ
significantly in their productivity. Among various co-ligands,
the addition of imidazolium salts proved to be beneficial for
the reaction. By gradually increasing the amount of IM-
es¥HCl as a precursor for the carbene ligand, the yield of
the desired telomers increased to 91% (TON=910000)
(Table 5, entries 4 9). By adding IMes¥HCl in the presence
&
^
Figure 3. Activity of complexes 8 11 at 708C; : complex 8; : com-
~
*
plex 9; : complex 10; : complex 11.
Scheme 3. Modeling of the carbene ligand exchange reaction of 8’; DH
(9’)=À2.1 kcalmolÀ1, DH (10’)=+1.6 kcalmolÀ1, DH (11’)=+2.4 kcal
molÀ1, DH (12’)=+6.5 kcalmolÀ1
.
tion. In contrast, two chlorine substitutions at the backbone
destabilize complex (10’) by 1.6 kcalmolÀ1, and this reveals
that complex (10’) should display enhanced destabilization
and activity. These results are in agreement with the ob-
served reactivity profiles shown in Figure 3.
3896
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 3891 3900

Palladium-Catalyzed Telomerizations
3891 3900
Table 5. Optimization of catalyst productivity for the telomerization of 1,3-butadiene with methanol.[a]
lyst and reduces the reactivity
of the corresponding intermedi-
ate (most likely 4) in the rate
determining step, while electron
withdrawing substitution of
chlorine destabilizes the cata-
lyst and promote the reactivity.
By carefully optimizing the
Entry Cat. Catalyst
[mol%][a]
Ligand
[mol%]
Yield
[%][b]
n:iso
[%]
Chemosel.
[%][c]
TON
TOF[d]
(1+2)[d]
1
2
3
4
5
6
7
8
9
0
0.004
0
20
19
17
40
69
87
91
89
77
0
89
90
88
95
97
98
99
98
99
0
0
12500
11875
10625
25000
43125
54375
56875
55625
96250
8
13
8
8
8
8
8
13
8
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.00005
98:2
98:2
98:2
98:2
98:2
98:2
98:2
98:2
98:2
200000
190000
170000
400000
690000
870000
910000
890000
1540000
0.0002
0.0004
0.001
0.002
0.004
0.004
0.004
telomerization
reaction
of methanol unprecedented cat-
alyst
>1500000)
productivity
and
(TON
activity
10
[a] General conditions: 16 h, 908C, 1.0mol% NaOMe, MeOH/1,3-butadiene 2:1, L = IMesHCl. [b] Yield of
1+2. [c] Chemoselectivity=(1+2)/(1+2+6+VCH) î 100; VCH=4-vinylcyclohexene. [d] Calculated with re-
spect to 1,3-butadiene.
(TOF=100000 hÀ1) are ob-
served in the presence of an
excess of imidazolium salt. For
the first time bulk telomeriza-
tion reactions appear to be
commercially viable and are
of only 0.5 ppm of 8 a good product yield (77%) is observed
(Table 5, entry 10). This corresponds to the highest catalyst
productivity (TON=1540000) and activity (TOF=
96250h À1 after 16 h) reported for any telomerization reac-
tion. Interestingly, the observed regioselectivity does not
change with respect to the ligand concentration. This result
is contradictory to our findings with palladium/phosphine
catalysts.[7] Apparently during the catalytic cycle there is
always only one carbene ligand bound to the central metal
atom (reaction path A in Scheme 2).
not limited by catalyst costs. The industrial applicability of
our catalyst system has been already demonstrated by the
production of telomers on a ton scale. In addition to metha-
nol other aliphatic alcohols and substituted phenols react
highly selectively in good to excellent yields, to produce the
corresponding telomers. The reactions described in this
study constitute prime examples of green chemistry.
Having demonstrated the excellent performance of 8 and
13 in the telomerization reaction of methanol, we were in-
terested in the coupling of 1,3-butadiene with other alcohols.
