[(NHC)Fe(CO)4]Efficient pre-catalyst for selective hydroboration of alkenes

Article abstract of DOI:10.1002/cctc.201301062

[(IMes)Fe(CO)₄] [IMes=1,3-bis(2,4,6-trimethylphenyl) imidazol-2-ylidene] complex was found to be an efficient pre-catalyst for the hydroboration of functional alkenes in the presence of pinacolborane at room temperature. Notably, UV irradiation

Full text of DOI:10.1002/cctc.201301062

CHEMCATCHEM
COMMUNICATIONS
DOI: 10.1002/cctc.201301062
[(NHC)Fe(CO)4] Efficient Pre-catalyst for Selective
Hydroboration of Alkenes
Jianxia Zheng, Jean-Baptiste Sortais,* and Christophe Darcel*[a]
[(IMes)Fe(CO)4] [IMes=1,3-bis(2,4,6-trimethylphenyl) imidazol-
2-ylidene] complex was found to be an efficient pre-catalyst
for the hydroboration of functional alkenes in the presence of
pinacolborane at room temperature. Notably, UV irradiation
(350 nm) is important to promote this catalytic transformation.
Interestingly, high chemo- and regioselectivities were observed
as only anti-Markovnikov boronate derivatives were obtained
and various functional groups can be tolerated.
idine iron(0) and bis(imino)pyridine iron dinitrogen, respective-
ly, for efficient hydroboration of terminal and disubstituted al-
kenes.[13]
In the continuation of our investigation on iron-catalyzed
transformations,[14] we report that the [(IMes)Fe(CO)4] com-
plex[15] catalyzed the hydroboration of olefins in the presence
of pinacolborane at room temperature under UV irradiation at
l=350 nm.
We started by investigating the hydroboration of 1-decene
(1a) as the model substrate in the presence of pinacolborane
(1.25 equiv) using several well defined known iron complexes
as the catalyst (5 mol%) (Table 1). We have first selected [Fe-
(cod)(CO)3] (cod=1,5-cycloocta-diene) and [Fe(PBO)(CO)3]
(PBO=trans-4-phenylbut-3-en-2-one), which have shown ex-
cellent activities and selectivities for the hydrosilylation of car-
boxylic acids.[16] Unfortunately, either under thermal conditions
(508C) or UV irradiation (l=350 nm at room temperature)
after 24 h, disappointing yields of the hydroboration product
Organometallic-catalyzed hydroelementation processes using
transition metal complexes represent nowadays an efficient
and greener approach for the introduction of heteroelements
such as boron,[1] phosphorus,[2] and silicon[3] into unsaturated
carbon-based structures. More precisely, in the field of transi-
tion metal catalyzed hydroboration, organoboronates have
emerged as a significant class of organic reagents, owing to
their good stability toward atmospheric oxidation, associated
with their widespread use as synthons in selective transforma-
tions for carbon–carbon or carbon–heteroatom bond forma-
tion.[4] To perform such transformations, rhodium is usually the
metal of choice.[5] On another hand, iron is an earth-abundant
element and, therefore, is a sustainable, economical alternative
source for developing new catalytic process. Iron has emerged
as a powerful surrogate in transition metal catalysis,[6] in partic-
ular in hydrosilylation reactions.[7] By contrast, to promote hy-
droboration, borylation, and diborylation, iron complexes have
been scarcely used. In a stoichiometric version, Hartwig[8] re-
ported a pioneering activation of arenes leading to arylcate-
cholborane using the complex [CpFe(Bcat)(CO)2] as the pro-
moter. (cat=catechol) Then, Ritter described in 2010 the first
iron catalyzed hydroboration of 1,3-dienes in the presence of
pinacolborane (HBpin, pin=O2C2Me4), using a catalyst generat-
ed from a well-defined [(iminopyridine)FeCl2] complex in the
presence of magnesium.[9] In 2011, Enthaler succeeded in hy-
droboration of alkynes using [Fe2(CO)9] complex as the catalyst
yielding vinylboronate derivatives with high stereoselectivity.[10]
Recently, Huang[11] and Chirik[12] reported in situ generated
pincer iron catalysts, bis-tert-butylphosphino-methyl-2,2’-bypyr-
Table 1. Optimization of iron-catalyzed hydroboration of 1-decene with
pinacolborane.[a]
Entry
Catalyst
[mol%]
Conditions
Conv.[b]
[%]
Yield 2a[b]
[%]
1
2
3
4
5
6
7
8
[Fe(cod)(CO)3] (5)
[Fe(cod)(CO)3] (5)
