Ruthenium catalysed reduction of alkenes using sodium borohydride

Article abstract of DOI:10.1016/j.tetlet.2006.10.026

The reduction of alkenes has been achieved using NaBH₄ as the reducing agent using 0.5-1.0 mol % Ru(PPh₃)₄H₂ in the presence of water.

Full text of DOI:10.1016/j.tetlet.2006.10.026

Tetrahedron Letters 47 (2006) 8943–8944
Ruthenium catalysed reduction of alkenes using sodium borohydride
Gareth R. A. Adair, Kamal K. Kapoor,ꢀ Alexandre L. B. Scolan and
Jonathan M. J. Williams*
Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK
Received 8 September 2006; revised 28 September 2006; accepted 5 October 2006
Abstract—The reduction of alkenes has been achieved using NaBH4 as the reducing agent using 0.5–1.0 mol % Ru(PPh3)4H2 in the
presence of water.
Ó 2006 Elsevier Ltd. All rights reserved.
Table 1. Study of ruthenium catalysts for the reduction of indene 1
The transition metal catalysed reduction of alkenes is
generally acheived using hydrogen gas or by transfer
hydrogenation from suitable hydrogen donor.
Catalyst
(5 mol % in Ru)
Reducing agent
(4 equiv)
Conversion
(%)a
a
Amongst the various transition metals that are able to
catalyse the reduction of alkenes, homogeneous ruthe-
nium complexes play a significant role.1 We have
recently been interested in the use of ruthenium com-
plexes in oxidation and reduction processes, and wanted
to identify alternative reducing agents for alkenes.2
Herein, we report the ruthenium catalysed reduction
of alkenes using sodium borohydride. Sodium borohy-
dride is normally associated with the reduction of polar
functional groups, such as carbonyl compounds. How-
ever, it is known that in the presence of simple metal
salts, sodium borohydride can be used to reduce a wider
range of functionality, including alkenes.3 Transition
metals have also been shown to catalyse the generation
of hydrogen from sodium borohydride.4
CpRuCl(PPh3)2
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
Bu3NÆBH3
C5H5NÆBH3
H3NÆBH3
NaBH4
0
100
97
100
90
100
31b
100
100
77
CpRuCl(PPh3)2/KOH
[(p-cymene)RuCl2]2
[(p-cymene)RuCl2]2/KOH
PhCH = Ru(PCy3)2Cl2
Ru(PPh3)3CO(H)2
Ru(PPh3)3CO(H)2
Ru(PPh3)3CO(H)2
Ru(PPh3)4(H)2
Ru(PPh3)4(H)2
Ru(PPh3)4(H)2
None
100
0
NaBH4
a Conversion was determined from analysis of the 1H NMR spectra.
b No water added.
Alternative proton sources including ammonium chlo-
ride, pyridinium p-toluenesulphonate and carboxylic
acids were less successful, in the latter cases leading to
the reduction of the carboxylic acid into an alcohol
under the reaction conditions.
Preliminary experiments were performed with 5 mol %
of a ruthenium catalyst using an excess reducing agent
in order to convert indene 1 into indane 2 (Scheme 1,
Table 1).
Optimisation experiments led to the use of significantly
reduced catalyst loading (down to 0.5 mol %) for the
reduction of indene 1 and stilbene 3. However, some
other substrates gave either incomplete reduction or
by-products under these conditions. For example, the
reduction of p-allylanisole 4 afforded the reduction
product 6a along with the isomerised product 6b
(Scheme 2).
Catalyst (5 mol% in Ru)
Reducing agent (5 eq.)
H2O (2 eq.)
Toluene, 100 °C, 1 h
2
1
Scheme 1. Ruthenium catalysed reduction of indene.
*
Corresponding author. E-mail: j.m.j.williams@bath.ac.uk
ꢀ Permanent address: Department of Chemistry, University of Jammu,
In subsequent reductions, we used 1 mol % catalyst for
22 h, in order to ensure a complete reduction. Using
Jammu 180006, India.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.026

