In(OTf)3-catalyzed highly chemo- and regioselective head-to-tail heterodimerization of vinylarenes with 1,1-diarylethenes

Article abstract of DOI:10.1002/chem.201101190

Cross combination: The olefinic C-H bond in 1,1-diarylethenes selectively adds to the C=C double bond of substituted styrenes in a head-to-tail fashion under In(OTf)₃ (OTf=triflate) catalysis (see scheme) while homo- and cyclodimerization reactions are suppressed. The high chemo- and regioselectivity of this intermolecular hydroalkenylation originates from the high stability and steric bulk of the diaryl-substituted tertiary carbocation intermediate. Copyright

Full text of DOI:10.1002/chem.201101190

DOI: 10.1002/chem.201101190
InACHTUNGTRENNUNG(OTf)3-Catalyzed Highly Chemo- and Regioselective Head-to-Tail
Heterodimerization of Vinylarenes with 1,1-Diarylethenes
Jing Dai,[a] Jinlong Wu,[a] Guanglian Zhao,[a] and Wei-Min Dai*[a, b]
Dedicated to Professor Wei-Yuan Huang on the occasion of his 90th birthday
The dimerization of alkenes highlights an important in-
dustrial process for accessing longer chain alkenes from
abundant and inexpensive petrochemical feedstocks.[1] Ho-
modimerization may be considered simpler than heterodi-
merization between two different partners and yet a mixture
of homodimers is often generated due to versatile dimeriza-
tion modes.[1,2] As illustrated in Scheme 1, catalytic homodi-
merization of vinylarenes 1, such as styrene (Ar=Ph), gives
the alkenes 2–4 through head-to-tail (h–t),[3] tail-to-tail
(t–t),[4] and head-to-head (h–h)[5] homodimerization, respec-
tively. Moreover, compounds 1 undergo cyclodimerization
to produce the indan derivatives 5.[6] With recent advance-
ments in catalyst development and an increasing under-
standing of the reaction mechanisms, selective head-to-tail
homodimerization of 1 has been achieved.[3] For example,
the combination of a transition metal complex with a Lewis
acid,[7] such as Pd
ACHTUNGTRNENUG(OAc)2/PPh3/InCAHTUNGTREN(NUGN OTf)3 (OTf=triflate),
forms an efficient catalyst, affording dimers 2 at 20–508C in
73–98% yields.[3a] The Lewis acid is thought to activate 1 to-
wards nucleophilic attack by the Pd0 complex, followed by
insertion of another vinylarene with the formation of a pal-
ladium oxidative adduct. Alternatively, cationic ironACHTUNGTRENNUNG(III)
salts, formed in situ from FeCl3 and AgOTf or AgNTf2,
afford dimers 2 at 808C in 51–97% yields.[3e] Selective for-
mation of dimers 3 through tail-to-tail homodimerization
has also been reported[4] and the N-heterocyclic carbene
(NHC) nickel(II) hydride complex [(IPr)NiH]OTf (IPr=
1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene) promotes
both homo- and heterodimerization of 1 in high chemo- and
regioselectivity.[4d] A ruthenium(0) complex, [Ru(h6-cot)(h2-
ACTHNUTRGENNUG ACHTUNGTRENNUNG
dmfm)2] (cot=1,3,5-cyclooctatriene, dmfm=dimethyl fuma-
rate), has been found to be efficient for the formation of al-
kenes 4 at 80–1008C in alcoholic solvents in 53–66% isolat-
Scheme 1. The homodimerization pathways of 1: a) the formation of 2–4
through head-to-tail (h–t), tail-to-tail (t–t), and head-to-head (h–h) dime-
rization modes; b) the formation of cis and trans indans 5 through cyclo-
dimerization, involving 2 as the intermediate; and c) the formation of
ed yields.[5] Moreover, a rhodium(I) complex, [RhCl
ACHTUNGTRENNUNG(cod)]2
(cod=1,5-cyclooctadiene), in combination with tBuOH and
Na2CO3 at 1608C facilitates the formation of trans-stilbenes
ꢀ
trans-stilbenes 6 through dimerization by cleavage of the ortho C H
bond.
6 (24–63% yields) from vinylarenes 1 through cleavage of
[8]
ꢀ
the ortho C H bond.
