C O M M U N I C A T I O N S
however, further investigation is required to fully ascertain their
exchange behavior. The extension of these studies to include the
dynamics of substituted metallacycles derived from 5 also remains
to be investigated.15
Scheme 5. Dynamics of Ruthenium Metallacycles
In summary, we have shown evidence supporting the bottom-
face orientation of ruthenium(IV) metallacycles derived from both
ethylene and propene. The metallacycle structures derived from
propene represent the first observed examples of substituted
ruthenacyclobutanes and offer new insight into the preferred
stereochemical orientation of metathesis intermediates. In addition,
we have presented new evidence that suggests that ruthenacyclobu-
tanes derived from ethylene and propene are dynamic structures
that proceed through a series of nonproductive metallacycle
formations/cycloreversions prior to olefin exchange. Current efforts
are underway to apply these data to the rational design of additional
olefin metathesis catalysts, particularly those directed at the still
unsolved problem of olefin diastereocontrol.
Figure 1. Reaction of catalyst 3b with propene.
sufficient line broadening of the metallacycle peaks had occurred
such that they disappeared into the baseline. Thinking that a similar
scenario might be in effect, we observed the reaction of 3b with
propene at lower temperatures. Gratifyingly, three new peaks at
-2.11, -2.35, and -2.84 ppm were observed in the upfield region
1
of the H NMR upon cooling the reaction to -95 °C (Figure 1b).
The formation of these new species was found to be reversible, as
warming the reaction mixture back to -40 °C afforded solely
metallacycle 4 without any visible decomposition. In addition, the
presence of these new peaks was found not to be solely due to
ethylene, as cooling a solution of 4 to -95 °C afforded no new
species. Instead, these new complexes were ultimately attributed
to metallacycles derived from propene (Scheme 4).
Acknowledgment. The authors thank Dr. Phillip Dennison and
the U.C. Irvine NMR Facility for the generous donation of
instrument time for the propene VT NMR experiments. Theodor
Agapie is acknowledged for his instruction on Schlenk techniques
for quantitated gas additions. Special thanks also goes to Professor
Daniel O’Leary of Pomona College for invaluable research discus-
sion. Funding was provided by NIH. Postdoctoral funding for
A.G.W. was provided by NIH (NRSA GM070147-02) and UNCF-
Pfizer.
2D COSY, ROESY, and HMQC analysis of the reaction mixture
identified the protons at -2.11 and -2.84 ppm to occupy the same
carbon on propene-derived metallacycle 7, which was found to be
present in 29% conversion. The doublet observed at -2.35 ppm
was attributed the â-hydrogens of the C2-symmetric metallacycle
8. As with 4, metallacycles 7 and 8 lie in a bottom-face orientation.
Repeating the reaction in the presence of 3-13C-labeled propene
confirmed that no metallacycles derived from either cis- or trans-
2-butene are observable at these reaction temperaturessa result
consistent with what Piers had observed in the reaction of 3a with
butene at -50 °C.7 It is important to note that the trans orientation
of the methyl groups in metallacycle 8 is in marked contrast to the
preferred cis orientation that had been previously postulated in
analogy to cyclobutane puckering.1b As the structure of 8 indicates,
a cis/trans argument based solely upon the configurational prefer-
ences of cyclobutane is not entirely valid, particularly in view of
experimental evidence that suggests the metallacycle ring is a
distorted kite shape due to a M‚‚‚Câ interaction.7
Supporting Information Available: Experimental procedures and
characterization data. This material is available free of charge via the
References
(1) (a) Handbook of Metathesis, Vols. 1-3; Grubbs, R. H., Ed.; Wiley-
VCH: Weinheim, Germany, 2003. (b) Ivin, K. J.; Mol, J. C. Olefin
Metathesis and Metathesis Polymerization; Academic Press: San Diego,
CA, 1997.
(2) (a) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118,
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(4) He´risson, J. L.; Chauvin, Y. Makromol. Chem. 1971, 141, 161-176.
(5) Adlhart, C.; Chen, P. J. Am. Chem. Soc. 2004, 126, 3496-3510.
(6) (a) Tallarico, J. A.; Bonitatebus, P. J., Jr.; Snapper, M. L. J. Am. Chem.
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(7) Romero, P.; Piers, W. E. J. Am. Chem. Soc. 2005, 127, 5032-5033.
(8) We have subsequently confirmed the presence of 4 under these conditions
via mass spectrometry. Refer to the Supporting Information.
(9) Refer to the Supporting Information.
At this point, we had acquired significant insight into the Ru-
NHC ligand dynamics and the configurational preferences of
ruthenium metallacycles. During the course of our 2D NMR
investigations, however, we had additionally found the R- and
â-positions of metallacycles 4 and 6 to possess exchange cross-
peaks,13,9 raising the issue of metallacycle dynamics. Taking
previous mechanistic studies into account,1,4 the presence of
exchange cross-peaks was strongly indicative that metallacycle
cycloreversion and re-formation was occurring on the NMR time
scale, leading to an equilibrium between B and B′ (Scheme 5). In
addition, as no exchange was observed between the metallacycle
and free olefin within this time period, exchange between the R-
and â-positions must be attributed to rotation of the π-bound olefin
between successive reaction cycles. Two-dimensional EXSY
experiments on 4 were utilized to determine the rate constant
(10) Mashima, K.; Kaidzu, M.; Nakayama, Y.; Nakamura, A. Organometallics
1997, 16, 1345-1348.
(11) Howard, T. R.; Lee, J. B.; Grubbs, R. H. J. Am. Chem. Soc. 1980, 102,
6878-6880.
(12) Relative to anthracene as an internal standard.
(13) Neuhaus, D.; Williamson, M. P. The Nuclear OVerhauser Effect in
Structural and Conformational Analysis, 2nd ed.; Wiley-VCH: New York,
2000; Chapter 8.
(14) The kBTB′ rate constant includes the rates of metallacycle cycloreversion,
olefin rotation, and metallacycle formation to interconvert positions 1 and
2 in Scheme 5.
(15) Examples of additional olefins that we have thus far investigated: 1,1-
difluoroethylene, vinyl fluoride, ethyl vinyl ether, neohexene, acenaph-
thylene, 2-butyne, and methyl acrylate.
(16) Propene was added in excess (35 equiv) to both minimize the conversion
of 3b into a bis-ruthenium µ-Cl decomposition product and prevent the
reaction mixture from freezing.
(kBTB′)
14 for this process to be 26 ( 2 s-1, corresponding to ∆Gq
) 12.18 ( 0.04 kcal/mol. Experimental evidence indicates that
233K
metallacycles derived from propene are also dynamic structures;
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