[Fe(PBO)(CO)3] (5)
[Fe2(CO)9] (5)
3 (5)
4 (5)
5a (5)
5a (2.5)
5a (5)
RT, UV
508C[c]
508C[c]
RT, UV
RT, UV
RT, UV
RT, UV
RT, UV
RT, UV[e]
RT, UV[f]
RT, UV
RT, UV
97[d]
97[d]
73
30
25
<5
30
0
75
80
80
25
97[d]
0
97[d]
97[d]
97[d]
97[d]
97
97[d]
92[d]
9
10
11
12
5a (5)
5b (5)
5c (5)
94 (78[g]
55
)
[a] J. Zheng, Dr. J.-B. Sortais, Prof. Dr. C. Darcel
UMR 6226 CNRS-Universitꢀ de Rennes 1
“Institut des Sciences Chimiques de Rennes”
Team “Organometallics:Materials and Catalysis”
Centre for Catalysis and Green Chemistry
Campus de Beaulieu, 35042 Rennes Cedex (France)
E-mail: jean-baptiste.sortais@univ-rennes1.fr
christophe.darcel@univ-rennes1.fr
40
[a] 1-decene (0.25 mmol), pinacolborane (0.313 mmol, 1.25 equiv), iron
complex (0.013 mmol, 5 mol%), toluene (1 mL) under UV irradiation (l=
350 nm) at RT for 24 h. [b] Conversion of the alkene and yield of borylat-
ed derivative 2a, determined by 1H NMR spectroscopy. [c] Reaction per-
formed without UV irradiation at 508C. [d] The observed by-products are
resulting from the isomerization of the terminal C=C bond. [e] Reaction
performed in THF. [f] Reaction performed in neat conditions. [g] Isolated
yield.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201301062.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 763 – 766 763

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
2a were observed with these catalysts even at full conversion,
owing to the isomerization of the terminal alkene into a mix-
ture of decenes (entries 1–3). Such isomerization of alkenes
was not surprising as it has been already reported for iron car-
bonyl catalysts, such as [Fe(cod)(CO)3][17] or [Fe(CO)5][18] under
UV irradiation. The complex [CpFe(CO)2(IMes)][I] 3 acts as an ef-
ficient pre-catalyst for the hydrosilylation of aldehydes, ketones
and amides.[14] However, under UV irradiation at room temper-
ature for 24 h, no conversion occurred (entry 5). On the other
hand, [(NHC)Fe(CO)4] complexes were found to be active pre-
catalyst for chemo-selective hydrosilylation of esters to aldehy-
des.[15a] First of all, with the complex bearing a phosphine,
[(PhMe2P)Fe(CO)4], the reactivity and the regioselectivity were
enhanced as full conversion was obtained with the anti-Mar-
kovnikov boronate 2a in 75% yield (entry 6). More
with a moderate 52% yield (entry 13). It must be pointed out
that in all the cases, the dehydrogenative borylation reaction
led to less than 5% of side-product detected in the crude mix-
ture.
a-Olefins containing an epoxide, an acetal, or a cyano group
on the lateral chain are also hydroborated, but the correspond-
ing boronate esters are isolated with lower yields (21–37%),
mainly due to isomerization processes without hydroboration
(entries 14–16).[19] Hydroboration of internal alkenes using
[(IMes)Fe(CO)4] as the catalyst (5 mol%) with pinacolborane is
also possible. Starting from 1-(p-methoxyphenyl)prop-2-ene,
50% conversion was reached after 24 h at room temperature
under UV irradiation (l=350 nm) leading to the 3-phenylpro-
pylboronate ester 2e, with 35% isolated yield, resulting from
significantly, the use of [(NHC)Fe(CO)4] complexes as
the pre-catalysts (5 mol%) in toluene gave higher
Table 2. Scope of [(IMes)Fe(CO)4]-catalyzed hydroboration of alkenes with pinacol-
yields than with the other complexes. [(IPr)Fe(CO)4]
5b and [(iPr2NHC)Fe(CO)4] 5c gave similar results: 55
and 40% of 2a, respectively, in mixture with isomer-
ized decenes (entries 11 and 12). Finally, the complex
[(IMes)Fe(CO)4] 5a was found to be the best catalyst
to perform the hydroboration of 1-decene with a full
conversion and leading to the anti-Markovnikov bor-
onate ester 2a as the major product in 80% yield
(entry 7). Notably, working with 2.5 mol% of 5a as
the catalyst led to similar result, whereas performing
the reaction in THF instead of toluene had a deleteri-
ous effect as only 25% of 2a was obtained under
similar conditions (entries 8 and 9). Interestingly, in
neat conditions, full conversion was observed and
the boronate ester 2a was obtained as the sole prod-
uct in 74% isolated yield (entry 10).
