Reduction of olefins using ruthenium carbene catalysts and silanes

Article abstract of DOI:10.1055/s-2005-872695

Ruthenium carbene complexes are able to mediate reduction of olefins in the presence of trialkylsilanes. Under these reduction conditions, when kinetically favorable ring-closing metathesis is possible, a one-pot cyclization-reduction sequence can be performed. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-872695

LETTER
2449
Reduction of Olefins Using Ruthenium Carbene Catalysts and Silanes
Reduction of
Ole
a
fins ndice Menozzi, Peter I. Dalko,* Janine Cossy*
Laboratoire de Chimie Organique associé au CNRS, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
E-mail: peter.dalko@espci.fr; E-mail: janine.cossy@espci.fr
Received 21 June 2005
yield.11 Similar results were observed when TESH was re-
placed by triphenylsilane (Ph3SiH) or tert-butyldimethyl-
silane (TBSH), even though compounds 7b and 7c were
not formed, the reduction was more sluggish, and com-
pounds 5b and 5c were obtained after 60 hours accompa-
nied by the O-silylated products 6b and 6c, respectively,
in a ratio of 32/68 and 86/14 (Scheme 2). As the best re-
sults were obtained with triethylsilane, this reagent was
selected for further studies.
Abstract: Ruthenium carbene complexes are able to mediate re-
duction of olefins in the presence of trialkylsilanes. Under these
reduction conditions, when kinetically favorable ring-closing
metathesis is possible, a one-pot cyclization–reduction sequence
can be performed.
Key words: reduction, hydrosilylation, stereoselective synthesis
One of the most important advancements in synthetic or-
ganic chemistry in the last ten years is the development of
efficient catalysts for metathesis reactions.1 While an im-
pressive array and variety of catalysts have been devised
for carbon–carbon double bond formation, some attention
has been paid to the non-metathetic reactions of such
complexes.2 Ruthenium carbene complexes were distin-
guished in mediating halogenation,3 cyclopropanation,4
alkylative5a and radical cyclizations,5b,c intramolecular
[3+2]-cycloaddition of alkynilidene cyclopropanes,6
isomerization7 and reduction of olefins,8a including
hydrosilylation of carbonyl compounds8a and alkynes.8b,c
Somewhat surprisingly, the capacity in mediating these
reactions sequentially was seldom considered.2,4,7e,8
1 (3–5 mol%)
R3SiH (3–4 equiv)
OH
CH2Cl2, 40 °C, 60 h
4
SiR3
+
+
OSiR3
5a–c
Reagent
OSiR3
7a–c
OSiR3
6a–c
Ratio 5/6/711
Yield
a R3SiH = Et3SiH
b R3SiH = t-BuMe2SiH
c R3SiH = Ph3SiH
91/4/5
(94%)
(74%)
(79%)
32/68/0
86/14/0
Scheme 2
P(Cy)3
Ru
N
N
Mes
Mes
Cl
N
N
Mes
Mes
Cl
Cl
Cl
Ru
In order to optimize the reaction conditions, different ru-
thenium complexes 1–3 (5 mol%) were compared in the
reduction of eugenol acetate 8 in the presence of TESH
(2.5 equiv) in refluxing CH2Cl2 (Scheme 3). In the pres-
ence of catalyst 1 (5.0 mol%) and after 16 hours, com-
pound 8 was transformed to a mixture of 9 and 10 in 79%
global yield, and in a ratio of 96/4.11 When catalyst 2 was
used, the reaction was also complete after 16 hours and
compounds 9 and 10 were isolated in 71% yield (95/5).11
Ru
Cl
Cl
(Cy)3P
i-PrO
(Cy)3P
1
2
3
Scheme 1
As ruthenium complexes are generally considered to be
poor hydrogen-transfer agents toward olefins and
alkynes,9 these mild hydrogen donors can be exploited ad-
vantageously in selective transformations. Here, we wish
to describe the selective hydrogenation of olefins using
complexes 1–3 in the presence of silanes (Scheme 1).10
catalyst (5.0 mol%)
MeO
TESH (2.5 equiv)
AcO
CH2Cl2, 40 °C
MeO
8
When olefin 4 was treated with a mixture of triethylsilane
(TESH, 3–4 equiv) and ruthenium complex 1 (3–5 mol%),
in refluxing CH2Cl2, a slow conversion of olefin 4 was
observed (Scheme 2). After 60 hours, the saturated O-
silylated phenol 5a was obtained, accompanied by a small
amount of unsaturated O-silylated product 6a and hydro-
silylated compound 7a in a ratio of 91/4/5 and in 94%
MeO
RO
SiEt3
+
RO
9 R = Ac
9' R = H
10 R = Ac
10' R = H
NaOMe / MeOH
NaOMe / MeOH
Ratio 9/1011
Catalyst
Time
Yield of 9+10
(79%)
1
2
3
16 h
16 h
3 h
96/4
95/5
68/32
(71%)
(74%)
SYNLETT 2005, No. 16, pp 2449–2452
Advanced online publication: 21.09.2005
0
4
.1
0
.2
0
0
5
DOI: 10.1055/s-2005-872695; Art ID: G16905ST
© Georg Thieme Verlag Stuttgart · New York
Scheme 3

