Synthesis of Linear Allylsilanes via Molybdenum-Catalyzed Regioselective Hydrosilylation of Allenes

Article abstract of DOI:10.1021/acscatal.6b00627

A simple molybdenum-based catalytic system for hydrosilylation of allenes has been developed. The reactions of mono- and disubstituted allenes with secondary and tertiary silanes proceeded smoothly and selectively to afford linear allylsilanes. The origin

Full text of DOI:10.1021/acscatal.6b00627

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Article
Synthesis of Linear Allylsilanes via Molybdenum-
Catalyzed Regioselective Hydrosilylation of Allenes
Sobi Asako, Sae Ishikawa, and Kazuhiko Takai
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00627 • Publication Date (Web): 14 Apr 2016
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Synthesis of Linear Allylsilanes via Molybdenum-Catalyzed Regiose-
lective Hydrosilylation of Allenes
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Sobi Asako,* Sae Ishikawa, and Kazuhiko Takai*
Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University,
3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530
ABSTRACT: A simple molybdenumꢀbased catalytic system for hydrosilylation of allene has been developed. The reactions of
monoꢀ and disubstituted allenes with secondary and tertiary silanes proceeded smoothly and selectively to afford linear allylsilanes.
The origin of the unprecedented linear selectivity was investigated by density functional theory studies to reveal that the reaction
consists of the following steps; 1) concerted hydromolybdation/Si–H oxidative addition from a Mo(CO)4/allene/silane adduct to
form πꢀallylmolybdenum, 2) allyl rotation from the initially formed πꢀallylmolybdenum to a thermodynamically more stable isoꢀ
mer, and 3) reductive elimination at the lessꢀhindered allyl carbon to afford a linear allylsilane.
KEYWORDS: molybdenum, hydrosilylation, allenes, linear allylsilanes, allyl rotation, density functional theory
Previous work
INTRODUCTION
cat. Pd
R'3Si
•
•
The hydrosilylation of unsaturated molecules such as alkenes,
alkynes, and dienes has matured into one of the most efficient
synthetic methods for the preparation of organosilanes, which are
versatile synthetic intermediates that participate in a wide range of
transformations.1,2 The major difficulty of the reaction resides in
the control of regioꢀ and stereoselectivity. For instance, the regioꢀ
and stereoselective hydrosilylation of internal alkynes is still a
nontrivial challenge,3 and it is only recently that the first 1,2ꢀ
hydrosilylation of 1,3ꢀdienes has been achieved.4 Hydrosilylation
of allenes is no exception. Despite the potential utility for the
selective synthesis of allylsilanes and alkenylsilanes, such reacꢀ
tions have been largely ignored, partly because of the difficulty in
selectively obtaining a single isomer among others. To date, only
a few examples of selective hydrosilylation reactions of allenes
that afford branched allylsilanes under Pd5a–c catalysis (Scheme
1a) and vinylsilanes under Ni,5a Pd,5b,c Al,5e or Au5f catalysis
(Scheme 1b) have been reported. Among them, the Pdꢀ and Niꢀ
based catalytic systems developed by Montgomery et al. are parꢀ
ticularly attractive and have recently been expanded to hydrosiꢀ
lylation of 1,3ꢀdisubstituted allenes.5d The following density funcꢀ
tional theory (DFT) studies on the mechanism of the Pdꢀ and Niꢀ
catalyzed regioselective hydrosilylations revealed that both the
metals and the size of NHC ligands are crucial for the reactivity
and regioselectivity of the reactions.6 Despite these recent adꢀ
vances, no general methods to access linear allylsilanes via allene
hydrosilylation are available.7 We report here a molybdenumꢀ
catalyzed hydrosilylation of allenes that selectively produces lineꢀ
ar allylsilanes with high Z selectivity (Scheme 1c). The developꢀ
ment of simple and efficient syntheses of Zꢀallylsilanes is still a
challenging task.8 We also performed DFT calculations and reꢀ
vealed the origin of the unique linear selectivity.
