DOI: 10.1002/anie.201007752
Supramolecular Catalysis
Controlled Assembly of Chiral Tetraaminophosphonium Aryloxide–
Arylhydroxide(s) in Solution**
Daisuke Uraguchi, Yusuke Ueki, and Takashi Ooi*
The chemistry at the interface between homogeneous catal-
ysis and supramolecular chemistry, namely supramolecular
catalysis, has attracted sustained attention over the last
decade.[1] In a departure from early endeavors to create
preorganized molecular receptors as an artificial enzyme, the
current approach toward understanding and mimicking
enzymatic catalysis has focused on the design of systems
capable of spontaneously generating well-defined, functional
supramolecular architectures by self-assembly. The inner
spaces of these assemblies serve as an isolated reaction
vessel that can confine the substrates, thus delivering other-
wise unattainable reactivity and selectivity.[2] In parallel, new
possibilities of supramolecular catalysis have been demon-
strated by the marriage of multicomponent assemblies with
homogeneous transition-metal catalysis and organic molec-
ular catalysis since the seminal studies of Breit, von Leeuwen,
and Reek in 2003.[3] We recently reported that chiral P-spiro
triaminoiminophosphorane 1 and three equivalents of aryl-
hydroxides (ArOH) spontaneously assembled into the highly
organized molecular structure 1·[ArOH]3 by the formation of
anion equivalent. Although the discrete three-dimensional
structure of 1·[PhOH]3 in solid state was determined by
single-crystal X-ray diffraction analysis, the actual behavior of
this type of supramolecular catalyst in solution remains an
important issue to be resolved. This situation prompted us to
conduct a series of spectroscopic analyses of a solution of 1
and 2 in an organic solvent, which revealed not only the
effectiveness of the low-temperature 31P NMR spectroscopy
measurement for tracing the solution structure, but also
unexpected, yet intriguing, phenomenon regarding the mode
of molecular association. Herein, we disclose the stepwise and
exclusive generation of three types of molecular assemblies,
1·[2]n (n = 1–3), in solution by simply adjusting the stoichi-
ometry of 2. Each structure was unequivocally verified in the
solid state by X-ray crystallographic analysis. The finding that
the mode of the spontaneous assembly of 1 and 2 can be
precisely controlled in solution suggests the possibility of
selective use of 1·[ArOH]n (n = 1–3) as a requisite catalyst for
target organic transformations and could also provide a
expedient means to gain insights into the structural integrity
of the reactive intermediate.
an
ion-pair-assisted
hydrogen-bonding
network
(Scheme 1).[4,5] Furthermore, the resultant network 1·
[ArOH]3, and particularly that with a 3,5-dichlorophenol
component (3,5-Cl2C6H3OH; 2), exerted efficient cooperative
catalysis in the stereoselective conjugate addition of an acyl
Initially, 31P NMR spectroscopic analysis of isolated 1·[2]3
was performed in toluene (10.0 mm) at À988C, and a sharp
*
singlet was observed at d = 32.4 ppm (Figure 1a, ). To assess
the validity of assigning this signal to 1·[2]3 even in solution,
the stoichiometry in generating the same signal in situ from
iminophosphorane 1 and 2 was probed by varying the amount
of 2 to 1; this method led to some rather surprising yet
interesting observations. For a toluene solution of 1 (Fig-
~
ure 1b, ) treated with a half equivalent of 2, two separate
signals were detected, at d = 45.0 and 35.5 ppm (Figure 1c).
&
The new signal that appeared upfield ( ) was essentially
different from that observed in the case of 1·[2]3. As this signal
grew as a single peak at d = 35.4 ppm upon introducing one
equivalent of 2 (Figure 1d), it could be assigned to 1·[2]1, that
is, the simplest ion pair of aminophosphonium cation 1·H with
aryloxide (Figure 2a). Furthermore, treatment of 1 with one
and a half equivalents of 2 resulted in the appearance of a new
upfield signal with a decrease in the resonance corresponding
to 1·[2]1 (Figure 1e), and it became an only detectable peak at
d = 33.9 ppm when the amount of 2 was increased to two
Scheme 1. Chiral P-spiro tetraaminophosphonium aryloxide assembly
1·[ArOH]3.
[*] Dr. D. Uraguchi, Y. Ueki, Prof. Dr. T. Ooi
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University
Furo-cho B2-3(611), Chikusa, Nagoya 464-8603 (Japan)
Fax: (+81)52-789-3338
E-mail: tooi@apchem.nagoya-u.ac.jp
!
equivalents (Figure 1 f, ). This signal was still different from
that observed in the case of 1·[2]3, and it could correspond to
!
another molecular assembly such as 1·[2]2 ( , Figure 2b).
[**] This work was supported by the Global COE program in Chemistry
of Nagoya University, Grants of JSPS for Scientific Research, the
Ogasawara Foundation for the Promotion of Science & Engineer-
ing, the Asahi Glass Foundation, and the Takeda Science Founda-
tion. Y.U. acknowledges the JSPS for financial support.
Indeed, a similar tendency was observed when two and a half
equivalents of 2 were added (Figure 1g), and the spectrum
measured after the treatment of 1 with three equivalents of 2
*
showed a sharp singlet at d = 32.4 ppm (Figure 1h, ), which
was identical to the chemical shift observed for isolated 1·[2]3.
Consequently, there seems to exist a stepwise equilibrium
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2011, 50, 3681 –3683
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3681