Catalytic Asymmetric Hydroalkenylation of Vinylarenes: Electronic Effects of Substrates and Chiral N-Heterocyclic Carbene Ligands

Article abstract of DOI:10.1002/anie.201411882

An asymmetric tail-to-tail cross-hydroalkenylation of vinylarenes with terminal olefins was achieved by catalysis with NiH complexes bearing chiral N-heterocyclic carbenes (NHCs). The reaction provides branched gem-disubstituted olefins with high enantioselectivity (up to 94% ee) and chemoselectivity (cross/homo product ratio: up to 99:1). Electronic effects of the substituents on the vinylarenes and on the N-aryl groups of the NHC ligands, but not a π,π-stacking mechanism, assist the steric effect and influence the outcome of the cross-hydroalkenylation.

Full text of DOI:10.1002/anie.201411882

Angewandte
Chemie
DOI: 10.1002/anie.201411882
N-Heterocyclic Carbenes Very Important Paper
Catalytic Asymmetric Hydroalkenylation of Vinylarenes: Electronic
Effects of Substrates and Chiral N-Heterocyclic Carbene Ligands**
Chun-Yu Ho,* Chun-Wa Chan, and Lisi He
Abstract: An asymmetric tail-to-tail cross-hydroalkenylation
of vinylarenes with terminal olefins was achieved by catalysis
with NiH complexes bearing chiral N-heterocyclic carbenes
(NHCs). The reaction provides branched gem-disubstituted
olefins with high enantioselectivity (up to 94% ee) and
chemoselectivity (cross/homo product ratio: up to 99:1).
Electronic effects of the substituents on the vinylarenes and
on the N-aryl groups of the NHC ligands, but not a p,p-
stacking mechanism, assist the steric effect and influence the
outcome of the cross-hydroalkenylation.
hydroalkenylation reaction. We began our investigation with
C2-symmetric NHC ligands A1–A4 (Figure 1), which were
derived from (1S,2S)-1,2-diphenyl-1,2-ethanediamine.[6,7]
While both [1,3-bis(2,6-di-iso-propylphenyl)]-imidazolidin-2-
ylidene (SIPr) and imidazol-2-ylidene (IPr) were effective
ligands in the racemic tail-to-tail cross-hydroalkenylation,[7]
the use of the corresponding NHC ligand rac-A1 led to
a significant drop in reactivity (Table 1, entry 1).[8] We
suspected the vicinal Ph groups to have an unfavorable
T
he transition-metal-catalyzed cross-hydroalkenylation is an
atom-economical method for the synthesis of substituted
olefins,[1] and has undergone an incredible growth over the
past decade, accompanied by the development of important
new concepts to solve challenging problems.[2,3] Recently, we
have reported the tail-to-tail (t-t) hydroalkenylation of vinyl-
arenes, catalyzed by in situ generated [IPr-NiH]+
(Scheme 1).[4] Such processes directly employ simple olefins
and vinylarenes, and give an assortment of unsymmetrical
branched gem-disubstituted olefins.[5]
As part of our continued efforts to discover new
applications for N-heterocyclic carbenes (NHCs) in transi-
tion-metal catalysis, we became interested in identifying
chiral NHC ligands to promote an asymmetric version of the
Figure 1. Structures of NHC ligands. Cy=cyclohexyl, 9-Phen=9-phe-
nanthracenyl.
steric effect, by pushing the N-aryl groups with their bulky
isopropyl substituents in ortho position too close to the
reaction center, thereby increasing the activation energy for
the coordination of the substrate and lowering the reactivity.
Further studies showed that both the size and the number of
N-aryl ortho substituents played very important roles in
determining the reactivity and selectivity of the hydroalke-
nylation (Table 1, entries 2–4). A higher reactivity could be
achieved by using NHC ligands with less than four N-aryl
ortho substituents. The cross-product (S)-3aa was obtained in
21% yield and with 73% ee by using the C2-symmetric NHC
A4 with two cyclohexyl substituents in ortho positon (one on
each of the two N-aryl groups),[9] and the reaction also gave
the cross-product 4aa,[10] a tail-to-head isomer of 3aa, in 47%
yield. The success of NHCs of general structures B and C
(Figure 1, B1–B5 and C1–C3) with their various N-aryl
substituents was limited (Table 1, entries 5–12).[6] The best
balance of ee value, reactivity, and selectivity was achieved
with B2, the C1-symmetric structure of which is closely related
to that of the C2-symmetric NHC A4 (Table 1, entry 6).
