Efficient synthesis of rotaxane based on complexation of acetylene and dicobalt hexacarbonyl: Introduction of a transformable functional group

Article abstract of DOI:10.1246/cl.2006.766

An efficient end-capping method for preparation of rotaxanes by complexation of dicobalt hexacarbonyl and acetylene moiety of the axle end is described here. The end-capping reaction of pseudorotaxanes proceeds under mild conditions and furnishes the expected rotaxanes bearing a transformable functional group in high yield. Copyright

Full text of DOI:10.1246/cl.2006.766

766
Chemistry Letters Vol.35, No.7 (2006)
Efficient Synthesis of Rotaxane Based on Complexation of Acetylene
and Dicobalt Hexacarbonyl: Introduction of a Transformable Functional Group
Yuji Tokunaga,ꢀ1 Go Ohta,1 Yuji Yamauchi,1 Tatsuhiro Goda,1 Nobuhiko Kawai,1 Takumichi Sugihara,2 and Youji Shimomura1
1Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, Bunkyo, Fukui 910-8507
2Faculty of Pharmaceutical Sciences, Niigata University of Pharmacy and Applied Life Sciences, Kamishinei-cho, Niigata 950-2081
(Received April 12, 2006; CL-060437; E-mail: tokunaga@matse.fukui-u.ac.jp)
An efficient end-capping method for preparation of rotax-
1)
-
OHC
PF6
anes by complexation of dicobalt hexacarbonyl and acetylene
moiety of the axle end is described here. The end-capping reac-
tion of pseudorotaxanes proceeds under mild conditions and fur-
nishes the expected rotaxanes bearing a transformable functional
group in high yield.
Ar
NH2
+
2
Ar
N
H2
2) NaBH4
3) HCl; NH4PF6
1
R
3
R
O
O
O
O
O
O
Rotaxanes, which consist of a macrocycle and dumbbell-
like axle, have received much attention as a component of mo-
lecular machines. Many methods for rotaxane synthesis have
been developed, and various functionalities to rotaxanes has
been introduced.1 Recently, end-capping or clipping approaches
to rotaxanes and the subsequent stopper-modification have been
reported. Transformation of phosphonium groups as a stopper
part of the axle into bulky alkenes by the Wittig reaction was
completed by Stoddart,2 and Leigh et al. succeeded in replace-
ment of a mechanically interlocking auxiliary via transesterifica-
tion reaction.3 Kihara et al. also reported the exchange of a stop-
per part by the Tsuji–Trost allylation reaction.4 However, these
methods have been capable of each accepting only one type of
reaction until now. Therefore, it might be difficult to introduce
various functionalities into a main skeleton of rotaxane using
the same methodology.
Co2(CO)8
O
O
DB24C8
O
O
-
O
O
O
O
PF6
+
Ar
N
H2
Co(CO)3
Co(CO)3
O
O
R
4
The use of the bimetallic alkyne complex has prevailed in
the literature on organic synthesis and organometallic chemistry
for a couple of decades. The complexes are commonly exploited
for unique reactivity. Among these examples, the Pauson–Khand
reaction,5 a familiar reaction that employs the acetylene–Co2-
(CO)6 complex, is useful to construct cyclopentenone. The Nich-
olas reaction6 using acetylene–Co2(CO)6 complex as a substrate
is a powerful method introducing functionalities at the ꢀ-posi-
tion of acetylene. Furthermore, hydrosilylation and decomplex-
ation of acetylene–Co2(CO)6 complex7 and a utility of the com-
plex as a protective group of acetylene8 have also been reported.
This paper describes an efficient method for preparation of rotax-
anes using acetylene–Co2(CO)6 complexation as a primary step
of end-capping and the subsequent stopper-modification of ro-
taxane.
Self-assembly formed from secondary ammonium salt and
crown ether is known to be effective for construction of rotax-
anes.1a,1b,1f,9 Synthesis of the requisite ammonium salts 3 used
in this procedure begins with condensation of aldehydes 1
and benzylamines 2 to generate the corresponding imines
(Scheme 1). Reduction of the imines, followed by salt formation,
then produces the key intermediates, ammonium salts 3, which
possess bulky aryl groups on one end and alkyne moieties at
the other.
