Oxidative coupling reactions of grignard reagents with nitrous oxide

Article abstract of DOI:10.1002/anie.201302471

Catalysis with laughing gas: N₂O in combination with transition-metal catalysts allow the oxidative homo- and cross-coupling of Grignard reagents. The reactions can be performed under mild conditions despite the inert character of N₂O. Copyright

Full text of DOI:10.1002/anie.201302471

Angewandte
Chemie
Communications
Nitrous oxide
Catalysis with laughing gas: N2O in
combination with transition-metal cata-
lysts allow the oxidative homo- and cross-
coupling of Grignard reagents. The reac-
tions can be performed under mild con-
ditions despite the inert character of N2O.
G. Kiefer, L. Jeanbourquin,
K. Severin*
&&&& — &&&&
Oxidative Coupling Reactions of Grignard
Reagents with Nitrous Oxide
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
DOI: 10.1002/anie.201302471
Nitrous oxide
Oxidative Coupling Reactions of Grignard Reagents with Nitrous
Oxide**
Gregor Kiefer, Loꢀc Jeanbourquin, and Kay Severin*
Table 1: Metal-catalyzed homocoupling of phenyl magnesium chloride
with N2O or O2 as oxidant.
Nitrous oxide (“laughing gas”, N2O) is a potent oxidation
agent, from a thermodynamic point of view.[1] Moreover, it is
an environmentally benign oxidant, because the side product
is dinitrogen. An obstacle in using N2O in oxidation reactions
Entry
Catalyst (mol%)
Oxidant
t [h]
Yield [%][a]
is the inert nature of the gas. Heterogeneous catalysts have
been used with good success for the activation of N2O, but
high temperatures and/or pressures are typically required to
achieve acceptable reaction rates.[2] Thus far, N2O-based
oxidation reactions with homogeneous catalysts in solution
have met with only limited success. Many transition-metal
complexes are known to react with N2O under mild con-
ditions,[3] but catalytic turnover is difficult to achieve. Some
polyoxometalates[4] and ruthenium complexes[5] were shown
to catalyze oxidation reactions with N2O, but the reactions
require high temperatures (100–2008C) and often elevated
pressures.[6] Furthermore, the reported turnover numbers are
modest (ꢀ 100). Herein, we describe oxidative carbon–carbon
coupling reactions with N2O, which can be performed under
mild conditions with good selectivity and unprecedented
turnover numbers.
Oxidative homo- and cross-coupling reactions of
Grignard reagents[7,8] in the presence of metal catalysts can
be achieved with different oxidants, including 1,2-dihalo-
ethanes[9] and dioxygen.[10] The reactions are believed to
involve low-valent organometallic complexes.[7–10] We
hypothesized that these low-valent, nucleophilic complexes
might be susceptible to oxidation by N2O. As a model
reaction, we studied the homocoupling of phenylmagnesium
chloride. The reactions were performed in THF at room
temperature under an atmosphere of N2O using different
transition-metal salts as potential catalysts (Li2CuCl4,
Li2MnCl4, CoCl2, FeCl3, [Fe(acac)3]). To avoid reactions
caused by traces of dioxygen, we have used N2O of high
purity (99.999%). Test reactions with metal salt (1 mol%)
gave the oxidative coupling product biphenyl after 1 h in
yields of 30–95% (Table 1, entries 1–5). The best results were
found for FeCl3 (94% yield), [Fe(acac)3] (94% yield) and
CoCl2 (95% yield). The latter two complexes were used for
further studies.
1
2
3
4
5
6
7
8
9
Li2CuCl4 (1.0)
Li2MnCl4 (1.0)
CoCl2 (1.0)
N2O
N2O
N2O
N2O
N2O
N2O
N2O
O2
1
1
1
1
1
1
18
1
30
32
95
FeCl3 (1.0)
94
[Fe(acac)3] (1.0)
[Fe(acac)3] (0.1)
CoCl2 (0.004)
[Fe(acac)3] (0.1)
Li2MnCl4 (0.1)
94
94[b]
83
traces[c]
9[d]
O2
1
[a] Yields were determined by GC-MS analysis. [b] The the product was
isolated in 92% yield. [c] A 41% yield of phenol was formed. [d] A 10%
yield of phenol was formed.
