yield of 32%, presumably as a result of templating action10 during the cyclization of the tricationic precursor to afford the tetracationic "bead".11The 'H NMR spectrum (400 MHz) of 5-4PF6 exhibits temperature dependence (Figure 1 Chem. Soc. 1990, 112, 5525-5534) between the 7r-electronrich hydroquinol rings and the ir-electron-deficient bipyridinium units and (ii) electrostatic "T-type" edge-to-face interactions (cf.: Jorgensen, W. L.; Severance, D. L. J. Am. Chem. Soc. 1990, 112, 4768-4774) 0002-7863/91/1513-5131S02

-90°C). When the temperature was near -60°C. quenching of the solution in water and standard workup afforded crude reaction products in ca. 90Yo yield (GC). Purification (recrystallization or column chromatography) gave pure products.[9] Only the Markownikoff regioisomer 9 (by GC and I3C NMR analyses) was fund to be present in yields of 69%-91% after quenching in water.Academic Press, Orlando, FL, USA 1984, Chapter 5, p. 341. Chem. 88 (1976 Cyclobis(paraquat-p-phenylene). A Tetracationic Multipurpose ReceptorBy Barbara Odell, Mark V. Reddington, Alexandra M. Z . Slawin, Neil Spencer, J. Fraser Stoddart,* and David J . WilliamsRecently, we reported''] on cyclophane-like molecular receptors, such as bisparaphenylene-34-crown-10 1, which contains n-electron-rich hydroquinone units for face-to-face complexation ( Fig. 1) with paraquat 2, a n-electron-deficient dication. These observations led us to propose that it might be possible to reverse constitutionally the roles of the receptor and the substrate and so bind a n-electronrich diphenol ether, such as 1,4-dimethoxybenzene 3a, inside a tetracationic molecular receptor (Fig. 1). Such a receptor is the tetracation cyclobis(paraquat-p-phenylene) 4 which incorporates two n-electron-deficient paraquat residues connected in a phane-like manner by means of two para-phenylene groups.[21 Here, we describe (i) the synthesis of 4 as its tetrakis(hexafluorophosphate) 4 . 4 PF,, (ii) its X-ray structural characterization as the acetonitrile solvate 4 . 4 PF6.3 MeCN, (iii) the UV and 'H-NMR spectroscopic evidence for 1 : 1 complex formation (a) between 4 . 4 PF6 and 1,2-, 1,3-, and 1,4-dimethoxybenzene 3a/3b/ 3c in acetonitrile (CD,CN) and (b) between 4 . 4 C I and 1,4-dihydroxybenzene 6 in water (D20)J3' and (iv) the results of molecular and semiempirical quantum mechanical calculations on 4 and on 4 . 2 MeCN.[ Fig. 1 Schematic representation of the basis for the proposal that the cyclobis(paraquat-p-phenylene) 4 should act as a tetracationic receptor for 1,4-dimethoxybenzene 3a.The procedure (cf. Ref.[2]) employed for the preparation of 4 -4 P F 6 starting from the bis(pyridinium) salt 5 and 1,4-bis(bromomethyl)benzene is outlined in Scheme 1. (Fig. 2, top) that 4 adopts a rigid centrosymmetric rectangular box-like conformation with the two paraquat units forming the longer sides and the two para-xylylene residues, the shorter ones. There is a 19" twist angle between the two pyridinium rings of each paraquat unit: in addition, there are deformations of both the paraquat and para-xylylene components producing a bowing of the sides of the cyclophane. Thus, the strain within the molecule is relieved by out-of-plane bending of the six aromatic rings and is distributed throughout the macrocycle with the maximum deviations associated with the exocyclic C-CH2 bonds emanating from the para-phenylene residues: these bonds subtend an angle of 14" with respect to each other whilst the two Ne-CH, bonds associated with the paraquat units subtend an angle of 23 O . There is also a con...

tided as ca. 99% unreacted 'H NMR (vide supra). The components of the aqueous phase were identified by 'H NMR as paraquat (S 4.35 (s, 3 H), 8.37 (d, J = 1,2 H), 8.90 (d, J = 1,2 H)) and 2-(ethylamino)-2-methylpropanol hydrochloride (spectrum identical to that described above). Integrals of the NMR signals for paraquat and the amino alcohol indicated a 2.5% conversion of DEM-3 dimer to the amino alcohol.B. Buffered Methanol Medium. The reaction was performed in pH 7, Tris-buffered methanol. This time the reaction mixture turned dark blue. Again, the residue from solvent evaporation was extracted into 1 mL of D20 and 1 mL of CDC13. The components of the organic phase were identified by 'H NMR as DEM-3 dimer and 2-(ethylamino)-2methylpropanol (6 1.06 (s, 6 H), 1.07 (t, J = 7.2, 3 H), 2.44 (q, J = 1.2, 2 H), 3.33 (s, 2 H)). The components of the aqueous phase were identified as paraquat and a small amount of 2-(ethylamino)-2-methylpropanol hydrochloride, also from the 'H NMR spectrum. Integrals of the NMR signals indicated a 41% conversion of DEM-3 dimer to amino alcohol.Attempted Reduction of Daunomycin with DEM-3 Dimer. The reaction vessel was a 9 mm X 20 cm Pyrex tube equipped with a 2.5-cm side arm. The side arm was charged with 2.65 X 10"* mol of DEM-3 dimer dissolved in methylene chloride, and the methylene chloride was evaporated with a stream of nitrogen. The main tube was charged with 2 mL of 2 X 10~3 M 1:1 Tris/Tris-HCl buffered methanol containing 2.66 X 10"6 mol of daunomycin. The methanol solution was freeze-thaw-degassed, and the tube was sealed with a torch. After mixing the reagents, the solution was heated at 36 °C for 18 h. C-18 reverse-phase HPLC analysis as described earlier35 showed no formation of 7-deoxydaunomycinone.

A very simple self-assembling system, which produces inclusion complexes with pseudorotaxane geometries, is described. The self-assembly of eight pseudorotaxanes with a range of stoichiometries-I : I , 1 :2, 2:1, and 2:2 (host:guest)-has been Keywords achieved. These pseudorotaxanes self-assemble from readily available componentscrown ethers -dialkylammonium well-known crown ethers, such as dibenzo [24]crown-8 and bis-p-phenylene[34lcrown-salts 9 hydrogen bonding -molecular 10, and secondary dialkylammonium hexafluorophosphate salts, such as (PhCH,),-recognition -pseudorotaxanes * NHiPF; and (nBu),NHlPF;-and have been characterized not only in the solid state, self-assembly but also in solution and in the "gas phase". The pseudorotaxanes are stabilized largely by hydrogen-bonding interactions and, in some instances, by aryl-aryl interactions.

A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecu...