Reactions and mechanistic studies of rhenium-catalyzed insertion of α,β-unsaturated carbonyl compounds into a C-H bond

Article abstract of DOI:10.1246/bcsj.81.1393

A rhenium complex, [ReBr(CO)₃(thf)]₂, catalyzes the insertion of α, β-unsaturated carbonyl compounds into a C-H bond of aromatic compounds having nitrogen-containing directing groups. In this reaction, Re₂(CO)₁₀ can also be used as a catalyst. When imines are employed as the aromatic substrates, sequential cyclization proceeds and indene derivatives are obtained in good to excellent yields. This reactivity is in contrast to those of ruthenium and rhodium complexes, which are usually used as catalysts in the insertion reactions of unsaturated molecules into a C-H bond. Investigations on the reaction mechanism indicate that the rhenium catalyst promotes C-H bond activation of aromatic compounds, the insertion of α, β-unsaturated carbonyl compounds into a Re-C bond, and intramolecular nucleophilic cyclization followed by reductive elimination and the elimination of an amine.

Full text of DOI:10.1246/bcsj.81.1393

ꢀ 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 11, 1393–1401 (2008) 1393
BCSJ Award Article
Reactions and Mechanistic Studies of Rhenium-Catalyzed Insertion
of ꢀ,ꢁ-Unsaturated Carbonyl Compounds into a C–H Bond
ꢀ
Makoto Shouho, and Kazuhiko Takai
Yoichiro Kuninobu, Yuta Nishina, Kayo Okaguchi,
ꢀ
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology,
Okayama University, Tsushima, Okayama 700-8530
Received March 31, 2008; E-mail: kuninobu@cc.okayama-u.ac.jp, ktakai@cc.okayama-u.ac.jp
A rhenium complex, [ReBr(CO)3(thf)]2, catalyzes the insertion of ꢀ,ꢁ-unsaturated carbonyl compounds into a
C–H bond of aromatic compounds having nitrogen-containing directing groups. In this reaction, Re2(CO)10 can also
be used as a catalyst. When imines are employed as the aromatic substrates, sequential cyclization proceeds and indene
derivatives are obtained in good to excellent yields. This reactivity is in contrast to those of ruthenium and rhodium
complexes, which are usually used as catalysts in the insertion reactions of unsaturated molecules into a C–H bond.
Investigations on the reaction mechanism indicate that the rhenium catalyst promotes C–H bond activation of aromatic
compounds, the insertion of ꢀ,ꢁ-unsaturated carbonyl compounds into a Re–C bond, and intramolecular nucleophilic
cyclization followed by reductive elimination and the elimination of an amine.
Transformations via the insertion of unsaturated molecules
into an inactivated C–H bond are efficient and useful methods
to synthesize more complex molecules.1 Previously, ruthenium
and rhodium complexes have usually been employed as cata-
lysts in such transformations.1 These reactions proceed via
C–H bond activation, insertion of unsaturated molecules into
the metal–hydrogen bond, and reductive elimination.2–4 We
have recently reported on rhenium-catalyzed C–H bond activa-
tion followed by the insertion of unsaturated molecules, such as
acetylenes,5,6 isocyanates,6 aldehydes,7 and acrylates.8 Howev-
er, the reaction style is quite different from that with ruthenium
and rhodium catalysts. Although only the insertion of unsatu-
rated molecules into a C–H bond proceeds in the case of the
ruthenium- and rhodium-catalyzed transformations, the rheni-
um catalyst can promote both the insertion of unsaturated mole-
cules and intramolecular nucleophilic cyclization.9 Thus, we
have been interested in the reaction mechanism of the rheni-
um-catalyzed transformations. Here, we focus on the reactions
of aromatic compounds having a directing group with acrylates.
When aromatic ketimines were used as the aromatic com-
pounds, indene derivatives were synthesized in one operation.8
and the reaction and its mechanistic studies are discussed.
We show that when an imine moiety is used as the directing
group, intramolecular cyclization occurs after insertion of
ꢀ,ꢁ-unsaturated carbonyl compounds.
Insertion of Ethyl Acrylate into an Aromatic C–H Bond
at the Ortho Position of a Directing Group. Survey of
Catalysts:
Using the reaction between 2-phenylpyridine
(1a) with ethyl acrylate (2a) as a probe, the catalytic activity
of several transition-metal complexes was surveyed. Treat-
ment of 1a with acrylate 2a in the presence of a catalytic
amount of a rhenium complex, Re2(CO)10, at 150 ꢁC for
24 h, gave mono-adduct 3a and di-adduct 4a in 61% and
39% yields, respectively (Table 1, Entry 1). Rhenium com-
plexes, [Re(CO)4(PPh3)]2 (3a, 88%; 4a, 8%, Entry 2), ReBr-
(CO)5 (3a, 86%; 4a, 12%, Entry 3), and [ReBr(CO)3(thf)]2
(3a, 81%; 4a, 19%, Entry 4), also showed high catalytic activ-
ꢀ
ities. In contrast, ReCp (CO)3 and ReMeO3 were ineffective
(Entries 5 and 6). Although manganese complexes, Mn2(CO)10
and MnBr(CO)5, have catalytic activities, the yields of 3a
and 4a were low (Entries 7 and 8). No reaction was observed
with ruthenium complexes, Ru3(CO)12 and RuH2(CO)(PPh3)3,
which are usually employed as catalysts to promote the inser-
tion of unsaturated molecules into a C–H bond under the same
reaction conditions (Entries 9 and 10). In contrast, by using a
rhodium complex, RhCl(PPh3)3, insertion of 2a into a C–H
bond of 1a proceeded and mono-alkylated product 3a was
formed in 18% yield (Entry 11).10
Results and Discussion
In the first half of the paper, we discuss the rhenium-cata-
lyzed insertion of ꢀ,ꢁ-unsaturated carbonyl compounds into
an aromatic C–H bond of 2-phenylpyridine. Several mechanis-
tic studies using H/D scrambling between an aromatic com-
pound and an acrylate are also described. In the second half
of the paper, we employ aromatic ketimines as the substrate,
Survey of Directing Groups: A nitrogen atom of sp2 hy-
bridization proved to be effective as the direction group for the
Published on the web November 12, 2008; doi:10.1246/bcsj.81.1393

1394 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) BCSJ AWARD ARTICLE
Table 1. Investigation of CatalystsaÞ
dine i with acrylates at the ortho position of i are shown in
Scheme 1. In the case of a rhenium catalyst (Mtl = Re), two
main routes can be considered for the reaction: 1) C–H bond
activation is accelerated by the coordination of a nitrogen
atom of i to the rhenium center (step [1]), and insertion of
an acrylate occurs, and 2) Friedel–Crafts-type electrophilic
addition (steps [15] and [16]).
