A new alternative to Stryker's reagent in hydrosilylation: Synthesis, structure, and reactivity of a well-defined carbene-copper(II) acetate complex

Article abstract of DOI:10.1039/b509964a

A new, air stable and well-defined carbene-copper(II) complex has been prepared, which is an efficient precatalyst for the 1,2- and 1,4-reduction of carbonyl compounds under hydrosilylation conditions. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b509964a

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A new alternative to Stryker’s reagent in hydrosilylation: synthesis,
structure, and reactivity of a well-defined carbene–copper(II) acetate
complex{
Jaesook Yun,*a Daesung Kimb and Hoseop Yunb
Received (in Cambridge, UK) 18th July 2005, Accepted 23rd August 2005
First published as an Advance Article on the web 19th September 2005
DOI: 10.1039/b509964a
Herein, we wish to report the successful synthesis of the new
copper(II) complex 1, its X-ray structure, and a direct comparison
of its activity to Stryker’s reagent.
A new, air stable and well-defined carbene–copper(II) complex
has been prepared, which is an efficient precatalyst for the 1,2-
and 1,4-reduction of carbonyl compounds under hydrosilyla-
tion conditions.
The synthesis of 1 was achieved by the reaction of a free
carbene, IPr12 and copper(II) acetate in toluene at room
temperature (Scheme 1).{ Single crystals for X-ray diffraction
were obtained by the slow diffusion of pentane into a concentrated
CH2Cl2 solution of catalyst 1. A representation of the X-ray
structure is shown in Fig. 1, with selected bond lengths and bond
Copper hydride mediated chemoselective conjugate reduction
of a,b-unsaturated carbonyl compounds is an important trans-
formation for organic synthesis.1 Since the first report on the use
of the organocopper hydride, [(PPh3)CuH]6, in the conjugate
reduction of enones by Stryker and coworkers,2 the copper
complex, known as Stryker’s reagent, has been widely employed in
organic reductions. A major disadvantage of this reagent is its
sensitivity to air;2 the quality of the commercially available
Stryker’s reagent is quite variable3 and special precautions are
needed for its storage.
˚
angles.§ The Cu–Ccarbene distance of 1.942(4) A confirms the
absence of p-character in the bond.13 The Cu–O distance for
˚
the bound carboxylate oxygen (1.941(3) A) is shorter than
those in three- or four-coordinate copper(I) bis(triphenylphos-
phine)carboxylates,14 and longer than those in two-coordinate
linear copper carboxylates.15 A relatively strong interaction exists
˚
between the Cu and the distal oxygen O(2) (2.259(3) A). This
We have recently described a new copper(II) system employing
Cu(OAc)2 as a precatalyst that can be easily activated in situ
by hydrosilanes without using strong bases4 and that efficiently
reduces aromatic ketones4 and a,b-unsaturated nitriles5 in
combination with a chelating phosphine. Based on our previous
studies, we envisioned that a well-defined copper(II) complex
incorporating the acetate moieties and
a strong s-donor
N-heterocyclic carbene (NHC) ligand would work as an efficient
and convenient precatalyst for hydrosilylation and might be a
good alternative to Stryker’s reagent. N-heterocyclic carbenes have
emerged as a new family of ligands for homogeneous catalysis
during the past decade.6 However, their use as ligands for copper
complexes remains relatively rare.7 The first NHC–Cu(I) complex
was reported by Arduengo et al. in 19938 and such copper
complexes were shown to catalyze conjugate additions of diethyl
zinc to enones by other groups.9 Recently, Buchwald et al.10 and
Nolan et al.11 independently reported the synthesis of a carbene–
copper(I) complex, (IPr)CuICl (IPr 5 1,3-bis(2,6-diisopropyl-
phenyl)-imidazol-2-ylidene). The latter complex efficiently
catalyzes the conjugate reduction of enones and esters10 and
hydrosilylation of ketones11 when activated, however, the use of
excess base is required for catalyst activation.
Scheme 1 Synthesis of (IPr)CuII(OAc)2.
aDepartment of Chemistry and Institute of Basic Sciences,
Sungkyunkwan University, Suwon 440-746, Korea.