no effect on the yield. Further reduction to 0.01 mol% gave
a poor yield in the case of [Fe(acac)3], even if the reaction
time was prolonged. With CoCl2, however, the catalyst
loading could be reduced to 0.004 mol% and biphenyl was
still obtained in 83% yield (Table 1, entry 7). Taking into
account the small amount of product formed without catalyst
(8% after 18 h), and assuming that one catalytic cycle
produces one biphenyl molecule, we can calculate a turnover
number of 9.4 ꢀ 103. This value greatly exceeds what has been
reported thus far for metal-catalyzed oxidation reactions with
N2O in homogeneous solution.[4,5]
The groups of Lei[10c] and Cahiez[10d] have shown that Fe
complexes are able to catalyze the oxidative coupling of aryl
Grignard reagents with O2. In the case of 4-methylphenyl-
magnesium bromide, a yield of only 46% was reported for
experiments using [Fe(acac)3] (5 mol%) under conditions
related to our own (THF, room temperature).[10c] The
difficulty in performing oxidative coupling reactions with O2
is due to the reactivity of the Grignard reagent itself towards
O2. Consequently, the catalytic process has to be fast and high
catalyst loadings are needed. This issue is illustrated by the
attempted synthesis of biphenyl using O2 and [Fe(acac)3]
(0.1 mol%). Only traces of the desired coupling product were
obtained and phenol was formed in 41% yield (Table 1,
entry 8). Li2MnCl4 is another catalyst that is known to
promote the coupling of Grignard reagents in the presence
of O2.[10b,d] Good yields for aryl–aryl coupling reactions were
reported with Li2MnCl4 (5 mol%).[10d] When only a 0.1 mol%
loading was used, however, this catalytic system failed to give
an acceptable yield (Table 1, entry 9) and a 10% yield of
phenol was obtained. These experiments reveal an intrinsic
advantage of N2O over O2: as Grignard reagents are reluctant
First, we examined the efficiency of the reaction. Low-
ering the amount of catalyst from 1.0 mol% to 0.1 mol% had
[*] G. Kiefer, L. Jeanbourquin, Prof. K. Severin
Institut des Sciences et Ingꢀnierie Chimiques, Ecole Polytechnique
Fꢀdꢀrale de Lausanne (EPFL), 1015 Lausanne (Switzerland)
E-mail: kay.severin@epfl.ch
[**] The work was supported by funding from the Swiss National
Science Foundation and by the EPFL.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302471.
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
to react with N2O,[1] metal-independent side reactions are less
problematic.
N2O based procedure. By increasing the catalyst loading to
5 mol%, it was possible to obtain the coupling products in
50% and 32% yield, respectively (Table 2, entries 6 and 7).
Interestingly, CoCl2 gave better results for the ester, whereas
[Fe(acac)3] gave superior results in the case of the cyano-
containing substrate (Table S4). The substrate 2-thienylmag-
nesium chloride coupled to produce 2,2’-dithienyl in 79%
yield after 6 h with only 1 mol% of [Fe(acac)3] (Table 2,
entry 8).
Next, we examined the substrate scope. Aryl Grignard
reagents containing different groups in the para position
(CH3, F, OMe) or a sterically congesting methyl group in the
ortho position could be coupled cleanly with [Fe(acac)3]
(0.1 mol%; Table 2, entries 1–4) or CoCl2 (Table S4). For
reactions with CoCl2, high turnover numbers of more than 103
could be achieved as well.
Alkenyl Grignard reagents also homodimerize readily:
2,5-dimethylhexa-2,4-diene and 2,3-diphenylbutadiene were
both obtained from the corresponding Grignard reagents in
77% yield using [Fe(acac)3] (1 mol%; Table 2, entries 9 and
10). For 2,3-diphenylbutadiene, CoCl2 was significantly less
effective than [Fe(acac)3] (Table S4).
Attempts to homocouple sp-hybridized Grignard reagents
failed, regardless of the catalyst employed (Table S2). Only
low conversions were observed with CoCl2, Li2CuCl4, or
Li2MnCl4. Phenylethynylmagnesium bromide was consumed
when iron(III) salts were present, but the yield of the
homodimer was below 5%. The homocoupling of propynyl-
magnesium chloride was also not successful.
