Mechanistic study of a switch in the regioselectivity of hydroheteroarylation of styrene catalyzed by bimetallic Ni-Al through C-H activation

Article abstract of DOI:10.1002/chem.201400303

We previously reported a highly efficient protocol for bimetallic Ni-Al-catalyzed hydroheteroarylation of styrene with benzimidazole based on C-H bond activation. We have now delineated the mechanism of this process, providing a rationale for an observed switch in regioselectivity in the presence of the Lewis acid, AlMe₃. The present mechanistic study gives insights for the rational development of catalysts that exhibit required linear/branched selectivity. Lewis acid switches regioselectivity: The mechanism underpinning a highly efficient protocol for bimetallic Ni-Al-catalyzed hydroheteroarylation of styrene with benzimidazole based on C-H bond activation has been unraveled (see scheme). The study gives insights that can be used for the rational development of catalysts that give the required linear/branched selectivity.

Full text of DOI:10.1002/chem.201400303

DOI: 10.1002/chem.201400303
Full Paper
&
Arylation
Mechanistic Study of a Switch in the Regioselectivity of
Hydroheteroarylation of Styrene Catalyzed by Bimetallic Ni–Al
through CÀH Activation
Wen-Ching Chen,[a] Ying-Chieh Lai,[b] Wei-Chun Shih,[a] Ming-Shiuan Yu,[c] Glenn P. A. Yap,[d]
and Tiow-Gan Ong*[a, e]
Abstract: We previously reported a highly efficient protocol
for bimetallic Ni–Al-catalyzed hydroheteroarylation of sty-
rene with benzimidazole based on CÀH bond activation. We
have now delineated the mechanism of this process, provid-
ing a rationale for an observed switch in regioselectivity in
the presence of the Lewis acid, AlMe3. The present mecha-
nistic study gives insights for the rational development of
catalysts that exhibit required linear/branched selectivity.
Introduction
Lewis-acid-assisted transition metal catalysis has slowly
caught the interest of the synthetic community for its coopera-
tive effect of enhancing the rate and controlling the selectivity
of the respective transition metal catalyst.[3] For example, the
groups of Hartwig[3a] and Nolan[3b] have reported that Lewis
acids such as AlCl3 and BEt3 accelerate palladium-mediated re-
ductive elimination reactions. Knochel reported the direct pal-
ladium-catalyzed arylation of methyl-substituted pyridine pro-
moted by ZnCl2, Sc(OTf)3, and BF3·OEt2 with excellent regiose-
lectivity.[3c] Similarly, our group,[4] as well as those of Nakao and
Hiyama[5] and Kanai,[6] have observed the beneficial effect of
added AlMe3 in promoting nickel-mediated CÀH bond activa-
tion of heteroarenes. More recently, we reported a switch in
the regioselectivity for CÀH functionalization of benzimidazole
derivatives with styrene and allylbenzene to afford either linear
or branched products when using AlMe3.[7] In this work, we
present a detailed mechanistic investigation of this branched/
linear regioselectivity with regard to the synergistic interaction
between Ni and Al bimetallic catalysts bearing amino-NHC (ni-
trogen heterocyclic carbene) ligand 1a.
Transition-metal-mediated CÀH bond activation and functional-
ization has garnered significant interest in the past decade.
The ability to circumvent additional steps associated with pre-
functionalized coupling partners renders the synthetic strategy
appealing for a myriad of applications.[1] From the perspective
of synthetic and catalytic utility, judiciously controlling CÀH
bond functionalization for site selectivity would be highly de-
sirable. Recently, several research groups have reported the
use of specific directing groups on the substrates or alteration
of the structural morphology of metal-bound ligands to facili-
tate the desired regioselectivity.[2] However, because of the re-
quirement of tedious synthetic manipulations for the installa-
tion of directing groups, these strategies are inherently limited.
[a] Dr. W.-C. Chen, W.-C. Shih, Prof. Dr. T.-G. Ong
Institute of Chemistry, Academia Sinica
No. 128, Sec. 2, Academia Road, Nangang, Taipei (Taiwan, R.O.C.)
Fax: (+886)2-2783-1237
E-mail: tgong@gate.sinica.edu.tw
[b] Y.-C. Lai
Department of Chemistry and Biochemistry
National Chung Cheng University
Results and Discussion
No. 168, University Road, Min-Hsiung, Chiayi 621 (Taiwan, R.O.C.)
