Cationic ruthenium allenylidene complexes as a new class of performing catalysts for ring closing metathesis

Article abstract of DOI:10.1039/a803286f

Cationic allenylidene ruthenium complexes [Ru=C=C=CR₂(L)(Cl)(arene)]PF₆ (L = PCy₃, PPrⁱ₃), easily prepared from RuCl₂(L)(p-cymene), prop-2-yn-1-ol and NaPF₆, are found to be exce

Full text of DOI:10.1039/a803286f

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Cationic ruthenium allenylidene complexes as a new class of performing
catalysts for ring closing metathesis
A. Fu¨rstner,*a† M. Picquet,b C. Bruneau,b and P. H. Dixneuf*b‡
a Max-Planck-Institut fu¨r Kohlenforschung, D-45470 Mu¨lheim/ Ruhr, Germany
b UMR 6509: CNRS - Universite´ de Rennes, Laboratoire de Chimie de Coordination et Catalyse, Campus de Beaulieu, F-35042,
Rennes, France
Cationic allenylidene ruthenium complexes [RuN
PCy3, PPri3), easily
prepared from RuCl2(L)(p-cymene), prop-2-yn-1-ol and
NaPF6, are found to be excellent catalyst precursors for ring
closing olefin metathesis.
situ irradiation with UV light.9 A route has been designed in
order to straightforwardly introduce around a Ru(+2) site only
one bulky phosphine, one cumulenylidene ligand attached to the
metal via a RuNC bond, and one easy to displace p-cymene
group, three likely conditions to produce an active olefin
metathesis catalyst precursor. As the activation of prop-2-yn-
1-ols with selected ruthenium precursors has been shown to
give the RuNCNCNCR2 moiety,10 complexes 3 were reacted
with a prop-2-ynyl alcohol such as 4 in the presence of NaPF6
in MeOH at ambient temperature and cleanly led to the
formation of the cationic allenylidene complexes 5a-c, isolated
in high yields as violet powders (Scheme 1). It is noteworthy
that the corresponding salts with less bulky R3P groups existed
only as transient allenylidene intermediates in situ adding
methanol to give methoxycarbene [RuNC(OMe)CHNCR2] com-
plexes11 which are inactive as olefin metathesis catalysts.
A preliminary assessment of the performance of these
complexes in ring closing metathesis revealed a strong
correlation with the nature of the chosen phosphine. In line with
CNCNCR2(L)(Cl)(arene)]PF6 (L
=
The recent development of a new generation of well-defined
catalysts precursors for olefin metathesis has had a tremendous
impact on progress in this field.1,2 Most popular among them are
the neutral 16-electron ruthenium carbene complexes
Cl2(PCy3)2RuNCHR (1a: R = Ph, 1b R = 2CHNCPh2)
described by Grubbs3 which are distinguished by excellent
activity as well as good compatibility with various functional
groups in the substrates. Since these catalysts are fairly stable
against oxygen and moisture, they represent a simple yet
efficient tool for advanced organic synthesis and polymer
chemistry. A major drawback, however, resides in the use of
either diazoalkanes or diphenylcyclopropene for the preparation
of these complexes, i.e. reagents which are hazardous or rather
difficult to make, respectively. However, an improvement of the
synthesis of complexes 1 synthesis has just been achieved
starting from Ru(H)(H2)Cl(PCy3)2.4
i
[(p-cymene)RuCl2]2
We now describe a new class of efficient single-component
catalyst for ring closing olefin metathesis: the cationic 18-elec-
tron allenylidene ruthenium complexes [RuNCNCNCR2(PR3)-
(Cl)(arene)]PF6 5, easily available in three steps from
RuCl3.xH2O, which also provide an unprecedented example of
the involvement of metal allenylidene complexes in cataly-
sis.5
2
Ru
Cl
Cl
R3P
3
ii
+
Ph
Ph
The 18-electron complexes of the general formula
OH
Ph
Ph
6
(h -arene)RuCl2(PR3) such as 3, which are readily obtained
–
PF6
Ru
•
•
4
6
Cl
from commercially available [(h -arene)RuCl2]2 2 upon addi-
R3P
tion of a phosphine,6 are very active in catalytic transformations
of alkynes,7 but exhibit only very low catalytic activity for ring
opening metathesis polymerization (ROMP) of strained cy-
cloalkenes and ring closing metathesis (RCM). However, they
can be activated either upon addition of diazoalkanes8 or by in
5a R = Cy
b R = Pri
c R = Ph
Scheme 1
Table 1 Screening of the catalytic activity of the allenylidene complexes 5a
Ts
N
Ts
N
5 (2.5 mol%)
7
6
Entry
Catalyst
Solvent
Additive
T/°C
t/h
Yield (%)b
1
2
3
4
5
6
5c
5b
5b
5a
5a
5a
toluene
toluene
CH2Cl2
toluene
toluene
toluene
—
—
—
—
80
80
40
80
80
80
3
3
26
3
4
3
2
66
95 (76)
79
100 (83)
31
—
PCy3 (5%)c
a General conditions: diene (1 mmol), catalyst (0.025 mmol), solvent (5 ml). b GC yield (isolated yield). c Based on the diene.
Chem. Commun., 1998
1315

