Staircase Inclusion Compounds Formed by Tetrahalodiquinoline Hosts

The tetrabromodiquinoline 2, and its tetraiodo counterpart 3, are members of the new tetrahalo aryl host family. They form lattice inclusion compounds...
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CRYSTAL GROWTH & DESIGN

Staircase Inclusion Compounds Formed by Tetrahalodiquinoline Hosts

2002 VOL. 2, NO. 5 421-426

A. Noman M. M. Rahman, Roger Bishop,* Donald C. Craig, Christopher E. Marjo, and Marcia L. Scudder School of Chemical Sciences, The University of New South Wales, UNSW Sydney New South Wales 2052, Australia Received May 1, 2002

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: The tetrabromodiquinoline 2, and its tetraiodo counterpart 3, are members of the new tetrahalo aryl host family. They form lattice inclusion compounds when crystallized from many organic liquids, and the X-ray crystal structures of five such compounds are reported and analyzed in crystal engineering terms. Host molecules assemble into parallel staircases by means of aryl offset face-face and halogen-halogen interactions both within, and between, the staircases. The guests are situated in parallel interstitial channels. Variation of the type of included guest can result in significant changes to the construction and symmetry of the host staircase assemblies. Introduction Many well-known organic hosts (such as crown ethers, cyclodextrins, cryptands, and calixarenes) have a preformed receptor structure that can combine with a guest species to produce an inclusion compound.1,2 The behavior and properties of such molecules often may be predicted or modeled comparatively easily. This is no longer true, however, for lattice inclusion (clathrand) hosts (for example thiourea, tetraaryl porphyrins, and bile acids). For this alternative group of compounds, molecular inclusion occurs as a result of the interactions present between the many hosts and guests comprising their crystal lattice. Hence prediction, design, and crystal engineering development of new lattice inclusion hosts generally have been more difficult than for the former group of hosts.3 Historically, chance discovery has uncovered many lattice inclusion hosts, and their subsequent systematic modification into new structures has also been productive. Now our rapidly developing understanding of intermolecular forces, lattice packing, and supramolecular chemistry can be applied to such problems.4-6 A significant proportion of known clathrand hosts incorporate hydrogen bonding functionality (especially the hydroxy group) as part of their structure. Hydrogen bonding is a relatively strong intermolecular attractive force that provides some degree of predictability in lattice inclusion host design.7-11 In this paper, we describe the inclusion properties of a new type of lattice inclusion host 2, the structure of which does not usually hydrogen bond with either itself or guest molecules. Instead, combinations of different weak intermolecular attractions such as aryl offset face-face (OFF), aryl edge-face (EF), C-H‚‚‚halogen, nitrogen-halogen, and halogen-halogen interactions can be involved in the resulting inclusion structures. * To whom correspondence should be addressed. Fax: 61-2-93856141. E-mail: [email protected].

Scheme 1

Results and Discussion Synthesis, Inclusion, and Crystallization. The preparations of the tetrabromo host 2, and its tetraiodo analogue 3, are illustrated in Scheme 1. We have reported earlier that Friedla¨nder condensation12,13 of bicyclo[3.3.1]nonane-2,6-dione14 with two equivalents of o-aminobenzaldehyde15 affords the diquinoline adduct 1.16 Quinoline is known to be preferentially brominated,17,18 or iodinated,19 at its 5 and 8 positions if the halogenation is conducted in concentrated sulfuric acid in the presence of silver sulfate. These sites correspond to the 1, 4, 9, and 12 positions of the diquinoline molecule 1, and we found that bromination of this compound did indeed afford the 1,4,9,12-tetrabromo derivative 2 (46%).20 A similar iodination experiment yielded the tetraiodide 3 in 55% yield. The tetrabromide 2 proved to be an excellent host for inclusion of many small molecules such as dichloromethane, benzene, tetrahydrofuran, 1,1,2,2-tetrachloroethane, 1,1,1-trichloroethane, 1,1,2-trichloroethane, toluene, and ethyl acetate. Furthermore, an allyl cyanide-water mixture was also included by 2. Although solid inclusion compounds could be obtained by evaporation of solvent from solutions of 2, in all cases it proved exceptionally difficult to grow crystals of suitable quality for X-ray structural investigations. This proved to be even more difficult for the tetraiodide host 3, which includes guests such as toluene and chloroform. None-

10.1021/cg020017o CCC: $22.00 © 2002 American Chemical Society Published on Web 08/03/2002