Hydroheteroarylation of styrenes by CÀH bond activation of
[c] M.-S. Yu
benzimidazole
Department of Chemistry
National Tsing Hua University
No. 101, Sec. 2, Kuang-Fu Road, Hsinchu (Taiwan, R.O.C.)
Our group has previously reported the direct amino-NHC
nickel-catalyzed hydroheteroarylation of styrene with benzimi-
dazole by CÀH bond activation, as summarized in Table 1.[7a]
High selectivity in favor of the branched product with good
yield was observed with various styrene substrates. Interesting-
ly, addition of AlMe3 to the reaction mixture resulted in com-
plete reversal of the site selectivity in favor of the linear prod-
uct (anti-Markovnikov). Moreover, electronic modulation of the
styrene component did not seem to perturb the linear selectiv-
ity. For instance, complete linear regioselectivity and excellent
[d] Dr. G. P. A. Yap
Department of Chemistry and Biochemistry
University of Delaware
Newark, Delaware, 19716 (USA)
[e] Prof. Dr. T.-G. Ong
The Department of Applied Chemistry
National Chiao Tung University
No. 1001, Ta Hsueh Road, Hsinchu (Taiwan, R.O.C.)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400303.
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the resulting reaction profiles are shown in Figure 1.
The formation of linear product 20ab in the presence
of AlMe3 (diamonds) reached completion (90% yield)
within 150 min, whereas the reaction without AlMe3
(circles) was still in the induction period at this time.
Clearly, the presence of AlMe3 had a remarkable
impact on the reaction rate. Close inspection of the
1H NMR spectra for the stoichiometric reaction of
AlMe3 with 1a revealed the generation of an Al–ben-
zimidazole adduct (3), and the methyl signal of ben-
zimidazole was shifted upfield from d=2.61 to
2.20 ppm. Furthermore, the identity of 3 was con-
firmed by a single-crystal X-ray study, which revealed
that AlMe3 was datively coordinated to the benzimi-
dazole nitrogen (Figure 2).[7a,8] Thus, we postulated
that coordination of the Lewis acid to benzimidazole
would increase the acidity of the C2ÀH bond, result-
ing in the observed higher activity. It is imperative to
mention the recent advances in the use of Lewis acid
additives to enhance reaction rates; for example, the
groups of Hartwig, Nolan, and Knochel have em-
ployed the Lewis acids AlCl3 and BEt3 for reductive
elimination reactions mediated by palladium.[3]
Table 1. Scope of the reaction in terms of various styrenes with benzimidazole:
branched and linear selectivities in the absence and presence of AlMe3.[a]
Entry Styrene
Without AlMe3, T=1508C
10 mol% AlMe3, T=1008C
Yield [%]
Selectivity B/L
Yield [%]
Selectivity B/L
1
2
3
4
70
100:0
85
0:100
50
69
67
100:0
100:0
100:0
88
96
93
0:100
0:100
0:100
To understand the nature of the CÀH bond cleav-
age process in the present (amino-NHC) nickel-cata-
lyzed hydroarylation reaction, we first performed
a series of competition experiments with various
benzimidazole derivatives. Our findings revealed that
neutral benzimidazole 10a was functionalized only
0.93 times that of its electron-rich counterpart 10i
[Eq. (1)], thus suggesting that an electrophilic aro-
5
6
97
72
100:0
100:0
90
66
4:96
55:45
[a] The reactions were carried out using 10a (0.50 mmol) and styrene (0.75 mmol) and
products were determined by 1H NMR analysis with isolated yields based on 10a as
the limiting reagent. 10 mol% of [Ni(cod)2], AlMe3, and NHC ligand 1a.
yield were observed for 4-methylstyrene (11 b), 4-methoxystyr-
ene (11 c), 4-fluorostyrene (11 d), and 4-phenylstyrene (11 e) de-
Table 2. Scope of the reaction in terms of various heterocycles in the
presence of AlMe3 as an additive.[a]
rivatives. An exception to this trend was found with 2-vinyl-
naphthalene (11 f), for which no chemical bias for site selectivi-
ty was observed, resulting in a product distribution of equal
amounts of the linear and branched isomers. The lack of selec-
tivity with 11 f was attributed to more effective p-conjugated
stabilization due to the naphthalene ring.