Catalyst
+
N
CO2Et
toluene, 150 °C, 24 h
1a
2a
CO2Et
Although the rhenium complex shows Lewis acidity in
some cases,12 the insertion of acrylates into a C–H bond of
2-phenylpyridine should not proceed via the latter electrophilic
addition pathway (steps [15] and [16]) from the following
observations. (1) The insertion of acrylates took place similar-
ly even in the presence of stoichiometric amounts of tributyl-
amine, which is a Lewis base. (2) If the reaction proceeds via
electrophilic addition, the reaction should also proceed at meta
and para positions. However, we observed only the reaction
products at the ortho positions. (3) The corresponding coupling
product was not formed from anisol, an electron-rich aromatic
compound, and ethyl acrylate under the same reaction condi-
tions. (4) In the case of 2-naphthylpyridine (1d), a less reactive
C–H bond for electrophilic addition (3-position of the naphtha-
lene ring) was selectively functionalized in the reaction
(Table 2, Entry 5).
Once rhenium inserts into a C–H bond at the ortho position,
an ꢀ,ꢁ-unsaturated carbonyl compound can insert into the
Re–H bond (step [3] or [7]) or the Re–C bond (step [5] or
[9]) of an aryl–rhenium intermediate ii. Reductive elimination
from intermediate iii or iv gives adduct vii via steps [11] or
[12], respectively. Similarly, intermediates v or vi could give
inserted product viii, a regioisomer of vii; however, the forma-
tion of viii was not observed.
We then examined the following H/D scrambling reaction
to clarify the difference in the reactivity between the rhenium
complex, [ReBr(CO)3(thf)]2, and a rhodium complex, RhCl-
(PPh3)3 (Table 3). The rhenium catalyst promoted the cou-
pling reaction between 1a and 2b leading to 3a0 faster than
the rhodium catalyst. However, the complete scrambling was
only observed in the case of the rhodium catalyst. Therefore,
steps [3], [4], [7], and [8] proceed faster than the reductive
elimination step [11] in the case of the rhodium catalyst
(Mtl = Rh). Although the formation of v was suggested by
H/D scrambling of acrylate at the internal olefin moiety, viii
was not obtained. This indicates that the rate of the reductive
elimination step [13] is much slower than that of step [11].
It has been suggested that the rhodium complex promotes
the insertion of unsaturated molecules into a Mtl–H bond of
arylmetal intermediate ii (Mtl = Rh).10 Rhodium-catalyzed
H/D scrambling supports this suggestion. On the other hand,
the rhenium catalyst, [ReBr(CO)3(thf)]2, did not promote
H/D scrambling.13 This suggests that the rhenium catalyst pro-
motes other pathways; (1) insertion of unsaturated molecules
into a Re–C bond of ii (step [5]) followed by reductive
elimination (step [12]), and/or (2) insertion of unsaturated
molecules into a Re–H bond of ii (step [3]) and reductive elim-
ination (step [11]), which occurs faster than ꢁ-elimination
(step [4]). No H/D scrambling also suggests that the equilib-
rium between ii and v (steps [7] and [8]) should be slow
compared to the sequential steps leading to vii ([5] ! [12]
and/or [3] ! [11]).
N
N
+
CO2Et
CO2Et
3a
4a
Yield/%bÞ
Entry
Catalyst (mol %)
Re2(CO)10 (3.0)
3a + 4a
3a
4a
1
2
3
4
5
6
7
8
9
100
96
98
100cÞ
0
0
23
6
61
88
86
81
0
0
23
6
39
8
12
19
0
0
<1
0
[Re(CO)4(PPh3)]2 (3.0)
ReBr(CO)5 (6.0)
[ReBr(CO)3(thf)]2 (3.0)
ꢀ
ReCp (CO)3 (6.0)
ReMeO3 (6.0)
Mn2(CO)10 (3.0)
MnBr(CO)5 (6.0)
Ru3(CO)12 (2.0)
RuH2(CO)(PPh3)3 (6.0)
RhCl(PPh3)3 (6.0)
0
0
18
0
0
18
0
0
0
10
11
a) 1 (1.0 equiv); 2a (1.5 equiv). b) The yield was determined
1
by H NMR. c) Isolated yield.
rhenium-catalyzed C–H activation. However, neither N,N-
dimethylbenzylamine nor 2,5-diphenylfuran gave the desired
adduct upon treatment with acrylate 2a in the presence of a
catalytic amount of ReBr(CO)5. In contrast, a pyridyl group
was a good directing group for the transformation; for exam-
ple, 2-phenylpyridine gave a mixture of mono- and di-adducts
3a and 4a in quantitative yield (Table 2, Entry 1). 2-o-Tolyl-
pyridine (1b) afforded the corresponding adduct 3b in 34%
yield (Entry 2); however, a rhenium complex, Re2(CO)10,
showed higher reactivity, and gave 3b quantitatively (Entry 3).
By using 3-methyl-2-phenylpyridine (1c), only mono-adduct
3c was formed in 77% yield selectively (Entry 4).11 In the
case of 2-naphthalen-2-ylpyridine (1d), mono- and di-adducts
3d and 4d were obtained in 53% and 14% yields, respec-
tively, and the 3-position was functionalized regioselectively
(1-adduct:3-adduct = 9:91) (Entry 5). The regioselectivity
deserves further comment; if the reaction proceeded via the
Friedel–Crafts type electrophilic addition, the main product
should be 1-adduct, so the result also supports a C–H activa-
tion pathway (vide infra). The effectiveness of oxazolinyl
groups depends on the substituent. For example, di-adduct 4f
was formed quantitatively with 1f having a 4,4-dimethyloxa-
zolinyl group (Entry 7); however, a reaction of 1e having a
simple oxazoline with 2a gave a complex mixture (Entry 6).