Again our molecular defined carbene complexes proved to
be excellent catalysts.[11] Other alcohols, which were tested
applying the standard conditions as described above, include
n-butanol, n-hexanol isopropanol, benzyl alcohol, 2-methox-
yethanol, 2-methylphenol, and 2,6-dimethylphenol. The re-
sults are summarized in Table 6. The activities and chemose-
lectivities for the linear telomers are also excellent using
higher aliphatic alcohols in the presence of the mesityl-sub-
stituted complexes 8 and 9 (Table 6, entries 2 and 3, 8 and 9,
12 17). In addition to aliphatic alcohols, the telomerization
of phenols proceeds reasonably well in the presence of pal-
ladium carbene catalysts. Interestingly, the more-substituted
phenol gave the best yield (86%) in this reaction, which
might be attributed to the higher basicity of 2,4,6-trimethyl-
phenol compared to o-cresol (Table 6, entries 18 20).
Experimental Section
General procedure for telomerization reaction: [IMesPd0dvds] (8; 1.6 mg,
2.77î10À6 mol) was dissolved in methanol (17.8 g, 0.555 mol). Subse-
quently NaOMe (149.5 mg, 2.77î10À3 mol) was added. The mixture was
transferred under argon into a secured 100 mL stainless steel Parr auto-
clave. The autoclave was cooled with dry ice and 1,3-butadiene (15.0g,
2.77î10À1 mol) was condensed in a separate 75 mL pressure cylinder
(mass control). The given amount of 1,3-butadiene was condensed into
the cooled autoclave and the vessel was heated to the desired reaction
temperature. After 16 h the autoclave was cooled to room temperature
and the remaining 1,3-butadiene was condensed. Isooctane (5 mL) as in-
ternal standard was added. The yield of telomerization products was de-
termined by GC using an HP 6869A gas chromatograph. In order to iso-
late the different octadienyl ethers the reaction mixture was distilled in
vacuo.
cis/trans 1-Methoxyocta-2,7-diene (1): b.p. (5 Torr) 358C; 1H NMR
(CDCl3, 300 MHz): d=1.49 (q, J=8.0Hz, 2H), 2.68 2.76 (m, 4H), 3.31
(s, 3H), 3.86 (dd, J=6, 2 Hz, 2H), 4.95 4.99 (m, 2H), 5.49 5.61 (m, 1H),
5.64 5.87 (m, 2H); 13C NMR (CDCl3, 75 MHz): d=26.97, 28.30, 28.73,
31.69, 33.21, 57.67, 57.90, 68.13, 73.25, 114.58, 126.30, 126.47, 133.29,
134.44, 138.54, 138.62.
3-Methoxyocta-2,7-diene (2): b.p. (5 Torr) 358C; 1H NMR (CDCl3,
300 MHz): d=1.34 1.68 (m, 4H), 2.01 2.10 (m, 2H), 3.26 (s, 3H), 3.46
3.54 (m, 1H), 4.90 5.20 (m, 2H), 5.14 5.22 (m, 1H), 5.58 5.87 (m, 2H);
13C NMR (CDCl3, 75 MHz): d=24.62, 33.70, 34.81, 56.14, 82.92, 114.53,
117.03, 138.72, 138.85.
8-Butoxyocta-1,6-diene: b.p. (5 Torr) 808C; 1H NMR (CDCl3, 400 MHz):
d=5.7 5.5 (m, 2H), 5.45 5.36 (m, 1H), 4.9 4.7 (m, 2H), 3.7 (dd, J=6,
1 Hz, 2H), 3.26 (t, J=6.7 Hz, 2H), 1.9 (m, 4H), 1.42 (quint, J=7.1 Hz,
2H), 1.42 (quint, J=7.1 Hz, 2H), 1.39 (q, 3J5,4,6 =7 Hz, 2H), 1.25 (sext,
J=7.1 Hz, 2H), 0.75 (t, J=7.3 Hz, 3H); 13C NMR (CDCl3, 100 MHz):
d=138.2, 133.5, 126.7, 114.3, 71.3, 69.1, 32.9, 31.6, 31.4, 28.05, 19.1, 13.6;
MS: m/z (%): 182 (1.4) [M +], 139 (4.3), 126 (10.6), 108 (24), 101 (3.9),
97 (11), 93 (27), 82 (35), 67 (72), 57 (100); HRMS: m/z: calcd for
C12H22O: 182.16707, found: 182.16460.
