Synthesis of a silica-based heterogeneous second generation grubbs catalyst

Article abstract of DOI:10.1002/1615-4169(200208)344:6/7<712::AID-ADSC712>3.0.CO;2-W

The synthesis of a second generation Grubbs catalyst immobilized onto non-porous silica is described. For this purpose, a polymerizable cationic NHC precursor, 1,3-bis(1-mesityl)-4-{[(bicyclo[2.2.1]hept-5-en-2-ylcarbonyl)oxy] methyl}-4,5-dihydro-1H-imidaz

Full text of DOI:10.1002/1615-4169(200208)344:6/7<712::AID-ADSC712>3.0.CO;2-W

FULL PAPERS
Synthesis of a Silica-Based Heterogeneous Second Generation
Grubbs Catalyst
Monika Mayr,a Michael R. Buchmeiser,a,* Klaus Wurstb
a
Institute of Analytical Chemistry and Radiochemistry, University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
Fax: ( ‡ 43)-512-507-2677; e-mail: michael.r.buchmeiser@uibk.ac.at
b
Institute of General, Inorganic and Theoretical Chemistry. University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
Received: January 31, 2002; Accepted: April 18, 2002
Abstract: The synthesis of a second generation ing initiator. This allowed the controlled synthesis of
Grubbs catalyst immobilized onto non-porous silica oligomers via stoichiometry. Using both non-porous
is described. For this purpose, a polymerizable and porous silica, degrees of derivatization of 0.04
cationic NHC precursor, 1,3-bis(1-mesityl)-4-{[(bicy- and 0.02 mmol, respectively, of cationic NHC precur-
clo[2.2.1]hept-5-en-2-ylcarbonyl)oxy]methyl}-4,5-di-
sor/g silica were obtained. These precursors were
hydro-1H-imidazol-3-ium tetrafluoroborate (5) was converted into the corresponding NHC×s by standard
prepared and characterized by X-ray analysis. procedures and used for the generation of a hetero-
Oligomers were prepared therefrom using both the geneous second-generation Grubbs catalyst. Ruthe-
well-defined Schrock initiator Mo(N-2,6-i-Pr2C6H3) nium loadings of 5.3 and 1.3 mmol/g, corresponding to
(CHCMe2Ph)[OCMe(CF3)2]2 and the first generation 0.5and 0.1 weight-% of catalyst were realized. Addi-
Grubbs catalyst Cl2Ru(CHPh)(PCy3)2. Ru-initiated tionally, coating techniques were applied, where C18-
oligomerizations were terminated with ethyl vinyl derivatized silica-60 was loaded with oligo-5. Con-
ether, Mo-initiated oligomerizations were terminated version into the corresponding heterogeneous catalyst
ˆ ˆ
by addition of (EtO)3SiCH2CH2CH2N C O. (EtO)3Si- revealed 4.1 mmol/g, corresponding to 0.4 weight-%
terminated oligomers obtained by the Wittig-like of catalyst. All supported catalysts prepared by this
reaction between the Mo-containing oligomer and approach were successfully used in RCM in slurry
the isocyanate were used for the immobilization of type reactions.
the NHC-precursor containing oligomers on non-
porous silica. Both oligomerizations were character- Keywords: heterogeneous catalysis; high throughput
ized by quantitative consumption of the correspond- screening; metathesis; polymers; ruthenium
Introduction
and large-scale reactor technology. As others,[16] we
focus on synthetic protocols that use N-heterocyclic
Besides of transition metal-catalyzed C-C coupling carbene (NHC)-based ligands rather than the alkyli-
reactions such as the Heck,[1,2] Sonogashira Hagihara, dene moiety for immobilization.[17 20] In this context, we
À
or Suzuki coupling,[3 6] N- and O-arylations or alkyla- already reported on the synthesis of a second gener-
tions, olefin metathesis represents one of the most ation, NHC-based Grubbs catalyst immobilized onto a
powerful reactions for catalytic C-C bond formation.[7] monolithic system.[21] In the following, we describe the
In particular, ring-closing metathesis (RCM) has be- synthetic protocol for the synthesis of a system immo-
come a highly attractive tool for the synthesis of bilized onto non-porous and porous silica. These
complex architectures.[8,9] So far, a broad variety of supported catalysts can be used for ring-closing meta-
homogeneous catalytic systems is available.[10] Among thesis (RCM) in batch experiments and give access to
the numerous binary and ternary systems, the most catalyst-free products.
outstanding ones are high oxidation state Mo-based
Schrock catalysts[11 14] and the ruthenium-based Grubbs
initiators.[15] In spite of, or rather because of, their highly
favorable (complementary) properties, we have started
Results and Discussion
the search for analogous heterogeneous systems that
combine both the properties of the parent homogeneous Monomer Synthesis
catalysts and the advantages of heterogenization. In
case of realization, one would gain access to supported 2,3-Dibromo-1-propanol was converted into the dime-
catalysts for both small-scale combinatorial chemistry sityl derivative 1 by reaction with 2,4,6-trimethylaniline
712
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1615-4150/02/3446+7-712 719 $ 17.50-.50/0
Adv. Synth. Catal. 2002, 344, No. 6+7

Silica-Based Heterogeneous Second Generation Grubbs Catalyst
FULL PAPERS
OH
Br
OH
NH
OSi(CH3)3
NH
(i)
(ii)
Br
NH
NH
1
2
(iii)
(iv)
OH
OH
–
Cl–
BF4
N
N
N
N
3
4
(vi)
(v)
O
X = BF4– (6a)
X = Cl– (6b)
O
O
O
X-
–
BF4
N
N
N
N
Figure 1. X-ray structure of 5 (disordered part C18A-C24A is
omitted for clarity). Selected bond distances [ä] and angles [o]:
N1-C1 1.311(5), N1-C2 1.491(5), N1-C4 1.445(5), N2-C1
1.305(5), N2-C3 1.481(6), N2-C10 1.439(5), C2-C3 1.525(6),
N2-C1-N1 114.2(4), C1-N1-C2 109.7(3), C1-N2-C3 110.0(3),
N1-C2-C3 102.8(3), N2-C3-C2 103.3(3).