Inspired by the electronic effects of NHC ligands on Ru-
catalyzed olefin metathesis, we were also interested in the
Scheme 1. Catalytic tail-to-tail hydroalkenylation. [IPr-NiH]+ catalyst
generated in situ from [Ni(cod)2], IPr, 1-octene, p-anisaldehyde,
TESOTf/NEt3, in toluene, under N2 atmosphere, at RT, for 24 h.
cod=1,5-cyclooctadiene, TES=triethylsilyl.
[*] Prof. Dr. C.-Y. Ho
Department of Chemistry, South University of Science and
Technology of China (SUSTC).
Prof. Dr. C.-Y. Ho, C.-W. Chan, L. He
Center of Novel Functional Molecules, Shenzhen Research Institute
The Chinese University of Hong Kong (CUHK).
E-mail: jasonhcy@sustc.edu.cn
jasonhcy@cuhk.edu.hk
[**] CYH thanks the Thousand Young Talents Program. LH thanks HK
PhD Fellowship. Support for this work was provided by SUSTC,
NSFC project 21102122, Guangdong Province Matching Fund
K12213101, CUHK TBF 3120090, and SZRI CUHK.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411882.
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Table 1: Screening of chiral NHC ligands for the asymmetric cross-
hydroalkenylation of styrene (1a) with 1-octene (2a).[a]
Table 2: Influence of NHC ligands with different electronic properties
and of substituents X of vinylarenes 1 on the hydroalkenylation.
Entry
NHC
X
ee [%][a]
Yield [%][b]
3
3
4
5
Entry
NHC
ee [%][b]
3aa
Yield [%][c]
4aa
1
2
3
B2
B2
B2
F
H
OMe
60
73
77
73
82
80
8
11
16
1
3
2
3aa
5aa
1
2
3
4
rac-A1
A2
A3
–
5
76
11
21
<5
<5
<5
47
<5
26
13
73
9[d]
4[c]
5[c]
B1
B1
F
38
73
65
69
23
15
5
8
1
1
A4
OMe
5
6
7
8
9
B1
B2
B3
B4
B5
63
73
69
58
15
67
82
77
43
43
24
11
12
5
5[e]
3
3
6
6
7
8
9
C4
C4
C5
C5
F
H
H
51
69
75
82
77
80
81
84
10
14
7
1
3
3
2
OMe
13
10
<5
[a] Determined by HPLC analysis of the corresponding epoxides.
[b] Determined by GC analysis. [c] Yield determined by H NMR
spectroscopy.
1
10
11
12
13
14
C1
C2
C3
C4
C5
60
72
73
69
75
55
11
84
80
81
15
<5
<5
14
4
<5
<5
3
gradually from X = F to X = OMe in a given NHC carboske-
leton (Table 2, entries 1–5). This tendency is interesting, as it
defies typical explanations that are entirely based on the trans
influence,[16] which would normally predict the opposite
tendency (i.e. gradual increase of the ee value from X =
OMe to X = F). Our results thus suggest that the p character
of the NHC Ni bond might be more pronounced than we
would normally expect from a first-row transition metal in
a high oxidation state. Alternatively, p-face electron dona-
tion, as observed in NHC-RuII complexes,[11] might be
operative in our NHC-NiII complexes. It should be noted
that the trans coordination of electron-deficient substrates
might not lenghten the NHC Ni bond (or move the chiral
steric element on the NHC further away from the metal
center) any more than the pure s-interaction of an electron-
rich substrate.
Second, vinylarenes and NHCs with different electronic
properties (Table 2, entries 6–9) were employed for a system-
atic study. This study showed that the above-described
changes in ee values were not a direct result of a) p–p-
stacking or attractive electrostatic interactions between the
NHC N-aryl groups and the vinylarene, or b) steric effects
between the F and OMe groups. The best ee values were
obtained with an OMe group at para position of the vinyl-
arene and an F substituent at the NHC N-aryl group (Table 2,
entry 9). Exchanging the position of these two electronic
activators, that is, having the F group at para position of the
vinylarene and an OMe substituent on the NHC N-aryl group,
resulted in the worst ee value (Table 2, entry 6 vs. entry 9,
more than 30% difference in ee values).
7
3
[a] Catalyzed by in situ generated presumed NHC-Ni hydride complex.
See Experimental Section for preparation procedure and separation of
3aa. Product 4aa was obtained as an inseparable mixture of internal
olefins, and product 6aa was not obtained with any of the examined
catalysts. [b] Determined by HPLC analysis of the corresponding
epoxides. [c] Determined by 1H NMR spectroscopy. [d] 46% ee.
