Synthesis of new bulky isocyanide ligands and their use for Rh-catalyzed hydrosilylation

Article abstract of DOI:10.1246/cl.2006.1038

Bulky isocyanide ligands having meta-terphenyl backbone were developed. Their usefulness as a supporting ligand in catalysis was illustrated by the application to the rhodium-catalyzed hydrosilylation of cyclohexanone. The combined effect of the high liga

Full text of DOI:10.1246/cl.2006.1038

1038
Chemistry Letters Vol.35, No.9 (2006)
Synthesis of New Bulky Isocyanide Ligands and Their Use for Rh-catalyzed Hydrosilylation
Hajime Ito,ꢀ Takayuki Kato, and Masaya Sawamuraꢀ
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810
(Received June 12, 2006; CL-060674; E-mail: hajito@sci.hokudai.ac.jp; sawamura@sci.hokudai.ac.jp)
Bulky isocyanide ligands having meta-terphenyl backbone
R
R
O
4a: R = H
4b: R = Me
4c: R = Me3Si
4d: R = t-BuMe2Si
were developed. Their usefulness as a supporting ligand in
catalysis was illustrated by the application to the rhodium-
catalyzed hydrosilylation of cyclohexanone. The combined ef-
fect of the high ligand affinity to the rhodium atom and the ligand
bulkiness to facilitate the formation of a catalytically active,
monoligated isocyanide–rhodium species was proposed.
H
NH2
H
N
Br
Br
t-BuCO2CHO
Br
Br
B(OH)2
Pd(OAc)2 (5 mol %)
PPh3 (10 mol %)
Na2CO3, toluene/EtOH/H2O
reflux
Et2O, rt
Me
2
Me
3 (89%)
R
R
R
R
O
C
N
H
POCl3, Et3N
CH2Cl2
N
H
Isocyanides are isoelectronic with carbon monoxide and co-
ordinate to various transition metals at the terminal isocyano car-
bon atom.1 In general, both isocyanides and carbon monoxide
are the ligands of extreme compactness. Unlike CO, however,
the electronic and steric properties of the isocyanides are tunable
by the substituent at the nitrogen atom. In fact, many different
isocyanides exist and have been used as a ligand of transition-
metal complexes with interesting properties in various aspects.
One can also enjoy diverse reactivities of the isocyanides for
the applications to organic and organometallic chemistries.2,3
While cases are rare, some isocyanides having a bulky N-sub-
stituent were used as a supporting ligand of a transition-metal
catalyst.4–7 For example, Nile et al. and Yamazaki et al. used
2,6-disubstituted phenyl isocyanides in the rhodium- and plati-
num-catalyzed hydrosilylations.4 Ito and co-workers developed
the bis-silylations of carbon–carbon unsaturated bonds and relat-
ed reactions by the use of tert-alkyl isocyanide–palladium com-
plexes as a catalyst precursor.5 Recently, Nagashima et al. re-
ported that a nickel complex of 2-biphenyl isocyanide is highly
active for the ethylene polymerization.6
R
R
R
R
0 °C
Me
Me
5a: R = H (90%)
5b: R = Me (67%)
5c: R = Me3Si (89%)
1a: R = H (91%)
1b: R = Me (80%)
1c: R = Me3Si (89%)
1d: R = t-BuMe2Si (80%)
5d: R = t-BuMe2Si (91%)
Scheme 1. Synthesis of substituted m-terphenyl isocyanides.
POCl3/Et3N resulted in clear conversion to the desired isocya-
nides (1a–1d) in high yields (80–91%).8 The new isocyanides
are odorless and colorless crystals, and stable against oxidation
and hydrolytic decomposition in the air. Therefore, they are easy
to handle. The space-filling representation of the optimized
structures (MMFF94, MacSpartan Pro 1.0.4.) of 1a and 1c
(Figure 1) visualizes that the meta-terphenyl backbone is an ide-
al platform to create a concave steric environment around the
isocyano carbon atom.