E-mail: jaesook@skku.edu; Fax: +82-31-290-7075;
Tel: +82-31-299-4561
bDepartment of Molecular Science and Technology, Ajou University,
Suwon 443-749, Korea. E-mail: hsyun@ajou.ac.kr;
Fax: +82-31-219-1615; Tel: +82-31-219-2605
{ Electronic supplementary information (ESI) available: Experimental
details and spectroscopic data of all products. See http://dx.doi.org/
10.1039/b509964a
Fig. 1 X-Ray structure of 1. Thermal ellipsoids are shown at their 30%
˚
angles (u): Cu–C(2) 5 1.942(4), Cu–O(1), (Cu–O(1)i) 5 1.941(3), Cu–O(2),
(Cu–O(2)i) 5 2.259(3), O(1)–Cu–O(1)i 5 160.36(19), O(1)–Cu–C(2),
(O(1)i–Cu–C(2)) 5 99.82(9).
values. Symmetry code: (i) 2x, y, 0.5 2 z. Selected bond distances (A) and
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Table 1 1,2-/1,4-Reductions catalyzed by 1 vs. Stryker’s reagent
Catalyst 1 (1 mol%)
Stryker’s reagent (3 mol%)
Substrate
Product
Silane (equiv.)
PMHS (2.5)
Time/ha
Yield (%)
Time/ha
Yield (%)
,0.1
97
3
3
98
94b
PMHS (2.5)
TMDS (1.5)
0.3
0.5
96
10
97b
79c
24
85b,d
TMDS (0.55)
6
93
14
91b,d
a
b
c
d
Time needed for complete conversion. Taken from ref. 3 and 17. Combined yields of monomeric and dimeric products. 5 mol% Stryker’s
reagent was used.
complex is air stable and storable at ambient temperature without
any special precautions for months, but decomposes at tempera-
tures above 180 uC.
tolerant to steric hindrance at the b-carbon and reduces even
bulkier nitrile substrates (2f, 2g) that were not reactive under our
previous reduction conditions employing the Cu–Xantphos-type
ligand.
The activation of precatalyst 1 was tested in the presence of
various hydrosilanes in toluene. Diphenylsilane and polymethyl-
We are at present uncertain of the exact nature of the active
copper hydride species. Currently, we assume that an NHC–
Cu(I)H is the active catalytic species and the silane effects the
generation of the Cu(I)–H from the Cu(II) precursor.21 The
active copper hydride species reacts with a,b-unsaturated nitrile
forming a new organocopper species and the intermediate
undergoes rapid deprotonation by t-BuOH to yield the protonated
product and a copper alkoxide. The latter then regenerates the
active catalyst Cu–H with PMHS.
hydrosiloxane (PMHS) instantly activate
1 upon mixing.
Activation is indicated by the color change of the solution from
blue to yellow. The activation of 1 by tetramethyldisiloxane
(TMDS) was slow in comparison to diphenylsilane and PMHS.16
In order to compare the catalytic activity of 1 with that of Stryker’s
reagent in hydrosilylation reactions, a few examples of 1,2-/1,4-
reductions were carried out. As shown in Table 1, 1 performs
better than Stryker’s reagent both in the 1,2-reduction of aldehydes
and ketones17 and in the conjugate reduction of enones.3 The
conjugate reduction of a,b-, b,b-disubstituted enones was com-
pleted at faster reaction rates with lower catalyst loading (1 mol%)
of 1 than with Stryker’s reagent (5 mol%).
In conclusion,
a robust and highly effective copper(II)
precatalyst for 1,2- and 1,4-hydrosilylations has been prepared
and isolated in good yield. This well-defined and easily-handled
precatalyst is effectively activated by hydrosilanes and displays
great reactivity in reduction. Considering the convenience and
efficiency offered by 1, this precatalyst can be a good alternative to
the widely-used Stryker’s reagent under hydrosilylation conditions.
