Visible-light photocatalytic radical alkenylation of a-carbonyl alkyl bromides and benzyl bromides

Article abstract of DOI:10.1002/chem.201203694

Through the use of [Ru- (bpy)₃Cl₂] (bpy=2,2'-bipyridine) and [Ir(ppy)₃] (ppy=phenylpyridine) as photocatalysts, we have achieved the first example of visible-light photocatalytic radical alkenylation of various acarbonyl alkyl bromides and benzyl bromides to furnish a-vinyl carbonyls and allylbenzene derivatives, prominent structural elements of many bioactive molecules. Specifically, this transformation is regiospecific and can tolerate primary, secondary, and even tertiary alkyl halides that bear b-hydrides, which can be challenging with traditional palladium-catalyzed approaches. The key initiation step of this transformation is visible-light-induced single-electron reduction of CBr bonds to generate alkyl radical species promoted by photocatalysts. The following carboncarbon bond-forming step involves a radical addition step rather than a metal-mediated process, thereby avoiding the undesired b-hydride elimination side reaction. Moreover, we propose that the Ru and Ir photocatalysts play a dual role in the catalytic system: they absorb energy from the visible light to facilitate the reaction process and act as a medium of electron transfer to activate the alkyl halides more effectively. Overall, this photoredox catalysis method opens new synthetic opportunities for the efficient alkenylation of alkyl halides that contain b-hydrides under mild conditions.

Full text of DOI:10.1002/chem.201203694

DOI: 10.1002/chem.201203694
Visible-Light Photocatalytic Radical Alkenylation of a-Carbonyl Alkyl
Bromides and Benzyl Bromides
Qiang Liu,[a] Hong Yi,[a] Jie Liu,[a] Yuhong Yang,[a] Xu Zhang,[a] Ziqi Zeng,[a] and
Aiwen Lei*[a, b]
Abstract: Through the use of [Ru-
(bpy)3Cl2] (bpy=2,2’-bipyridine) and
[Ir(ppy)3] (ppy=phenylpyridine) as
traditional palladium-catalyzed ap-
proaches. The key initiation step of this
transformation is visible-light-induced
dride elimination side reaction. More-
over, we propose that the Ru and Ir
photocatalysts play a dual role in the
catalytic system: they absorb energy
from the visible light to facilitate the
reaction process and act as a medium
of electron transfer to activate the
alkyl halides more effectively. Overall,
this photoredox catalysis method opens
new synthetic opportunities for the ef-
ficient alkenylation of alkyl halides
that contain b-hydrides under mild
conditions.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
À
photocatalysts, we have achieved the
first example of visible-light photocata-
lytic radical alkenylation of various a-
carbonyl alkyl bromides and benzyl
bromides to furnish a-vinyl carbonyls
and allylbenzene derivatives, promi-
nent structural elements of many bio-
active molecules. Specifically, this
transformation is regiospecific and can
tolerate primary, secondary, and even
tertiary alkyl halides that bear b-hy-
drides, which can be challenging with
single-electron reduction of C Br
bonds to generate alkyl radical species
promoted by photocatalysts. The fol-
À
lowing carbon carbon bond-forming
step involves a radical addition step
rather than a metal-mediated process,
thereby avoiding the undesired b-hy-
Keywords: alkenylation · halides ·
photocatalysis · radical reactions ·
transition metals
Introduction
Theoretically, in addition to the oxidative addition path-
way, alkyl halides can also be activated by means of single-
electron reduction process to generate the key intermediate
The palladium-catalyzed Heck reaction of alkenes with aryl
or vinyl halides (or sulfonates) has been widely applied in
organic synthesis and related disciplines.[1] Although consid-
erable efforts have been made towards expanding the syn-
thetic applications of this reaction, alkyl halides still pose a
major challenge,[2] especially for secondary and tertiary alkyl
halides.[3] The difficulty is mainly attributed to two prob-
lems: sp3-hybridized alkyl halides are generally reluctant to
undergo oxidative addition processes with low-valent palla-
dium catalyst,[4] and the alkyl-palladium intermediates nor-
mally undergo b-hydride elimination more facilely than
olefin insertion.[5] Both of them are inherent conundrums
for palladium-catalyzed Heck reaction of alkyl halides.
