Cobalt-catalyzed regioselective synthesis of indenamine from o-lodobenzaldimine and alkyne: Intriguing difference to the nickel-catalyzed reaction

Article abstract of DOI:10.1002/chem.200801457

A new and convenient method for the regioselective synthesis of indenamine from the corresponding o-halobenzyaldehyde, amine, and alkyne in the presence of zinc powder catalyzed by cobalt complexes, was reported. The indenamines and indene-enamine gave the corresponding imine derivatives upon treatment with TBAF in THF at 40°C. The results show that chelation-assisted oxidative addition of 2-iodo-benzaldinine to Cobalt^I, which is formed from the reduction of Co^III by zinc, gives a five-membered ring cobaltacycle. The nickel-catalyzed reaction and reductive elimination takes place to afford an isoquinolinium salt and then the isoquinoline product. The regiochemistry for the cobalt-catalyzed indenamine and nickel-catalyzed isoquinoline formation is opposite to each other when alkynes with an electron-withdrawing group are used.

Full text of DOI:10.1002/chem.200801457

COMMUNICATION
DOI: 10.1002/chem.200801457
Cobalt-Catalyzed Regioselective Synthesis of Indenamine from o-
Iodobenzaldimine and Alkyne: Intriguing Difference to the Nickel-Catalyzed
Reaction
Chuan-Che Liu, Rajendra Prasad Korivi, and Chien-Hong Cheng*[a]
Indene derivatives[1,2] are known to show various pharma-
cological properties, such as enzyme inhibitors,[1a] reuptake
blocking,[1b] biogenic transporters,[1c] and anti-arrhythmic ac-
tivity.[1d] In addition, they have also found applications in
material science as discotic liquid crystals,[1e] coating materi-
als for steel,[1f] conducting polymers,[1g] ligands for metallo-
cene complexes,[1h,i] and so forth. Indenamine derivatives
were recently synthesized by reacting chloroindan[1c] with
the desired amines and by nucleophilic addition into nitrile
derivatives,[3a] whereas indenones were prepared by oxida-
tion of the corresponding indene derivatives[3b] and by cata-
lytic intramolecular isomerization–aldolization[3c] of allyl al-
cohol with aldehyde. In this paper, we report a new conven-
ient method for the regioselective synthesis of indenamine
from the corresponding o-halobenzaldehyde, amine, and
alkyne in the presence of zinc powder catalyzed by cobalt
complexes. Moreover, these compounds can be conveniently
converted to the corresponding substituted indenone deriva-
tives.
Recently, we and other groups had found methods for the
synthesis of isoquinoline derivatives[4] from 2-iodobenzaldi-
mines (1) and alkynes catalyzed by nickel[4a] and palladium
complexes, respectively.[4b–f] Our interests in the cobalt-cata-
lyzed reactions[5] and the recent observation of activation of
arylhalides by cobalt complexes[5] have prompted us to in-
vestigate the use of cobalt complexes for the reaction of N-
tert-butyl-2-iodobenzaldimine (1a) with alkyne. To our sur-
prise, when we treated 1a with 4-octyne in the presence of
Scheme 1. Nickel- and cobalt-catalyzed cyclization of N-tert-butyl-2-iodo-
benzaldimine with alkyne.
el-[4a] and palladium-catalyzed[4b–f] reactions that gave the
corresponding isoquinoline derivative as product. The struc-
ture of the indenamine was established based on its 1H,
13C NMR, and MS spectral data.
To simplify the reaction, we observed that by using the
corresponding o-iodobenzaldehyde and amine to replace 1a,
the same indenamine product can be obtained. Thus, o-iodo-
benzaldehyde (2a), p-toluidine (3a), and 4-octyne under-
went three-component reaction in the presence of [CoCl2-
AHCTRE(UNG dppe)] in an acetonitrile and THF mixture (optimization
studies are given in the Supporting Information) to afford
indenamine 5b in essentially quantitative yield (entry1,
Table 1). In a similar manner, diphenylacetylene (4b) af-
forded product 5c in 78% yield. The three-component reac-
tions of unsymmetrical alkynes with 2a and 3a were careful-
ly studied by using the same catalyst system. Terminal
alkyne 4c gave a single regioisomer 5d with the propyl
group adjacent to the amino group in 82% yield (entry 3).