[e] 76% ee.
II
À
electronic effects of our catalysts on the hydroalkenyla-
tion.[11,12] We thus prepared NHC ligands with activating
substituents at the N-aryl group (C4 and C5) and investigated
their influence on the reaction. Unlike in related studies with
Ru catalysts, the electronic nature of the N-aryl substituents
had no significant effect on the catalytic activity and yield of
the hydroalkenylation (Table 1, compare entry 6 with entries
13 and 14).[13] A slight change in the ee values of the products
as a function of the NHC N-aryl substituents prompted us to
investigate the electronic effect of the vinylarene substituent
on the ee values (Table 2). Interestingly, the asymmetric
induction ability of the NHC N-aryl substituents and the
vinylarene substituents has until now not been studied
systematically.
II
À
First, we examined para-substituted vinylarenes with
different electronic properties. Typical NHC ligands, such as
B1 or B2, lack p-conjugation at their heterocyclic core and are
generally considered to have a strong s-donating character
with limited p-accepting ability compared to phosphorus-
based ligands.[14] Therefore, the lengths of the NHC Ni bond
and the N aryl bonds (and thus the steric effect of the NHC
ligand on the hydroalkenylation) are not expected to change
significantly with substrates that have different electronic
properties.[15] However, the ee values of products 3 increased
À
À
Further analysis of the results showed that the electronic
effects of both the NHC N-aryl group and the vinylarene
might be synergistic. This can be deduced from the extend
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Table 3: Scope and selectivity of the hydroalkenylation using C5 as the
ligand to generate the presumed [C5-NiH]+ catalyst.
with which the ee value increased with the introduction of
both electronic activators in the favorable combination
(Table 2, entry 2 vs. entry 9). This increase is higher than the
sum of the increases, when only one of the electronic
activators was present (entries 3 and 8). Moreover, this
tendency can also be observed with the unfavorable combi-
nations (Table 2, compare entries 1, 2, 6, and 7).
Overall, a comparison of the results in Table 1 (entries 6,
13, and 14) and Table 2 shows that the coordination of an
external p-system, such as a vinylarene, could effectively
Entry X, Y
R
ee[a] Yield[b]
Ratio[b]
À
change the asymmetric induction ability of an NHC Ni
[%]
[%]
complex at a high oxidation state. The synergistic effect we
observed is remarkable, especially with regard to a) the effect
of electronic changes on the asymmetric induction ability of
the NHC ligand, and b) in an asymmetric Ni-catalyzed
hydrocyanation with bis-1,2-diarylphosphinites, only an aryl
group with a para-OMe substituent had an effect on the
ee value of the product, whereas an aryl group with a para-F
substituent had no effect.[17] The p-face electron donation and
related effects of NHC substituents, such as their impact on
bond lengths and back bonding, and the inductive and
resonance effects of NHCs have been known for years.[11,12]
Yet, new synthetic strategies based on the influence of the
electronic properties of the substrate on the ee value of the
product have not been reported until now, probably because
those products and NHCs that were used were not chiral, and
the alkenes used in those pioneering studies had similar
electronic properties.[11,12] At this stage of our investigation,
we can only attribute our overall observations to the changing
electron density at the Ni center, however, the exact origin of
the observed synergistic effect still needs to be investigated.
We should also note that there is no clear correlation
between the discussed electronic effects and the ratio, in
which cross- and homo-products are formed (Table 2,
entries 6–9). We attribute the significant improvement of
the stereoselectivity of the hydroalkenylation (from former
9:1 with IPr, to a range of 96:4 to 99:1 with NHCs) to the
increased steric demand at the reaction center during vinyl-
arene insertion. The improved ratio of cross- to homo-
products is presumably a result of increased steric repulsions
between the vicinal Ph group and the N-aryl ortho substitu-
ents.[18]
With the above-described observations in mind, we next
studied the scope of the hydroalkenylation of various vinyl-
arenes with C5 as the ligand (Table 3). We were pleased to
achieve high yields, as well as good chemo- and regioselec-
tivities in most cases. The ee values improved with the use of
sterically more demanding terminal olefins. The system
accepted vinylarenes with substituents in para, ortho, and
meta position (Table 3, entries 1–4). Apart from vinylanisoles,
a thioanisole and a bulky vinylarene were also compatible
substrates for this reaction (Table 3, entries 5 and 6). The
scope of the terminal olefin was studied by using 4-tert-
butoxy-styrene (Table 3, entries 7–12). Steric effects resulted
in ee values up to 94% when we used allylbenzene instead of
1-octene or related non-aromatic vinylalkanes, including
cycloalkyl or acyclic alkyl compounds.
