Catalytic organocopper chemistry from organosiloxane reagents

Article abstract of DOI:10.1002/chem.200901438

The coupling of organosilanes with vinyl epoxides in the presence of a copper catalyst has been reported. Functional-group-tolerant synthetic methods of organosilanes can be developed through activation and transmetalation with a transition-metal catalyst

Full text of DOI:10.1002/chem.200901438

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DOI: 10.1002/chem.200901438
Catalytic Organocopper Chemistry from Organosiloxane Reagents
Jessica R. Herron,[a] Vincenzo Russo,[a] Edward J. Valente,[b] and Zachary T. Ball*[a]
The search for increasingly selective and environmentally
benign methods continues to be of importance to synthetic
methodology. Organosilane reagents are important in this
regard because they are typically stable to a wide range of
functional groups and reaction conditions, allowing selective
chemical transformations in multifunctional molecules.[1] In
addition, organosilanes are generally inexpensive and pro-
duce innocuous and easily removed polysiloxane byproducts.
However, the classical use of organosilanes as carbon nucle-
metal organometallics, such as those of copper, are unstable
with respect to protonolysis in the presence of water, thus
preventing the addition of water to allow access to silanoate
intermediates.
Focussing on copper-catalyzed activation of organosilanes,
important early work demonstrated the feasibility of this ap-
proach with readily transferrable groups such as alkynyl-
and allylsilanes.[8] Beyond these substrate classes, demon-
stration of organocopper reactivity from organosilanes is
generally limited to specially activated substrates.[9,10]
À
ophiles for C C bond formation is generally limited to in-
À
tramolecular processes or reactions with extremely reactive
electrophiles. We here report the coupling of organosilanes
with vinyl epoxides in the presence of a copper catalyst.
Functional-group-tolerant synthetic methods of organosi-
lanes can be developed through activation and transmetala-
tion with a transition-metal catalyst to produce new, more
reactive organometallic intermediates. This approach has
been successfully applied to palladium-catalyzed cross-cou-
pling chemistry, typically utilizing fluoride activation of the
Copper-based organosilane C C bond formation processes
include alkenylsilane dimerization with stoichiometric
copper[11] and catalytic 1,2-addition to aldehydes.[12] In these
cases, organocopper intermediates can be inferred on the
basis of available evidence, but care must be taken since
À
fluoride can activate organosilanes for C C bond formation
and it can be difficult to distinguish transmetalation path-
ways from silicate reactivity.[3,13] Copper salts have come
into use as additives in palladium cross coupling, and they
show beneficial effects with organosilane substrates as
well;[14] the role of copper in these palladium-catalyzed reac-
tions is not well understood. Stoichiometric fluoride activa-
tors are typically employed in silane activation chemistry,
yet few stable, anhydrous fluoride sources are soluble in
apolar, aprotic solvents.[15] Studying copper(I) chemistry in
the presence of fluoride is further challenging due to dispro-
portionation of CuF in the absence of appropriate stabilizing
ligands.[16]
silane.[2–4] However, the kinetic stability and strong C Si
À
bonds of organosilanes have limited the use of organosilane
precursors for catalytic generation of other organometallic
intermediates.[5] A few reports of rhodium-catalyzed reactiv-
ity have appeared,[4,6] but rhodium and palladium processes
typically occur in the presence of water, allowing the inter-
mediacy of silanolates which are thought to facilitate trans-
metalation.[7] Aqueous conditions are possible since these
oganometallics are relatively stable to proteolytic decompo-
sition. Extension of these ideas to other transition metals is
challenging because many synthetically important transition-
To develop silane-based methods for organocopper reac-
tivity, such as addition to electron-deficient olefins, we previ-
ously examined the transmetalation of organosilanes with a
well-defined copper(I) fluoride complex 1.[17] In that initial
report, we were able to demonstrate clean transmetalation
of arylsilanes to afford stable aryl copper compounds and to
[a] J. R. Herron, V. Russo, Prof. Z. T. Ball
Department of Chemistry MS-60
Rice University, Houston, TX 77005 (USA)
Fax : (+1)713-348-5155
À
demonstrate the feasibility of C C bond-forming reactions
E-mail: zb1@rice.edu
of these intermediates. However, the sluggish reactivity of
[b] Prof. E. J. Valente
copper complexes with the IPr ligand (IPr=1,3-bis(2’,6’-di-
AHCTUNGTREGiNNUN sopropylphenyl)imidazol-2-ylidene) in transmetalation and
Department of Chemistry/Swindells 108
University of Portland
Portland, OR 97203 (USA)
À
C C bond-forming reactions prevented its extension into a
general catalytic process.[18] Treatment of complex 1 with
phenyltrimethoxysilane in THF results in the quantitative
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901438.