As illustrated in Table 2, catalytic CÀH bond activation pro-
moted by the Al-Ni bimetallic system is also conceivable with
other benzimidazole derivatives. High levels of linear selectivity
and good yields were consistently obtained with various benzi-
midazoles.
Mechanistic studies on the CÀH bond activation
The accelerating effect of AlMe3 on Ni-mediated CÀH bond ac-
tivation of benzimidazole is clearly evident from the fact that
the yields of reactions in the presence of AlMe3 (85–96%,
Table 1) are generally higher than those in the absence of
Lewis acid additives (50–70%, Table 1). To delineate the role of
AlMe3 in promoting the reaction, product conversions in the
presence and absence of AlMe3 were monitored as a function
of time by sampling small aliquots of the reaction mixtures;
[a] The reactions were carried out using 10 (0.50 mmol) and styrene
(0.75 mmol) and products were determined by 1H NMR analysis with iso-
lated yields based on 10 as the limiting reagent. 10 mol% [Ni(cod)2],
AlMe3, and NHC ligand 1a.
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contrast to our previous CÀH bond activation studies on pyri-
dine, which displayed only a small primary KIE of 1.25,[4] high-
lighting the requirement of p-coordination of pyridine prior to
the CÀH bond breaking.
1
Close monitoring by H NMR of the catalytic hydroarylation
of styrene with benzimidazole 10a mediated by the nickel-alu-
minum catalyst system revealed a very weak upfield signal at
d=À12.05 ppm, indicating the appearance of nickel hydride
as a result of CÀH bond breaking by oxidative addition. Un-
fortunately, however, we were unable to isolate this intermedi-
ate, thus preventing us from further elucidating the identity of
this species in the catalytic process. Nonetheless, the benzimi-
dazolium salt 13a, with its two substituents on the nitrogen
atoms, should bear considerable resemblance to the Lewis pair
adduct 3 and thus serve as an appropriate model compound
to delineate the nature of the reaction intermediates. When
[Ni(cod)2] (cod=1,5-cyclooctadiene) was treated with stoichio-
metric amounts of 1a and benzimidazolium salt 13a, an NiÀH
signal appeared at d=À12.92 in the 1H NMR spectrum
[Eq. (2)], which could be assigned to a new nickel hydrido
complex, 4c. Unequivocal confirmation of the structure of 4c
as a nickel hydride was provided by X-ray analysis of a single
crystal grown from a solution in CH3CN/hexane at À208C
(Figure 3). Complex 4c exhibits a distorted square-planar ge-
ometry, in which the two amino-NHC moieties have a trans dis-
position to avoid any unfavorable steric interaction, thus forc-
Figure 1. The reaction rate profile for the reaction of 10a and 11 a in the
presence (diamonds) and absence of AlMe3 (circles).
Figure 2. The molecular structure of Al–benzimidazole adduct 3.[7a]
matic CÀH bond activation pathway is less likely to be opera-
tive in the Ni–Al bimetallic system.[9]
Subsequently, we examined the CÀH bond activation pro-
cess by employing benzimidazole isotopically labeled at C2,
[D1]-10a. Intermolecular competition in the presence of AlMe3
resulted in a kinetic isotope effect (KIE) of kH/kD ꢀ3.56, diag-
nostic of CÀH bond breaking in an oxidative addition mecha-
nism, most probably as a reversible rate-limiting step. A posi-
tive KIE of 4.61 was also observed for the reaction performed
in the absence of AlMe3. These KIE outcomes are in striking
Figure 3. ORTEP representation of Ni hydrido complex 4c with thermal ellip-
soids drawn at the 30% probability level. All other hydrogen atoms, with
the exception of NiH, have been omitted for clarity. Selected bond lengths
(ꢁ) and angles (deg.): Ni–C1 1.893(2), Ni–C19 1.907(3), N1–C1 1.365(3), N1–C2
1.394(3); C1-Ni-C1A 167.25(13), C1-Ni-C19 96.38(6).
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the intermediate resembling complex 4b bearing adduct 3
have thus far not been fruitful. We therefore undertook
a series of indirect experiments to confirm our hypothesis of
steric-based linear selectivity induced by AlMe3 as a chemical
switch.