+
+
R'3SiH
R'3SiH
(a)
(b)
R
R
cat. Ni, Pd, Al, Au
SiR'3
R
R
This work
cat. Mo
•
+
R'3SiH
(c)
SiR'3
R
R
RESULTS AND DISCUSSION
Reaction Development.
We commenced our investigation with the identification of an
effective catalyst for hydrosilylation of 3ꢀbutylꢀ1,2ꢀheptadiene
with Ph2SiH2 (Table 1). We first examined rhenium carbonyl
complexes, Re2(CO)10 and ReCl(CO)5, because of our recent inꢀ
terest in their use as catalysts for organic synthesis,9 to find that
they showed catalytic activity for hydrosilylation, albeit in low
yields (entries 1 and 2). Further screening of various metal carꢀ
bonyl complexes led us to identify Mo(CO)6 as a particularly
efficient catalyst for the hydrosilylation of the allene that selecꢀ
tively afforded the linear allylsilane in 88% yield after performing
the reaction in toluene at 120 °C for 20 h (entry 5). While molybꢀ
denum carbonyl complexes have been reported to be efficient
catalysts for hydrosilylation of aldehydes, ketones,
α,βꢀ
unsaturated carbonyl compounds, norbornadiene, and 1,3ꢀ
dienes,10,11 to the best of our knowledge they have never been
used for the hydrosilylation of allenes.
Table 1. Screening of Metal Carbonyl Complexesa
Scheme 1. Regioselective Hydrosilylation of Allene
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nBu
nBu
nBu
catalyst (10 mol %)
toluene, 120 °C, 20 h nBu
stituted allene with a primary alkyl chain produced branched alꢀ
Ph2SiH2
(1.2 equiv)
•
+
1
2
3
4
5
6
7
8
lylsilane as the major product under the standard conditions (entry
13), the simple replacement of Mo(CO)6 catalyst with
Mo(CO)3(MeCN)3 restored the linear selectivity and the linear
allylsilane was selectively obtained together with a small amount
of other isomers (entry 14). It should be noted that the silylꢀ and
borylꢀsubstituted allylsilanes serve as versatile bifunctional reaꢀ
gents for the synthesis of complex molecules (entries 15 and
16).12 Branched allylsilanes, vinylsilanes, and other isomers were
either not observed or detected only in a trace amount except in
entries 8, 13, and 14.
SiHPh2
product / %b
entry
catalyst
1c
2
Re2(CO)10
ReCl(CO)5
Mn2(CO)10
Cr(CO)6
3
22
10
42
88
5
3c
4
5
Mo(CO)6
W(CO)6
9
6
10
11
12
13
14
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7
Fe(CO)5
2
Table 3. Molybdenum-Catalyzed Hydrosilylation of Al-
lenesa
8d
Ru3(CO)12
11
aThe reaction was performed on a 0.2 mmol scale in toluene (0.25 M) at
120 °C for 20 h. Determined by 1H NMR using 1,1,2,2ꢀtetrachloroethane
b
Yieldb / % (E / Z)c
entry
substrate
nBu
nBu
R3SiH
product
as an internal standard. cCatalyst (5 mol %) dCatalyst (3.3 mol %).
nBu
nBu
1
2
Ph2SiH2
80
63
•
PhMe2SiH
With this initial discovery, further optimization of the reaction
conditions was conducted using Mo(CO)6 as the catalyst (Table
2). First, the influence of reaction temperature was examined to
find that 100 °C was necessary for high conversion, probably to
remove carbonyl ligands and create open coordination sites for the
allene and silane substrates (entries 1–5). The catalyst loading and
reaction time could be reduced to 5 mol % and 12 h, respectively,
without decreasing the reaction efficiency, when the concentration
was increased from 0.25 M to 1.0 M, giving the allylsilane prodꢀ
uct in 91% yield (entry 9).