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Particularly noteworthy are the smooth cyclizations of the
conformationally flexible dienes 12 and 14 to 16- and
18-membered cycloalkenes 13 and 15, respectively. The
hydrogenation of compound 13 under standard conditions leads
to the macrocyclic musk exaltolide,14 which is used as a
valuable perfume ingredient. Disaccharide 17 is an advanced
intermediate en route to tricolorin A, a carcinostatic resin
glycoside isolated from Ipomoea tricolor.2d The examples
summarized in Table 2 clearly highlight the excellent compati-
bility of the allenylidene catalyst 5a with various functional
groups, including even unprotected secondary hydroxy
groups.
Although the mode of action of these new metathesis
catalysts and the nature of the actual intermediates involved in
the catalytic cycle require further indepth studies, we have
shown that the addition of an excess of PCy3 strongly retards the
metathetic conversion.
Current work in our laboratories is aiming at investigating the
mechanism as well as to further explore the preparative scope of
these readily accessible and highly promising metathesis
catalysts, since they allow substantial possible variations of
their basic structural motive.
Table 2 Ring closing metathesis employing Ru-allenylidene catalyst 5aa
Substrate
Product
Yield
(%)
Ts
N
75
40
N
Ts
8
9
O
O
O
O
O
O
10
11
O
O
O
O
90
The authors are grateful to Deutscher Akademischer Aus-
tauschdienst (DAAD) for a stipend to M. P., covering his stay in
Mu¨lheim for a three month period. We also thank K.
Langemann and T. Mu¨ller, Mu¨lheim, for providing various
starting materials.
12
13
O
O
NH
NH
Notes and References
73
fuerstner@mpi-muelheim.mpg.de
† E-mail: pierre.dixneuf@univ-rennes1.fr
14
15
1 K. J. Ivin and J. C. Mol, Olefin Metathesis and Metathesis Polymeriza-
tion, Academic Press, New York, 1997; R. H. Grubbs, S. J. Miller and
G. C. Fu, Acc. Chem. Res., 1995, 28, 446; M. Schuster and S. Blechert,
Angew. Chem., Int. Ed. Engl., 1997, 36, 2036; A. Fu¨rstner, Top. Catal.,
1997, 4, 285; S. K. Armstrong, J. Chem. Soc., Perkin Trans. 1, 1998,
371.
2 (a) A. Fu¨rstner and K. Langemann, Synthesis, 1997, 792; (b) A. Fu¨rstner
and K. Langemann, J. Org. Chem. 1996, 61, 8746; (c) A. Fu¨rstner and
K. Langemann, J. Org. Chem., 1996, 61, 3942; (d) A. Fu¨rstner and T.
Mu¨ller, J. Org. Chem., 1998, 63, 424.
O
O
O
O
O
85
O
O
Ph
O
O
O
O
O
O
O
O
O
Ph
O
O
OH
O
3 S. T. Nguyen, R. H. Grubbs and J. W. Ziller, J. Am. Chem. Soc. 1993,
115, 9858; P. Schwab, M. B. France, J. W. Ziller and R. H. Grubbs,
Angew. Chem., Int. Ed. Engl. 1995, 34, 2039; E. L. Dias, S. T. Nguyen
and R. H. Grubbs, J. Am. Chem. Soc., 1997, 119, 3887.
OH
O
