Intramolecular cyclization of aminoalkynes catalyzed by PdMo 3S4 cubane clusters

Article abstract of DOI:10.1246/cl.2006.590

PdMo₃S₄ cubane clusters [(Cp*Mo) ₃(μ₃-S)₄Pd(L)][PF₆] (Cp* = η⁵-C₅Me₅; L = dba (2), ma (3); dba = dibenzylideneacetone, ma = maleic anhydride) showed high catalytic activity for the intramolecular hydroamination of aminoalkynes to afford the corresponding cyclic imines in good yields under mild conditions. A molecular structure of 3 has been determined by X-ray diffraction study. Copyright

Full text of DOI:10.1246/cl.2006.590

590
Chemistry Letters Vol.35, No.6 (2006)
Intramolecular Cyclization of Aminoalkynes Catalyzed by PdMo3S4 Cubane Clusters
Izuru Takei,1 Yutaka Enta,1 Youhei Wakebe,1 Toshiaki Suzuki,2 and Masanobu Hidaiꢀ1
1Department of Materials Science and Technology, Faculty of Industrial Science and Technology,
Tokyo University of Science, Noda 278-8510
2RIKEN, Wako 351-0198
(Received March 1, 2006; CL-060247; E-mail: hidai@rs.noda.tus.ac.jp)
PdMo3S4 cubane clusters [(CpꢀMo)3(ꢀ3-S)4Pd(L)][PF6]
(Cpꢀ = ꢁ5-C5Me5; L = dba (2), ma (3); dba = dibenzylidene-
acetone, ma = maleic anhydride) showed high catalytic activity
for the intramolecular hydroamination of aminoalkynes to afford
the corresponding cyclic imines in good yields under mild
conditions. A molecular structure of 3 has been determined by
X-ray diffraction study.
Pd
S
S
Mo
Mo
S
S
Mo
Although the chemistry of transition-metal sulfide clusters
with the M4S4 cubane core has been receiving much attention
in relevance to the active sites of metalloproteins such as ferre-
doxins and nitrogenases as well as industrial hydrodesulfuriza-
tion catalysts,1 catalytic activities of such cubane clusters are
still left undeveloped.2 In the course of our extensive studies
on multimetallic sulfur-bridged complexes, we have developed
several rational synthetic routes to various cubane clusters with
a desired metal composition, some of which show intriguing
reactivities toward small molecules.3 Thus, MMo3S4 (M = Pd,
Ni) cubane clusters exhibit remarkable catalytic activity for
some reactions of alkynes, including intramolecular cyclization
of alkynoic acids to afford enol lactones.4 Further, M2Mo2S4
Figure 1. ORTEP drawing of the cationic part of cluster 3.
comparable to or slightly shorter than those in Pd(II) alkene
complexes such as [PdCl2(L)] (L = 1,4- or 1,5-cyclooctadiene;
9
˚
˚
1.37–1.39 A) and [PdClMe(L)(ma)] (1.410(7) A; L = 2,9-di-
methyl-1,10-phenanthroline).10 This indicates that the palladium
atom in cluster 2 or 3 behaves as an unique tetrahedral Pd(II)-
˚
like center. The Pd–Mo distances at 2.894(2)–2.949(2) A as well
˚
as the Mo–Mo distances at 2.826(2)–2.848(2) A are diagnostic of
metal–metal bonds between these atoms, and the latter are
6
˚
slightly longer than those in cluster 1 (2.819(5) A, average).
The core structure with six metal–metal bonds for 3 is consistent
with its total electron count of 60 eꢁ.
5b
(M = Rh, Ir)5a and RuMo3S4 cubane clusters are effective
for the catalytic N–N bond cleavage of hydrazines. As an exten-
sion of these studies, we report here the intramolecular hydro-
amination of aminoalkynes catalyzed by PdMo3S4 cubane clus-
ters.