Conclusion
In conclusion, we have synthesized monocarbene palladi-
um(0) complexes, which constitute excellent catalysts for the
telomerization of 1,3-butadiene with various alcohols. Four
new palladium(0) carbene complexes have been character-
ized by X-ray crystallography, which allows for a systematic
comparison of structure and catalyst performance. Density
functional theory calculations of different palladium car-
bene complexes show that the electron donating substitution
of methyl group at the carbene backbone stabilizes the cata-
Chem. Eur. J. 2004, 10, 3891 3900
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3897

M. Beller et al.
FULL PAPER
Table 6. Telomerization of 1,3-butadiene with different alcohols.[a]
Entry
1
ROH
Cat.
Cat.[mol%]
0.001
Yield [%][b]
36
n:iso[%]
Chemosel. [%][c]
47
TON[d]
76000
TOF
4750
Pd(OAc)2/3 equiv PPh3
95:5
2
3
4
5
6
8
9
0.001
0.001
0.001
0.001
0.001
97
97
63
89
34
98:2
99:1
97
97
86
89
43
99500
99500
73000
99500
78000
6218
6218
4562
6218
4875
10
11
12
96:4
89:11
89:11
7
Pd(OAc)2/3 equiv PPh3
0.005
0.005
0.005
0.005
0.005
3
82
85
68
37
97:3
98:2
99:1
98:2
99:1
16
82
85
68
37
3800
20000
20000
20000
19800
238
1250
1250
1250
1238
8
8
9
9
10
11
10
11
12
13
8
9
0.005
0.005
96
96
97:3
98:2
96
96
19200
20000
1200
1250
14
15
16
17
8
9
8
9
0.001
0.001
0.001
0.001
90
93
98
95
97:3
98:2
98:2
99:1
95
93
98
98
95000
99800
99800
96500
5938
6238
6238
6031
18
19
8
8
0.005
0.005
37
56
98:2
97
97
7600
475
718
84:16
11500
20
8
0.005
86
84:16
92
18600
1162
[a] General conditions: 16 h, 708C, 1.0mol% NaOR, MeOH to 1,3-butadiene =2 to 1. [b] Yield of telomers. [c] Chemoselectivity=(yield of telomers)/
(yield of telomers+octatriene+VCH) î 100; VCH=4-vinylcyclohexene. [d] Calculated with respect to 1,3-butadiene.
8-Hexyloxyocta-1,6-diene: b.p. 908C; 1H NMR (CDCl3, 400 MHz): d=
5.8 5.4 (m, 3H), 5.1 4.9 (m, 2H), 3.9 (brd, J=6.1 Hz, 2H), 3.3 3.4 (m,
2H), 2.1 2.0(m, 4H), 1.9 (m, 4H), 1.6 1.4 (m, 4H), 1.4 1.2 (m, 6H),
0.9 0.8 (m, 3H); 13C NMR (CDCl3, 100 MHz): d=138.4, 133.7, 126.9,
114.4, 71.4, 70.1, 33.1, 31.6, 29.7, 28.2, 25.8, 22.5, 13.9; MS: m/z (%): 210
(0.6) [M +], 126 (4.8), 109 (13), 97 (4), 93 (9), 82 (18), 67 (38), 57 (34), 43
(100); elemental analysis calcd (%) for C14H26O: C 79.94, H 12.46;
found: C 79.88, H 12.76; HRMS: m/z: calcd for C14H26O: 210.19836,
found: 210.19477.
8-Isopropoxyocta-1,6-diene: b.p. 508C; 1H NMR (CDCl3, 400 MHz): d=
5.7 5.5 (m, 2H), 5.4 5.5 (m, 1H), 4.9 4.8 (m, 2H), 3.8 (dd, J=5, 1 Hz,
2H), 3.5 (sept, J=6.1 Hz, 1H), 2.0, 1.9 (m, 4H), 1.4 (quint, J=7.5 Hz,
2H), 1.05 (d, J=6.1 Hz, 6H); 13C NMR (CDCl3, 100 MHz): d=138.1,
132.8, 114.2, 70.2, 68.5, 32.9, 31.3, 27.9, 21.7, 127.7, 114., 133.7, 126.1,
3898
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2004, 10, 3891 3900

Palladium-Catalyzed Telomerizations
3891 3900
114.1, 71.5, 68.5, 58.4, 58.3, 32.7, 31.1, 27.8; MS: m/z (%): 168 (0.11)
[M +], 126 (12.5), 109 (30.6), 97 (13), 93 (25), 82 (68), 67 (95), 55 (76), 43
(100); elemental analysis calcd (%) for C11H20O: C 78.51, H 11.98;
found: C 78.56, H 11.95.