To the best of our knowledge, 1,1-diarylethenes
8
(Scheme 2) usually give indan products similar to 5 through
cyclodimerization in the presence of Lewis acids, such as
[a] J. Dai, Dr. J. Wu, G. Zhao, Prof. Dr. W.-M. Dai
Laboratory of Asymmetric Catalysis and Synthesis
Department of Chemistry, Zhejiang University
Hangzhou 310027 (P.R. China)
Fax : (+86)571-87953128
E-mail: chdai@zju.edu.cn
[b] Prof. Dr. W.-M. Dai
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR (P.R. China)
Fax : (+852)23581594
E-mail: chdai@ust.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101190.
Scheme 2. Possible products of the Lewis acid catalyzed heterodimeriza-
tion of vinylarenes 7 with 1,1-diarylethenes 8.
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Chem. Eur. J. 2011, 17, 8290 – 8293

COMMUNICATION
EtAlCl2,[9a] RuCl3·nH2O,[9b] InBr3,[9c] and BiCl3,[9d] or amini-
um hexachloroantimonates.[10] There is one example of ami-
nium salt promoted heterodimerization of 1,1-diphenyle-
thene 8a (Ar=Ph) with vinylferrocene, which occurs in
62% yield, along with a 10% yield of the head-to-tail dimer
of vinylferrocene.[10d] We report, herein, a general and effi-
cient head-to-tail heterodimerization of vinylarenes 7 with
These results on the In
tion of styrene suggest that the InAHCUTNGTRENNUNG
carbocation 12a (X=H) can regioselectively add to vinylar-
ene 7a (X=H) to form a new carbocation 13a (X=H)
without the action of palladium(0),[3a] albeit at higher tem-
peratures (Scheme 3). Therefore, we envisioned a heterodi-
ACHTUNGTRENNUNG
1,1-diarylethenes 8 catalyzed by InACTHNUTRGNENUG(OTf)3, affording alkenes
9 in good to excellent yields with high chemo- and regiose-
lectivity (Scheme 2). Our results support a Lewis acid medi-
ated cationic mechanism and the role of 8 in this highly se-
lective heterodimerization[11,12] is discussed.
During our screening of FeCl3-catalyzed reactions, we
found that styrene underwent homodimerization to give the
head-to-tail dimer 2 (Ar=Ph) in 22–65% yields in the pres-
ence of FeCl3·6H2O (10 mol%) in 1,4-dioxane at 80–1208C
(Table 1, entries 1, 3, and 4). The temperature-dependent
Table 1. The Lewis acid catalyzed formation of 2 (Ar=Ph) from styre-
ne.[a]
Lewis acid ([mol%]) T [8C] t [h] Conversion [%] Yield [%][b]
1
2
3
4
5
6
7
8
9
FeCl3·6H2O (10)
FeCl3·6H2O (50)
FeCl3·6H2O (10)
FeCl3·6H2O (10)
FeCl3·6H2O (10)
CrCl3 (10)
80
48
24
36
22
24
24
24
19
24
19
19
39
40
95
100
100
40
56
20
100
100
100
22
25
62
65
80[c]
100
120
120[d]
120
120
120
120
120
120
48
trace
trace
20
83
82
Zn
Cu
U
Scheme 3. The possible mechanisms of the highly selective heterodimeri-
zation reaction.
Sn
(OTf)3 (10)
11 Yb(OTf)3 (10)
A
10 In
N
N
66
[a] Reaction conditions: styrene (1.0 mmol) and Lewis acid (10–
50 mol%) in 1,4-dioxane (1 mL) at 80–1208C in a sealed vial. [b] Isolated
yield of 2. [c] In THF. [d] In PEG-400.
merization process by selecting terminal alkenes that could
compete with 7 in the addition reaction with 12 and for
which the homodimerization process would be much slower.
We found that 1,1-diarylethenes 8 meet these criteria ac-
cording to their electronic and steric properties: 1) the terti-
conversion of styrene suggests a relatively weak Lewis acidi-
ty of FeCl3.[3e] We also found 1,4-dioxane to be a better sol-
vent than THF and PEG-400 (entries 1 vs. 2 and entries 4 vs.
5, Table 1) when using a lower catalyst loading or for obtain-
ing a higher yield of the dimer. Other solvents including tol-
uene, CH2Cl2, ClCH2CH2Cl, dimethylacetamide (DMA),
and DMF were totally ineffective, giving none of the dimer
product. The conversion of styrene approached completion
at temperatures above 1008C, indicating that formation of
polymeric materials is the competitive pathway. We exam-
ined Cr3+, Zn2+, and Cu2+ salts for catalysis of the same di-
merization at 1208C and found that both the styrene conver-
sion and the dimer yield were reduced (entries 6–8, Table 1).
