A most convenient and atom-economic preparation of a highly active ring-closing metathesis catalyst

Article abstract of DOI:10.1021/om1005929

A new olefin metathesis catalyst has been prepared in situ by a one-step method utilizing an easy-to-prepare organic precursor and only 1 equiv of tricyclohexylphosphine ligand per ruthenium atom. The activity of the new catalytic system for the formation of five-, six-, and seven-membered disubstituted cycloalkenes via ring-closing metathesis under standard conditions is comparable to that of related highly active commercial catalysts.

Full text of DOI:10.1021/om1005929

Organometallics 2010, 29, 3471–3473 3471
DOI: 10.1021/om1005929
A Most Convenient and Atom-Economic Preparation of a Highly Active
Ring-Closing Metathesis Catalyst
Leonel R. Jimenez, Benjamin J. Gallon, and Yann Schrodi*
Department of Chemistry and Biochemistry, California State University Northridge,
Northridge, California 91330
Received June 16, 2010
Summary: A new olefin metathesis catalyst has been prepared
in situ by a one-step method utilizing an easy-to-prepare
organic precursor and only 1 equiv of tricyclohexylphosphine
ligand per ruthenium atom. The activity of the new catalytic
system for the formation of five-, six-, and seven-membered
disubstituted cycloalkenes via ring-closing metathesis under
standard conditions is comparable to that of related highly
active commercial catalysts.
Olefin metathesis has become an exceptionally powerful
and applicable method for the formation of carbon-carbon
Figure 1. Ruthenium-based olefin metathesis catalysts.2
bonds in organic and polymer synthesis.1 Ruthenium-based
complexes (1-3; Figure 1)2 are the most commonly em-
ployed olefin metathesis catalysts in academic and industrial
laboratories, because they can be handled in air and are
tolerant of various organic functional groups.3 However, the
syntheses of these complexes are relatively cumbersome.
They usually involve more than one step and require isola-
tion of the catalysts to remove catalyst-inhibiting byproducts
such as liberated phosphine ligands.2 Therefore, we became
interested in developing a one-step procedure that forms
highly active olefin metathesis catalysts and does not require
purification or isolation. Ideally, the new procedure should
be as atom economic as possible. In particular, a new method
that requires only 1 equiv of expensive ligands (e.g., tricyclo-
hexylphosphine) per ruthenium center is desirable.
Scheme 1. One-Step Synthesis of Ruthenium Allenylidene 45c
Previous attempts to generate olefin metathesis catalysts in
situ focused on the preparation of ruthenium vinylidene4 and
allenylidene5 species. However, these types of complexes
proved less active in olefin metathesis than their ruthe-
nium-alkylidene counterparts. For example, the ruthenium
allenylidene complex 4 can be very conveniently prepared in
a one-step procedure involving the treatment of [RuCl2(p-
cymene)]2 with 1,1-diphenylprop-2-yn-1-ol in the presence of
2 equiv of tricyclohexylphosphine (Scheme 1).5c Unfortu-
nately, 4 is inactive in olefin metathesis, although its chemical
isomer;the ruthenium indenylidene complex 3a6,7;shows
good activity.2g,5c
*To whom correspondence should be addressed. E-mail: yann.schrodi@
csun.edu.
(1) (a) Chauvin, Y. Angew. Chem., Int. Ed. 2006, 45, 3740–3747. (b)
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R. H. Angew. Chem., Int. Ed. 2006, 45, 3760–3765. (d) Nicolaou, K. C.;
Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4490–4527. (e)
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r
2010 American Chemical Society
Published on Web 07/22/2010
pubs.acs.org/Organometallics