It is interesting to note that the catalytic reaction of benzox-
azole (12) under similar conditions with 10 mol% AlMe3 fur-
nished solely the branched product in good yield (91%) and
failed to produce any detectable amount of the linear isomer.
1H NMR spectroscopy revealed no such Lewis acid adduct (7)
for benzoxazole. Moreover, we also observed that the extent
of the reaction after 30 min was not significantly different in
either the presence (47%) or absence (44%) of AlMe3
(Scheme 1). This negative result highlights the importance of
interaction between the Lewis acid and the heterocycle for
promoting both linear selectivity and high reaction rate.
Figure 4. Proposed mechanism for CÀH bond activation in the hydrohet-
eroarylation of styrene.
ing the hydride and benzimidazole into a mutually trans ar-
rangement. This isolated solid-state structure provided further
evidence in support of an oxidative addition mechanism for
CÀH bond breaking. A similar Ni hydride complex has been re-
ported by Cavell’s group for Ni-mediated CÀH activation/
alkene insertion to produce a linear 2-alkyl imidazolium salt.[10]
Based on our experimental observations, we were able to
confirm our previously proposed mechanistic pathway, as illus-
trated in Figure 4.[7a] The CÀH bond functionalization in hydro-
arylation proceeds through the following mechanistic steps:
(1) concerted oxidative addition of the CÀH bond to generate
intermediate NiÀH species 4, (2) facile coordination of styrene
to the metal followed by migratory insertion to yield Ni alkyl
species 5, and (3) reductive elimination of 5a (pathway A)
under electronic control and 5b (pathway B) under steric con-
trol to afford branched and linear products, respectively.
Scheme 1. Reaction of 12 and 11 a.
Steric influence on linear selectivity
At this stage, our focus was to understand the influence of
steric variations of the heteroaromatic substrates on selectivity.
Again, variation of the substituents on the nitrogen atoms of
the benzimidazolium salt was considered to provide a useful
molecular model for elucidating the steric effects of both
AlMe3 and the side arm of the imidazole on the selectivity of
the reaction. Initial efforts were focused on optimizing the
range of benzimidazolium salts with different counter anions
(Table 3). The best optimized yield (69%) with a high linear se-
À
lectivity of 4:1 for 13a·PF6 was achieved under conditions
Mechanistic implications of branched/linear selectivity
similar to those used previously.
In the proposed mechanism, the selectivity for the branched
product is dictated by electronic preference for the p-benzyl-
nickel species 5a, a feature that is well documented.[5f,11] As
a consequence, the catalytic cycle “A” under electronic control
is manifested in formation of the Markovnikov product. On the
other hand, we believe that AlMe3 plays a critical role in favor-
ing selectivity for the linear product. Based on the isolation of
the Al–benzimidazole adduct (3) (see above), we postulate
that the dormant reaction pathway “B” is activated, thus avoid-
ing steric repulsion at the Ni center in 4b. In our previous
studies on Ni-mediated CÀH bond activation of pyridine, we
found that AlMe3 not only activates the CÀH bond towards
functionalization, but also invokes a steric environment pro-
moting para selectivity.[4] Attempts to isolate and characterize
Under the optimized conditions, the substrate scope in
À
terms of benzimidazolium salts with PF6 as counterion in the
hydroheteroarylation of styrene was probed, and the results
are summarized in Table 4. The 1,3-dimethyl benzimidazolium
salt 13a performed quite well, giving a moderate yield (69%)
with a linear/branched selectivity of 8:2 (entry 1). AlMe3 is not
required to induce linear selectivity in this catalytic reaction.
Replacing the methyl groups with larger pendants such as
ethyl (13b, entry 2; 13 f, entry 6) and isopropyl (13c, entry 3)
increased the linear/branched selectivity to 8.5:1.5 and 9:1.
Benzyl derivative 13d gave complete linear selectivity with
a yield of 68% (entry 4). Unfortunately, a more sterically en-
cumbered benzimidazolium salt (13g, entry 7) bearing two iso-
propyl side arms failed to react, thus representing the upper
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Table 3. Screening process.[a]
Table 4. Hydroheteroarylations of styrene with various benzimidazolium
salts.[a]
Entry
Anion XÀ
L/B
Yield [%]
Entry
1
13
L/B
Yield [%]
69
À
1
2
3
BF4
IÀ
7:3
–
4:1
28
0
69
À
8:2
PF6
[a] The reactions were carried out using 13a (0.5 mmol) and 11 a
1
(0.75 mmol) and products were determined by H NMR analysis. 10 mol%
[Ni(cod)2] and 10 mol% NHC ligand 1a.