These results imply that, to a large extent, Fe3+ is the true
catalyst for the formation of the dimerization product in
entry 4, Table 1 and other metal impurities, if any are pres-
ent, are not likely to be involved in the catalysis. Additional-
ly, three metal triflates were tested in the styrene dimeriza-
ary carbocation 11, formed from 8 and InACTHNURTGNENG(U OTf)3, is very
stable and yet sterically bulky, preventing reactions with al-
kenes 7 and 8 to give the hetero- and homodimerization
products 10 and 16; and 2) the formation of the diaryl-stabi-
lized tertiary carbocation 15 is much faster than the forma-
tion of benzylic counterpart 13, allowing the selective forma-
tion of 15 from 12 and 8, followed by decomposition of 15
to form heterodimers 9 with regeneration of the Lewis acid,
InACHTUGNTRENNNUG
7
AHCTUNGTRENNUNG
1208C for 24 h the heterodimer 9aa was isolated in 80%
yield, along with a 15% yield of homodimer 14a (X=H;
entry 1, Table 2). Most of the excess of 1,1-diphenylethene
was recovered after the reaction and its homodimer 16a
(Ar=Ph) was not obtained. The reactions of 3- and 4-halo-
phenylethenes with 1,1-diphenylethene under the same reac-
tion conditions afforded 9af–ai in 70–81% yields (entries 6–
tion (entries 9–11, Table 1). Among them, InACTHNUGTRENUNG(OTf)3 was
found to be a slightly better catalyst in terms of both reac-
tion time and yield, affording 2 (Ar=Ph) in 82% yield in
19 h.
Chem. Eur. J. 2011, 17, 8290 – 8293
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8291

W.-M. Dai et al.
Table 2. The synthesis of heterodimerization products 9 from 7 and 8.[a]
To explore the reaction of 1,1-diarylethenes 8 with other
alkenes (Scheme 4), we found that allyl alcohol 17 (2 equiv)
reacted with 1,1-diphenylethene (8a) to form the diene 18a
7: X
T [8C]
Ar=Ph
Ar=tolyl
yield [%][b]
yield [%][b]
1
2
3
4
5
6
7
8
9
H
2-Me
2-F
7a
7b
7c
7d
7e
7 f
7g
7h
7i
120
140
140
140
140
120
120
120
120
120
120
9aa: 80 (15)[c]
9ab: 91
9ac: 94
9ad: 93
9ae: 94
9ba: 90
9bb: 94
9bc: 96
9bd: 95
9be: 90
9bf: 82
9bg: 83
9bh: 93
9bi: 94
2-Br
2-Cl
3-Br
3-Cl
4-Br
4-Cl
4-Me
4-MeO
9af: 76
9ag: 70
9ah: 75
9ai: 81 (17)[c]
9aj: 71
10
11
7j
7k
9bj: 84
9ak: trace[d]
9bk: trace[d]
Scheme 4. The CuACHTNUGTRENUNG(OTf)2-catalyzed reaction of allyl alcohol 17 with 1,1-
diarylethenes 8 under microwave heating.
[a] Reaction conditions: 7 (0.5 mmol), 8a (Ar=Ph, 1.0 mmol) or 8b
(Ar=4-tolyl, 0.75 mmol), and In(OTf)3 (10 mol%) in 1,4-dioxane (1 mL)
AHCTUNGTRENNUNG
at 120–1408C for 24 h in a sealed vial. Byproduct 14 was observed in en-
tries 1 and 6–10. [b] Isolated yield of 9. [c] Isolated yield of byproduct 14
given in parentheses. [d] Polymerization might occur.