SiR3
•
3
4
Ph2SiH2
Ph2SiH2
65
SiHPh2
•
71 (45 / 55)
SiHPh2
Ph
Ph
•
5
Ph2SiH2
SiHPh2
65 (21 / 79)
•
Table 2. Optimization of Reaction Conditionsa
6
7
Ph2SiH2
Ph2SiH2
65 (7 / 93)
Si
Si
SiHPh2
SiHPh2
nBu
nBu
nBu
nBu
Mo(CO)6 (x mol %)
toluene
nC5H11
•
+
Ph2SiH2
nC5H11
nC5H11
•
70 (15 / 85)
SiHPh2
nC5H11
(1.2 equiv)
83 (13 / 87)d
53 (8 / 92)
product / %b
8
9
Ph2SiH2
Ph3SiH
Et3SiH
x
entry
1
conc / M
time / h temp / °C
•
10
10
10
10
10
0.25
0.25
0.25
0.25
0.25
20
20
20
20
20
80
13
SiR3
10
11
12
52 (10 / 90)
2
3
4
5
90
100
110
120
57
87
91
88
BnMe2SiH
PhMe2SiH
72 (15 / 85)
78 (9 / 91)
13
14
Ph2SiH2
Ph2SiH2
SiHPh2 21 (<1 / >99) + 45e
53 (8 / 92) + 3f,g
•
R
6
7
8
9
10
5
0.25
0.25
0.5
12
12
12
12
110
110
110
110
81
75
83
91
+
R
Ph
SiHPh2
5
•
15
16
PhMe2SiH
PhMe2SiH
78 (20 / 80)
PhMe2Si
SiMe2Ph
PhMe2Si
5
1.0
•
aThe reaction was performed on a 0.2 mmol scale. bDetermined by 1H
NMR using 1,1,2,2ꢀtetrachloroethane as an internal standard.
B
SiMe2Ph
B
O
O
61 (30 / 70)
O
O
aReaction conditions: allene (0.2 mmol), R3SiH (1.2 equiv), and Mo(CO)6
(5 mol %) in toluene (0.2 mL) at 110 °C for 12 h. bIsolated yields. cDeterꢀ
Scope of the Allene Hydrosilylation.
With the optimized conditions in hand, the scope of the moꢀ
lybdenumꢀcatalyzed hydrosilylation reaction was investigated
(Table 3). The reactions of 1,1ꢀdisubstituted allenes proceeded
smoothly and selectively to afford linear allylsilanes (entries 1–6).
When unsymmetrically substituted allenes were used, allylsilanes
were obtained as a mixture of E and Z isomers, where the ratio of
the Z product increased as the size difference between the two
substituents increased (entries 4–6). A 1,3ꢀdisubstituted allene also
took part in this reaction (entry 7). Monosubstituted allenes bearꢀ
ing an alkyl, silyl, or boryl substituent reacted equally well with
secondary and tertiary silanes to give the corresponding linear
allylsilanes with Z selectivity (entries 8–16). Although monosubꢀ
1
d
mined by H NMR and GLC. Branched allylsilane was obtained in 3%
e
yield. Linear allylsilane (21%) was obtained as a mixture with branched
f
allylsilane (45%). Mo(CO)3(MeCN)3 was used instead of Mo(CO)6 at
60 °C. gLinear allylsilane (53%) was obtained as a mixture with branched
allylsilane (3%) and alkenylsilane (5%).
Allene Hydrosilylation with Alkynylsilane.
The present reaction conditions enabled the unprecedented
chemoselective hydrosilylation of allene with alkynylsilane (Eqs 1
and 2). Thus, the reactions of cyclohexylallene with monoꢀ and
bis(alkynyl)silanes proceeded equally efficiently to afford the
alkynylallylsilane and bis(alkynyl)allylsilane, respectively, with
linear and Z selectivity. These reactions constitute a rare example
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under UV irradiation (250–385 nm). bDetermined by 1H NMR in the presꢀ
ence of 1,1,2,2ꢀtetrachloroethane as an internal standard. Determined by
GLC.
