Tributylphosphine-catalyzed acylation of alcohol by active ester directed toward effective end-capping of pseudorotaxane consisting of ammonium group and crown ether

Article abstract of DOI:10.1246/cl.2002.924

According to the results of a model study using benzyl alcohol, pseudorotaxane with terminal hydroxy group on the axle was acylated with S-2-pyridyl 3,5-dimethylthiobenzoate in the presence of tributylphosphine to produce rotaxane in 85% yield. Optically

Full text of DOI:10.1246/cl.2002.924

9
24
Chemistry Letters 2002
Tributylphosphine-catalyzed Acylation of Alcohol by Active Ester Directed toward
Effective End-capping of Pseudorotaxane Consisting of Ammonium Group and Crown Ether
ꢀ
ꢀ
Nobuhiro Kihara, Naohisa Nakakoji, and Toshikazu Takata
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531
(Received June 17, 2002; CL-020505)
According to the results of a model study using benzyl
alcohol, pseudorotaxane with terminal hydroxy group on the axle
was acylated with S-2-pyridyl 3,5-dimethylthiobenzoate in the
presence of tributylphosphine to produce rotaxane in 85% yield.
Optically active [2]rotaxane and [3]rotaxane were readily
synthesized by this method.
Scheme 1.
[
2]Rotaxane 1 was prepared in 85% yield from ammonium
salt 2 and 2-pyridylthio ester 3 in the presence of dibenzo-24-
The end-capping of pseudorotaxane is one of the most easily
accessible methods to prepare rotaxanes consisting of secondary
ammonium salt and crown ether.1 Although various synthesiz-
ing strategies have been proposed, limitations have arisen
stemming from the fact that the reaction must be carried out
crown-8 (DB24C8) and a catalytic amount of tributylphosphine
4
(Scheme 2). The reaction proceeded more slowly than that of the
model system, presumably because of steric hindrance of
;2
DB24C8. The NMR and IR spectra of 1 were identical to those
1
reported elsewhere. Since the yield by the active ester method is
comparable with that by the acid anhydride method (90%), the
method constitutes an effective alternate for the simple and
1
without the neutralization of the ammonium group. We have
reported that the tributylphosphine-catalyzed acylation of the
hydroxy group with acid anhydride is a highly effective and
5
practical preparation of rotaxanes.
1
practical end-capping method. However, this method cannot be
applied to rotaxanes with functional groups that are incompatible
with acid anhydride: for instance, the preparation of acid
anhydride of amino acids is accompanied by racemization.
Further, since acid anhydride has two reactive points, half of the
anhydride is thrown away during the acylation. Thus, the
acylation method is unsuited for the preparation of higher
interlocked compounds such as [3]rotaxane. We found that these
limitations could be overcome by the use of active ester as
acylating agent. In this paper, we report that the acylation of
alcohol with 2-pyridylthio ester takes place under neutral
conditions and constitutes an effective means of producing
rotaxanes that could not be prepared using the acid anhydride
method.
While Busch reported the preparation of a rotaxane via the
3
end-capping of amine with an active ester, the end-capping of a
hydroxy group poses great difficulty because amines, which are
considered necessary for esterification with active esters, are
incompatible with secondary ammonium salt-based rotaxane
Scheme 2.
1
The effectiveness of the active ester method can be
demonstrated by the synthesis of some functional rotaxanes and
interlocked compounds having higher topology. First, 2 and 2-
pyridylthio ester 4 derived from l-phenylalanine were reacted in
the presence of DB24C8 and tributylphosphine to produce
synthesis. Thus, various catalysts for esterification under neutral
conditions were examined in a model system. Reactions of benzyl
alcohol with active esters of benzoic acid were carried out in
dichloromethane at room temperature. Although most of the
active esters, such as phenyl, p-nitrophenyl, succinimidoyl, and
