Structure and reactivity of alkyne-chelated ruthenium alkylidene complexes

Article abstract of DOI:10.1021/ja907155r

(Chemical Equation Presented) The isolation and structural characterization of alkyne-bound Ru alkylidene complexes has been elusive. However, a sequential enyne ring-closing metathesis of a diyne moiety and metallotropic [1,3]-shift followed by a second

Full text of DOI:10.1021/ja907155r

Published on Web 10/02/2009
Structure and Reactivity of Alkyne-Chelated Ruthenium Alkylidene
Complexes
Kung-Pern Wang, Sang Young Yun, Daesung Lee,* and Donald J. Wink
Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061
Received August 24, 2009; E-mail: dsunglee@uic.edu
The coordination of an alkene or alkyne to the ruthenium metal
center is one of the key steps in metathesis reactions catalyzed by
Grubbs-type complexes.1 Snapper and co-workers reported a crystal
structure of alkene-chelated ruthenium complex 1 in which the
Scheme 1. Ring-Closing Metathesis of 1,3-Diynes and the
Formation of an Alkyne-Chelated Ru Alkylidene Complex
tricyclohexylphosphine ligand and an alkene are bound to the
ruthenium in trans fashion.2 On the other hand, vinyl alkylidene
complex 2, an N-heterocyclic carbene (NHC)-containing ruthenium
complex reported by Grubbs, shows a cis relationship between the
NHC ligand and the bound alkene.3 Recently, Grubbs also reported
crystal structures of both 3a and 3b in which the coordinated alkene
and the NHC ligand have a cis relationship.4 In solution, 3a and
3b interconvert, providing a 2:3 equilibrium ratio at 25 °C.
In contrast, neither a crystal structure nor a spectroscopy-based
structural characterization of alkyne-coordinated Grubbs-type ru-
thenium complexes such as 4a and 4b has been reported to date.
This discrepancy may be due to the lack of efficient methods for
preparing Ru complexes containing proper elements that stabilize
the alkyne-coordinated structures. It is expected that the charac-
terization of alkyne-coordinated ruthenium complexes would
provide important insight into the mechanism of enyne metathesis.5
This structural information would also provide a clue concerning
the unproductive metathesis reactions of substrates containing
multiple alkyne moieties. Herein we report the structure and
reactivity of second-generation Grubbs-type ruthenium complexes
containing an alkyne as a ligand.
On the basis of the reactivity of Grubbs-type Ru alkylidenes in
enyne metathesis and the facile metallotropic [1,3]-shift,6 we
envisioned that an alkynyl Ru alkylidene A, formed via an initial
enyne ring-closing metathesis (RCM) or cross-metathesis (CM),
would induce a metallotropic [1,3]-shift to provide a new alkylidene
B (eq 1).7 Subsequent RCM of B would be expected to provide a
new vinyl alkylidene C, which, because of the proximity of the
cis-oriented alkyne moiety and the metal center, would form a
chelated ruthenium complex stable enough to be isolated if
appropriate stabilizing elements are present.
stoichiometric amount of the second-generation Grubbs complex8
were not successful (Scheme 1). Only the final metathesis products
7a and 7b were isolated. Gratifyingly, however, the reaction of
substrate 5c containing a gem-dimethyl group at the propargylic
carbon provided a deep-green crystalline material 6c without the
formation of metathesis product 7c. X-ray diffraction analysis of
6c clearly showed that the alkyne moiety is chelated to the
ruthenium center trans to the NHC (Figure 1). The overall ligand
array of 6c around the ruthenium center is similar to that in the
Grubbs-Hoveyda complex,9 in which an isopropoxide instead of
an alkyne moiety is chelated to the ruthenium. To the best of our
knowledge, 6c is the first alkyne-bound Grubbs-type complex ever
isolated and fully characterized.10
We surmised that one of the key structural features for formation
of a stable alkyne chelate is the gem-dimethyl group, because a
similar substrate 5b lacking this group provided only the metathesis
product 7b. To gain further insight into stable alkyne-coordinated
ruthenium complexes, we examined other substrates with variation
of the propargylic substituents and the incipient ring size (Scheme
2). We expected that the propargylic substituents in alkylidenes
6d and 6e may affect their stability. The five-membered ring in 6f
and the seven-membered ring in 6g would change the strength of
the interaction between the alkyne and the ruthenium center because
Initial attempts to isolate the expected alkyne-coordinated
complexes 6a and 6b generated from substrates 5a and 5b and a
9
15114 J. AM. CHEM. SOC. 2009, 131, 15114–15115
10.1021/ja907155r CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Reactivity of Alkyne-Chelated Ru Alkylidenes
ligand. We believe that the observed alkyne-ruthenium chelate
formation will not only provide insight into the mechanism of enyne
metathesis but also guide us in the design of more efficient and
sophisticated tandem processes.
Figure 1. Molecular structure of 6c in the solid state. Selected bond lengths
(Å) and angles (deg): Ru1-C1, 1.851(3); Ru1-C21, 2.049(3); Ru1-C8,
2.370(4); Ru1-C9, 2.402(4); Ru1-Cl1, 2.3378(9); Ru1-Cl2, 2.3128(10);
C1-Ru1-C21, 98.67(19); C1-Ru1-Cl1, 99.07(11); C1-Ru1-Cl2,
98.75(11); C1-Ru1-C9, 102.60(19).
Acknowledgment. We thank UIC and NIH (CA106673) for
financial support of this work.
of the different bond angles. Indeed, under identical reaction
conditions, substrates 5d-g showed markedly dissimilar behaviors.
Substrates 5d and 5e provided metal complexes 6d and 6e
exclusively without forming 7d and 7e, whereas the reaction of 5f
gave a mixture of complex 6f and metathesis product 7f.11 It is
disappointing that the expected seven-membered-ring closure did
not happen in the reaction of 5g, which gave only the prematurely
terminated product 7g, preempting the formation of 6g.
Supporting Information Available: An acknowledgment for the
mass spectrometry facility at UIUC, general procedures, a CIF for 6c,
and characterization data for representative compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
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Scheme 2. Formation of Alkyne-Chelated Ru Alkylidenes
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To examine whether these alkyne-complexed ruthenium species
are viable catalysts for metathesis reactions, 6c and 6f were treated
with ethylene (Scheme 3). As expected, complex 6c containing the
six-membered ring was recovered unchanged,12 but complex 6f
containing the five-membered ring was readily converted to 7f and
methylidene complex 8.13
In conclusion, we have demonstrated that certain structural
elements of Ru alkylidenes can effectively modulate their reactivity
and stability by showing that alkyne-chelated ruthenium complexes
can be isolated by introducing a gem-dimethyl group near the metal
center. For the first time, we have obtained the X-ray crystal
structure of a Ru alkylidene complex containing an alkyne as a
(11) Complex 6f was characterized by 1H NMR monitoring without isolation
(because of its instability) and by its conversion to 7f. The reported yield
is based on an internal standard (1,2,4,5-tetrabromobenzene).
(12) In the reaction of dimethyl-2,2-diallyl malonate with 6c (10 mol % in
CDCl3), both compounds remained unchanged after 3 h at 40 °C.
(13) Ethylene was introduced into the reaction mixture after complete consump-
tion of the starting material 5f. Therefore, PCy3 in complex 8 originated
from the starting Grubbs II complex.
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