Macrocyclization on the fullerene core: Direct regio- and diastereoselective multi-functionalization of [60]fullerene,and synthesis of fullerene-dendrimer derivatives

The macrocyclization between buckminsterfullerene, C₆₀, and bis-malonate derivatives in double Bingel reaction provides a versatile and simple method for the preparation of covalent bis-adducts of C₆₀ with high regio- and diastereoselectivity. A combination of spectral analysis, stereochemical considerations, and X-ray crystallography (Fig. 2) revealed that out of the possible in-in, in-out, and out-out stereoisomers, the reaction of bis-malonates linked by o-, m-, or p-xylylene tethers afforded only the out-out ones (Scheme 1). In contrast, the use of larger tethers derived from 1,10-phenanthroline also provided a first example, (±)- 19 (Scheme 2), of an in-out product. Starting from optically pure bis-malonate derivatives, the new bis-functionalization method permitted the diastereoselective preparation of optically active fullerene derivatives (Schemes 4 and 5) and, ultimately, the enantioselective preparation (enantiomeric excess ee > 97%) of optically active cis-3 bis-adducts whose chirality results exclusively from the addition pattern (Fig. 6). The macrocyclic fixation of a bis-malonate with an optically active, 9,9′-spirobi[9H-fluorene]-derived tether to C₆₀ under generation of 24 and ent- 24 with an achiral addition pattern (Scheme 4) was found to induce dramatic changes in the chiroptical properties of the tether chromophore such as strong enhancement and reversal of sign of the Cotton effects in the circular dichroism (CD) spectra (Figs. 4 and 5). By the same method, the functionafized bis-adducts 50 and 51 (Schemes 10 and 11) were prepared as initiator cores for the synthesis of the fullerene dendrimers 62 , 63 , and 66 (Schemes 12 and 13) by convergent growth. Finally, the new methodology was extended, to the regio- and diastereoselective construction of higher cyclopropanated adducts. Starting from mono-adduct 71 , a clipping reaction provided exclusively the all-cis-2 tris-adduct (±)- 72 (Scheme 14), whereas the similar reaction of bis-adduct 76 afforded the all-cis-2 tetrakis-adduct 77 (Scheme 15). Electrochemical investigations by steady-state voltammetry (Table 2) in CH₂Cl₂ (+0.1M Bu₄NPF₆) showed that all macroeyciic bis(methano)fullerenes underwent multiple reduction steps, and that regioisomerism was not much influencing the redox potentials, All cis-2 bis-adducts gave an instable dianion which decomposed during the electrochemical reduction. In CH₂Cl₂, the redox potential of the fullerene core in dendrimers 62, 63 , and 66 is not affected by differences in size and density of the surrounding poly(ether-amide) dendrons. The all-cis-2 tris- and tetrakis(meihano)fullercnes (±) -72 and 77 , respectively, are reduced at more negative potential than previously reported all-e tris- and tetrakis-adducts with methane bridges that are also located along an equatorial belt. This indicates a larger perturbation of the original fullerene π-chromophore and a larger raise in LUMO energy in the former derivatives.     

Polyphospha

The Eglinton reaction of diethynyl(2,4,6-tri-tert-butylphenyl)phosphane (7a), that is, the oxidative coupling of 3, 4, 5, or 6 of these phosphane units, affords a mixture of the 15-, 20-, 25-, and 30-membered macrocycles 8, 9, 10, and 11. Pure triphosphacyclopentadecahexayne 8 and pentaphosphacyclopentacosadecayne 10 were isolated by HPLC, while the mixture of 9 and 11 could not be separated. Multistep syntheses of open-chain polyphosphapolyynes are described, whose intra- or intermolecular coupling yields the phosphamacrocycles 8, 9, and 11. Eglinton coupling of bis(ethynylphosphanyl)butadiyne (17) gave a mixture of the 20-membered tetraphosphacycloicosaoctayne 9, the 30-membered hexaphosphacyclotriacontadodecayne 11, and the 40-membered octaphosphacyclotetracontahexadecayne 23 as result of a di-, tri-, and tetramerization, respectively. Intramolecular coupling of bis[(ethynylphosphanyl)butadiynyl]phosphane 25a gave 8, while intermolecular coupling gave 11; these two compounds were isolated by chromatography to give yields of 70 and 5%, respectively. The open-chain tetraphosphaeikosaoctayne 28 couples intramolecularly to give 9 and intermolecularly to give the 40-membered octaphosphacyclotetracontahexadecayne 23, which was isolated in the pure form. Octaphosphatetracontahexadecayne 32 cyclized to give 23, exclusively. The temperature-dependent 1H and 31P NMR spectra of the open-chain and cyclic ethynylphosphanes indicated a lowering of the inversion barrier of the tertiary phosphanes from the usual 130-140 kJ mol(-1) to 65-75 kJ mol(-1). Ab initio calculations proved that the dramatic reduction of the inversion barriers results from the interaction of the lone pair on phosphorus with the pi orbitals of the triple bonds in the planar transition state during inversion. The situation is comparable with the dramatic reduction of the P inversion barrier in phospholes, because of the planar, aromatic transition state. The polyphospha[m]cyclo[n]carbons may be considered as precursors to cyclic PmCn systems.     