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Table 1. Catalytic coupling of organosiloxanes with vinylepoxides.[a]
formation of [(IPr)CuPh], but the process is rather slow, re-
quiring about 2 h to reach complete conversion. More trou-
blesome, the [(IPr)CuPh] complex exhibits limited reactivi-
ty: [(IPr)CuPh] (5a) does not react at all with butadiene
monoepoxide at ambient temperature, and eventually de-
composition is observed at elevated temperatures.
Entry[a] Silane
Epoxide[b] Product
Yield
[%],
(E/Z
ratio)
72
(2.6:1)
1
2
70
(2.0:1)
90
(2.4:1)
3
4
65
(1.8:1)
81
(2.0:1)
To improve reactivity, we examined other ligands, settling
on the saturated ligand dicyclohexylimidazolin-2-ylidene
(SICy).[19] The choice was based on the reasoning that the
decreased steric demand and increased electron-donating
ability might improve reactivity. Stoichiometric reactivity
studies confirm this straightforward hypothesis. The complex
[(SICy)CuF] (3) is readily available upon treatment of the
dimeric tert-butoxide complex 2 with Et3N·3HF.[20] In a stoi-
5
6
80
(2.2:1)
82
(1.5:1)
7
45
(1.5:1)
1
8
9
chiometric study analyzed by H NMR spectroscopy, trans-
metalation of trimethoxyphenylsilane with the [(SICy)CuF]
complex (3) proceeds in less than 5 min at ambient tempera-
ture, and the organocopper compound 5b reacts cleanly,
though slowly, with butadiene monoepoxide, also at ambient
temperature (Scheme 1). The stoichiometric reaction of the
68
(2.4:1)
70
(3.9:1)
10
11
89
(7.8:1)
82
(1:1)
12
13
80
(3:1)[b]
Scheme 1. Reactivity of N-heterocyclic carbene–copper complexes in
transmetalation and conjugate addition reactions.
[a] Unless otherwise noted, all reactions began at À788C, and were immedi-
ately allowed to warm to room temperature. The reaction time is 48 h. Prod-
ucts were isolated as a mixture of E/Z isomers, and the isomeric ratio was
determined by 1H NMR spectroscopy or GC-MS analysis. [b] Ratio of 1,4-
product to 1,2-product (7a/7b). Only products of anti stereochemistry are
observed.
phenylcopper species 5b with butadiene monoepoxide took
À
8 h to reach completion, indicating that C C bond forma-
tion may well be turnover-limiting in catalytic reactions (see
the Supporting Information for details).
Despite concerns about our ability to generate turnover
in the absence of a stoichiometric fluoride activator, we
were overjoyed to find that complex 3 is a competent cata-
lyst for the formation of alcohol 6 (Table 1, entry 1), pro-
ceeding in good yield at ambient temperature.[21] The reac-
tion appears general for aryl and heteroaryl silanes. Substi-
tution at the ortho position is well tolerated (Table 1, en-
tries 6 and 7), and electron-poor as well as electron-rich
arene rings succeed as nucleophiles (Table 1, entries 3 and
4). In addition, the use of organosilane reagents allows sig-
nificant functional group tolerance to the coupling reaction.