alkyl radical species.[6] The following carbon carbon bond-
À
forming step involves a radical addition step rather than a
metal-mediated process, thereby avoiding the undesired b-
hydride elimination side reaction.[7] However, only a very
few radical alkenylation reactions catalyzed by transition
metals other than palladium by means of the single-electron
transfer (SET) process have been reported.[8] These kinds of
radical alkenylation reactions promoted by titanocene and
cobalt catalysts are normally stoichiometric in highly reac-
tive alkylmagnesium reagents, thus limiting their potential
applications.[8a–c,9] In addition, nickel-catalyzed alkenylation
of benzyl chlorides and a-carbonyl alkyl bromides has also
been developed.[8e,f] In spite of the significant progress of-
fered by this method, issues of substrate scope limitation,[10]
high catalyst loading, the use of air-sensitive Ni0 catalyst,
and high temperature still remain concerns. To achieve the
radical alkenylation reaction of these activated alkyl halides
in a more facile manner, we sought to identify other easy-
to-handle reagents that could accomplish catalytic formation
of electron-deficient alkyl radicals by the single-electron re-
[a] Dr. Q. Liu,+ H. Yi,+ J. Liu, Y. Yang, X. Zhang, Z. Zeng, Prof. A. Lei
The College of Chemistry and Molecular Sciences
Wuhan University, 430072, Wuhan, Hubei (P.R. China)
Fax : (+86)27-68754672
E-mail: aiwenlei@whu.edu.cn
[b] Prof. A. Lei
Beijing National Laboratory for Molecule Sciences (BNLMS)
Key Laboratory for Molecular Recognition and Function
Institute of Chemistry, the Chinese Academy of Sciences
100191, Beijing (P.R. China)
À
ductive cleavage of C X bonds under mild conditions.
As we know, Ru2+– or Ir3+–polypyridyl complexes are
suggested to have the capability of photocatalytic generation
of electron-deficient alkyl radicals from activated alkyl hal-
ides irradiated with visible light.[11] In addition, photocatalyt-
ic alkenylation of aryl diozanium salts has been reported
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203694.
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FULL PAPER
very recently.[12] Accordingly, we envisioned that alkenyla-
tion of activated alkyl halides could be realized by means of
the photoredox catalysis process that takes visible light as
the terminal energy source. With this idea in mind, we de-
veloped a novel method to accomplish the alkenylation of
various a-carbonyl alkyl bromides and benzyl bromides
under mild conditions through visible-light photoredox cat-
alysis (Scheme 1a and b), which is emerging as a promising
(1a), [RuCl2ACTHNGUTERN(NUG bpy)3] (2 mol%; bpy=2,2’-bipyridine), Et3N
(20 mol%), and a 25 W household fluorescent lamp in the
presence of Na2CO3 (1 equiv) in DMF (Table 1, entry 1;
42% yield). Importantly, no alkenylation product was ob-
served only with either [RuCl2ACHTNUTRGNENUG(bpy)3] catalyst or light, which
indicated that the photoredox catalysis is essential to this
process. A brief screen of bases revealed that NaHCO3 af-
forded the best yield of the alkenylation product (Table 1,
entries 1–5). Subsequent investigations uncovered the im-
portance of solvent (Table 1, entries 3 and 6–10). Other
tested solvents were less effective than DMF for this trans-
formation. However, at this stage, the reaction was still inef-
ficient with less than 50% conversion. To further increase
the reaction conversion, pyridine derivatives were employed
as the additives to improve the reactivity, which made the
substrate more electron-deficient and introduced a better
leaving group (Table 1, entries 14–16).[14] To our delight, 4-
methoxypyridine clearly increased the reaction yield
(Table 1, entries 11–14). Actually, the combination of photo-
catalysis and organocatalysis has been exploited since 2008,
and it will substantially broaden the booming field of visi-
ble-light photocatalytic synthesis.[11a,c,14,15] Finally, the opti-
mized result was obtained by raising both the amount of
sacrificial reductant Et3N and the reaction concentration
(Table 1, entry 14; 90% yield).
