Rhenium-catalyzed insertion of terminal alkenes into a C(sp2)-H bond and successive transfer hydrogenation

Article abstract of DOI:10.1016/j.jorganchem.2010.09.064

Treatment of aromatic aldimines with terminal alkenes in the presence of a rhenium catalyst, [HRe(CO)₄]_n, gives 2-alkenylbenzylamines in good to excellent yields. This reaction proceeds via the insertion of the alkene into a C-H bond

Full text of DOI:10.1016/j.jorganchem.2010.09.064

Journal of Organometallic Chemistry 696 (2011) 348e351
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rhenium-catalyzed insertion of terminal alkenes into a C(sp2)eH bond
and successive transfer hydrogenation
*
*
Yoichiro Kuninobu , Takahiro Nakahara, Peng Yu, Kazuhiko Takai
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Kita-ku, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of aromatic aldimines with terminal alkenes in the presence of
[HRe(CO)4]n, gives 2-alkenylbenzylamines in good to excellent yields. This reaction proceeds via the
insertion of the alkene into a CeH bond at the ortho-position of the imino group of the aromatic aldimine
a rhenium catalyst,
Received 27 July 2010
Received in revised form
24 September 2010
Accepted 27 September 2010Available
online 7 October 2010
followed by sequential
b-hydride elimination from the formed alkyl rhenium intermediate and then by
hydrogenation of the imino group of the aldimine.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Rhenium
Insertion
Hydrogen transfer
Aldimine
Alkene
1. Introduction
fourth route, which we also report herein, is the insertion of alkenes
into a CeH bond of aromatic compounds followed by dehydrogena-
tion of the formed alkylated aromatic compounds (Fig. 1(d)) [6].
CeH functionalization is one of the most efficient and direct
methods in synthetic organic chemistry. There have, therefore,
recently been many efforts to develop new transformations via
CeH bond activation [1]. In such transformations, insertion of
unsaturated molecules into a CeH bond is well known, and the
reactions usually stop at the insertion step. Our group has also
reported on transformations via CeH bond activation using
rhenium complexes as catalysts [2]. In some of these reactions,
intramolecular nucleophilic cyclization occurs after the insertion of
unsaturated molecules into a formed C(sp2)eRe bond [2]. During
investigations of rhenium-catalyzed CeH transformations, we
discovered the alkenylation reaction of aromatic compounds.
There have been several reports of alkenylation reactions of
aromatic CeH bonds. The first approach involves the cross
coupling reaction between aromatic compounds and alkenyl halides
(Fig. 1(a)) [3], and the second strategy is based on the insertion of
alkynes into a CeH bond of aromatic compounds (Fig. 1(b)) [4]. The
third process is oxidativeedehydrogenative alkenylation of CeH
bonds using a stoichiometric amount of an oxidant (Fig.1(c)) [5]. The
X
(a)
(b)
R
− HX
R
DG
DG
oxidant
(c)
(d)
R
R
R
− H2
DG
dehydrogenation
R
Fig. 1. Four methods for alkenylation of aromatic CeH bonds.
2. Results and discussion
First, we investigated the reaction between aromatic aldimine
1a and styrene (2a) using the rhenium complex, [ReBr(CO)3(thf)]2,
as the catalyst. Quinoline derivative 3 was formed in 31% yield (Eq.
(1)) [7]. In this reaction, the rhenium complex worked as a Lewis
acid and promoted a regioselective aza-DielseAlder reaction [8].
N-Benzylaniline was also formed as a side product. This result
indicated that aldimine 1a also worked as a hydrogen acceptor to
produce quinoline 3 from tetrahydroquinoline, which is part of
* Corresponding authors. Tel.: þ81 86 251 8097; fax: þ81 86 251 8094.
E-mail addresses: kuninobu@cc.okayama-u.ac.jp (Y. Kuninobu), ktakai@cc.
okayama-u.ac.jp (K. Takai).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.064