borane.[a]
Entry
Alkene
Product
Conv[b]
[%]
Yield[c]
[%]
1
2
3
n=7, 2a
n=5, 2b
n=3, 2c
97
95
95
80
78
66
4
2d
97
65
5
2e
97
71
6
2 f
97
64
7
8
9
2g
2h
2i
97
97
97
72
70
43
With these optimized conditions in hand (5 mol%
of 5a, 1 equiv of alkene, 1.25 equiv of pinacolborane
at room temperature for 24 h under UV irradiation at
l=350 nm), we then studied the scope and limita-
tions of this transformation with a special interest for
functional group tolerance (Table 2). Firstly, non-func-
tional linear terminal alkenes such as 1-hexene, 1-
octene, and 3-phenyl-1-propene were selectively hy-
droborated and the corresponding anti-Markovnikov
alkylboronate esters were isolated with 65-80%
yields (Table 2, entries 1–4). Functionalized aryl substi-
tuted terminal olefins with fluoride, trifluoromethyl,
methoxy or acetal can be also successfully hydrobo-
rated yielding the corresponding boronates with 43–
72% yields (Entries 5–9). The olefins bearing ether
moieties can be also hydroborated using pinacolbor-
ane: 3-trimethylsiloxy-1-propene and 3-benzyloxy-1-
propene led to the derivatives 2j and 2k with satis-
factory yields (58 and 62%, respectively (entries 10
and 11). Surprisingly, 3-phenoxy-1-propene, even if
the conversion was full, the borylated compound 2l
was isolated with only 26% yield (entry 12). Interest-
ingly, ester moiety can be tolerated under such con-
ditions, the resulting compound 2m being isolated
10
11
12
2j
2k
2l
97
97
97
58
62
26[d]
13
14
2m
2n
97
97
52
37[e]
15
16
2o
2p
97
90
32
21[e]
17
2e
50
35
18
19
2c
2a
97
93
50
73
[a] alkene (0.5 mmol), pinacolborane (0.625 mmol, 1.25 equiv), [(IMes)Fe(CO)4] 5a
(11.8 mg, 0.025 mmol, 5 mol%), under UV irradiation (l=350 nm) at room tempera-
ture for 24 h. [b] conversion determined by 1H NMR spectroscopy. [c] Isolated yield
after flash chromatography on silica. [d] Mixture of uncharacterized products and 2l
were observed. [e] Main of the by-products results from the isomerization of the
olefins.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 763 – 766 764

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
stirred at the room temperature for 5 min and then irradiated
under UV (l=350 nm) for 24 h. At the end of the reaction, the
crude reaction mixture was filtered through a plug of silica gel
with CH2Cl2 as the eluent and the filtrate was evaporated and was
a tandem iron-catalyzed isomerization/hydroboration of the
obtained terminal olefin. Similarly, using internal alkenes such
as (3E)-hexene and (5E)-decene, 1-hexylboronate and 1-decyl-
boronate esters 2c and 2a are obtained in 80 and 73%,
respectively (entries 18 and 19).
1
then analyzed by H NMR spectroscopy to determine the conver-
sion. The residue was purified by column chromatography on silica
using ethyl acetate-pentane mixture to achieve the desired
product.