Scheme 1. Visible-light photocatalytic alkenylation of a) a-carbonyl alkyl
halides and b) benzyl halides.
tool for bond breaking and bond making in organic synthe-
sis. Moreover, the recent developments in photoredox catal-
ysis further demonstrate the broad potential of these meth-
ods to promote various useful chemical transformations by
irradiation with visible light.[13]
With the optimal reaction conditions established, we then
examined the scope of this reaction by coupling a variety of
alkenes 2 with 1a (Scheme 2). Alkenylation of 1a with
varied electronically diverse 1,1-diarylethenes proceeded
under mild conditions to give modest to excellent yields
(Scheme 2, 3a–g). Both electron-donating and electron-
withdrawing substituents on 1,1-diarylethenes were satisfac-
torily tolerated. In particular, the transformation is compati-
ble with aryl bromide and aryl chloride functional groups,
Results and Discussion
As shown in Table 1, our initial studies confirmed the feasi-
bility of the proposed alkenylation process when 1,1-diphe-
nylethene (2a) was exposed to ethyl a-bromopropionoate
Table 1. Optimization studies of the alkenylation of 1a with 2a.[a]
Entry
Base
Solvent
Additive
Yield
[%][b]
AHCTUNGTRENNUNG
1
2
3
4
5
6
7
8
Na2CO3
NaHCO3
NaOAc
NaOH
K3PO4
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
DMF
DMF
DMF
DMF
DMF
MeCN
MeOH
CH2Cl2
PhNO2
DMA
DMF
DMF
DMF
DMF
DMF
DMF
none
none
none
none
none
none
none
none
none
42
46
9
0
0
19
0
0
9
0
10
11
12[c]
13[d]
14[c,e]
15[c,e]
16[c,e]
none
33
64
78
73
90
84
59
4-methoxypyridine (40)
4-methoxypyridine (40)
4-methoxypyridine (40)
4-methoxypyridine (40)
pyridine (40)
DMAP (40)
[a] Conditions: 1a (0.5 mmol), 2a (0.25 mmol), [RuCl2ACTHNUTRGNEN(UG bpy)3]
(0.005 mmol), additive, Et3N (0.05 mmol), base (0.25 mmol), solvent
(2 mL), irradiation with 25 W household light bulb, 36 h. [b] GC yields.
[c] Et3N (0.1 mmol). [d] Et3N (0.15 mmol). [e] DMF (1 mL).
Scheme 2. Scope of alkenes 2 for visible-light photocatalytic radical alke-
nylation reaction with 1a.
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A. Lei et al.
which are normally reactive in the Pd- or Ni-catalyzed cou-
pling reactions (Scheme 2, 3 f and 3g). However, it was diffi-
cult to control the stereoselectivity for the reactions of un-
symmetrical 1,1-disubstituted ethylenes. Mixed E/Z isomers
were obtained from these substrates (Scheme 2, 3d–g). Styr-
enes substituted with electron-donating groups were also
suitable substrates (Scheme 2, 3h and 3i). Moreover, the
stereoselectivity for alkylation of substituted styrenes was
selective to be trans. The isolated trans products were stable
under the standard reaction conditions for 5 h without cis–
trans isomerization (Scheme S1 in the Supporting Informa-
tion). The electron-deficient alkyl radical intermediate gen-
erated from 1a is more readily trapped by electron-rich ole-
fins.[16] As a result, styrenes with electron-withdrawing sub-
stituents did not work in the visible-light-induced radical al-
kenylation reaction.
We next extended this alkenylation reaction to other a-
carbonyl alkyl bromides as well (Schemes 3 and 4). Under
the optimized reaction conditions described above, secon-
dary a-carbonyl alkyl bromides bearing ester and cyano
groups were shown to react with p-methoxystyrene 2h and
1,1-diphenylethene 2a in 63–92% yield (Scheme 3, 3h–k).
In addition, tertiary a-carbonyl alkyl bromides underwent
the alkenylation transformation with 2a and 2h smoothly as
well (Scheme 3, 3l–n).
Scheme 4. Scope of primary a-carbonyl alkyl bromides 2 for visible-light
photocatalytic radical alkenylation reaction with 1.
ing agent than [RuACHTUNGTRENNUNG
SCE in CH3CN).[18] It could therefore perform the reductive
À
cleavage of C Br bonds of less-reactive primary alkyl hal-
ides without the assistance of organocatalyst.