Next, we applied this catalyst system to the conjugate reduction
of a,b-unsaturated nitriles. a,b-Unsaturated nitriles are a class
of compounds that react very slowly compared to other related
unsaturated carbonyl compounds. Even stoichiometric amounts
of Stryker’s reagent cannot complete the conjugate reduction of
b-monosubstituted unsaturated nitriles,18 and our previous reduc-
tion system based on Cu–H with a chelating Xantphos-type
ligand19 is not effective for the reduction of highly sterically
demanding nitriles.5 As shown in Table 2, 1 mol% 1 is sufficient
to effect 1,4-reductions of a series of a,b-unsaturated nitriles.20 In
general, reactions catalyzed by 1 require smaller amounts of
catalyst, hydrosilane, and t-BuOH and shorter reaction times than
our previous system. For example, b-monosubstituted unsaturated
nitriles, 2a and 2b, were reduced to the corresponding saturated
nitriles in less than 20 min. with 1 mol% 1, which previously took
1 h with 3 mol% Cu(II)–Xantphos. b,b-Disubstituted unsaturated
nitriles (2c–2e) were successfully reduced in shorter reaction times
as well. It is noteworthy that the present catalytic system is quite
Table 2 Conjugate reduction of a,b-unsaturated nitriles using 1
Entry Substrate R
R9 Time/h Yield (%)a
1
2
3
4
5
6
7
a
2a
2b
2c
2d
2e
2f
Ph
Furanyl
CH3(CH2)5
Ph
p-CNC6H4
1,2,3,4-Tetrahydronaphthyl
t-Bu
H
H
,0.3 91
,0.3 87
CH3 1.5 88
CH3 88
CH3 4.5 94
1.5 87
CH3 1.5 82
3
2g
Isolated yield.
5182 | Chem. Commun., 2005, 5181–5183
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9 (a) F. Guillen, C. L. Winn and A. Alexakis, Tetrahedron: Asymmetry,
2001, 12, 2083; (b) J. Pytkowicz, S. Roland and P. Mangeney,
Tetrahedron: Asymmetry, 2001, 12, 2087.
10 V. Jurkauskas, J. P. Sadighi and S. L. Buchwald, Org. Lett., 2003, 5,
2417.
Studies aimed at investigating the scope and mechanism of this
catalyst system and devising more efficient NHC–Cu(II) acetate
complexes are currently ongoing in our laboratories.
This work was supported by Korea Research Foundation grant
funded by the Korean Government (R08-2004-000-10429-0).
11 H. Kaur, F. K. Zinn, E. D. Stevens and S. P. Nolan, Organometallics,
2004, 23, 1157.
12 A. N. Arduengo III, H. V. R. Dias, R. L. Harlow and M. Kline, J. Am.
Chem. Soc., 1992, 114, 5530; IPr is also now commercially available
from Strem as is its imidazolium salt precursor.
Notes and references
˚
13 A Cu–Csp3 single bond distance (1.90–1.96 A) is expected if the carbene
binds through a pure s-lone pair. See ref. 7c.
{ Synthesis of 1: Cu(OAc)2 (90.8 mg, 0.5 mmol) and IPr (214.8 mg,
0.55 mmol) were placed in a Schlenk flask and 3 mL of toluene were added.
The reaction mixture was stirred at room temperature for 12 h and a blue
precipitate formed. The resulting precipitate was isolated by filtration,
washed with toluene, and dried in vacuo to afford [Cu(IPr)(OAc)2] as a blue
solid (229 mg, 80%). Crystals suitable for X-ray diffraction studies were
obtained by slow diffusion of pentane to a concentrated CH2Cl2 solution of
the complex. (Found: C, 65.34; H, 7.48; N, 4.98%. Calc. for
C31H42CuN2O4: C, 65.30; H, 7.42; N, 4.91%.)
14 (a) D. J. Darensbourg, E. M. Longridge, M. W. Holtcamp,
K. K. Klausmeyer and J. H. Reibenspies, J. Am. Chem. Soc., 1993,
115, 8839; (b) D. J. Darenbourg, M. W. Holtcamp, B. Khandelwal and
J. H. Reibenspies, Inorg. Chem., 1994, 33, 531.
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J. H. Reibenspies, Inorg. Chem., 1992, 31, 3951; (b) N. P. Mankad,
T. G. Gray, D. S. Laitar and J. P. Sadighi, Organometallics, 2004, 23,
1191.
16 The following order of silane reactivity was observed PhSiH3 . Ph2SiH2
y PMHS . TMDS ..PhMe2SiH; when TMDS was employed,
the reaction mixture was stirred for 20–30 min for catalyst activation
before the addition of substrate. The activation of 1 with PhMe2SiH
failed and no reduction occurred with PhMe2SiH as the stoichiometric
reducing agent.
17 B. H. Lipshutz, W. Chrisman and K. Noson, J. Organomet. Chem.,
2001, 624, 367.
18 P. Chiu, C.-P. Szeto, Z. Geng and K.-F. Cheng, Org. Lett., 2001, 3,
1901.