8944
G. R. A. Adair et al. / Tetrahedron Letters 47 (2006) 8943–8944
Ru(PPh ) H (0.5 mol%)
No catalyst
3 4
2
E
E
Ph
Ph
E =
CN
CO2Et
NaBH (0.5 eq.)
4
NaBH4 (0.5 eq.)
H2O (2.0 eq.)
Toluene,
CN
CN
H O (2 eq.)
2
2
1
3
Toluene, 100 °C, 2.5 h
E =
CN
100% (71%)
16 97%
17 86%
14
15
100 °C, 22 h
CO2Et
Ph
Ph
Ph
"
Ph
Scheme 4. Uncatalysed reduction of electron deficient alkenes.
5
100% (95%)
position,5 and alkyne 10 was also fully reduced to the
alkane. The more highly substituted alkenes triphenyl-
ethene and tetraphenylethene were found to be essen-
tially inert to reduction under these conditions.
"
MeO
MeO
6a
+
4
Control experiments demonstrated that no reduction of
the alkenes in Scheme 3 was observed in the absence of a
catalyst. However, the electron deficient alkenes 14 and
15 were readily reduced without a catalyst (Scheme 4).
MeO
6b
(
)
100%
6a:6b 30:70
Scheme 2. Reduction (and isomerisation) of alkenes. Conversions,
with isolated yields in parentheses.
Overall the reduction procedure allows hydrogenation
reactions to take place without the need for an external
supply of hydrogen.6 We assume that the ruthenium
complex catalyses both the formation of hydrogen from
borohydride/water and the subsequent reduction
process.
Ph
Ph
Ru(PPh ) H (1.0 mol%)
3 4
2
Ph
Ph
NaBH (1 eq.)
4
5
3
H O (2 eq.)
2
100% (94%)
Toluene, 100 °C, 22 h
In summary, we have shown that ruthenium catalysts
are capable of reducing alkenes using sodium boro-
hydride/water as an alternative reducing agent.
"
MeO
Ph
6a
MeO
4
100% (78%)
Acknowledgements
Ph
N
"
N
O
We wish to thank the EPSRC for studentship funding
(to G.R.A.A.) and INSA, New Delhi, and the Royal
Society for a visiting fellowship (to K.K.K.).
O
7
11
100% (92%)
OAc
OAc
"
AcO
AcO
8
12
100%
References and notes
1. Ruthenium in Organic Synthesis; Murahashi, S.-I., Ed.;
Wiley-VCH: Weinheim, 2004.
2. Adair, G. R. A.; Williams, J. M. J. Chem. Commun. 2005,
5578.
3. (a) Wade, R. C. J. Mol. Catal. 1983, 18, 273; Cobalt salts:
(b) Osby, J. O.; Heinzman, S. W.; Ganem, B. J. Am. Chem.
Soc. 1986, 108, 67.
4. Brown, H. C.; Brown, C. A. J. Am. Chem. Soc. 1962, 84,
1493.
OH
OH
9
"
"
13
85%
Ph
Ph
Ph
Ph
5
10
5. Takaya, H.; Ohta, T.; Sayo, N.; Kumobayashi, H.; Aku-
tagawa, S.; Inoue, S.; Kasahara, I.; Noyori, R. J. Am.
Chem. Soc. 1987, 109, 1596.
100% (82%)
Scheme 3. Reduction of other alkenes.
6. Experimental procedure for the reduction of stilbene: (E)-
stilbene (450 mg, 2.5 mmol), sodium borohydride (47.5 mg,
1.25 mmol), water (90 mg, 5 mmol) and Ru(PPh3)4H2
(14 mg, 0.5 mol %) in toluene (5 mL) were heated at
100 °C for 2.5 h in a pressure tube (Caution: hydrogen
evolved). After cooling, acetone (0.5 mL) and ethyl acetate
(20 mL) were added. After washing with brine, drying over
MgSO4 and concentration, the residue was distilled to give
1,2-diphenylethane as a colourless solid (432 mg, 2.37
mmol, 95%).
these conditions, a range of alkenes was reduced
(Scheme 3).
The longer reaction times allowed alkene 4 to be reduced
without any isomerisation product remaining. Amide
and ester functionalities in substrates 7 and 8 were toler-
ated. Geraniol 9 was reduced selectively at the allylic