6
5
(vii)
O
O
N
N
Cl
norborn-5-ene-2-carboxylic chloride. Since compound 5
represents a key intermediate, it was characterized by X-
ray analysis. Its structure is shown in Figure 1. The
compound is obtained in form of two diastereomers,
with all enantiomers (RR, SS, RS, SR) present. Interest-
ingly, only the endo-form is obtained. Though endo-
isomers are generally considered to be less active in ring-
opening metathesis polymerization (ROMP) compared
to their exo-analogues, the polymerization of 5 proceeds
smoothly with both Mo- and Ru-based initiators (vide
infra).
Ru
Ph
Cl
PCy3
7
Scheme 1. Synthesis of compounds 1 6. (i) a) 2,4,6-trime-
thylaniline, 1208C, 19 h; b) CH2Cl2, 15% NaOH. (ii) CH2Cl2,
(CH3)3SiCl, NEt3. (iii) a) (NH4)BF4, HC(OEt)3, 2 h, 1108C;
b) acetic acid. (iv) a) (NH4)BF4, HC(OEt)3, 2 h, 1108C; b)
HCl. (v) CH2Cl2, NEt3, norborn-5-ene-2-carboxylic chloride.
(vi) CH2Cl2, NEt3, acetyl chloride. (vii) a) KO-t-Bu, THF; b)
Cl2Ru(CHPh)(PCy3)2, toluene, 808C.
(mesitylamine) and subsequent deprotonation with
aqueous sodium hydroxide (Scheme 1).
In order to prepare a non-polymerizable analogue to 5
that can be used for the synthesis of a monomeric,
In order to rule out the formation of even small soluble second-generation Grubbs initiator, compound
amounts of a 4,5-dihydro-1H-oxazolium derivative in- 6 was synthesized from 3 by reaction with acetyl
stead of the desired 4,5-dihydro-1H-imidazolium salts 3 chloride. This NHC precursor 6 was converted to the
À
and 4, the hydroxymethyl group in 1 was protected by a free carbene by reaction with KO-t-Bu in THFat 408C.
trimethylsilyl group via reaction with (CH3)3SiCl and In situ NMR experiments carried out in THF-d8
triethylamine in CH2Cl2 to form the O-protected species revealed that carbene formation was complete at this
2. Compound 2 can be converted into the desired 4,5- temperature within minutes. Without isolation, the free
dihydroimidazolium salts 3 and 4 by reaction with carbene was reacted with Cl2Ru(CHPh)(PCy3)2 in
HC(OEt)3 and ammonium tetrafluoroborate[22] and THF[22] at 08C to yield Cl2Ru(CHPh)(Imes×)(PCy3), 7,
subsequent deprotection with acetic acid, respectively, [Imes× ˆ 1,3-di(1-mesityl)-4,5-dihydro-4-acetoxymeth-
hydrochloric acid. Both deprotected 4,5-dihydro-1H- yl-1H-imidazolydene]. Similar to the systems reported
imidazolium salts 3 and 4 can be used for the synthesis of by F¸rstner et al.,[23] comparably low temperatures are
the polymerizable NHC precursor 5 by reaction with required during synthesis in order to avoid carbene
norborn-5-ene-2-carboxylic chloride. Nevertheless, tak- decomposition. So far we were not able to obtain 7 in
ing advantage of the high free energy that is entailed pure form, yet its formation can be monitored by
with the formation of a Si-Cl bond, the most straightfor- 1H NMR in THF-d8. Relevant chemical shifts are
ward approach to 5 is reaction of TMS-protected 3 with summarized in the Experimental Section.
Adv. Synth. Catal. 2002, 344, 712 719
713

Monika Mayr et al.
FULL PAPERS
Ph
initiation is quantitative. Since alkylsilanes react with
aldehydes via Si-O rearrangement[25], there exist no
stable silylaldehydes that could be used for endcapping.
We therefore investigated the extent to which w-
(trialkoxysilyl)alkyl isocyanates might be used for this
purpose. In principle, an isocyanate group should react
in the same way than an aldehyde. Preliminary experi-
ments entailed the reaction of Mo(N-2,6-i-Pr2-C6H3)
(CHCMe2Ph)(OCMe(CF3)2)2 with (EtO)3SiCH2CH2
O
n
(i)
R =
N
O
R
–
BF4
5
N
(ii)
(EtO)3Si
CMe2Ph
N
n
ˆ ˆ
CH2N C O. With a ten-fold excess of isocyanate, the
R
original alkylidene proton resonances at d ˆ 12.1 and
12.8 ppm, respectively, disappeared and a new signal,
corresponding to the new olefinic proton of the keteni-
mine appeared at d ˆ 5.28 ppm. Additionally, GC-MS
investigations were carried out and confirmed the
Scheme 2. Reaction of the Schrock initiator with the isocya-
nate. (i) a) Cl2Ru(CHPh)(PCy3)2 in ClCH2CH2Cl; b) ethyl
vinyl ether. (ii) a) Mo(N-2,6-i-Pr2-C6H3)(CHCMe2Ph)
(OCMe(CF3)2)2 in CH2Cl2; b) (EtO)3SiCH2CH2CH2NCO.
formation of the proposed product (EtO)3SiCH2CH2
‡
ˆ ˆ
CH2NH C CHCMe2Ph (m/z ˆ 364) and the corre-
.
.
‡
Polymerization and Synthesis of Telechelic Polymers
sponding fragments (EtO)3Si (m/z ˆ 163) and CH2
‡
ˆ ˆ
CH2CH2NH C CHCMe2Ph (m/z ˆ 201) (Scheme 3).
The NHC precursor 5 can be polymerized using both
Based on these results, 5 was initiated with the Mo-
ruthenium- and molybdenum-based initiators. Thus, based catalyst and oligomers with a DP of 7 were
reaction of 5 with Cl2Ru(CHPh)(PCy3)2 in methylene prepared. The living oligomer was reacted with a ten-
chloride at 458C results in complete consumption of the fold excess of the isocyanate at 458C to yield the
initiator and formation of an oligomer with a theoretical trialkoxysilyl-telechelic oligomer (Scheme 3).
degree of polymerization (DP) of 7. A reaction time of
1 hour was necessary in order to achieve complete
Mo(NAr')(CHCMe2Ph)[OCMe(CF3)2]2 + (EtO)3SiCH2CH2CH2N=C=O
monomer consumption. The actual DP as determined
by endgroup analysis using 1H NMR was 7 Æ 3.
Endcapping was accomplished by reaction with ethyl
δ = 5.25 ppm
vinyl ether (Scheme 2).
