Rational design of chiral nanoscale adamantanoids

Article abstract of DOI:10.1021/ol005781n

(equation presented) Utilizing coordination as a motif, the self-organization of six ditopic and four tritopic building blocks leads to the formation of nanoscale adamantanoid frameworks.

Full text of DOI:10.1021/ol005781n

ORGANIC
LETTERS
2
000
Vol. 2, No. 9
255-1257
Rational Design of Chiral Nanoscale
Adamantanoids
1
Manuela Schweiger, S. Russell Seidel, Marion Schmitz, and Peter J. Stang*
Department of Chemistry, UniVersity of Utah, 315 S. 1400 E.,
Salt Lake City, Utah 84112
stang@chemistry.utah.edu
Received March 9, 2000
ABSTRACT
Utilizing coordination as a motif, the self-organization of six ditopic and four tritopic building blocks leads to the formation of nanoscale
adamantanoid frameworks.
4
Self-assembly via rational design has led to a fascinating
variety of two- and three-dimensional supramolecular struc-
tures. Such species are formed and held together by the
noncovalent or dative interactions of predesigned building
blocks. One approach exploits transition-metal-mediated self-
organizing processes, in which the coordination of suitable
The hydrocarbon adamantane has been well-known since
its discovery in petroleum residues in the 1930s. Its
framework is composed of four six-membered rings that are
fused nearly strain-free in their thermodynamically most
favored chair conformations. With its crystal lattice consisting
of an infinite adamantanoid network, the high-pressure
1
5
precursors serves as a viable recognition motif. This
carbon allotrope diamond is also structurally related to this
methodology allows for the preparation of complex, nano-
compound.
2
scopic supramolecules of predetermined shape, size, and
To date, the only examples of self-assembled adaman-
tanoids have been reported by Saalfrank, et al. These
3
6
functionality.
systems exploit chelating effects between bidentate angular
subunits and various alkaline earth and transition metals. In
our approach, supramolecular adamantanoids are designed
(
1) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100,
8
1
3
53. (b) Caulder, D. L.; Raymond, K. N. J. Chem. Soc., Dalton Trans.
999, 8, 1185. (c) Caulder, D. L., Raymond, K. N. Acc. Chem. Res. 1999,
2(11), 975. (d) Chambron, J.-C.; Dietrich-Buchecker, C.; Sauvage, J.-P.
Transition Metals as Assembling and Templating Species. In ComprehensiVe
Supramolecular Chemistry; Lehn, J.-M., Chair Ed.; Atwood, J. L., Davis,
J. E. D., MacNicol, D. D., V o¨ gtle, F., Exec. Eds.; Pergamon Press: Oxford,
1a
according to the “Molecular Library” model. By combining
2
six angular ditopic units (A ), such as 1, with four angular
3
2
3
6 4
tritopic units (A ), such as 2, an A A entity arises. Provided
1
996; Vol. 9, Chapter 2, p 43. (e) Baxter, P. N. W. Metal Ion Directed
that the angles are all approximately 109°, an adamantanoid
Assembly of Complex Molecular Architectures and Nanostructures. In
ComprehensiVe Supramolecular Chemistry; Lehn, J.-M., Chair Ed.; Atwood,
J. L., Davis, J. E. D., MacNicol, D. D., V o¨ gtle, F., Exec. Eds.; Pergamon
Press: Oxford, 1996; Vol. 9, Chapter 5, p 165. (f) Fujita, M.; Ogura, K.
Coord. Chem. ReV. 1996, 148, 249. (g) Fujita, M. Chem Soc. ReV. 1998, 6,
(4) Fort, R. C.; Schleyer, P. R. Chem. ReViews 1964, 64, 277.
(5) Angus, J. C.; Hayman, C. C. Science 1988, 4868, 913.
(6) (a) Saalfrank, R. W.; Stark, A.; Peters, K.; von Schnering, H. G.
Angew. Chem., Int. Ed. Engl. 1988, 27, 851. (b) Saalfrank, R. W.; Stark,
A.; Bremer, M.; Hummel, H.-U. Angew. Chem., Int. Ed. Engl. 1990, 29,
311. (c) Saalfrank, R. W.; Burak, R.; Breit, A.; Herbst-Irmer, R.; Daub, J.;
Porsch, M.; Bill, E.; M u¨ ther, M.; Trautwein, A. X. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1621. (d) Saalfrank, R. W.; Burak, R.; Reihs, S.; L o¨ w, N.;
Hampel, F.; Stachel, H.-D.; Lentmaier, J.; Peters, K.; Peters, E.-M.; von
Schnering, H. G. Angew. Chem., Int. Ed. Engl. 1995, 34, 993. (e) Saalfrank,
R. W.; L o¨ w, N.; Demleitner, B.; Stalke, D.; Teichert, M. Chem. Eur. J.
1998, 4, 1305.
4
17. (h) Saalfrank, R. W.; Bernt, I.; Uller, E.; Hampel, F. Angew. Chem.,
Int. Ed. 1997, 36, 2482. (i) Baxter, P. N. W.; Lehn, J.-M.; Baum, G.; Fenske,
D. Chem. Eur. J. 1999, 5, 102. (j) Stang, P. J.; Olenyuk, B. Acc. Chem.
Res. 1997, 30, 502.
(
2) Philp, D.; Stoddard, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35,
154.
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b) Ibukuro, F.; Kusukawa, T.; Fujita, M. J. Am. Chem. Soc. 1998, 120,
561.
1
(
(
8
1
0.1021/ol005781n CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/08/2000

