Versatile dehydrogenative alcohol silylation catalyzed by Cu(I)-phosphine complex

Article abstract of DOI:10.1021/ol050559+

(Chemical Equation Presented) Cu(I) complexes of xanthane-based diphosphines were versatile catalysts for dehydrogenative alcohol silylation, exhibiting high activity and broad substrate scope. Highly selective silylation of 1-decanol over 2-decanol is possible even with a silylating reagent of small steric demand such as HSiMe₂Ph or HSiEt₃.

Full text of DOI:10.1021/ol050559+

ORGANIC
LETTERS
2005
Vol. 7, No. 9
1869-1871
Versatile Dehydrogenative Alcohol
Silylation Catalyzed by Cu(I)
Complex
−Phosphine
Hajime Ito,* Akiko Watanabe, and Masaya Sawamura*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity,
Sapporo 060-0810, Japan, and PRESTO, Japan Science and Technology Agency
hajito@sci.hokudai.ac.jp; sawamura@sci.hokudai.ac.jp
Received March 15, 2005
ABSTRACT
Cu(I) complexes of xanthane-based diphosphines were versatile catalysts for dehydrogenative alcohol silylation, exhibiting high activity and
broad substrate scope. Highly selective silylation of 1-decanol over 2-decanol is possible even with a silylating reagent of small steric demand
such as HSiMe2Ph or HSiEt3.
Development of environmentally benign chemical processes
is a challenge in modern synthetic organic chemistry.
Substituting a process that employs an organic halide for a
halogen-free process should have an impact. Silyl ether
formation is not only a fundamental process in the synthesis
of functional organosilicon compounds but also an important
technique for protection of reactive hydroxy groups during
multistep organic syntheses.1 From the standpoint of “green
chemistry”, this transformation should be conducted through
catalytic dehydrogenative silylation with a hydrosilane rather
than through electrophilic silylation with a silyl electrophile
(R3Si-X) in combination with a stoichiometric base.2-6 The
former forms H2 as the sole byproduct instead of a HX‚base
in the latter. Here, we report that copper complexes of
xanthene-based diphosphines (Figure 1, 1a, 1b)7 are highly
efficient catalysts for dehydrogenative alcohol silylation,
which provides an unprecedented broad substrate scope and
a high level of chemoselectivity.
Various transition metal complexes have been reported as
catalysts for dehydrogenative silylation.2-5 Because of poor
activity, however, most are applicable only with a limited
Figure 1. Xanthene-based ligands.
range of substrate sets. Moreover, some possess CdC
hydrosilylation, hydrogenation, and isomerization activities.
Therefore, unsaturated alcohols cannot be utilized. While
[(Ph3P)CuH]6 is a chemoselective catalyst for the dehydro-
genative alcohol silylation of unsaturated alcohols, its low
activity prevents its use in practical syntheses.4
To find an efficient and versatile catalyst for dehydro-
genative silylation, we screened various copper salt-ligand
(1) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic
Synthesis, 3rd ed.; Wiley & Sons: New York, 1999.
10.1021/ol050559+ CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/07/2005

combinations for reaction of 1-phenylethanol (2a) with
HSiEt3 (3a) and found that the copper complex generated
in situ from t-BuOCu and Xantphos (1a) possessed high
catalytic activity (Table 1, entry 1). With 0.5 mol % catalyst
phosphine ligands such as PPh3, dppe, dppp, dppf, and
(R)-BINAP resulted in a drastic decrease in activity (1-34%
conversion at 2 h, entries 3-7). The superior activity of the
t-BuOCu-Xantphos system compared to previously reported
silylation catalysts such as RhCl(PPh3)3,3a Rh2(OCOC4F7)4,3b
RuCl2(p-cymene)2,3d and Ru3(CO)12,3c is apparent as shown
in Table 1 (<6% conversion at 2 h, entries 8-11).3
Table 1. Dehydrogenative Silylation with Various Catalystsa
The scope and limitations of the present Cu-catalyzed
silylation are summarized in Table 2. Simple primary and
secondary alkanols (2b, 2c) also were silylated with Et3SiH
(3a) at 23 and 24 °C, but this catalyst was not effective for
tertiary alcohol 2d (entries 1-3). HSiMe2t-Bu (3b) under-
went silylation with primary alcohol 2b at room temperature
and with secondary alcohol 2e at 50 °C (entries 4 and 5).