By contrast, when reacting 3-(dibenzylamino)prop-1-ene, the
product resulting from the full reduction of the alkene, namely,
dibenzylpropylamine was obtained in 65% isolated yield, with
low trace amount of hydroborated compound, which enlight-
ens that hydroboration of vinyl-functionalized compounds
seems to be difficult. (Scheme 1) We also note that under our
conditions, the hydroboration of trisubstituted alkene such as
2-methylbut-2-ene was not observed.[20]
Acknowledgements
We are grateful to the CNRS, the University of Rennes 1, the Min-
istꢁre de l’Enseignement Supꢀrieur et de la Recherche, the ANR
program ANR-12-BS07-0011 “IRONHYC” and Axa Research Funds
for a PhD grant to JZ.
Keywords: alkenes · alkylboronates · hydroboration · iron ·
pinacolborane
Scheme 1.
[1] For selected reviews on hydroboration reactions, see: a) K. Burgess,
M. J. Ohlmeyer, Chem. Rev. 1991, 91, 1179–1191; b) I. Beletskaya, A.
Pelter, Tetrahedron 1997, 53, 4957–5026; c) N. Miyaura in Catalytic Het-
erofunctionalization (Eds.: A. Togni, H. Grꢁtzmacher), Wiley-VCH, Wein-
heim, 2001, pp. 1–46; d) C. M. Vogels, S. A. Westcott, Curr. Org. Chem.
2005, 9, 687–699; e) C. M. Crudden, D. Edwards, Eur. J. Org. Chem.
2003, 4695–4712; f) A.-M. Carroll, T. P. O’Sullivan, P. J. Guiry, Adv. Synth.
Catal. 2005, 347, 609–631; g) J. F. Hartwig, Chem. Soc. Rev. 2011, 40,
1992–2002; h) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M. Murphy,
J. F. Hartwig, Chem. Rev. 2010, 110, 890–931; i) J. F. Hartwig, Acc. Chem.
Res. 2012, 45, 864–873.
[2] For reviews on hydrophosphination reactions, see: a) I. P. Beletskaya,
V. P. Ananikov, L. L. Khemchyan in Phosphorus Compounds, Advanced
Tools in Catalysis and Material Sciences, Vol. 37 (Eds.: M. Peruzzini, L.
Gonsalvi), Springer, Dordrecht, NL, 2011, pp. 213–264; b) V. P. Ananikov,
I. P. Beletskaya, Chem. Asian J. 2011, 6, 1423–1430; c) O. Delacroix, A.-C.
Gaumont, Curr. Org. Chem. 2005, 9, 1851–1882; d) M. Tanaka, Top. Curr.
Chem. 2004, 232, 25–54; e) F. Alonso, I. P. Beletskaya, M. Yus, Chem. Rev.
2004, 104, 3079–3160.
[3] For books on hydrosilylation reactions, see: a) Comprehensive Handbook
on Hydrosilylation (Ed.: B. Marciniec), Pergamon Press, Oxford, 1992;
b) Hydrosilylation: A Comprehensive Review on Recent Advances (Ed.: B.
Marciniec), Springer, Heidelberg, 2009.
[4] For selected reviews, see: a) D. G. Hall, Boronic Acids, Wiley-VCH, New
York, 2005; b) N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457–2483;
c) A. Suzuki, J. Organomet. Chem. 1999, 576, 147–168; d) S. R. Chemler,
D. Trauner, S. J. Danishefsky, Angew. Chem. Int. Ed. 2001, 40, 4544–
4568; Angew. Chem. 2001, 113, 4676–4701; e) H. Doucet, Eur. J. Org.
Chem. 2008, 2013–2030; f) R. Jana, T. P. Pathak, M. S. Sigman, Chem.
Rev. 2011, 111, 1417–1492.
[5] For selected reviews, see: a) J. M. Brown in Modern Rhodium-Catalyzed
Organic Reactions (Ed.: P. A. Evans), Wiley-VCH, Weinheim, 2005,
pp. 33–54; b) A. G. Coyne, P. J. Guiry in Modern Reduction Methods
(Eds.: P. G. Andersson, I. J. Munslow), Wiley-VCH, Weinheim, 2008,
pp. 65–86.
[6] For representative recent reviews on iron catalysis, see: a) K. Gopalaiah,
Chem. Rev. 2013, 113, 3248–3296; b) C.-L. Sun, B.-J. Li, Z.-J. Shi, Chem.
Rev. 2011, 111, 1293–1314; c) B. A. F. Le Bailly, S. P. Thomas, RSC Adv.