3
3
3:4
93:7
89:11 95:5
85:15 87:13
3:5
1
2
3
4
OMe, H
H, 3-OMe
H, 2-OMe
Bn
Bn
Bn
89
87
81
57
87
96:4
86
88[c]
90
OMe, 3-OMe Bn
91:9
96:4
5
6
SMe, H
tBu, H
Bn
Bn
86
70
75
91:9
90:10 90:10
95:5
85[c]
7
8
OtBu, H
OtBu, H
nHex
iBu
84[c]
86
81
82
86:14 96:4
89:11 96:4
9
OtBu, H
86
75
88:12 93:7
10
11
OtBu, H
OtBu, H
89
94
79
85
89:11 95:5
Bn
92:8
93:7
96:4
96:4
12
OtBu, H
93
87
[a] Determined by HPLC analysis. [b] Determined by GC analysis.
[c] Determined by HPLC analysis of the corresponding epoxides or diols
after epoxidation or dihydroxylation, respectively.
experiment that supported the presence of NiH (aldehyde
reduction by an in situ generated [SIPr-NiH]OTf),[19] we
formulated a tentative working hypothesis to explain the
major stereochemical outcome of the hydroalkenylation with
steric effects (Figure 2). An analysis of the results obtained
with a series of closely related NHCs (A3, A4, and B2 in
Table 1) showed that the N-aryl ortho-Me substituent might
play an important role with regard to reactivity and regiose-
lectivity. It should be noted that the theoretical investigation
of the P-NiII-catalyzed hydrovinylation of styrene suggested
an alternative b-hydrogen transfer mechanism,[20] and without
taking the above-discussed electronic properties into account,
our steric model alone cannot explain the large differences
observed with different vinylarenes.
In summary, a catalytic asymmetric intermolecular tail-to-
tail hydroalkenylation of vinylarenes with terminal olefins
offers a direct way to chiral unsymmetrical gem-disubstituted
olefins from electron-rich monoenes. The result showed that
the asymmetric induction ability of the NHC-Ni complex is
tunable by the electronic properties of the NHC N-aryl
substituents and those of external p-systems, such as vinyl-
arenes. The obtained information might also be of general use
Based on prior results of asymmetric reactions that were
catalyzed by P-NiII and NHC-NiII complexes,[2] and an
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[3] For a review on hemilabile ligands, see: a) P. Braunstein, F.
Naud, Angew. Chem. Int. Ed. 2001, 40, 680; Angew. Chem. 2001,
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Chem. 2010, 122, 9368.
[5] For a short review of [NHC-NiH]+-catalyzed hydroalkenylation
reactions, see: a) C.-Y. Ho, L. He, C.-W. Chan, Synlett 2011,
1649; for their applications in organometalloid syntheses, see:
b) C.-Y. Ho, L. He, Chem. Commun. 2012, 48, 1481; for
a vinylether dimerization, see: c) C.-Y. Ho, L. He, Synlett 2014,
25, 2738; for an unsymmetrical cycloisomerization, see: d) C.-Y.
Ho, L. He, J. Org. Chem. 2014, 79, 11873.
Figure 2. Tentative transition-state models for the hydroalkenylation in
order to explain the observed stereochemical outcome when C5 is
used as the ligand.
[6] For reviews on the preparation of chiral NHC ligands, see: a) V.
Cꢀsar, S. Bellemin-Laponnaz, L. H. Gade, Chem. Soc. Rev. 2004,
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2012, 256, 804; for the use of 1,2-diphenyl-1,2-ethanediamine as
the chiral element in the preparation of a C1-symmetrical NHC,
see: d) K.-S. Lee, A. H. Hoveyda, J. Org. Chem. 2009, 74, 4455.
[7] For recent advances in chiral NHC-Ni catalysis: a) W. A.
Herrmann, L. J. Goossen, C. Koecher, G. R. J. Artus, Angew.
Chem. Int. Ed. Engl. 1996, 35, 2805; Angew. Chem. 1996, 108,
2980; b) S.-S. Ng, T. F. Jamison, Tetrahedron 2006, 62, 11350;
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D. R. Jensen, M. S. Sigman, Tetrahedron 2005, 61, 6418; Recent
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D. H. Sherman, J. Montgomery, Chem. Sci. 2012, 3, 892.
[9] Product 3aa, obtained by using an (S,S)-configured NHC, was
converted to the corresponding chiral ketones in order to
determine the absolute configuration: a) A. Yada, T. Yukawa, Y.