Table 2: Oxidative homocoupling of aryl and alkenyl Grignard reagents.[a]
Entry
1
Catalyst (mol%)
[Fe(acac)3] (0.1)
t [h]
Substrate
Yield [%][b]
93
1
2
[Fe(acac)3] (0.1)
1
92
3
4
[Fe(acac)3] (0.1)
[Fe(acac)3] (0.1)
1
1
92
99 (96)[c]
To date, there are few reports about the oxidative
coupling of alkyl Grignard reagents.[9a,c,10d,11e,12] These sub-
strates are problematic because they are prone to undergo
side reactions (such as eliminations, isomerizations, or, in the
case of O2, reactions with the oxidant[9a]). When we screened
different transition-metal salts for the catalytic oxidative
homocoupling of phenethylmagnesium chloride with N2O, we
found that manganese, iron, and cobalt salts led to consid-
erable amounts of styrene (Table S1). Phenethylmagnesium
chloride is a “difficult” substrate, because b-hydride elimi-
nation of the corresponding transition metal alkyl complex is
particularly easy. However, with Li2CuCl4 (1.0 mol%), an
84% yield of the homocoupling product 1,4-diphenylbutane
was obtained (isolated in 80% yield) and only traces of
styrene were observed (Table 3, entry 1). The catalyst loading
could be lowered to 0.5 mol% without compromising the
yield, but further reduction to 0.2 mol% gave a yield of only
57% (Table 3, entry 2) and an elevated amount of styrene
5
CoCl2 (5.0)
1.5
87[d]
6
7
8
CoCl2 (5.0)
18
18
6
50[e]
32[e]
79
[Fe(acac)3] (5.0)
[Fe(acac)3] (1.0)
9
[Fe(acac)3] (1.0)
[Fe(acac)3] (1.0)
1
1
77 (72)[c]
77
10
[a] Unless noted otherwise, reactions were performed at RT. [b] Yields
were determined by GC or GC-MS analysis. [c] Yields of isolated products
are given in parentheses. [d] The reaction temperature was 508C and the
Grignard reagent was slowly added to the catalyst solution with a syringe
pump (over 1 h). [e] The Grignard reagents were prepared according to
a literature procedure, the reaction was started at À208C and the mixture
was then slowly warmed to RT.
Table 3: Oxidative homocoupling of alkyl Grignard reagents.
Mesitylmagnesium bromide, a sterically very demanding
substrate, provided bimesityl in 87% yield with CoCl2
(5 mol%) after 1.5 h at 508C (Table 2, entry 5). The success
of this reaction is in sharp contrast to what has been reported
for reactions with dioxygen: the attempted coupling of
mesitylmagnesium bromide gave only traces of product,
despite the fact that a relatively large amount (20 mol%) of
Li2MnCl4 catalyst was employed.[10b] The homocoupling of
sterically demanding arylmagnesium halides is also not
possible with the TEMPO-based method developed by the
Studer group.[11b,c] Comparable yields for the homocoupling of
mesitylmagnesium halides were only achieved when stoichio-
metric amounts of 1,2-dihaloethanes[9d,e] or 3,3’,5,5’-tetra-tert-
butyldiphenoquinone[11d] were used as oxidants.
Entry
x [mol%]
Oxidant
substrate
Yield [%][a]
1
2
3
1.0
0.2
1.0
N2O
N2O
O2
84 (80)[b]
57
17[c]
4
5
1.0
1.0
N2O
N2O
85 (81)[b]
78
6
7
1.0
1.0
N2O
N2O
79
44
[a] Yields were determined by GC or GC-MS analysis. [b] Yields of
isolated products are given in parentheses. [c] Other products: 2-
phenylethanol (58%) and styrene (4%).
Aryl Grignard reagents with reactive ester or cyano
groups were found to be more challenging substrates for our
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
Table 4: Oxidative cross-coupling of Grignard reagents.
(10%). As expected, it was not possible to replace N2O by O2:
when the reaction was carried out with Li2CuCl4 (1 mol%)
under an O2 atmosphere, a 58% yield of 2-phenylethanol was
obtained, along with a 4% yield of styrene and only a 17%
yield of the coupling product (Table 3, entry 3).