Even HSiPh2t-Bu (3c) and HSiPh3 (3d), which are more
hindered than 3a and 3b, reacted smoothly with primary and
secondary alcohols (2f, 2a) (entries 6-9). Only a trace of
the product was detected in the reaction of HSi(i-Pr)3 (3e)
even with primary alcohol 2f after 24 h at 70 °C (entry 10).
Silyl ethers of 9-decen-1-ol (2g) and 3-hexyn-1-ol (2h) were
obtained in good yields with unsaturated bonds intact (entries
11 and 12). No carbonyl hydrosilylation was observed in
the alcohol silylation of 5-hydroxy-2-pentanone (2i) (entry
13). The â-alkoxy group, which is a potential coordination
site for the metal center, exerted virtually no influence on
reactivity (entry 14). Entry 15 in Table 2 demonstrates the
practical advantage of this method in large-scale preparation
(see, Supporting Information for experimental procedures).
It is an additional characteristic feature of the present
catalytic silylation that the selective silylation of a sterically
less congested hydroxy group over a more congested one is
possible with rather small silyl groups such as PhMe2Si and
Et3Si groups. Such silylation is generally difficult with the
conventional electrophilic silylation.8 Results for the selective
silylation of primary alcohol 1-decanol (2b) in the presence
of secondary alcohol 2-decanol (2c) are summarized in Table
3. The electrophilic method using chlorosilanes in combina-
tion with Et3N and DMAP (method A) required the bulkiness
of the t-BuMe2Si group to obtain a reasonably high selectiv-
ity; 4bx:4cx ) 51:49 (ClSiMe2Ph, 5f), 68:32 (ClSiEt3, 5a),
95:5 (ClSiMe2t-Bu, 5b) (entries 1-3). In sharp contrast,
dehydrogenative silylation with Cu(I)-Xantphos (1a) cata-
lyst (method B) exhibited selectivity for the primary alcohol
as high as 90:10, even with HSiMe2Ph (3f) (entry 4). Higher
selectivities were obtained with sterically more demanding
hydrosilanes HSiEt3 (3a) (93:7) and HSiMe2t-Bu (3b)
(96:4) (entries 5, 6). Use of a new xanthene-based ligand
1b with larger steric demand (method C) further improved
temp time yieldb
entry
catalyst
solvent
(°C)
(h)
(%)
1
2
3
4
5
6
7
8
9
t-BuOCu, 1a
[(Ph3P)CuH]6
toluene
toluene
toluene
toluene
toluene
toluene
24
23
24
23
22
23
23
22
23
25
26
1
2
2
2
2
2
2
2
2
2
2
99
trace
1
2
34d
12
5
trace
3
c
t-BuOCu, PPh3
t-BuOCu, dppe
t-BuOCu, dppp
t-BuOCu, dppf
t-BuOCu, (R)-BINAP toluene
RhCl(PPh3)3
Rh2(OCOC4F7)4
RuCl2(p-cymene)2
Ru3(CO)12
toluene
CH2Cl2
neat
10
11
6
neat
trace
a 2a, 0.5 mmol; 3a, 1.0 mmol; solvent, 1.0 mL unless otherwise noted.
b Determined by GC. c Performed with 0.5 mol % for Cu(I). d After 24 h,
yield ) 40%.
loading, the reaction was complete in 1 h at 24 °C, while
almost no reaction occurred with [(Ph3P)CuH]6 under the
same conditions (entry 2). Replacing Xantphos with other
(2) For selected references, see: (a) Sommer, L. H.; Lyons, J. E. J. Am.
Chem. Soc. 1969, 91, 7061-7067. (b) Chalk, A. J. J. Chem. Soc., Chem.
Commun. 1970, 847-848. (c) Oehmichen, U.; Singer, H. J. Organomet.
Chem. 1983, 243, 199-204. (d) Lukevics, E.; Dzintara, M. J. Organomet.
Chem. 1985, 295, 265-315. (e) Luo, X. L.; Crabtree, R. H. J. Am. Chem.
Soc. 1989, 111, 2527-2535. (f) Yamamoto, K.; Takemae, M. Bull. Chem.
Soc. Jpn. 1989, 62, 2111-2113. (g) Gregg, B. T.; Cutler, A. R. Organo-
metallics 1994, 13, 1039-1043. (h) Chung, M. K.; Ferguson, G.; Robertson,
V.; Schlaf, M. Can. J. Chem. 2001, 79, 949-957. (i) Chung, M. K.; Orlova,
G.; Goddard, J. D.; Schlaf, M.; Harris, R.; Beveridge, T. J.; White, G.;
Hallett, F. R. J. Am. Chem. Soc. 2002, 124, 10508-10518. (j) Maifeld, S.