When an effective oxazolinyl group was placed at the 1-
position of naphthalene, the reaction proceeded only at the
ortho position of the directing group (Entry 8).
Mechanistic Studies of Rhenium-Catalyzed Insertion of
Acrylates into a C–H Bond of 2-Phenylpyridine. Possible
routes for metal-catalyzed coupling reactions of 2-phenylpyri-

Y. Kuninobu et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1395
Table 2. Reactions of Aromatic Compounds Having a Directing Group R with Ethyl
AcrylateaÞ
CO2Et
R
[ReBr(CO)3(thf)]2
(3.0 mol%)
R
R
+
CO2Et
+
toluene, 150 °C, 24 h
CO2Et
R'n
R'n
1
2a
CO2Et
R'n
3
4
Yield %b)
Entry
1
3 + 4
3
4
1
100
81
19
4a
3a
N
N
1a
1b
2
3c)
34 (35)
34 (35)
3b
3b
99 (>99)
99 (>99)
4
77 (83)
67 (77)
77 (83)
3c
N
1c
1
N
5d)
N
14 (18)
4d
3
1d
CO2Et
53 (59)
3d
[1-adduct:3-adduct = 9:91]
O
6
a complex mixture
N
1e
1f
O
7e)
99 (>99)
0 (0)
99 (>99)
4f
N
O
N
O
N
8
CO2Et
1g
3g
99 (>99)
a) 1 (1.0 equiv); 2a (1.5 equiv). b) Isolated yield. The yield determined by 1H NMR is
reported in parentheses. c) Re2(CO)10 (3.0 mol %) was used as a catalyst. 180 ꢁC. d) 2a
(1.0 equiv). e) 2a (3.0 equiv).
[ReBr(CO)3(thf)]2
(3 mol%)
H
Because the H/D scrambling between 2-phenylpyridine
1a-d5 and acrylate 2b was not observed in the case of a
rhenium complex, we next examined the difference between
C–H and C–D bond insertion reactions. If the C–H activation
is a rate-determining step, a kinetic isotope effect (KIE)14
using a mono-deuterated 2-phenylpyridine 1a-d1 should be
observed. Treatment of 1a-d1 with acrylate 2b in the presence
of a rhenium complex, [ReBr(CO)3(thf)]2, in toluene at
115 ꢁC for 24 h gave inserted products 3a0-H and 3a0-D
in 61% yield (3a0-H:3a0-D = 73:27, KIE = 2.6) (eq 1).15
This result suggests that C–H bond activation is a rate-
determining step of the reaction between 2-phenylpyridine
and acrylate.
N
+
CO2R
toluene, 115 °C, 24 h
D
2b
(1 equiv)
(97% D)
1a-d1
R =
ð1Þ
H
RO2C
N
H
N
+
D
D
CO2R
3a'-D
3a'-H
61% (3a'-H : 3a'-D = 73 : 27)

1396 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) BCSJ AWARD ARTICLE
CO2R3
+ Mtl
N
N
OR3
Mtl
[ 15 ]
i
H
H
O
ix
− Mtl
− Mtl
− Mtl
[ 16 ]
[ 11 ]
+ Mtl
N
N
H
[ 2 ]
[ 1 ]
CO2R3
Mtl
CO2R3
vii
CO2R3
iii
iv
H
[ 3 ]
[ 12 ]
[ 4 ]
CO2R3
N
Mtl H
[ 5 ]
[ 7 ]
CO2R3
[ 6 ]
CO2R3
N
Mtl H
ii
N
N
CO2R3
[ 8 ]
CO2R3
[ 9 ]
CO2R3
[ 13 ]
[ 14 ]
− Mtl
− Mtl
Mtl
v
viii
H
H
[ 10 ]
N
Mtl H
R3O2C
vi
Scheme 1. Possible routes for the insertion of an acrylate into a C–H bond.
Table 3. H/D Scrambling Experiment Using Rhenium and Rhodium Catalysts
(>99%)
D
D
(>99%)
N
+
CO2R
toluene, 115 °C, 24 h
D
D
(>99%)
(>99%)
(1 equiv)
D
R =
(>99%)
2b
1a-d5
a
b
D
D
(>99%)
e
D
D
g
N
N
(>99%)
d
+
+
CO2R
D
D
D
CO2R
a
(>99%)
(>99%)
f
D
(>99%D)
c
(>99%)
1a-d5'
3a'-d4
2b'
Yield/%
1a-d5
3a0-d4
1H NMR integration value
Catalyst
a
b
c
d
e
f
g
[ReBr(CO)3(thf)]2 (3.0 mol %)
RhCl(PPh3)3 (6.0 mol %)
38
90
62
6
0.07
0.58
0.04
0.54
1.99
1.08
1.01
1.11
0.99
0.57
0.99
0.56
0.97
0.57

Y. Kuninobu et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1397
In order to obtain information for the initial arylrhenium
intermediate ii, we conducted a stoichiometric reaction of
2-phenylpyridine with [ReBr(CO)3(thf)]2 in toluene-d8 at
150 ꢁC for 1.5 h, and measured the resulting mixture by
1H NMR. No proton signal connected to the rhenium atom
was observed, but the shifted signals of 2-phenylpyridine
were seen.16 This observation suggests that the C–H activation
step with the rhenium complex is an uphill process, which is
consistent with the KIE experiment. It also suggests that the
C–H activation step is the rate-determining step.
