An air-stable, reusable, bimetallic version of Grubbs' catalyst for alkene metathesis

Article abstract of DOI:10.1055/s-2002-34867

RCM, cross-metathesis and ring-opening cross-metathesis are efficiently catalyzed by a new, air-stable, bimetallic analog of Grubbs' Ru-based metathesis catalyst, which can be recovered and recycled.

Full text of DOI:10.1055/s-2002-34867

LETTER
1925
An Air-Stable, Reusable, Bimetallic Version of Grubbs’ Catalyst for Alkene
Metathesis
B
T
imetallic Vers
a
ion of
G
rub
r
bs’ Cata
u
lyst n K. Maishal, Amitabha Sarkar*1
Division of Organic Chemistry (Synthesis), National Chemical Laboratory, Pune – 411 008, India
Fax +91(33)4732805; E-mail: ocas@mahendra.iucs.res.in
Received 13 August 2002
tron-rich and bulky substituent on the carbene ligand is
Abstract: RCM, cross-metathesis and ring-opening cross-metathe-
sis are efficiently catalyzed by a new, air-stable, bimetallic analog
of Grubbs’ Ru-based metathesis catalyst, which can be recovered
and recycled.
likely to promote dissociation of phosphine. At the same
time, it would also favor coordination of an alkene over
recombination of a free phosphine ligand on steric
grounds. Combining this argument with our interest in
ferrocene-based materials, we have described herein syn-
thesis and metathesis reactions of a novel analog of cata-
lyst I (Figure 1) that features a ferrocene substituent on
the carbene carbon. Ferrocene is electron-rich and has a
considerable steric bulk – thus it addresses both electronic
and steric criteria to promote metathesis. Preliminary re-
sults indicate that the catalytic efficiency of the new com-
plex, III, is comparable to Grubbs’ catalyst I, and the
robust, air-stable complex can be very conveniently
stored, recovered and recycled.4
Key words: metathesis, ruthenium, carbene complexes, catalysis,
organometallic reagents
2
Grubbs’ ruthenium-based catalysts (I, II) for alkene-me-
tathesis received wide attention owing to their versatility
in promoting different kinds of transformations like ring-
closing metathesis (RCM), cross-metathesis (CM), ring-
opening-cross-metathesis (ROCM) and ring-opening
metathetic polymerization (ROMP). The first three have
been extensively exploited in synthetic organic chemistry.
Attempted base-induced decomposition of the tosylhydra-
zone of ferrocenecarboxaldehyde failed to produce ferro-
cenyldiazomethane; thermolysis produced only 1,2-
diferrocenylethene. The complex III was, therefore,
Based on kinetic studies, Grubbs and coworkers
concluded that the first step is dissociative: one of the
3
phosphine ligands dissociates to provide a 14e complex.
Rate of alkene coordination to this intermediate (initia-
tion) is competitive with the rate of phosphine ‘re-associ-
ation’; for an efficient catalyst, former must predominate.
Steric and electronic attributes of the carbene ligand tacit-
ly influence this initiation process. For instance, an elec-
5
prepared by treatment of vinylferrocene with complex I
at ambient temperature (Scheme 1), and isolated (87%) as
6
an air-stable, dark purple solid. It is soluble in common
solvents for alkene metathesis like benzene, toluene and
dichloromethane.
Scheme 1
Figure 1
Synlett 2002, No. 11, Print: 29 10 2002.
Art Id.1437-2096,E;2002,0,11,1925,1927,ftx,en;G22802ST.pdf.
©
Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

