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Self Assembly Through Hydrogen Bonding Preparation of a Supramolecular Aggregate Composed of Ten Molecules.

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rac4 a-f
rac-7a (R = Ph)
rac-7b (R = Me)
Scheme 2. Reactions of the glycine derivative 1b with electrophiles (only one
enantiomer shown). After addition of one equivalent of the electrophile the
yellow solution of the Li compound C decolorizes. Boc = ieri-butyloxycarbonyl. Compounds 4 are specified in Table 1.
sible stereoisomers (35: 1 and 7.6: 1 , respectively, according
to NMR analysis). Again the depicted products having trans
configuration are formed; the hydrolysis product of 7 b correlates to ah-threonine. Similar results are obtained when
Z-protected alkoxydihydroimidazoles 1 a and 1c are used
(Z = benzyloxycarbonyl).
Table 1 Alkylation of 1b with alkyl halides. Only one diastereomer is formed
according to analysis by N M R spectroscopy. The yields given refer to chromatographically purified (SiO,. penlanejether) samples
Prod. RX
Yield [%I
Prod. RX
5 [a]
6 [bl
Yield [Yo]
[a] Prepared from 4e. [b] Prepared from 4b.
As expected, hydrolysis of the dihydroimidazoles to give
r-amino acid methyl esters is very easy (Scheme 3) and provides pure products directly, as evidenced by NMR spectroscopy. The mild hydrolysis conditions employed should
allow for the synthesis of many sensitive amino acid derivatives previously not accessible to us,r3,43for example those
containing double bonds, three-membered rings, and other
acid-labile groups in the side chain, as well as racemizationprone aryl glycines.
1) CF,CO,H
4e, 4d, 5, 7b
/CH2Clz(l:l), I h , RT
2) solvent removed
3) 0.2N aq HCI, 0.25-3h, RT
70 -85%
For the preparation of enantiomerically pure alkoxydihydroimidazoles 1 a and 1 b, the enantiomers of the stable
imidazolidinone 2 were separated by crystallization of their
diastereoisomeric salts. Camphorsulfonic acid (CSA), available in both enantiomeric
turns out so far to be best
suited for this.["' A preliminary experiment using enantiomerically enriched (S)-1b17c1(87 % ee) gave the L-valine
methyl ester after isopropylation and hydrolysis with 81 Yo
ec. Further investigations with enantiomerically pure material are currently in progress.
Since the imidazolidinones 2 and 3 are readily available,
we now have a route to the glycyl glycine building block 8 in
enantiomerically pure form.[8n1Previous work has demonstrated the utility of racemic 8.18']
Received: June 24. 1993 [Z6166IE]
German version: Angew. C/vni.1993, 105. 1780
[ l ] For an overview that I S still up to date see: R. M. Williams. Sjwhcsi.s of
Op/icuiiy A c i i ~r-Ainino A ~ TPergamon,
Oxford, 1989.
[2] U. Schollkopf. Asynmevic .s~nr/iesse.%
i'iu hrteroq ciic iniermed!ure.s, in. Orgunic S~ntlitsiv,m i Intrrdw rp/innr:r CiiuNeng~:5th IUPAC Symposium on
Organic Synthesis, Freiburg I. Br.. 1984;J. Streith. H. Prinzbach. G. Schtll.
Blackwell, Oxford. 1985, pp. 101-112; K. Busch, U. M. Groth. W. Kiihnle.
U. Schollkopf. Girdiedrun 1992. 48. 5607-5618.
131 For more recent discussions and comparison of different heterocycles B see:
K . Suzuki. D. Seebach, Lkhig.%Ann. C/zrni. 1992. 51-61: D. Seebach. T.
Gees, F. Schuler. ihiri. 1993, 785.
[4] The noncovalently hound chiral "auxiltary" in this case is a commercial
carboxylic acid or the stationary phase used for chromatography. R. Fit&
D. Seebach. E~iiuhedron1988, 44. 5277-5292 (there refs. [16,17,25]): D.
Seebach. S. G . Miiller, U. Gysel, J. Zimmermann. Hdi'. Chhn. Acru 1988,
71. 1303-1318; D. Blaser. D. Seebach. L R h i g ~Ann. Ciieiii. 1991. 1067-1078:
W. Miiller. D. A. Lowe. H. Neijt, S. Urwyler. P. L. Hurling. D. Blaser. D.
Seebach, Hell). C/iini. A<.[u1992, 75, 855-864.