4 T. E. Wilhelm, T. R. Belderrain, S. N. Brown and R. H. Grubbs,
Organometallics, 1997, 16, 3867.
16
17
5 A ruthenium allenylidene intermediate has been suggested in the
catalytic coupling of propy-2-yn-1-ols with allylic alcohols, cf B. M.
Trost and J. A. Flygare, J. Am. Chem. Soc., 1992, 114, 5476.
6 M. A. Bennett and A. K. Smith, J. Chem. Soc., Dalton Trans., 1974,
233; R. A. Zelonka and M. C. Baird, Can. J. Chem. 1972, 50, 3063.
7 C. Bruneau and P. H. Dixneuf, Chem. Commun., 1997, 507.
a All reactions using 5a were carried out in toluene at 80 °C using a catalyst
loading of 5 mol%.
previous observations with ruthenium-based initiators,3,8,9 their
catalytic activity decreases in the order PCy3 > PPri3 >> PPh3
(Table 1). With 5a (2.5 mol%) as the catalyst, diene 6 is
quantitatively cyclized to dihydropyrrole 7 after 4 h in toluene
at 80 °C (entry 5). Dichloromethane can also be used, although
the turnover frequency of 5 is slightly lower in this particular
reaction medium at 40 °C (cf. entries 2 and 3).
Having established the optimum reaction conditions, we
applied catalyst 5a to RCM of a set of representative diene
substrates. As can be seen from the results compiled in Table 2,
this catalyst nicely applies to the formation of essentially all ring
sizes ! 5, including macrocyclic and medium sized products.
The isolated yields obtained were found to be good to excellent
and are comparable to those previously obtained using the
Grubbs carbenes 1 [9: 75% vs. (68%);12 13: 90% vs. (79%);2c
15: 73% vs. 83%;2a 17: 85% vs. 77%2d]. Only in the case of the
10-membered ring of jasmine ketolactone 11, did the allenyli-
dene complex 5a turn out to be somewhat less efficient [11:
40% vs. (86%)13].
8
A. W. Stumpf, E. Saive, A. Demonceau and A. F. Noels, J. Chem.,
Soc. Chem. Commun., 1995, 1127; A. Demonceau, A. W. Stumpf, E.
Saive and A. F. Noels, Macromolecules, 1997, 30, 3127.
9 A. Hafner, A. Mu¨hlebach and P. A. van der Schaaf, Angew. Chem., Int.
Ed. Engl., 1997, 36, 2121.
10 P. H. Dixneuf and C. Bruneau, in Organic Synthesis via Organome-
tallics, OSM 5, ed. G. Helmchen, Vieweg, Wiesbaden, 1997, p. 1; D.
Pe´ron, A. Romero and P. H. Dixneuf, Organometallics, 1995, 14,
3319.
11 D. Pilette, L. Ouzzine, H. Le Bozec, P. H. Dixneuf, C. E. F. Rickard and
W. R. Roper, Organometallics, 1992, 11, 809.
12 M. S. Visser, N. M. Heron, M. T. Didiuk, J. F. Sagal and A. H. Hoveyda,
J. Am. Chem. Soc., 1996, 118, 4291.
13 A. Fu¨rstner and T. Mu¨ller, Synlett, 1997, 1010.
14 Exaltolide is a trademark of Firmenich SA, Geneva, Switzerland; for a
general review see: G. Ohloff, Riechstoffe und Geruchssinn, Springer,
Berlin, 1990.
Received in Cambridge, UK, 1st May 1998; 8/03286F
1316
Chem. Commun., 1998