2
3
8.5:1.5
9:1
97
99
limiting case for steric control of this reaction.[12] Surprisingly,
the linear/branched selectivity decreased to 7:3 for the 1-
methyl-3-(p-tolyl)benzimidazolium salt (13e), even though the
steric bulk of tolyl (13e) is clearly larger than that of methyl
(13a). This outcome may seem counterintuitive to convention-
al wisdom about steric effects. It should be borne in mind,
however, that simplified steric demand concepts offer only
a snapshot of a molecule without considering the level of flexi-
bility exhibited in different parts of the molecular framework.
Considering the anisotropic steric nature of the mesityl groups,
we believe that they may freely rotate in a certain direction to
minimize steric repulsion with the nickel complex, thus leading
to a less sterically encumbered environment within the com-
plex and favoring branched selectivity.
4
5
6
10:0
7:3
68
54
99
9:1
Nevertheless, the experimental outcomes illustrated in
Table 4 are indicative of a high correlation between steric
demand and linear selectivity. More importantly, two substitu-
ents or side arms on the heteroarene in the vicinity of the CÀH
activation site are required to ensure high linear selectivity;
this is further validated by observations on benzimidazole 10j
bearing a pyridinyl group. We speculated that the pyridine
moiety in 10j could compete with the imidazole nitrogen cen-
ters for the incoming Lewis acid, thus reducing the efficacy of
AlMe3 as a second steric control site for promoting high linear
selectivity. As expected, the linear/branched selectivity de-
creased dramatically to only 33:16 when the reaction was per-
formed under similar conditions in the presence of 10 mol%
AlMe3 (Scheme 2). In fact, a ten-fold increase in the amount of
AlMe3 (100 mol%) was required to obtain high linear selectivi-
ty. This outcome with 10j again demonstrates that a single
pendant substituent on the benzimidazole is insufficient to
ensure a high linear/branched selectivity.
no
reaction
7
--
[a] The reactions were carried out using 13 (0.5 mmol) and styrene
(0.75 mmol) and product yields were determined by 1H NMR analysis.
10 mol% [Ni(cod)2], AlMe3, and NHC ligand 1a.
linear selectivity in the reaction. Unfortunately, no reactivity
was observed when IPr was employed as a ligand under similar
reaction conditions, further reflecting the steric limitations of
the Ni system. For a more in-depth understanding of the steric
properties of the NHC ligands around the nickel center, the
percent buried volumes (%Vbur) of both 1a and IMes were cal-
culated using the SambVca method developed by Cavallo and
co-workers.[13] In this method, the %Vbur value indicates the
portion of a sphere around the metal occupied by the ligand.
For this study, we considered complex 4c and compared it
with the previously reported 4d used by Cavell’s group as
a benchmark ligand. The buried volume of the amino-NHC
ligand 1a (28.1%) was apparently smaller than that of IMes
(32.1%). However, the reaction of 1a gave higher linear selec-
tivity than that with the IMes ligand, which would appear to
be at variance with steric control of the linear/branched selec-
Next, we monitored reactions with a range of NHC ligands
in the hope of understanding their effect on selectivity. The re-
sults of the CÀH bond activation of benzimidazolium salt 13a
using different NHC ligands are summarized in Figure 5. The
amino-NHC framework 1a (L/B=4:1) appeared to be a more
effective scaffold than IMes (L/B=51:49) for exerting a high
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Conclusions
In summary, without resorting to functional group manipula-
tions on the substrate or ligand, we have disclosed a new
mode of chemically regioselective switching for CÀH bond
functionalization of benzimidazole derivatives. The catalytic
method involves a cooperative interaction between Ni and Al
to create a steric environment for obtaining the linear product.
Omission of the AlMe3 co-catalyst switches the reaction to-
wards branched selectivity. Equally importantly, AlMe3 has a re-
markable impact on the rate of the reaction. Based on a molec-
ular model of a benzimidazolium salt, we have successfully iso-
lated Ni hydride intermediate 4c involved in the CÀH transfor-
mation. The key feature that ensures high linear over branched
selectivity in the hydroheteroarylation of styrene is the pres-
ence of two substituents or side arms on the heteroarene com-
ponents in the vicinity of the CÀH activation site. We anticipate
that further detailed studies of this bimetallic catalysis may fa-
cilitate its possible wider application in CÀH bond activation.
