Rate enhancement with a bowl-shaped phosphane in the rhodium-catalyzed hydrosilylation of ketones

Article abstract of DOI:10.1002/anie.200390331

Up to 154-fold rate enhancement (see picture) and higher yields are achieved with the bowl-shaped triarylphosphane ligand P(tm-tp)₃ compared with common phosphane ligands in Rh-catalyzed hydrosilylation of ketones. The superior performance of P(tm-tp)₃ over the similar P(tp)₃ is apparently related to the deeper bowl formed by the methyl-bearing terphenyl substituents in the former.

Full text of DOI:10.1002/anie.200390331

Angewandte
Chemie
R
R
B
B
R
R
R= Me: P(tm-tp)3
R= H: P(tp)3
P
A
3
Table 1: Effects of ligands in the hydrosilylation of cyclohexanone.[a]
Entry
Ligand[b]
Yield [%][c]
d]
1
2
P(tm-tp)3
P(tp)3
97 (13)[
[
[
d]
25 (<1)
7 (<1)
22
d]
3
4
5
6
7
8
9
PPh
3
P(2-furyl)3
P(o-tol)3
PMes3
PEt3
PCy3
PtBu3
32
25
<2
<2
31
High-Speed Hydrosilylation
[a] Reaction conditions: cyclohexanone (1.0 mmol), HSiMe Ph
2
(
1.2 mmol), [{RhCl(C H ) } ] (0.0050 mmol), ligand (0.020 mmol), tri-
2 4 2 2
decane (0.25 mmol, as an internal standard), benzene (1.0 mL), RT, 3h.
b] tol=tolyl, Mes=mesityl, Cy=cyclohexyl. [c] Yield of cyclohexanol by
GC after desilylation with HCl/MeOH. [d] In CH Cl as solvent.
Rate Enhancement with a Bowl-Shaped
Phosphane in the Rhodium-Catalyzed
Hydrosilylation of Ketones**
[
2
2
2
002.[6b] In the presence of catalytic amounts of P(tm-tp) and
3
Osamu Niyomura, Makoto Tokunaga, Yasushi Obora,
Tetsuo Iwasawa, and Yasushi Tsuji*
[
{RhCl(C H ) } ] (P/Rh = 2), the reaction proceeded
2 4 2 2
smoothly in benzene at room temperature over 3 h, and
cyclohexanol was obtained in 97% yield after desilylation
(Table 1, entry 1). In contrast, the same reaction with P(tp)3
afforded the product in only 25% yield (entry 2). Further-
more, other representative triarylphosphanes (entries 3–6)
and trialkylphosphanes (entries 7–9) were also much less
Since the nature of P ligands is very important in transition-
metal-catalyzed reactions, a wide variety of these ligands has
been designed to realize high catalytic activity and selectiv-
[
1]
ity. So far, most P ligands are rather small, and their design
and modification have hitherto been performed within close
proximity of the P atom. Recently, several large (nanosized)
phosphorus ligands were developed for transition-metal-
effective than P(tm-tp) . With these ligands (entries 2–9), the
3
reactions were sluggish at room temperature, and much
longer reaction times (40–500 h) were required to obtain the
products in good yields (70–95%). A kinetic study indicated
[
2,3]
[3]
catalyzed reactions.
In the course of our studies, we
found that a bowl-shaped[ phosphane ligand markedly
4]
that the P(tm-tp) catalyst system (entry 1) realized 154, 31,
3
enhances the rate of rhodium-catalyzed hydrosilylation of
ketones.
and 28 times faster reactions than PPh3 (entry 3), P(tp)
3
[
5]
[7]
(entry 2), and P(o-tol) (entry 5), respectively. Benzene is
3
The two triarylphosphanes tris(2,2’’,6,6’’-tetramethyl-m-
terphenyl-5’-yl)phosphane[ (denoted as P(tm-tp)3) and
a better solvent than CH Cl in the reactions of entries 1–3.
2
2
6]
The rate enhancement with P(tm-tp) was further con-
3
tris(m-terphenyl-5’-yl)phosphane (denoted as P(tp) ) were
prepared and compared with common phosphanes in the
rhodium-catalyzed hydrosilylation of cyclohexanone with a
firmed with various silanes and ketones, and compared with
P(tp) and PPh (Table 2). With HSiEt (Table 2, entries 1–3)
3
3
3
3
or HSiMePh (entries 4–6), the hydrosilylation of cyclohex-
2
trisubstituted silane (Table 1). P(tm-tp) was first prepared in
anone proceeded much faster with P(tm-tp) (entries 1 and 4)
3
3
[
6a]
0
2
001 and its Pd complex [Pd{P(tm-tp) } ] was reported in
than with P(tp) (entries 2 and 5) and PPh (entries 3 and 6).