The Rh–isocyanide complexes were examined for catalytic
activity in the hydrosilylation of cyclohexanone with dimethyl-
phenylsilane.8 Results of the hydrosilylation experiments carried
out in benzene at rt in the presence of 1 mol % each of [Rh-
(cod)2]BF4 and the isocyanide ligand (1a–1h) are summarized
in Table 1. Results with conventional phosphine ligands (PPh3
and dppp) are also shown for comparison. The acceleration
effect with each ligand was evaluated by the GC yield of the
corresponding silyl ether after 1 h. The acceleration with isocya-
nides varied considerably depending on the ligand bulkiness. In
particular, the Me3Si-substituted terphenyl isocyanide (1c) ex-
hibited the most significant effect, giving the silyl ether product
in 97% yield (Table 1, Entry 4). The t-BuMe2Si-substituted iso-
cyanide (1d) showed slightly decreased acceleration effect (79%
yield, Entry 5), suggesting that the bulkiness is too much. The
terphenyl isocyanides (1a and 1b) with smaller steric demand
We report here the synthesis of new bulky isocyanides (1a–
1d, Chart 1) having a meta-terphenyl backbone and their use in
the Rh-catalyzed hydrosilylation of cyclohexanone. The bulky
isocyanide ligands markedly accelerated the reaction as com-
pared with the effect of the conventional isocyanide and phos-
phine ligands, demonstrating the usefulness of the designed iso-
cyanides as a supporting ligand in transition-metal catalysis.
C
R
R
N
1a: R = H
1b: R = Me
R
R
1c: R = Me3Si
1d: R = t-BuMe2Si
Me
Chart 1.
Synthesis of the terphenyl isocyanides (1a–1d) is straight-
forward and easy (Scheme 1). It starts with N-formylation of
2,6-dibromotoluidine (2) to give N-formyl-2,6-dibromotoluidine
(3) (89%). Suzuki–Miyaura coupling of dibromoarene 3 with
3,5-disubstituted phenylboronic acid (4a–4d) in the presence
of 5 mol % of Pd(OAc)2 and 10 mol % of PPh3 gave N-terphenyl
formamides 5a–5d in 67–91% yields. Treatment of 5a–5d with
1a
1c
Figure 1. Space-filling models of isocyanides 1a and 1c.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.9 (2006)
1039
Table 1. Rh-catalyzed hydrosilylation with various isocyanide
and phosphine ligandsa
ligands having a meta-terphenyl backbone. Their usefulness as
a supporting ligand in the catalysis was illustrated by the appli-
cation to the rhodium-catalyzed hydrosilylation. Efforts aimed
at further developing efficient transition metal-catalyzed
reactions influenced by the isocyanide ligands are ongoing in
our laboratory.
O
OSiMe2Ph
[Rh(cod)2]BF4 (1.0 mol %)
ligand (1.0 mol %)
+ HSiMe2Ph
benzene, rt., 1 h
Entry
Ligand
Yield/%b
This work was supported by PRESTO program, JST and
Grant-in-Aid for Scientific Research (B), JSPS.
1
2
—
1a
13
36 (85, 4 h)
3
4
1b
1c
22 (89, 4 h)
97
References and Notes
1
a) L. Malatesta, Prog. Inorg. Chem. 1959, 1, 283. b) P. M.
Treichel, Adv. Organomet. Chem. 1973, 11, 21. c) E. Singleton,
H. E. Oosthuizen, Adv. Organomet. Chem. 1983, 22, 209.
For multicomponent reactions with isocyanides, see: A.
Domling, I. Ugi, Angew. Chem., Int. Ed. 2000, 39, 3169.
For metal-mediated synthetic reactions with isocyanides, see:
a) Y. Ito, M. Sawamura, T. Hayashi, J. Am. Chem. Soc. 1986,
108, 6405. b) S. Kamijo, T. N. Jin, Y. Yamamoto, J. Am. Chem.
Soc. 2001, 123, 9453.
a) K. P. Adams, J. A. Joyce, T. A. Nile, A. I. Patel, C. D. Reid,
J. M. Walters, J. Mol. Catal. 1985, 29, 201. b) T. Hagiwara,
K. Taya, Y. Yamamoto, H. Yamazaki, J. Mol. Catal. 1989,
54, 165.
a) Y. Ito, M. Suginome, M. Murakami, J. Org. Chem. 1991, 56,
1948. b) M. Suginome, T. Iwanami, Y. Ohmori, A. Matsumoto,
Y. Ito, Chem.—Eur. J. 2005, 11, 2954. c) M. Suginome, T.
Matsuda, Y. Ito, J. Am. Chem. Soc. 2000, 122, 11015.
M. Tanabiki, K. Tsuchiya, Y. Kumanomido, K. Matsubara, Y.
Motoyama, H. Nagashima, Organometallics 2004, 23, 3976.
For other reactions with metal-isocyanide catalysts, see: a)
T. Saegusa, Y. Ito, Synthesis 1975, 291. b) V. Cadierno, P.
Crochet, J. Diez, S. E. Garcia-Garrido, J. Gimeno, Organo
metallics 2004, 23, 4836. c) B. M. Trost, C. A. Merlic, J. Am.
Chem. Soc. 1990, 112, 9590. d) U. Kazmaier, D. Schauss, M.