Based on the investigations using a reaction between
2-phenylpyridine and acrylate with the rhenium complex
(Mtl = Re), we conclude that the coordination of a nitrogen
atom of 2-phenylpyridine i to the rhenium center, and arylrhe-
nium intermediate ii is generated via C–H activation (step [1]),
and the step is the rate-determining step. However, at this
point, we cannot determine if either or both pathways (1)
and (2) occur after the C–H activation: (1) the insertion of
an ꢀ,ꢁ-unsaturated compound into the Re–C bond of the aryl-
rhenium intermediate ii (step [5]) and reductive elimination
(step [12]); (2) the insertion of an ꢀ,ꢁ-unsaturated compound
into the Re–H bond of the intermediate iv (step [3]) and reduc-
tive elimination (step [11]).
ent, t-butyl acrylate, did not afford an indene derivative. Vinyl
ketone 2e also produced the corresponding indene derivative
6h in 44% yield (Table 4, Entry 7). Replacing the ketimine
with aromatic aldimine 5f promoted the reaction in low yield
(Table 4, Entry 8). By using a rhenium complex, Re2(CO)10,
as a catalyst, aminoindane 7b was obtained in good yield
(Table 4, Entry 9).19
Mechanistic Studies of Rhenium-Catalyzed Synthesis of
Indene Derivatives. Scheme 2 shows possible routes for
the rhenium-catalyzed synthesis of indene derivatives from ar-
omatic ketimines and ꢀ,ꢁ-unsaturated esters. Two reaction
pathways can be considered; carbon–carbon bond formation
initiated by C–H bond activation (via step [1]) and electrophil-
ic addition (via step [3]). Similar to the reaction between 2-
phenylpyridine and acrylates, we conclude that the reaction
proceeds via the C–H bond activation pathway from the fol-
lowing results: (1) stoichiometric amount of base did not in-
hibit the reaction, (2) only the ortho position of the aromatic
ring was functionalized, and (3) the 3-position of the naphtha-
lene skeleton, a less reactive position for electrophilic addition,
was functionalized regioselectively (Table 4, Entry 4).
When ketimines are used instead of 2-phenylpyridine, fur-
ther intramolecular cyclization leading to indene derivatives
occurs, because the imine moiety is a good acceptor for nu-
cleophilic addition, and the Re–C bond is relatively polarized.
The cyclization to ix should proceed from vi (step [15]), and
so, if insertion of an acrylate into the Re–H bond of iii occurs
(step [6]), indene derivative xi should be produced from vi via
viii (step [14]) because reductive elimination of v generates
viii (step [12]). Thus, we prepared the key intermediate viii,
which would be formed via the C–H activation followed by
the insertion of an acrylate into the Re–H bond (steps [6]
and [12]),20 and examined the reaction (eq 3). Treatment of
11 with a catalytic amount of [ReBr(CO)3(thf)]2 under the
same reaction conditions gave indene 6a in only 9% yield
and most of 11 remained unchanged. In addition, we did not
observe 11 in the reaction mixture between 5a and ethyl acry-
late (2a) (eq 2). These results suggest that the main pathway
does not go through viii (via steps [6] and [12]), and the
pre-cyclization intermediate vi should be generated directly
by the insertion of an acrylate into the Re–C bond (step [8]).
Synthesis of Indene Derivatives by the Treatment
of Aromatic Imines with ꢀ,ꢁ-Unsaturated Carbonyl
Compounds. By the reaction between aromatic ketimine
5a and acrylate 2a in the presence of a rhenium complex,
[ReBr(CO)3(thf)]2, indene derivative 6a was obtained in
87% yield (eq 2).17,18 The formation of 6a with the rhenium
catalyst is in sharp contrast to the result with a rhodium com-
plex, RhCl(PPh3)3, which catalyzes only C–H bond activation
and insertion of an acrylate.10
[ReBr(CO)3(thf)]2 (3.0 mol%)
Ph
OEt
N
+
toluene
150 °C, 24 h
O
2a
5a
(1.0 equiv)
(1.5 equiv)
ð2Þ
O
OEt
6a 87%
Ph
[ReBr(CO)3(thf)]2 (3.0 mol%)
N
Next, we investigated the substituent on a nitrogen atom
of the imine moiety. An aromatic ketimine bearing a benzyl
group 5b gave indene derivative 6a and aminoindane 7a in
11% and 62% yields, respectively (Table 4, Entry 1). On the
other hand, ketimines having a methoxy group on a nitrogen
atom and hydrazones did not afford the indene or aminoindane
derivatives.
Aromatic ketimines bearing a methoxy or trifluoromethyl
group at the para position gave indene derivatives 6b or 6c
in 75% and 33% yields, respectively (Table 4, Entries 2 and
3). Treatment of naphthylketimine 5e with ethyl acrylate
(2a), produced a mixture of 6d and 6e in 90% yield
(6d:6e = 80:20, Table 4, Entry 4). By using acrylates 2c and
2d, the reaction proceeded and indene derivatives 6f and 6g
were obtained in 63% and 72% yields, respectively (Table 4,
Entries 5 and 6). However, an acrylate having a bulky substitu-
toluene, 150 °C, 24 h
CO2Et
11
ð3Þ
CO2Et +
11
6a 9%
91%
recovery
Treatment of deuterated ketimine 5g with acrylate 2b in the
presence of a rhenium complex, [ReBr(CO)3(thf)]2, in toluene
at 150 ꢁC for 5 h gave a mixture of indene derivatives 6j-H and
6j-D in 27% yield (6j-H:6j-D = 66:34, KIE = 2.0) (eq 4).15
This is in accordance with the result obtained by the KIE ex-
periment shown in eq 1. The C–H activation step is also the
rate-determining step in the formation of indene derivatives.