[5] Pivalaldehyde now is aVdlkibk from the Shell Chemical Corporation.
161 The ci.5 configuration of the benzyl and the tert-butyl groups in 5 has been
established by X-ray analysis.
[7] a) (IS)-CSA purchased commercially costs about $40 per mol. the ( 1 R )
isomer about $100 per mol. b) The id salt is less soluble in acetone/
methanol: we obtained 2 98% enantiomerically pure ( S ) -and (R)-2(each
in 20% yield) from the crystalltne solid and the mother liquor after two
recrystalhrations. c) An X-ray analysis of ( R ) - 2 . ( S ) - C S Awas obtained.
[8] a) Reaction of 3 with bromoacetic acid esters gives 8 quantitatively. b) R.
Polt, D. Seebach. Heir. C/lIn7. Actu 1987. 70. 1930.1936; R. Polt, D. Seebach. J. A n ] . Chmi. So<. 1989. 111, 2622-2632.
Self-Assembly Through Hydrogen Bonding :
Preparation of a Supramolecular Aggregate
Composed of Ten Molecules**
By John P. Mathias, Eric E. Simanek, Christopher 7: Seto,
and George M. Whitesides*
Learning how to control the association of many molecules into single, highly-structured supramolecular aggregates is a current objective in molecular self-assembly.~'-51
In this communication we report the self-assembly of a
supramolecular aggregate based on a compound containing
[*] Prof. G. M. Whitesides. Dr. J. P. Mathias, E. E. Simanek. Dr. C. T. Seto
Department of Chemistry, Harvard University
12 Oxford Street, Cambridge, MA 02138 (USA)
Telefax: Int. code + (617)495-9857
m.p. 121°C
[a],= -20.1
(C = 0.8, CHCI,)
m.p. 121°C
[a],= +20.9
(C = 0.8, CHCI,)
[ale = -9.3
(C = 1.5, CHCIJ
Scheme 3. Transformation of dihydroimidazoles 4a.d. 5. and 7b [o provide
r-amino acid methyl esters.
This work was supported by the National Science Foundation (Grants
CHE-91-22331 to G.M.W. and DMR-89-20490 to the Harvard University
Materials Research Laboratory). N M R instrumentation was supported by
the National Science Foundation Grant CHE-88-14019 and the National
Institutes of Health Grant 1 S10 RR 4870. Mass spectrometry was performed by Dr. A. Tyler. The Harvard University Mass Spectrometry Facility was supported by The National Science Foundation Grant CHE-9020043 and The National Institutes of Health Grant 1 S10 RR 06716-01
J P. M. was an SERCINATO Postdoctoral Fellow, 1991- 1993.
nine melamine rings (M) that we have called Hub(MMM),
(2). This molecule associates with nine equivalents of
neohexylisocyanurate (neohex(CA) 3) to form a hydrogenbonded supramolecular aggregate of composition Hub(MMM), . 9neohex(CA) (4).This aggregate is composed of
ten molecules in three parallel CA, . M, "rosettes",[61 and is
stabilized by 54 hydrogen bonds.
The nonamelamine derivative 2 was synthesized by using
the procedure shown in Scheme 1. This synthesis extends a
strategy we have described previously.[', ' I The m-xyIyl spacers between adjacent melamine rings in each arm of 2 match
those we have used in aggregates based on two parallel
CA, . M, rosettes.[3.
H 37
1) CF3COzH/CHzCI~0+ 25°C
2) 1,3,5-C,H3(COCI),
"fully eclipsed"
"fully staggered"
Hub(MMM), SNeohex(CA) 4
+ 14 additional
Schcinc 1. Synthesis of Hub(MMM), (2)
Scheme 2. Self-assembly of Hub(MMM), . 9neohex(CA) (4). The conformational isomers shown are just two of at least 16 possibilities.