The new conditions under UV irradiation were applied to othꢀ
er substrates, all of which afforded the corresponding linear alꢀ
lylsilanes with improved Z selectivity (Figure 1).
of hydrosilylation of a carbon–carbon double bond in the presence
c
of an alkyne13 and provide a novel synthetic entry to alkynylalꢀ
lylsilanes, which serve as useful substrates for further transforꢀ
mations.14 To date, there have been few methods available for the
synthesis of alkynylallylsilanes bearing a substituent at the allyl
moiety.14,15
1
2
3
4
5
6
Me2
Si
•
Mo(CO)6 (5 mol %)
B
O
SiMe2Ph
O
Ph
SiMe2Ph
+
Ph
SiHMe2
(1.2 equiv)
(1)
SiHPh2
toluene, 110 °C, 12 h
7
8
66% (E / Z = 6 / 94)
73% (4 / 96)
67% (18 / 82)
62% (27 / 73)
9
Me
Si
•
Mo(CO)6 (5 mol %)
Ph
10
11
12
13
14
15
16
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45
46
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50
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52
53
54
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56
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58
59
60
Figure 1. Examples of the MolybdenumꢀCatalyzed Allene Hyꢀ
drosilylation under UV Irradiation. Reactions in Et2O at 25 °C for
1 h under UV Irradiation.
2
+
(2)
Ph
SiHMe
2
toluene, 110 °C, 12 h
(1.2 equiv)
63% (E / Z = 8 / 92)
Reactions under UV irradiation.
The optimized conditions using Mo(CO)6 as catalyst require
high temperature for smooth reaction (vide supra). To lower the
reaction temperature and thereby improve the selectivity, we perꢀ
formed the reaction at room temperature under UV irradiation for
the removal of carbonyl ligands (Table 4).16 As expected, the
reaction proceeded much faster in toluene and reached completion
within 1 h with improved Z selectivity, albeit in lower yield (enꢀ
tries 1–3). The reaction under UV irradiation promoted product
isomerization, and prolonged reaction time resulted in lower Z
selectivity (entry 3). The milder conditions at room temperature
allowed us to examine the reactions in solvents possessing lower
boiling points, among which Et2O and CH2Cl2 showed the best
results in terms of yield and selectivity (entries 6 and 7).
Computational Studies.
To understand the origin of the unprecedented selectivity for
linear allylsilanes, DFT calculations (B3LYPꢀD3 and M06) were
performed using cyclohexylallene and Me3SiH as a model subꢀ
strate. The predicted energy trends were consistent between the
two methods. Therefore, we discuss the free energies calculated at
the B3LYPꢀD3/SDD:6ꢀ31G(d) level, unless otherwise noted.
First, relevant molybdenum carbonyl complexes and their adducts
with an allene and a silane at the early stage of the catalytic cycle
were investigated (Table 5). The dissociation of one and two CO
ligands is endergonic by 31.2 and 62.0 kcal/mol, respectively.17
The resulting Mo(CO)5 and Mo(CO)4 complexes can be stabilized
either by coordination of an allene (1, 2) or a silane (3, 4) but only
to a small extent, insufficient for compensating the loss of CO
ligands. This would explain why high temperature is necessary for
the thermal reaction, while room temperature is enough once CO
ligands are removed by UV irradiation. We could not locate both
an allene and a silane on Mo(CO)5 at the same time to form a
sevenꢀcoordinate complex, nor find a transition state leading to a
Table 4. Molybdenum-Catalyzed Hydrosilylation under UV
Irradiationa
•
Mo(CO)6 (5 mol %)
SiMe2Ph
+
PhMe2SiH
(1.2 equiv)
hν, 25 °C
product / %b
E / Zc
πꢀallyl intermediate such as Mo(CO)5(allyl)(SiMe3). Instead, a
entry
solvent
time
concerted Si–H oxidative addition/allene hydromolybdation tranꢀ
sition state (TS) from the Mo(CO)4/allene/Me3SiH adduct 6 was
found (vide infra), which has been similarly proposed in the Pdꢀ
catalyzed hydrosilylation of allenes.6 The oxidative addition of the
Si–H bond in Mo(CO)4/Me3SiH complex 4 forms molybdenum
hydride 5. Attempts to locate an allene hydromolybdation TS
from 5 and an allene resulted in the same TS as the concerted one
from 6. Therefore, in the following sections, we used
Mo(CO)4/allene/Me3SiH adduct 6a and its analogues as a starting
point of the reaction.