(Ar=Ph) in PEG-400 at 1208C after 24 h in the presence of
a Lewis acid (50 mol%). The isolated yield of 18a increases
9, Table 2). Formation of homodimers 14 was observed in all
reactions and 14i (X=4-Cl) was isolated in a 17% yield for
entry 9, Table 2. For the electron-rich styrene 7j (X=4-Me),
the desired heterodimer 9aj was produced in 71% yield, al-
though homodimer 14j (X=4-Me) was not observed, pre-
sumably due to the formation of an oligomer/polymer
(entry 10, Table 2). The polymerization pathway became
dominant for styrene 7k (X=4-MeO) and no separable
products were obtained (entry 11, Table 2).[3e] In contrast, 4-
vinylpyridine failed to react via either hetero- or homodime-
rization even at 1408C and was recovered after heating for
24 h (data not shown in Table 2). Moreover, the reactivity of
2-substituted styrenes deserves special comment. The addi-
tional steric hindrance arising from the ortho substituent
makes it difficult for carbocation 12 to react with both 7 and
8 at 1208C. After raising the temperature to 1408C, the het-
erodimerization took place in much higher selectivity to fur-
nish 9ab–ae in 91–94% isolated yields (entries 2–5, Table 2).
We also examined the reaction of 4-chlorophenylethene
(7i) with 1,1-diphenylethene (8a) in 1,4-dioxane (1208C,
24 h) by using Brønsted acids [H2SO4 (80 mol%), HCl
(100 mol%), and CF3CO2H (20 mol%)]. Heterodimer 9ai
was isolated in a 15% yield under H2SO4 catalysis, whereas
only trace amounts were detected for reactions with the
other two Brønsted acids. These data clearly demonstrate
the importance of the bulky indium coordination sphere in
carbocations 11 and 12 for deactivating 11 and directing the
heterodimerization process (12!15!9). Presumably the
indium-bound carbocations are much less reactive than their
corresponding metal-free counterparts so that the undesired
homo- and cyclodimerization of vinylarenes 7[3f,4c,6] and 1,1-
diarylethenes 8[9,10] are eliminated or suppressed. We further
enhanced the selective formation of carbocation 15 by incor-
porating electron-rich aryl groups, such as 4-tolyl, generally
providing a roughly 10% increase in the yield for the heter-
odimers 9ba and 9bf–bj (entries 1 and 6–10, Table 2). This
also allowed the ratio of 7 to 8b (Ar=4-tolyl) to be reduced
to 1:1.5. These results are consistent with the cationic mech-
anisms depicted in Scheme 3.
in the order AgOTf (18%)<Sc
ACHUTGTNRNE(NUG OTf)3 (32%)<InACHTUNGTRENNUNG
(47%)<Sn(OTf)3 (58%)<FeCl3·6H2O (67%)<CuACHTUNGTRENNUNG
G
(88%). Other solvents, including DMA, DMF, DMSO, tolu-
ene, and 1,4-dioxane, were examined with FeCl3·6H2O
(50 mol%) at 1208C for 24 h, but diene product 18a was
barely formed under these conditions except for a 46%
yield obtained for the reaction in 1,4-dioxane. If the reaction
was carried out under microwave heating at 1708C for 2 h,
the loading of CuACHTNUTRGNE(NUG OTf)3 could be reduced to 10 mol% and
product 18a was formed in 83% yield. Moreover, by using
electron-rich 8b (Ar=4-tolyl), the yield of diene 18b was
improved to 92%.[13]
In conclusion, we have established a general and efficient
hydroalkenylation of vinylarenes 7 through addition to an
ꢀ
olefinic C H bond in 1,1-diarylethenes 8, catalyzed by In-
(OTf)3, to furnish trisubstituted alkenes 9 in good to excel-
lent yields. The highly chemo- and regioselective head-to-
tail heterodimerization likely takes place through a cationic
mechanism initiated by the indium-bound carbocation 12.
This cation, as compared to its corresponding metal-free
counterparts, contributes to the heterodimerization in a syn-
ergetic manner with 1,1-diarylethenes 8. The unique carbo-
cation stabilizing ability and steric bulk of the diaryl groups
in 8 are the indispensable factors responsible for the high se-
lectivity. Our results present a new method for highly selec-
tive alkene heterodimerization[4d,10d,11,12b–d] without relying
on the action of transition metals.
Acknowledgements
The Laboratory of Asymmetric Catalysis and Synthesis is established
under the Cheung Kong Scholars Program of The Ministry of Education
of China. This work is supported by Zhejiang University and the Zhe-
jiang University Education Foundation.
ꢀ
Keywords: alkenes
·
carbocations
·
C H activation
·
hydroalkenylation · Lewis acids
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Head-to-Tail Heterodimerization
COMMUNICATION
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Received: April 18, 2011
Published online: June 7, 2011
Chem. Eur. J. 2011, 17, 8290 – 8293
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8293