framework results. As building block 2 has a stereogenic
center in its carbon backbone, its assembly with tecton 1
leads to the first optically active adamantanoid. Moreover,
Scheme 2. Preparation of Chiral, Tritopic 109° Tecton
7
this aggregate serves as the first example of an adamantanoid
to have a nanoscale internal cavity.
To test the limits of the structural design and the allowable
flexibility of the ditopic building block, the reaction of tecton
3
with tritopic linker 2 was probed. Although 3, with its 120°
angle, does not fulfill the ideal requirements, the deviation
is apparently not substantial enough to prevent the self-
organization of the subunits into an adamantanoid, as
spectroscopic data reveal.
Platinum containing linker 1 is prepared as shown in
Scheme 1. A halogen exchange is carried out by reacting
and can be monitored by multinuclear NMR. Of particular
1
31
1
diagnostic value are the H and P{ H} NMR spectra of 6
and 7, which indicate the formation of highly symmetrical
structures. Furthermore, in both cases the relative shifts of
these resonances are comparable in both direction and
absolute value. The protons in the positions R and â to the
pyridine-N both exhibit downfield shifts due to the loss of
electron density upon coordination. The shifts for the
R-pyridyl protons were 0.19 ppm for 6 and 0.26 ppm for 7,
while the resonances of the â-pyridyl protons experience a
stronger influence with shifts of 0.66 ppm in the case of 6
Scheme 1. Synthesis of Ditopic 109° Corner Unit
Scheme 3. Self-Assembly of the Adamantanoids
8
2
,2-bis(4-bromophenyl)propane with tert-butyllithium and
I
2
to yield compound 4. Via 2-fold oxidative addition,
3 4
Pt(PEt ) is inserted (5), followed by subsequent reaction with
AgOTf. As a result, bistriflate 1 can be isolated in good yield.
Precursor 3 has been prepared according to previously
published literature.9
To synthesize an adamantanoid, a complementary tritopic
3
subunit A is required. Compound 2, which fills this role, is
1
0
synthesized by reacting lithiated tris(4-pyridyl)methanol
11
with 2-(R)-phenylbutyryl chloride. The corresponding chiral
ester 2 emerges in 25% yield as depicted in Scheme 2.
2 2
By combining dilute CD Cl solutions of 1 or 3 with 2 in
the proper 6:4 ratio, adamantanoids 6 and 7 form, respec-
tively, in quantitative yield as determined by NMR (Scheme
3). The self-organization of these species is instantaneous
(7) (a) Stang, P. J.; Olenyuk, B.; Muddiman, D. C.; Smith, R. D.
Organometallics 1997, 16, 3094. (b) Fuss, M.; Siehl, H.-U.; Olenyuk, B.;
Stang, P. J. Organometallics 1999, 18, 758. (c) M u¨ ller, C.; Whiteford, J.
A.; Stang, P. J. J. Am. Chem. Soc. 1998, 120, 9827. (d) Fan, J.; Whiteford,
J. A.; Olenyuk, B.; Levin, M. D.; Stang, P. J.; Fleischer, E. B. J. Am. Chem.
Soc. 1999, 121, 2741.
(8) Van Walree, C. A.; Roest, M. R.; Schuddeboom, W.; Jenneskens, L.
W.; Verhoeven, J. W.; Warman, J. M.; Kooijman, H.; Spek, A. L. J. Am.
Chem. Soc. 1996, 118, 8395.
(
9) Leininger, S.; Schmitz, M.; Stang, P. J. Org. Lett. 1999, 1, 1921.
(10) Oleynuk, B.; Levin, M. D.; Whiteford, J. A.; Shield, J. E.; Stang,
P. J. J. Am. Chem. Soc. 1999, 121, 10434.
11) Jones, P. R.; Goller, E. J.; Kauffman, W. J. J. Org. Chem. 1971,
6, 3311.
(
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Org. Lett., Vol. 2, No. 9, 2000