8-(2-Methoxyethoxy)octa-1,6-diene: 1H NMR (CDCl3, 400 MHz): d=
5.7 5.5 (m, 2H), 5.4 5.5 (m, 1H), 4.9 4.7 (m, 2H), 3.8 (dd, J=5.4, 1 Hz,
2H), 3.45 3.35 (m, 4H), 3.2 (s, 3H), 1.85 1.75 (m, 4H), 1.3 (quint, J=
7.5 Hz, 2H); 13C NMR (CDCl3, 100 MHz): d=137.9, 133.7, 126.1, 114.1,
71.5, 68.5, 58.4, 58.3, 32.7, 31.1, 27.8; MS: m/z (%): 183 (1.6) [M +À1],
125 (4.7), 115 (9.1), 103 (6.3), 93 (23), 81 (23), 77 (29), 67 (100), 59 (47),
45 (23), 29 (17); HRMS: m/z: calcd for C11H20O2: 184.14633, found:
184.14468.
Octa-2,7-dienyloxybenzene: 1H NMR (CDCl3, 400 MHz): d=7.6 7.5 (m,
2H), 7.25 7.15 (m, 3H), 6.2 5.9 (m, 3H), 5.35 5.25 (m, 2H), 4.75 (dd, J=
5, 1 Hz, 2H), 2.5 2.3 (m, 4H), 1.8 (quint, J=7.5 Hz, 2H); 13C NMR
(CDCl3, 100 MHz): d=158.5, 138.3, 134.8, 129.2, 129.1, 125.1, 120.5,
114.5, 68.4, 33.1, 31.6, 27.9; MS: m/z (%): 202 (2.4) [M +], 108 (9.9), 94
(100), 79 (10.9), 67 (55), 58 (11), 55 (24), 43 (40); HRMS: m/z: calcd for
C14H18O: 202.13577, found: 202.13485.
(10); elemental analysis calcd (%) for C35H55N2OPdSi2: C 61.69, H 7.99,
N 4.11; found: C 61.82, H 8.13, N 4.21.
1,3-Bis-(2,6-diisopropylphenyl)-4,5-dimethylimidazol-2-ylidine-palladi-
um(0) h2,h2-1,1,3,3-tetramethyl-1,3-divinyl-disiloxane (12): 1H NMR
([D8]THF, 400 MHz): d=7.38 (dd, J=8.1, 8.5 Hz, 2H), 7.25 (d, J=8 Hz,
4H), 3.0(sept, J=7 Hz, 4H), 2.3 (dd, J=11.9, 2 Hz, 2H), 2.2 (dd, J=
15.5, 2 Hz, 2H), 2.05 (dd, J=15.5, 11.9 Hz, 2H), 2.0(s, 6H), 1.22 (d, J=
6.9 Hz, 12H), 1.15 (d, J=6.9 Hz, 12H), À0.4 (brs, 12H); 13C NMR
([D8]THF, 100 MHz): d=197.9, 147.1, 137.1, 129.9, 127.6, 124.7, 60.3,
59.2, 29.1, 24.9, 24.3, 10.9, 0.0; MS: m/z (%): 708 (2.9) [M +], 522 (13),
415 (58), 401 (26), 385 (5), 255 (10), 171 (100), 159 (18), 143 (42), 117
(80), 103 (13), 73 (28), 59 (29)elemental analysis calcd (%) for
C37H58N2OPdSi2: C 61.64, H 8.24, N 3.95; found: C 62.48, H 8.38, N 3.81.
X-ray crystallographic studies of the complexes: Data were collected
with a STOE-IPDS diffractometer using graphite-monochromated MoKa
radiation. The structures were solved by direct methods (SHELXS-86:
G. M. Sheldrick, Acta Crystallogr. 1990, A46, 467) and refined by full-
matrix least-squares techniques against F2 (SHELXL-93: G. M. Shel-
drick, University of Gˆttingen (Germany), 1993) All nonhydrogen atoms
were refined anisotropically. XP (BRUKER AXS) was used for structure
representations.
