Catalyst-free syntheses of [2]rotaxanes utilizing a pentacoordinated hydrosilane as an end-capping agent

Article abstract of DOI:10.1021/ol100786a

A pentacoordinated hydrosilane activated by an intramolecular nitrogen-silicon dative bond was utilized as an end-capping agent for catalyst-free syntheses of [2]rotaxanes. The end-capping reaction of a pseudo[2]rotaxane bearing a salicylic acid terminus

Full text of DOI:10.1021/ol100786a

ORGANIC
LETTERS
2010
Vol. 12, No. 11
2586-2589
Catalyst-Free Syntheses of [2]Rotaxanes
Utilizing a Pentacoordinated Hydrosilane
as an End-Capping Agent
Yuya Domoto,† Akihiro Fukushima,‡ Yousuke Kasuga,‡ Shohei Sase,†
Kei Goto,*,† and Takayuki Kawashima*,‡
Department of Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551,
Japan, and Department of Chemistry, Graduate School of Science,
The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
goto@chem.titech.ac.jp; takayuki@chem.s.u-tokyo.ac.jp
Received April 6, 2010
ABSTRACT
A pentacoordinated hydrosilane activated by an intramolecular nitrogen-silicon dative bond was utilized as an end-capping agent for catalyst-
free syntheses of [2]rotaxanes. The end-capping reaction of a pseudo[2]rotaxane bearing a salicylic acid terminus with the pentacoordinated
hydrosilane readily proceeded at room temperature to produce the corresponding silyl-capped [2]rotaxane.
Extensive efforts have been devoted to developing efficient
syntheses of mechanically interlocked molecules by utilizing
a variety of synthetic methods.1 Among them, the reactions
featuring high chemoselectivity and broad functionality
tolerance, such as the Huisgen 1,3-dipolar cycloaddition2 and
olefin metathesis,3 have been successfully applied to the
construction of various interlocked architectures, promoting
rapid progress in this field. To date, most of the reactions
employed in the syntheses of interlocked molecules involve
the use of catalysts or activating agents. In view of
simplifying purification procedures as well as atom economy,
a new synthetic method that demands no such additive is
still desired. Here, we report the efficient syntheses of
[2]rotaxanes that can be performed without loading any
catalyst and activating agent.
Scheme 1. Catalyst-Free Reactions of 1 with Oxygen
Nucleophiles
† Tokyo Institute of Technology.
‡ The University of Tokyo.
(1) (a) Arico´, F.; Badjic, J. D.; Cantrill, S. J.; Flood, A. H.; Leung, K. C.-
F.; Liu, Y.; Stoddart, J. F. Top. Curr. Chem. 2005, 249, 203–259. (b)
Haussmann, P. C.; Stoddart, J. F. Chem. Rec. 2009, 9, 136–154, and
references cited therein.
ˇ
(2) (a) Aprahamian, I.; Miljanic´, O. S; Dichtel, W. R.; Isoda, K.; Yasuda,
T.; Kato, T.; Stoddart, J. F. Bull. Chem. Soc. Jpn. 2007, 80, 1856–1869,
and references cited therein. (b) Spruell, J. M.; Dichtel, W. R.; Heath, J. R.;
Stoddart, J. F. Chem.sEur. J. 2008, 14, 4168–4177.
10.1021/ol100786a  2010 American Chemical Society
Published on Web 05/04/2010