2450
C. Menozzi et al.
LETTER
When catalyst 3 (2.5 mol%) was employed the reaction issued from the competing oxidative addition of TESH to
was faster although less selective than with catalysts 1 and the double bond (Scheme 4, eq 3). Furthermore, the re-
2, as after 3 hours, compounds 9 and 10 were isolated in duction conditions are compatible with the presence of a
74% yield in a ratio of 68/32.11 It is worth noting that com- benzyl protecting group as D-glucal (14) was converted to
pounds 9 and 10 could not be separated. However, after the corresponding dideoxy D-glucose derivative 19 in
cleavage of the acetate using a catalytic amount of 92% yield without detectable loss of the benzyl groups
NaOMe in MeOH, the corresponding phenol derivatives (Scheme 4, eq 4). The fact that trisubstituted olefins are
9¢ and 10¢ were obtained in quantitative yield, separated inert under the reaction conditions can be used advanta-
and characterized (Scheme 3).
geously in chemoselective reductions (Scheme 4, eq 5).
For example, triene 15 was converted to the selectively re-
duced product 20 with the concomitant silylation of the
free alcohol in 63% yield (Scheme 4, eq 5). We have to
point out that the stereochemistry of the trisubstituted
double bond remained unchanged under the reaction
conditions.11
1 (2.5 mol%)
OH
TESO
TESH (3.0 equiv)
O
O
O
CH2Cl2, 40 °C, 16 h
(72%)
O
11
16
(eq 1)
(eq 2)
1 (15 mol%)
The reduction conditions of alkenes by TESH in the
presence of catalyst 1 can be applied to the selective trans-
formation of a,b-unsaturated carbonyl compounds to the
corresponding saturated products (Scheme 5).13 In these
transformations a catalytic amount of 1 (2.5 mol%) and
TESH (2.5 equiv) was used either in refluxing CH2Cl2 or
at room temperature for 12–48 hours.14
TESH (7.5 equiv)
AcO ( )17
AcO ( )8
( )8
CH2Cl2, 40 °C, 24 h
(76%)
12
17
1 (2.5 mol%)
TESH (2.5 equiv)
N
N
Ts
Ts
CH2Cl2, 40 °C, 4 h
Cl
Cl
18a
(60%)
1 (2.5 mol%)
13
O
O
TESH (2.5 equiv)
N
TES
N
TES
CH2Cl2, 40 °C, 24 h
(70%)
MeO
MeO
Ts
Ts
25
21
(eq 1)
Cl
Cl
18c
(eq 3)
18b
18b/18c = 10/1
(23%)
1 (2.5 mol%)
O
O
1 (2.5 mol%)
TESH (2.5 equiv)
O
O
TESH (2.5 equiv)
BnO
BnO
CH2Cl2, r.t., 12 h
(73%)
Ph
NHSO2Ph
Ph
NHSO2Ph
BnO
BnO
CH2Cl2, 40 °C, 72 h
(92%)
OBn
26
OBn
22
14
19
syn/anti = 78/22
(eq 4)
(eq 2)
1 (5.0 mol%)
1 (2.5 mol%)
O
O
TESH (5.0 equiv)
TESH (4.0 equiv)
MeO
MeO
TESO
HO
(Z/E = 28/72)
CH2Cl2, r.t., 48 h
(64%)
Ph OTES
CH2Cl2, 40 °C, 2.5 h
(63%)
Ph OH
23
15
27
20
(Z/E = 27/73)
(eq 5)
syn/anti = 20/80
(eq 3)
Scheme 4
1 (5.0 mol%)
O
O
TESH (5.0 equiv)
MeO
The reaction is general and the results are reported in
Scheme 4. Olefins 11–15 were treated with catalyst 1
(2.5–15 mol%) and TESH (3.0–7.5 equiv) in refluxing
CH2Cl2.12 Compound 11, having a terminal olefin, was
converted to the corresponding saturated compound 16 in
72% yield [1 (2.5 mol%), TESH (3.0 equiv), 16 h
