Nickel-catalyzed heck-type alkenylation of secondary and tertiary α-carbonyl alkyl bromides

Article abstract of DOI:10.1002/anie.201108350

Ni made it! A novel Heck-type reaction of secondary and tertiary α-carbonyl alkyl bromides, most likely involving a radical process, was achieved through the use of a nickel catalyst. Various substituted styrenes and 1,1-diaryl alkenes were utilized as substrates to easily construct α-alkenyl carbonyl compounds with tertiary or quaternary carbon centers. A catalytic cycle involving Ni^I/Ni^II is proposed based on our experimental results. EWG=electron-withdrawing group.

Full text of DOI:10.1002/anie.201108350

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Angewandte
Communications
DOI: 10.1002/anie.201108350
Cross-Coupling
Nickel-Catalyzed Heck-Type Alkenylation of Secondary and Tertiary
a-Carbonyl Alkyl Bromides**
Chao Liu, Shan Tang, Dong Liu, Jiwen Yuan, Liwei Zheng, Lingkui Meng, and Aiwen Lei*
Since its development in the early 1970s, the palladium-
catalyzed Heck reaction has become a powerful tool for the
alkenylation of aryl and alkenyl electrophiles in chemical
synthesis [Eq. (1)].[1] Thus far, alkyl halides have had limited
application in Heck-type reactions, with only isolated exam-
metal enolates.[7] Compared to the widely available alkenes,
alkenyl halides are normally expensive and not easy to obtain.
On the other hand, a-carbonyl alkyl halides are bulk
chemicals and readily available. Obviously, the direct utiliza-
tion of alkenes as coupling partners with a-carbonyl alkyl
halides to construct a-alkenyl carbonyls would be an appeal-
ing approach.
ples available for primary alkyl halides[1b,2] and both secon-
dary and tertiary alkyl halides remaining almost unstudied.[3]
This may be because of facile b-hydride elimination on
palladium catalysts with such species. To solve this problem,
other transition metal catalysts would be potential choices,
but very few results have been reported to date. Cobalt and
titanocene catalysts were among the few reports to achieve
the reaction of alkyl halides with styrenes. However, highly
reactive alkyl magnesium reagents were needed as a base in
these transformations,[4] thus limiting their potential applica-
tion. Although nickel catalysts have been applied in various
cross-couplings of alkyl halides with organometallic
reagents,[5] the application of such catalysts in the Heck-type
reaction of alkyl halides is rare.[6] Herein, we report the first
nickel-catalyzed Heck-type reaction of secondary and tertiary
a-carbonyl alkyl halides with olefins to construct a-alkenyl
carbonyls [Eq. (2)].
Recently, the direct coupling of secondary a-carbonyl
alkyl halides with arylboronic acids to achieve a-arylation of
carbonyls in the presence of a Ni catalyst has been accom-
plished in our group.[8] To achieve a-alkenylation of carbon-
yls, we decided to attempt the coupling of alkenes with a-
carbonyl alkyl halides. We started our research by applying
ethyl 2-bromopropanoate and p-methoxystyrene in a model
reaction to test different reaction conditions. Selected data
from this study is listed in Table 1.
First, various Ni catalyst precursors were screened in
toluene at 608C in the presence of K3PO4 as base. No reaction
occurred with NiII precursors (Table 1, entries 2–5), however,
the reaction did proceed with the Ni0 precursor [Ni(PPh3)4]
and afforded the coupling product in 4% yield (Table 1,
entry 6). This indicated that NiII could not be reduced to
a lower valent nickel species to initiate the coupling process in
those reaction systems. This promising result encouraged us to
do further optimization with [Ni0(PPh3)4] as the catalyst
precursor. It was interesting to find that the addition of PPh3
to the [Ni(PPh3)4] system improved the yield to 11% (Table 1,
entry 7). We then investigated the influence of several
bidentate ligands. Both bis(diphenylphosphino)methane
(dppm) and 1,2-bis(diphenylphosphino)ethane (dppe)
afforded trace amounts of the coupling product (Table 1,
entries 8 and 9). However, the greatest improvement came
from 1,3-bis(diphenylphosphino)propane (dppp), which
afforded the desired coupling product in 80% yield
(Table 1, entry 1). Further lengthening the carbon chain in
the phosphine decreased the yield dramatically, with 1,4-
bis(diphenylphosphino)butane (dppb) giving only 7% yield
(Table 1, entry 10). Utilizing the rigid bidentate ligand 1,1’-
bis(diphenylphosphino)ferrocene (dppf) afforded the desired
product in only 17% yield (Table 1, entry 11). The influence
of the base was also tested. When other potassium salts, such
as K2HPO4 or K2CO3, were used in place of K3PO4 the desired
product was obtained in low yields (Table 1, entries 12 and
13), and the sodium salt Na3PO4 afforded no desired product
at all (Table 1, entry 14). Solvent screening showed that the
The a-alkenylation of carbonyls is normally accomplished
by palladium-catalyzed cross-coupling of alkenyl halides with
[*] C. Liu, S. Tang, D. Liu, J. Yuan, L. Zheng, L. Meng, Prof. A. Lei
College of Chemistry and Molecular Sciences
Wuhan University, Wuhan 430072 (P. R. China)
E-mail: aiwenlei@whu.edu.cn
Homepage: http://www.chem.whu.edu.cn/GCI_WebSite/
main.htm
Prof. A. Lei
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, Lanzhou 730000 (P. R. China)
[**] This work was supported by the National Natural Science
Foundation of China (20772093, 20972118, and 20832003), the
Fundamental Research Funds for the Central Universities, and the
Program for New Century Excellent Talents in University and the
Program for Changjiang Scholars and Innovative Research Team in
University (IRT1030). We thank Mr. Maomao He from Prof.