For disubstituted unsymmetrical alkyne 4d, two regioisom-
ers 5e (R1 =Ph, R2 =Et) and 5eꢀ (R1 =Et, R2 =Ph) in 60
and 36% yields, respectively, were observed. The applica-
tion of alkynes with electron-withdrawing substituents 4e–g
in the cyclization reaction with 2a and 3a gave highly regio-
selective indene–enamine derivatives 6a–c in moderate
yields (entries 5–7, Table 1). It is worth noting that product
6 arises from a double-bond migration of the corresponding
product 5. Always, the electron-withdrawing ester or keto
groups of alkynes are adjacent to the amine group. The re-
[CoCl2ACHTREUNG(dppe)] and zinc powder in acetonitrile, the corre-
sponding indenamine (5a; Scheme 1, R1 =R2 =nPr) was ob-
tained in 78% yield. This is in sharp contrast to the nick-
[a] C.-C. Liu, Dr. R. P. Korivi, Prof. Dr. C.-H. Cheng
Department of Chemistry, National Tsing Hua University
Hsinchu, 30043 (Taiwan)
Fax : (+886)35724698
E-mail: chcheng@mx.nthu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801457.
Chem. Eur. J. 2008, 14, 9503 – 9506
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9503

Table 1. Results of cobalt-catalyzed three-component reaction of o-iodo-
benzaldehyde, p-toluidine and alkynes.
forded the corresponding indenamine derivative 5h in
nearly quantitative yield, while sterically bulky 2,4,6-trime-
thylaniline provided 5i in 73% yield. Other substituted ani-
lines with electron-donating (3d,e) and electron-withdraw-
ing substituents (3 f,g) all gave the corresponding indena-
mines, 5j–m, in excellent yields. However, the reaction with
8-aminoquinoline 3h as the amine substrate failed to pro-
ceed to provide the expected product 5n. Possibly, the che-
lation of both nitrogen atoms in 3h inhibits the reactivity of
the catalyst. For benzylamine (3i), the three-component re-
action afforded indenamine 5o in 68% yield, but n-butyla-
mine failed to give the expected indenamine. It appears that
other unknown products were produced and further investi-
gation is necessary to unravel the reaction of n-butylamine
with 2a and 4a. The cyclization reaction can be applied to
various substituted 2-iodoaldehydes 2. Treatment of benzox-
yl-substituted o-iodobenzaldehyde 2b with 3a and 4a and of
2c with 3a and 4 f afforded the expected indenamines 5q
and indene-enamine 6d in 89 and 78% yields, respectively.
Entry[a]
Alkyne
Product
Yield[b]
1
2
3
4
5
6
7
8
9
4a: R1 =R2 =(CH2)2CH3
4b: R1 =R2 =Ph
5b
5c
5d
5e+5e’
6a
6b
6c
5 f
5g
99
78
82
60+36
73 (85)[c]
64 (93)[c]
45 (90)[c]
95
4c: R1 =(CH2)2CH3; R2 =H
4d: R1 =Ph; R2 =Et
4e: R1 =CO2Me; R2 =C6H13
4 f: R1 =CO2Et; R2 =Ph
4g: R1 =COMe; R2 =Ph
4h: R1 =Me; R2 =TMS
4i: R1 =Ph; R2 =TMS
82
[a] All reactions were carried out under one nitrogen atmosphere using
aldehyde 2 (0.30 mmol), amine 3 (0.30 mmol), alkyne 4 (0.60 mmol),
[CoCl2ACHTREUNG(dppe)] (0.0210 mmol) and Zn (0.60 mmol) at 1008C for 12 h. [b]
Isolated yields. [c] Additional Hacac (penta-2,4-dione, 0.021 mmol) was
added to the catalyst solution for the yield in the parenthesis.
gioselectivity is opposite to the nickel-catalyzed annulation
of tert-butyl-2-iodobenzaldimines with these alkynes to give
the corresponding isoquinolines. The yields of products 6a–c
were greatly improved, when Hacac (Hacac=penta-2,4-
dione) was added to the catalytic solutions. The regiochem-
istry of these products was determined based on their NOE
data. In addition, the structure of compound 6c was further
confirmed by the X-ray crystallographic analyses. The reac-
tions of TMS-substituted alkynes 4h and 4i gave indena-
mine derivatives 5 f and 5g with excellent regioselectivity
1
and yields. These yields were measured by the H NMR in-
tegration method with mesitylene as the internal standard in
the crude mixtures. These compounds are difficult to obtain
in pure form from a silica gel column due to decomposition.