A homogeneous solution of Hub(MMM), . Bneohex(CA)
(4)was prepared by mixing one equivalent of 2 and nine
equivalents of 3 in chloroform (Scheme 2). We have ennumerated at least 16 different geometrical conformers as being possibilities for 4.19*We believe that the aggregate exists
initially as a mixture of the many available conformers on
mixing the two different components. The conversion of this
mixture to a single supramolecular aggregate of composition
4 occurs over about 48 h at room temperature (or 1 min at
reflux) in chloroform, as judged by 'H N M R spectroscopy.
The supramolecular aggregate 4 was characterized by
' H N M R spectroscopy (COSY, NOE, and NOESY experiments), gel permeation chromatography (GPC), and vapor
pressure osmometry (VPO). The ' H N M R spectrum of 4
(Fig. 1 a) shows a sharp set of resonances that can be assigned to a single conformer of the supramolecular aggregate. These sharp resonances contrast with the broad, poorly-defined resonances observed in the spectrum obtained
from uncomplexed 2 in CDCI, (Fig. 1c); the broadening of
the resonances in this spectrum may reflect hindered rotation
about amide bonds in 2 and/or self-association of 2. Even in
a strong hydrogen-bonding solvent such as dimethylsulfoxide (DMSO), however, there is little detail in the 'H N M R
spectrum of 2. The spectrum in Figure 1 b shows the aggregate immediately after mixing the components 2 and 3. The
progression from a mixture of conformers of 4 (Fig. 1 b) to
a single conformer (Fig. 1 a) is clear in these 'H N M R spectra. The observation of discrete resonances for the different
conformers confirms that exchange between them is slow on
the N M R time scale. The six resonances with equal intensities ( 0 , in Fig. 1 a) between 6 = 14 and 16 correspond to
three sets of the two unsymmetrical hydrogen-bonded isocyanurate protons seen in each CA, . M, rosette in 4. The
observation of four discrete singlets (v, Fig. 1 a) between
6 =1.8 and 2.1 for the four methyl substituents [(a)-(d),
Scheme 21 provides further support for the proposed structure of 4. Cooling the sample has no effect on the signals
observed in the 'H N M R spectrum. This feature strengthens
the inference that the resonances observed a t 298 K in
Figure l a belong to a single conformer and not to a
rapidly-equilibrating mixture of conformations.['01Nuclear
Overhauser effects between the imide protons on the isocyanurate molecules (3) and those of the melamine rings in 2 confirm the geometry of the hydrogen-bonded regions, and are
consistent with the structure that we propose for 4. We do
not see NOE interactions between protons in adjacent
CA, . M, rosettes. This feature means that we cannot deduce,
unambiguously, which of the possible conformational
isomers is the thermodynamically preferred (observed) one.
(a) Hub(MMM),
. Sneohex(CA)
in these traces is a consequence of dissociation of the aggregate that occurs during the analysis by GPC, and more tailing is seen in CHCI, than in CH,CI,. This difference indicates that the stability of 4 is lower in CHCl,, the stronger
hydrogen-bonding solvent, than it is in CH,CI, .I1 Although dissociation occurs during analysis of 4, the GPC
results establish that the dissociation is slow and indicate,
therefore, that 4 is surprisingly stable.
The molecular weight m for 4 has been obtained by VPO
in chloroform. using four different molecular weight standards.f’2J Each standard gives an observed m that is within
15% of the calculated value of 4 (6.435 kDa). Observed
molecular weights of 4 are 5.6 kDa (standard: N,N’-bistert-butoxycarbonyl-gramicidin S). 6.4 kDa (sucrose octaacetate), 6.5 kDa (polystyrene), 7.5 kDa (perbenzoyl-pcyclodextrin).
Addition of only three equivalents of 3 to one equivalent
of 2 leads to formation of only fully assembled 4; excess 2
remains uncomplexed. We d o not observe any intermediates
single layer
(such as Hub(MMM), . 3neohex(CA)-the
aggregate %or Hub(MMM), . 6neohex(CA)-the double
layer aggregate 6) on the pathway between 2 and 4. This
observation indicates that the self-assembly of 4 displays
positive cooperativity.
