Self-Folding MetallocaVitand
J. Am. Chem. Soc., Vol. 123, No. 41, 2001 9931
Scheme 1
here the synthesis and host-guest properties of a self-folding,
metal-containing cavitand 6, a metallocaVitand. The structure
of 6 combines a deepened (8 × 10 Å) self-folding cavity and
a Lewis acidic metal center presented to its interior (Figure 2).
The arrangement offers an additional binding site and a well-
positioned, potentially catalytic functionality.9
Spectroscopic Features. Extensive FTIR and NMR spec-
troscopic studies, single-crystal X-ray analysis and molecular
modeling,8,11 have established that the upper rim of self-folding
cavitands and their derivatives feature a seam of intramolecular
1
hydrogen bonds. The H NMR spectra of 6 and 9 are sharp in
deuterated, chlorinated solvents at room temperature (Figure 3),
and modeling13 suggests that its amides are stitched in a seam
of five intramolecular hydrogen bonds within a vase-like
structure. A unique hydrogen bond is formed between the
benzimidazole NH and its neighboring amide CdO oxygen. All
amide NH resonances are singlets for 6 and 9 and they are
situated downfield of 8 ppm in CD2Cl2 as expected for strong
hydrogen bonding; the benzimidazole NH is seen at ∼12 ppm.
The head-to-tail seam results in two cycloenantiomers,14 with
clockwise and counterclockwise orientation of the HN-CO
bonds, and the interconversion between those is apparently slow
Results and Discussion
Synthesis (Scheme 1). Condensation of 1,10-phenanthroline-
2-carbaldehyde 710 with previously reported diamine cavitand
811 in hot nitrobenzene as solvent and oxidant gave metal-free
cavitand 9 in 40% yield. Compound 9 was quantitatively
converted to metallocavitand 6 with ZnCl2 in boiling CHCl3/
MeOH, (4:1).12 The simpler Zn-phenanthroline complex 10
was likewise prepared for comparison purposes.
1
on the NMR time scale. The H NMR spectra indicated that
(9) Several other cavities functionalized with metal centers are known.
Cyclodextrins: (a) Kuroda, Y.; Hiroshige, T.; Sera, T.; Shiroiwa, Y.; Tanaka,
H.; Ogoshi, H. J. Am. Chem. Soc. 1989, 111, 1912-1913. (b) Kuroda, Y.;
Ito, M.; Sera, T.; Ogoshi, H. J. Am. Chem. Soc. 1993, 115, 7003-7004.
(c) Breslow, R.; Dong, S. D. Chem. ReV. 1998, 98, 1997-2011. (d)
Engeldinger, E.; Armpach, D.; Matt, D. Angew. Chem., Int. Ed. 2001, 40,
2526-2529. (e) Armpach, D.; Matt, D.; Kyritsakas, N. Polyhedron 2001,
20, 663-668. Steroid-capped porphyrins: (f) Bonar-Law, R. P.; Sanders,
J. K. M. J. Am. Chem. Soc. 1995, 117, 259-271; Bonar-Law, R. P.;
Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1995, 3085-3096.
Cavitand-porphyrins: (g) Starnes, S. D.; Rudkevich, D. M.; Rebek, J., Jr.
J. Am. Chem. Soc. 2001, 123, 4659-4669. (h) Calix[4]salenes: Rudkevich,
D. M.; Verboom, W.; Reinhoudt, D. N. J. Org. Chem. 1994, 59, 3683-
3686.
the molecules 6 and 9 have no mirror plane, and all NH protons
are magnetically nonequivalent (Figure 3). This has also been
observed for other benzimidazole-containing self-folding cav-
itands.11 Since the benzimidazole NH proton is involved, the
seam’s cycloracemization would require the tautomerization of
this heterocycle.
The 1H NMR spectra also showed the spectroscopic features
of the vase conformation of the cavities in 6 and 9 (CD2Cl2,
295 K), with the characteristic8,11 methine CH signals at ∼6
ppm.
(10) Weijnen, J. G.; Koudijs, A.; Engbersen, J. F. J. J. Chem. Soc., Perkin
Trans. 2 1991, 1121-1126.
(13) MacroModel 7.0; Amber* Force Field; Mohamadi, F.; Richards,
N. G.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.;
Hendrickson, T.; Still, W. C. J. Comput. Chem. 1990, 11, 440-467.
(14) Cycloenantiomerism: (a) Prelog, V.; Gerlach, H. HelV. Chim. Acta
1964, 47, 2288-2294. (b) Goodman, M.; Chorev, M. Acc. Chem. Res. 1979,
12, 1-7. (c) Yamamoto, C.; Okamoto, Y.; Schmidt, T.; Ja¨ger, R.; Vo¨gtle,
V. J. Am. Chem. Soc. 1997, 119, 10547-10548. In agreement with the
Prelog’s terminology, enantiomers 5 possess the clockwise and counter-
clockwise “directionality” of the eight secondary amide “building blocks”
in an otherwise identical “distribution pattern”, or the sequence of
connectivity.
(11) Lu¨cking, U.; Tucci, F. C.; Rudkevich, D. M.; Rebek, J., Jr. J. Am.
Chem. Soc. 2000, 122, 8880-8889.
(12) A structural analogue of 9 with longer alkyl chains (R )
(CH2)6CH3) was also prepared: 1H NMR (CD2Cl2, 295 K): 12.82 (s, 1
H), 10.02 (s, 1 H), 9.76 (s, 1 H), 9.56 (s, 1H), 9.10 (d, J ) 2.8 Hz, 1 H),
8.97 (s, 1 H), 8.95 (s, 1 H), 8.77 (s, 1 H), 8.40-8.20 (m, 3 H), 7.95-7.60
(m, 8 H), 7.55-7.10 (m, 12 H), 5.80-5.62 (4 × t, J ) 8 Hz, 4 H), 2.80-
0.50 (m, 182 H); MALDI-TOF m/z 2470 ([M + H]+, calcd for C157H216N10O14
) 2469).