V.; Miller, R. L.; Lee, D. Tetrahedron Lett. 2002, 43, 6363-6366. (k) Field,
L. D.; Messerle, B. A.; Rehr, M.; Soler, L. P.; Hambley, T. W.
Organometallics 2003, 22, 2387-2395. (l) Schmidt, D. R.; O’Malley, S.
J.; Leighton, J. L. J. Am. Chem. Soc. 2003, 125, 1190-1191. (m) Biffis,
A.; Braga, M.; Basato, M. AdV. Synth. Catal. 2004, 346, 451-458.
(3) (a) Ojima, I.; Kogure, T.; Nihonyanagi, M.; Kono, H.; Inaba, S. Chem.
Lett. 1973, 501-504. (b) Doyle, M. P.; High, K. G.; Bagheri, V.; Pieters,
R. J.; Lewis, P. J.; Pearson, M. M. J. Org. Chem. 1990, 55, 6082-6086.
(c) Funatsu, A.; Kubota, T.; Endo, M. (Shin-Etsu Chemical Industry Co.,
Ltd., Japan). Jpn. Kokai Tokkyo Koho JP2001-114788, 2001. (d) Miller,
R. L.; Maifeld, S. V.; Lee, D. Org. Lett. 2004, 6, 2773-2776.
(4) (a) Lorenz, C.; Schubert, U. Chem. Ber. 1995, 128, 1267-1269.
(b) Mahoney, W. S.; Stryker, J. M. J. Am. Chem. Soc. 1989, 111, 8818-
8823. For the synthesis of optically active silanes by asymmetric silane
alcoholysis catalyzed by a chiral Cu(I)-complex, see ref 2l.
(5) [IrH2(THF)2(PPh3)2]SbF6 was reported as the most active catalyst
for Et3SiH. However, this complex also promotes isomerization of C-C
double bonds and slow hydrosilylation of ketones. See ref 2e.
(6) For Lewis acid and base catalysts, see: (a) Tanabe, Y.; Okumura,
H.; Maeda, A.; Murakami, M. Tetrahedron Lett. 1994, 35, 8413-8414.
(b) Blackwell, J. M.; Foster, K. L.; Beck, V. H.; Piers, W. E. J. Org. Chem.
1999, 64, 4887-4892. See also ref 2d.
(7) 1b is a new compound. For 1a, see: (a) Kranenburg, M.; Vanderburgt,
Y. E. M.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje,
J. Organometallics 1995, 14, 3081-3089. (b) Kamer, P. C. J.; van Leeuwen,
P. W. N. M.; Reek, J. N. H. Acc. Chem. Res. 2001, 34, 895-904.
(8) Moderate selectivity between primary and secondary alcohols was
reported for the Rh2(OCOC4F7)4-catalyzed silylation with HSiEt3; 1-butanol:
2-butanol ) 79:21, see ref 3b.
(9) Observed ligand effect is in sharp contrast to that in the Cu-catalyzed
(asymmetric) 1,2-hydrosilylation of ketones and 1,4-hydrosilylation of R,â-
unsaturated carbonyl compounds. It has been reported that the hydro-
silylations were remarkably accelerated by some diphosphines of normal
bite angles. To the contrary, we observed that Xantphos was less effective
than the normal diphosphines for the hydrosilylations, see: (a) Lipshutz,
B. H.; Noson, K.; Chrisman, W. J. Am. Chem. Soc. 2001, 123, 12917-
12918. (b) Lipshutz, B. H.; Caires, C. C.; Kuipers, P.; Chrisman, W. Org.
Lett. 2003, 5, 3085-3088. (c) Moritani, Y.; Appella, D. H.; Jurkauskas,
V.; Buchwald, S. L. J. Am. Chem. Soc. 2000, 122, 6797-6798. (d) Chen,
J. X.; Daeuble, J. F.; Stryker, J. M. Tetrahedron 2000, 56, 2789-2798.
(e) Chen, J. X.; Daeuble, J. F.; Brestensky, D. M.; Stryker, J. M. Tetrahedron
2000, 56, 2153-2166.