Y. Kuninobu et al. / Journal of Organometallic Chemistry 696 (2011) 348e351
349
Table 1
Ph
δ+
Ph
Reactions of aldimines 1 with styrene (2a).a
δ−
δ+
δ−
cat. Re
Ph
HN
H
N
N
Ph
[HRe(CO)4]n (Re: 5.0 mol%)
N
+
Ph
(1)
toluene, 180 °C, 24 h
Ph
R1
1
2a
R1
4
(3)
Ph
N
Ph
2
(2)
N
H
Entry
1c
2
R1
Yield (%)b
N
H
4-MeO 1b
4-Me 1c
4-CF3 1d
4b 70 (73)
4c 76 (78)
4d 82 (87)
H
2
N
3
Ph
HN
Scheme 1. Proposed mechanism for the formation of quinoline derivative 3.
4
3-Me 1e
2-Me 1f
Ph
Ph
an aza-DielseAlder adduct between 1a and 2a. Therefore, by
increasing the amount of 1a (3.0 equiv), the yield of 3 was improved
to 66% (Eq. (1)).
4e 75 (79)
4f 80 (84)
5c
Ph
H
HN
H
[ReBr(CO)3(thf)]2 (2.5 mol%)
Ph
N
(1)
+
N
Ph
N
toluene, 150 °C, 24 h
6d
Ph
1a
2a
3
1g
4g 78 (79)
Equiv of 1a Yield (%)
a
2a (2.0 equiv).
b
c
Isolated yield. Yield determined by 1H NMR is reported in parentheses.
30 h.
1.0
3.0
31
66
d
After 24 h, [HRe(CO)4]n (Re: 5.0 mol%) and 2a (2.0 equiv) were added, and the
reaction mixture was stirred at 180 ꢀC for 24 h.
The possible mechanism for the formation of a quinoline
derivative is shown in Scheme 1: (1) Aza-DielseAlder reaction
between an N-arylaldimine and styrene; (2) tautomerization; (3)
transfer dehydrogenation of the formed tetrahydroquinoline with
2 equiv of N-arylaldimine. In this mechanism, the regioselectivity
must be determined by electron densities of N-arylaldimine and
styrene.
when aromatic aldimine bearing an electron-withdrawing group,
1d, was employed (entry 3). In the case of aromatic aldimine with
a methyl group at the meta-position, 1e, only one isomer 4e was
formed regioselectively in 75% yield (entry 4). Furthermore, the
reaction was not inhibited by steric hindrance at the ortho-position;
the corresponding 2-alkenylbenzylamine 4f was produced in 80%
yield when aromatic aldimine 1f with a methyl group at the ortho-
position was employed (entry 5). Only one product 4g was obtained
with (E)-N-((naphthalen-2-yl)methylene)benzenamine (1g), which
has two possible reaction points (entry 6). However, reaction
between the aromatic ketimine N-(1-phenylethylidene)benzen-
amine and styrene (2a) did not provide the corresponding
alkenylated product, but produced alkylated aromatic ketimine [(E)-
N-(1-(2-phenethylphenyl)ethylidene)benzenamine] in 7% yield.
Next, we investigated the reactivity of several alkenes
(Table 2). Styrene with an electron-donating group, 2b, gave
2-alkenylbenzylamine 4h in 90% yield (entry 1). The corresponding
2-alkenylbenzylamine 4i was obtained using styrene bearing
a fluorineatom atthepara-position,2c (entry2). 2-Vinylnaphthalene
(2d) also afforded 2-alkenylbenzylamine 4j in 63% yield (entry 3). A
mixture of 2-alkenylbenzylamines 4k, 4k0 and 4k00 was formed in
85% total yield when aliphatic alkene 2e was used as an olefinic
substrate (entry 4) [16]. By investigating the ratios between 4k
(trans-form) and 4k0 (cis-form) during the reaction, it was clarified
that 4k0 should be formed from 4k by isomerization [17].
By changing the rhenium catalyst, [ReBr(CO)3(thf)]2, to [HRe
(CO)4]n, the product of the reaction between aromatic aldimine 1a
and styrene (2a) changed dramatically. Treatment of 1a with 2a in
the presence of a catalytic amount of the rhenium-hydride
complex, [HRe(CO)4]n, gave N-(2-styrylbenzyl)benzenamine (4a) in
80% yield (Eq. (2)) [9e12]. The catalytic activity for the formation of
4a is much higher than that of other rhenium complexes that we
have previously reported as being effective in CeH bond trans-
formations [13]. In addition, a rhenium complex, [HRe(CO)4]n, has
a lower Lewis acidity than [ReBr(CO)3(thf)]2, [HRe(CO)4]n did not
promote aza-DielseAlder reaction. There have been several reports
of the transition metal-catalyzed insertion of styrene (or other
alkenes) into an aromatic CeH bond; however, transfer of dihy-
drogen did not occur after the insertion step [14]. In this reaction,
the loss of the imino group 1a may be disadvantage. However, it
must be valuable to synthesize 2-alkenylbenzylamines from aldi-
mines and alkenes because the synthesis can be achieved without
addition of any oxidant or hydrogen acceptor [15].
Ph
H
HN
Ph
[HRe(CO)4]n (Re: 5.0 mol%)
toluene, 180 °C, 24 h
N
(2)
+
The proposed mechanism for reaction is as follows (Scheme 2):
(1) oxidative addition of an aromatic aldimine to a rhenium center
(CeH bond activation) [2]; (2) insertion of an alkene into the
Ph
Ph
1a (1.0 equiv) 2a (2.0 equiv)
4a 80%
We next investigated the scope and limitations of the reaction,
specifically evaluating the imines that are suitable for the trans-
formation (Table 1). Aromatic aldimines with an electron-donating
group, 1b and 1c, produced 2-alkenylbenzylamines 4b and 4c in
70% and 76% yields, respectively (entries 1 and 2). The corre-
sponding 2-alkenylbenzylamine 4d was generated in 82% yield
formed rheniumecarbon bond; (3) b-hydride elimination [6]; (4)
insertion of a carbonenitrogen double bond of the imino group into
a rheniumehydrogen bond [18]; (5) and reductive elimination to
give 2-alkenylbenzylamines 4 and regenerate the rhenium catalyst.
It is interesting to note that
b-elimination from 5 proceeds prefer-
entially instead of intramolecular nucleophilic cyclization, which