Cluster 2 or 3 showed high catalytic activity for the
intramolecular cyclization of aminoalkynes to afford cyclic
imines (Table 1). Treatment of 5-phenyl-4-pentyn-1-amine
(4a; 1.0 mmol) in THF in the presence of cluster 2 (0.010 mmol)
at 60 ꢂC for 0.5 h afforded the corresponding cyclic imine, 2-
benzyl-1-pyrroline (5a) in almost quantitative yield (98%
GLC). Cluster 3 can also promote the intramolecular cyclization
with almost the same catalytic activity as cluster 2. On the other
hand, the PdMo3S4 cluster [(CpꢀMo)3(ꢀ3-S)4Pd(PPh3)][PF6]
prepared previously6 exhibited low catalytic activity, which is
probably owing to some difficulty in the substitution of substrate
4a for the PPh3 ligand. The cyclization did not proceed by using
cluster 1, [Pd(dba)2], or [Pd(nbd)(ma)].
Recently, the catalytic hydroamination of alkynes based on
early transition metals,11 especially lanthanide metals,12 has
been extensively reported. In contrast, the hydroamination reac-
tions using late transition metals have been relatively limit-
ed.13,14 Mononuclear Pd(II) complexes such as [PdCl2],14a
[Pd(CH3CN)4][BF4]2,14b,14c and [Pd(triphos)][BF4]2 (triphos =
bis(diphenylphosphinoethyl)phenylphosphine)14b,14c are known
to be effective for the intramolecular cyclization of aminoal-
Previously, we reported that the incomplete cubane-type
cluster [(CpꢀMo)3(ꢀ2-S)3(ꢀ3-S)][PF6] (1) serves as a versatile
precursor for synthesis of heterobimetallic MMo3S4 (M = Ru,
Ni, Pd) cubane clusters.4d,6 Thus, the PdMo3S4 cluster bearing
an alkene ligand dba on the palladium center, [(CpꢀMo)3-
(ꢀ3-S)4Pd(dba)][PF6] (2), is readily available from the reaction
of 1 and [Pd(dba)2] in 48% yield. In a similar manner, cluster 1
reacted with the Pd(0) ꢁ2-alkene complex, [Pd(nbd)(ma)]7
(nbd = 2,5-norbornadiene), to afford the PdMo3S4 cluster
[(CpꢀMo)3(ꢀ3-S)4Pd(ma)][PF6] (3) as dark brown crystals in
70% yield (Eq 1).
[PF6]
[PF6]
L
S
Mo
S
S
Mo
Pd
[Pd(dba)2] or
S
S
[Pd(nbd)(ma)]
THF
(1)
Mo
Mo
S
S
Mo
S
Mo
1
L = dba (2), ma (3)
kynes. Muller et al. reported that the intramolecular cyclization
¨
The X-ray analysis of cluster 3 unambiguously shows the
coordination of ma to the tetrahedral Pd site in a side-on fashion
(Figure 1).8 The C=C bond distance of the bound ma in cluster 3
of 4a by using [Pd(CH3CN)4][BF4]2 as catalyst at 90 ꢂC for 2 h
in THF gives imine 5a in 48% yield (substrate/catalyst = 40).14c
When these Pd(II) complexes were actually used as catalyst for
the reaction of 4a under the reaction conditions described in
Table 1, the yield of 5a was quite low. This indicates that the cat-
˚
(1.36(3) A) is shorter than that in the Pd(0) alkene complex
7
˚
[Pd(py)2(ma)] (1.430(3) A; py = pyridine), however, almost
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.6 (2006)
591
Table 1. Catalytic activity of several palladium complexes and
clusters for intramolecular cyclization of 4a to 5aa
containing an unique Pd(II)-like center show high catalytic
activity for the intramolecular cyclization of aminoalkynes to
afford cyclic imines. Studies aiming at optimization of the
transformation and elucidation of the reaction mechanism are
now in progress.
H2N
1.0 mol % cat.