The crude mixture was used for the removal of the TMS
functionality (vide infra).
The scope of amine used for the present cobalt-catalyzed
three-component reaction was carefully studied. The reac-
tions of 2a and 4a with various aromatic amines, 3b–g, pro-
ceed smoothly to give products 5h–m, respectively, in good
to excellent yields (entries 1–6, Table 2). 1-Napthylamine af-
Similarly, the reaction of 2-iodopiperonal (2d) with alkynes
4 f and 4a gave the corresponding indene-enamine 6e and
indenamine 5r in 79 and 68% yields, respectively. Even 2-
bromo-5,6-(methylenedioxy)benzaldehyde (2e) gave indena-
mine derivative 5s in 73% yield. Finally, the reaction of 2-
iodo(4,5-dimethoxy)benzaldehyde (2 f) with 4a also resulted
in the corresponding indenamine 5t in 86% yield.
The indenamines prepared by the present three-compo-
nent reaction can be readily converted to the corresponding
indenimines (Scheme 2). Thus, the indenamines 5b, 5c, 5 f,
and 5g and indene-enamine 6b gave the corresponding
imine derivatives 7a–e upon treatment with TBAF (tetrabu-
tylammonium fluoride) in THF at 408C. The isolated yields
of these imine derivatives after flash chromatography were
essentially quantitative. The dehydrogenation of indena-
mines in the presence of TBAF is interesting, as this is a
mild and convenient procedure, but the mechanism for the
dehydrogenation is not yet clear.[5f] The structure of com-
pound 7b was confirmed by X-ray crystallographic data (see
the Supporting Information). It is interesting to note that
compound 5g underwent both desilylation and dehydrogen-
ation to give indenimine 7d upon treatment with TBAF.
These indenimines can be used for the preparation of substi-
Table 2. Synthesis of indenamines using various amine derivatives and o-
iodobenzaldehyde.[a]
Entry
2
R3-NH2
4
5
Yield
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2a
2a
2a
2a
2a
3b: 1-napthylamine
3c: 2,4,6-(Me)3C6H2NH2
3d: 4-OMeC6H4NH2
3e: 3,4,5-(OMe)3C6H2NH2
3 f: 3,5-(F)2C6H3NH2
3g: 4-CF3C6H4NH2
3h: 8-aminoquinoline
3i: Bn-NH2
4a
4a
4a
4a
4a
4a
4a
4a
4a
5h
5i
5j
5k
5l
5m
5n
5o
5p
99
73
94
92
98
97
0
68
0
3j: nBuNH2
[a] The same reaction reaction conditions were used as those in Table 1.
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Cobalt Catalysis
COMMUNICATION
C can be reduced by Zn to give CoI and an intermediate
similar to C, but with the CoIII replaced by ZnII. During
workup, this intermediate undergoes hydrolysis to give in-
denamine 5.
It is worth noting that both cobalt and nickel complexes
can catalyze the cyclization reaction of 2-iodobenzaldimine
1 with alkyne 4, but the products, as shown in Scheme 4, are
Scheme 2. Synthesis of indenimine derivatives and indenones from the
corresponding amine and TBAF.
tuted indenones. Thus, acid hydrolysis of 7b and 7e gave in-
denones 8b and 8e in 96 and 92% yields, respectively.
It is known that ester-substituted indenone derivatives are
useful as agonists for peroxisome proliferator-activated re-
ceptor g (PPARg)[3b,6a]. One of these indenone derivatives, 9
(Scheme 2), can be prepared by the present oxidation
method from indene-enamine 6d in 95% yield. Previously
these substrates were prepared by oxidizing the correspond-
ing indene derivatives with SeO2 in dioxane.[3b] Other bio-
logically active 2,3-disubstituted indenone derivatives were
also described in literature.[3c.6b]
A proposed mechanism for the present cobalt-catalyzed
three component cyclization reaction was shown in
Scheme 3. Chelation-assisted oxidative addition of 2-iodo-
Scheme 4. Pathways for Ni- and Co-catalyzed insertion of alkynes.
different. These two reactions are likely all through a five-
membered azametalacycle
A followed by insertion of
alkyne into A to give B. For the cobalt-catalyzed reaction,
intramolecular nucleophilic attack of the carbon anion at
the imine carbon in B occurs to give finally the indenamine
product. On the other hand, for the nickel-catalyzed reac-
tion, reductive elimination probably takes place to afford an
isoquinolinium salt and then the isoquinoline product. The
observed different pathways are likely due to the difference
of oxidation states of the two metals in intermediate B. One
is CoIII, while the other is NiII. We think that in the cobalt-
catalyzed reaction, the CoIII center in intermediate
B
(Scheme 3) is higher in Lewis acidity compared with NiII.