1398 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) BCSJ AWARD ARTICLE
Table 4. Rhenium-Catalyzed Synthesis of Indene Derivatives from Aromatic Ketimines and ꢀ,ꢁ-Unsaturated
Carbonyl CompoundsaÞ
R3
R3
R1
[ReBr(CO)3(thf)]2 (3.0 mol%)
N
R4
R4
+
toluene
150 °C, 24 h
R2
R2
5
2
6
Entry
1
Product
Yield %b)
11 (15)
Olefin
2a (R4 = CO2Et)
Ketimine
5b (R1 = Bn; R2 = H; R3 = Me)
6a
Bn
NH
62 (75)
CO2Et
7a
2
3
5c (R1 = Ph; R2 = OMe; R3 = Me)
5d (R1 = Ph; R2 = CF3; R3 = Me)
2a
2a
6b (R2 = OMe; R3 = Me; R4 = CO2Et)
6c (R2 = CF3; R3 = Me; R4 = CO2Et)
75 (77)
33 (47)
Ph
N
4
CO2Et
5e
2a
90 (91)
6d
[6d:6e = 80:20]
CO2Et
6e
5
6
7
8
5a (R1 = Ph; R2 = H; R3 = Me)
2c (R4 = CO2Me) 6f (R2 = H; R3 = Me; R4 = CO2Me)
2d (R4 = CO2Ph) 6g (R2 = H; R3 = Me; R4 = CO2Ph)
63 (72)
72 (76)
44 (53)
8 (21)
5a
6h (R2 = H; R3 = Me; R4 = COEt)
5a
2e (R4 = COEt)
5f (R1 = Bn; R2 = H; R3 = H)
2a
6i (R2 = H; R3 = H; R4 = CO2Et)
Bn
NH
H
75 (80)d)
9c)
5f
2a
CO2Et
7b
1
a) Olefin (1.5 equiv). b) Isolated yield. The yield determined by H NMR is reported in parentheses. c) Re2(CO)10
(3.0 mol %) was used as a catalyst. d) Yield of 7b.
Ph
Bn
N
D
D
NH
D
D
[ReBr(CO)3(thf)]2 (3.0 mol%)
27%
+
CO2R
2b
catalyst (3.0 mol%)
6j-H : 6j-D = 66 : 34
toluene, 150 °C, 5 h
D
CO2Et
CO2Et
ð5Þ
5g
toluene, 150 °C, 24 h
R =
6i
7b
ð4Þ
D
none, 0%; [ReBr(CO)3(thf)]2, 85%; FeCl3, 84%
D
[0.94 H]
[0.95 H]
D
D
D
D
D
D
CO2R
Based on our investigations, the mechanism for the forma-
tion of indene derivatives is proposed as bold arrows in
Scheme 2.21 C–H bond activation is accelerated by the
coordination of a nitrogen atom of the ketimine derivative
i to the rhenium center, and the arylrhenium intermediate
iii is generated (step [1]). After C–H bond activation, an
ꢀ,ꢁ-unsaturated compound inserts into the Re–C bond of the
arylrhenium intermediate iii to generate vi (step [8]). The
intramolecular nucleophilic cyclization of a Re–C bond to
the imine moiety of vi (step [15]) followed by reductive
elimination (step [16]) and the elimination of aniline (step
[17]) affords the indene derivative xi. The elimination of
aniline is accelerated by the rhenium catalyst.
D
CO2R
[0.98 H]
(recovery)
CO2R
2b
6j-H
6j-D
The rhenium catalyst was found to act as a Lewis acid to
promote the elimination of amines from x (step [16]). Treat-
ment of aminoindane derivative 7b without any catalyst at
150 ꢁC for 24 h resulted in recovery of 7b (eq 5). In contrast,
addition of a catalytic amount of [ReBr(CO)3(thf)]2 to the re-
action mixture gave indene derivative 6i in 85% yield. Similar-
ly, the elimination of benzylamine also proceeded with a cat-
alytic amount of FeCl3. From these experiments, the rhenium
catalyst acts as a Lewis acid to promote the elimination of
amine.

Y. Kuninobu et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1399
R1
R1
H
CO2R3
CO2R3
CO2R3
+ Re
R2
O
R2
[ 3 ]
[ 4 ]
N
N
OR3
Re
[ 5 ]
i
H
ii
v
R1
R1
+ Re
[ 1 ]
R2
R2
N
[ 2 ]
N
[ 12 ]
[ 6 ]
H
Re
CO2R3
R1
CO2R3
viii
H
[ 13 ]
[ 7 ]
[ 8 ]
R2
N
− Re
R1
Re H
+ Re
iii
R2
[ 14 ]
N
CO2R3
[ 10 ]
[ 9 ]
Re
H
[ 11 ]
R1
vi
CO2R3
[ 15 ]
R2
CO2R3
R2
N
R1
N
Re H
Re
CO2R3
vii
H
[ 16 ]
− Re
ix
R2
R1
R1
NH
CO2R3
− R2NH2
CO2R3
[ 17 ]
cat. Re
xi
x
Scheme 2. Possible routes for the formation of indene derivatives.
Industries, Tokyo Kasei Kogyo Co., Nacalai Tesque, and Aldrich
Co., and used after distillation.
Conclusion
1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded
using a JEOL JNM-LA400 spectrometer. Proton chemical shifts
are reported relative to Me4Si (CDCl3) at ꢂ 0.00 or residual sol-
vent peak (CDCl3 at ꢂ 7.26). Carbon chemical shifts are reported
relative to CDCl3 at ꢂ 77.00. IR spectra were recorded on a
´ ´
Nicolet Protege 460 spectrometer. HR-MS spectra were measured
using a Waters LCT (ESI-TOF MS).
Indene derivatives 6a–6i are already known. The structures of
the reaction products were determined by a comparison of the
spectrum data reported previously.8
Ethyl 3-[2-(2-Pyridyl)phenyl]propionate (3a). A mixture of
1a (77.6 mg, 0.500 mmol), 2a (75.1 mg, 0.750 mmol), and [ReBr-
(CO)3(thf)]2 (6.2 mg, 0.0063 mmol) was dissolved in toluene
(1.0 mL), and stirred at 150 ꢁC for 24 h in a screw-capped test
tube. The crude product was purified by column chromatography
on silica gel to give 3a in 81% yield (103 mg).
The insertion of ꢀ,ꢁ-unsaturated carbonyl compounds into
a C–H bond of aromatic rings using a rhenium catalyst,
[ReBr(CO)3(thf)]2 or Re2(CO)10, proceeded. The reactions
between aromatic imines and ꢀ,ꢁ-unsaturated carbonyl com-
pounds provided indene derivatives in good to excellent yields.
Such cyclization reactions have not occurred with ruthenium
or rhodium complexes, which are usually employed as cata-
lysts to promote the insertion of unsaturated molecules into a
C–H bond. The difference in the reaction with the rhenium
complex derives from the insertion of the unsaturated bond in-
to the Re–C bond22 instead of the Re–H bond. In addition, the
nucleophilic reactivity of the carbon connected to rhenium
promotes further cyclization from the insertion intermediate.