N
Ph
5a
Ph
4a
THF, 60 °C
Run
Catalyst
Time/h Conv./%b Yield/%b
1
2
3
4
5
6
7
8
9
[(CpꢀMo)3(ꢀ3-S)4Pd(dba)][PF6] (2)
[(CpꢀMo)3(ꢀ3-S)4Pd(ma)][PF6] (3)
[(CpꢀMo)3(ꢀ3-S)4Pd(PPh3)][PF6]
[(CpꢀMo)3(ꢀ2-S)3(ꢀ3-S)][PF6] (1)
[Pd(dba)2]
[Pd(nbd)(ma)]
[PdCl2]
[Pd(CH3CN)4][BF4]2
[Pd(triphos)][BF4]2
0.5
1
20
20
20
20
20
20
20
98
96
15
<1
<1
<1
5
98 (85)c
References and Notes
95 (84)c
1
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6
<1
<1
<1
<1
14
2
3
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15
10
8
aAll of the reactions of 4a (1.0 mmol) with catalyst (0.010 mmol) were
carried out in THF (8.0 mL) at 60 ꢂC. bDetermined by GLC. cIsolated yield.
alytic activity of cubane clusters 2 and 3, which contain an
unique Pd(II)-like center embedded in the Mo3S4 aggregate, is
higher than that of conventional Pd(II) complexes.
Table 2 shows the results of the catalytic intramolecular
cyclization of various aminoalkynes by using cluster 2. Employ-
ment of an internal alkyne 4b gave the corresponding cyclic
imine 5b in high yield after 3 h, while the reaction of a terminal
alkyne 4c proceeded relatively smoothly and 5c was obtained in
high yield after 0.1 h. When 6-aminohex-1-yne (4d) was em-
ployed, a six-membered cyclic imine, 2-methyl-1,2-dehydropi-
peridine (5d) was obtained.
4
5
a) H. Seino, T. Masumori, M. Hidai, Y. Mizobe, Organo-
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¨
Angew. Chem., Int. Ed. 2003, 42, 3501.
A synthetic procedure and crystallographic data for cluster 3 are
described in the Supporting Information.
Table 2. Intramolecular cyclization of several aminoalkynes
catalyzed by 2a
Run
Aminoalkyne
Time/h Conv./%b Yield/%b
Product
6
7
8
9
N
H2N
Me
Me
1
4b
3
>98
>98
96
95
5b
5c
Me
N
N
H2N
2
3
4c
0.1
H
H2N
Me
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4d
4
>98
96
5d
H
aAll of the reactions of aminoalkynes (1.0 mmol) with 2 (0.010 mmol) were
carried out in THF (8.0 mL) at 60 ꢂC. bDetermined by GLC.
10 V. G. Albano, C. Castellari, M. E. Cucciolito, A. Panunzi, A.
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To gain insight into the active species, the UV–vis spectra of
the reaction solutions were measured. The UV–vis spectrum of a
THF solution of cluster 2 showed two bands at ꢂmax 482 nm
(" ¼ 5600 Mꢁ1 cmꢁ1) and 608 nm (sh, " ¼ 3250), while that
of cluster 3 also exhibited similar bands at 476 nm (" ¼ 4050)
and 602 nm (sh; " ¼ 750). When cluster 2 or 3 was employed
as catalyst for the hydroamination of aminoalkynes, analogous
bands at ꢂmax 490 and 615 nm were observed in the UV–vis
spectrum of the reaction solution during the reaction. These find-
ings suggest that the PdMo3S4 cubane core is maintained
throughout the catalytic reaction. On the basis of our previous in-
vestigations on the reactivities of PdMo3S4 cubane clusters to-
ward alkynes,4a–4c the most plausible mechanism for this hydro-
amination reaction is considered as follows. The coordination of
an aminoalkyne via the CꢃC bond initially occurs at the unique
Pd(II)-like atom in the cubane core, and the subsequent intramo-
lecular nucleophilic attack of the amino group on the activated
CꢃC bond results in the formation of a cyclic imine.
11 a) A. M. Baranger, P. J. Walsh, R. G. Bergman, J. Am.
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In summary, we have found that PdMo3S4 cubane clusters
Published on the web (Advance View) April 29, 2006; doi:10.1246/cl.2006.590