So far, with Ru-based systems we were not able to
(EtO)3SiCH2CH2CH2N=C=CHCMe2Ph
introduce an endgroup suitable for heterogenization in a
Scheme 3. Synthesis and end-functionalization of oligomers.
satisfactory (i.e., in a quantitative) way. We therefore
performed polymerizations with the Schrock initiator
Mo(N-2,6-i-Pr2-C6H3)(CHCMe2Ph)(OCMe(CF3)2)2
[24]
Grafting of the Telechelic Oligomer onto Silica and
in methylene chloride at ambient temperature. With this Synthesis of the Heterogeneous Catalyst
initiator, oligomerization was complete within 30 mi-
nutes. The theoretical DP of 7 was in excellent accord- Telechelic oligo-5 was reacted with non-porous and
ance with a DP of 7 Æ 1 found via endgroup analysis porous silica (Scheme 4). Non-porous silica was chosen
using 1H NMR. Again, this strongly suggests that in order to avoid diffusion-controlled reactions within
1)
O
O
OEt
Si
O
O
O
O
O
O
OH
O
O
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
(EtO)3Si
CMe2Ph
3
CMe2Ph
3
OH
OH
N
N
OH
n
O
n
O
OH
OH
OH
O
R
O
Me
O
O
O
Si
O
Me
OH
2) (CH3)3Si(OCH3)/(CH3)2Si(OCH3)2
3) KO-t-Bu/THF
4) Cl2Ru(CHPh)(PCy3)2
OH
OSiMe3
OH
OH
O
O
N
N
OSiMe3
Cl
OH
Ru
O
Ph
Cl
PCy3
R =
N
O
–
BF4
N
Scheme 4. Synthesis of heterogeneous ruthenium catalyst.
714
Adv. Synth. Catal. 2002, 344, 712 719

Silica-Based Heterogeneous Second Generation Grubbs Catalyst
FULL PAPERS
medium sized pores. The potentially low catalyst load- reaction times, temperature, and solvents), the max-
ing that is usually entailed with the use of low-specific imum turnover number (TON) that was achieved was
surface area supports was compensated via the grafting 75. Interestingly, basically identical results (TON ˆ 80)
approach by the factor of 7 (i.e., the DPof the oligomer). were obtained with catalyst immobilized onto porous
Endcapping of the residual silanol groups was accom- silica. In principle, second generation Grubbs-type
plished by reaction with (CH3)3SiOCH3 and initiators immobilized on non-porous silica should
(CH3)2Si(OCH3)2. It should be noted that such endcap- behave similar to those immobilized on monolithic
ping reactions of silica are never quantitative, typical supports.[21] In fact, catalysts immobilized onto mono-
amounts of residual Si-OH groups are around 7 8%.[26] lithic supports give similar maximum TON×s ( 65) in the
ˆ
Nevertheless, due to the functional group tolerance of absence of any chain transfer agent (CTA). Neverthe-
Ru-based systems and the low accessibility of these less, the low TON×s ( < 5!) obtained with other, less
remaining Si-OH groups, this does usually not entail any reactive compounds such as 1,7-octadiene, N,N-diallyl-
problems. Recently, our group demonstrated the low trifluoroacetamide, diallyl ether, and diallyldiphenylsi-
accessibility of such Si-OH moieties. Thus, even highly lane suggest that the stirred batch setup with this type of
active Mo-based Schrock initiators can be used in a support is highly diffusion controlled, where reaction is
grafting from-approach with such silica.[27]
too slow and decomposition of the intermediary ruthe-
Reaction of the grafted support with KO-t-Bu in THF nium methylidene dominates. Apart from these sober-
À
at 308C yielded the free carbene which was subse- ing results, those obtained with coated supports deserve
quently reacted with Cl2Ru(CHPh)(PCy3)2 to yield the special consideration. TON×s for diethyl diallylmalo-
immobilized second generation Grubbs catalyst. After nate, 1,7-octadiene, anddiallyl ether were 210, 55, and 16
leaching of the support with aqua regia under micro- in the absence of any CTA, thus exceeding data for TON
wave conditions, the ruthenium content of the solution of all other silica supports by far. Since coating
was measured by inductively coupled plasma-optical techniques are known to reduce the specific surface
emission spectroscopy (ICP-OES). In terms of catalyst area by clogging of the micropores, catalytic results can
loading it is worth mentioning that only 13% of the NHC be interpreted in a way that a support containing only
ligand were converted into the corresponding catalyst, macropores, which facilitate diffusion, is generated. It is
leading to a catalyst loading of 0.5weight-%. This value
worth mentioning that in all cases ruthenium measure-
is much lower than the one found in systems based on ments by means of ICP-OES revealed quantitative
monolithic supports, where roughly 40% of the NHC retention of the original amount of ruthenium at the
precursor could be used for immobilization, resulting in support within experimental error (Æ5%), thus offering
1.4 weight-% catalyst loading.[21]. Although a non- access to metal-free products. In view of these data, our
porous support should facilitate the accessibility of any current research emphasizes on improved conversion of
surface-bound groups, this particular silica shows a NHC precursors into the desired catalytic sites as well as
reduced accessibility of the corresponding NHC sites. on the use of cartridge systems. Results will be reported
We attribute this fact to the strong tendency of this in due course.
support to agglomerate. Concerning the implications for
catalysis, vide infra. In analogy, porous silica 60 was
surface functionalized with 0.02 mmol of NHC precur- Conclusion
sor in the form of (EtO)3Si-telechelic oligo-5. Conver-
sion into the initiator was accomplished as described Both Ru- and Mo-based initiators can polymerize
above, leading to a Ru-loading of 0.013%. Finally, cationic NHC precursors containing norborn-5-ene-2-
coating techniques[28] using again (EtO)3Si-telechelic yl groups. Mo-initiated polymerizations can be termi-
oligo-5 were applied, leading to a surface-derivatized nated with isocyanates in a Wittig-type reaction. In the
silica 60 containing 0.09 mmol of NHC precursor. case of w-(alkoxysilyl)alkyl isocyanates, the correspond-
Conversion into the initiator resulted in a support ing w-(trialkoxy)silane-containing polymers can be
containing 4.1 mmol Ru/g.
grafted onto silica by employing standard derivatization
procedures. Subsequent conversion of the NHC pre-
cursors into the corresponding carbenes and reaction
with Cl2Ru(CH Ph)(PCy3)2 yields the desired hetero-
genized second generation Grubbs-type initiators. The
silica-based systems can be used both in slurry reactions
ˆ
Reactivity and Use of the Heterogeneous Catalyst in
RCM
Preliminary RCM experiments were carried out with and for the fabrication of cartridge systems for RCM.
diethyl diallylmalonate. The catalyst immobilized onto
non-porous silica was added to a solution of this
monomer in 1,2-dichlorobenzene and the mixture was
heated to 508C for 2 hours. Irrespective of the reaction
conditions used (i.e., ultrasound, microwave, changing
Adv. Synth. Catal. 2002, 344, 712 719
715

Monika Mayr et al.
FULL PAPERS
Experimental Section
N,N'-Dimesityl-2,3-diamino-1-propoxytrimethylsilane
(2)
General Remarks
Compound 1 (4.000 g, 12.25mmol) was dissolved in 70 mL of
methylene chloride. Triethylamine (1.8 mL, 13 mmol) and
trimethylchlorosilane (1.6 mL, 13 mmol) were added sequen-
tially and the mixture was stirred at room temperature for two
hours. After this reaction time, thin layer chromatography
(TLC) on silica (mobile phase diethyl ether:pentane ˆ 1:1)
showed no remaining educt. The volatiles were removed under
vacuum leaving a yellow oil. Diethyl ether was added to extract
the product while the triethylammonium chloride formed
during the reaction precipitated. The salt was removed by
filtration through celite. Removal of the solvent gave the
product as yellow oil; yield: 4.88 g (100%). 1H NMR (CDCl3):
d ˆ 6.83 (d, 4H, Harom), 3,67 (dd, 1H, O-CH2, 2J ˆ 10.2 Hz, 3J ˆ
NMR data were obtained at 300 MHz in the indicated solvent
at 258C on a Bruker Spectrospin 300 and are listed in parts per
million downfield from tetramethylsilane for proton and
carbon. Coupling constants are listed in Hertz. IR spectra
were recorded on a Nicolet 510 FT-IR. GC-MS investigations
were carried out on a Shimadzu GCMS QP5050, using a SPB-5
fused silica column (30 m  0.25mm  25 mm film thickness).
Elemental analyses were carried out at the Institute of Physical
Chemistry, University of Vienna. A Jobin Yvon JY 38 plus was
used for ICP-OES measurements, a MLS 1200 mega for
microwave experiments. Further instrumentation is described
elsewhere.[29]
2
3
2.1 Hz), 3.60 (dd, 1H, O-CH2, J ˆ 10.2 Hz, J ˆ 3.7 Hz), 3.49
Syntheses of the ligands and polymerizations were per-
formed under an argon atmosphere by standard Schlenk
techniques or in an N2-mediated dry-box (Braun, Germany)
unless stated otherwise. Reagent grade diethyl ether, pentane,
THF, and toluene were distilled from sodium benzophenone
ketyl under argon. Reagent grade dichloromethane and 1,2-
dichloroethane were distilled from calcium hydride under
argon. Other solvents and reagents were used as purchased.
Deionized water was used throughout. Cl2Ru(CHPh)(PCy3)2,
was purchased from Fluka. endo-Norborn-2-ene-5-carboxylic
(m, 1H, CH), 3.75 3.45(b, 2H, NH), 3.29 (dd, 1H, CH , 2J ˆ
2
11.8 Hz, 3J ˆ 5.6 Hz), 2.98 (dd, 1H, CH2, 2J ˆ 11.8 Hz, 3J ˆ
6.2 Hz), 2.82 (m, 18H CH3), 0.1 (s, 9H, Si-CH3); 13C NMR
(CDCl3): d ˆ 143.8, 141.7, 131.1, 130.5, 129.7, 129.5, 129.4, 129.0
(Carom), 62.7, 57.4, 50.6 (2 Â CH2, CH), 20.5, 18.9, 18.8, 18.4
(CH3), 1.0 (Si-CH3).
1,3-Bis(1-mesityl)-4,5-dihydro-4-hydroxymethyl-1H-
imidazol-3-ium Tetrafluoroborate (3)
chloride[29]
and
Mo{N-2,6-(i-Pr2-C6H3)(CHCMe2Ph)
[OCMe(CF3)2]2},[24] were prepared according to literature
procedures and checked for purity by means of NMR. Non-
porous silica (Aerosil) was purchased from Degussa-Huels,
Germany. Silica 60 was purchased from Merck, Germany. C18
derivatization was carried out according to the literature.[26]
Compound 2 (4.880 g, 12.25mmol) and ammonium tetrafluo-
roborate (1.284 g, 12.25mmol) were dissolved in triethyl
orthoformate (5mL, 30 mmol) and the reaction mixture was
heated and stirred for two hours. After this time, TLC on silica
(mobile phase diethyl ether:pentane ˆ 1:1) showed no remain-
ing educt. The reaction mixture was cooled to 408C and the
volatiles were removed under vacuum. Ayellow oil remained.
It was treated with acetic acid to remove the trimethylsilyl
protecting group. Finally, all volatiles were removed under
vacuum and the residue was washed with pentane, dried, and
recrystallized from ethanol/pentane; yield: 1.951 g (36.5%,
N,N'-Dimesityl-2,3-diamino-1-propanol (1)
2,3-Dibromopropanol (20.000 g, 91.785mmol) was treated
with 2,4,6-trimethylaniline (32.267 g, 238.64 mmol). The mix-
ture was stirred at 1208C for 19 hours. A dark brown solid
precipitated. Methylene chloride and an aqueous 15% sodium
hydroxide solution (100 g, 400 mmol) were added and the
mixture was stirred until all solid had completely dissolved.
The organic layer was separated, washed with water and dried
over sodium sulfate. The solvent was removed under vacuum
and some dark red oil remained. The crude product was
cleaned by column-chromatography on silica gel 60 using a 1:1
mixture of diethyl ether and pentane; yield: 14.870 g (49.6%,
Ä
white solid). IR (KBr): n ˆ 3200 b (nOH), 2923 m (nasCH3),
‡
ˆ
ˆ
2880 m (nsCH3), 1624 s (nC N ), 1481 s, 1470 sh (all C Carom),
1389 cmÀ1 m (dsCH3); 1H NMR (CDCl3): d ˆ 7.80 (s, 1H, C2-H),
6.89 (s, 4H, m-Harom), 4.80 (m, 1H, CH), 4.4231 (d, 2H, CH2),
3.72 (d, 1H, CH2, 2J ˆ 11.9 Hz), 3.53 (d, 1H, CH2, 2J ˆ 11.9 Hz),
2.4 2.1 (m, 18 H, CH3); 13C NMR (CDCl3): d ˆ 158.1 (C2),
140.4, 140.2, 136.4, 135.0, 130.2, 128.4 (all Carom), 65.3, 58.6, 52.1
(2 Â CH2, CH), 20.9, 20.8, 17.9, 17.5, 17.4 (all CH3).