1,3,5-Trimethyl-2-octa-2,7-dienyloxybenzene:
1H
NMR
(CDCl3,
400 MHz): d=6.9 (s, 2H), 5.9 5.8 (m, 3H), 5.1 5.0(m, 2H), 4.3 (d, J=
5 Hz, 2 H), 2.35 (s, 6H), 2.3 (s, 3H), 2.2 2.1 (m, 4H), 1.6 (quint, J=
7.5 Hz, 2H); 13C NMR (CDCl3, 100 MHz): d=153.7, 138.5, 134.9, 132.8,
131.6, 129.2, 126.2, 114.5, 73.1, 33.1, 31.6, 28.1, 20.6, 16.3; MS: m/z (%):
244 (2.5) [M +], 137 (56), 135 (50), 121 (74), 109 (11), 107 (11), 105 (14),
91 (83), 67 (39), 55 (35), 41 (100); HRMS: m/z: calcd for C17H24O:
244.18271 found: 244.18105.
1-Methyl-2-octa-2,7-dienyloxybenzene: 1H NMR (CDCl3, 400 MHz): d=
7.45 7.4 (m, 2H), 7.15 (t, J=7.3 Hz, 1H), 7.1 (d, J=6.3 Hz, 1H), 6.2 6.0
(m, 3H), 5.35 5.25 (m, 2H), 4.7 (dd, J=5.5, 1.2 Hz, 2H), 2.55 (s, 3H),
2.35 2.25 (m, 4H), 1.8 (quint, J=7.5 Hz, 2H); 13C NMR (CDCl3,
100 MHz): d=156.8, 138.5, 134.1, 130.6, 126.8, 126.6, 125.6, 120.3, 114.6,
111.3, 68.6, 33.1, 31.7, 28.2, 16.3; MS: m/z (%): 216 (2.7) [M +], 108 (100),
90(21), 79 (45), 67 (31), 55 (14), 53 (23), 51 (23), 41 (17); HRMS: m/z:
calcd for C15H20O: 216.15141, found: 216.15059.
1,3-Dimesitylimidazol-2-yliden-palladium(0)-h2,h2-1,1,3,3-tetramethyl-1,3-
divinyldi-siloxane (8): 1H NMR ([D8]THF, 400 MHz): d=7.34 (s, 2H),
6.9 (d, J=0.6 Hz, 4H), 2.62 (dd, J=12.1, 1.4 Hz, 2H), 2.37 (dd, J=15.3,
1.4 Hz, 2H), 2.25 (s, 6H), 2.1 (dd, J=15.3, 12.1 Hz, 2H), 2.14 (s, 12H),
0.0 (s, 6H), À0.7 (s, 6H); 13C NMR ([D8]THF, 100 MHz): d=199.3,
138.8, 138.5, 136.0, 129.4, 123.8, 58.3, 57.2, 20.9, 18.3, 1.5, À1.5; MS
(FAB): m/z (%): 714 (20) [Pd(IMes)2]+, 596 (25) [M +], 412 (50), 305
(100), 187 (15); elemental analysis calcd (%) for C29H42N2OPdSi2: C
58.32, H 7.09, N 4.69; found: C 58.5, H 7.35, N 4.69.
CCDC-231917 231920( 9 12) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (+44)1223-336-033; or deposit@ccdc.cam.uk).
Computation: All calculations were carried out by using the Gaussian 98
program.[19] All structures were first optimized at B3LYP density func-
tional level of theory with the LANL2DZ[20] basis set, and the nature of
the optimized structures on the potential energy surface was character-
ized by the calculated number of imaginary frequency (NImag) at the
same level of theory (B3LYP/LANL2DZ), i.e., minimum structures with-
out (NImag=0), and transition states with only one imaginary frequency
(NImag=1),[21] The related frequency calculations provided at the same
time zero-point energies (ZPE) The obtained structures at B3LYP/
LANL2DZ were further refined at the B3LYP level of theory with the
LANL2DZ basis set including a set of polarization functions (B3LYP/
LANL2DZp[20c]). The structures and energies at the B3LYP/LANL2DZp
level were used for discussion. In the calculation, the real sized imidazoli-
um ions were used, while the diolefin ligand was modeled by two ethyl-
ene molecules.