4 (t = 48 h)
(b) Hub(MMM), . Sneohex(CA) 4 (t = 0 h)
(c) Hub(MMM),
1 6 1 5 1 4 1 3
- 6
Fig. 1. ‘H N M R spectra (500 MHz. CDCI,) o f 4 (a) after equilibration and (h)
on initial mixing of the components. The spectrum of uncomplexed 2 is shown
in (c).
Retention times and shapes of the peaks for 4 (hatched) in
CHCI, and CH,CI, as the eluent are consistent with observations from other self-assembled aggregates (Fig. 2). In
each trace, p-xylene (shaded peak) was used as an internal
standard. The trace in CH,CI, shows a single peak for the
aggregate. The trace in CHCI, shows a much broader peak
for the aggregate with a larger degree of “tailing” toward
longer retention time. In each case, the peaks for the aggregate have sharp leading edges. This feature indicates that the
solutions d o not contain stable self-assembled o r associated
supramolecular aggregates that are larger than 4. The tailing
The self-assembly of ten molecules into a single
supramolecular aggregate (4) that is stabilized by fifty four
hydrogen bonds demonstrates further the potential of
molecular self-assembly as a stategy for the preparation of
well-defined chemical nanostructures. In particular, these results illustrate that parallel hydrogen-bonded CA, . M,
rosettes are a structural motif that is well-suited to the preparation of large, structurally complex supramolecular aggregates.
4 in CH,CI,
Experimental Procedure
4 in CHCI,
polystyrene in CH,CI,
t [rnin]
Fig. 2. Top and middle: Gel permeation chromatograms of 4 (hatched peaks).
The shaded peaks are p-xylene. which is used as an internal standard. Bottom:
Gel permeation chromdtogram of polystyrene (PS) (F.W. 5050).
VCH V c r l u ~ . ~ ~ e s e l l s ~mbH,
l i n f t 0-69451 Weinhem,IY93
Hub(MMM), (2): TriflUOrodCetiC acid (2.5 mL) was added dropwise to a solution of 1 (451 mg, 0.29 mmol) in CH,CI, (10 mL) at 0 ’C. The reaction mixture
was warmed to 25 C and stirred for 2 h. This solution was diluted with toluene
(20 mL) and concentrated in vacuo. The residue was partitioned between
EtOAc (50 mL) and aqueous Na,CO, ( 5 % solution, 25 mL). The organic extract was washed with aqueous Na,CO, ( 5 % solution, 25 mL), brine
(2 x 25 mL), dried over MgSO,, filtered, and concentrated in vacuo to give
402 mg (0.276 mmol, 95%) of the deprotected amine as a white foam [high-res([M + HI’) 1454.0920. found:
olution FAB-MS: m/zcalcd for C,,H,,,N,,O,:
1454.09591.This amine (399 mg. 0.274 mmol) was dissolved in CH,CI, (10 mL)
and di-isopropylethylamine (DIPEA) (0.19 mL) and the solution was cooled to
0 “ C .I .3,5-Benzene tricarbonyl chloride (24.2 mg, 0.091 3 mmol) was added and
the solution was allowed to warm to 25 -C. After 90 mins, the reaction mixture
was diluted with CH,CI, (25 mL) and washed with aqueous Na,CO, ( 5 %
20 mL), brine (2 x 35 mL). dried over MgSO,. filtered, and concentrated in vacuo. The residue was purified by column chromatography (eluted
with a solution of 7 . 5 % NH,OH/MeOH in CH,CI, [7.5:92.5 v:v]) to give
342 mg (0.0757 mmol. 83%. two steps) of the product (2) as a white solid.
correct elemental analysis.
Received: July 12, 1993 [Z 6207 IE]
German version: Angrw. Cliem. 1993, f05, 1848
$ 10.00 + ,2510
Angel!.. Cliem. Int.
Ed. Engl. 1993. 32. N o . 12
[ l ] G . M Whitesides.J. P.Mathias.C.T.Seto,Scrc,nc.e1991.254. 1312-1319.