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Table 1. Numerical Details of the Solution and Refinement of the Tetrahalodiquinoline Inclusion Compound X-ray Crystal Structures empirical formula asymmetric unit formula M crystal system space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Dc/g cm-3 Z µMo/mm-1 2θmax crystal decay min trans. factor max trans. factor unique refl. observed refl. Rmerge R Rw a

(2)4‚(C6H6)

(3)4‚(C7H8)

(2)2‚(CH2Cl2)

(2)4‚(C4H8O2)

(2)2‚(C4H5N)‚(H2O)

2(C23H14Br4N2)‚ 0.5(C6H6) 1315.0 triclinic P1 h 13.290(5) 13.753(6) 14.598(6) 80.53(3) 66.83(3) 65.85(3) 2238(0) 1.95 2 7.137 46 none 0.21 0.36 6199 3067 0.016 0.055 0.078

2(C23H14I4N2)‚ 0.5(C7H8) 1698.1 triclinic P1 h 13.861(8) 14.305(9) 14.969(9) 77.85(3) 64.18(3) 66.84(3) 2454(2) 2.30 2 5.042 44 5% 0.50 0.60 6357 3516 0.024 0.058 0.086

C23H14Br4N2‚ 0.5(CH2Cl2) 680.5 monoclinic C2/c 27.417(6) 12.202(2) 13.819(3) 90 100.824(9) 90 4541(2) 1.99 8 9.982 (Cu)* 120 (Cu)* 4% 0.30 0.60 3356 1946 0.071 0.069 0.099

4(C23H14Br4N2)‚ C4H8O2 2640.1 triclinic P1 h 14.044(6) 14.997(7) 22.395(9) 77.35(3) 84.20(3) 81.51(3) 4540(3) 1.93 2 7.038 40 none 0.24 0.50 8458 3539 0.024 0.057 0.065

C23H14Br4N2‚ 0.5(C4H5N)‚0.5(H2O) 680.5 monoclinic C2/c 23.275(8) 16.383(6) 14.475(6) 90 121.77(2) 90 4693(3) 1.93 8 6.814 50 4% 0.40 0.46 4123 1876 0.044 0.062 0.073

Copper radiation (Cu KR, λ ) 1.5418 Å) was used for this structure.20

theless, adequate crystals were obtained in five instances. The numerical details pertaining to the data collection, data processing, and refinement of these inclusion compound structures are listed in Table 1. General Structural Features. Most of the inclusion compounds formed by racemic 2 (or 3) have structures that are broadly similar to each other, but they differ in detail. Molecules of the host have a twisted V-shaped geometry with the two planar aromatic wings inclined at an angle to each other due to the pseudo-C2 symmetry generated by the central alicyclic ring. A good measure of the relationship between the wings in the various structures is afforded by the angle defined by the three centroid positions 2/3, 6/14, and 10/11 (see structure 1 for ring numbering). These angles cover the small range 95.3 to 101.8°, but there is no correlation between individual angles and the overall structural types described below. The non-hydrogen bonded structure is unusual among lattice inclusion compounds in having the host molecules stacked into infinite staircase-like columns.21 The staircase is built up using one of the aryl wings of successive host molecules as the steps. These steps assemble through aryl offset face-face stacking. The second aromatic wings of the hosts form the staircasesurrounds, which point upward on one side of the staircase and downward on the other. There are also aryl edge-face interactions between the ends of the parallel steps and the aryl wings of the staircasesurrounds. Although interstep symmetry varies across the series of inclusion compounds (described case-by-case later), each compound contains centrosymmetric step pairs as part of its staircase construction. The association between these pairs of host molecules is such that one halogen atom from each is situated in the cleft of its V-shaped partner and is located over the pyridine ring of both aromatic wings, making effective Br‚‚‚π interactions. The halogen‚‚‚ring centroid distances range from 3.5 to 4.3 Å across the series of inclusion compounds.

