Transcript 3. Olefin Metathesis: Ruthenium Indenylidene Complexes and

Olefin Metathesis
•
1950s – In the presence of various organometallo compounds, olefin
metathesis (where the R groups of olefins were swapped with the other)
occurred
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1970s – Chauvin and colleagues proposed mechanism that seemed to fit
– Like a dance
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1980s – Schrock and Grubbs synthesized the first metallo-carbene
compounds that were air sensitive and shown to catalyze olefin metathesis
Three types of olefin metathesis
– Ring closing metathesis (RCM)
– Ring opening metathesis (ROM)
– Cross metathesis (CM)
Image from Greco, G.E. Nobel Chemistry in the Laboratory. J. Chem. Ed. 2007,84(12), 1996
Selectivity Model for CM
• Experiments done by Chatterjee et al showed that the various olefin
metathesis substrates could be classified by their relative reactivity
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Type I: formed homodimers rapidly
Type II: formed homodimers slowly
Type III: didn’t form homodimers, participate in CM
Type IV: spectator olefin, no CM reaction
• Reacting two olefins from different groups could yield stereospecific,
easily predictable products in good yield
• Reactivity of olefins depended on things such as sterics as well as
deactivating electron-withdrawing groups
Chatterjee, A. K. A General Model for Selectivity in Olefin Metathesis. J. Am. Chem. Soc. 2003, 125, 11360-11370.
Hypothesis
• In testing out the selectivity model, by reacting a type I
(allyl chloride) and type II olefin (4-fluoro-β-nitrostyrene),
a predictable product can be obtained.
Olefin Metathesis Catalysts
• Schrock and Grubbs
– Air sensitive
– Initially were molybdenum and ruthenium based, respectively
– Both won 2005 Nobel Prize in Chemistry for work in olefin metathesis,
along with Chauvin
Images from Pappenfus, T. M. Synthesis and Catalytic Activity of Ruthenium-Indenylidene Complexes for Olefin
Metathesis, J. Chem. Ed. 2007, 84 (12), 1998-2000.
Ruthenium Catalysts, continued
• Ruthenium indenylidene complexes
– Can be synthesized from commercially available diphenyl propargyl
alcohol, synthetic precursors are all relatively air stable, when not in
solution
– Catalytic properties similar to classic Grubbs, if not superior
– Synthesis methods are also relatively simple
Images from Pappenfus, T. M. Synthesis and Catalytic Activity of Ruthenium-Indenylidene Complexes for Olefin
Metathesis, J. Chem. Ed. 2007, 84 (12), 1998-2000.
Methods
• Synthesis of RuCl2(PPh3)3
– Reflux RuCl3·3H2O with triphenylphosphine under argon in a 1:6 molar
ratio for 3 hours, filter out black crystals, wash with anhydrous ether.
• Synthesis of 1a and 1b
– 1a:
• Reflux diphenyl propargyl alcohol with RuCl2(PPh3)3 (2:1 equivalents) in a
positive argon atmosphere for 2.5 hours, with THF as the solvent. Remove
solvent via rotary evaporation, redissolve dark red residue in CH2Cl2,
recrystallize with hexanes, slowly. Filter out solid, store in desiccator.
– 1b:
• Stir 1a and tricyclohexane in a 1:3.3 equivalent ratio under a positive argon
atmosphere for 1.5 hours, with dichloromethane as the solvent. Remove
solvent via rotary evaporation, added hexanes and stir for another 30
minutes. Filter out brown-orange solid, store in desiccator.
• CM reaction
– Reflux styrene, allyl chloride and 1b in a 1:1:0.01 ratio overnight.
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Characterization methods
– NMR & IR, as well as TM for the RuCl2(PPh3)3
Synthesis methods taken from Parry, R. W. Tris(triphenylphospine)dichlororuthenium(II) Inorganic Syntheses. 1970, XII,
238-239 as well as Pappenfus, T. M. Synthesis and Catalytic Activity of Ruthenium-Indenylidene Complexes for Olefin
Metathesis, J. Chem. Ed. 2007, 84 (12), 1998-2000.
Data: NMR spectra for 1a and 1b, literature
1a in chloroform-d
1b in chloroform-d
All reference spectra obtained from Pappenfus, T. M. Synthesis and Catalytic Activity of Ruthenium-Indenylidene Complexes
for Olefin Metathesis, J. Chem. Ed. 2007, 84 (12), 1998-2000.
