136
F. Touchard et al. / Journal of Organometallic Chemistry 567 (1998) 133–136
Table 4
that useful asymmetric catalytic systems could be ob-
tained with such derivatives. Considering the mecha-
nism of the hydride transfer reduction of acetophenone
using rhodium and diamine ligand, our dual approach
by theoretical calculation and practical experiments
gave us serious evidence to propose a modified catalytic
cycle with one diamine ligand and one cod bound to
the rhodium. Further studies are in progress to find
whether the same model is still valid for the dithiourea/
ruthenium system.
Results obtained with the thioureas
5
. Experimental
5
.1. Typical procedure for the reduction of
Conditions: [Ru]/[acetophenone]=5%; [t-BuOK]/[Ru]=4; [acetophe-
none]=6×10 mol l in iPrOH; 82°C; the synthesis of the ligands
−
2
−1
acetophenone
has been described in Ref. [9].
The appropriate amount of ligand was added to the
−
3
Rhodium gave the best results and two equivalents of
ligand were used for the reasons stated before, even if
one equivalent only was bound to the metal. Results
obtained with the urea series are reported in Table 3.
Rhodium was the metal of choice for the diureas
catalyst precursor (6×10
mmol) in 2 ml of a solu-
tion of potassium terbutoxide in 2-propanol (0.012 mol
−
1
l
) and stirred for 90 min under an inert atmosphere
(t-BuOK/Rh=4). After addition of acetophenone (0.12
mmol) the mixture was kept overnight at room temper-
ature. The solution was then heated (82°C) unless over-
wise stated in order for the reaction to proceed. The
reaction was monitored and the e.e. was measured by
GC using a capillary column CYDEX-B from SGE.
(
Table 3, entries 1 and 2). Iridium allowed work at
room temperature with monourea (Table 3, entry 3)
whereas with the other ligands, it was necessary to
work at 82°C, whatever the substituents on the
nitrogens.
As for thioureas (Table 4), ruthenium gave the best
results and only two equivalents of ligand per metal
atom were necessary.
References
[
1] S. Gladiali, L. Pinna, G. Delogu, S. De Martin, G. Zassinovich,
G. Mestroni, Tetrahedron Asymmetry 1 (1990) 635.
In the case of thioureas again, the compound bearing
a methyl group on the nitrogen gave the best result.
If we compare the three families of ligands, the
dithioureas with ruthenium gave the best e.e. and led to
a far more active catalytic system than the others with
only two equivalents of ligand/metal. With this last
family of compounds, ruthenium was preferred whereas
for the alkyldiamines and diureas rhodium was better.
[2] J. Takehara, S. Hashiguchi, A. Fujii, S.-I. Inoue, T. Ikariya, R.
Noyori, J. Chem. Soc., Chem. Commun. (1996) 233.
[
3] K-J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya, R. Noyori,
Angew. Chem. Int. Ed. Engl. 36 (1997) 285.
[
4] P. Gamez, F. Fache, M. Lemaire, Tetrahedron Asymm. 6 (1995)
7
05.
[5] P. Gamez, B. Dunjic, M. Lemaire, J. Org. Chem. 61 (1996) 5196.
[
[
6] F. Touchard, P. Gamez, F. Fache, M. Lemaire, Tetrahedron
Lett. 38 (1997) 2275.
7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G.
Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Pe-
tersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham,
V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B.
Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y.
Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R.
Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J.
Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. People,
Gaussian 94 (Revision D.1), Gaussian Inc., Pittsburgh, PA,
4. Conclusion
We have shown that diamines but also diamides,
ureas and thioureas (mono- or di-) could be efficient
and selective ligands fot asymmetric catalysis and that
substitution of the nitrogen atoms has a large effect on
both activity and selectivity of the catalytic system,
probably due to a different chelation pattern. Thus, the
chemistry of nitrogen containing ligands being far eas-
ier than the phosphorus counterpart, there is no doubt
1
995.
[
[
8] M. Bernard, V. Guiral, F. Delbecq, F. Fache, P. Sautet, M.
Lemaire, J. Am. Chem. Soc. 120 (1998) 1441.
9] F. Touchard, F. Fache, M. Lemaire, Tetrahedron Asymm. 8
(1997) 3319.
.