These standard staircases have an approximately hexagonal cross-section in projection and pack parallel to each other. Four of the hexagon edges constitute aromatic surrounds, and these interact with their neighbors through centrosymmetric aryl OFF interactions. The remaining two hexagon edges are slightly concave. Guest molecules occupy interstitial channels between concave edges belonging to adjacent staircases. In the case of (2)4‚(ethyl acetate), the standard staircases are accompanied by a second staircase-like structure, which is truncated rather than being infinite. The compound (2)2‚(allyl cyanide)‚(water) contains staircases of a different construction-type to those present in the other compounds. Structures (2)4‚(Benzene) and (3)4‚(Toluene). These compounds, in the triclinic space group P1 h , are approximately isostructural.22 There are two host molecules (A and B) in the asymmetric unit, and both enantiomers are present in the structures. The arrangement of staircase steps is -A-B-B*-A*-A-B-B*-A*(where A and B have the same chirality, and * indicates enantiomers of opposite handedness). In other words, adjacent steps are related by a center of symmetry, no symmetry, etc., along each infinite staircase (Figure 1). The staircases run parallel to each other along b, and have a hexagonal projection in the ac plane (Figure 2). Structure (2)2‚(Dichloromethane). This compound crystallizes in the monoclinic space group C2/c, and its structure20 is closely related to those described above. Now there is only one host molecule in the asymmetric unit, and the adjacent steps are alternately related by a center of symmetry and then a 2-fold axis. The staircase projection in the ab plane is very similar to the previous ones. Only the compound stoichiometry, the orientations of the staircases, and the arrangement of the guest molecules have changed (Figure 3a,b). The staircases now run parallel to each other along c, and have a hexagonal projection in the ab plane. Structure (2)4‚(Ethyl Acetate). This compound, which crystallizes in the triclinic space group P1 h,

Staircase Inclusion Compounds

Crystal Growth & Design, Vol. 2, No. 5, 2002 423

Figure 1. Standard staircase assembly present in the structure (2)4‚(benzene) using framework representation with H atoms omitted. The steps formed by one aromatic wing of the host molecules are edge-on in the center of the diagram, with the surrounds formed by the second wings pointing up (left) and down (right) along b. Symbols used: C (green), N (dark blue), Br (brown), and center of symmetry (*). W A 3D rotatable image in PDB format is available.

Figure 3. Comparative views of one standard staircase (here shown horizontally) and its surrounding guest molecules: (a) for (2)2‚(dichloromethane), and (b) for (2)4‚(benzene), indicating the origins of the difference in crystal stoichiometry.

Figure 2. Packing of the host staircases and benzene guests in (2)4‚(benzene). The staircases, with their steps in the center, have a hexagonal cross-section when projected in the ac plane. Four of the staircase faces are formed by aromatic surrounds and the remaining two opposed faces are slightly concave. Guest molecules occupy channels between the concave edges of two adjacent staircases. Additional color-coding: guest C (purple) and guest H (light blue).

contains four 2 molecules (A-D) in the asymmetric unit. Two of these (A and B) form infinite staircases of identical symmetry to those described above for (2)4‚ (benzene) and (3)4‚(toluene). In addition, truncated staircase structures are present, constructed from the remaining two independent molecules C and D. These comprise only four steps which are arranged C-D-D*C* (where C and D have the same chirality, and * indicates enantiomers of opposite handedness). In other words, the four adjacent steps are related by no symmetry, then a center of symmetry, and finally no symmetry.

This structure, when viewed down a, indicates that there are two distinct layers. One type of layer (top and bottom in Figure 4a) contains the standard infinite staircases. These have the usual hexagonal cross-section when projected in the bc plane. The second layer incorporates the ethyl acetate guest molecules and the truncated staircases. The projection of this truncated staircase is very similar to that of the infinite analogues (Figure 4b). Structure (2)2‚(Allyl Cyanide)‚(Water). This compound crystallizes in the monoclinic space group C2/c with only one host molecule in the asymmetric unit. This material has a different type of staircase with a different projection (Figure 5). The interstep symmetry, inversion then 2-fold, is the same as that present in (2)4‚ (dichloromethane).20 However, there is lateral displacement across the center of inversion, so that these steps are too far apart for an OFF interaction to operate. The 2-fold related steps do take part in a good OFF interaction (Figure 6). Host-Host Interactions. The dominant feature of these structures is aryl-aryl interaction. Aryl offset face-face π-π interactions (OFF) not only permit the host molecules to stack into staircases, but also play a key role in inter-staircase assembly. In addition, aryl edge-face (EF) interactions are involved in the staircase construction. Halogen-halogen interactions also are of major importance. Within the standard staircases, there are two

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Figure 4. (a) Packing of the host staircases and guest molecules in (2)4‚(ethyl acetate) when projected in the bc plane. The top and bottom rows show the standard infinite staircases (A and B molecules), while the central row depicts a side-view of the truncated staircases (C and D molecules) and the ethyl acetate guests. (b) The comparative projection of one truncated staircase when viewed along b.