2011, 1, 1435–1445; d) E. Nakamura, N. Yoshikai, J. Org. Chem. 2010,
75, 6061–6067; e) W. M. Czaplik, M. Mayer, J. Cvengrꢂs, A. J. von Wange-
lin, ChemSusChem 2009, 2, 396–417; f) R. H. Morris, Chem. Soc. Rev.
2009, 38, 2282–2291; g) E. B. Bauer, Curr. Org. Chem. 2008, 12, 1341–
1369; h) A. Correa, O. G. MancheÇo, C. Bolm, Chem. Soc. Rev. 2008, 37,
1108–1117; i) B. D. Sherry, A. Fꢁrstner, Acc. Chem. Res. 2008, 41, 1500–
1511; j) A. Fꢁrstner, R. Martin, Chem. Lett. 2005, 34, 624–629; k) C. Bolm,
J. Legros, J. Le Paih, L. Zani, Chem. Rev. 2004, 104, 6217–6254.
NMR studies were conducted to obtain insights into the
mechanism of this iron-catalyzed hydroboration reaction. Simi-
larly with the case of reaction with hydrosilane,[15a] the reaction
of [(IMes)Fe(CO)4] with 5 equiv of pinacolborane or catechol-
borane in C6D6 under UV irradiation at l=350 nm, for 3 h at
room temperature, even if the conversion did not exceed 10%,
led to the formation of a new species which was identified in
1H NMR as a singlet at d=À9.04 ppm and À8.91 ppm, respec-
tively,[21,24] and in 11B NMR with broad signals at d=47.8 ppm
and 50.2 ppm, respectively.[8a,22] These results imply that
[(IMes)Fe(CO)3(H)(BR2)] [BR2 =Bcat or Bpin) intermediates result
from the oxidative addition of the BÀH bond of the hydrobor-
onate on to unsaturated [(IMes)Fe(CO)3] species.[15a,23] Such
data should indicate that in a first step the UV-assisted decoor-
dination of a CO ligand was followed by the oxidative addition
of the H-B(OR)2 reagent leading to a plausible iron(hydride)-
(boryl) intermediate.[25]
In conclusion, we have shown that [(IMes)Fe(CO)4] complex
can be an effective catalyst for hydroboration of alkenes at
room temperature under UV irradiation at l=350 nm. Notably,
the catalyst is well-suitable for the hydroboration of terminal
alkenes leading to anti-Markovnikov boronate esters. In the
case of internal olefins, a tandem isomerization/hydroboration
permits to obtain the linear anti-Markovnikov products. More
interestingly, the reaction can tolerate various functional
groups such as ether, ester, silylether, epoxide, acetal, and ni-
trile, even if in some cases, yields are moderate.
Experimental Section
General procedure for the iron-catalyzed hydroboration reactions: A
10 mL oven dried Schlenk tube containing a stirring bar, was
charged with [(IMes)Fe(CO)4] (11.8 mg, 0.025 mmol). After purging
with argon (argon-vacuum three cycles), alkene was added fol-
lowed by HBpin (90 mL, 0.625 mmol). The reaction mixture was
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 763 – 766 765

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
[7] a) M. Zhang, A. Zhang, Appl. Organomet. Chem. 2010, 24, 751–757;
b) K. Junge, K. Schrçder, M. Beller, Chem. Commun. 2011, 47, 4849–
4859.
[8] a) J. F. Hartwig, S. Huber, J. Am. Chem. Soc. 1993, 115, 4908–4909;
b) K. M. Waltz, X. He, C. Muhoro, J. F. Hartwig, J. Am. Chem. Soc. 1995,
117, 11357–11358.
[17] Example of [Fe(cod)(CO)3] catalyzed alkene isomerization: H. Fleckner,
F.-W. Grevels, D. Hess, J. Am. Chem. Soc. 1984, 106, 2027–2032.
[18] Representative examples of [Fe(CO)5]-catalyzed alkene isomerization:
a) E. Frankel, E. Emken, V. Davison, J. Am. Oil Chem. Soc. 1966, 43, 307–
311; b) R. Jennerjahn, R. Jackstell, I. Piras, R. Franke, H. Jiao, M. Bauer, M.
Beller, ChemSusChem 2012, 5, 734–739.