Results: NMR spectra of 1a and 1b, experimental
1a in benzene-d
1b in benzene-d
Data: Literature IR spectra of 1a and 1b
Result: Experimental IR spectra of 1a and 1b
1a with nujol
1b with nujol
Data: Allyl Chloride impurities
Used allyl chloride in benzene-d
Allyl chloride without impurities, benzene-d
Results: Product vs 1b + reagents
Product in benzene-d
1b + reagents in benzene-d
Results: % Yields
• RuCl2(PPh3)3 (Yield: 0.5059 g)
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RuCl3·H2O used: 0.1658 g (0.641 mmoles, limiting reagent)
triphenylphosphine used: 0.9986 g (3.807 mmoles)
% yield: 82.3%
TM: 130-134˚C, literature indicates 132-134˚C
• 1a (Yield: 0.1944 g)
– RuCl2(PPh3)3 used: 0.3621 g (0.378 mmoles, limiting reagent)
– Diphenyl propargyl alcohol used: 0.1513 g (0.727 mmoles)
– % yield: 58.0%
• 1b (Yield: 0.1315 g)
– 1a used: 0.1524 g (0.172 mmoles, limiting reagent)
– Tricyclohexylphosphine used: 0.1672 g (0.596 mmoles)
– % yield: 89.9%
Literature value of Tm obtained from Parry, R. W. Tris(triphenylphospine)dichlororuthenium(II) Inorganic Syntheses. 1970, XII,
238-239.
Discussion
• The ruthenium indenylidene complexes
synthesized, based on the NMRs, seem to be
the desired complexes
• The IRs are less conclusive
– Nujol absorptions seem to drown out any
characteristic absorptions (peaks at 2950-2800, 14651450 and 1380-1370 cm-1)
Nujol peaks referenced from http://en.wikipedia.org/wiki/Nujol
Did a reaction occur?
• NMR seems to indicate it did
• However, it’s also possible that the difference in NMRs is
due to the presence of liquid allyl chloride in the “prereaction” NMR taken
• Theoretically, the allyl chloride is highly reactive and
should have reacted with the styrene
– Normally, styrenes are part of the Type II group, and is also
reactive in CM reactions. However, it’s possible that the
presence of the nitro group directly attached to the C=C bond
highly reduced its reactivity
Other possible reasons for no reaction
• Many of the intermediates, though fairly air stable, will
react with air in the presence of water.
• The purity of the catalyst is unknown, possible that it was
much less than 1% molar of the reagents
• Even if the catalyst were pure, it’s possible that 1%
molar is insufficient to catalyze the reaction overnight.
• When the CM reaction beaker was refluxed, the solvent
evaporated very quickly, had to turn off the heating
mantle
• It’s possible that since allyl chloride is fairly volatile, it
evaporated before the reaction could take place.
• The effectiveness of the ruthenium indenylidene
complexes has only been shown in RCM and ROM.
Conclusions
• Chatterjee et al’s model of CM selectivity is still valid
• Although the results of this experiment are not
conclusive, the model is also not disproven
• This model of CM selectivity could open paths to new
synthetic routes to important organic compounds, such
as various drugs (i.e. epothilones, antitumor agents)
Conclusions: ways to improve
• Find olefins of different reactivities that are both solid
– Also, olefins that are not deactivated by electron withdrawing
groups
– Product of CM should have distinctive properties from the
reagents, either physical or spectral
• Possibly work with different catalysts which are known to
have high reactivity in CM reactions
• Procure correct NMR solvent, to compare with literature
References
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Casey, C. P. 2005 Nobel Prize in Chemistry: Development of the Olefin
Metathesis Method in Organic Synthesis. J. Chem. Ed. 2006, 82 (2),
192-195.
Chatterjee, A. K. A General Model for Selectivity in Olefin Metathesis. J.
Am. Chem. Soc. 2003, 125, 11360-11370.
Fürstner, A. Indenylidene Complexes of Ruthenium: Optimized
Synthesis, Structure Elucidation, and Performance as Catalysts
for Olefin Metathesis – Application to the Synthesis of the ADE- Ring
System of Nakadomarin A. Chem. Eur. J. 2001, 7 (22), 48114820.
Pappenfus, T. M. Synthesis and Catalytic Activity of RutheniumIndenylidene Complexes for Olefin Metathesis, J. Chem. Ed. 2007, 84
(12), 1998-2000.
Parry, R. W. Tris(triphenylphospine)dichlororuthenium(II) Inorganic
Syntheses. 1970, XII, 238-239.