1,3-Bis(1-mesityl)-4,5-dihydro-4-hydroxymethyl-1H-
imidazol-3-ium Chloride (4)
Ä
white solid). IR (KBr): n ˆ 3200 b (nOH), 3419 s (nN1H), 3345
m (nN2H), 3030 m (nCaromH), 2965m ( nasCH3), 2855 m (nsCH3),
ˆ
1615w, 1593 w, 1483 s, 1445s (all nC Carom), 1432Àm (dasCH3),
Compound 3 (4.631 g, 11.62 mmol) and ammonium tetrafluo-
roborate (1.218 g, 11.62 mmol) were dissolved in triethyl
orthoformate (4 mL, 20 mmol) and the reaction mixture was
heated and stirred for two hours. After that reaction time, TLC
on silica (mobile phase ˆ diethyl ether:pentane ˆ 1:1) showed
no remaining educt. The reaction mixture was cooled to 408C
and the volatile compounds were removed under vacuum. A
yellow solid remained. It was washed with water to remove
unreacted ammonium tetrafluoroborate and dried. After-
wards the residue was washed with pentane and dried again.
In order to remove the trimethylsilyl protective group the
1
1
1374 m (dsCH3), 1248 s, 1099 s, 972 s, 872 s cm ; H NMR
(CDCl3): d ˆ 6.80, 6.81 (2  s, 4H, m-Harom.), 3.95(dd, 1H, CH 2,
2J ˆ 10.9 Hz, 3J ˆ 2.6 Hz), 3.83 (dd, 1H, CH2, 2J ˆ 10.1 Hz, 3J ˆ
4.1 Hz), 3.4 3.8 (sb, 2H, NH), 3.38 (m, 1H, CH), 3.22 (dd, 1H,
CH2, 2J ˆ 11.9 Hz, 3J ˆ 5.0 Hz), 2.98 (dd, 1H, CH2, 2J ˆ 11.9 Hz,
3J ˆ 4.0 Hz), 2.1 2.3 (m, 19H, CH3, OH); 13C NMR (CDCl3):
d ˆ 142.5, 141.8, 132.5, 130.9, 130.5, 129.8,128.5, 129.7, (Carom.),
52.3, 56.9, 66.0 (2 Â CH2, CH), 17.8, 20.5, 20.4, 18.9 (CH3); anal.
calcd. for C21H30N2O: C 77.26, H 9.26, N 8.58%; found: C 76.86,
H 9.66, N 8.38%.
716
Adv. Synth. Catal. 2002, 344, 712 719

Silica-Based Heterogeneous Second Generation Grubbs Catalyst
FULL PAPERS
residue was stirred with 20% hydrochloric acid for several
hours. With this step the anion changes from tetrafluoroborate
to chloride. The solution was neutralized with 15% sodium
hydroxide. Then the product was extracted with methylene
chloride and dried over sodium sulfate. The product was dried
under vacuum; yield: 90%. 1H NMR (CDCl3): d ˆ 8.20 (s, 1H,
C2-H), 6.9 6.8 (m, 4H, Harom), 4.80 (m, 2H, CH2), 4.35(t, 1H,
(s, 1H, C2-H), 6.93 (s, 4H, Harom), 5.22 (m, 1H, CH), 4.71 (t, 1H,
CH2), 4.21 (m, 2H, CH2), 3.96 (d, 1H, CH2), 2.35 2.20 (m, 18 H,
Carom-CH3), 2.03 (s, 3H, C OCH3); 13C NMR (CDCl3): d ˆ
ˆ
ˆ
169.8 (C O), 159.1 (C2), 140.7, 140.5, 135.3, 135.2, 130.5,
130.2, 129.8, 128.0 (all Carom), 62.0, 61.3, 52.5 (2 Â CH2, CH),
21.0, 20.9, 20.6, 18.0, 17.4, 17.2 (all CH3); anal. calcd. for
C24H31BF4N2O2: C 61.77, H 6.7, N 6.01%; found: C 61.23, H
6.86, N 5.81%.
2
2
CH), 3.78 (d, 1H, CH2, J ˆ 12.81 Hz), 3.45(d, 1H, CH , J ˆ
2
12.81 Hz), 2.4 2.2 (m, 18H, CH3); 13C NMR (CDCl3): d ˆ
156.6 (C2), 138.8, 138.6, 135.2, 133.1, 128.8, 128.5, 127.1 (all
Carom), 64.4, 56.6, 51.1 (2 Â CH2, CH), 19.4, 16.8, 16.4 (all CH3).
1,3-Bis(1-mesityl)-4-acetoxymethyl-4,5-dihydro-1H-
imidazol-3-ium Chloride (6b)
1,3-Bis(1-mesityl)-4,5-dihydro-4-hydroxymethyl-1H-imida-
zol-3-ium chloride (0.500 g, 1.32 mmol) was dissolved in
methylene chloride and then triethylamine (0.2 mL,
1,3-Bis(1-mesityl)-4-{[(bicyclo[2.2.1]hept-5-en-2-
ylcarbonyl)oxy]methyl}-4,5-dihydro-1H-imidazol-3-
ium Tetrafluoroborate (5)
À
1.4 mmol) was added. The mixture was chilled to 508C and
treated with acetyl chloride (55 mL, 0.77 mmol) and acetic
anhydride (71 mL, 0.75mmol). The reaction mixture was
stirred. After removal of the cooling bath after two hours, the
mixture was stirred for another 6 hours. The volatile com-
pounds were removed under vacuum and a brownish solid
remained. It was dissolved in methylene chloride and extracted
with water. The water phase was concentrated to a few
milliliters and sodium chloride was added. The product was
reextracted with methylene chloride (5 Â 60 mL) and dried
over sodium sulfate. After removal of the solvent the product
was recrystallized from ethanol/pentane; yield: 0.090 g (17%,
white solid). 1H NMR (CDCl3): d ˆ 9.44 (s, 1H, C2-H), 6.82 (s,
4H, Harom), 5.39 (m, 1H, CH), 4.86, 4.30, 4.08, 3.92 (all m, all 1H,
Compound 4 (1.952 g, 4.524 mmol) was dissolved in methylene
chloride, then triethylamine (1 mL, 7 mmol) was added. The
À
mixture was chilled to 508C and treated with norborn-2-ene-
5-carboxylic acid chloride (0.85 mL, 5.4 mmol, endo:exo ˆ
8:1). The reaction mixture was stirred for 2 hours, the cooling
bath was removed and the mixture was stirred for another
5hours. The volatiles were removed under vacuum and a
pinkish solid remained. This residue was washed with water
and dried. Afterwards the residue was washed with pentane
and dried again. The crude product was recrystallized from
ethanol/pentane; yield: 1.296 g (52.3%, white crystals). IR
ˆ
Ä
(KBr): n ˆ 3060 w (nCNBE C-H), 2973 s (nasCH3), 2869 m (ns
‡
ˆ
ˆ
1
CH3), 1736 s (nC O), 1628 s (C N ), 1481 m, 1460 m (all
nC Carom), 1383 m (dsCH3), 713 cmÀ m (da
NBE-H); 1H NMR
ˆ
2 ÂCH2),2.35 2.10(m, 18H, Carom-CH3), 1.97(s, 3H, C OCH3).
