836
Chemistry Letters Vol.35, No.8 (2006)
Selective Hydrosilylation of 1-Alkynes Using Iridium Catalyst with Biphosphinine Ligand
Yoshihiro Miyake, Eigo Isomura, and Masahiko Iyodaꢀ
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397
(Received April 28, 2006; CL-060506; E-mail: iyoda-masahiko@c.metro-u.ac.jp)
The iridium-catalyzed hydrosilylation of alkynes in the
typical results are shown in Table 1. After some preliminary
experiments, we found that [Ir(COD)2][BF4] and [Ir(ꢃ5-C9H7)
(COD)] (ꢃ5-C9H7 = ꢃ5-indenyl) are ideal catalysts for the hy-
drosilylation of alkynes. Thus, the treatment of 1 equiv. of 1a
with 0.5 equiv. of PhMe2SiH (7) in THF at 40 ꢁC for 24 h in
the presence of [Ir(COD)2][BF4] (0.005 equiv.) produced corre-
sponding vinylsilanes (4a, 5a, and 6a) in 94% total yield with ꢀ-
(Z)-5a as the major product (Table 1, Entry 1). However, 3
retarded the iridium-catalyzed hydrosilylation of 1a, and the
reaction of 1a with 7 under similar conditions led to the recovery
of 1a (95%) (Entry 2). In contrast, the hydrosilylation of 1a with
7 in the presence of [Ir(ꢃ5-C9H7)(COD)] gave different results.
Although the reaction of 1a with 7 in the presence of [Ir(ꢃ5-
C9H7)(COD)] (0.005 equiv.) in toluene at 80 ꢁC for 24 h formed
the vinyl silanes 4a–6a in 34% total yield with Z-selectivity
(Entry 3), a similar reaction in the presence of 0.006 equiv. of
tmbp 3 produced the vinyl silanes in 96% total yield with 4a
as the major product (Entry 4). The reactions of 1a with 7
(1–2 equiv.) in the presence of [Ir(ꢃ5-C9H7)(COD)] (0.005
equiv.) and 3 (0.006 equiv.) also produced the vinyl silanes in
96–97% with a high selectivity of 4a (89–93%) (Entries 5 and
6). The results in Table 1 show that the combination of [Ir(ꢃ5-
C9H7)(COD)] with 3 in toluene at 80 ꢁC produces vinylsilanes
in high yield with high E-selectivity, whereas the iridium-
catalyzed hydrosilylation of 1a with 7 in the absence of 3 forms
vinylsilanes in moderate to high yield with Z-selectivity.
presence of 4,40,5,50-tetramethylbiphosphinine (tmbp) has been
explored. The hydrosilylation of alkynes in the presence of tmbp
proceeds effectively to give ꢀ-(E)-vinylsilanes highly selective-
ly in moderate to high yields, whereas a similar hydrosilylation
in the absence of tmbp produces ꢀ-(Z)-vinylsilanes selectively.
The stereoselectivity of these reactions suggests the importance
of the electron-withdrawing properties of tmbp coordinated to
iridium.
The hydrosilylation of alkynes with transition-metal cata-
lysts is a simple and valuable method for the synthesis of alkenyl
silanes, which are widely used for organic synthesis.1 Thus,
regio- and stereoselective reactions are indispensable for the
synthetic use of alkenyl silanes. For the hydrosilylation of the
terminal alkyne 1 with the silane 2, three possible alkenylsilanes
can be obtained, i.e., the ꢀ-(E)-adduct 4 (syn-addition), the ꢀ-
(Z)-adduct 5 (anti-addition), and the ꢁ-adduct 6 (Scheme 1).
As has been reported,2 the anti-addition product 5 is formed
by the insertion of the alkyne into an M–Si bond, followed by
the isomerization of a vinyl complex2a to give 5 via a less steri-
cally hindered intermediate. In addition to the steric effect, elec-
tron density at metal centers strongly affects the regioselectivity
of the hydrosilylation of alkynes.2b,3,4
Ligands containing sp2-hybridized phosphorus atom have a
strong ꢂ-acceptor property because of their remarkable tendency
to engage in metal-to-phosphorus ꢂ back donation, comparable
to that of the carbonyl ligand.5 Thus, these compounds are
expected to decrease the electron density at metal centers.
Recently, the characteristic catalytic activities of transition-
metal complexes with diphosphinidene–cyclobutene ligands
have been reported.6 Although phosphinines, which are phos-
phorus analogues of pyridines, function as ꢂ-acceptor ligands,
catalytic transformations using transition-metal complexes
bearing phoshinine ligands have been limited until now.5a,5e,7
We report herein the iridium-catalyzed hydrosilylation of 1
with a high stereoserectivity using 4,40,5,50-tetramethyl-2,20-bi-
phosphinine (tmbp, 3).8,9
On the basis of the optimized conditions (Table 1, Entry 5),
the hydrosilylation of various alkynes was examined (Table 2).10
Similarly to that of 1a, the reaction of phenylacetylene (1b) and
its derivatives 1c and 1d with 7 in the presence of [Ir(ꢃ5-
Table 1. Iridium-catalyzed hydrosilylation of 1-octynea
[Ir] cat.
α
6a
β
4a
-(E)
β
-(Z)
5a
PhMe2SiH
+
n-C6H13
(tmbp 3)
+
+
1a
7
Silane ratio/%b
Temp.
/°C
Yieldc
/%
Entry
Ir catalyst
Solvent
CH2Cl2
4a 5a 6a
1d
2
40
40
98
1
94
0
[Ir(COD)2][BF4]
1
First, the hydrosilylation of 1-octyne (1a) was attempted and
[Ir(COD)2][BF4]-3 CH2Cl2
Toluene
3d Ir(η5-Ind)(COD)
80
67 12
34
21
Ir(η5-Ind)(COD)-3 Toluene
Ir(η5-Ind)(COD)-3 Toluene
[Ir],
4
80
80
96
96
94
93
4
4
2
3
tmbp (3)
P
P
+
R
H
H
SiR'3
5e
6f
2
1
Ir(η5-Ind)(COD)-3 Toluene 80
H
H
SiR'3
89
7
4
97
+
+
H
SiR'3
H
R
aReaction conditions: 1a (1.0 mmol), 7 (0.5 mmol), and Ir com-
plex (0.005 mmol) in the presence or absence of 3 (0.006 mmol)
for 24 h. Ratio was determined on the basis of H NMR data.
R
R
SiR'3
-(Z)-5
H
H
b
1
β
-(E)-4
β
α
-6
d
e
cGLC yield based on 7. Without 3. 7 (1.0 mmol) was used.
Scheme 1. Iridium-catalyzed hydrosilylation.
f7 (2.0 mmol) was used.
Copyright Ó 2006 The Chemical Society of Japan
Miyake, Yoshihiro