Electrophilic carbonyl groups (Table 1, entry 4) succeed in
the present method, as does an aryl chloride (Table 1,
entry 8). In addition to trialkoxysilanes, catalytic activation
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Catalytic Organocopper Chemistry from Organosiloxane Reagents
COMMUNICATION
proceeds cleanly with monoalkoxysilanes (Table 1, entries 2,
8, 11). This is a significant synthetic benefit, as monoalkoxy-
silanes have increased stability towards hydrolysis and are
readily purified by silica gel chromatography.
Experimental Section
General procedure: Synthesis of 4-(4-methoxyphenyl)-2-buten-1-ol
(Table 1, entry 3): In a glovebox, (SICy)CuF (50 mg, 0.158 mmol) and
THF (1.0 mL) were added to a 20 mL scintillation vial equipped with a
Teflon-coated stirbar. The vial was sealed with a rubber septum and re-
moved from the glovebox. The silane, (4-methoxyphenyl)-triethoxysilane
(510 mg, 1.89 mmol) and butadiene monoepoxide (110 mg, 1.57 mmol)
were added by using a syringe at À788C. The contents of the vial were al-
lowed to warm to room temperature and were stirred for 48 h. The reac-
tion was quenched with HCl (1 mL, 2 N aq soln). The organic layer was
diluted with ether (15 mL), washed with water (15 mL), dried over
MgSO4, filtered, and concentrated in vacuo. The crude reaction mixture
was taken up in hexane (10 mL) and filtered through Celite. The product
was purified by silica gel chromatography (eluent: 400:20:20:1 hexanes/
acetone/dichloromethane/methanol) to afford 252 mg (90% yield) of a
2.4:1 E/Z mixture of 4-(4-methoxyphenyl)-2-buten-1-ol.
A variety of vinyl epoxide substitution patterns are also
tolerated. In acyclic cases, only the 1,4-additon product is
observed, but a cyclic epoxide (Table 1, entry 13) gave
minor amounts of the 1,2-addition product 7b in addition to
the expected product 7a. Both products 7a and 7b are iso-
lated with strictly anti stereochemistry, resulting from clean
attack on the face opposite the epoxide leaving group.[22]
In addition to aryl and heteroaryl silanes, activated sp3-or-
ganosilanes also participate. Benzyl- and 2-furylmethylsi-
lanes (Table 1, entries 10, 11) couple successfully in the cata-
lytic process. Whereas aryl silane reagents afford mixtures
of olefin isomers, reactions of these sp3-organosilanes exhibit
significant bias for the formation of the (E)-olefin product.
Benzyltriethoxysilane is an example of the increased reactiv-
ity of [(SICy)CuF] (1) relative to [(IPr)CuF] (3) in transme-
Acknowledgements
À
talation reactions in addition to C C bond-forming steps.
Treatment of [(IPr)CuF] with benzyltriethoxysilane does not
result in any benzyl group transmetalation, yet clean catalyt-
ic reactivity is observed with [(SICy)CuF] (Table 1,
entry 10).
The authors gratefully acknowledge financial support provided by the
Robert A. Welch Foundation research grant C-1680 and Rice University.
À
Keywords: C C coupling reactions · copper · epoxides ·
homogeneous catalysis · silicon
The tert-butoxide complex 2 is quite unstable, decompos-
ing within hours at ambient temperature. The crystal struc-
ture of tert-butoxide 2 confirms the presence of a dimer in
the solid state (see the Supporting Information),[23] consis-
tent with the similar copper tert-butoxide complex, [{(SIiPr)-
CuOtBu}2].[9b] By contrast, the fluoride complex [(SI-
Cy)CuF] (3) is monomeric[20] and is stable indefinitely at
room temperature. The dimeric nature of tert-butoxide 2
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catalytically active species is a copper fluoride or copper
alkoxide is an unanswered question; preliminary NMR evi-
dence indicates that fluoride/ethoxide exchange is faster
than transmetalation under relevant reaction conditions.
We describe a mild and general catalytic method for ac-
cessing organocopper chemistry from organosilane reagents
without the need for stoichiometric fluoride additives. Cata-
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on silicon surfaces in addition to small-molecule synthesis.
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Received: May 28, 2009
Published online: July 20, 2009
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