1
926
T. K. Maishal, A. Sarkar
LETTER
Ring-closing metathesis catalyzed by complex III was
studied with several substrates and the results are
7
summarized in Table 1. The yields are generally excel-
lent while the reaction time is short. About 2–2.5 mol% of
catalyst was sufficient for quantitative conversion in most
instances. Efficiency of the catalyst compares well with
Grubbs’ first-generation catalyst I – the propagation steps
are identical for both the catalysts. Substrates with free
hydroxyl groups seem to require longer period for com-
plete conversion than corresponding acetates. For sub-
strate 6a, oligomeric side-products (via competitive
ADMET) accounted for diminished yield.
Table 1 RCM using Catalyst IIIa
Scheme 2
En- Substrates
try (Concn)
Product
Catalyst Time/
Yield
(%)
(mol%)
Temp
that is released after the ring-closing step is similar to
original catalyst I, yet the catalyst requirement is relative-
ly high. Again, use of 5 mol% of catalyst augmented with
7.5 mol% of vinylferrocene added after 20 min, afforded
the RCM product in 91% yield after 4 hours at ambient
temperature (Scheme 3), indicating in situ regeneration of
catalyst III.
1
2.5
15 min/r.t.
98
1
b
1
a (0.1 M)
2
2.5
2 h/r.t.
90
2
3
4
b
b
b
2
3
4
a (0.1 M)
a (0.1 M)
a (0.1 M)
3
4
5
2.5
2.5
2.5
15 min/r.t.
1.5 h/ r.t.
97
92
98
30 min/r.t.
5
6
b
b
5
6
a (0.05 M)
a (0.05 M)
6
7
2.5
2
1.5 h/r.t.
67
97
Scheme 3
30 min/r.t.
Unlike RCM, cross-metathesis worked best when heated
under reflux in dichloromethane (Table 2). Cross-met-
athesis of the substrate 10a with catalyst I was recently re-
ported. Yields obtained with the catalysts I and III are
comparable. Ring-opening cross-metathesis of bicyclic
substrate 12a also worked well with catalyst III.
7
a (0.1 M)
7b
10
a
All the reactions were carried out in CH Cl at room temperature un-
der Ar.
2
2
Attempts were made to recover the catalyst III from the
reaction mixture at the end of the reaction. In a prelimi-
nary attempt, after the substrate 7a was completely con-
verted by 2.5 mol% catalyst to RCM product 7b,
vinylferrocene (4 mol%) was added to the solution and it
was heated under reflux for 40 min. Precipitation by meth-
anol afforded 45% of the catalyst III in sufficiently pure
form (NMR) and without any loss of activity as tested by
subsequent runs. Improvement of catalyst recovery proto-
col, anchoring the catalyst on insoluble support and
The carbene complex 8a, which closely resembles the or-
ganic substrate 7a, requires more quantity of catalyst (108
mol% of I or III) than 7a, and longer time for completion
(
Scheme 2). Thermal activation does not compensate for
use of less amount of catalyst. When 7.5 mol% of vinyl-
ferrocene was added after 30 min and the solution was
heated under reflux, yield improved to 99%. This indicat-
ed that it was indeed possible to regenerate the active cat-
alyst III under this reaction condition.9
Another organometallic substrate, 9a, required 10 mol% ROMP activity are currently being studied in this labora-
of catalyst III to provide a high yield of corresponding tory.
RCM product, 9b. Interestingly in this case, the complex
Synlett 2002, No. 11, 1925–1927 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Bimetallic Version of Grubbs’ Catalyst
1927
Table 2 Cross-Metathesis using Catalyst IIIa
(4) Recycling catalyst: (a) Kingsbury, J. S.; Harrity, J. P. A.;
Bonitatebus, P. J. Jr.; Hoveyda, A. H. J. Am. Chem. Soc.
1999, 121, 791. (b) Dowden, J.; Savovic, J. Chem. Commun.
2001, 37. (c) Fürstner, A.; Ackermann, L.; Gabor, B.;