3
2
3
3
Furthermore, in the hydrosilylation of various ketones such as
acetophenone (entries 7–9), 2-octanone (entries 10–12), and
(ꢀ)-menthone (entries 13–15), rate enhancement with P(tm-
[
*] Prof. Dr. Y. Tsuji, Dr. O. Niyomura, Dr. M. Tokunaga, Dr. Y. Obora,
T. Iwasawa
Catalysis Research Center and Division of Chemistry
Graduate School of Science
Hokkaido University
Sapporo 060-0811 (Japan)
Fax: (+081)11-706-3698
E-mail: tsuji@cat.hokudai.ac.jp
tp) was also evident. As catalyst precursor, the cationic
3
rhodium complex [Rh(cod) ]BF4 (cod = cyclooctadiene)
2
showed
entries 16–18).
P(tm-tp) is a much more efficient than P(tp) , although
a
similar rate enhancement with P(tm-tp)3
(
3
3
the two ligands strongly resemble each other. The structures
of P(tm-tp)3 and P(tp)3 were optimized by HF/6-31G(d)
calculations[8a] on initial structures generated by CON-
[
**] Financial support from the Asahi Glass Foundation is gratefully
acknowledged. O.N. is grateful to the JSPS Research Fellowships for
Young Scientists.
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Table 2: Hydrosilylation with various substrates.[a]
Entry Ketone Silane Ligand
cyclohexanone HSiEt3 P(tm-tp)3 21
depth of the bowl: P(tm-tp) (depth: 0.208 nm) is a much
3
t [h] Yield [%][b]
deeper bowl than P(tp)3 (0.132 nm). The dihedral angles
between the phosphanated phenyl ring (ring A) and the
phenyl rings in the meta position (rings B) are 85.3 ꢁ 0.58 for
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
81
36
13
97
37
27
96
15
15
91
41
3
P(tp)3
21
21
P(tm-tp)3 and 44.7 ꢁ 0.38 for P(tp) . This difference in
3
PPh
3
dihedral angle alters the depth of the bowl.
cyclohexanone HSiMePh2 P(tm-tp)3 20
P(tp)3
PPh3
20
20
5
5
5
6
6
6
While the mechanism of the rate enhancement is not
completely understood, the high catalytic activity realized
acetophenone
2-octanone
HSiMe Ph
P(tm-tp)3
2
with P(tm-tp) might be attributable to the prevalence of the
3
P(tp)3
PPh3
HSiMe Ph P(tm-tp)3
monophosphane species. In rhodium-catalyzed hydrosilyla-
tion of alkenes, the rate of the reaction also increased in the
0[
c]
2
[
11]
[
c]
c]
order P/Rh = 3 < 2 < 1. Bulky phosphanes such as PCy and
3
1
2
P(tp)3
PPh3
[
12]
[
PtBu are also known to afford low-coordinate species.
3
1
[
d]
d]
d]
3(
4
ꢀ)-menthone HSiMe Ph P(tm-tp)3 20
92 (45/55)
However, severe congestion around the phosphorus atom
must lower the efficiency of ligands in the hydrosilylation
reaction. In contrast, the deeper bowl-shaped P(tm-tp)3
ensures substantial empty space around the phosphorus
atom and, at the same time, is bulky enough to provide low-
coordinate species (long-range steric effect). This unique
bowl-shaped structure could facilitate the rate enhancement.
Further studies are now in progress to elucidate the
mechanism of the acceleration and to explore the benefit of
nanosized well-defined phosphane ligands in transition-
metal-catalyzed reactions.
2
20
20
6
6
6
8 (43/57)[
P(tp)3
5
PPh
3
12 (38/62)[
[
[
[
e]
6
7
8
cyclohexanone HSiMe Ph P(tm-tp)3
95
65
9
2
e]
e]
P(tp)3
PPh3
[a] Reaction conditions: ketone (1.0 mmol), silane (1.2 mmol),
[
{RhCl(C H ) } ] (0.0050 mmol), ligand (0.020 mmol), tridecane
2
4 2 2
(
0.25 mmol, as internal standard), benzene (1.0 mL), RT. [b] Yield of
alcohol by GC after desilylation with HCl/MeOH. [c] At 508C. [d] Product
ratio (neomenthol/menthol). [e] [Rh(cod) ]BF (0.010 mmol) was used
2
4
instead of [{RhCl(C H ) } ], in CH Cl as solvent.
2
4
2
2
2
2
FLEX[8b]/MM3.[8c] They showed that P(tm-tp)3 and P(tp)3
both have nanosized bowl-shaped structures with the phos-
phorus atom at the bottom and diameters of 1.99 and 1.95 nm,
Experimental Section
Synthesis of P(tp) : n-Butyllithium (1.59m in n-hexane, 0.982 mL,
3
[
13]
1.56 mmol) was added to a solution of 5’-bromo-m-terphenyl
respectively (Figure 1). The cone angles of P(tm-tp) and
3
(
0.482 g, 1.56 mmol) in THF (9 mL) at ꢀ788C over 10 min. The
P(tp) , determined for the optimized structures in Figure 1,
3
solution was maintained at this temperature for an additional 1 h.
