S. Naskar, M. Bhattacharjee / Tetrahedron Letters 48 (2007) 465–467
467
Scheme 4.
acted substrates were recovered. Thus, all the control
reactions confirmed that, 1 catalyzes the regiospecific
transfer hydrogenation of unsaturated ketones and for-
mic acid is a specific hydrogen donor in the reduction of
double bonds. Also it is clear that 2-propanol is a more
effective hydrogen donor in the reduction of carbonyl
groups. A similar observation was made by Selvam
et al. in the PdMCM-41 catalyzed hydrogenation of
a,b-unsaturated ketones and it has been suggested that,
when 2-propanol is used as the hydrogen donor, a metal
alkoxide is formed and this has an affinity for the car-
bonyl groups.
Acknowledgement
S.N. thanks the Council of Scientific & Industrial
Research, Govt. of India, New Delhi, for a Fellowship.
References and notes
1
5
1
. Tenorio, M. J.; Mereiter, K.; Puerta, M. C.; Valerga, P.
J. Am. Chem. Soc. 2000, 122, 11230–11231.
2. Miecznikowski, J. R.; Crabtree, R. H. Organometallics
2004, 23, 629–631.
. Darensbourg, D. J.; Joo, F.; Kannisto, M.; Katho, A.;
3
Reibenspies, J. H. Organometallics 1992, 11, 1990–
However, it was reported that, [Ru(PPh ) Cl ] can selec-
3
3
2
1
993.
4. Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97–
02, and references cited therein.
tively reduce a,b-unsaturated ketones when heated with
various high boiling alcohols such as benzyl alcohol.16 It
1
may be noted that, except in the case of dibenzylidene
acetone, we did not observe any saturated alcohol for-
mation, when formic acid was used as the hydrogen
donor. Thus it is clear that the catalytic behaviour of
5
. Comprehensive Organometallic Chemistry; Trost, B. M.,
Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 8.
. Hudlicky, M. Reductions in Organic Chemistry; American
Chemical Society: Washington, DC, 1996.
6
1
is different to the parent compound from which
7. Sharma, A.; Kumar, V.; Sinha, A. K. Adv. Synth. Catal.
2006, 348, 354–360.
1
4
it is generated.
8
. Arterburn, J. B.; Pannala, M.; Gonzalez, A. M.; Cham-
berlin, R. M. Tetrahedron Lett. 2000, 41, 7847–7849.
. Doi, T.; Fukuyama, T.; Horiguchi, J.; Okamura, T.; Ryu,
I. Synlett 2006, 721–724.
Our repeated attempts to identify the catalytically active
species and the reaction intermediates by NMR spec-
troscopy failed. However, there were striking similarities
in reactivity and stereoselectivity in rhodium catalyzed
enantioselective reduction of C@C double bonds by
both molecular hydrogen and the transfer hydrogena-
9
1
1
0. Sakaguchi, S.; Yamaga, T.; Ishii, Y. J. Org. Chem. 2001,
6, 4710–4712.
1. Wu, X.; Vinci, D.; Ikariya, T.; Xiao, J. Chem. Commun.
005, 4447–4449.
6
2
1
7
17,18
tion protocol. Based on reported mechanisms
on
12. Wang, F.; Liu, H.; Cun, L.; Zhu, J.; Deng, J.; Jiang, Y.
J. Org. Chem. 2005, 70, 9424–9429.
13. Xing, Y.; Chen, J.-S.; Dong, Z.-R.; Li, Y.-Y.; Gao, J.-X.
Tetrahedron Lett. 2006, 47, 4501–4503.
rhodium catalyzed transfer hydrogenation of C@C dou-
ble bonds, a possible reaction pathway is proposed and
is shown in Scheme 4.
1
4. Naskar, S.; Bhattacharjee, M. J. Organomet. Chem. 2005,
90, 5006–5010.
6
In summary, the cationic ruthenium compound,
+
À
15. Selvam, P.; Sonavane, S. U.; Mohapatra, S. K.; Jayaram,
[
Ru(PPh ) (CH CN) Cl] [BPh ] has been found to be
3 2 3 3 4
R. V. Tetrahedron Lett. 2004, 45, 3071–3075.
an effective catalyst for regiospecific reduction of double
bonds in a,b-unsaturated ketones when formic acid is
used as the hydrogen donor. However, when 2-propanol
is used as the hydrogen donor, the conversions were
found to be poor and non-specific.
1
6. Sasson, Y.; Blum, J. Tetrahedron Lett. 1971, 24, 2167–
170.
7. Lange, S.; Leitner, W. Dalton Trans. 2002, 752–758.
2
1
18. Leitner, W.; Brown, J. M.; Brunner, H. J. Am. Chem. Soc.
1993, 115, 152.