ˆ
ˆ
C
(CDCl3): d ˆ 8.13, 8.11 (2  s, 1H, C2-H endo/exo), 6.92 (m, 4H,
Harom), 6.14, 6.04, 5.80 [all m, 2H, HNBE(db)], 5.17 (m, 1H, CH),
4.68 (m, 1H, CH2), 4.30 3.92 (m, 3H, CH2), 3.04, 2.89, 2.82 (all
m, 3H, all HNBE), 2.4 2.2 (m, 18H, CH3), 1.85, 1.63, 1.41, 1.25
Tricyclohexylphosphine[1,3-bis(1-mesityl)-4-
acetoxymethyl-4,5-dihydroimidazol-2-
ylidene][benzylidene]ruthenium(IV) Dichloride (7)
(all m, 4H, all HNBE); 13C NMR (CDCl3): d ˆ 175.6, 174.1 (C O
ˆ
exo/endo), 159.7 (C2), 141.0, 140.8, 138.4, 135.8, 135.7, 136.6,
135.5 132.1, 132.0, 130.8, 130.4, 130.2, 128.7, 128.5, (all Carom
‡
A suspension of 7 (10.5mg, 22.8 mmol) in THF-d8 (0.5mL) was
chilled to 408C and a solution of potassium t-butoxide
CNBE), 62.7, 62.6, (CH, CH2), 53.0, 52.9, 50.1, 49.9, 46.7, 46.1,
46.0, 43.3, 42.8, 41.8 (all CNBE ‡ CH2), 29.9, 29.6 (CNBE), 21.3,
21.2, 18.3, 18.0, 17.9, 17.8, 17.7 (all CH3); anal. calcd. for
C30H37BF4N2O2: C 66.18, H 6.85, N 5.15%; found: C 65.87, H
6.90, N 5.03%. Crystals suitable for X-ray analysis were
obtained from ethanol:pentane.
À
(2.6 mg, 22.8 mmol) in THF (0.5mL) was added. The reaction
À
mixture was stirred for 10 minutes at 408C. A solution of Cl2
Ru(CHPh)(PCy3)3 (13.1 mg, 15.9 mmol) in THF-d8 (0.5mL)
was added, the solution was stirred for another 2 hours at 08C
and then warmed to room temperature. All attempts to isolate
this compound in a pure form have failed so far. Nevertheless,
the formation of 8 in approximately 35% yield can be
monitored by 1H NMR: 1H NMR (THF-d8): d ˆ 19.2
1,3-Bis(1-mesityl)-4-acetoxymethyl-4,5-dihydro-1H-
imidazol-3-ium Tetrafluoroborate (6)
ˆ
(Ru CHPh), 7.3 7.0 (Harom).
Compound 4 (0.500 g, 1.16 mmol) was dissolved in methylene
chloride and then triethylamine (0.2 mL, 1.4 mmol) was added.
The mixture was chilled to 508C and treated with acetyl
Oligo-5
À
chloride (55 mL, 0.77 mmol) and acetic anhydride (71 mL,
0.75mmol). The reaction mixture was stirred for 2 hours, the
cooling bath was removed, and the mixture was stirred for
another 6 hours. The volatile compounds were removed under
vacuum. The residue was washed with pentane and dried. Then
the residue was washed with water and dried again. Recrystal-
lization from ethanol/pentane gave the product as white
Method A: Compound 5 (80 mg, 0.15mmol) was dissolved in
methylene chloride and treated with a solution of Mo(N-2,6-i-
Pr2-C6H3)[CHC(CH3)2Ph][OC(CH3)(CF3)2]2 (17 mg; 22 mmol)
in methylene chloride. The yellow reaction mixture was stirred
for half an hour at room temperature. The mixture was
endcapped with a solution of ferrocene aldehyde (40 mg,
184 mmol) in methylene chloride and stirred for 5hours at
room temperature. The oligomer was precipitated by addition
of pentane, filtered off, thoroughly washed with pentane and
dried; yield: 69 mg (86%). 1H NMR (CD2Cl2): d ˆ 8.3 7.9 (sb,
Ä
crystals; yield: 0.103 g (19.2%). IR (KBr): n ˆ 2956 m (nasCH3),
‡
ˆ
ˆ
2859 w ((nsCH3), 1750 (nC O), 1628 (nC N ), 1481 m, 1462 m
(all nC Carom), 1383 cmÀ1 (dsCH3); 1H NMR (CDCl3): d ˆ 8.09
ˆ
Adv. Synth. Catal. 2002, 344, 712 719
717

Monika Mayr et al.
FULL PAPERS
n  1H, NC-HN), 7.2 6.9 (m, n  4Harom ‡ 5Hendgroup), 5.4 5.0
(b, n  2Hdb ‡ n  1HCH), 4.8 3.7 (b, n  4HCH2 ‡ 9Hendgroup),
Afterwards the derivatized silica was filtered off, washed
with methylene chloride and dried. Analysis found: C 2.68, H
0.51, N 0.12%, corresponding to a loading of 40 mmol 5/g silica.
3.0  2.6 (b, n  3HNBE), 2.5 1.0 (b, n  18HCH3 ‡ n  4HNBE
‡
6Hendgroup).
Method B: Compound 5 (80 mg, 0.15mmol) was dissolved
in dichloroethane and treated with solution of
a
Generation of the Immobilized Ru-Catalyst
Cl2Ru(PCy3)2(CHPh) (18 mg, 22 mmol) in dichloroethane.