Trichlorophosphane (0.071 g, 0.52 mmol) in toluene (1.4 mL) was
added dropwise to the cooled solution over 15 min. The reaction
mixture was further stirred at ꢀ788C for 1.5 h, and at room
temperature for 15 h. After evaporation of the solvent, toluene
(8 mL) and water (3 mL) were added to the residue and the mixture
was stirred vigorously. The organic layer was separated, and the
aqueous layer was extracted with toluene (4 ” 2 mL). The organic
extracts were combined and dried over anhydrous MgSO , and then
4
filtered through a glass filter under argon. The solvent was removed
from the filtrate in vacuo to give a wet solid. Recrystallization of the
residue from CHCl /MeOH (1/4) yielded white microfine crystals.
3
+
Yield: 0.310 g, 83%. M.p. 219–2208C; FD-MS: m/z 718 [M ];
1
4
H NMR(400.13 Hz, CDCl , 238C): d = 7.83 (t, J(H,H) = 1.5 Hz,
3
Figure 1. Optimized structures of a) P(tm-tp) and b) P(tp) .
3
3
3
4
3
7
7
H; H2’), 7.75 (dd, J(P,H) = 7.9 Hz, J(H,H) = 1.5 Hz, 6H; H4’,6’),
.61–7.59 (m, 12H; H2,2’’,6,6’’), 7.46–7.41 (m, 12H; H3,3’’5,5’’), 7.37–
.33 ppm (m, 6H; H4,4’’); C{ H} NMR(100.61 Hz, CDCl , 238C):
1
3
1
3
are 205 and 1938, respectively.[9a] Hence, they are more bulky
than PCy and PtBu , and less congested than P(o-tol) and
PMes3. It is well-known that J( P, Se) coupling constants
3
d = 142.0 (d, J(C,P) = 7.2 Hz; C1’,3’), 140.7 (C1,1’’), 138.0 (d,
1
2
J(C,P) = 11.5 Hz; C5’), 131.5 (d, J(C,P) = 20.0 Hz; C4’,6’), 128.8
3
3
3
[
9a]
1
31 77
(C3,3’’,5,5’’), 127.6 (C4,4’’), 127.3 (C2,2’’,6,6’’), 127.0 ppm (C2’);
3
1
1
P{ H} NMR(161.98 Hz, CDCl 3, 238C, H PO ): d = ꢀ2.0 ppm;
3
4
of phosphane selenides Se¼PR are reliable measures of the
3
elemental analysis (%) calcd for C H P: C 90.22, H 5.47; found: C
5
4
39
basicity of the parent phosphane PR : larger coupling
3
9
0.21, H 5.51.
General procedure for hydrosilylation of ketones (Tables 1 and
2): A typical reaction procedure is described for entry 1 in Table 1.
[{RhCl(C (1.9 mg, 0.0050 mmol) and P(tm-tp) (18 mg,
.020 mmol) were placed in a 5-mL flask under argon. Anhydrous
constants mean lower basicities.[
10a–c]
The J( P, Se) coupling
1
31 77
[
10d]
constants of Se¼P(tm-tp) and Se¼P(tp)3 of 769.7
and
3
[
10d]
7
65.5 Hz,
respectively, indicate that P(tm-tp) and P(tp)
2
H
4
)
2
}
]
2
3
3
3
0
have comparable basicities. They are less basic than PPh
3
[
10b]
[10d]
benzene (1.0 mL) was added by syringe, and then the mixture was
stirred at room temperature in a water bath for 2 h. Cyclohexanone
(
735 Hz)
and P(o-tol) (732 Hz),
and more basic than
3
[
10a]
P(2-furyl) (793 Hz).
3
(98 mg, 1.0 mmol), tridecane (46 mg, 0.25 mmol), and dimethylphe-
Thus, the cone angles and basicities cannot explain the
difference in reactivity between P(tm-tp) and P(tp) . The
nylsilane (163 mg, 1.2 mmol) were then added by syringe. The
reaction mixture was stirred at room temperature. Hydrolysis was
performed by adding 1% HCl/MeOH, and yields of cyclohexanol
3
3
most important difference between P(tm-tp) and P(tp) is the
3
3
1
288
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Chemie
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9] a) Tolman determined the cone angles using “folded-back” CPK
molecular models.[ Therefore, they are considerably smaller
than those of the corresponding real free ligands or ligands in
metal complexes. In the present study, we determined the cone
9b]
Received: October 7, 2002 [Z50320]
Keywords: homogeneous catalysis · hydrosilylation · ligand
effects · P ligands · rhodium
.
angles using structures optimized by CONFLEX/MM3 and HF/
6
-31G(d).[ The cone angles of the representative phosphanes
8]
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3
3
3
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[10a]
1
31 77
prepared by a literature method,
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and the J( P, Se) values
31
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[
7] The rate is first order in both the ketone and the silane: rate =
kobs[ketone][silane]. Selected observed rate constants kobs at
2
48C: 4.79 ꢁ 0.36 for P(tm-tp) (Table 1, entry 1), 0.154 ꢁ 0.004
3
for P(tp)3 (entry 2), 0.0311 ꢁ 0.0013 for PPh (entry 3), and
3
ꢀ1
3
ꢀ1
0.169 ꢁ 0.003 mol dm h for P(o-tol) (entry 5).
3
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