Xuechuan Hong’s group for help with HRMS detection.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108350.
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Table 1: Impact of reaction parameters on the Ni-catalyzed Heck-type
reaction of ethyl 2-bromopropanoate with p-methoxystyrene.[a]
Entry
[M]/Ligand
Base
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
[Ni(PPh3)4]/dppp
[NiCl2(PPh3)2]/dppp
[Ni(acac)2]/–
[Ni(acac)2]/PPh3
[NiCl2(PPh3)2]/–
[Ni(PPh3)4]/–
[Ni(PPh3)4]/PPh3
[Ni(PPh3)4]/dppm
[Ni(PPh3)4]/dppe
[Ni(PPh3)4]/dppb
[Ni(PPh3)4]/dppf
[Ni(PPh3)4]/dppp
[Ni(PPh3)4]/dppp
[Ni(PPh3)4]/dppp
[Ni(PPh3)4]/dppp
[Ni(PPh3)4]/dppp
[Ni(PPh3)4]/dppp
[Ni(PPh3)4]/dppp
[Ni(PPh3)4]/dppp
[Pd(PPh3)4]/dppp
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2HPO4
K2CO3
Na3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
80
n.r.
n.r.
n.r.
n.r.
4
11
trace
trace
7
17
35
43
n.d.
21
9
10
11
12
13
14
15
16
17
18
19
20
dioxane
EtOH
DMF
CH3CN
toluene
56
n.d.
n.d.
n.d.
n.d.
[a] Unless otherwise noted, the reaction was carried out with 1a
(0.50 mmol), 2a (0.75 mmol), base (1.0 mmol), catalyst (0.025 mmol),
ligand (0.030 mmol), solvent (2 mL), 608C, 16 h. [b] The yield was
determined by GC analysis with biphenyl as the internal standard.
n.r.=no reaction, n.d.=no desired product.
Scheme 1. Substrate scope for the Ni-catalyzed Heck-type reaction of
secondary a-carbonyl alkyl bromides with olefins. [a] The reactions
were carried out with 1 (0.50 mmol), 2 (0.75 mmol), [Ni(PPh3)4] (5
mol%), dppp (6 mol%), K3PO4 (1.0 mmol), toluene (2 mL), 608C,
16 h. Yields shown are of isolated products. [b] The reactions were
carried out with 1 (0.50 mmol), 2 (1.0 mmol), [Ni(PPh3)4] (10 mol%),
dppp (12 mol%), K3PO4 (1.0 mmol), toluene (2 mL), 1008C, 16 h.
Yields shown are of isolated products. [c] Acetanilide (1.0 mmol) was
added, 1008C.
alkenylation product could be obtained in THF or dioxane,
but only in low yields (Table 1, entries 15 and 16). No product
could be detected in the polar protic solvent EtOH or the
polar aprotic solvent DMF (Table 1, entries 17 and 18). Also,
no desired product was obtained in CH3CN (Table 1,
entry 19). In addition, [Pd(PPh3)4] was tested, but gave no
desired product (Table 1, entry 20). From these experiments,
we determined the optimized conditions to be: [Ni(PPh3)4]
(5 mol%), dppp (6 mol%), K3PO4 (2 equiv), toluene, 608C,
16 h.
With the optimized conditions in hand, we started
employing other substrates in this Ni-catalyzed Heck-type
reaction. First of all, various secondary a-carbonyl alkyl
bromides were employed to couple with olefins (Scheme 1).