[2] C. T. Seto. G. M. Whitesides, J. An?. Cliem. Soc. 1993, 115. 905-916.
[3] C. T. Seto. J. P. Mathias. G . M. Whitesides. J Ain. Clrein. Sot. 1993. 115.
1321 li29.
[4] C. T. Seto. G. M. Whitesides, J Am. Chrm. Soc. 1993, 115. 1330-1340.
[5] a ) P. Baxter. J.-M. Lehn, A. DeCian, J. Fischer. Angew. Chem. 1993, 105.
92-96: ,'fl7gPii. C h w i i . I ~ Ed.
I . D i g / . 1993. 32. 69-72: b) S. Bonazzi. M. M.
DeMorais. G. Gottarellt. P. Mariani, G. P. Spada. hid. 1993. 105, 251253 brw 1993.32.240-25O;c) S. J. Geib. C. Vicent, E. Fan. A. D. Hamilton. I / J U / . 1993. f05.83-85 and 1993,32. 119-121; d) S. C. Zimmerman.
B. F. Duerr. J. Orx. Chrni. 1992. 57. 2215-2217: e) J. F Stoddart et al.
Sj.i7hvc. 1992. 914-918. 919-922. 923- 926; f ) E. C. Constable. fiwuhrh.on 1992. 48, 10013- 10059.
[6] J. A. Zerkowski. C. T. Seto. G. M. Whitesides. J. An?. C/7en7. Soc. 1992.
114. 5413 - 5475.
[7] C. T. Seto. G M. Whitesides, J Am. Clrrm. SOL..1990, 112. 6409-6411.
[X] C. T. Seto. G. M. Whitesides, J Am. Chern. So(.. 1991. 113. 712-713.
[9] I n principle, there are least four conformations in which the linker arm can
join the uppermost melamine ring in 4 to the central benzene "hub' 121.
Additionally. adjacent melamine rings in each arm of the Hub(MMM),
unit can lie in eclipsed or staggered conformations, resulting in a total of
at l e i i h t 16 discrete conformers.
[lo] Supi-amoleculai-aggregates based on a single CA, . M, rosette often exist
as mixtures of different geometrical isomers. The exchange between these
structures can be slowed to reveal the separate isomers by ' H N M R at
temperatures below ambient: M. Wdzeer, J. P. Mathias. E. E. Simanek.
G. M . Whitesides. unpublished results.
I l l ] The traces from 4 in the GPC are significantly broader than those of
previously reported double-layer aggregates. such as that between the
hexamelamine derivative Hub(MM), and six equivalent of neohexylisocyanurate. Hub(MM), . 6neohex(CA). The reduction in stability for4 suggested by this observation places this aggregate close to the lower limit of
stability that can be observed successfully by GPC.
(121 Chloroform was Aldrich HPLC grade. No attempt was made to monitor
its moisture content during analysis by VPO.
aqueous NaOH along two different paths: the first path
involves the degradation of the heterocycle AH to give
RNCS and azide; the RNCS is subsequently hydrolyzed to
the corresponding amine. The second path involves the isomerization of the thiatriazole AH to the thioxotetrazolyl
anion (B-). Path 1 is favored when an excess of O H - ions
and H,O is used. I n contrast, our recent study"' showed
that in the presence of HMPA ligands, the use of stoichiometric amounts of solid Ba(OH), and AH prevent the hydrolysis step in path 1, and indeed, constructively favor path
2: the rearrangement increases the distance of the R group
(naphthyl) from the (N=C--S)unit which chelates the
large Ba(HMPA):' fragment.
Here we report a third, and rather more spectacular, pathway for the deprotonation of aminothiatriazoles AH: extrusion of N, and S at ambient temperature lead to metal complexes of the anion of R-N(H)-C=N. Thus, in this way
complex 1, whose solid-state structure-the first of a lithiat(PhNCNLi HMPA),
The First Structure of a Lithiated Cyanamide;
Synthesis of (PhNCNLi HMPA), by Extrusion
of N, and S from 5-Phenylamino-1,2,3,4thiatriazole with Li Reagents and HMPA **
By David R. Armstrong, E: Adele Banbury, Ian Cragg-Hine,
Matthew G. Davidson, Francis S . Mair, Ehinke Pohl,
Paul R. Raithby, and Ronald Snuith"
We described recently the product resulting from the reaction of solid Ba(OH), with aminothiatriazole (AH; R =
naphthyl) and HMPA [O=P(NMe,),] dissolved in toluene!']
