of the ruthenium family of complexes by increasing their
activity, we recently prepared ruthenium-based complexes
coordinated with 1,3-dimesitylimidazol-2-ylidene ligands 3.5
These complexes exhibited a high ring-closing metathesis
activity similar to that of the molybdenum complex 1, yet
have also retained the remarkable air and water stability
characteristic of the parent benzylidene ruthenium complex
2.
easily made via the condensation of a variety of aromatic
and aliphatic amines with glyoxal, followed by a reduction
of the resulting Schiff bases with either NaCNBH3 or with
H2 and Pd/C (Scheme 1).11 The diamines 6 were obtained
Scheme 1. Precursor Synthesis
In search for even more efficient ligands for our ruthenium
olefin metathesis catalysts, we have focused on the related
4,5-dihydroimidazol-2-ylidenes 4.6-8 We reasoned that due
to the lack of carbene stabilization provided by the absence
of π-interactions, these saturated imidazole ligands might
be more basic then their unsaturated analogues.9 The higher
basicity of these ligands should in turn translate into an
increased activity of the desired catalysts.10
in one step via the palladium-catalyzed amination reaction
of various aryl bromides with 1,2-disubstituted ethane-1,2-
diamines.12
The diamines 5 and 6 were subsequently converted to the
corresponding imidazolium salts through treatment with
triethyl orthoformate in the presence of 1 equiv of ammonium
tetrafluoroborate at 120 °C (Scheme 1).13 The resultant
imidazolium tetrafluoroborate salts 7 were obtained quan-
titatively and could be purified further by recrystallization
from ethanol/hexanes.
We had hoped that these precursors could be readily
deprotonated with metal hydrides to yield the desired ligands
4 directly,7 especially since similar procedures have worked
well for many unsaturated imidazolium salts.14 Unfortunately,
we were unsuccessful at extending the published proce-
dures7,14 to our saturated systems.
Parallel work in our group, however, revealed the pos-
sibility of using protected carbenes as precursors to generate
the free carbene ligands in situ. Specifically, it was found
that 5-methoxytriazoles could be reacted directly with metal
complexes at elevated temperatures via the in situ conversion
to the free triazol-5-ylidenes.15 Correspondingly, we prepared
the similar 2-alkoxy-4,5-dihydroimidazoles 8 by treatment
of tetrafluoroborate salts 7 with sodium methoxide in
methanol or potassium tert-butoxide in THF at ambient
temperature (Scheme 2).
We began the synthesis of ligands 4 with the preparation
of ethane-1,2-diamines 5 (R′ ) H) and 1,2-disubstituted
ethane-1,2-diamines 6 (R′ ) Ph, alkyl). The diamines 5 were
(7) For the first report of isolation of 1,3-dimesityl-4,5-dihydroimidazol-
2-ylidenes, see: Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J. J. Am.
Chem. Soc. 1995, 117, 11027-11028.
(8) For other examples of saturated carbene ligands, see: (a) Alder, R.
W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew. Chem., Int. Ed. Engl.
1996, 35, 1121-1123. (b) Alder, R. W.; Blake, M. E.; Bortolotti, C.; Bufali,
S.; Butts, C. P.; Linehan, E.; Oliva, J. M.; Orpen, A. G.; Quayle, M. A.
Chem. Commun. 1999, 241-242.
(9) Ding, S.; Feldmann, M. T.; Grubbs, R. H.; Goddard, W. A. J. Am.
Chem. Soc. 1999, submitted.
(10) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1997,
119, 3887-3897.
(11) For examples, see: (a) Hutchins, R. O.; Su, W.-Y.; Sivakumar, R.;
Cistone, F.; Stercho, Y. P. J. Org. Chem. 1983, 48, 3412-3422. (b) Dieck,
H. t.; Verfu¨rth, U.; Diblitz, K.; Ehlers, J.; Fendesak, G. Chem. Ber. 1989,
122, 129-131.
(12) For example, see: Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J.
Am. Chem. Soc. 1996, 118, 7215-7216. For a recent review of palladium-
catalyzed amination of aryl halides, see: Yang, B. H.; Buchwald, S. L. J.
Organomet. Chem. 1999, 576, 125-146.
(13) Saba, S.; Brescia, A.-M.; Kaloustain, M. K. Tetrahedron Lett. 1991,
32, 5031-5034.
(14) Herrmann, W. A.; Ko¨cher, C.; Goossen, L. J.; Artus, G. R. J. Chem.
Eur. J. 1996, 2, 1627-1636.
(15) (a) Trnka, T. M.; Grubbs, R. H. Manuscript in preparation. For
reports of preparation and isolation of triazol-5-ylidenes starting from the
5-methoxytriazoles, see: (b) Enders, D.; Breuer, K.; Raabe, G.; Runsink,
J.; Teles, J. H.; Melder, J. P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1021-1023. (c) Teles, J. H.; Melder, J. P.; Ebel, K.;
Schneider, R.; Gehrer, E.; Harder, W.; Brode, S.; Enders, D.; Breuer, K.;
Raabe, G. HelV. Chim. Acta 1996, 79, 61-83.
The alkoxy-protected species 8 did not react with ben-
zylidene ruthenium complex 2 in benzene at ambient
temperature. However, they readily reacted with complex 2
954
Org. Lett., Vol. 1, No. 6, 1999