Organic Letters
Letter
Scheme 1. Hydrosilylation of Esters by Mn(I) Catalysts
similar complexes fac-[Mn(iPr P(CH ) PiPr )(CO) Br and
2
2 2
2
3
2
1
fac-[Mn(nPr P(CH ) PnPr )(CO) Br].
2
2 2
2
3
We set out to examine the catalytic activity of 1 under
various reaction conditions for the hydrosilylation of ester,
using benzyl benzoate as the standard substrate (Table 1). A
a
Benzoate under Different Reaction Conditions
b
en. 1 (mol %) silane (equiv) temp (°C) time (h)
yield (%)
c
1
2
3
4
5
6
7
8
9
1
1
2
1
1
1
1
1
1
1
1
1
1
PhSiH (2)
100
100
100
100
100
100
100
80
6
6
5
4
4
4
4
4
4
5
4
>99 (92)
>99 (91)
>99 (91)
95
51 (>99)
45 (>99)
46 (>99)
46 (>99)
53 (>99)
>99 (92)
93
3
c
c
PhSiH (2)
3
PhSiH (2)
conditions (Scheme 1). Thus pyrophoric hydride reagents for
conventional reduction can be avoided by using silane as a
much milder reagent. Because classical hydrogenation requires
an expensive high-pressure reactor, the present hydrosilylation
protocol is much more economical and is safer. In addition,
this hydrosilylation is highly selective, whereas the hydro-
genation of ester suffers poor selectivity.
3
PhSiH (2)
3
d
d
d
d
d
c
Ph SiH (2)
2
2
Ph SiH (2)
3
Et SiH (2)
3
PhSiH (2)
3
PhSiH (2)
90
100
100
3
0
1
PhSiH (1)
We selected xantphos for the synthesis of the corresponding
Mn(I) complex because it is readily available and is one of the
most used ligands for the development of a large variety of
3
PhSiH (1)
3
a
Reactions conducted in a pressure tube (10 mL) with 0.5 mmol S ,
1
2
0
b
metal catalysts. The neutral Mn(I) complex fac-[Mn-
xantphos)(CO) Br] (1) was synthesized in high isolated
0.5/1.0 mmol of silane, and 1/2 mol % of 1. Yields of P
determined by H NMR spectroscopy using ferrocene (0.1 mmol) as
an external standard. Isolated yields of P . NMR yields of P for 12
h reaction. Key parameters for each entry are indicated in bold.
1
were
1
(
3
c
d
yield (95%) by reacting the commercially available precursor
1
1
Mn(CO) Br with the commercially available bisphosphine
5
ligand xantphos at r.t. (Scheme 2). The resulting yellow-orange
complete conversion of benzyl benzoate (S ) to benzyl alcohol
1
3
(P ) was observed when S (0.5 mmol) was heated with
1
1
(
Molecular Structure Showing 50% Ellipsoid)
phenylsilane (1.0 mmol) to 100 °C for 6 h in the presence of 2
mol % catalyst 1 (entry 1). Reducing the catalyst loading to 1
mol % (entry 2) led to the complete conversion of S to P in
1
1
just 6 h with >90% isolated yield. Decreasing the reaction time
to 4 h gave P in 95% NMR yield (entry 4). We also tested
1
various secondary and tertiary silanes such as diphenylsilane
(
entry 5), triphenylsilane (entry 6), and triethylsilane (entry
7
), and we obtained P in approximately 40−50% yield in 4 h.
1
Therefore, phenylsilane proved to be the best one under the
reaction conditions. Thereafter, the hydrosilylation of S was
1
tested at lower temperatures. Upon lowering the temperature
to 80 (entry 8) and 90 °C (entry 9), a poorer yield
(
approximately 40−50%) of P1 was obtained in 4 h.
Thereafter, we varied the amount of silane used for the
hydrosilylation of S . The use of 1 equiv of phenylsilane led to
1
complex 1 is diamagnetic and was characterized by H, 13C,
1
the complete conversion of S to P (92% isolated yield) in just
1
1
3
1
and P NMR spectroscopy. The NMR spectra of 1 are
consistent with the overall C symmetry. The bisphosphine
ligand framework was clearly seen in the H and C NMR
spectra of 1. A broad resonance at 26.14 ppm appears in the
5 h at 100 °C with 1 mol % catalyst loading (entry 10).
Thereafter, we explored various additional esters for the
hydrosilylation of esters to alcohols based on the optimized
conditions (1 mol % catalyst loading, neat, 100 °C, 6 h) to
expand the substrate scope (Scheme 3).
s
1
13
3
1
P NMR spectrum, which is shifted downfield compared with
the free xantphos ligand (−17.97 ppm). Complex 1 was also
characterized by IR spectroscopy. Three strong CO stretching
vibrations at 2023, 1950, and 1909 cm− were observed in the
First, we tested various aromatic esters. Methyl and ethyl
benzoate derivatives (1a, 1b, 2a, 2b, 3, 4a, 4b) were reduced to
benzyl alcohol derivatives in high isolated yields (1a′: 93%,
1b′: 92%, 2a′: 83%, 2b′: 81%, 3′: 87%, 4a′: 87%, 4b′: 94%).
Another aromatic ester, methyl 2-furoate (5), was also easily
reduced to furfuryl alcohol (5′: 79%). This catalytic system is
also very effective for the reduction of phenyl acetates (6a, 6b)
to phenols (6a′: 96%, 6b′: 93%). Therefore, the catalytic
system is compatible with both electron-donating and electron-
1
21
IR spectrum, which is consistent with the previous reports.
The mass analysis of 1 shows that the peak at 716.1161
+
corresponds to [M − Br]. Complex 1 was further
characterized by single-crystal X-ray analysis (Scheme 2).
The geometry around manganese in 1 is distorted octahedral,
and the bond distances and bond angles are consistent with
B
Org. Lett. XXXX, XXX, XXX−XXX