Instead of the expected aqua complex, (A),Ba . 2 H , O . s
HMPA."' a rearrangement occurs to give the thioxotetrazole derivative, (B) ,Ba . 3 HMPA, i n 60 O/Q yield. This
rearrangement has been known for a long timeL3]as has the
fact that compounds of the type AH react with excess
Dr. R . Snaitli. F. A. Banbury. I. Cragg-Hine. M. G. Davidson.
Dr. E S. Mair, Dr. P. R. Raithby
University Chemical Laboratory
Lensfield Road. GB-Cambridge, CB2 1EW (UK)
Telefax: Int. code + (223) 336-362.
Dr. D. R . Armstrong
Department of Pure and Applied Chemistry
University of Strathclyde. GB-Glasgow. G1 IXL (UK)
E. Pohl
lnstitut fur Anorganische Chemie der Universitzt
Tammanstrasse 4. D-37077 Gottingen (FRG)
This work was supported by the Science and Engineering Research Council (SERC) (quota award. F.A.B.), the SERC and Associated Octel (CASE
awards. I . C-H.. M.G.D.), Queens' College, Cambridge and Associated
Octel (Research Fellowship. F.S.M.), and St. John's College. Cambridge
(Research Fellowship, M.G.D.). We thank Professor G. M. Sheldrick
(University of Gottingen) for providing access to crystallographic facilities.
A n ~ e i i . .Chcnr. In/. Ed. En,?/. 1993. 32. No. I2
ed cyanamide-was determined, was formed from AH (R =
phenyl). Deprotonations of 5-(phenylamino)-l,2,3,4-thiatriazole (AH; R = Ph) were performed with a range of lithium
bases. In general, equimolar amounts of AH and HMPA are
dissolved in toluene at -78 "C, and then one equivalent of
the chosen base (LiOH, MeOLi, LiNH,, iPr,NLi as solids;
nBuLi as a solution in hexane; see the Experimental Procedure for specific details) is added. Warming the mixture to
room temperature causes dissolution of any solid, usually
accompanied by visible gas evolution. Refrigeration of the
resulting solution gives crystals of 1 (20-60% yield) except
when the base used for the reaction is solid LiOH. In this
case crystals of (B)Li . H,O . HMPA are isolated initially;
that is those containing the rearranged, rather than the extruded, anion. However, subsequent treatment of these crystals with additional LiOH/HMPA, or with nBuLi/HMPA,
affords 1. Interestingly, the reaction of BH with nBuLi/HMPA affords (B)Li. HMPA rather than 1. The most immediate evidence for an extrusion product [(RNCN), . M"' . y
ligand] is the appearance of a strong v(N-C'-N)stretch
in the IR spectrum; for 1, this band appears at 2147 cm-'.
The products of the reactions of AH (R = naphthyl) with
HMPA and all the above-mentioned lithium bases show
a similar absorption. However, treatment of HMPA and
AH (R = Ph) in toluene with the highly reactive system
Ba/Ba(NH,), (obtained by evaporating a solution of metallic Ba in liquid NH,) affords a product having no such band.
In this case the rearrangement product (B) ,Ba . 3 HMPA
(R = Ph) is formed. This product is akin to that [R = naphthy1 in B] obtained from a similar reaction in which rather
unreactive solid Ba(OH), is used.['] Thus it seems that formation of extrusion products is dependent on the metal atom
attached to the base (switch from Ba to Li) rather than on the
R group (switch from R = naphthyl to R = Ph in AH), or on
a change in the general reactivity of the metalating reagent
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preparation, hydrogen, bonding, self, assembly, composer, supramolecular, molecules, ten, aggregates
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