Organometallics
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giving better enantiodiscrimination with respect to 3 (42% vs
8% ee) and mirroring the best results (in terms of conversion
and enantioselectivity) obtained by Collins with modified
versions of 3 (95% conversion, 43% ee).6d The ARCM of 27
carried out with 6 and 8 in the presence of NaI gave no reaction
(entries 8 and 10, Table 2). Very likely, the substitution of Cl−
bound to Ru by I− ligands inhibits the reactivity of the catalysts
toward substrates with increased steric bulk, such as 27.
On the basis of these preliminary results, it appears that
complexes 6 and 8 have catalytic properties that are
intermediate between those of the Grubbs and Collins catalysts,
deserving therefore deeper investigation. Further studies on the
catalytic behavior of 6 and 8 in asymmetric olefin metathesis
are underway.
REFERENCES
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(1) For reviews, see: (a) Handbook of Metathesis; Grubbs, R. H., Ed.;
Wiley-VCH: Weinheim, Germany, 2003. (b) Vougioukalakis, G. C.;
Grubbs, R. H. Chem. Rev. 2010, 110, 1746−1787. (c) Samojlowicz, C.;
Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708−3742.
(2) (a) Stewart, I. C.; Ung, T.; Pletnev, A. A.; Berlin, J. M.; Grubbs,
R. H.; Schrodi, Y. Org. Lett. 2007, 9, 1589−1592. (b) Berlin, J. M.;
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(3) (a) Colacino, E.; Martinez, J.; Lamaty, F. Coord. Chem. Rev. 2007,
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In summary, we have presented the first example of stable Ru
metathesis catalysts bearing unsymmetrical N-alkyl/N-aryl
NHCs that differ in the relative orientation of phenyl groups
on the backbone (syn or anti). For the Hoveyda−Grubbs type
complexes 7 and 8, an investigation of their crystal structure
revealed rare C−H···Ru anagostic interactions, which were
confirmed in solution by NMR spectral analysis. Preliminary
studies on the catalytic behavior of complexes 5−8 revealed
that chiral anti isomers 6 and 8 were highly efficient catalysts
for sterically demanding RCM reactions and furthermore were
able to successfully accomplish the challenging ARCM reaction,
forming a tetrasubstituted olefin with moderate enantioselec-
tivity. On the other hand, syn catalysts showed improved
activities and Z selectivities in the CM reaction. These findings
clearly demonstrate that different backbone configurations,
allowing uNHCs to modulate their encumbrance around the
metal, dramatically affect the properties of the catalyst.
Accordingly, the present study opens new perspectives in fine
tuning the steric and electronic properties of unsymmetrical
NHC-Ru catalysts for specific applications.
(5) Selected examples of N-alkyl/N-aryl NHC-Ru complexes:
(a) Vehlow, K.; Maechling, S.; Blechert, S. Organometallics 2006, 25,
25−28. (b) Ledoux, N.; Linden, A.; Allaert, B.; Mierde, H. V.;
Verpoort, F. Adv. Synth. Catal. 2007, 349, 1692−1700. (c) Vehlow, K.;
Wang, D.; Buchmeiser, M. R.; Blechert, S. Angew. Chem., Int. Ed. 2008,
47, 2615−2618. (d) Endo, K.; Grubbs, R. H. J. Am. Chem. Soc. 2011,
133, 8525−8527. (e) Thomas, R. M.; Keitz, B. K.; Champagne, T. M.;
Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 7490−7496. (f) Ablialimov,
O.; Kedziorek, M.; Toborg, C.; Malinska, M.; Wozniak, K.; Grela, K.
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Organometallics 2012, 31, 7316−7319. (g) Rouen, M.; Borre, E.;
Falivene, L.; Toupet, L.; Berthod, M.; Cavallo, L.; Olivier-Bourbigou,
H.; Mauduit, M. Dalton Trans. 2014, 43, 7044−7049.
(6) (a) Collins, S. K.; Fournier, P.-A. Organometallics 2007, 26,
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2945−2949. (b) Fournier, P.-A.; Savoie, J.; Bedard, M.; Stenne, B.;
Grandbois, A.; Collins, S. K. Chem.Eur. J. 2008, 14, 8690−8695.
(c) Savoie, J.; Stenne, B.; Collins, S. K. Adv. Synth. Catal. 2009, 351,
1826−1832. (d) Stenne, B.; Timperio, J.; Savoie, J.; Dudding, T.;
Collins, S. K. Org. Lett. 2010, 12, 2032−2035.
(7) (a) Grisi, F.; Costabile, C.; Gallo, E.; Mariconda, A.; Tedesco, C.;
Longo, P. Organometallics 2008, 27, 4649−4656. (b) Grisi, F.;
Mariconda, A.; Costabile, C.; Bertolasi, V.; Longo, P. Organometallics
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Longo, P.; Bertolasi, V.; Ragone, F.; Grisi, F. Chem. Eur. J. 2011, 17,
8618−8629. (d) Perfetto, A.; Costabile, C.; Longo, P.; Bertolasi, V.;
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(8) Complexes 5−8 are stable in the solid state over a period of 5−6
months and in C6D6 solution over 1−2 weeks..
(9) syn: N-alkyl group located on the same side as the benzylidene
unit (complex 5 syn:anti 0.3:1; complex 6 syn:anti 0.8:1). 2D-EXSY
experiments at various mixing times as well as variable-temperature 1H
NMR experiments carried out in the range 25 °C < T < 70 °C showed
no exchange between syn and anti rotamers.
(10) A mixture of two conformational isomers was observed in
solution for both 7 (major:minor 1:0.29) and 8 (major:minor 1:0.09).
Very likely, these confomers originate in restricted rotation around the
N−C(aryl) bond.
(11) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6908−6914.
(12) Ritter, A. T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H.
Organometallics 2006, 25, 5740−5745.
More in general, the synthetic routes employed for the
preparation of the backbone-substituted NHC ligand pre-
cursors should allow for easy modification of both the N-alkyl
and the N-aryl moieties, thus offering new opportunities for
various applications in organometallic catalysis as well as in
organocatalysis.
ASSOCIATED CONTENT
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S
* Supporting Information
Text, figures, tables, and CIF files giving experimental details,
compound characterization data, and crystallographic data. This
material is available free of charge via the Internet at http://
AUTHOR INFORMATION
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Corresponding Author
(13) (a) Seiders, T. J.; Ward, D. W.; Grubbs, R. H. Org. Lett. 2001, 3,
3225−3228. (b) Funk, T. W.; Berlin, J. M.; Grubbs, R. H. J. Am. Chem.
Soc. 2006, 128, 1840−1846.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The authors thank Dr. Chiara Costabile for helpful discussions
and Dr. Patrizia Iannece and Patrizia Oliva for technical
assistance. Financial support from the Ministero dell’Universita
e della Ricerca Scientifica e Tecnologica is gratefully acknowl-
edged.
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dx.doi.org/10.1021/om500731k | Organometallics XXXX, XXX, XXX−XXX