1,3-Dimesityl-4,5-dimethylimidazol-2-ylidene-palladium(0) h2,h2-1,1,3,3-
tetramethyl-1,3-divinyl-disiloxane (9): H NMR ([D8]THF, 400 MHz): d=
Acknowledgement
1
This work was supported by Degussa AG (Oxeno Olefinchemie GmbH)
and the state of Mecklenburg-Vorpommern. Analytical services were
provided by Mrs. S. Buchholz and Dr. C. Fischer (both IfOK). For cata-
lyst development of the complexes 13 and 14 performed at the University
of New Orleans, the National Science Foundation and the Louisiana
Board of Regents are gratefully acknowledged.
6.9 (s, 4H), 2.6 (dd, J=12.3, 1.4 Hz, 2H), 2.39 (dd, J=15.3, 1.4 Hz, 2H),
2.25 (s, 6H), 2.1 (s, 12H), 2.05 (dd, J=15.3, 12.3 Hz, 2H), 1.9 (s, 6H), 0.0
(s, 6H), À0.8 (s, 6H); 13C NMR ([D8]THF, 100 MHz): d=196.4, 138.8,
136.9, 136.5, 129.7, 126.7, 58.6, 57.1, 21.1, 18.3, 9.2, 1.6, À1.4; MS: m/z
(%): 666 (20) [M +], 480 (25), 373 (15), 337 (50), 301 (20), 171 (100), 143
(30), 117 (80), 59 (25).
1,3-Dimesityl-4,5-dichloroimidazol-2-ylidene-palladium(0) h2,h2-1,1,3,3-
tetramethyl-1,3-divinyl-disiloxane (10): 1H NMR ([D8]THF, 400 MHz):
d=7.34 (s, 4H), 2.7 (dd, J=12.3, 1.6 Hz, 2H), 2.45 (dd, J=15.4, 1.6 Hz,
2H), 2.2 (dd, J=15.4, 12.3 Hz, 2H), 2.3 (s, 6H), 2.14 (s, 12H), 0.0 (s, 6H),
À0.8 (s, 6H); 13C NMR ([D8]THF, 100 MHz): d=191.9, 140.3, 136.7,
134.9, 129.9, 60.1, 59.7, 21.1, 18.2, 1.5, À1.4; MS: m/z (%): 666 (20) [M +],
480 (25), 373 (15), 337 (50), 301 (20), 171 (100), 143 (30), 117 (80), 59
(25); elemental analysis calcd (%) for C29H40Cl2N2OPdSi2: C 52.29, H
6.05, N 4.21; found: C 52.30, H 6.2, N 4.12.
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1,3-Bis-(2,6-diisopropylphenyl)imidazol-2-ylidine-palladium(0)
h2,h2-
1,1,3,3-tetra-methyl-1,3-divinyl-disiloxane (11): 1H NMR ([D8]THF,
400 MHz): d=7.66 (s, 2H), 7.55 (t, J=7.7 Hz, 2H), 7.43 (d, J=7.7 Hz,
4H), 3.23 (sept, J=6.7 Hz, 4H), 2.66 (d, J=12.7 Hz, 2H), 2.49 (d, J=
15.3 Hz, 2H), 2.28 (dd, J=12.7, 15.3 Hz, 2H), 1.45 (d, J=6.7 Hz, 12H),
1.34 (d, J=6.9 Hz, 12H), 0.18 (s, 6H), À0.8 (s, 6H); 13C NMR ([D8]THF,
100 MHz): d=200.8, 146.7, 138.5, 130.0, 125.2, 124.3, 59.5, 58.2, 29.3, 26.0,
23.2, 1.8, À1.2; MS: m/z (%): 681 (10) [M +], 494 (33), 387 (100), 186
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Chem. Eur. J. 2004, 10, 3891 3900
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3899

M. Beller et al.
FULL PAPER
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Received: February 23, 2004
Published online: June 21, 2004
3900
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www.chemeurj.org
Chem. Eur. J. 2004, 10, 3891 3900