Pohlman, Org. Lett. 1999, 1, 1017. e) A. Efraty, I. Feinstein,
Inorg. Chem. 1982, 21, 3115.
General procedure for the Rh(I)-catalyzed hydrosilylation:
[Rh(cod)2]BF4 (6.1 mg, 0.015 mmol) and an isocyanide (0.015–
0.045 mmol) were placed in a reaction tube. The tube was evac-
uated and backfilled with argon. After addition of anhydrous,
degassed benzene (1.5 mL), the mixture was stirred at room
temperature for 1 h. Cyclohexanone (147 mg, 1.5 mmol), 1,4-
diisopropylbenzene (66.9 mg, 0.412 mmol, as an internal stand-
ard for GC analysis), and dimethylphenylsilane (245 mg, 1.8
mmol) were then added. The yield of product was determined
by GC.
5
6
7
8
9
10
11
1d
1e
1f
1g
1h
PPh3
dppp
79
7
14 ð32; 4 hÞ
39 (61, 4 h)
36
2
3
34–71 (46–81, 3 h)
trace
aConditions: ligand (0.015 mmol), [Rh(cod)2]BF4 (0.015
mmol), cyclohexanone (1.5 mmol), and Me2PhSiH (1.8
4
5
mmol) in benzene (1.5 mL) at rt (21–24 ꢁC). GC yield of
the silyl ether.
b
C
N
C
N
C
N
C
N
6
7
1e
1f
1g
1h
Chart 2.
than 1c were much less effective (Entries 2 and 3). 2-Biphenyl
isocyanide (1e)6 and 2,6-xylyl isocyanide (1f) (Chart 2)4 showed
no acceleration effect, compared with the rate without added li-
gand (Entries 6 and 7). 2,6-Diisopropylphenyl isocyanide (1g) is
comparable with the non-substituted terphenyl isocyanide (1a)
in the acceleration effect (Entry 8),4 suggesting that the steric ef-
fect of the i-Pr substituents at the positions ortho to the isocyano
group is only comparable with the Ph substituents in simple ter-
phenyl isocyanide 1a. tert-Butyl isocyanide (1h)5,7 gave a simi-
lar result (Entry 9). The yields with the small isocyanides (1a,
1b, 1f, and 1g) increased reasonably by prolonging reaction time
to 4 h (Entries 2, 3, 7, and 8), indicating that the low yields is re-
sponsible for low catalytic activity rather than catalyst deactiva-
tion. These results strongly suggest that the accelerating effect
with 1c and 1d is due to their concave steric feature.9
8
Although the reaction with PPh3 gave a good yield at 1 h,
it suffered from poor reproducibility and substantial catalyst
deactivation over time (Entry 10). Bidentate phosphine ligand
dppp inhibited the reaction almost completely (Entry 11).
Next, correlations between the catalytic activities and
the Rh:1c ratio were examined. The hydrosilylation with
[Rh(cod)2]BF4 and 1c in 1:1 ratio proceeded with the highest
rate among the reactions carried out with five different Rh:1c
ratios ranging from 1:0–1:3 (13% at 1 h (Rh:1c = 1:0), 71%
(1:0.5), 97% (1:1), 80% (1:2), 55% (1:3)). According to these
results, we postulate that the active species is a 1:1 Rh–isocya-
nide complex, and that the coordination of the second molecule
of isocyanide inhibits the catalysis.10,11
9
Similar rate acceleration effect was observed when [RhCl-
(C2H4)2]2 was used as a catalyst precursor (yields at 3h, 1a:
48%, 1c: 97%, 1g: 15%, PPh3: 24%).
10 For the importance of mono(phosphine) complexes as an active
species in palladium-catalyzed reactions, see: a) M. Miura,
Angew. Chem., Int. Ed. 2004, 43, 2201. b) U. Christmann, R.
Vilar, Angew. Chem., Int. Ed. 2005, 44, 366.
11 For related studies with phosphine ligands, see: a) O.
Niyomura, M. Tokunaga, Y. Obora, T. Iwasawa, Y. Tsuji,
Angew. Chem., Int. Ed. 2003, 42, 1287. b) O. Niyomura, T.
Iwasawa, N. Sawada, M. Tokunaga, Y. Obora, Y. Tsuji,
Organometallics 2005, 24, 3468. c) T. Matsumoto, T. Kasai,
K. Tatsumi, Chem. Lett. 2002, 346. c) Y. Ohzu, K. Goto, T.
Kawashima, Angew. Chem., Int. Ed. 2003, 42, 5714.
In conclusion, we have developed the new bulky isocyanide
Published on the web (Advance View) August 12, 2006; doi:10.1246/cl.2006.1038