Scheme 2. Reaction of 10j and 11 a.
Experimental Section
Synthesis of Ni-hydrido complex (4c)
A solution of amino-NHC (1.43 g, 5.0 mmol) in THF (35.0 mL) was
added to
a solution of [Ni(cod)2] (0.69 g, 2.5 mmol) in THF
(35.0 mL) at room temperature. After stirring for 1 h, the orange so-
lution was added dropwise to a vial containing 1,3-dimethyl-benzi-
midazolium hexafluorophosphate (0.70 g, 2.4 mmol) and the mix-
ture was stirred for 3 d at room temperature. The resulting mixture
was filtered and the collected solid was washed with hexanes. The
crude solid was further purified by recrystallization from acetoni-
trile/hexanes (80:20, v/v) to give complex 4c (1.47 g, 66%) as col-
orless crystals. 1H NMR (400 MHz, CD3CN): d=7.31 (s, 2H), 7.28–
7.25 (m, 2H), 7.22–7.19 (m, 2H), 6.81 (br, 6H), 4.36 (t, J=6.2 Hz,
4H), 3.04 (q, J=6.7 Hz, 4H), 2.85 (s, 6H), 2.33 (s, 6H), 1.45 (br, 12H),
1.07 (s, 20H), À12.92 ppm (s, 1H); 13C NMR (100 MHz, CD3CN): d=
200.5, 183.7, 140.3, 137.1, 137.0, 135.8, 130.2, 123.8, 123.6, 122.9,
110.6, 53.2, 51.3, 43.1, 33.6, 29.7, 21.5, 17.6 ppm; HR-MS (ESI): calcd
for [C45H65N8Ni]+: 775.4686; found: 775.4689.
Figure 5. Dependence of reaction selectivity on NHC ligand.
General procedure for nickel-mediated hydroheteroarylation
of styrene
tivity. This apparent anomaly may be rationalized by consider-
ing that, unlike typical monodentate Arduengo NHC ligands
such as IMes, 1a has a bidentate framework bearing a free
dangling tert-butylamine arm, which may fleetingly coordinate
to the metal center in a rapid manner. Overall, this intermittent
coordination behavior with the metal would be likely to in-
crease the average steric demand in complexes and thus favor
linear product formation. In support of this rationale, we calcu-
lated the steric buried volume based on a known bidentate
structure from our previous work, the amino-NHC nickel(II)
complex 5.[14] Indeed, a bidentate feature in amino-NHCs gen-
erates a large buried volume of 48.1%, thus making a compel-
ling case for sterically controlled selectivity. Furthermore, the
theoretical findings on cobalt-mediated hydroarylation of styr-
enes by Fu’s group further support our observation of ligand-
controlled regioselectivity based on steric effects.[15]
Styrene 11 a (0.75 mmol) was added to a solution of [Ni(cod)2]
(14 mg, 0.05 mmol), amino-NHC (1a; 14 mg, 0.05 mmol), and the
requisite benzimidazolium salt derivative 13[x] (0.5 mmol) in tolu-
ene. After sealing the reaction vessel, it was removed from the glo-
vebox and the solution was heated at 1308C for 16 h. The resulting
mixture was filtered through Celite, which was then washed with
acetone. The combined filtrate and washings were concentrated in
vacuo to afford the crude product, which was further purified by
flash chromatography eluting with ethyl acetate/acetone.
Crystallography
Details of the crystal structure determination are given in the Sup-
porting Information. CCDC-988238 (4c) contains the supplementa-
ry crystallographic data for this paper. These data can obtained
free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
The authors declare no competing financial interests. We are
grateful to the Taiwan National Science Council (NSC: 101-
2628M-001-004-MY3) and the Academia Sinica for their gener-
ous financial support. We would also like to thank Dr. Titel
Jurca (University of Bristol) for his valuable suggestions and
discussions.
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Keywords: alkenes · CÀH bond activation · homogeneous
catalysis · nickel · regioselectivity
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Received: January 24, 2014
Published online on May 13, 2014
Chem. Eur. J. 2014, 20, 8099 – 8105
www.chemeurj.org
8105
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