1
2
3
4
toluene
toluene
toluene
hexane
10 min
1 h
34
70
67
58
6 / 94
6 / 94
12 h
1 h
16 / 84
6 / 94
5
6
7
THF
Et2O
1 h
1 h
1 h
27
73
74
3 / 97
4 / 96
4 / 96
CH2Cl2
aReaction conditions: allene (0.2 mmol), PhMe2SiH (1.2 equiv), and
Mo(CO)6 (5 mol %) in solvent (0.2 mL) were stirred at 25 °C for 1 h
Table 5. Calculated Energies of Molybdenum Carbonyl Complexes and Their Adducts with Cyclohexylallene and Me3SiHa
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aGibbs free energies and enthalpies (italicized) in kcal/mol calculated at the B3LYPꢀD3/SDD:6ꢀ31G(d) level.
1
2
Scheme 2. Reaction Pathways of Molybdenum-Catalyzed Allene Hydrosilylation
3
4
5
6
7
8
9
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(44.1 kcal/mol). Thus, it is lowꢀbarrier rotational isomerization to
the more stable ꢀallyl intermediate that plays a key role in deterꢀ
mining the selectivity for linear allylsilane.
Scheme 2 shows four major reaction pathways that we considꢀ
ered in this study. The first two paths (A and B) involve a hydroꢀ
molybdation TS concerted with Si–H bond oxidative addition,
whereas the latter two (C and D) involve a silamolybdation TS
concerted with Si–H bond oxidative addition. Depending on the
regioselectivity of the allene insertion, we further divided each of
π
In contrast to the linear selectivity observed in the hydrosilylaꢀ
tion with cyclohexylallene, branched allylsilane was selectively
obtained when an allene bearing a smaller substituent was used in
the presence of Mo(CO)6 (Table 3, entry 13). To explain this selecꢀ
tivity switch, similar calculations were performed using methylꢀ
allene as a substrate. Important parts of the energy profiles for
hydrosilylation of cyclohexylallene and methylallene starting from
7a (R = Cy) and 7a′ (R = Me) are shown in Figure 3 (see SI for
the complete results). When the Cy substituent was replaced with
Me, the barrier for reductive elimination leading to branched alꢀ
them into two: one leading to a πꢀallylmolybdenum intermediate
and the other leading to an alkenylmolybdenum intermediate. It
should be noted that each of the four paths still has several isomerꢀ
ic structures when the allene has a substituent. We found that the
reaction takes place preferentially via path A and affords a linear
allylsilane through reductive elimination from the thermodynamiꢀ
cally more stable
πꢀallyl intermediate, which is accessible by allyl
lylsilane decreased by 5.1 kcal/mol (ꢀG‡
= 14.6
RE_branch_Cy
rotation of the initially formed
πꢀallyl intermediate.