We proposed a plausible mechanism for the radical alke-
nylation reaction of a-carbonyl alkyl bromides (Scheme 5).
Irradiation of [RuACTHNUTRGNEUNG
(bpy)3]2+ with visible light engenders its
Scheme 5. Proposed mechanism of [RuACHTNUGRTNEUNG(bpy)3Cl2]-catalyzed alkenylation
reaction of 1.
excited [Ru
fer (MLCT),[19] which will be initially quenched by sacrificial
reductant Et3N to form [Ru
(bpy)3]+. 4-Methoxypyridine can
react with 1a to afford the corresponding electron-poor pyr-
idinium salt.[20] Single-electron transfer from [Ru(bpy)3]+ to
pyridinium salt furnishes the electron-deficient radical I and
(bpy)3]2+* through metal-to-ligand charge trans-
ACHTUNGTRENNUNG
Scheme 3. Scope of secondary and tertiary a-carbonyl alkyl bromides 2
for visible-light photocatalytic radical alkenylation reaction with 1.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
It is known that primary a-carbonyl alkyl bromides are
[14,21]
À
less likely to undergo single-electron reduction of C Br
regenerates [Ru
G
.
Since electron-deficient radi-
bonds than secondary and tertiary ones. This is due to the
relatively unstable primary alkyl radicals produced.[17]
Hence, the previous Ni-catalyzed approach could not afford
the corresponding alkenylation products of primary a-car-
bonyl alkyl bromides.[8f] To our delight, this photocatalytic
reaction system could solve this problem and furnish the de-
sired Heck-type products of primary alkyl bromides in good
yields (Scheme 4), although the reaction conditions are dif-
ferent from the optimized conditions with the change of
cals are known to react with electron-rich olefins to form
[16a]
À
C C bonds,
the key bond-formation step through rapid
addition of radical I to the electron-rich olefin 2 forms the
g-carbonyl radical II. It then undergoes single-electron
transfer (SET) with [Ru
(bpy)3]2+* to deliver cation inter-
ACHTUNGTRENNUNG
mediate 4 and [Ru
AHCTUNGTRENNUNG
talytic cycle. The desired alkenylation product 3 was ac-
quired by means of the deprotonation of 4 with NaHCO3.
Et3N displayed luminescence quenching of the [RuACHTUNGTRENNUNG
(bpy)3]2+
photocatalyst from [RuCl
phenylpyridine). Excitation of fac-[Ir
light generates its excited state *fac-[Ir
À1.73 V versus SCE in CH3CN), which is a stronger reduc-
G
E
* excited state, whereas no emission quenching was ob-
served in the presence of substrates 1, thus illustrating that
this reaction was achieved through the reductive quenching
of the photocatalyst (Figure S1 in the Supporting Informa-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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Photocatalytic Radical Alkenylation
FULL PAPER
tion). Furthermore, this alkenylation process is completely
shut down in the presence of radical inhibitor (2,2,6,6-tetra-
methylpiperidin-1-yl)oxyl (TEMPO), which indicates that
the single-electron transfer process exists within the catalytic
cycle (see Scheme S2 in the Supporting Information).
As mentioned above, fac-[IrACTHUNGTRENNU(G ppy)3] could generate a stron-
ger reducing agent to perform the reductive cleavage of the
À
C Br bond more efficiently. Therefore, we started to inves-
tigate the impact of different parameters on the efficiency of
this reaction by using fac-[IrACHTNUGRTENUNG(ppy)3] as the photocatalyst
Meanwhile, we cannot rule out the radical-chain propaga-
tion mechanism, but this Ru-catalyzed transformation has
been proven to require continuous irradiation of visible
light (Figure 1), which indicates that chain propagation is
(Table 2), and we ultimately found that the combination of
Table 2. Optimization studies of the alkenylation of 5a with 2h.[a]
Entry
Solvent
Base
Et3N
[mol%]
Yield
[%][b]
AHCTUNGTRENNUNG
1
2
3
4
5
6
7
8
DMSO
MeCN
MeOH
DCE
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
KHCO3
K2CO3
10
10
10
10
10
10
10
10
10
10
10
0
trace
60
46
45
66
60
64
56
65
41
76
60
70
80
70
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Cs2CO3
Li2CO3
9
10
11
12
13
14[c]
15[d]
lutidine
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
20
10
10
[a] Conditions: 5a (0.5 mmol), 2h (0.25 mmol), [IrACTHNURGTNEUNG(ppy)3] (0.0025 mmol),
Et3N, base (0.25 mmol), solvent (2 mL), irradiation with 25 W household
light bulb, 36 h. [b] GC yields. [c] DMF (1 mL). [d] DMF (3 mL).