The high acidity of CoIII greatly activates the imine group
and enhances the intramolecular nucleophilic attack.
Scheme 3. Proposed mechanism for the Co-catalyzed formation of inden-
amines.
Surprisingly, the regiochemistry for the cobalt-catalyzed
indenamine and nickel-catalyzed isoquinoline formation is
opposite to each other when alkynes with an electron-with-
drawing group, such as COOR and COR, were used
(Scheme 4). The observation may be explained based on the
benzaldimine 1 to CoI, which is formed from the reduction
of CoIII by zinc, gives a five-membered ring cobaltacycle A.
À
Regioselective insertion of alkyne into C Co bond of inter-
mediate A yields a seven-membered[7] azacobaltacycle B.
Subsequent intramolecular nucleophilic addition of the al-
kenyl group to C=N bond in B results in intermediate C.
Upon hydrolysis, indenamine 5 and CoIII species were pro-
duced. The reduction of CoIII by zinc generates the CoI
needed for the next catalytic cycle. Generally, the alkyne in-
sertion takes place in such a way that the electron-deficient
carbon atom is near the metal.[8] Alternatively, intermediate
alkyne insertion into the M C and M N bonds in the five-
membered metalacycle intermediate. In the cobalt-catalyzed
À
À
À
reaction, the insertion of alkyne is all into the Co carbon
bond by Michael addition. Thus, the electron-withdrawing
group is adjacent to the amine group of the indenamine
product. However, for the nickel-catalyzed reaction, the
alkyne insertion has two types. For normal alkynes such as
TMSCꢀCPh, the alkyne is inserted into the nickel–carbon
Chem. Eur. J. 2008, 14, 9503 – 9506
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Experimental Section
Procedure for the synthesis of 5a: A screw-cap seal tube initially fitted
with septum containing 1a (0.070 g, 0.30 mmol) was evacuated and
purged with nitrogen gas three times. Then 4-octyne (0.066 g, 60 mmol),
a mixture of freshly distilled CH3CN and THF (2.0 mL, 1:1 by volume)
were added to the system by syringe. The septum was removed and
[CoCl2ACHTREUNG(dppe)] (0.012 g, 0.0210 mmol) and zinc (0.040 g, 0.60 mmol) were
added to the system and the tube was quickly sealed with a screw cap.
The resultant reaction mixture was stirred at 1008C for 12 h. After com-
pletion of reaction, the mixture was cooled, diluted with dichlorometh-
ACHTREUNGane, filtered through a small silica-gel pad, and then concentrated. Sepa-
ration on a silica gel column using hexane/EtOAc as eluent gave pure in-
denamine derivative 5a in 78% yield. 1H NMR (CDCl3, 500 MHz): d=
0.97–1.01 (m, 6H), 1.30 (s, 9H), 1.42–1.50 (m, 1H), 1.52–1.63 (m, 3H),
2.36–2.45 (m, 4H), 4.23 (s, 1H), 7.12 (t, J=7.0 Hz, 1H), 7.16 (d, J=
7.5 Hz, 1H), 7.23 (t, J=7.0 Hz, 1H), 7.48 ppm (d, J=7.0 Hz, 1H);
13C NMR (CDCl3, 125 MHz): d=14.4, 14.5, 22.0, 23.6, 27.6, 27.7, 30.9,
50.7, 60.7, 118.5, 123.0, 124.3, 127.0, 136.0, 145.1, 147.2, 148.5 ppm;
HRMS (EI+): m/z calcd for C19H29N: 271.2300; found: 271.2300; IR
(KBr): n˜ =1226, 1465, 1604, 1704, 2869, 2931, 2954, 3355 cmÀ1
.
Acknowledgements
We thank the National Science Council of Republic of China (NSC 96-
2113M-007-020-MY3) for support of this research.
Keywords: cobalt · nickel · nucleophilic addition · reaction
mechanisms
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Received: July 18, 2008
Published online: September 22, 2008
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