The reaction mixture was heated to 458C and stirred for two
hours. The reaction mixture was endcapped by adding ethyl
vinyl ether. Then the oligomers were precipitated by addition
of pentane, filtered, washed with pentane and dried; yield:
Poly-5 grafted silica (1.50 g, corresponding to 60 mmol NHC
precursor) was suspended in THF (10 mL), chilled to 358C
and KO-t-Bu (8.1 g, 72 mmol) was added. While warming to
À
À
208C, the suspension was stirred for 10 minutes. A solution of
1
Cl2Ru(CHPh)(PCy3)2 (49 mg, 72 mmol) in THF (5mL) was
added and the reaction mixture was sonicated for 2 hours at
ambient temperature. The silica was filtered off, washed with
THF, methylene chloride and pentane and dried under
vacuum. Analysis found: Ru 0.054% (5.3 mmol/g).
70.1 mg (87%). H NMR (CD2Cl2): d ˆ 8.2 7.9 (sb, n  1H,
NC-HN), 7.4 6.9 (m, n  4Harom ‡ 5Hendgroup), 5.5 5.0 (b, n Â
2Hdb ‡ n  1HCH), 4.7 4.4 (b, n  1HCH2), 4.3 3.9 (b, n Â
3HCH2), 3.0 2.6 (b, n  3HNBE), 2.5 2.1 (b, n  18HCH3), 1.93,
1.72, 1.57, 1.26 (n  4HNBE).
Oligo-5 Loaded Porous Silica 60
Reaction of Schrock Initiator with Isocyanate
Silica 60 was dried under vacuum at 2008C for two days.
Si(OCH2CH3)-telechelic oligo-5 (169 mg) was dissolved in
methylene chloride. Silica 60 (4.0 g) was added and the
reaction mixture was refluxed at 508C for 6 hours. In order to
cap the excess of silanol groups, a mixture of methoxytrime-
thylsilane (420 mg, 4.03 mmol) and dimethoxydimethylsilane
(350 mg, 2.91 mmol) in methylene chloride was added. The
mixture was refluxed at 508C for 14 hours. Finally, the silica
was filtered off, washed with methylene chloride and dried
under vacuum. Loading with NHC precursors: 0.06 wt-% N,
corresponding to 20 mmol NHC precursor/g silica. The prep-
aration of the heterogeneous catalyst was carried out as
described for non-porous silica. Analysis found: Ru 0.013%
(1.3 mmol/g).
Mo(N-2,6-i-Pr2-C6H3)[CHC(CH3)2Ph][OC(CH3)(CF3)2]2
(5mg, 7 mmol) and 3-isocyanatopropyltriethoxysilane (9 mg,
40 mmol) were combined in CDCl3. The mixture was stirred
for eight hours at 458C. The color of the solution changed from
yellow to dark brown. The alkylidene signal at d ˆ 12.1/
12.8 ppm disappeared, a new signal at d ˆ 5.28 ppm appeared.
Oligo-5 Containing an R-Si(OCH2CH3)3 Endgroup
For immobilization purposes, 5 (150 mg, 0.274 mmol) was
dissolved in methylene chloride and treated with a solution of
Mo(N-2,6-i-Pr2-C6H3)(CHCMe2Ph)[OC(CH3)(CF3)2]2
(14 mg; 18 mmol) in methylene chloride. The yellow reaction
mixture was stirred for half an hour at room temperature. A
solution of 3-isocyanatopropyltriethoxysilane (39 mg,
160 mmol) in methylene chloride was added to terminate the
reaction. The mixture was stirred and heated to 458C for four
hours. The color of the solution changes from yellow to dark
brown. To precipitate the oligomer, pentane was added to the
solution. The oligomers were filtered off, thoroughly washed
with pentane and dried; yield: 0.145g (97%, slightly salmon-
Oligo-5 Coated Porous Silica 60
The Si(OCH2CH3)-telechelic oligo-5 (200 mg) was dissolved in
methylene chloride and the solution was added to C18
derivatized silica 60 (4.0 g, particle size 43 60 mm). The
solvent was slowly removed under vacuum (700 torr) and the
residue was dried under high vacuum. 5weight-% coating
correspond to a loading of 90 mmol NHC-precursor/g silica.
The preparation of the heterogeneous catalyst was carried out
as described for non-porous silica. Analysis found: Ru 0.041%
(4.10 mmol/g).
1
colored solid). H NMR (CD2Cl2): d ˆ 8.3 7.9 (sb, n  1H,
NC-HN), 7.2 6.8 (b, n  4Harom ‡ 5Hendgroup), 5.5 5.0 (b, n Â
2Hdb ‡ n  1H, CH), 4.7 4.4 (b, n  1H, CH2), 4.3 3.9 (b, n Â
3H, CH2), 3.8 (m, 6Hendgroup), 3.3 2.6 (b, n Â3HNBE ‡2Hendgroup),
2.5 2.1 (b, n  18HCH3), 2.0 1.0 (n  4HNBE ‡ 9Hendgroup
‡
4Hendgroup); triethoxysilyl endgroup: d ˆ 3.68 (OCH2) and 1.14
(CH3); 13C NMR (CD2Cl2): d ˆ 175(C O), 160 (N-C-N), 140,
ˆ
135, 130 (all Carom ‡ CNBE), 62, 61, 59 (CH ‡ CH2), 48, 41, 37
RCMExperiments (Slurry Reactions)
(CNBE), 21, 18, 17 (CH3).
Diethyl diallylmalonate (20 mg, 0.08 mmol) was dissolved in
1,2-dichlorobenzene (2 mL) and the supported catalyst
(40 mg) was added. The reaction mixture was heated to 508C
for 2 hours. After removal of the catalyst by filtration, the yield
was determined by 1H NMR in CDCl3.
Oligo-5 Loaded Non-Porous Silica
Non-porous silica was dried under vacuum at 2008C for two
days. Poly-5 containing Si(OCH2CH3) endgroups (120 mg) was
dissolved in methylene chloride. Non-porous silica (4 g) was
added and the reaction mixture was refluxed at 458C for
4 hours. In order to cap the excess of silanol groups a mixture of
methoxytrimethylsilane (120 mg, 1.15mmol) and dimethoxy-
dimethylsilane (90 mg, 0.75mmol) in methylene chloride was
added. The mixture was refluxed at 458C for 72 hours.