Experimental
General. All reactions were carried out in dry toluene under
an argon atmosphere. Toluene was purchased from Wako
Pure Chemical Industries and was dried and degassed before
use. [ReBr(CO)3(thf)]2 was prepared by heating a THF solution
of ReBr(CO)5 at reflux temperature for 16 h.23 The resulting solu-
tion was concentrated in vacuo and recrystallized from THF/
hexane to give [ReBr(CO)3(thf)]2 as a white solid in 75% yield.24
Ketimines were prepared by condensation of the corresponding
ketones with aniline in the presence of molecular sieves (4A) in
toluene under reflux conditions for 10 h, and were used after dis-
tillation. Ketones, ꢀ,ꢁ-unsaturated carbonyl compounds, anilines,
and bromobenzene-d5 were purchased from Wako Pure Chemical
1H NMR (CDCl3): ꢂ 1.19 (t, J ¼ 7:1 Hz, 3H), 2.53 (t, J ¼
7:9 Hz, 2H), 3.05 (t, J ¼ 7:9 Hz, 2H), 4.06 (q, J ¼ 7:1 Hz, 2H),
7.23–7.38 (m, 5H), 7.41 (d, J ¼ 7:8 Hz, 1H), 7.76 (t, J ¼
7:8 Hz, 1H), 8.66–8.71 (m, 1H); 13C NMR (CDCl3): ꢂ 14.0,
28.4, 35.6, 60.1, 121.7, 123.8, 126.3, 128.4, 129.6, 129.8, 136.3,
138.6, 140.3, 149.0, 159.8, 172.9; IR (nujol, ꢃ/cmꢂ1): 1734,
1585, 1563, 1460, 1424, 1374, 1289, 1182, 1027, 789, 752; Anal.
Calcd for C16H17NO2: C, 75.27; H, 6.71; N, 5.49%. Found: C,
75.38; H, 6.80; N, 5.46%.
Ethyl 3-[2-(2-Pyridyl)-3-methylphenyl)propionate (3b).
1H NMR (CDCl3): ꢂ 1.18 (t, J ¼ 7:2 Hz, 3H), 2.03 (s, 3H), 2.43
(br, 2H), 2.67 (br, 2H), 4.04 (q, J ¼ 7:2 Hz, 2H), 7.13–7.18 (m,

1400 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) BCSJ AWARD ARTICLE
2H), 7.22–7.30 (m, 3H), 7.72–7.73 (m, 1H), 7.78 (td, J ¼ 7:5,
1.5 Hz, 1H); 13C NMR (CDCl3): ꢂ 14.0, 20.1, 28.6, 35.3, 60.1,
121.8, 124.4, 126.3, 128.0, 135.9, 136.2, 138.2, 140.2, 149.5,
159.1, 172.8; IR (nujol, ꢃ/cmꢂ1): 1734, 1585, 1563, 1460,
1424, 1374, 1289, 1182, 1027, 789, 752; HRMS Calcd for
C17H19NO2 [Mþ]: 269.1415. Found: 269.1409.
Ethyl 3-[2-(3-Methyl-2-pyridyl)phenyl]propionate (3c).
1H NMR (400 MHz, CDCl3): ꢂ 1.17 (t, J ¼ 7:2 Hz, 3H), 2.12 (s,
3H), 2.43 (t, J ¼ 8:1 Hz, 2H), 2.68–2.83 (m, 2H), 4.04 (q,
J ¼ 7:2 Hz, 2H), 7.16 (d, J ¼ 7:2 Hz, 1H), 7.21 (dd, J ¼ 4:8,
7.5 Hz, 1H), 7.24–7.34 (m, 3H), 7.60 (d, J ¼ 7:8 Hz, 1H), 8.50
(d, J ¼ 3:9 Hz, 1H); 13C NMR (100 MHz, CDCl3): ꢂ 14.1 (1C),
19.2 (1C), 28.3 (1C), 35.1 (1C), 60.2 (1C), 122.3 (1C), 126.3
(1C), 128.2 (1C), 128.9 (1C), 129.2 (1C), 131.5 (1C), 137.9
(1C), 138.2 (1C), 139.9 (1C), 146.6 (1C), 159.1 (1C), 172.9
(1C); IR (nujol, ꢃ/cmꢂ1): 1738 (s), 1566 (m), 1348 (w), 1157
(m), 1119 (m), 1024 (m), 908 (w), 793 (m), 756 (m), 627 (m);
HRMS Calcd for C17H19NO2 [Mþ] 269.1415. Found: 269.1422.
C21H29NO5 [Mþ]: 375.2046. Found: 375.2049.
Aromatic Ketimine-d5 (5g). To a mixture of magnesium
(0.21 g, 8.40 mmol) and ether (5.0 mL), bromobenzene-d5
(1.24 g, 7.60 mmol) was added dropwise. After gentle reflux, the
reaction mixture was stirred for 20 min and cooled to ꢂ78 ꢁC.
To this mixture, a solution of benzaldehyde (891 mg, 8.40 mmol)
in ether (3.0 mL) was added, and stirred for 10 min. The mixture
was allowed to warm to room temperature before being quenched
with aqueous NH4Cl. The mixture was extracted with ether, and
the organic layer was dried with Na2SO4 to give crude diphenyl-
methanol-d5. Then CH2Cl2 and MnO2 (4.80 g, 55.0 mmol) were
added, and stirred vigorously for 18 h. The mixture was filtered,
washed with CH2Cl2, concentrated under reduced pressure, and
purified by column chromatography on silica gel to give benzo-
phenone-d5 in 76% yield (1.08 g). A mixture of benzophenone-
d5 (800 mg, 4.27 mmol), aniline (550 mg, 5.91 mmol), molecular
sieves 4A (5.00 g), and toluene (6.0 mL) was heated at reflux for
22 h. The mixture was filtered, washed with toluene, and concen-
trated to about 3.0 mL under reduced pressure. Then hexane
(5.0 mL) was added and cooled to 0 ꢁC. The precipitate was
filtered and washed rapidly with hexane to give 5g as a yellow
solid in 42% yield (470 mg).