(Scheme 4, eq 1)]. The reduction of disubstituted olefins
such as 12 was more sluggish (24 h) and required 15
mol% of catalyst to afford 17 in 76% yield (Scheme 4, eq
MeO
CH2Cl2, r.t., 48 h
(60%)
OTES
OH
24
28
syn/anti = 13/87
(eq 4)
Scheme 5
2). It is worth noting that unsaturated aryl chloride deriv- After 24 hours, compound 21 was transformed to ketone
atives can be reduced without concomitant dehalogena- 25 in 70% yield (Scheme 5, eq 1). The reduction of the
tion under the reaction conditions. The reduction of olefin ketone under the reaction conditions was not observed.
13 afforded the saturated compound 18a in 60% yield, The diastereoselectivity of the transformation was studied
accompanied by a small amount of 18b and 18c (23%) in the reduction of acyclic enone 22 and a,b-unsaturated
Synlett 2005, No. 16, 2449–2452 © Thieme Stuttgart · New York

LETTER
Reduction of Olefins
2451
esters 23 and 24 (Scheme 5, eq 2–4). The saturated way. Accordingly, the addition will provide 48, which in
products 26, 27 and 28 were formed in 73%, 64% and the presence of Si–H is reduced, and gives raise to the
60% yield, respectively. We have to point out, that syn se- alkane and LnRuSiR¢3. The starting RuH is regenerated by
lectivity was observed in reducing amide 22 (dr = 78/22), further reduction with the silane, forming disilane as a by-
while the anti products were formed as the major isomers product.
from alcohols 23 and 24 (dr = 20/80 and 13/87).15
P(Cy)3
Ru
H
Furthermore, the ability in mediating tandem metathesis
reaction–reduction under the reaction conditions was test-
ed. Even in the presence of trialkylsilanes the metathetic
activity of ruthenium complex 1 was retained, as 29 was
transformed to tetrahydropyran 30 in 75% yield in a one-
pot cyclization–reduction sequence using a catalytic
amount of 1 (5 mol%) and TESH (5 equiv) in refluxing
CH2Cl2 for 24 hours (Scheme 6, eq 1).16 Likewise, N-
tosyl-N,N-diallylamine (31) was converted to the pyrrol-
idine derivative 32 in a one-pot sequence in 76% yield
(Scheme 6, eq 2). It is interesting to compare the efficien-
cy of this transformation with the sequential addition of
the carbene catalyst 1 followed by the addition of the tri-
ethylsilane reagent. The two processes afforded 32 in a
comparable yield (76% vs. 88%, Scheme 6, eq 2 and 3).
SiR'3
Cl
Cl
R'3SiH
slow
LnRu
R
R'3SiH
(Cy)3P
1'
46
R'3SiH
H
H
H
RuLn
R
R
49
48
R'3SiRuLn
R'3SiH
HRuLn
R'3SiRuLn
R'3SiSiR'3
Scheme 7
1 (5 mol%)
A secondary reaction is the dimerization of the silanes
forming disilanes or disiloxanes and molecular H2, when
water is present (Scheme 8). In fact, in all the reactions the
corresponding disiloxane was the major secondary prod-
uct, suggesting that water is required for the process. Due
to this competing reaction an excess of silane is necessary
for completing the desired reduction.
