CuI/Pd0 cooperative dual catalysis: Tunable stereoselective construction of tetra-substituted alkenes

Article abstract of DOI:10.1002/chem.201304284

This paper describes a tunable and stereoselective dual catalytic system that uses copper and palladium reagents. This cooperative silylcupration and palladium-catalyzed allylation readily affords trisubstituted alkenylsilanes. Fine-tuning the reaction conditions allows selective access to one stereoisomer over the other. This new methodology tolerates different substituents on both coupling partners with high levels of stereoselectivity. The one-pot reaction involving a Cu^I/Pd⁰ cooperative dual catalyst directly addresses the need to develop more time-efficient and less-wasteful synthetic pathways.

Full text of DOI:10.1002/chem.201304284

DOI: 10.1002/chem.201304284
Communication
&
Catalysis
CuI/Pd0 Cooperative Dual Catalysis: Tunable Stereoselective
Construction of Tetra-Substituted Alkenes
[a]
¨
Sꢀbastien Vercruysse, Loıc Cornelissen, Fady Nahra, Laurent Collard, and Olivier Riant*
formations.[5] In particular, trisubstituted alkenylsilanes could
be useful for the construction of tetrasubstituted alkenes. Re-
cently, particular attention has been paid to silylmetalation re-
actions catalyzed by transition metals, such as rhodium and
copper complexes.[6] This new trend has gained considerable
interest because of the readily available silicon pronucleo-
philes, such as the Suginome reagent (PhMe2SiBpin, pin=pina-
col).[7]
Abstract: This paper describes a tunable and stereoselec-
tive dual catalytic system that uses copper and palladium
reagents. This cooperative silylcupration and palladium-
catalyzed allylation readily affords trisubstituted alkenylsi-
lanes. Fine-tuning the reaction conditions allows selective
access to one stereoisomer over the other. This new meth-
odology tolerates different substituents on both coupling
partners with high levels of stereoselectivity. The one-pot
reaction involving a CuI/Pd0 cooperative dual catalyst di-
rectly addresses the need to develop more time-efficient
and less-wasteful synthetic pathways.
Applying this methodology to alkynes to form b-silyl alkenyl
carbonyl compounds has been long neglected. Although the
first example was reported by Oshima et al. in 1983,[8] this
methodology was only revisited in 2011, by Loh et al.[9] Gener-
ally, these b-silyl alkenyl carbonyl compounds are still being ac-
cessed by means of the hydrosilylation of electron-deficient al-
kynes by using ruthenium catalysts.[10] Tetrasubstituted b-silyl
alkenyl carbonyl compounds are even more difficult to form
and, therefore, their synthetic accessibility is limited.[3c,11]
In recent years, our group and others have developed effi-
cient catalytic systems for the generation of silyl copper(I) in-
termediates that could react with electron-deficient double
bonds, such as acrylates (Scheme 1 a).[6d,h] Furthermore, we
have recently developed a CuI/Pd0 cooperative dual catalytic
process to access enantioenriched a-allylated ketones.[12] In
this strategy, a copper(I)-catalyzed 1,4-reduction reaction was
combined with a palladium-catalyzed asymmetric allylic alkyla-
tion reaction (Scheme 1 b). We envisioned that merging the
catalytic silylcupration with the palladium allylation on an in-
ternal alkyne would provide access to tetrasubstituted vinylsi-
lanes (Scheme 1 c).
In recent decades, the importance of catalysis has grown sub-
stantially. Nevertheless, until recently, efforts have focused on
traditional catalysis, involving a single catalyst. In general, for
this single-catalyst approach the catalyst is activated by one
substrate, lowering the energetic barrier of bond formation
with the unactivated substrate.[1] Recently, the spectrum of in-
terest has shifted towards the emerging concept of coopera-
tive dual catalysis. This type of catalysis is based on the use of
two catalysts working together by activating each substrate
simultaneously. This concept is not new, and is prevalent in
nature. Numerous examples of the development and improve-
ment of chemical transformations have been recently reported
by the MacMillan group and others.[1,2] Nevertheless, the ste-
reocontrol of multisubstituted double bonds has been some-
what neglected.
Regio- and stereocontrol of tetrasubstituted alkenes remains
an important challenge in organic chemistry. During the last
decade, organic chemists have developed strategies to efficiently
synthesize tri- and tetrasubstituted alkenes.[3] The most widely
used methodologies consist of carbometalation strategies using
palladium, copper, zinc, boron, tin, or magnesium reagents.[2l,3b,d,4]
Alkenylsilanes are useful synthetic intermediates in organic
synthesis owing to their occurrence in various chemical trans-
[a] S. Vercruysse, L. Cornelissen, F. Nahra, L. Collard, Prof. O. Riant
Institute of Condensed Matter and Nanosciences–Molecules
Solids and Reactivity (IMCN/MOST)
Universitꢀ Catholique de Louvain
Place Louis Pasteur 1, bte L4.01.03
1348 Louvain-la-Neuve (Belgium)
Fax: (+32)10 47-41-68
E-mail: Olivier.riant@uclouvain.be
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304284.