Nakao, T. Hiyama, Chem. Commun. 2009, 3931; b) S. Takeuchi,
A. Ohira, N. Miyoshi, H. Mashio, Y. Ohgo, Tetrahedron:
Asymmetry 1994, 5, 1763.
in the development of asymmetric NHC/transition-metal
catalysis and the asymmetric co-dimerization of other olefins.
Experimental Section
Typical procedure of the catalyst preparation: In a glove box,
[Ni(cod)2] (0.05 mmol, 10 mol%) and NHC (10 mol%) were added
to an oven-dried test tube equipped with a stir bar. The tube was
sealed with a septum, taken out of the glove box, and connected to an
N2 line. The mixture was dissolved in dried degassed toluene (2 mL)
and stirred at room temperature for 1 h. 1-Octene (15 mol%), NEt3
(60 mol%), p-anisaldehyde (10 mol%), and TESOTf (22 mol%)
were then added sequentially and stirred at room temperature for 5 h.
Standard hydroalkenylation: The vinylarene (0.5 mmol) and the
terminal alkene (1.5 mmol) were added to the solution containing the
in situ prepared “NHC-NiH” catalyst. The reaction mixture was
stirred at room temperature for 40 h. After the addition of n-hexane
(4 mL), the solution was stirred for 30 min under air. The reaction
mixture was then filtered through a short plug of silica gel and rinsed
with diethyl ether (75 mL). The solvents were removed under
reduced pressure and the resulting residue was purified by flash
column chromatography on silica gel to afford the hydroalkenylation
products.
Received: December 10, 2014
Published online: && &&, &&&&
Keywords: asymmetric catalysis · hydroalkenylation ·
.
N-heterocyclic carbenes · nickel · olefins
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4
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b) K. M. Anderson, A. G. Orpen, Chem. Commun. 2001, 2682.
[17] For the effect of the P ligand on the NiII p-donating ability in the
catalytic hydrocyanation of vinylarenes, see: A. L. Casalnuovo,
T. V. RajanBabu, T. A. Ayers, T. H. Warren, J. Am. Chem. Soc.
1994, 116, 9869.
[18] As such, the dimerization reactivity of vinylarenes (i.e. reaction
without a terminal olefin) is expected to be much lower with the
new ligands. The use of a Ni catalyst with C5 as the ligand led to
the conversion of 4-vinylanisole to products 5 and 6 in 29 and
9% yield, respectively, and with 78 and 72% ee, respectively. For
an example of an asymmetric tail-to-head dimerization of
vinylarenes, see: R. B. Bedford, M. Betham, M. E. Blake, A.
Garces, S. L. Millar, S. Prashar, Tetrahedron 2005, 61, 9799.
[19] After the formation of a catalyst with SIPr, the treatment with p-
anisaldehyde and TESOTf gave TESOCH2C6H4OMe (45%
yield at room temperature, 95% at 358C for 16 h). This observed
reduction suggests the presence of [NHC-NiH]OTf. For evi-
dence of the existence of P-NiH species, see: K. Fischer, K.
Jonas, P. Misbach, R. Stabba, G. Wilke, Angew. Chem. Int. Ed.
Engl. 1973, 12, 943; Angew. Chem. 1973, 85, 1001.
[20] a) L. Fan, A. Krzywicki, A. Somogyvari, T. Ziegler, Inorg. Chem.
1996, 35, 4003; b) see Ref [2a]; c) J. Joseph, T. V. RajanBabu,
E. D. Jemmis, Organometallics 2009, 28, 3552; d) I. Nikiforidis,
A. Gçrling, W. Hieringer, J. Mol. Catal. A 2011, 341, 63.
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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.
Angewandte
Communications
Communications
N-Heterocyclic Carbenes
C.-Y. Ho,* C.-W. Chan,
L. He
&&&& — &&&&
Catalytic Asymmetric Hydroalkenylation
of Vinylarenes: Electronic Effects of
Substrates and Chiral N-Heterocyclic
Carbene Ligands
Crossed: An asymmetric tail-to-tail cross-
hydroalkenylation of vinylarenes with ter-
minal olefins was catalyzed by NiH
complexes with chiral N-heterocyclic car-
benes (NHCs). Depending on steric and
electronic effects of the substrates and
NHC ligands, the reaction can provide
branched gem-disubstituted olefins with
high enantio- (up to 94% ee) and che-
moselectivity (cross/homo product ratio:
up to 99:1).
6
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!