phenylthio ester, showed no or faint acylation activity without
triethylamine, 2-pyridylthio ester quantitatively acylated benzy-
lalcohol within 3 h in the presence of tributylphosphine as
catalyst. TMSOTf, silver nitrate, triphenylphosphine, 1,2-bis(di-
phenylphosphino)ethane, triethyl phosphite, and triphenyl phos-
phite showed no catalytic activity. Thus, the combination of 2-
pyridylthio ester and tributylphosphine appears suitable for the
preparation of various interlocked compounds consisting of
crown ether and secondary ammonium salt by acylative end-
capping.
6
optically active [2]rotaxane 5 in a high yield (Scheme 3).
Second, 2 was reacted with bifunctional active ester 6 in the
7
presence of DB24C8 to generate [3]rotaxane 7 in a good yield
(
Scheme 4). It should be pointed out that [3]rotaxane cannot be
obtained by the acid anhydride method because acid anhydride
has two equally reactive carbonyl groups.
In summary, we developed the tributylphosphine-catalyzed
esterification of alcohol with 2-pyridylthio ester, and prepared
higher functional rotaxanes using this novel acylation reaction.
Further applications using these higherfunctional rotaxanes are
under development.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
925
2
a) N. Kihara and T. Takata, J. Syn. Org. Chem., Jpn., 59, 206
(
2
2001). b) T. J. Hubin and D. H. Busch, Coord. Chem. Rev.,
00–202, 5 (2000). c) T. Takata and N. Kihara, Rev. Heteroat.
Chem., 22, 197 (2000). d) M. C. T. Fyfe and J. F. Stoddart,
Adv. Supramol. Chem., 5, 1 (1999). e) F. M. Raymo and J. F.
Stoddart in ‘‘Templated Organic Synthesis,’’ ed. by F.
Diederich and P. J. Stang, Wiley-VCH, Weinheim (2000),
p 75. f) F. M. Raymo and J. F. Stoddart in ‘‘Molecular
Catenanes, Rotaxanes and Knots,’’ ed. by J.-P. Sauvage and
C. Dietrich-Buchecker, Wiley-VCH, Weinheim (1999),
p 143. g) Y. Furusho, T. Hasegawa, A. Tsuboi, N. Kihara,
and T. Takata, Chem. Lett., 2000, 18.
3
4
A. G. Kolchinski, D. H. Busch, and N. W. Alcock, J. Chem.
Soc., Chem. Commun., 1995, 1289.
Typical procedure: To a solution of 40 mg (0.10 mmol) of 2
and 47 mg (0.105 mmol) of DB24C8 in dichloromethane
Scheme 3.
(
3
0.2 mL) were added 36 mg (0.15 mmol)of active ester 3 and
ꢀL (0.01 mmol) of tributylphosphine, and the reaction
mixture was allowed to stand at room temperature for 5 days.
The reaction mixture was purified by preparative HPLC
(eluent: CHCl3) to isolate 83 mg (85%) of 1 as a colorless
solid.
5
Potentially basic 3 and 2-mercaptopyridine formed during the
reaction may disturb pseudorotaxane formation from 2 and
DB24C8. Thus, 3 and 2-mercaptopyridine were added to a
CDCl3 solution of 2 and DB24C8 to evaluate the effect of the
1
additives. Since no change was observed in the H-NMR
spectra of the pseudorotaxane by the addition, neither 3 nor 2-
mercaptopyrdine disturbs hydrogen-bonding between 2 and
DB24C8 that stabilize the pseudorotaxane.
1
6
7
H NMR (270 MHz, CDCl3): ꢁ 7.58 (br, 2H), 7.4–7.1 (m,
1
1
2
2H), 6.9–6.7 (m, 8H), 5.10 (d, J ¼ 13:2 Hz, 1H), 5.02 (br,
H), 5.00 (d, J ¼ 13:2 Hz, 1H), 4.7–4.6 (m, 3H), 4.5–4.4 (m,
H), 4.2–4.0 (m, 8H), 3.8–3.7 (m, 8H), 3.47 (s, 8H), 3.1–3.0
(
1
m, 2H), 2.15 (s, 6H), 1.39 (s, 9H); IR (KBr): 3151, 2927,
À1
Scheme 4.
743, 1714, 1506, 1254, 841, 557 cm ; FAB-MS (m-NBA):
25
þ
ꢂ
We acknowledge financial supports by a Grant-in-Aid for
Scientific Researchon Priority Areas (A) (No. 11133258) from
the Ministry of Education, Science, Sports and Culture and a grant
from The Association for the Progress of New Chemistry.
m=z 951.5 ([M-PF6] ); ½ꢂꢁ 34.2 (c 0.1, CH2Cl2).
D
1
H NMR (270 MHz, CDCl3): ꢁ 8.09 (d, J ¼ 8:5 Hz, 4H), 7.58
(br, 4H), 7.37 (d, J ¼ 8:1 Hz, 4H), 7.26 (d, J ¼ 8:1 Hz, 4H),
7.10 (d, J ¼ 8:5 Hz, 4H), 6.9–6.7 (m, 22H), 5.26 (s, 4H), 4.6–
4.5 (m, 4H), 4.5–4.4 (m, 4H), 4.2–4.0 (m, 16H), 3.9–3.7 (m,
16H), 3.46 (s, 16H), 2.14 (s, 12H); IR (KBr): 3273, 1718,
References and Notes
1
À1
H. Kawasaki, N. Kihara, and T. Takata, Chem. Lett., 1999,
015.
1595, 1504, 1248, 1109, 834, 557 cm ; FAB-MS (m-NBA):
þ
1
m=z 1630.7 ([M-2PF6] ).