make a molecular sensor; emission switching of a BBI-
namely, the exo binding sites [N(1) and N(1′)] and the endo
binding sites [N(3) and N(3′)]. The schematic lines made
2
6
ruthenium complex was shown in the presence of various
20
metal ions. Additionally, it was demonstrated that supramo-
lecular assemblies of BBI complexes could be formed
by two aromatic moieties are parallel, and the distance
2
7
between these should be ca. 8 Å. This cleftlike structure
creates the possibility of controlling complex formations at
the exo or endo sites. Less sterically congested complexes,
such as tetrahedral complexes, may be formed at either the
exo or endo binding sites (A). In contrast, only the exo
binding sites could participate in sterically congested com-
plexes, such as octahedral complexes (B). Addition of aryl
substituents will allow for further structural elaboration.
Introduction of heterocyclic aromatics may form multi-
dentate ligands (C). Ring closure between the two ends of
the aryl groups may generate a new class of macrocyclic
ligands (D). Moreover, a series of large, macrocyclic ligands
may be generated by fusion of more than one 4,4′-bisaryl-
BBI (E).
21
through hydrogen bonding in the solution and solid
phases.22
Although the chemistry of simple BBIs has been studied
widely, the knowledge of substituted BBIs, such as aryl
23
derivatives, is limited. From our own experience with
24
supramolecular metal complexes, the extent of study for a
given system depends on the ease of the synthesis of a series
2
5
of derivatives.
4,4′-Bisaryl-BBI has the potential to be a building block
for a variety of more complex ligands (Figure 1). Taking a
Transition-metal-catalyzed coupling reactions have been
shown to be an efficient method to synthesize functionalized
28
heteroaromatic ligands. These methods have advantages for
the synthesis of 4,4′-bisaryl-BBI (Scheme 1). Retrosyntheti-
Scheme 1. Synthetic Plan of 4,4′-Bisaryl-BBI
Figure 1. Schematic representation of 4,4′-bisaryl-BBI and its
derived complexes and structures.
schematic approach, 4,4′-bisaryl-BBI can adopt a U-shaped
conformation which has two differentiated binding sites,
(
15) Us o´ n, R.; Gimeno, J.; Oro, L. A.; Aznar, M. A.; Cabeza, J. A.
Polyhedron 1983, 2, 163.
16) Haddad, M. S.; Hendrickson, D. N. Inorg. Chem. 1978, 17,
622.
17) Lemos, S. S.; Deflon, V. M.; Bessler, K. E.; Abbott, M. P.; Niquet,
E. Transition Met. Chem. 2004, 29, 46.
18) (a) Us o´ n, R.; Vicente, J.; Chicote, M. T. J. Organomet. Chem. 1981,
09, 271. (b) Tzeng, B.-C.; Li, D.; Peng, S.-M.; Che, C.-M. J. Chem. Soc.,
Dalton Trans. 1993, 2365. Also see ref 12a.
(
cally, a biaryl linkage at the 4,4′-positions is disconnected
to 4,4′-dihalo-BBI and aryl organometals. Our plan was to
substitute these halogens with a variety of aryl groups via
coupling reactions at the latter stage of the synthesis. In this
context, Boc-protected 4,4′-dibromo-BBI 1 was chosen as a
2
(
(
2
(
19) See refs 4, 7, 9c, 12, and 14-18.
(
20) Rau, S.; B u¨ ttner, T; Temme, C.; Ruben, M.; G o¨ rls, H.;
Walther, D.; Duati, M.; Fanni, S.; Vos, J. G. Inorg. Chem. 2000, 39,
621.
21) Rau, S.; Sch a¨ fer, B.; Schebesta, S.; Gr u¨ âing, A.; Poppitz, W.;
Walther, D.; Duati, M.; Browne, W. R.; Vos, J. G. Eur. J. Inorg. Chem.
003, 1503.
22) (a) Rau, S.; Ruben, M.; B u¨ ttner, T.; Temme, C.; Dautz, S.; G o¨ rls,
H.; Rudolph, M.; Walther, D.; Brodkord, A.; Duati, M.; O’Connor, C.; Vos,
J. G. J. Chem. Soc., Dalton Trans. 2000, 3649. (b) Rau, S.; B o¨ ttcher, L.;
Schebesta, S.; Stollenz, M.; G o¨ rls, H.; Walther, D. Eur. J. Inorg. Chem.
(25) (a) Loren, J. C.; Siegel, J. S. Angew. Chem., Int. Ed. 2001, 40, 754.
(b) Toyota, S.; Woods, C. R.; Benaglia, M.; Siegel, J. S. Tetrahedron Lett.
1998, 39, 2697. (c) Benaglia, M.; Toyota, S.; Woods, C. R.; Siegel, J. S.
Tetrahedron Lett. 1997, 38, 4737.
(26) The discussion on the topology of 4,4′-bisaryl-BBI is based on an
assumption that two aryl groups are pointing the same direction (syn
conformation), though it is known that the anti conformer is dominant in
the solid state and likely preferred in the solution phase as well. See: Rath,
N.; Mohanty, R. R.; Jena, S.; Hemling, H. Indian J. Heterocycl. Chem.
1997, 6, 303. Torsional studies of 1,1′-dimethyl-2,2′-bisbenzimidazole have
calculated the difference in the heats of formation from the anti to the syn
conformer to be 12.3 kcal/mol, giving an indication in related systems.
Studies of N1,N1′-bridged-4,4′-bisaryl-BBIs are currently under investiga-
tion.
1
(
2
(
2
002, 2800.
23) For reported substituted BBIs, see: (a) Shi, Z.; Thummel, R. P. J.
(
Org. Chem. 1995, 60, 5935. (b) M u¨ ller, E.; Bernardinelli, G.; Reedijk, J.
Inorg. Chem. 1995, 34, 5979. (c) Bates, G. B.; Parker, D.; Tasker, P. A. J.
Chem. Soc., Perkin Trans. 2 1996, 1117. (d) K o¨ nig, B.; Pitsch, W.;
Thondorf, I. J. Org. Chem. 1996, 61, 4258. (e) Edlin, C. D.; Parker, D.
Tetrahedron Lett. 1998, 39, 2797. (f) Zhang, L.; Carroll, P.; Meggers, E.
Org. Lett. 2004, 6, 521.
(27) The distance between C(4) and C(7′) of trans-oriented 4,4′-di-
bromo-6,6′-dimethyl-2,2′-bisbenzimidazole was measured from its X-ray
structure.
(
24) (a) Woods, C. R.; Benaglia, M.; Toyota, S.; Hardcastle, K.; Siegel,
(28) For selected examples, see: (a) ref 24. (b) Tzalis, D.; Tor, Y.
Tetrahedron Lett. 1995, 36, 6017. (c) Savage, S. A.; Smith, A. P.; Fraser,
C. L. J. Org. Chem. 1998, 63, 10048. (d) Lehmann, U.; Schl u¨ ter, A. D.
Eur. J. Org. Chem. 2000, 3483. (e) Goeb, S.; Nicola, A. D.; Ziessel, R. J.
Org. Chem. 2005, 70, 6802.
J. S. Angew. Chem., Int. Ed. 2001, 40, 749. (b) Benaglia, M.; Ponzini, F.;
Woods, C. R.; Siegel, J. S. Org. Lett. 2001, 3, 967. (c) Loren, J. C.;
Yoshizawa, M.; Haldimann, R. F.; Linden, A.; Siegel, J. S. Angew. Chem.,
Int. Ed. 2003, 42, 5701.
4990
Org. Lett., Vol. 8, No. 22, 2006