Figure 5. Packing of the host staircases and guest molecules in (2)2‚(allyl cyanide)‚(water) when projected in the ab plane. The different types of staircases present in this compound are clearly apparent when compared with Figures 2 and 4. The various guest disorder positions are shown, with water molecules simply shown as spheres.

types of Br‚‚‚Br (or I‚‚‚I) interactions. First, between the steps that are either not symmetry-related or are related by a 2-fold axis, there is a Br‚‚‚Br contact (3.90 Å, benzene compound), (3.84 Å, dichloromethane compound), (3.94 Å standard staircase and 3.76 Å truncated staircase, ethyl acetate compound), and an I‚‚‚I contact of 4.01 Å in (3)4‚(toluene). The second type of halogen-halogen contact is from a step to a staircase-surround. In (2)2‚(dichloromethane), there is only one independent Br‚‚‚Br interaction (3.93 Å), but for the less symmetrical staircases there are two (3.80 and 4.02 Å, benzene compound), (4.15 and 4.46 Å,

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Figure 6. Two orthogonal views of the different staircase assembly present in structure (2)2‚(allyl cyanide)‚(water) using framework representation and with H atoms omitted. (a) Twofold axis normal to the plane, and (b) 2-fold axis in the plane of the diagram. Symbols used: center of symmetry (*) and 2-fold axis (ellipse or arrow, respectively). W A 3D rotatable image in PDB format is available.

ethyl acetate compound), and I‚‚‚I interactions of 4.05 and 4.22 Å in (3)4‚(toluene). There are also interactions between staircases that range from 3.57 Å, in the (2)4‚ (benzene) case, upward. For (2)2‚(allyl cyanide)‚(water), there are Br‚‚‚Br interactions of 3.82 Å between the steps which form the offset face-face assembly and are 2-fold related. There are also steps to staircase-surround Br‚‚‚Br interactions of 3.80 and 4.09 Å. The latter is accompanied by an aryl edge-face interaction between the same pair of wings. In addition, there are Br‚‚‚Br interactions between staircases of 3.87 Å. Molecules of the parent diquinoline 1 are noteworthy for assembling into edge-edge pairs by means of the Ar-H‚‚‚N dimer interaction.16 Its halogenated derivatives 2 and 3 can no longer form this motif due to replacement of the key hydrogen atoms. Nonetheless, it is significant that less common C-H‚‚‚N interactions recently reported by us23,24 are also absent in the present crystal structures. The only significant participation by nitrogen is hydrogen bonding to a water molecule in the compound (2)2‚(allyl cyanide)‚(water). Host-Guest Interactions. The most significant host-guest interactions occur in the benzene and toluene compounds. In (2)4‚(benzene), the guest is located exactly around a center of inversion, so the contacts it makes with the two surrounding host lattice staircases are symmetrical. The benzene guest is located between two staircase-surround wings of host molecules, so that it makes rather long EF interactions with both (C‚‚‚C 4.24 Å). These interactions are supported by C-H‚‚‚Br interactions. In all, each benzene guest makes a total of 14 such interactions, 7 with each of the two adjacent staircases. The interactions to one staircase span, and interact with, the Br atoms of four contiguous steps. In addition, there are interactions to two staircasesurround wings. The H‚‚‚Br distances range from 3.01 to 3.39 Å.