TESH (5 equiv)
(eq 1)
CH2Cl2, 40 °C, 24 h
O
O
(75%)
O
O
30
29
O
O
1 (10 mol%)
TESH (2.5 equiv)
ROH
N
N
CH2Cl2, 40 °C, 10 h
(76%)
(eq 2)
HRuLn
Ts
Ts
32
31
ROSiR'3
R'3SiH
R'3SiRuLn
HRuLn
1 (10 mol%)
TESH (2.5 equiv)
31
32
R'3SiH
H2
N
CH2Cl2
CH2Cl2, 40 °C, 5 h
(88%)
HRuLn
Ts
40 °C, 1 h
(eq 3)
33
R'3SiSiR'3
Scheme 6
Scheme 8
The mechanism of the transformation is the subject of
speculation. It is difficult to account all the observed
events to a single organometallic complex, and they are
probably the consequence of competing processes. Unfor-
tunately, we were unable to characterize any of these com-
plexes by different 1H NMR techniques, and at present we
can only speculate on the plausible intermediates involved
in the process. The first step of the mechanism can be the
addition of the silane to the metal-carbene, which produc-
es 46 (Scheme 7). The methylidene complex 1¢ derived
from 1 reacts probably slowly with silanes under the reac-
tion conditions, which would explain the sustained meta-
thetic activity of the mixture even after extended reaction
time. The addition of LnRuH to olefins can then follow a
classical oxidative addition–reductive elimination path-
In summary, a method allowing the selective reduction of
non-activated, and activated olefins in the presence of ru-
thenium–carbene catalysts and silanes was presented. The
selectivity of the reduction is essentially steric: terminal
olefins are reduced preferentially in the presence of di-,
tri- or tetrasubstituted olefins. This selectivity, and the fact
that no molecular hydrogen at higher pressure is required
should be of interest in synthesis. The ability in mediating
sequential transformations such as ring-closing metathe-
sis and reductions contributes to the fascinating array of
reactions of the ruthenium carbene complexes. The study
of the mechanism and the structure of the active complex
are the subject of further investigations and will be report-
ed in due course.
Synlett 2005, No. 16, 2449–2452 © Thieme Stuttgart · New York

2452
C. Menozzi et al.
LETTER
(11) Product ratio was determined by GC-MS.
References
(12) 2,6-Dimethyl-8-triethylsilyloxyundec-2,6-diene (20).
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0.65 mmol, 63%); Rf = 0 .57 (pentane–EtOAc, 95:1). IR
(neat): 1475, 1400, 1260, 1080, 760 cm–1. 1H NMR (300
MHz, CDCl3, E- and Z-isomers): d = 5.10–5.00 (m, 2 H),
4.30–4.20 (m, 1 H), 2.05–1.89 (m, 4 H), 1.61 (dd, J = 4.9, 1.1
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0.87 (t, J = 7.9 Hz, 9 H), 0.84 (t, J = 6.4 Hz, 3 H), 0.49 (q,
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134.4, 131.4, 129.5, 124.1, 69.3, 40.9, 39.6, 26.3, 25.6, 18.6,
17.5, 16.4, 14.1, 6.8, 5.0. Z-Isomer: d = 134.6, 131.7, 130.3,
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(14) 4-(4-Methoxyphenyl)butan-2-one (25).
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(0.10 mL, 0.70 mmol, 2.5 equiv) and catalyst 1 (5 mg, 7 mol,
2.5 mol%). The resulting solution was stirred until total
conversion of the starting material. The solution was
concentrated under reduced pressure, and the crude product
was purified by flash chromatography on silica gel using a
gradient of eluent (pentane–EtOAc). Colorless oil (30 mg,
0.17 mmol, 70%); Rf = 0.63 (pentane–EtOAc, 4:1). IR
(neat): 1720, 1610, 1510, 1250, 1035 cm–1. 1H NMR (300
MHz, CDCl3): d = 7.05 (d, J = 8.7 Hz, 2 H), 6.75 (d, J = 8.7
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[M+], 121 (100).
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Synlett 2005, No. 16, 2449–2452 © Thieme Stuttgart · New York