Scheme 1. New strategy to access b-silyl alkenyl carbonyl compounds.
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Herein, we report our results concerning the development
of a cooperative dual catalysis reaction for the formation of
functionalized stereodefined tetrasubstituted alkenes.
Methyl-3-phenylpropiolate 1 was chosen as a model sub-
strate for the optimization process. Conditions that have previ-
ously been used to access a-allylated ketones[12] were initially
used, substituting the hydride source (Me(EtO)2SiH) by the silyl
boronic ester (PhMe2SiBPin). With these conditions in hand,
preliminary testing revealed that the reaction proceeded
smoothly and the desired product, 2, was isolated as a mixture
of two stereoisomers (Scheme 2). Accessing either one or the
other of these stereoisomers presented an exciting challenge.
During the preliminary optimization, (Z)-2 was the major ste-
reoisomer derived from this reaction. NOESY-2D experiments
unambiguously showed the major stereoisomer to be the Z
isomer.[13]
Scheme 3. The effect of the ligand on the reaction. [a] Yield of 2 determined
1
by H NMR spectroscopy with dinitrobenzene as an internal standard.
1
[b] Ratio of the two stereoisomers determined by H NMR analysis of the
crude mixture. [c] 3.5 mol% of PPh3 was used.
entry 5). However, when CuCl was used with PPh3 as a ligand,
a similar yield was obtained with a slight improvement in ste-
reoselectivity (Table 1, entry 7). These reagents are commercial-
ly available and, therefore, this new simplified version of the
reaction provided a cheaper route to the desired product. It
should be noted that when 1 mol% of PPh3 was used, instead
of 3.5 mol%, 2 was obtained in 83% yield and a ratio of 70:30
(Z)-2/(E)-2 was observed (Table 1, entry 6). Intriguingly, this
ratio was the lowest observed during the optimization for se-
lectively obtaining the (Z)-2 stereoisomer.
Scheme 2. Preliminary results obtained by using our previous optimal condi-
The above results indicate that on lowering the amount of
PPh3, the (E)-2 stereoisomer becomes slightly more favored.
Even though (Z)-2 remains as the major stereoisomer, we
tions.
Heating the reaction to 458C was sufficient to afford a good
yield (see the Supporting Information, S3). Subsequent optimi-
zation of the reaction solvent revealed dichloromethane to be
the optimal solvent, yielding 80% of 2 and a 88:12 ratio of (Z)-
2/(E)-2. Altering the CuI/Pd0 ratio, as well as the amount of
tBuOK used, showed no improvements in the reaction yield or
selectivity (see the Supporting Information, S3).
Table 1. Optimization of the copper and palladium source.
A quick screening of different phosphine ligands, well
known in p-allyl palladium chemistry, allowed us to optimize
the reaction even further (Scheme 3). Using PPh3, instead of
1,2-bis(diphenylphosphino)ethane (dppe), as the palladium
ligand gave a similar yield, but slightly increased the stereoiso-
meric ratio to 91:9 (Scheme 3).
Entry
Cu source
Pd source
Yield 2 [%][a]
(Z)-2:(E)-2[b]
1
2
3[c]
4
[(IMes)Cu(DBM)]
[(IMes)Cu(DBM)]
[(IMes)Cu(DBM)]
[(IPr)Cu(DBM)]
[(IMes)CuCl]
Pd(OAc)2
[PdCl2PPh3]
[Pd2dba3]
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
79
78
68
77
53
83
80
91:9
88:12
87:13
90:10
83:17
70:30
92:8
When the palladium source was varied, no improvement in
the yield or stereoselectivity of the desired product was ob-
served (Table 2, entries 1–3). In a last attempt to optimize the
system, different copper complexes were tested (Table 2, en-
tries 4 and 5). On replacing the 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene (IMes) ligand on the copper complex with
the bulkier 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
(IPr) ligand no significant effect was observed (Table 1, entry 4).
Changing the counter ion from dibenzoylmethanate (DBM) to
a chloride ion did not improve the reaction either (Table 1,
5
6[d]
7[e]
[(IMes)Cu(DBM)]
CuCl/PPh3
[a] Yields determined by 1H NMR spectroscopy with dinitrobenzene as an
internal standard. [b] Ratio of the two stereoisomers determined by
1H NMR analysis of the crude mixture. [c] 0.5 mol% of [Pd2dba3] (1 mol%
of Pd, dba=dibenzylideneacetone) was used. [d] 1 mol% of Pd(OAc)2
was used with 1 mol% of PPh3. [e] 5 mol% of CuCl and a total of
23.5 mol% of PPh3 was used.