350
Y. Kuninobu et al. / Journal of Organometallic Chemistry 696 (2011) 348e351
Table 2
Acknowledgment
Reactions of aldimine 1a with alkenes 2.a
This work was partially supported by the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
Ph
H
HN
Ph
[HRe(CO)4]n (Re: 5.0 mol%)
toluene, 180 °C, 24 h
+
N
R
R
Appendix. Supplementary data
1a
2
4
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.09.064.
Entry
R
Yield (%)b
1c
2
4-MeC6H4 2b
4-FC6H4 2c
4h 90 (91)
4i 66 (76)
References
3
4j 63 (66)
[1] There have been several reviews on transition metal-catalyzed trans-
formations via CeH bond activation. See: (a) F. Kakiuchi, S. Murai, Top.
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(b) Y. Guari, S. Sabo-Etienne, B. Chaudret, Eur. J. Inorg. Chem. (1999) 1047;
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(d) F. Kakiuchi, T. Kochi, Synthesis (2008) 3013.
[2] We have already reported several rhenium-catalyzed transformations that
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K. Takai, J. Am. Chem. Soc. 128 (2006) 202;
2d
4d
nC8H17 2e
Ph
Ph
Ph
HN
HN
HN
n
C8H17
+
+
n
C8H17
(b) Y. Kuninobu, Y. Nishina, M. Shouho, K. Takai, Angew. Chem. Int. Ed. 45
(2006) 2766;
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n
4k''
C8H17
4k
4k'
4kþ4k0þ4k00 85 (86) [77:11:12]e
a
2 (2.0 equiv).
b
c
Isolated yield. Yield determined by 1H NMR is reported in parentheses.
2 (4.0 equiv).
After 24 h, [HRe(CO)4]n (Re: 5.0 mol%) and 2a (2.0 equiv) were added, and the
d
reaction mixture was stirred at 180 ꢀC for 24 h.
e
The ratios between 4k, 4k0 and 4k00 are reported in square brackets.
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Lett. (1995) 681;
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8146;
formations [2].
(c) Y. Kuninobu, K. Kikuchi, Y. Tokunaga, Y. Nishina, K. Takai, Tetrahedron 64
(2008) 5974;
(d) Y. Kuninobu, Y. Fujii, T. Matsuki, Y. Nishina, K. Takai, Org. Lett. 11 (2009)
2711.
3. Summary
We have succeeded in developing the rhenium complex
[HRe(CO)4]n, which catalyzes the synthesis of 2-alkenylbenzyl-
amines from aromatic aldimines and alkenes. This reaction
proceeds via aromatic CeH bond activation, insertion of the alkene
[5] (a) Y. Fujiwara, I. Moritani, S. Danno, R. Asano, S. Teranishi, J. Am. Chem. Soc.
91 (1969) 7166;
(b) J. Tsuji, H. Nagashima, Tetrahedron 40 (1984) 2699;
(c) T. Yokota, M. Tani, S. Sakaguchi, Y. Ishii, J. Am. Chem. Soc. 125 (2003) 1476;
(d) M. Dams, D.E. De Vos, S. Celen, P.A. Jacobs, Angew. Chem. Int. Ed. 42 (2003)
3512;
(e) E.M. Beck, N.P. Grimster, R. Hatley, M.J. Gaunt, J. Am. Chem. Soc. 128 (2006)
2528.
into a CeH bond of the aromatic compound,
and hydrogenation of the imino group of the aromatic aldimine.
The -hydride elimination and hydrogenation steps are rare
b-hydride elimination
b
[6] There have been a few reports of
b-hydride elimination following the inser-
examples in transformations that proceed via aromatic CeH bond
activation. By changing the rhenium catalyst to [ReBr(CO)3(thf)]2,
a quinoline derivative was produced from an aromatic aldimine and
styrene via an aza-DielseAlder reaction.
tion of an alkene into an aromatic CeH bond. See: (a) P. Hong, H. Yamazaki,
Chem. Lett. (1979) 1335;
(b) F. Kakiuchi, T. Sato, M. Yamauchi, N. Chatani, S. Murai, Chem. Lett. (1999) 19;
(c) F. Kakiuchi, T. Tsujimoto, M. Sonoda, N. Chatani, S. Murai, Synlett (2001) 948.
[7] Investigation of several rhenium catalysts and temperatures: Re2(CO)10 at
180 ꢀC, 0%; ReBr(CO)5 at 150 ꢀC, 28% and at 180 ꢀC, 30%; [ReBr(CO)3(thf)]2 at
150 ꢀC, 31% and at 180 ꢀC, 28%; [HRe(CO)4]n at 150 ꢀC, 0% and at 180 ꢀC, 0%.
[8] Iron-promoted and rhodium-catalyzed synthesis of quinoline derivatives via
an aza-DielseAlder reaction between N-aryl aldimines and styrenes has been
reported. See: (a) R. Leardini, D. Nanni, A. Tundo, G. Zanardi, F. Ruggieri, J. Org.
Chem. 57 (1992) 1842;
Ph
HN
H
Ph
N
R2
(b) M. Beller, O.R. Thiel, H. Trauthwein, C.G. Hartung, Chem. Eur. J. 6 (2000)
2513.
[9] The reaction did not proceed using the following transition metal complexes:
R1
R1
4
Re
(1)
H
(5)
Re
Mn2(CO)10
Ir4(CO)12
,
MnBr(CO)5, RuH2(CO)(PPh3)3, Ru3(CO)12
,
RhCl(PPh3)3 and
.
Ph
H
[10] Investigation of catalytic amounts: 0.50 mol%, 7%; 1.0 mol%, 9%; 2.5 mol%, 22%.
[11] Investigation of several solvents: neat, 7%; octane, trace; toluene, 80%; 1,2-
dichloroethane, trace; n-hexanenitrile, 10%.
[12] Following the reviewer’s suggestion, we have examined the reactions
between aldimine 1a and styrene (2a) using 5.0 equiv of a hydrogen acceptor,
such as norbornene or 3,3-dimethyl-1-butene, under the reaction conditions
shown in Eq. (2). As a result, alkenylated product 4a was obtained in 4% and
64% yields, respectively.
N
Ph
N
Re
H
R1
R2
R1
H
H
Re
Ph
R2
H
(4)
Ph
N
N
[13] The reaction did not proceed using Re2(CO)10, ReBr(CO)5 or [ReBr(CO)3(thf)]2.
[14] (a) S. Murai, F. Kakiuchi, S. Sekine, Y. Tanaka, A. Kamatani, M. Sonoda,
N. Chatani, Nature 366 (1993) 529;
H
(2)
Re
H
(3)
R2
R1
R1
R2
5
(b) F. Kakiuchi, M. Sonoda, T. Tsujimoto, N. Chatani, S. Murai, Chem. Lett.
(1999) 1083;
Scheme 2. Proposed mechanism for the formation of 2-alkenylbenzylamines 4.
(c) Y.-G. Lim, J.-S. Han, S.-S. Yang, J.H. Chun, Tetrahedron Lett. 42 (2001) 4853;