Scheme 1.
with dicobalt octacarbonyl in the presence of two equivalents of
dibenzo[24]crown8 (DB24C8) afforded the corresponding ro-
taxane 4a10 in good yield. Typical results for various acetylenes
3 are shown in Table 1. High-temperature conditions and using
CH2Cl2–CH3CN (1:1) as a solvent, which enfeeble the hydrogen
bonding between the crown and the ammonium ion, decrease
the rotaxane yield (Runs 1–3). Other reactions are therefore
performed at 0 ꢁC in dichloromethane, even though the reaction
mixtures are heterogeneous. The neighboring steric effects of
substrates were observed. Monosubstituted acetylenes, including
2-propyn-1-ol derivative 5,11 give the corresponding rotaxanes
in excellent yields (Table 1, Runs 1, 4, 5, and 9). In contrast, di-
substituted substrates afforded products 4 in moderate yields
(Table 1, Runs 6–8).12
The association constants of pseudorotaxanes are examined
using 3a and 3d as axles to obtain more information on the reac-
tion. The 1H NMR titration (500 MHz, CD3CN–CDCl3 (1:1),
25 ꢁC) of equimolar mixtures of 3 and DB24C8 (10 mM) reveals
formation of pseudorotaxanes whose sharp signals are observed
independently. Integrations of the complexes and ammoniums 3
afford the apparent association constants Kexp,
13 respectively, as
in eq 1. The Kexp of the ammonium salt 3a, monosubstituted
acetylene, is 860 ꢂ 190 Mꢃ1. That of disubstituted acetylene
3d is 1700 ꢂ 440 Mꢃ1. Because these values are not related to
Synthesis of rotaxanes by complexation of acetylene and di-
cobalt octacarbonyl was investigated. Treatment of acetylene 3a
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
767
Table 1. Synthesis of rotaxanes by complexation of dicobalt
hexacarbonyl and acetylenesa
Grant-in-Aid for Scientific Research on Encouraged Areas
(No. 15750116) from Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Run
1
Axle
Product Yield/%b
References and Notes
99 (61)c
66 (44)d
45 (40)e
94 (63)c
3a: Ar = 3,5-dimethylphenyl,
4a
4a
4a
4b
1
a) V. Balzai, A. Credi, F. M. Raymo, J. F. Stoddart, Angew.
Chem., Int. Ed. 2000, 39, 3348. b) R. Ballardini, V. Balzani,
A. Credi, M. T. Gandolfi, M. Venturi, Acc. Chem. Res.
2001, 34, 445. c) A. Harada, Acc. Chem. Res. 2001, 34,
R = H
2
3
4
3a: Ar = 3,5-dimethylphenyl,
R = H
3a: Ar = 3,5-dimethylphenyl,
456; C. A. Schalley, K. Beizai, F. Vogtle, Acc. Chem. Res.
¨
R = H
2001, 34, 465. d) J.-P. Collin, C. Dietrich-Buchecker, P.
Gavina, M. C. Jimenez-Molero, J.-P. Sauvage, Acc. Chem.
Res. 2001, 34, 477. e) J.-P. Sauvage, C. Dietrich-Buchecker,
Molecular Catenanes, Rotaxanes and Knots, Wiley-VCH,
Weinheim, 1999. f) T. Takata, N. Kihara, Rev. Heteroat.
Chem. 2000, 22, 197.
3b: Ar = 4-tert-butylphenyl,
R = H
89 (53)c
78 (55)c
5
6
3c: Ar = 9-anthryl, R = H
4c
3d: Ar = 3,5-dimethylphenyl,
4d
2
S. J. Rowan, J. F. Stoddart, J. Am. Chem. Soc. 2000, 122, 164;
S. J. Rowan, S. J. Cantrill, J. F. Stoddart, A. J. P. White, D. J.
Williams, Org. Lett. 2000, 2, 759; S.-H. Chiu, S. J. Rowan,
S. J. Cantrill, L. Ridvan, P. R. Ashton, R. L. Garrell, J. F.
Stoddart, Tetrahedron 2002, 58, 807; S. J. Rowan, J. F.
Stoddart, Polym. Adv. Technol. 2002, 13, 777.
R = CH3
77 (59)c
78 (30)c
7
8
3e: Ar = 3,5-dimethylphenyl,
4e
4f
R = phenyl
3f: Ar = 9-anthryl, R = phenyl
+
Ar
N
3
4
5
J. S. Hannam, S. M. Lacy, D. A. Leigh, C. G. Saiz, A. M. Z.
Slawin, S. G. Stitchell, Angew. Chem., Int. Ed. 2004, 43, 3260.
N. Kihara, S. Motoda, T. Yokozawa, T. Takata, Org. Lett.
2005, 7, 1199.
For recent reviews on Pauson–Khand reaction, see: K. M.
Brummonda, J. L. Kentb, Tetrahedron 2000, 56, 3263; A. J.