Entry
t [h]
Product B
Yield B [%][a]
A/B/C
Using N2O, other primary (benzyl, n-decyl) and secondary
Grignard reagents (cyclohexyl) could be coupled in yields of
78–85% (Table 3, entries 4–6). The sterically demanding tert-
butylmagnesium bromide gave a lower yield, only 44%
(Table 3, entry 7). The success of copper as a catalyst for these
reactions is in line with the well-known propensity of
1
2
2
2
59
61
8:84:8
11:80:9
3[c]
18
67 (57)[b]
5:52:43
4
5
2
2
83
16
0:76:24
À
organocuprates to undergo C C coupling reactions upon
53:31:16
oxidation.[13,14] However, a stoichiometric amount of copper
salts are typically used for these reactions.
6
2
87
0:79:21
Having established that homocoupling reactions of
Grignard reagents can be achieved with N2O, we examined
whether this method is also suitable for cross-coupling
reactions. Cahiez et al. have reported oxidative cross-coupling
reactions between sp- and sp2-hybridized RMgX compounds
with O2 as oxidant and Li2MnCl4 (20 mol%) as catalyst.[10b]
For some substrate combinations, they were able to achieve
a good selectivity for the cross-coupling product.
7
2
2
82 (76)[b]
65
0:100:–
0:66:34
8[d]
[a] Yields were determined by GC-MS analysis. [b] Yields of isolated
products are given in parentheses. [c] The reaction was started at 08C
and was then allowed to slowly warm to RT. [d] The E/Z ratio of the
product (86:14) was similar to that of the starting material (89:11).
First test reactions with two different sp3-hybridized or
two different sp2-hybridized Grignard reagents yielded an
almost statistic distribution of the possible coupling products.
The formation of the mixed product could be favored by using
a 1:2 ratio of the starting materials, but the final yield of the
cross-coupling product was still below 50%. A different
behavior was observed for oxidative cross-coupling reactions
between sp2- and sp3-hybridized Grignard reagents. The
coupling of phenylmagnesium chloride with phenethylmag-
nesium chloride gave biphenyl, bibenzyl and diphenylbutane
in the molar ratio 11:85:4, which is quite distinct from the
statistical distribution of 1:2:1. Further optimization was
achieved by lowering the reaction temperature to 08C and
using a 1:2 ratio of the starting materials (Table S3). Under
these conditions, it was possible to obtain the cross-coupling
products of phenylmagnesium chloride and different primary
(n-butyl, n-decyl, phenethyl) and secondary alkyl Grignard
reagents (cyclohexyl) in yields of 59–83% (Table 4, entries 1–
4). In line with the low reactivity of tert-butylmagnesium
bromide in the homocoupling reactions, the cross-coupling
with phenylmagnesium chloride was not very efficient
(Table 4, entry 5), whereas very good selectivities were
obtained for oxidative alkenyl–alkyl cross-coupling reactions
(Table 4, entries 6–8).
suggested that oxidation of a [CuR2]À complex preceeds the
reductive elimination,[13] and a similar mechanism can be
proposed for our system. Whitesides et al. had observed that
the oxidation of Li[CuPh(nBu)] with O2 gave a nearly
statistical mixture of biphenyl, phenylbutane, and octane.[14a]
Our catalytic cross-coupling reactions of aryl and alkyl
Grignard reagents with N2O, on the other hand, displayed
good selectivity for the mixed product. Interestingly, we
observed that the selectivity for the cross-coupling product
was lower when higher catalyst loadings were employed
(Table S3).
It is evident that N2 is a likely side product for all N2O
reactions described above. To demonstrate that N2O is indeed
converted into N2 during the catalytic cycle, we analyzed the
gas headspace before, during, and after completion of the
reaction of octylmagnesium bromide with Li2CuCl4
(1 mol%). The chromatograms clearly show the formation
of N2 (Figure S1). Along with N2, one expects the formation of
equal amounts of MgO (Scheme 1). The latter can aggregate
The results summarized in Tables 1–4 demonstrate that
aryl, alkenyl and alkyl Grignard reagents (RMgX) can be
efficiently coupled using N2O as the oxidant and simple
transition metal salts as catalysts. The mechanism of these
reactions likely involves the formation of a diorganyl metal
complex MR2Ln, which undergoes a reductive elimination
before or after oxidation by N2O. The order of these two steps
may depend on the substrate and the catalyst. In the case of
iron-catalyzed cross-coupling reactions of Grignard reagents
with electrophiles, catalytic cycles shuttling between FeI/FeIII,
Fe0/FeII or FeÀII/Fe0 have been proposed.[15] At the moment,
we do not have experimental evidence in favor of a particular
scenario. For the oxidation of organocuprates, it has been
Scheme 1. General reaction of Grignard reagents with N2O.