Goddard, R.; Lehmann, C. W.; Mynott, R.; Stelzer, F.; Thiel,
O. R. Chem.–Eur. J. 2001, 7, 3236. (d) Ahmed, M.; Barrett,
A. G. M.; Braddock, D. C.; Cramp, S. M.; Procopiou, P. A.
Tetrahedron Lett. 1999, 40, 8657. (e) For a survey of Ru-
based initiators with other alkylidene groups, see: Frenzel,
U.; Nuyken, O. J. Polym. Sci. A 2002, 40, 2895.
En- Substrate
try (Concn)
Product
Cat. III Time/
(mol%) Temp. (%)
Yield
1.
5
6 h/
reflux
83
1
0a (0.5 M)
1
0b
1b
(5) We have followed the procedures described in (a) Schwab,
P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118,
2
.
.
5
5
6 h/
reflux
81
72
100. (b) Trnka, T. M.; Day, M. W.; Grubbs, R. H. Angew.
Chem. Int. Ed. 2001, 40, 3441.
1
1
1a (0.5 M)b
2a (0.5 M)c
(6) Vinylferrocene (61 mg, 0.29 mmol) in 1 mL
1
dichloromethane was added to a solution of Ru-benzylidene
complex, I, (200 mg, 0.243 mmol) in dichloromethane (5
mL). The reaction mixture was stirred at room temperature
for 40 min when color of the solution turned from purple to
red-violet. Volume of the solvent was reduced followed by
addition of dry methanol at 0 °C, which afforded
3
1.5 h/
reflux
d
(
8
Pcy ) Cl Ru=C(H)Fc, III, as dark brown solid powder in
3 2 2
1
2b
1
7% yield (198 mg). H NMR (CD Cl ): 19.00 (s, 1 H,
2 2
a
All reactions were carried out in dichloromethane and in each case
Ru=CH), 4.70 (s, 2 H, Cp), 4.43 (s, 2 H, Cp), 4.14 (s, 5 H,
Cp), 2.54 (broad s, 6 H, Pcy ), 1.21–1.74 (m, 60 H, Pcy ). C
NMR (CD Cl ): 292.14, 104.23, 71.55, 71.21, 69.77,
3
E/Z >9:1.
13
3
3
b
3
2
equiv allyltrimethylsilane was used.
0% molar excess of vinylferrocene was used.
No ROMP product was detected.
2
2
c
31
2.66, 32.04, 30.11, 28.40, 26.98. P NMR (CD Cl ): 34.4
2 2
d
(
s, Pcy ). Details of preparation and complete
3
characterization including crystal structure will be reported
separately.
(
7) Reactions were performed in 1–2 mmol scale in
dichloromethane. Yields represent isolated yields of
products after purification by chromatography. All
compounds were characterized by their IR, H and 13C NMR
spectra, and satisfactory elemental analyses were obtained
for all new compounds.
Acknowledgement
1
Financial support from Reliance Industries Ltd. and grant of a rese-
arch fellowship (TKM) from CSIR, India, are gratefully acknow-
ledged.
(8) For examples of RCM on organometallic substrates, see:
(
a) Sültemeyer, J.; Dötz, K. H.; Hupfer, H.; Nieger, M. J.
References
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M.; Sarkar, A. Tetrahedron Lett. 2001, 42, 4373.
(
1) Current address: Department of Organic Chemistry, Indian
Association for the Cultivation of Science, Kolkata – 700
0
32, India. E-mail: ocas@mahendra.iacs.res.in.
2) (a) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34,
8. (b) Fürstner, A. Angew. Chem. Int. Ed. 2000, 39, 3012.
(
(
9) The Ru=CH complex, IV, does not readily react with
2
1
vinylferrocene at room temperature. However, on heating
under reflux in dichloromethane with 1.2–1.5 equivalent of
vinylferrocene, complex IV affords >80% yield of complex
III.
(
(
c) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413.
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1997, 36, 2036. (e) Armstrong, S. K. J. Chem. Soc., Perkin
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(
10) Maishal, T. K.; Sinha-Mahapatra, D. K.; Paranjape, K.;
Sarkar, A. Tetrahedron Lett. 2002, 43, 2263.
(
3) (a) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem.
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Synlett 2002, No. 11, 1925–1927 ISSN 0936-5214 © Thieme Stuttgart · New York