3472 Organometallics, Vol. 29, No. 16, 2010
Jimenez et al.
Figure 2. RCM of diethyl diallylmalonate.
Scheme 2. In Situ Formation of an Active RCM Catalyst
Table 1. RCM with Catalyst Prepared in Situ by Methods 1 and 2a
entry
X
n
time (min)
yieldb (%)
1c
2d
3d
4d
5d
6d
C(CO2Et)2
C(CO2Et)2
NTs
NTs
NTs
1
1
1
2
3
4
30
60
60
60
60
150
>97
>95
>97
>97
>95
0
Such a rearrangement most likely involves an electrophilic
aromatic substitution of one of the allenylidene’s phenyl
groups (eq 1). Therefore, we hypothesized that an activated
derivative of 1,1-diphenylprop-2-yn-1-ol would favor the for-
mation of olefin metathesis active indenylidene over olefin
metathesis inactive allenylidene complexes.
NTs
a Conditions: 0.1 M. b Determined by 1H NMR analysis of the crude
reaction mixture. c Catalyst prepared in situ according to method 1.
d Catalyst prepared in situ according to method 2.
under standard conditions,11 regardless of which method
was used to generate it (Figure 2).
The new in situ RCM catalyst effectively promotes the full
conversion of different substrates into five-, six-, and seven-
membered cycloalkenes within 1 h under mild conditions and
at low ruthenium loadings (Table 1; entries 1-5). However,
the cyclization of N-allyl-N-(hex-5-enyl)-4-methylbenzene-
sulfonamide to yield an eight-membered-ring olefin could
not be achieved (Table 1; entry 6), as previously reported for
other RCM catalysts.12
The new in situ catalyst proves to be quite stable. Indeed, a
solution of catalyst prepared by method 2 and stored in a vial
in air at room temperature for 2 weeks was shown to give
essentially the same conversion in the RCM of diethyl
diallylmalonate within 30 min at 40 °C as a freshly prepared
catalyst solution (Table 2).
Thus, 1-(3,5-dimethoxyphenyl)-1-phenylprop-2-yn-1-ol
(5) was reacted with RuCl2(p-cymene)(PCy3) (method 1;
Scheme 2a) or with [RuCl2(p-cymene)]2 in the presence of 1
equiv of tricyclohexylphosphine (method 2; Scheme 2b) to
yield the ruthenium indenylidene complex 6 as a major
product, according to NMR spectroscopy.9 The resulting
dark brown solution was subsequently used without any
further treatment to promote the ring-closing metathesis
(RCM) of diethyl diallylmalonate (eq 2). The in situ10
catalyst proved as effective in this RCM as highly active 2a
(9) See the Supporting Information.
(10) The term in situ is used herein to describe a catalyst that can be
used to effectively promote catalysis without having to be isolated.
(11) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H.
Organometallics 2006, 25, 5740–5745.
(12) Kirkland, T. A.; Grubbs, R. H. J. Org. Chem. 1997, 62, 7310–
7318.

Communication
Table 2. Stability Study of the in Situ Catalyst Solution
Organometallics, Vol. 29, No. 16, 2010 3473
formation of disubstituted five-, six-, and seven-membered-
ring cycloalkenes under standard conditions and is stable in
solution in air for at least 2 weeks. A complete discussion of the
development of this new method and of studies related to
isolating and fully characterizing the active species will be
reported in due course. Work is underway to apply this method
to the preparation of olefin metathesis catalysts that contain
N-heterocyclic carbene ligands.
duration of storage (days)a
yield (%)b
0
94
5
94
9
94
14
95
Acknowledgment. Y.S. is a College Cottrell Scholar.
We thank Prof. Thomas G. Minehan for helpful discus-
sions and Materia, Inc., for their generous gift of
[RuCl2(p-cymene)]2.
a The catalyst solution was prepared by method 2 and stored in a vial
in air at 22 °C. b Determined by 1H NMR analysis of the crude reaction
mixture.
In summary, we have utilized a derivative of 1,1-diphenyl-
prop-2-yn-1-ol that contains electron-donating substituents
in the meta positions of one phenyl group (5) to generate a
new olefin metathesis catalyst in situ via a simple one-step
procedure. The new catalyst is as active as the commercial first-
generation Hoveyda-Grubbs catalyst (2a) at promoting the
Supporting Information Available: Text and figures giving
experimental details for the preparation of 5, the methods for
the generation of the in situ catalyst, and the RCM reactions.
This material is available free of charge via the Internet at http://
pubs.acs.org.