Figure 1. Time profile of photocatalytic radical alkenylation reaction of
2a and 1a.
not the major reaction pathway. Atom-transfer radical addi-
tion (ATRA) followed by the elimination of HBr with the
assistance of base could be another possible reaction path-
way as well. Although we cannot completely exclude this
possibility, an ATRA-type product is not observed during
this radical alkenylation reaction process (Figure S2 in the
Supporting Information). Additionally, under the standard
conditions in the absence of base, there was no ATRA-type
product formed either (see Scheme S3 in the Supporting In-
formation). As a result, the ATRA reaction pathway is not
favored.[22]
[IrACHTNUGTRENNU(G ppy)3] (1 mol%)/Na2CO3 (1 equiv)/Et3N (10 mol%) in
DMF yielded most of the alkenylation product 6a (Table 2,
entry 14). DMF and Na2CO3 revealed superior performances
among all the solvents and bases examined (Table 2, en-
tries 1–11). In the absence of Et3N, the reaction yield was re-
duced from 76 to 60% (Table 2, entry 12). Increasing the
amount of Et3N to 20 mol% afforded lower yield as well
(Table 2, entry 13)
After the successful photocatalytic radical alkenylation of
5a with 2h, the reaction scope toward structurally diverse
primary substituted benzyl bromides
5 was examined
Encouraged by the positive results of a-carbonyl alkyl
bromides, we turned our attention to the investigation of the
photocatalytic radical alkenylation reaction of less-reactive
substrates, benzyl bromide derivatives. 1-Bromo-2-(bromo-
methyl) benzene 5a and 2h were chosen as model substrates
to determine the optimum reaction conditions. However,
(Scheme 6). Under the standard conditions, Ir-catalyzed al-
kenylation of electron-deficient substrates could generate
the desired allylbenzene derivatives smoothly, and moderate
to high yields were obtained for various substrates. A wide
À
À
À
À
range of functional groups, including C Br, C Cl, C F, C I,
nitro groups, cyano groups, and so forth, were compatible
with this method. In addition, the method is regiospecific as
well, with only trans-linear alkenylation products generated
in most cases (with the exception of fluoro-substituted prod-
ucts 6c and 6i).
there was no reaction in the presence of [RuCl
the photocatalyst [Eq. (1)].
2ACHTUNGTERN(NUNG bpy)3] as
Secondary benzyl bromides were also suitable substrates
for this photocatalytic radical alkenylation reaction. This Ir-
catalyzed radical alkenylation reaction could provide the
corresponding alkenylation products in moderate to good
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A. Lei et al.
Scheme 7. Scope of alkenes 2 for visible-light photocatalytic radical alke-
nylation reaction with 5.
Scheme 6. Scope of substituted benzyl bromides 5 for visible-light photo-
catalytic radical alkenylation reaction.
yields (Scheme 6, 6k–l). To the best of our knowledge, this
is the first example of the intermolecular alkenylation reac-
tion of secondary benzyl halides that bear b-hydrides.
As revealed in Scheme 7, this photocatalytic alkenylation
technology tolerated a wide range of olefins 2 as well, in-
cluding various substituted styrene derivatives (40 to 83%
yield; Scheme 7, 6n–q) and 1,1-diarylalkenes (58 to 80%
yield; Scheme 7, 6r–v). As was the case for benzyl coupling
partner, this transformation also proceeded with perfect re-
gioselectivity in the presence of a variety of olefin substrates
except unsymmetrical 1,1-diarylalkenes (Scheme 7, 6u and
6v).
Scheme 8. Proposed mechanism of fac-[Ir
nylation reaction of 5.