19 DPEphos 5 bis(2-diphenylphosphinophenyl)ether, Xantphos 5 9,9-
dimethyl-4,6-bis(diphenylphosphino)xanthene.
§ Crystal structure data for 1, C31H42CuN2O4, Mr 5 570.22, monoclinic,
˚
˚
space group C2/c, a 5 22.863(3) A, b 5 8.680(4) A, c 5 17.977(5) A, b 5
˚
3
˚
119.275(15)u, V 5 3111.8(16) A , T 5 150(1) K, Z 5 4, rcalc
5
1.217 Mg m23, F(000) 5 1212, crystal dimensions 0.10 6 0.10 6 0.10 mm3,
m(MoKa) 5 0.737 mm21, MoKa radiation (l 5 0.7107 A). Of 14788
˚
reflections collected in the 2h range 3.0u–55.0u using an v scan on a Rigaku
R-axis rapid diffractometer, 3554 were unique reflections (Rint 5 0.109).
The structure was solved and refined against F2 using SHELXS22 and
SHELXL9723, 164 variables, wR2 5 0.1450 (the 3554 unique reflections),
2
R1 5 0.0618 (the 1667 reflections having Fo . 2s(Fo2)), GOF 5 1.008,
and max/min residual electron density 0.336/20.267 e A23. CCDC 274568.
˚
See http://dx.doi.org/10.1039/b509964a for crystallographic data in CIF or
other electronic format.
20 General procedure for the conjugate reduction of a,b-unsaturated
nitriles: 1 (5.7 mg, 0.010 mmol) was placed in an oven-dried Schlenk
tube and PMHS (0.18 mL, 3.0 mmol) and toluene (0.50 mL) were added
under N2. The reaction mixture was stirred for 5 min at room
temperature and then, a,b-unsaturated nitrile (1.0 mmol) was added,
followed by t-BuOH (0.18 mL, 2.0 mmol). The reaction tube was
washed with toluene (0.50 mL), sealed, and stirred until no starting
nitrile was detected by TLC or GC. The reaction mixture was quenched
with water and transferred to a round-bottom flask with the aid of Et2O
(10 mL), and NaOH (2.5 M, 1.2 mL) was added. The biphasic mixture
was stirred vigorously for 0.5 h. The layers were separated and the
aqueous layer was extracted with Et2O (3 6 20 mL). The combined
organic layers were washed with brine, dried over MgSO4, and
concentrated. The product was purified by Kugelrohr distillation or
silica gel chromatography.
1 For a review, see: B. H. Lipshutz, in Modern Organocopper Chemistry,
ed. N. Krause, Wiley-VCH, Weinheim, 2002, pp. 167–187.
2 W. S. Mahoney, D. M. Brestensky and J. M. Stryker, J. Am. Chem.
Soc., 1988, 110, 291.
3 B. H. Lipshutz, W. Chrisman, K. Noson, P. Papa, J. A. Sclafani,
R. W. Vivian and J. M. Keith, Tetrahedron, 2000, 56, 2779.
4 D. Lee and J. Yun, Tetrahedron Lett., 2004, 45, 5415.
5 D. Kim, B.-M. Park and J. Yun, Chem. Commun., 2005, 1755.
6 For recent reviews, see: (a) W. A. Herrmann, Angew. Chem., Int. Ed.,
2002, 41, 1290; (b) D. Bourissou, O. Guerret, F. P. Gabba¨ı and
G. Bertrand, Chem. Rev., 2000, 100, 39.
7 (a) A. O. Larsen, W. Leu, C. N. Oberhuber, J. E. Campbell and
A. H. Hoveyda, J. Am. Chem. Soc., 2004, 126, 11130; (b) X. Hu,
I. Castro-Rodriguez and K. Meyer, J. Am. Chem. Soc., 2003, 125,
12237; (c) P. L. Arnold, A. C. Scarisbrick, A. J. Blake and C. Wilson,
Chem. Commun., 2001, 2340 and ref. 15b.
21 For the generation of phosphine-ligated Cu(I)–H from a copper(II)
precursor, see: D. Lee and J. Yun, Tetrahedron Lett., 2005, 46, 2037.
22 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
23 G. M. Sheldrick, SHELX97, Program for the Refinement of Crystal
Structure, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
8 A. J. Arduengo III, H. V. R. Dias, J. C. Calabrese and F. Davidson,
Organometallics, 1993, 12, 3405.
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Chem. Commun., 2005, 5181–5183 | 5183