Under the optimized conditions, p-methoxystyrene coupled
with a range of secondary a-carbonyl alkyl bromides in good
to excellent yields. A phenyl ester afforded the desired
product in 82% yield (Scheme 1, 3b). Methyl 2-bromohex-
anoate also proceeded well, giving the coupling product in
52% yield (Scheme 1, 3c). a-Bromoamides were similarly
found to be suitable substrates for this transformation and
gave the desired products in good yield (Scheme 1, 3d). In
contrast, an amide-bearing substrate with a free NH group
afforded only a low yield (Scheme 1, 3e). However, other
substituted styrenes proceeded well. An ortho-methoxy
substituent has little negative steric effect, affording the
coupling product in good yield (Scheme 1, 3 f), and NMe2 was
also well tolerated (Scheme 1, 3g). It is interesting to note
that a styrene substituted with an electron-withdrawing group
is also a suitable substrate, when acetanilide is used as an
additive (Scheme 1, 3h). Ortho- and para-methyl substituted
styrenes afforded the coupling product in good yields in the
presence of acetanilide (Scheme 1, 3i and 3j), but p-
methoxystyrene afforded the alkenylation product 3a in
a lower yield (54%), even at 1008C. 1,1-Diaryl substituted
ethylenes were also suitable substrates for this alkenylation
(Scheme 1, 3k–3t). 1,1-Diphenyl ethylene proceeded well,
coupling with various a-bromoesters and amides in the
presence of the Ni catalyst, including amides bearing free
NH groups (Scheme 1, 3m and 3n), with even an unsubsti-
tuted amide offering the desired product in 46% yield
À
(Scheme 1, 3o). Normally, Ni reacts with aromatic C Cl
À
bonds, however, the C Cl bond was well tolerated in this
transformation and the reaction proceeded chemoselectively
to afford the desired alkenylation product (Scheme 1, 3p).
Both electron-donating and electron-withdrawing groups
were introduced to the 1,1-diaryl substituted ethylenes,
providing the alkenylation products in good yields
(Scheme 1, 3q, 3r, 3t). A naphthalenyl group was also
employed (Scheme 1, 3s). No isomerized a,b-unsaturated
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ester or amide products have been detected in any of the cases
herein reported.
afforded the coupling product (Table 1, entry 6). The uni-
valent Ni complex [NiX(PPh3)3] (X = Cl, Br) is known to be
obtained from the reaction of an alkyl halide and [Ni-
(PPh3)4].[10] To determine whether NiI is present in these
transformations, a classic NiI complex [NiCl(PPh3)3] was
synthesized.[10a] This NiI complex was used in the reaction of
1a with 2a under the standard conditions [Eq. (3)]. To our
Encouraged by these promising results, we further applied
these Ni-catalyzed Heck-type alkenylation conditions to
tertiary a-carbonyl alkyl bromides. To our delight, good to
excellent yields were obtained with various alkenes
(Scheme 2). Both ethyl and p-tolyl 2-bromoisobutanoates
delight, the reaction proceeded well and afforded the product
3a in 77% yield, which indicates that the NiI complex could
further react with the alkyl bromide to initiate the radical
process. Based on this experimental data, we propose the
following mechanism for this Ni-catalyzed Heck-type reac-
tion of a-carbonyl alkyl bromides (Scheme 3).
Scheme 2. Substrate scope for the Ni-catalyzed Heck-type reaction of
tertiary a-carbonyl alkyl bromides with olefins. [a] The reactions were
carried out with 1 (0.50 mmol), 2 (0.75 mmol), [Ni(PPh3)4] (5 mol%),
dppp (6 mol%), K3PO4 (1.0 mmol), toluene (2 mL), 608C, 16 h. Yields
shown are of isolated products. [b] The reactions were carried out with
1 (0.50 mmol), 2 (1.0 mmol), [Ni(PPh3)4] (10 mol%), dppp (12
mol%), K3PO4 (1.0 mmol), toluene (2 mL), 1008C, 16 h. Yields shown
are of isolated products. [c] Acetanilide (1.0 mmol) was added, 1008C.
Scheme 3. Proposed mechanism.