Y. Kuninobu et al. / Journal of Organometallic Chemistry 696 (2011) 348e351
351
(d) R.K. Thalji, K.A. Ahrendt, R.G. Bergman, J.A. Ellman, J. Am. Chem. Soc. 123
(2001) 9692.
4k and 4k0 changed during the reaction. That is, at the beginning of the
reaction, the ratio of 4k is high; however, the ratio of 4k decreased and that of
4k0 increased as the reaction proceeded.
[15] There has been a report on palladium-catalyzed oxidative ortho-olefination of
N-substituted benzylamines using alkenes in the presence of a stoichiometric
amount of an oxidant. See: G. Cai, Y. Fu, Y. Li, X. Wan, Z. Shi, J. Am. Chem. Soc.
129 (2007) 7666.
[17] Yields of 4k and 4k0 (the ratios between 4k and 4k0): 1 h, 3%, 0% (100:0); 3 h,
16%, trace (>99:1); 8 h, 32%, 4% (89:11); 24 h, 63%, 10% (86:14).
[18] For an example of the insertion of a carbonenitrogen double bond into
a rheniumehydrogen bond, see: X.-Y. Liu, K. Venkatesan, H.W. Schmalle,
H. Berke, Organometallics 23 (2004) 3153.
[16] One possible reason for the formation of 4k0 (cis-form) is that 4k (trans-form)
is isomerized to 4k0 under the reaction conditions. In fact, the ratios between