with MgX2 to form magnesium oxohalide clusters, such as
[(MgO)(MgBr2)3(solv)4] (solv = solvent), which are known
oxidation products of Grignard reagents.[16]
In conclusion, we have shown that N2O can be used as an
oxidant for the oxidative coupling reactions of Grignard
reagents with [Fe(acac)3], CoCl2, or Li2CuCl4 as catalysts. For
most reactions, catalyst loadings of 0.1–1.0 mol% were
sufficient to obtain good yields. Some aryl–aryl coupling
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Lett. 2001, 922 – 923; e) T. Yamada, K. Hashimoto, Y. Kitaichi,
reactions could be performed with less than 0.01 mol%
catalyst loading. The corresponding turnover numbers are
unprecedented for solution-based oxidation reactions with
N2O. Compared to alternative procedures with O2 as the
oxidant, our new method offers some important advantages:
1) It is possible to use lower amounts of catalyst, as N2O is less
prone to undergo metal-independent side reactions. 2) Steri-
cally demanding arylmagnesium halides can be used as
substrates. 3) Reactive alkyl Grignard reagents can be
employed as substrates. 4) Aryl–alkyl and alkenyl–alkyl
cross-coupling reactions can be achieved with good selectiv-
ity. These features should be attractive for applications in
organic synthesis. But the implications of our work are
possibly broader. The N2O method is apparently compatible
with a variety of transition metals (Fe, Co, and Cu).
Complexes of these metals are used as catalysts in many
other oxidative coupling reactions of nucleophiles,[7] some of
which are thought to involve low-valent organometallic
species. It appears likely that N2O can be used as oxidant
for some of these reactions as well.
K. Suzuki, T. Ikeno, Chem. Lett. 2001, 268 – 269.
[6] For solution-based oxidation reactions without metal catalysts,
see: a) K. Banert, O. Plefka, Angew. Chem. 2011, 123, 6295 –
6298; Angew. Chem. Int. Ed. 2011, 50, 6171 – 6174; b) D. P.
Ivanov, K. A. Dubkov, D. E. Babushkin, S. V. Semikolenov, G. I.
Panov, Adv. Synth. Catal. 2009, 351, 1905 – 1911; c) I. Hermans,
B. Moens, J. Peeters, P. Jacobs, B. Sels, Phys. Chem. Chem. Phys.
2007, 9, 4269 – 4274; d) E. V. Starokon, K. A. Dubkov, D. E.
Babushkin, V. N. Parmon, G. I. Panov, Adv. Synth. Catal. 2004,
346, 268 – 274; e) F. S. Bridson-Jones, G. D. Buckley, L. H. Cross,
A. P. Driver, J. Chem. Soc. 1951, 2999 – 3008.
[7] a) C. Liu, H. Zhang, W. Shi, A. Lei, Chem. Rev. 2011, 111, 1780 –
1824; b) J. Hassan, M. Sꢂvignon, C. Gozzi, E. Schulz, M.
Lemaire, Chem. Rev. 2002, 102, 1359 – 1469.
[8] For early investigations, see: a) G. M. Bennett, E. E. Turner, J.
Chem. Soc. 1914, 1057 – 1062; b) H. Gilman, M. Lichtenwalter, J.
Am. Chem. Soc. 1939, 61, 957 – 959.
[9] a) S.-K. Hua, Q.-P. Hu, J. Ren, B.-B. Zeng, Synthesis 2013, 518 –
526; b) K. Kude, S. Hayase, M. Kawatsura, T. Itoh, Heteroat.
Chem. 2011, 22, 397 – 404; c) Z. Zhou, W. Xue, J. Organomet.
Chem. 2009, 694, 599 – 603; d) G. Cahiez, C. Chaboche, F.
Mahuteau-Betzer, M. Ahr, Org. Lett. 2005, 7, 1943 – 1946; e) T.
Nagano, T. Hayashi, Org. Lett. 2005, 7, 491 – 493.
[10] a) P. I. Aparna, B. R. Bhat, J. Mol. Catal. A 2012, 358, 73 – 78;
b) G. Cahiez, C. Duplais, J. Buendia, Angew. Chem. 2009, 121,
6859 – 6862; Angew. Chem. Int. Ed. 2009, 48, 6731 – 6734; c) W.