As the b-hydride elimination from the nickel species and
the reductive elimination of nickel hydride have been
suggested to be not as facile as for palladium, based on
DFT calculations,[11] these two mechanistic steps were not
suggested in this transformation. Instead, the following is
proposed: First, the reaction of 2 with [Ni(PPh3)4] generates
the active NiI species A, which then donates an additional
electron to 2 to generate the radical species 2I and a NiII
species B. The radical addition of 2I to olefin 1 then generates
benzyl radical 3I. The next step is believed to be accom-
plished through the direct oxidation of radical 3I by B to
regenerate a benzyl cation intermediate 3II.[12] This is
followed by deprotonation with the base K3PO4 to release
the final product 3, whereupon B is reduced to A to restart the
catalytic cycle. It is for this reason that an electron-donating
substituent, which is in favor of cation formation, is beneficial
for product formation. Moreover, when amides containing
a free NH group were used as substrates, a significant amount
of lactam cyclization products were observed (Supporting
Information, Scheme S1), which provides further support for
the formation of cation 3II.
reacted with p-methoxystyrene under the standard conditions
to afford the coupling products in good yields (Scheme 2, 4a
and 4b). Similarly, 2,4-dimethoxystyrene coupled well with
ethyl 2-bromoisobutanoates (Scheme 2, 4c). Acetanilide was
again added to promote the alkenylation of tertiary a-
carbonyl alkyl bromides with styrene, p-methyl styrene, and
styrenes substituted with electron-withdrawing groups
(Scheme 2, 4d–4g). Various 1,1-diaryl alkenes were also
shown to couple with tertiary a-bromo esters in good to
excellent yields (Scheme 2, 4i–4o). A range of functional
groups were well tolerated, including Cl (4k), CF3 (4n), and
OMe (4l and 4m).
Alkyl halides have been suggested to react with Ni0
complexes through a single electron transfer process.[9] Ni0
I
À
offers a single electron to the C X bond to generate Ni and
an alkyl radical. Although condition screening showed that
the NiII precursor [NiCl2(PPh3)2] could not initiate the
reaction (Table 1, entry 5), the Ni0 precursor [Ni(PPh3)4]
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Tetrahedron Lett. 2003, 44, 5751 – 5754; h) K. Higuchi, K.
In conclusion, we have demonstrated the first nickel-
catalyzed Heck-type reaction of secondary and tertiary a-
carbonyl alkyl bromides with olefins under mild conditions.
Several substituted styrenes and various 1,1-diaryl alkenes
were found to be suitable substrates for this a-alkenylation.
This work opens up a new approach for the construction of a-
alkenyl carbonyl compounds. Tentative mechanistic studies
suggest that this reaction is likely to proceed by a single
electron transfer mechanism, with a catalytic cycle involving
NiI and NiII complexes as the catalyst species. Further studies
into the use of other alkyl halides and olefins as substrates, as
well as more mechanistic investigations, are underway in our
laboratory.
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Experimental Section
General procedure for the synthesis of 3a: [Ni(PPh3)4] (27.7 mg,
0.025 mmol), dppp (12.4 mg, 0.030 mmol) and K3PO4 (212.3 mg,
1.0 mmol) were added to a Schlenk tube in an argon-filled glove box.
The tube was then sealed with a septum and taken out of the glove
box. Toluene (2 mL) was then injected into the tube by syringe. After
stirring for 5 min, 2a (135.8 mg, 0.75 mmol) and 1a (0.50 mmol) were
consecutively injected into the reaction tube. The reaction was then
heated to 608C and stirred for 16 h. Upon completion, the reaction
was quenched with dilute HCl solution (2m) and extracted with ethyl
ether (3 ꢀ 10 mL). The organic layers were combined and the pure
product was obtained by flash column chromatography on silica gel
(petroleum/ethyl acetate = 50:1). The yield of the isolated product
1
was 80%. H NMR (300 MHz, CDCl3): d = 7.30 (d, J = 7.8 Hz, 2H),
6.84 (d, J = 7.8 Hz, 2H), 6.42 (d, J = 15.9 Hz, 1H), 6.13 (dd, J = 15.7,
7.9 Hz, 1H), 4.27–4.01 (m, 2H), 3.80 (s, 3H), 3.40–3.10 (m, 1H), 1.34
(d, J = 6.6 Hz, 3H), 1.27 ppm (t, J = 6.8 Hz, 3H). 13C NMR (75 MHz,
CDCl3): d = 175.36, 159.73, 131.02, 130.33, 128.05, 127.30, 114.54,
61.25, 55.87, 43.88, 18.15, 14.82 ppm. HRMS (ESI) calcd for C14H18O3
[M+H]+: 235.1334; found: 235.1340.
Received: November 27, 2011
Revised: January 1, 2012
Published online: March 1, 2012
Keywords: alkyl bromides · a alkenylation · Heck reaction ·
.
nickel · radical reactions
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