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Table 2. Varying the amount of PPh3.
Entry
x [mol%]
Yield 2 [%][a]
(Z)-2:(E)-2[b]
1
2
3
4
5[c]
3.5
1
0.5
0
79
83
84
86
82
91:9
70:30
38:62
28:72
4:96
0
[a] Yields determined by 1H NMR spectroscopy with dinitrobenzene as an
internal standard. [b] Ratio of the two stereoisomers determined by
1H NMR analysis of the crude mixture. [c] 5 mol% of Pd(OAc)2 was used.
thought that this finding could be further exploited and opti-
mized to selectively obtain (E)-2. Therefore, the amount of
PPh3 was varied and it was found that lower amounts of PPh3
lead to an increased selectivity for the (E)-2 stereoisomer
(Table 2, entries 1–3). The optimal result was obtained when
no phosphine ligand was used (Table 2, entry 4). Finally, when
5 mol% of Pd(OAc)2 was used in the absence of any phosphine
ligand, a complete inversion of selectivity was observed and
the (E)-2 stereoisomer was obtained in a 4:96 (Z/E) ratio
(Table 2, entry 5).
Next, we examined the scope of the reaction by varying the
substitution patterns on both coupling partners. Initially, vari-
ous electron-deficient alkynes,[14] with different substituents,
were tested in the two methods mentioned previously (meth-
ods A and B, affording the Z and E stereoisomers, respectively)
by using methyl allylcarbonate as the model allylating reagent
(Scheme 4). The results showed that alkyl groups at the 4-posi-
tion were well tolerated and gave good yields and high stereo-
selectivities for the desired products, (Z)-5, (Z)-7, and (Z)-6.
Substituents bearing different functional groups also under-
went both reactions conditions to form the desired products,
(Z)-8, (E)-8, and (Z)-9, in good yields and high stereoselectivities
for both isomers. Different substituents on the ester moiety
((Z)-10, (Z)-12 and (E)-12) also gave satisfactory results.
Scheme 4. The scope of the alkynyl ester. Yields of the two stereoisomers
1
were determined by H NMR spectroscopy with dinitrobenzene as an inter-
1
nal standard. The ratio of the two stereoisomers was determined by H NMR
analysis of the crude mixture.
a secondary product, 3, derived from the hydrolysis of the
copper intermediates I–III, was observed as a mixture of two
stereoisomers. (Scheme 6).
Finally, different allylic carbonates[15] were employed, under
the reaction conditions in method A and B, with 1 as a model
substrate (Scheme 5). The results confirmed that different sub-
stituents, such as alkyl, heteroaryl (for example, thiophene),
and functionalized aryl moieties bearing a halide or even an al-
dehyde group, are well tolerated in method A. Method B was
also used successfully to obtain product (E)-13 in good yield
and stereoselectivity. Moreover, the reaction exhibits high re-
gioselectivity (no branched allylation product was observed).
No isomerization of (E)-2 to (Z)-2 occurs when (E)-2 is sub-
jected to the reaction conditions of method A or to triphenyl-
phosphine alone. This result indicates that no isomerization
through a Baylis–Hilman-type mechanism occurs with the reac-
tion conditions of method A and, thus, this mechanism cannot
account for the E/Z selectivity. Finally, in all of the reactions,
The typical cupration of propiolates is known to proceed by
means of a syn-addition pathway, leading to an alkenyl copper
intermediate.[16] The difference observed in the Z/E selectivity
could be attributed to the difference in reactivity of the p-allyl
palladium intermediates, which are generated in solution.[17]
Without the addition of a phosphine reagent, a less-electro-
philic dimeric p-allyl species, IV, is generated and the Z selec-
tivity could be explained by the direct transmetalation with Z-
vinyl copper intermediate I, through an inner-sphere mecha-
nism. However, under the conditions of method A, the mono-
meric cationic p-allyl palladium intermediate, V, which has
PPh3 ligands, inhibits the transmetalation pathway. The subse-
quently formed intermediate, I, could isomerize to III via the al-
lenolate species II, as previously reported for the related conju-
gate additions of organocopper reagents to alkynoates.[18] Cat-
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ionic palladium intermediate V promotes nucleophilic attack
from the less hindered side of the allenoate through an outer-
sphere mechanism, affording (E)-2 as the major product.
(Scheme 6). This hypothesis is supported by a similar chemose-
lectivity switch reported by the group of Echavarren[19]
.