Scheme 2. Synthesis of Axle Molecules
Highly coordinated hydrosilanes4 bearing an intramolecu-
lar dative bond between silicon and donor (nitrogen or
oxygen) atoms exhibit high reactivity toward nucleophiles.5
Recently, we have demonstrated that the neutral pentacoor-
dinated hydrosilane 16a smoothly reacts with phenol and
benzoic acid derivatives in the absence of any catalyst and
activating agent to produce the corresponding silyl ether and
silyl benzoates, respectively, with evolution of hydrogen as
the sole side product (Scheme 1).6b We also reported that
the reaction of 1 with benzoic acid derivatives, especially
salicylic acid, is much faster than that with phenol.6b This
type of reaction involving 1 is expected to be useful for the
syntheses of interlocked molecules under additive-free condi-
tions. To probe the utility of the reaction between 1 and
oxygen nucleophiles, it was applied to the end-capping step
for [2]rotaxane syntheses.
As axle molecules, we prepared two kinds of ammonium
salts 5·B(C6F5)4 and 10·B(C6F5)4 bearing a phenol and a
salicylic acid terminus, respectively (Scheme 2). Preparation
of 5·B(C6F5)4 began with 3,5-dimethylbenzoic acid (2), which
was converted via a chlorination-amination-reduction
sequence to amine 4. Treatment of 4 with hydrochloric acid
followed by anion exchange to tetrakis(pentafluorophe-
nyl)borate afforded 5·B(C6F5)4 in moderate yield. The axle
10·B(C6F5)4 was synthesized from the Boc-protected amine
6, which was easily obtained from 4. Introduction of a
salicylic acid moiety was accomplished via arylmethylation
of 6 with bromide 77 to give 8 in excellent yield. After
removal of the acetonide moiety of 8 under basic conditions,
deprotection of the Boc group with hydrochloric acid
followed by anion exchange afforded the ammonium salt
10·B(C6F5)4.
Scheme 3. Synthesis of [2]Rotaxanes 12·B(C6F5)4 and 14·B(C6F5)4 Utilizing Pentacoordinated Hydrosilane 1 as End-Capping Agent
Org. Lett., Vol. 12, No. 11, 2010
2587

With two types of axle molecules in hand, the synthesis
of rotaxanes utilizing 1 as an end-capping agent was
examined (Scheme 3). Solutions of pseudo[2]rotaxanes
11·B(C6F5)4 and 13·B(C6F5)4 were prepared by treatment of
the corresponding axle molecules with dibenzo-24-crown-8
(DB24C8) in chloroform. Slow addition of 11·B(C6F5)4 to
an equimolar amount of 1 in chloroform over 4 h under reflux
conditions, followed by continual refluxing for 24 h, afforded
the corresponding [2]rotaxane 12·B(C6F5)4 with a silyl ether
terminus. This rotaxane was formed in 78% yield, as
after heating for 24 h. This result indicates that disiloxane
15 also acts as an end-capping agent, although the reactivity
of 15 is much lower than that for 1 (Scheme 4). Involvement
of 15 in the formation of 12·B(C6F5)4 was confirmed by the
following control experiment: treatment of 15 with
11·B(C6F5)4 in 1,2-dichloroethane under reflux conditions
afforded 12·B(C6F5)4 (Scheme 5).
1
Scheme 5. Synthesis of 12·B(C6F5)4 from Disiloxane 15
estimated by H NMR, and isolated as a colorless solid in
57% yield. In contrast with the reaction of 11·B(C6F5)4 that
required heating, the end-capping of pseudorotaxane
13·B(C6F5)4 bearing a salicylic acid terminus with 1 pro-
ceeded much faster under milder conditions. The reaction
of 2 molar equiv of 1 with 13·B(C6F5)4 in chloroform was
completed within 1 h at room temperature, yielding [2]ro-
taxane 14·B(C6F5)4 with a silyl salicylate terminus in 75%
isolated yield. In neither case was addition of any catalyst
or activating agent necessary, which is due to the high
electrophilicity of the silicon atom in 1 enhanced by the
intramolecular coordination. Although both 12·B(C6F5)4 and
14·B(C6F5)4 have a pentacoordinated silicon atom, these
[2]rotaxanes were found to be stable in wet chloroform at
room temperature for 2 days.
The difference in the reactivity of 1 toward 11·B(C6F5)4
and 13·B(C6F5)4 is consistent with our previous obser-
vation;6b the reaction of 1 with phenol took 13 h under reflux
conditions in chloroform, while that with salicylic acid was
completed within 10 min at room temperature. Interestingly,
monitoring of the reaction progress by 1H NMR spectroscopy
during these rotaxane syntheses indicated that the formation
mechanisms of 12·B(C6F5)4 and 14·B(C6F5)4 are different.
In the case of the synthesis of 12·B(C6F5)4, the rotaxane was
formed only in ca. 30% yield just after the addition of
11·B(C6F5)4 to 1. At that moment, no unreacted 1 was
observed in the 1H NMR spectrum, and instead, a consider-
able amount of disiloxane 15 was detected, which was
presumably generated by the hydrolysis of 1 followed by
dehydrative condensation (Scheme 4). After further heating
In sharp contrast, monitoring of the reaction between 1
and 13·B(C6F5)4 by 1H NMR spectroscopy revealed that the
resonances due to [2]rotaxane 14·B(C6F5)4 immediately
appeared upon mixing both compounds at room temperature.
This observation confirms that the end-capping to form
14·B(C6F5)4 is achieved directly by the reaction between 1
and 13·B(C6F5)4. The reaction rate of 1 with 13·B(C6F5)4 is
comparable to that of 1 with salicylic acid, indicating that
the existence of the interlocked moiety does not significantly
affect the reactivity of the salicylic acid functionality in
13·B(C6F5)4. It is worth noting that this rapid end-capping
reaction can be performed at room temperature under
additive-free conditions. Recently, a rotaxane synthesis
utilizing copper-free 1,3-dipolar cycloaddition between an
azide and a strained bicyclic alkene has been reported.8
However, this reaction requires heating and longer reaction
time.
In summary, the efficient end-capping syntheses of [2]ro-
taxanes have been achieved by utilizing the high reactivity
of a pentacoordinated hydrosilane toward oxygen nucleo-
(3) (a) Wisner, J. A.; Beer, P. D.; Drew, M. G. B.; Sambrook, M. R.
J. Am. Chem. Soc. 2002, 124, 12469–12476. (b) Coumans, R. G. E.;
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Chem., Int. Ed. 2003, 42, 650–654. (c) Hannam, J. S.; Kidd, T. J.; Leigh,
D. A.; Wilson, A. J. Org. Lett. 2003, 5, 1907–1910. (d) Lilbinger, A. F. M.;
Cantrill, S. J.; Waltman, A. W.; Day, M. W.; Grubbs, R. H. Angew. Chem.,
Int. Ed. 2003, 42, 3281–3285. (e) Vignon, S. A.; Jarrosson, T.; Iijima, T.;
Tseng, H.-R.; Sanders, J. K. M.; Stoddart, J. F. J. Am. Chem. Soc. 2004,
126, 9884–9885. (f) Suzaki, Y.; Osakada, K. Dalton Trans. 2007, 2376–
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Lett. 2005, 7, 2129–2132. (h) Guidry, E. N.; Li, J.; Stoddart, J. F.; Grubbs,
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Scheme 4
. Mechanism for the Formation of [2]Rotaxane
12·B(C6F5)4 under Heated Conditions
(4) For reviews, see: (a) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young,
J. C. Chem. ReV. 1993, 93, 1371–1448. (b) Holmes, R. R. Chem. ReV. 1990,
90, 17–31. (c) Holmes, R. R. Chem. ReV. 1996, 96, 927–950.
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Royo, G. J. Organomet. Chem. 1986, 311, C39–C43. (b) Arya, P.; Corriu,
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(6) (a) Saruhashi, K.; Goto, K.; Kawashima, T. Chem. Heterocycl.
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of the reaction mixture, the amount of 12·B(C6F5)4 was
gradually increased along with the decrease of 11·B(C6F5)4
and 15, and the yield of 12·B(C6F5)4 finally reached 78%
(8) Gassensmith, J. J.; Barr, L.; Baumes, J. M.; Paek, A.; Nguyen, A.;
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Org. Lett., Vol. 12, No. 11, 2010

philes such as phenol and salicylic acid. This end-capping
procedure requires no catalyst and activating agent. Further-
more, the end-capping involving a salicylic acid functionality
enabled a quick synthesis of a [2]rotaxane at room temper-
ature. This procedure, which is performable under mild and
additive-free conditions, is expected to serve as a promising
tool for constructing a wide range of interlocked structures.
Sports, Science and Technology, Japan. K.G. is grateful to
Tokuyama Science Foundation for financial support. We also
thank Tosoh Finechem Corp. for the generous gifts of
alkyllithiums.
Supporting Information Available: Experimental pro-
cedures for the preparation of 4-10, disiloxane 15, and
rotaxanes 12·B(C6F5)4 and 14·B(C6F5)4. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was partly supported by the
Global COE Program (Chemistry) and Grants-in-Aid for
Scientific Research from the Ministry of Education, Culture,
OL100786A
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