Liu, A. Lei, Tetrahedron Lett. 2008, 49, 610 – 613; d) G. Cahiez,
A. Moyeux, J. Buendia, C. Duplais, J. Am. Chem. Soc. 2007, 129,
13788 – 13789.
[11] For the metal-free oxidation of Grignard reagents, see: a) M. S.
Maji, T. Pfeifer, A. Studer, Chem. Eur. J. 2010, 16, 5872 – 5875;
b) M. S. Maji, A. Studer, Synthesis 2009, 2467 – 2470; c) M. S.
Maji, T. Pfeifer, A. Studer, Angew. Chem. 2008, 120, 9690 – 9692;
Angew. Chem. Int. Ed. 2008, 47, 9547 – 9550; d) A. Krasovskiy,
A. Tishkov, V. del Amo, H. Mayr, P. Knochel, Angew. Chem.
2006, 118, 5132 – 5136; Angew. Chem. Int. Ed. 2006, 45, 5010 –
5014; e) T. Nishiyama, T. Seshita, H. Shodai, K. Aoki, H.
Kameyama, K. Komura, Chem. Lett. 1996, 549 – 550.
[12] T. Nagano, T. Hayashi, Chem. Lett. 2005, 34, 1152 – 1153.
[13] S. S. Surry, D. R. Spring, Chem. Soc. Rev. 2006, 35, 218 – 225.
[14] For early investigations, see: a) G. M. Whitesides, J. SanFilip-
po, Jr., C. P. Casey, E. J. Panek, J. Am. Chem. Soc. 1967, 89,
5302 – 5303; b) C. Glaser, Ber. Dtsch. Chem. Ges. 1869, 2, 422 –
424.
[15] R. Jana, T. P. Pathak, M. S. Sigman, Chem. Rev. 2011, 111, 1417 –
1492.
[16] a) H. Vitze, H.-W. Lerner, M. Bolte, Acta Crystallogr. Sect. E
2011, 67, m1614; b) G. Stucky, R. E. Rundle, J. Am. Chem. Soc.
1964, 86, 4821 – 4825.
Received: March 25, 2013
Published online: && &&, &&&&
Keywords: copper · Grignard reagents ·
.
homogeneous catalysis · iron · nitrous oxide
[1] A. V. Leontꢁev, O. A. Fomicheva, M. V. Proskurnina, N. S.
Zefirov, Russ. Chem. Rev. 2001, 70, 91 – 104.
[2] a) G. I. Panov, K. A. Dubkov, A. S. Kharitonov in Modern
Heterogeneous Oxidation Catalysis (Ed.: M. Noritaka), Wiley-
VCH, Weinheim, 2009, pp. 217 – 252; b) V. N. Parmon, G. I.
Panov, A. S. Noskov, Catal. Today 2005, 100, 115 – 131.
[3] W. B. Tolman, Angew. Chem. 2010, 122, 1034 – 1041; Angew.
Chem. Int. Ed. 2010, 49, 1018 – 1024.
[4] a) H. Goldberg, D. Kumar, G. N. Sastry, G. Leitus, R. Neumann,
J. Mol. Catal. A 2012, 356, 152 – 157; b) J. Ettedgui, R. Neumann,
J. Am. Chem. Soc. 2009, 131, 4 – 5; c) R. Ben-Daniel, R.
Neumann, Angew. Chem. 2003, 115, 96 – 99; Angew. Chem. Int.
Ed. 2003, 42, 92 – 95; d) R. Ben-Daniel, L. Weiner, R. Neumann,
J. Am. Chem. Soc. 2002, 124, 8788 – 8789.
[5] a) A. G. Tskhovrebov, E. Solari, R. Scopelliti, K. Severin,
Organometallics 2012, 31, 7235 – 7240; b) H. Tanaka, K. Hashi-
moto, K. Suzuki, Y. Kitaichi, M. Sato, T. Ikeno, T. Yamada, Bull.
Chem. Soc. Jpn. 2004, 77, 1905 – 1914; c) K. Hashimoto, H.
Tanaka, T. Ikeno, T. Yamada, Chem. Lett. 2002, 582 – 583; d) K.
Hashimoto, Y. Kitaichi, H. Tanaka, T. Ikeno, T. Yamada, Chem.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!