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Org. Lett., Vol. 7, No. 9, 2005

Table 2. Dehydrogenative Silylation with Cu(I)-1a Catalysta
catalyst
(mol %)
temp
(°C)
time
(h)
yieldb
(%)
entry
alcohol
hydrosilane
product
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CH3(CH2)8CH2OH (2b)
CH3(CH2)7CH(OH)CH3 (2c)
(CH3)3COH (2d)
CH3(CH2)8CH2OH (2b)
cyclo-C6H11OH (2e)
PhCH2CH2OH (2f)
PhCH(OH)CH3 (2a)
PhCH2CH2OH (2f)
PhCH(OH)CH3 (2a)
PhCH2CH2OH (2f)
CH2dCH(CH2)7CH2OH (2g)
EtCtCCH2CH2OH (2h)
CH3CO(CH2)2CH2OH (2i)
CH3OCH2CH2OH (2j)
geraniol (2k)c
HSiEt3 (3a)
4ba
4ca
4da
4bb
4eb
4fc
4ac
4fd
4ad
4fe
4ga
4ha
4ia
4jb
4ka
0.5
1.0
0.5
1.0
0.5
1.0
5.0
1.0
2.0
0.5
0.5
0.5
2.0
0.5
0.1
23
24
50
25
50
24
50
23
23
70
25
26
24
23
24
1
4
22
4
5
3
3
2
1
24
2
91
97
7
99
95
94
94
94
96
trace
95
95
84
89
3a
3a
t
HSiMe2 Bu (3b)
3b
t
HSiPh2 Bu (3c)
3c
HSiPh3 (3d)
3d
HSi(iPr)3 (3e)
3a
3a
3a
3b
3a
2
5
1
1
97
a 2, 0.5 mmol; 3, 1.0 mmol; solvent, 1.0 mL unless otherwise noted. b Isolated yield. c 2k, 50 mmol; 3a, 55 mmol, no solvent.
the selectivity, and very high selectivities were achieved
irrespective of the silyl group structures; 4bx:4cx ) 98:2
(HSiMe2Ph), 99:1 (HSiEt3), 99:1 (HSiMe2t-Bu) (entries
7-9).
reactivity of Xantphos catalyst is attributable at least in part
to the larger contribution of the monomeric hydride A in
the aggregation equilibria. However, this alone cannot
A mechanism for Cu-catalyzed silylation is proposed in
Scheme 1. Mixing t-BuOCu, the diphosphine, and HSiR′3
generates phosphine-chelated Cu(I) hydride A as an active
species, which is in equilibrium with less active or inactive
dimer B or higher aggregates C. Hydride A reacts with ROH
through σ-bond metathesis to produce alkoxocopper(I) D and
H2. Subsequent metathesis between D and HSiR′3 affords
ROSiR′3 and A. According to this mechanism, the high
Scheme 1. Proposed Mechanism for Cu(I)-Catalyzed
Silylation
Table 3. Selective Silylation of a Primary Alcohol in the
Presence of a Secondary Alcohol
explain the unique superiority of the Xantphos ligand
compared to other chelating diphosphines. A large P-Cu-P
bite angle expected for Xantphos should exert some salient
effect on the acceleration of σ-bond metathesis between A
and the alcohol.9
Having demonstrated pronounced reactivity, wide substrate
scope, and selectivity, we believe that this reaction is useful
not only for organosilicon chemistry but also for multistep
syntheses of complex organic molecules and represents a
significant step toward the development of green chemistry.
temp time yield (%)b ratioc
entry silyl reagent methoda (°C) (h) 4bx + 4cx 4bx:4cx
1
2
3
4
5
6
7
8
9
ClSiMe2Ph (5f)
ClSiEt3 (5a)
ClSiMe2 Bu (5b)
A
A
A
B
B
B
C
C
Cd
24
24
23
25
22
23
22
22
22
13
2
1
1
2
3
7
19
24
94
95
94
91
93
88
92
95
85
51:49
68:32
95:5
90:10
93:7
96:4
98:2
99:1
99:1
t
HSiMe2Ph (3f)
HSiEt3 (3a)
t
HSiMe2 Bu (3b)
Acknowledgment. H.I. thanks Meiji Seika for generous
financial support.
HSiMe2Ph (3f)
HSiEt3 (3a)
t
HSiMe2 Bu (3b)
Supporting Information Available: Experimental pro-
cedures and compound characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
a Method A: 2b, 0.5 mmol; CH2Cl2, 1.0 mL; 2b:2c:5:Et3N:DMAP )
1:1:1:1.2:0.04. Method B (C): 2b, 0.5 mmol; toluene, 1.0 mL; 2b:2c:3:t-
BuOCu:1a (1b for method C) ) 1:1:1:0.02:0.02. b Isolated yield. c Deter-
mined by GC. d 2b:2c:3b:t-BuOCu:1b ) 1:1:1:0.05:0.05.
OL050559+
Org. Lett., Vol. 7, No. 9, 2005
1871