Fletcher, S. D. R. Christie, J. Chem. Soc., Perkin Trans. 1
2000, 1657; Y. K. Chung, Coord. Chem. Rev. 1999, 188,
297; T. Sugihara, M. Yamaguchi, M. Nishizawa, Chem.
Eur. J. 2001, 7, 1589; L. V. R. Bon˜aga, M. E. Krafft, Tetra-
hedron 2004, 60, 9795.
H2
82 (63)c
9
5:
4g
-
PF6
OH
Ar = 3,5-dimethylphenyl
1
aAll new compounds were identified by H and 13C NMR, MS,
and IR spectra. bDetermind by 1H NMR spectra of crude products.
( ): isolated yield. Reactions were conducted at 0 ꢁC–room tem-
perature using 2 equiv of DB24C8 and 1.2 equivalents of dicobalt
octacarbonyl. cReactions were performed at 0 ꢁC in CH2Cl2.
e
dA reaction was performed at room temperature in CH2Cl2. A
6
7
For recent reviews on Nicholas reaction, see: K. M. Nicholas,
Acc. Chem. Res. 1987, 20, 207; B. J. Teobald, Tetrahedron
2002, 58, 4133.
K. Kira, H. Tanda, A. Hamajima, T. Baba, S. Takai, M. Isobe,
Tetrahedron 2002, 58, 6485; S. Hosokawa, M. Isobe, Tetra-
hedron Lett. 1998, 39, 2609.
reaction was performed at 0 ꢁC in CH2Cl2–CH3CN (1:1).
rotaxane yields (Table 1, Runs 1 and 6), the yields depend on the
reactivity of pseudorotaxanes consisting of DB24C8 and the
ammonium ion, not the percentage of pseudorotaxane, that is,
the difference of reactivity between the ammonium ion and the
pseudorotaxane might be important.14
8
9
For example, see: D. Seyferth, M. O. Nestle, A. T. Wehman,
J. Am. Chem. Soc. 1975, 97, 7417, and references given.
P. R. Ashton, P. J. Campbell, E. J. T. Chrystal, P. T. Glink,
S. Menzer, D. Philp, N. Spencer, J. F. Stoddart, P. A. Tasker,
D. J. Williams, Angew. Chem., Int. Ed. 1995, 34, 1865.
½pseudorotaxaneꢄ
½ammonium salt þ ammonium ionꢄ½DB24C8ꢄ
Kexp
¼
ð1Þ
Recently, Takata et al. reported an end-capping synthesis of
rotaxane using Ru-catalyzed hydrosilylation of acetylene.15
However, they were unable to synthesize the rotaxane using
acetylene 3a as an axle part because of the neighboring bulkiness
of DB24C8 of pseudorotaxane. Accordingly, the long spacer
part between ammonium and the acetylene moieties needed to
achieve synthesis of the rotaxane by their hydrosilylation meth-
od. This kind of rotaxane synthesis might result in the reactivity
of metal complexes (catalysts) to the acetylene moiety of pseu-
dorotaxane, and might be sensitive to the neighboring bulkiness
because of large ligands of the complexes.
10 See Supporting Information.
11 The ammonium salt 5 was synthesized from 3,5-dimethyl-
benzylamine and the corresponding aldehyde, which was ob-
tained from terephthalaldehyde and ethynylmagnesium bro-
mide.
12 The 1H NMR spectra of crude products of disubstituted
acetylenes revealed formation of the other products, which
might be produced by the reaction of 3 and dicobalt octacar-
bonyl. On the other hand, the certain signals were not ob-
served in 1H NMR spectra of crude products of monosubstitut-
ed acetylenes.
In conclusion, these results demonstrate that the reaction of
dicobalt octacarbonyl and acetylene of pseudorotaxanes affords
rotaxanes in high yields. This new technique for synthesis of
various rotaxanes is being explored through our ongoing studies
in this area.
13 F. Huang, J. W. Jones, C. Slebodnick, H. W. Gibson, J. Am.
Chem. Soc. 2003, 125, 7001.
14 The association constant of the ammonium salt 3c was also
examined; the value of Kexp is 6400 ꢂ 1400 Mꢃ1
.
15 H. Sasabe, N. Kihara, K. Mizuno, A. Ogawa, T. Takata,
Tetrahedron Lett. 2005, 46, 3851; H. Sasabe, N. Kihara, K.
Mizuno, A. Ogawa, T. Takata, Chem. Lett. 2006, 35, 212.
This work was performed with the financial support of a
Published on the web (Advance View) June 10, 2006; doi:10.1246/cl.2006.766