Chemistry Reference
In-Depth Information
9.2.2. Asymmetric Hydrosilylation of 1,3 - Dienes
Palladium-catalyzed asymmetric hydrosilylation of 1,3-dienes with hydrosilanes contain-
ing electron-withdrawing substituents on silicon atom gives another synthetically useful
chiral organosilanes (Scheme 9.8). The hydrosilylation proceeds in a 1,4-fashion, giving
chiral allylsilanes (
35
), which are converted to homoallyl alcohols (
36
) on the reaction
with aldehyde [3,23] .
HO
Pd/L*
*
PhCHO/DMF
SiRCl
2
+
HSiRCl
2
Ph
0 C
34
35
36
Scheme 9.8.
Chiral monodentate phosphine ligands have been proven to be effective for the asym-
metric hydrosilylation of 1,3-dienes, just as for the reaction of styrenes.
Various monodentate ligands were studied for the asymmetric hydrosilylation of
cyclopentadiene. In the fi rst report, chiral ferrocenylphosphine ligand, (
R
) - (
S
) - ppfa (
11
)
was used to give the corresponding allylsilane of 25% ee [24a]. The enantioselectivity
was increased up to 60% ee by using ferrocenyl ligands
37
and
38
containing perfl uoro-
alkyl groups on the side chain [24b]. Some of (β -
N
- sulfonylaminoalkyl)phosphines
12
and
39
were also used for the asymmetric hydrosilylation [7d,25]. Of those ligands having
axial chirality, MeO - mop (
14
) and H - mop (
13
) were not effective for the hydrosilylation
of cyclopentadiene. Instead, the MeO-mop analogue having biphenanthryl skeleton
(mop - phen,
40
) showed 80% ee [26]. Ar-mop ligands
41
, in which an aryl group is
substituted at the 2′-position of the binaphthyl skeleton, were more effective to give
90% ee at − 20 ° C [27] . The Ar - mop ligand
42
having
n
-octyl groups at the 6- and 6′ -
positions showed higher catalytic activity than Ar-mop, lacking the long alkyl group
by enhanced solubility of catalysts in the reaction media [28]. The reaction proceeded
at a lower temperature to give the highest enantioselectivity for the hydrosilylation of
cyclopentadiene (Table 9.2 ).
The use of ferrocenylphosphine
11
and
45
, ( β -
N
- sulfonylaminoalkyl)phosphines
12
and
39
, mop - phen
40
, and mop ligands was also reported for the asymmetric hydrosi-
lylation of 1,3-hexadiene (
43
) (Scheme 9.9) [23,29]. Of those ligands, the dioctylated
Ar - mop ligand
42
was the most enantioselective to give 83% ee [27,28]. Notably, the
enantioselectivity was higher with phenyldifl uorosilane than with trichlorosilane or
methyldichlorosilane in the asymmetric hydrosilylation of 1,3-hexadiene catalyzed by
chiral ferrocenylphosphine
45a
[29]. A plausible mechanism of palladium-catalyzed
hydrosilylation of 1,3 - diene, including 1,4 -
cis
- addition and π - allylpalladium intermediate
46
, was proposed by the reaction with deuterium-labeled silane (DSiF
2
Ph) (Table 9.3 ).
Regioselectivity is an issue in palladium-catalyzed asymmetric hydrosilylation of
linear 1,3 - dienes (Scheme 9.10 ) [30] . In the reaction of 1 - phenyl - 1,3 - butadiene (
48)
with
ferrocenyl ligand
11
, a mixture of regioisomeric allylsilanes
49
and
50
was obtained in a
ratio of 94 to 6, with 64% ee and 30% ee, respectively [30]. Higher enantioselectivities
and regioselectivities were achieved by the use of Ar-mop ligand
41
and its dioctylated
derivative
42
[28]. In the reaction of alkyl-substituted 1,3-diene
52a
with ferrocenyl
