Me
MN
Table 1 Enantioselective hydrogenation catalyzed by 4 and 4
Entry Complex
Substrate p (bar)
conversion (%)
ee (%)
Me
1
2
3
4
5
6
4
4
4
4
4
4
5
5
7
7
7
7
40
40
25
40
25
40
100
100
100
100
65
49
53
32
14
61
53
MN
Me
Me
MN
MN
100
1
Conversion was determined by H NMR spectroscopy, enantiomeric
excesses were extracted from chiral GC.
In conclusion, a straightforward synthesis for new chiral bis-
1,2,4-triazolium salts and their corresponding rhodium(I) com-
plexes was presented. The latter were successfully applied as
enantioselective catalysts for the hydrogenation of prochiral olefins
that yielded moderate to good ee values. The potential of the new
chiral ligands is currently being investigated for a variety of other
catalytic applications.
Fig. 1 Molecular structure of the cation in 4 . The counter ion PF -
Me
, and
6
hydrogen atoms are omitted for clarity. The thermal ellipsoids are drawn
13–20
Selected bond lengths (A) and angles
˚
at the 50% probability level.
(
◦
): Rh1–C9 2.036(7), Rh1–C11 2.042(7), Rh1–Cg 1 2.099, Rh1–Cg2
.111, Rh1–C1 2.212(6), Rh1–C2 2.200(5), Rh1–C5 2.212(6), Rh1–C6
.229(7), C9–Rh1–C11 83.2(3), Cg1–Rh1–Cg2 86.4, N1–C9–N2 102.5(5),
N2–C9–Rh1 129.7(5), N1–C9–Rh1 127.8(5), N4–C11–N5 104.8(6),
N4–C11–Rh1 116.8(5), N5–C11–Rh1 138.3(6).
2
2
Acknowledgements
The metallacycle arranges in a boat conformation and the bite
angle of the biscarbene ligand is 83.2(3) . The Rh–Ccarbene distances
◦
This work was generously supported by the Margarete Ammon
Stiftung (Ph.D. grant for S.K.U.R).
˚
˚
of 2.036(7) A and 2.042(7) A are found within the typical range
21
for rhodium NHC complexes based on 1,2,4-triazoles. More
crystallographic details to 4 are provided in the ESI.†
The rhodium(I) complexes 4 and 4 were applied as cata-
lysts in the hydrogenation of methyl-2-acetamidoacrylate 5 and
dimethylitaconate 7 (Scheme 3). The experiments were carried out
Me
Me
MN
Notes and references
1
2
(a) W. A. Herrmann, L. J. Gooßen, C. K o¨ cher and G. R. J. Artus,
Angew. Chem., Int. Ed. Engl., 1996, 35, 2805–2807; (b) D. Enders, K.
Breuer and J. H. Teles, Helv. Chim. Acta, 1996, 79, 1217–1221; (c) D.
Enders, K. Breuer, J. Runsink and J. H. Teles, Helv. Chim. Acta, 1996,
◦
in 1,2-dichloroethane at 60 C in the presence of 1 mol% catalyst.
7
9, 1899–1902.
(a) V. Cesar, S. Bellemin-Laponnaz and L. H. Gade, Chem. Soc. Rev.,
004, 33, 619–636; (b) L. Gade and S. Bellemin-Laponnaz, in N-
2
Heterocyclic Carbenes in Transition Metal Catalysis, 2007, pp. 117–
1
1
57; (c) M. C. Perry and K. Burgess, Tetrahedron: Asymmetry, 2003,
4, 951–961.
3
4
(a) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47,
3
1
122–3172; (b) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41,
290–1309.
(a) W.-L. Duan, M. Shi and G.-B. Rong, Chem. Commun., 2003, 2916–
2
7
2
917; (b) Q. Xu, X. Gu, S. Liu, Q. Dou and M. Shi, J. Org. Chem., 2007,
2, 2240–2242; (c) L.-j. Liu, F. Wang and M. Shi, Organometallics, 2009,
8, 4416–4420.
5
(a) T. Chen, J.-J. Jiang, Q. Xu and M. Shi, Org. Lett., 2007, 9, 865–868;
b) S.-J. Liu, L.-j. Liu and M. Shi, Appl. Organomet. Chem., 2009, 23,
83–190.
(
1
Me
Scheme 3 Catalytic hydrogenation performed with complexes 4
6 (a) T. Zhang and M. Shi, Chem.–Eur. J., 2008, 14, 3759–3764; (b) H.
MN
Clavier, J.-C. Guillemin and M. Mauduit, Chirality, 2007, 19, 471–476.
and 4
.
7
G.-N. Ma, T. Zhang and M. Shi, Org. Lett., 2009, 11, 875–
78.
8
The conversions and enantiomeric excesses were determined
after 20 h and are summarized in Table 1.
8 (a) D. S. Clyne, J. Jin, E. Genest, J. C. Gallucci and T. V. RajanBabu,
Org. Lett., 2000, 2, 1125–1128; (b) A. R. Chianese and R. H. Crabtree,
Organometallics, 2005, 24, 4432–4436; (c) A. Arnanz, C. Gonzalez-
Arellano, A. Juan, G. Villaverde, A. Corma, M. Iglesias and F. Sanchez,
Chem. Commun., 2010, 46, 3001–3003.
At a hydrogen pressure of 40 bar 5 was fully converted to 6
MN
by both catalysts, but 4 led to a slightly higher enantiomeric
excess (entry 2). Sterically more demanding groups seem to
positively influence the optical induction (entries 1 and 2). At
9 M. C. Perry, X. Cui and K. Burgess, Tetrahedron: Asymmetry, 2002,
1
3, 1969–1972.
1
0 L. G. Bonnet, R. E. Douthwaite and R. Hodgson, Organometallics,
003, 22, 4384–4386.
Me
MN
4
0 bar 7 was also quantitatively reduced by 4 and 4 (entries 4
2
Me
and 6) while at reduced pressure (25 bar) only 4 led to full
11 Y. Garcia, G. Bravic, C. Gieck, D. Chasseau, W. Tremel and P. Gutlich,
Inorg. Chem., 2005, 44, 9723–9730.
conversion (entry 3). The highest ee value, however, was found for
MN
12 C. K o¨ cher and W. A. Herrmann, J. Organomet. Chem., 1997, 532,
complex 4 , when the reaction was done at a hydrogen pressure
2
61–265.
of 25 bar with 61% although the conversion was incomplete
1
3 K. Brandenburg, DIAMOND Version 3.1, Crystal and Molecular
Structure Visualization, 2005, Bonn.
(
entry 5).
4
2 | Dalton Trans., 2011, 40, 41–43
This journal is © The Royal Society of Chemistry 2011