Leaching of the Support
Aqua regia (3.0 mL) was added to the heterogeneous catalyst
(30 mg). The mixture was placed inside high-pressure Teflon
tubes and leaching was carried out under microwave con-
718
Adv. Synth. Catal. 2002, 344, 712 719

Silica-Based Heterogeneous Second Generation Grubbs Catalyst
FULL PAPERS
ditions (50, 600, and 450 W pulses, respectively, t ˆ 32 mi-
nutes). After cooling to room temperature, the mixture was
filtered and measured by ICP-OES (l ˆ 240.272 nm, ion line).
The background was measured at l ˆ 240.287 and 240.257 nm,
respectively.
[6] A. F¸rstner, A. Leitner, Synlett 2001, 290 292.
[7] K. J. Ivin, J. C. Mol, Olefin Metathesis and Metathesis
Polymerization, Academic Press, San Diego, 1997, pp.
1
472.
[8] A. F¸rstner, Angew. Chem. 2000, 112, 3140 3172;
Angew. Chem. Int. Ed. 2000, 39, 3012 3043.
[9] A. F¸rstner, Ed., Alkene Metathesis in Organic Synthesis,
Springer, Berlin, Heidelberg, 1998, Vol. 1, pp. 230.
[10] M. R. Buchmeiser, Chem. Rev. 2000, 100, 1565 1604.
[11] R. R. Schrock, Acc. Chem. Res. 1990, 23, 158 165.
[12] R. R. Schrock, in Ring-Opening Polymerization, (Ed.:
D. J. Brunelle), Hanser, Munich, 1993, pp. 129.
[13] R. R. Schrock, Polyhedron 1995, 14, 3177.
[14] A. H. Hoveyda, R. R. Schrock, Chem. Eur. J. 2001, 7,
945 950.
[15] T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34,
18 29.
[16] S. C. Sch¸rer, S. Gessler, N. Buschmann, S. Blechert,
Angew. Chem. 2000, 112, 4062 4065; Angew. Chem. Int.
Ed. 2000, 39, 3898 3901.
[17] S. B. Garber, J. S. Kingsbury, B. L. Gray, A. H. Hoveyda,
J. Am. Chem. Soc. 2000, 122, 8168 8179.
[18] J. Dowden, J. Savovic, Chem. Commun. 2001, 37 38.
[19] A. G. M. Barrett, S. M. Cramp, R. S. Roberts, Org. Lett.
1999, 1, 1083 1086.
[20] Q. Yao, Angew. Chem. 2000, 112, 4060 4062; Angew.
Chem. Int, Ed. 2000, 39, 389 3898.
[21] M. Mayr, B. Mayr, M. R. Buchmeiser, Angew. Chem.
2001, 113, 3957 3960; Angew. Chem. Int. Ed. 2001, 40,
3839 3842.
X-Ray Crystallographic Study of 5
Monoclinic, space group P2(1)/c (No.14), a ˆ 21.031(2) ä, b ˆ
9.6800(4) ä,
c ˆ 14.660(1) ä,
b ˆ 101.907(3)o,
Vˆ
À
3
2920.3(4)Àä3, Z ˆ 4, 1calc ˆ 1.238 g cm , Tˆ 223 K, m ˆ
1
0.094 mm , Mo-Ka-radiation (l ˆ 0.71073 ä), colorless plate
0.4 Â 0.4 Â 0.06 mm3. 10430 reflections were collected on a
Nonius KappaCCD diffractometer with 2193 reflections >
2s(I). The structure was solved and refined using SHELXS83
and SHELXL97 to R1 ˆ 0.0695and wR2 ˆ 0.1778 for 2193
unique reflections. Non-hydrogen atoms were refined with
anisotropic displacement parameters, hydrogen atoms were
placed at calculated positions and refined isotropically using a
riding model. A 1:1 disorder of the norbornenyl group C18-
C24:C18A-C24A occurs with split and overlying positions. The
chiral centers C18 and C18A are enantiomeric. This disorder
itself is induced by the diastereomeric compound with two
chiral centers at C2 and C18, all 4 chiral combinations (RR, RS,
SR and SS) are equally represented in the centrosymmetric
unit cell. Further crystallographic data have been deposited
with the Cambridge Crystallographic Data Center as supple-
mentary publication no. CCDC 177467. Copies of the data can
be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: ( ‡ 44) 1223-336033;
e-mail: deposit@ccdc.cam.ac.uk ).
[22] M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett.
1999, 1, 953 956.
[23] A. F¸rstner, H. Krause, L. Ackermann, C. W. Lehmann,
Chem. Commun. 2001, 2240 2241.
Acknowledgements
[24] J. H. Oskam, H. H. Fox, K. B. Yap, D. H. McConville, R.
O×Dell, B. J. Lichtenstein, R. R. Schrock, J. Organomet.
Chem. 1993, 459, 185 197.
Financial support provided bythe Austrian Science Fund (FWF
Vienna, project Y-158) is gratefully acknowledged.
[25] O. W. Webster, J. Polym. Sci. A: Polym. Chem. 2000, 38,
2855 2860.
[26] M. R. Buchmeiser, J. Chromatogr. A 2001, 918/2, 233
266.
[27] M. R. Buchmeiser, F. Sinner, M. Mupa, K. Wurst,
Macromolecules 2000, 33, 32 39.
[28] M. R. Buchmeiser, M. Mupa, G. Seeber, G. K. Bonn,
Chem. Mater. 1999, 11, 1533 1540.
References
[1] A. de Meijere, F. E. Meyer, Angew. Chem. 1994, 106,
2473 2506; Angew. Chem. Int. Ed. 1994, 33, 2379 2411.
[2] J. G. de Vries, Can. J. Chem. 2001, 79, 1086 1092.
[3] A. F. Littke, C. Dai, G. C. Fu, J. Am. Chem. Soc. 2000,
122, 4020 4028.
[4] S.-Y. Liu, M. J. Choi, G. C. Fu, Chem. Commun. 2001,
2408 2409.
[5] M. R. Netherton, C. Dai, K. Neusch¸tz, G. C. Fu, J. Am.
Chem. Soc. 2001, 123, 10099 10100.
[29] F. Sinner, M. R. Buchmeiser, R. Tessadri, M. Mupa, K.
Wurst, G. K. Bonn, J. Am. Chem. Soc. 1998, 120, 2790
2797.
Adv. Synth. Catal. 2002, 344, 712 719
719