and 0.74 ppm in that of 7. As a result of increased back-
donation from the platinum to the phosphine moiety, the
protons of the methylene groups bonded to the phosphorus
show almost identical upfield shifts, 0.34 and 0.33 ppm, for
31
1
both compounds 6 and 7, respectively. The P{ H} signals
themselves are shifted upfield by 5.6 ppm (JPt-H ) 2652
Hz) for 6 and 5.3 ppm (JPt-H ) 2670 Hz) for 7 relative to
starting materials 1 and 3. 13C{ H} and F NMR spectra
1
19
are both consistent with the proposed structure.
Electrospray ionization mass spectrometry has proven to
be an indispensable tool in the corroboration of structural
1
2
assignments for self-assembled aggregates of this nature.
Both species 6 and 7 show similar peak and fragmentation
patterns, lending further evidence to their structural similarity.
Peaks attributable to the consecutive loss of triflate counter-
3+
ions, [M - 3OTf] {m/z ) 3107 for 6 (calcd 3105) and 3079
for 7 (calcd 3077)}, [M - 4OTf]4 {m/z ) 2293 for 6 (calcd
+
5
+
2
292) and 2272 for 7 (calcd 2271)}, and [M - 5OTf]
{m/z ) 1805 for 6 (calcd 1804) and 1788 for 7 (calcd 1787)},
where M represents the intact adamantanoid cage, were
detected. Furthermore, several fragments could be identified,
with the most dominant among them consisting of a +1
charged 1:1 composition of 2 and 1 or 3 minus one OTf
Figure 1. MM2 force field simulation of adamantanoid 6.
{m/z ) 1616 for 6 (calcd 1615) and 1601 for 7 (calcd 1601)}.
expected for achiral adamantanoids. The diameter of the inner
When combined with the information obtained from NMR
measurements, this mass spectral data provides strong support
for the proposed structures of 6 and 7.
Both 6 and 7 should be optically active, given that they
each contain four of the chiral building blocks 2. Optical
rotation studies indicate that this is indeed the case. More-
cavity of the cage along one of the C -axes is approximately
3 nm.
2
In conclusion, herein we report the first self-assembled
adamantanoid cages that are chiral and in the nanoscale
range. These molecular architectures represent further unique
members in the growing family of large supramolecular
entities, illustrating the versatility of an angle-directed,
transition-metal-mediated approach.
D
over, the molarity-based specific rotation, [φ] , is enhanced
compared to that of starting unit 2 (-122) with values of
-
261 for 6 and -1168 for 7.
As suitable X-ray quality crystals could not be obtained,
Acknowledgment. We thank the National Institute of
Health (5R01GM57052) for support, the Austrian Foundation
for Scientific Research (FWF) for an Erwin Schr o¨ dinger
Fellowship for Manuela Schweiger, and the Alexander von
Humboldt Foundation for a Feodor Lynen Fellowship for
Marion Schmitz.
MM2 force field simulations13 were employed to visualize
both the size and shape of these three-dimensional cages.
The calculated structure of adamantanoid 6 is displayed in
Figure 1. As a result of the stereogenic centers present in
the backbone of the ester moiety, the symmetry of 6 and 7
2 d
is reduced, with D symmetry as opposed to the T symmetry
Supporting Information Available: General experimen-
tal procedures and characterization data for compounds 6
and 7. This material is available free of charge via the Internet
at http://pubs.acs.org.
(
12) (a) Whiteford, J. A.; Lu, C. V.; Stang, P. J. J. Am. Chem. Soc. 1997,
1
19, 2524. (b) Stang, P. J.; Cao, D. H.; Chen, K.; Gray, G. M.; Muddiman,
D. C.; Smith, R. D. J. Am. Chem. Soc. 1997, 119, 5163. (c) Constable, E.
C.; Schofield, E. J. Chem. Soc., Chem. Commun. 1998, 403.
(13) Chem3D Pro 3.5.2; CambridgeSoft Corporation: U.S., 1996.
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