ACHTUNGERTN(NUNG ppy)3]-catalyzed radical alke-
The proposed mechanistic explanation for the Ir-catalyzed
radical alkenylation reactions is described in Scheme 8. fac-
[Ir
source (a household fluorescent bulb) to populate the *fac-
[Ir
(ppy)3] excited state,[18b] which underwent an SET process
ACHTUNGTRENNUNG(ppy)3] could readily accept a photon from a visible-light
ACHTUNGTRENNUNG
with benzyl bromides 5 to render the electrophilic radical
III and IrIV metal complex. As shown in Figure 2, various
substrates 5 displayed luminescence quenching of the *fac-
[Ir
ACHTUNGTRENNUNG
between *fac-[IrACHTUNGTRENNUNG
of more substrates, see the Supporting Information for de-
À
tails). The key C C bond-formation step occurred through
rapid addition of radical III to the p-rich olefin 2, thus form-
ing the radical IV. Given that radical IV should have a
Figure 2. Luminescence quenching by substrates 5 and Et3N.
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Photocatalytic Radical Alkenylation
FULL PAPER
(1 mL) was added into the reaction tube with a syringe. The reaction
mixture was degassed by the freeze–pump–thaw method and then irradi-
ated with a 25 W fluorescent household light bulb (distance approximate-
ly 10 cm) for 36 h. After completion of the reaction, it was quenched
with a diluted HCl solution and extracted with ethyl acetate (3ꢁ10 mL).
The organic layers were combined and the pure product was obtained by
flash column chromatography on silica gel.
lower barrier of oxidization due to the existence of the elec-
tron-rich substituted groups Ar’ and R’, a second electron-
transfer event with +fac-[Ir
ACTHNUTRGENUN(G ppy)3] would generate the
cation intermediate 7 and finally close the photoredox cata-
lytic cycle. The desired Heck-type product 6 would be pro-
duced through the deprotonation of cation 7 with Na2CO3.
In addition, the radical chain propagation mechanism
could be another possible reaction pathway as a substoichio-
metric amount of Et3N benefits this transformation. In this
case, Et3N is proposed to donate one single electron to IrIV
Acknowledgements
species, thus regenerating the fac-[IrACTHNUTRGENUG(N ppy)3] catalyst. Even
This work was supported by the 973 Project from the MOST of China
(2011CB808600) and the National Natural Science Foundation of China
(21025206, 20832003, 20973132, and 20972118). The authors also thank
the support of the Fundamental Research Funds for the Central Univer-
sities and the Program for Changjiang Scholars and Innovative Research
Team in Universities (IRT1030).
so, this transformation still requires continuous irradiation
with visible light (see Figure S3 in the Supporting Informa-
tion), thereby revealing that radical chain propagation is not
the major reaction route. Moreover, Et3N could react with
substrate 5 to produce quaternary ammonium salts 5’
[Eqs. (2) and (3)]. Hence, Et3N is also capable of acting as
an additive like 4-methoxypyridine to promote the reductive
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We have developed a mechanistically novel radical alkenyla-
tion reaction of various a-carbonyl alkyl bromides and
benzyl bromides under mild conditions, which is the first ex-
ample of visible-light photocatalytic radical alkenylation re-
action of alkyl halides. The key initiation step of this trans-
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À
of the C Br bond promoted by the photocatalysts. In partic-
ular, primary, secondary, and tertiary alkyl bromides are all
suitable substrates for this photocatalytic transformation,
which is a beneficial supplement to the previous Ni-cata-
lyzed protocols from a synthetic point of view.[8e,f] The appli-
cation of this powerful strategy to the synthesis of natural
products and further development of other related syntheti-
cally valuable reactions are continuing efforts in our labora-
tory.
Experimental Section
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General procedure for the photocatalytic radical alkenylation of a-car-
bonyl alkyl bromides: A dried Schlenk tube equipped with a stirrer bar
was loaded with a-carbonyl alkyl bromide (0.5 mmol), sodium bicarbon-
ate (0.25 mmol), and [Ru
evacuated and backfilled with nitrogen before alkene (0.25 mmol), Et3N
(0.1 mmol), and 4-methoxypyridine (0.1 mmol) were added. Then DMF
(bpy)3Cl2] (0.005 mmol, 2 mol%). The tube was
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(see Table S1 in the Supporting Information).
Received: October 16, 2012
Revised: January 22, 2013
Published online: February 20, 2013
5126
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Chem. Eur. J. 2013, 19, 5120 – 5126