kcal/mol from 7a and ꢀG‡
= 9.5 kcal/mol from 7a′)
RE_branch_Me
because of the reduced steric repulsion, whereas the barrier for
A. Hydromolybdation Concerted with Si–H Oxidative Addi-
tion ( -Allylmolybdenum)
rotational isomerization leading to linear allylsilane increased by
π
1.5 kcal/mol (ꢀG‡
= 10.9 kcal/mol and ꢀG‡
= 12.4
rot_Cy
rot_Me
A summary of the reaction mechanism via path A and the enꢀ
ergy profiles are shown in Figures 2 and 3, respectively. Four
routes were taken into consideration starting from
Mo(CO)4/allene/Me3SiH complexes 6a–6d via TSA featuring
concerted hydromolybdation/Si–H oxidative addition to the correꢀ
kcal/mol). Thus, the reaction with methylallene has a smaller barꢀ
rier for branched productꢀproducing reductive elimination than
that for allyl rotation, and therefore affords branched allylsilane,
which is consistent with the experiment. The two systems have
similar geometrical parameters during the allyl rotation
sponding
πꢀallylmolybdenum complexes 7a–7d. Among them, the
(7a/TS7a7c/7c vs 7a′/TS7a 7c /7c′) except that the Mo–Callyl_sub bond
′
′
path from 6a to 7a was found to have the lowest transition state,
TSA1 (52.5 kcal/mol), which is least sterically demanding with a
cyclohexyl substituent pointing away from the metal center. The
Si–H and C–H bond lengths in TSA1 (1.95 and 1.55 Å, respectiveꢀ
ly, Figure 4) indicate that the present system has an earlier transiꢀ
tion state compared with the Pd system (2.48 and 1.63 Å, respecꢀ
length is longer in 7a than 7a′ reflecting more steric crowding in
the former (2.54 and 2.48 Å, respectively, Figures 4 and 5). The
larger driving force for the allyl rotation with a Cy substituent
(ꢀGrot_Cy = –2.5 kcal/mol and ꢀGrot_Me = –0.7 kcal/mol) gained by
releasing the steric hindrance would explain the lower rotational
barrier with the Cy substituent.
Another experimental observation is that the use of
Mo(CO)3(MeCN)3 instead of Mo(CO)6 resumed linear selectivity
for the reaction using an allene with a small substituent (Table 3,
tively).6 The
πꢀallyl intermediates 7a–7d undergo reductive elimiꢀ
nation to afford branched allylsilane, linear Zꢀallylsilane, and lineꢀ
ar Eꢀallylsilane, respectively, where the transition states leading to
branched allylsilane from 7a and 7b are higher in energy by as
much as 10 kcal/mol than those leading to linear allylsilane from
entry
14).
DFT
calculations
with
the
Mo(MeCN)4/methylallene/Me3SiH system predicted a higher
activation barrier for branched productꢀproducing reductive elimiꢀ
nation (TS7a′′RE = 14.0 kcal/mol) than allyl rotation (TS7a′′7c′′ =
7c and 7d. Because πꢀallylmolybdenum 7a–7d are interconnected
via allyl rotation18 or anti–syn isomerization, 7a has a chance to
isomerize to the more stable 7c via TS7a7c (40.4 kcal/mol), which
then undergoes reductive elimination via the lower TS7cRE (34.3
kcal/mol), leading to an experimentally observed linear Zꢀ
allylsilane, before producing a branched allylsilane via TS7aRE
12.8 kcal/mol) leading to the linear product, and thus well reproꢀ
duced this phenomenon (see SI).19
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Figure 2. MolybdenumꢀCatalyzed Hydrosilylation of Cyclohexylallene and Me3SiH via Path A (Energies in kcal/mol)
Figure 3. Energy Profiles for MolybdenumꢀCatalyzed Allene Hydrosilylation via Path A (Energies in kcal/mol)
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Figure 4. Selected Optimized Structures of Molybdenum Complexes Involved in Path A (R = Cy) Calculated at the B3LYPꢀ
D3/SDD:6ꢀ31G(d) Level (Color code: gray, carbon; white, hydrogen; red, oxygen; blue, silicon; green, molybdenum)
Figure 5. Selected Optimized Structures of Molybdenum Complexes Involved in Path A (R = Me) Calculated at the B3LYPꢀ
D3/SDD:6ꢀ31G(d) Level (Color code: gray, carbon; white, hydrogen; red, oxygen; blue, silicon; green, molybdenum)
one of which has the substituent pointing away from the Mo cenꢀ
B. Hydromolybdation Concerted with Si–H Oxidative Addi-
tion (Alkenylmolybdenum)
ter and hence is the most favorable among the others (Figure 6).
Complexes 6e and 6f form alkenylmolybdenum 8e and 8f via
concerted hydromolybdation/Si–H oxidative addition, which ultiꢀ
Next, two reaction paths via path B were examined, the first
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mately affords the corresponding alkenylsilane and vinylsilane.