[19] For the hydroboration of 10-undecanenitrile under our standard condi-
tions, 64% of compounds resulted from the isomerization were ob-
tained in mixture with 21% of 11-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)undecanenitrile 2p.
[9] J. Y. Wu, B. Moreau, T. Ritter, J. Am. Chem. Soc. 2009, 131, 12915–12917.
[10] M. Haberberger, S. Enthaler, Chem. Asian J. 2013, 8, 50–54.
[11] L. Zhang, D. Peng, X. Leng, Z. Huang, Angew. Chem. Int. Ed. 2013, 52,
3676–3680; Angew. Chem. 2013, 125, 3764–3768.
[20] Using our standard conditions, the hydroboration of styrene led to
[12] J. V. Obligacion, P. J. Chirik, Org. Lett. 2013, 15, 2680–2683.
[13] During the preparation of the manuscript, S. P. Thomas described simi-
lar results than those described by P. Chirik (Ref. 12) using an in situ
generated catalyst from FeCl2, the free bis(imino)pyridine ligand and
EtMgBr, which has shown a good functional group tolerance. See: M. D.
Greenhalgh, S. P. Thomas, Chem. Commun. 2013, 49, 11230–11232.
[14] a) D. Bꢃzier, J.-B. Sortais, C. Darcel, Adv. Synth. Catal. 2013, 355, 19–33;
b) L. C. Misal Castro, J.-B. Sortais, C. Darcel, Chem. Commun. 2012, 48,
151–153; c) D. Bꢃzier, G. T. Venkanna, L. C. Misal Castro, J. Zheng, J.-B.
Sortais, C. Darcel, Adv. Synth. Catal. 2012, 354, 1879–1884; d) D. Bꢃzier,
F. Jiang, T. Roisnel, J.-B. Sortais, C. Darcel, Eur. J. Inorg. Chem. 2012,
1333–1337; e) D. Bꢃzier, G. T. Venkanna, J.-B. Sortais, C. Darcel, Chem-
CatChem 2011, 3, 1747–1750; f) F. Jiang, D. Bꢃzier, J.-B. Sortais, C.
Darcel, Adv. Synth. Catal. 2011, 353, 239–244.
a
mixture of products: Ph-CH2-CH2-Bpin (55%), (E)-Ph-CH=CH-Bpin
(11%), Ph-CH2-C(Bpin)2 (12%) and Ph-CH2-CH3 (19%) (1H NMR yields
with pyridine as the internal standard).
[21] Selected iron hydrido complexes: a) G. Bellachioma, G. Cardaci, E. Colo-
mer, R. J. P. Corriu, A. Vioux, Inorg. Chem. 1989, 28, 519–525; b) G. Rein-
hard, B. Hirle, U. Schubert, M. Knorr, P. Braunstein, A. DeCian, J. Fischer,
Inorg. Chem. 1993, 32, 1656–1661.
[22] Selected boron iron complexes, see Ref 8a and K. M. Waltz, C. N.
Muhoro, J. F. Hartwig, Organometallics 1999, 18, 3383–3393.
[23] UV activation was shown to easily release one CO from [(PR3)Fe(CO)4]-
type complexes, see: a) J. L. Graff, R. D. Sanner, M. S. Wrighton, Organo-
metallics 1982, 1, 837–842; b) D. K. Liu, C. G. Brinkley, M. S. Wringhton,
Organometallics 1984, 3, 1449–1457.
[24] Up to now, the plausible Fe(hydrido)(boryl) intermediate cannot be iso-
lated and further characterized.
[25] S. A. Westcott, T. B. Mader, R. T. Backer, J. C. Calabrese, Can. J. Chem.
1993, 71, 930–936.
[15] a) H. Li, L. C. Misal Castro, J. Zheng, T. Roisnel, V. Dorcet, J.-B. Sortais, C.
Darcel, Angew. Chem. Int. Ed. 2013, 52, 8045–8049; Angew. Chem. 2013,
125, 8203–8207; b) S. Warratz, L. Postigo, B. Royo, Organometallics
2013, 32, 893–897; c) G. Huttner, W. Gartzke, Chem. Ber. 1972, 105,
2714–2725.
[16] L. C. Misal Castro, H. Li, J.-B. Sortais, C. Darcel, Chem. Commun. 2012, 48,
10514–10516.
Received: December 11, 2013
Published online on February 20, 2014
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 763 – 766 766