In conclusion, we have developed a new way to access
highly substituted and functionalized vinylsilanes. The Z and E
stereoisomers could be obtained selectively and efficiently by
slightly changing the reaction conditions. This methodology
tolerates different substituents on both coupling partners,
such as halide, thiophene, carbonate, or even aldehyde moiet-
ies, with high levels of stereoselectivity. This one-pot reaction,
involving a CuI/Pd0 cooperative dual catalyst, directly addresses
the need to develop more time-efficient and less-wasteful syn-
thetic pathways.
Experimental Section
General procedure for the dual-catalysis reaction (method A,
optimized conditions for (Z)-2)
A flame-dried Schlenk flask was charged sequentially with palladi-
um acetate (1.2 mg, 0.005 mmol, 0.01 equivalents), PPh3 (4.6 mg,
0.0175 mmol, 0.035 equivalents), and dichloromethane (0.4 mL).
The reaction mixture was stirred at room temperature. After 5 min
the allylcarbonate (0.12 mL, 1 mmol, 2.0 equivalents) was added
and the mixture was stirred for an additional 5 min. Simultaneous-
ly, another flame-dried Schlenk flask was charged with CuCl
(2.5 mg, 0.025 mmol, 0.05 equivalents), PPh3 (26.2 mg, 0.1 mmol,
0.2 equivalents), and KOtBu (5.8 mg, 0.05 mmol, 0.1 equivalents) in
dichloromethane (0.4 mL) and stirred for 1 min. Once the palladi-
um acetate had completely dissolved, the palladium mixture was
transferred into the Schlenk flask containing the copper catalyst,
followed immediately by methylphenylpropiolate (80.1 mg in
0.4 mL dichloromethane, 0.5 mmol, 1 equivalent). The mixture was
then immediately heated to 458C and PhMe2SiBpin (0.21 mL,
0.75 mmol, 1.5 equivalents) was quickly added by using a syringe.
Scheme 5. The scope of the allyl carbonate. Yields of the two stereoisomers
1
were determined by H NMR spectroscopy with dinitrobenzene as an inter-
1
nal standard. The ratio of the two stereoisomers was determined by H NMR
analysis of the crude mixture.
After 24 h, 0.7 mL of petroleum ether/ether (4:1) was added to the
reaction mixture. The solution was then filtered through a plug of
silica gel (eluted with petroleum ether/Et2O 4:1). The filtrate was
concentrated in vacuo to obtain a colorless residue. TMS was se-
lected as internal calibration and 1,4-dinitrobenzene was used as
an internal standard. The yield and the diastereoisomeric ratio
were determined by NMR analysis. The residue was purified by
HPLC chromatography or flash chromatography to give the desired
product (Z)-2.
General procedure for the dual-catalysis reaction (method B,
optimised conditions for (E)-2)
A flame-dried Schlenk flask was charged sequentially with palladi-
um acetate (5.7 mg, 0.025 mmol, 0.05 equivalents) and dichloro-
methane (0.4 mL). The reaction mixture was stirred at room tem-
perature. After 5 min the allylcarbonate (0.12 mL, 1 mmol,
2.0 equivalents) was added and the mixture was stirred for an addi-
tional 5 min. Simultaneously, another flame-dried Schlenk flask was
charged with [(IMes)Cu(DBM)] (15 mg, 0.025 mmol, 0.05 equiva-
lents), and KOtBu (5.8 mg, 0.05 mmol, 0.1 equivalents) in dichloro-
methane (0.4 mL) and stirred for 1 min. Once the palladium acetate
had completely dissolved, the palladium mixture was transferred
into the Schlenk flask containing the copper catalyst, followed im-
Scheme 6. Proposed pathway for the observed products and selectivity.
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chloromethane, 0.5 mmol, 1 equivalent). The mixture was then im-
mediately heated to 458C and the PhMe2SiBpin (0.21 mL,
0.75 mmol, 1.5 equivalents) was quickly added. After 24 h, 0.7 mL
of petroleum ether/Et2O (4:1) was added to the reaction mixture.
The reaction mixture was then filtered through a plug of silica gel
(eluted with petroleum ether/Et2O 4:1). The filtrate was concentrat-
ed in vacuo to obtain a colorless residue. 1,4-dinitrobenzene was
selected for internal calibration and was added (0.21 mg,
0.125 mmol, 0.25 equivalents) to the colorless residue. A sample of
this residue was taken for NMR analysis to determine the yield and
the diastereoisomeric ratio. The residue was purified by HPLC chro-
matography or flash chromatography to give the desired product
(E)-2.
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Keywords: cooperative catalysis · copper · dual catalysis ·
palladium · silylboronate
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Received: November 1, 2013
Published online on January 21, 2014
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