Polyamine-based anion receptors: Extraction and structural studies

In the discussion that follows some of the more recent progress in the area of anion binding by synthetic polyamine receptors is presented, with emphasis given to work undertaken by the authors’ groups. A continuing theme in these studies has been the relationship between receptor structure and its anion extraction properties.Systematic solvent extraction and structural studies for halide and perrhenate complexes with polyamines of tripodal, macrocyclic and macrobicyclic architecture that contain both aromatic moieties and four to eight amine functions have been performed in order to derive relevant structure-binding/extractability relationships. The results demonstrate that the binding and extraction behaviour of the polyamines towards halides and perrhenate is a complex function of their structural features, degree of protonation and lipophilic properties. The extraction is characterized by the preferred formation of mono- and diprotonated amine species in the organic phase. X-ray structure studies of iodide and perrhenate complexes with open-chain tetraamino derivatives and octaamino cryptands in different protonation states lead to the conclusion that in the first case only limited chelation of the anion occurs and in the second only highly protonated species are able to encapsulate the anion. The structural patterns observed are strongly influenced by the presence of water molecules in the crystals.     

Ground-state properties and static dipole polarizabilities of the alkali dimers from K2 n to Fr2 n(n=0,+1) from scalar relativistic pseudopotential coupled cluster and density functional studies

The newly adjusted energy-consistent nine-valence-electron pseudopotentials for K to Fr are used to calculate spectroscopic properties for the neutral and positively charged alkali dimers using coupled cluster and density functional theory. For the neutral dimers the static dipole polarizability was calculated. The coupled cluster results are all in excellent agreement with experimental values. The density functionals used can give quite different spectroscopic properties especially for the dipole polarizability, with the Perdew-Wang PW91 functional performing best.     

Dynamic alterations of fibronectin layers on copolymer substrates with graded physicochemical characteristics

Desorption and exchange of preadsorbed fibronectin layers in pure buffer solution and solutions of human serum albumin or fibronectin, respectively, were studied in dependence on the physicochemical characteristics of maleic acid copolymer films used as substrates. Although the preadsorbed amount of fibronectin differed only slightly, the protein was found to exhibit a significantly enhanced anchorage at the more hydrophobic polymer surface as compared to the more hydrophilic and more negatively charged polymer surface. The preadsorbed fibronectin layer was most efficiently exchanged by fibronectin (i.e., in the homodisplacement process) while pure buffer solution and human serum albumin solutions induced desorption or exchange of fibronectin to lower and similar degrees. An increase of the total adsorbed amount of protein due to additional adsorption of fibronectin or human serum albumin accompanied the partial exchange of the preadsorbed fibronectin in the displacement experiments. Evaluation of the kinetics of desorption and exchange of fibronectin at any of the substrates revealed two kinds of surface-attached protein populations--a fast desorbing species and a species with a slow desorption and exchange rate. By a multivariate regression analysis the surface characteristics of the polymer substrate were confirmed to determine the degree of protein desorption and exchange while the dynamics of the layer alteration was found to solely depend on the diffusion behavior of the proteins.     

The Synthesis and Mechanistic Considerations of a Series of Ammonium Monosubstituted H-Phosphonate Salts

A series of ammonium monosubstituted H-phosphonate salts were synthesized by combining H-phosphonate diesters with amines in the absence of solvent at 80 °C. Variation of the ester substituent and amine produced a range of ionic liquids with low melting points. The products and by-products were analyzed by spectroscopic and spectrometric techniques in order to get a better mechanistic picture of the dealkylation and formal dearylation observed. For dialkyl H-phosphonate diesters, (RO)₂P(O)H (R=alkyl), the reaction proceeds via direct dealkylation with the reactivity increasing in the order R=iPr<Et<Me corresponding to DFT calculated activation enthalpies of 22.6, 20.8, and 17.9 kcal mol⁻¹. For the diphenyl H-phosphonate diesters, (PhO)₂P(O)H, the dearylation was found to proceed via phenol-assisted formation of a 5-coordinate intermediate, (PhO)₃PH(OH), from which P(OPh)₃ and water were eliminated. The presence of an equivalent of water then facilitated the formation of P(OH)₂OPh and the amine, R'NH₂, subsequently abstracted a proton from it to yield [(PhO)PH(O)O]^-[R'NH₃]⁺.