The highest transition states lie in the alkenyl–Si reductive elimiꢀ
lybdation/Si–H oxidative addition from Mo(CO)4/allene/silane
adduct to form ꢀallylmolybdenum via path A, 2) allyl rotation
from the initially formed ꢀallylmolybdenum to a thermodynamiꢀ
cally more stable isomer, and 3) reductive elimination at the lessꢀ
hindered allyl carbon to afford a linear allylsilane. Further studies
on molybdenumꢀcatalyzed selective hydrosilylation of unsaturated
molecules are ongoing.
1
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6
7
π
nation step (TS9eRE: 54.2 kcal/mol) and the initial step (TSB2: 56.7
kcal/mol), respectively, both of which are higher in energy than
TSA1 (52.5 kcal/mol) in path A. The computational result is conꢀ
sistent with the experimental observation that alkenylsilanes and
vinylsilanes were not produced under the current reaction condiꢀ
tions with Mo(CO)6 catalyst.
π
8
9
COMPUTATIONAL METHODS
All calculations were performed with the Gaussian 09 proꢀ
gram package.21 The density functional theory (DFT) method was
employed using the B3LYP22 and M0623 hybrid functionals. Strucꢀ
tures were optimized with a basis set consisting of the Stuttgart–
Dresden (SDD) basis set and effective core potential (ECP) for
Mo24 and 6ꢀ31G(d)25 for the rest. To take into account the disperꢀ
sion contributions, we performed B3LYP calculations with
Grimme’s D3 parameter set as implemented in Gaussian 09.26
Each stationary point was adequately characterized by normal
coordinate analysis (no imaginary frequencies for an equilibrium
structure and one imaginary frequency for a transition structure)
and thermal corrections were calculated under standard conditions
(298.15 K, 1 atm). Cartesian coordinates and energies of all the
reported structures are given in the SI. Visualizations of the comꢀ
plexes in Figures 4 and 5 were obtained using CYLView.27
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Figure 6. MolybdenumꢀCatalyzed Hydrosilylation of Cycloꢀ
hexylallene and Me3SiH via Path B (Energies in kcal/mol)
ASSOCIATED CONTENT
Supporting Information
C and D. Silamolybdation Concerted with Si–H Oxidative
Addition
Finally, concerted silamolybdation/Si–H oxidative addition
paths C and D were investigated (Figure 7). All four transition
states considered are much higher in energy than TSA1, excluding
the possibility of these mechanisms operating. Unlike the linear
alignment of Si–H–C atoms in TSA (Figure 4) and TSB, H–Si–C
bond angles are 73.7–89.2 degrees in TSC and TSD because of the
bulkiness of a SiMe3 group compared with a hydrogen atom. The
steric effect is also reflected in the skewed relation between the
Mo–Si bond and the C=C double bond, which makes their interacꢀ
tion weak and hence renders silamolybdation unfavorable.
Experimental procedures and physical properties of the comꢀ
pounds. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*asako@cc.okayamaꢀu.ac.jp
*ktakai@cc.okayamaꢀu.ac.jp
ACKNOWLEDGMENT
We thank MEXT for financial support (KAKENHI GrantꢀinꢀAid
for Scientific Research (A) No. 26248030 to K.T., GrantꢀinꢀAid
for Young Scientists (B) No. 15K21180 to S.A.). The generous
amount of computational time from the Research Center for Comꢀ
putational Science, Okazaki National Research Institute, is grateꢀ
fully acknowledged.
Figure 7. MolybdenumꢀCatalyzed Hydrosilylation of Cycloꢀ
hexylallene and Me3SiH via Paths C and D (Energies in
kcal/mol)
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In conclusion, we have developed a simple molybdenumꢀ
based catalytic system for the hydrosilylation of allene with unꢀ
precedented selectivity for linear allylsilane. The chemoselective
allene hydrosilylation in the presence of alkyne was demonstrated
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