Staircase Inclusion Compounds

For (3)4‚(toluene), the picture is a little different because the guest lacks inversion symmetry. The guest is disordered about the center, and is offset a little, so that there is no overlap, even of the atoms of the aryl ring of the two disorder components. The hydrogen atoms of the methyl group were not included in the refinement, so no comment can be made regarding Cmethyl-H‚‚‚I interactions. Considering just one disorder component, there are a total of 11 Caryl-H‚‚‚I interactions around each guest. These range from 3.23 to 3.63 Å (H‚‚‚I), and the interactions are to four contiguous steps and one staircase-surround from one staircase, and 3 of 4 contiguous steps and one surround from the other. Again, there are EF interactions between the face of the guest and two host wings of adjacent staircases. In both (2)4‚(ethyl acetate) and (2)2‚(allyl cyanide)‚(water), there is a lack of specific identifiable hostguest interactions. The ethyl acetate is located within the nonstandard staircase region, while in (2)2‚(allyl cyanide)‚(water), the guests are arranged end-to-end (and disordered) along the c axis. Conclusions Weber has observed that organic molecules with inclined aromatic planes show an enhanced probability of acting as lattice inclusion hosts.25,26 Recently, we have suggested the existence of a group of lattice inclusion hosts (the tetrahalo aryl hosts) that combine this structural feature along with tetrahalo substitution at their molecular extremities.20 Hosts 2 and 3 are members of this new family, along with the compounds 4b-e studied recently by Tanaka et al.27-29 A characteristic of these hosts is that their nonhalogenated parent substance may not itself exhibit inclusion properties. This has been demonstrated to be the case for compounds 1 and 4a. The inclusion compounds of 2 and 3 are unusual in utilizing no hydrogen bonding, conventional or otherwise, but instead involve weaker and less familiar interactions. Despite this, it is noteworthy that staircase compounds can be predicted, although new guests may result in modification of the staircase construction. There is, however, no requirement or expectation that different members of the tetrahalo aryl host family should necessarily share similar crystal packing or supramolecular behavior. Recently, we have synthesized further hosts of this family. These also form lattice inclusion compounds, but our initial work shows them to be of different types to those of hosts 2, 3, and 4b-d. We intend to describe their chemistry soon.30 Experimental Section NMR data were recorded using a Bruker ACF300 instrument at 25 °C, and carbon substitution information was determined using the DEPT procedure. MS data (EI) were recorded by Dr. J. J. Brophy using a VG Quattro triple quadrupole instrument. The microanalytical results were determined at the Australian National University, Canberra. 1,4,9,12-Tetrabromo-6,7,14,15-tetrahydro-6,14-methanocycloocta[1,2-b:5,6-b′]diquinoline (2). This compound was prepared from the diquinoline (1) as described by us previously.20 1,4,9,12-Tetraiodo-6,7,14,15-tetrahydro-6,14-methanocycloocta[1,2-b:5,6-b′]diquinoline (3). Diquinoline (1)16 (0.40 g, 1.24 mmol), Ag2SO4 (3.09 g, 9.91 mmol), and sulfuric acid

Crystal Growth & Design, Vol. 2, No. 5, 2002 425 (98%; 7.0 mL) were placed in a round-bottom flask fitted with a reflux condenser and drying tube. The stirred mixture was heated to 170-180 °C, and then diiodine (4.00 g) added in portions (0.50 g each) at intervals over 3 h. After cooling the sample to room temperature, the reaction mixture was poured into aqueous NaOH (2 M; 140 mL). Solid sodium sulfite was added until all excess diiodine had been reduced. The resulting suspension was extracted several times using dichloromethane, and the combined organic extracts were dried (Na2SO4). Solvent was evaporated from the filtrate to give a white solid. This crude product was eluted through a column of silica using 2:1 60-80° petrol-chloroform to yield the tetraiodide (3) (0.57 g, 55%), mp 294-295 °C; IR (paraffin mull) 1590w, 1120w, 1005s, 925m, and 815m cm-1; 1H NMR (300 MHz, CDCl3) δ 2.57 (br s, 2H), 3.44 and 3.49 (d, 2H, JAB ) 16.6 Hz), 3.59 and 3.64 (dd, 2H, JAB ) 16.6 Hz, JBX ) 4.1 Hz), 3.97 (br s, 2H), 7.65 (d, 2H, J ) 7.9 Hz), 7.84 (s, 2H), and 7.92 (d, 2H, J ) 7.9 Hz); 13C NMR (75.4 MHz, CDCl3) δ 28.4 (CH2), 35.7 (CH), 37.9 (CH2), 98.2 (C), 104.6 (C), 129.2 (C), 131.3 (C), 137.6 (CH), 139.6 (CH), 140.7 (C), 140.8 (CH), and 163.5 (C); m/z (>200 mass and >10% intensity, plus base peak): 827 (20%), 826 (M+, 85), 700 (17), 699 (10), 572 (15), 571 (14), 432 (63), 413 (22), 395 (30), 318 (13), 317 (29), 316 (21), 315 (14), 305 (23), 268 (18), 222 (17), and 139 (100); Anal. Calcd. for C23H14I4N2 (FW ) 826.0): C, 33.44; H, 1.71; N, 3.39. Found: C, 33.42; H, 1.73; N, 3.20. Solution and Refinement of the Crystal Structures. Reflection data were measured with an Enraf-Nonius CAD-4 diffractometer in θ/2θ scan mode using graphite monochromated molybdenum radiation (λ ) 0.7107 Å). Data were corrected for absorption.31 Reflections with I > 2σ(I) were considered observed. The structures were determined by direct phasing (SIR92)32 and Fourier methods. Hydrogen atoms for each structure were included in calculated positions. The atoms of each host molecule were refined with independent positional parameters, but for (2)4‚(ethyl acetate) reflection data were weak and severely restricted in range, with almost 60% being unobserved at θ ) 20°. Therefore, in this case, the quinolines of the host molecules were modeled as identical refineable planar groups, and the remaining atoms were refined with independent positional parameters. For the host molecules in all structures, individual anisotropic temperature parameters were assigned to the bromine atoms, and a 15parameter TLX rigid-body thermal parameter (where T is the translation tensor, L is the libration tensor, and X is the origin of libration) described the thermal motion of the remaining atoms.33 Guest modeling and refinement were as follows. Benzene compound: The guest was modeled as a planar group with refineable hexagonal symmetry, and its thermal motion was refined using a 12-parameter TL rigid-body thermal parameter with the center of libration at the center of the ring. Toluene compound: The guest was modeled as a planar group with refineable mm2 symmetry. Its thermal motion was refined using a 15-parameter TLX rigid-body thermal parameter. Ethyl acetate compound: The guest was refined using independent positional parameters, with distance and angle constraints to maintain reasonable geometry of the poorly defined molecule. Its thermal motion was refined using a 12parameter TL group thermal parameter with the center of libration at the center of mass. Allyl cyanide-water compound: The allyl cyanide guest was refined using independent positional parameters, with distance and angle constraints to maintain reasonable geometry of the poorly defined molecule. Its thermal motion was refined using a single overall anisotropic thermal parameter. Reflection weights used were 1/σ2(Fo), with σ(Fo) being derived from σ(Io) ) [σ2(Io) + (0.04Io)2]1/2. The weighted residual was defined as Rw ) (Σw∆2/ΣwFo2)1/2. Atomic scattering factors and anomalous dispersion parameters were from International Tables for X-ray Crystallography.34