Ethyl 1-Benzylamino-1-methylindane-2-carboxylate (7a).
1H NMR (CDCl3): ꢂ 1.32 (t, J ¼ 7:2 Hz, 3H), 1.60 (s, 3H), 1.60
(br, 1H), 3.04–3.13 (m, 2H), 3.23–3.35 (m, 1H), 3.44–3.58 (m,
2H), 4.20–4.35 (m, 2H), 7.12–7.34 (m, 9H); 13C NMR (CDCl3):
ꢂ 14.4, 27.1, 33.4, 47.5, 55.5, 60.5, 67.2, 123.4, 124.8, 126.7,
127.9, 128.0, 128.1, 141.1, 145.7, 172.7; Anal. Calcd for
C20H23NO2: C, 77.64; H, 7.49%. Found: C, 77.89; H, 7.65%.
Ethyl 1-Benzylaminoindane-2-carboxylate (7b). 1H NMR
(CDCl3): ꢂ 1.30 (t, J ¼ 7:2 Hz, 3H), 1.78 (br, 1H), 2.96–3.08
(m, 1H), 3.40–3.52 (m, 2H), 3.78–3.89 (m, 2H), 4.23 (q,
J ¼ 7:2 Hz, 2H), 4.44 (d, J ¼ 7:2 Hz, 1H), 7.18–7.40 (m, 9H);
13C NMR (CDCl3): ꢂ 14.3, 33.2, 49.5, 50.9, 60.5, 63.3, 124.5,
124.9, 126.4, 126.8, 127.9, 128.0, 128.3, 140.3, 141.5, 143.4,
173.0; HRMS Calcd for C19H21NO2 [Mþ]: 295.1572. Found:
295.1566.
Ethyl
3-[3-(2-Pyridyl)-2-naphthyl)propionate
(3d).
1H NMR (CDCl3): ꢂ 1.17 (t, J ¼ 7:2 Hz, 3H), 2.55 (t, J ¼
7:8 Hz, 2H), 3.24 (t, J ¼ 7:8 Hz, 2H), 4.07 (q, J ¼ 7:2 Hz, 2H),
7.22–7.32 (m, 1H), 7.39–7.57 (m, 3H), 7.70–7.89 (m, 5H), 8.70
(d, J ¼ 4:8 Hz, 1H); 13C NMR (CDCl3): ꢂ 14.1, 28.7, 35.4, 60.2,
121.8, 124.1, 125.7, 126.3, 127.1, 127.7, 128.2, 129.2, 131.9,
133.2, 136.4, 136.5, 149.0, 159.8, 173.0; IR (neat, ꢃ/cmꢂ1):
3057, 2980, 2907, 1728, 1701, 1585, 1558, 1477, 1458, 1436,
1371, 1182, 1039, 889, 785, 750; HRMS Calcd for C20H19NO2
[Mþ]: 305.3704. Found: 305.3702.
Ethyl 3-[1-(4,4-Dimethyloxazolinyl)-2-naphthyl]propionate
(3g). 1H NMR (CDCl3): ꢂ 1.24 (t, J ¼ 7:2 Hz, 3H), 1.52 (s,
6H), 2.71 (t, J ¼ 8:1 Hz, 2H), 3.16 (t, J ¼ 8:1 Hz, 2H), 4.14 (q,
J ¼ 7:2 Hz, 2H), 4.24 (s, 2H), 7.37 (d, J ¼ 8:4 Hz, 1H), 7.41–
7.52 (m, 2H), 7.77–7.84 (m, 2H), 7.90 (d, J ¼ 8:1 Hz, 1H); HRMS
Calcd for C20H23NO3 [Mþ]: 325.1678. Found: 325.1680.
Ethyl 3-[(3-Ethoxycarbonylethyl)-2-(2-pyridyl)phenyl]pro-
pionate (4a). 1H NMR (CDCl3): ꢂ 1.16 (t, J ¼ 7:1 Hz, 6H),
2.36–2.44 (m, 4H), 2.60–2.69 (m, 4H), 4.04 (q, J ¼ 7:1 Hz, 4H),
7.17 (d, J ¼ 7:5 Hz, 2H), 7.27–7.32 (m, 3H), 7.78 (t,
J ¼ 7:5 Hz, 1H), 8.71 (d, J ¼ 3:9 Hz, 1H); 13C NMR (CDCl3): ꢂ
14.0, 28.6, 35.3, 60.2, 122.1, 124.6, 127.0, 128.4, 136.3, 138.63,
149.62, 158.53, 172.84; HRMS Calcd for C21H25NO4 [Mþ]:
355.1784. Found: 355.1777.
Synthesis of Alkylated Aromatic Ketimine 11. Alkylated
aromatic ketimine 11 was prepared by a modified procedure.10
A mixture of ketimine 5a (97.6 mg, 0.500 mmol), ethyl acrylate
(2a, 75.0 mg, 0.750 mmol), and RhCl(PPh3)3 (23.1 mg, 0.025
mmol) was dissolved in toluene (1.0 mL) and stirred at 150 ꢁC
for 24 h. The crude product was purified by column chromatogra-
phy on silica gel and GPC to give 11 in 50% yield (73.8 mg).
Synthesis of Indene Derivative 6a from Alkylated Aromatic
Ketimine 11. A mixture of 11 (73.8 mg, 0.250 mmol) and [ReBr-
(CO)3(thf)]2 (6.2 mg, 0.0063 mmol) was dissolved in toluene
(0.5 mL), and stirred at 150 ꢁC for 24 h. The crude product was
purified by column chromatography on silica gel to give 6a in
9% yield (4.6 mg).
Ethyl 3-[(3-Ethoxycarbonylethyl)-2-(2-pyridyl)-1-naph-
thyl]propionate (4d).
1H NMR (CDCl3): ꢂ 1.17 (t, J ¼ 7:2
Hz, 6H), 2.44–2.55 (m, 3H), 2.58–2.69 (m, 1H), 2.69–2.84 (m,
2H), 2.95–3.07 (m, 1H), 3.07–3.21 (m, 1H), 4.06 (q, J ¼ 7:2 Hz,
4H), 7.30–7.38 (m, 2H), 7.45–7.58 (m, 2H), 7.66 (s, 1H), 7.77–
7.87 (m, 2H), 8.03 (d, J ¼ 7:8 Hz, 1H), 8.75 (d, J ¼ 4:5 Hz,
1H); 13C NMR (CDCl3): ꢂ 14.1, 25.2, 29.1, 35.0, 35.1, 60.2,
60.3, 122.3, 123.8, 124.7, 126.0, 126.1, 126.3, 128.4, 130.3,
133.6, 134.7, 136.2, 136.4, 138.8, 149.7, 159.1, 172.9, 172.9; IR
(neat, ꢃ/cmꢂ1): 2979, 2936, 1732, 1585, 1560, 1477, 1425,
1175, 1041, 889, 748; HRMS Calcd for C25H27NO4 [Mþ]:
405.4862. Found: 405.4853.
Synthesis of Aminoindane Derivative 7b.
A mixture of
aromatic aldimine 5f (97.6 mg, 0.500 mmol) and Re2(CO)10
(9.8 mg, 0.015 mmol) was dissolved in toluene (1 mL), and stirred
at 150 ꢁC for 24 h. The crude product was purified by column
chromatography on silica gel to give 7b in 75% yield (111 mg).
1H NMR (CDCl3): ꢂ 1.30 (t, J ¼ 7:2 Hz, 3H), 1.78 (br, 1H),
2.96–3.08 (m, 1H), 3.40–3.52 (m, 2H), 3.78–3.89 (m, 2H), 4.23
(q, J ¼ 7:2 Hz, 2H), 4.44 (d, J ¼ 7:2 Hz, 1H), 7.18–7.40 (m,
9H); 13C NMR (CDCl3): ꢂ 14.3, 33.2, 49.5, 50.9, 60.5, 63.3,
124.5, 124.9, 126.4, 126.8, 127.9, 128.0, 128.3, 140.3, 141.5,
143.4, 173.0; HRMS Calcd for C19H21NO2 [Mþ]: 295.1572.
Found: 295.1566.
Ethyl 3-[2-(4,4-Dimethyloxazolinyl)-3-ethoxycarbonylethyl-
phenyl]propionate (4f). 1H NMR (CDCl3): ꢂ 1.21 (t, J ¼ 7:2
Hz, 6H), 1.44 (s, 6H), 2.62 (t, J ¼ 7:8 Hz, 4H), 2.95 (t, J ¼ 7:8
Hz, 4H), 4.13 (q, J ¼ 7:2 Hz, 4H), 4.14 (s, 2H), 7.09 (d, J ¼
7:8 Hz, 2H), 7.24 (t, J ¼ 7:8 Hz, 1H); 13C NMR (CDCl3): ꢂ
14.1, 28.1, 29.0, 36.0, 60.2, 68.0, 127.1, 128.6, 129.7, 139.6,
161.4, 172.7; IR (nujol, ꢃ/cmꢂ1): 1734, 1664, 1464, 1374,
1348, 1297, 1184, 1096, 1040, 961, 761; HRMS Calcd for

Y. Kuninobu et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1401
Elimination of Benzylamine from Aminoindane Derivative
7b. A mixture of aminoindane derivative 7b (29.5 mg, 0.100
mmol) and [ReBr(CO)3(thf)]2 (2.5 mg, 0.30 mmol) was dissolved
in toluene (0.20 mL), and stirred at 150 ꢁC for 24 h. The crude
product was purified by column chromatography on silica gel to
give 6i in 85% yield (16.0 mg).
13 A little H/D scrambling at the ortho-positions was ob-
served. A 4% scrambling also occurred by treating 2-phenylpyri-
dine (1a-d5) with a catalytic amount of [ReBr(CO)3(thf)]2 under
the same conditions using toluene or toluene-d8 as a solvent.
The result suggests that the hydrogen came from water in the
reaction mixture.
14 a) W. D. Jones, Acc. Chem. Res. 2003, 36, 140. b) J. Ito,
H. Nishiyama, Eur. J. Inorg. Chem. 2007, 1114.
15 We neglected H/D scrambling between the ortho-positions
of an aromatic ring and water, because the values were considered
to be small enough.
16 After usual workup with aq HCl, 2-phenylpyridine was
recovered in 91%.
17 We have reported on the rhenium-catalyzed synthesis of
indene derivatives via C–H bond activation, in which aromatic
imines were prepared in situ. See Ref. 8.
This work was partially supported by a Grant-in-Aid for
Scientific Research (No. 18037049) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
We appreciate the Asahi Glass Foundation, Okayama Founda-
tion for Science and Technology, and Meiji Seika Co. for fi-
nancial support. We thank Professor Isao Kadota (Department
of Chemistry, Okayama University, Japan) for acquiring
HR-MS spectra.
18 A rhenium complex, Re2(CO)10, also produced indene
derivative 6a in 64% yield.
Supporting Information
1H NMR charts of compounds 1a-d5 (Table 3), 3a0-d4
(Table 3), 2b0 (Table 3), 1a-d1 (eq 1), 3a0-H + 3a0-D (eq 1), 2b
(eq 1), 6n0-D + 6n0-H (eq 4), and 2b (eq 4). This material is
available free of charge on the Web at: http://www.csj.jp/
journals/bcsj/.
19 Ethyl methacrylate, ethyl trans-crotonate, and t-butyl acry-
late did not afford the indene derivatives. Phenyl vinyl sulfone,
N,N-dimethylacrylamide and acrylonitrile did not give the indene
derivatives either, due to the polymerization of these ꢀ,ꢁ-unsatu-
rated compounds under the reaction conditions.
20 The electrophilic addition path (steps [3] and [5]) also
produces 11.
References
21 Although we propose the aryl–Re–H intermediate ii in
Scheme 1 and iii in Scheme 2, there is another possibility that
the formation of an aryl–Re species without a hydride ligand.
One possibility is as follows: (1) ortho metalation (cyclometala-
tion) via the elimination of HBr; (2) insertion of an acrylate
into a rhenium–carbon bond; and (3) protonation by HBr. This
mechanism is also consistent with the results obtained by H/D
scrambling and KIE experiments, and Ref. 16.
1
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