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Acknowledgment. We thank the Australian Research Council for financial support of this work. Supporting Information Available: Crystallographic data (CIF) for (2)2‚(dichloromethane)20 (QUJLAI) are available from the Cambridge Crystallographic Data Centre (deposition number CCDC 152353) by e-mail [email protected]. X-ray crystallographic information files (CIF) for the other four inclusion compounds (CCDC 190587-190590) are available from the same address and free of charge via the Internet at http://pubs.acs.org.

References (1) Inclusion Compounds; Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D., Eds.; Academic Press: London, 1984; Vols. 1-3; Oxford University Press: Oxford, 1991; Vols. 4-5. (2) Comprehensive Supramolecular Chemistry, Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D.; Vo¨gtle, F., Eds.; Pergamon: Oxford, 1996; Vols. 1-11. (3) Bishop, R. Chem. Soc. Rev. 1996, 25, 311-319. (4) Goldberg, I. In Inclusion Compounds; Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D., Eds.; Oxford University Press: Oxford, 1991; Vol. 4, Ch. 10, pp 406-447. (5) Mascal, M. Contemp. Org. Synth. 1994, 1, 31-46. (6) Aakero¨y, C. B. Acta Crystallogr., Sect. B 1997, 53, 569586. (7) Caira, M. R.; Nassimbeni, L. R.; Vujovic, D.; Weber, E. J. Chem. Soc., Perkin Trans. 2 2001, 861-863. (8) Toda, F. In Comprehensive Supramolecular Chemistry, Vol. 6; MacNicol, D. D.; Toda, F.; Bishop, R., Eds.; Pergamon: Oxford, 1996; Ch. 15, pp 465-516. (9) Sada, K.; Sugahara, M.; Kato, K.; Miyata, M. J. Am. Chem. Soc. 2001, 123, 4386-4392. (10) Yue, W.; Bishop, R.; Craig, D. C.; Scudder, M. L. Tetrahedron 2000, 56, 6667-6673. (11) Endo, K.; Ezuhara, T.; Koyanagi, M.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1997, 119, 499-505. (12) Cheng, C.-C.; Yan, S.-J. Org. React. 1982, 28, 37-201. (13) Thummel, R. P. Synlett 1992, 1-12. (14) Schaefer, J. P.; Honig, L. M. J. Org. Chem. 1968, 33, 26552659. (15) Smith, L. I.; Opie, J. W. Org. Synth. 1955, Coll. Vol. 3, 5658. (16) Marjo, C. E.; Scudder, M. L.; Craig, D. C.; Bishop, R. J. Chem. Soc., Perkin Trans. 2 1997, 2099-2104.

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