Sequential olefination-dimerisation of benzylic dithioacetals by the nickel-catalysed reaction with methyl Grignard or zinc reagent.

Products of main group elements from cross-coupling reactions have been shown to serve as Lewis acids, mediating further reactions of organic coupling products. Thus, the nickel-catalysed olefination of benzylic dithioacetal with MeMgI in benzene in a sealed Schlenk tube at 130 °C generates magnesium mercaptide which regioselectively converts 2-arylpropene into a dimer in good yield. Aryl iodide reacts with 2-propenylmagnesium bromide in the presence of 1,2-ethanedithiol and NiCl2(PPh3)2 to yield the same dimer. Replacement of the Grignard reagent by an organozinc reagent gives the dimers in a better yield.

A Fischer-Type Ruthenium Carbene Complex as a Metathesis Catalyst for the Synthesis of Enol Ethers.

The Grubbs G-I or G-II catalyst gives the ruthenium ethoxy carbene complex, which catalyzes ring-opening cross metathesis (ROCM) of a strained cyclic alkene to give a diene where one of the two alkene moieties in the product contains an ethoxy substituent. No polymeric products are detected. Hydrocarbons such as parent norbornene or substituted cyclopropenes can proceed with the reaction smoothly. Tertiary amines, N-alkylimides, esters, and aryl or alkyl bromides remain intact under the reaction conditions. In addition to vinyl ethers, vinylic esters can also be used. The time required to reach a 50% yield of the ROCM product t50 varies from 0.01 to 140 h depending on the strain and nucleophilicity of the double bond. Anchimeric participation of an electron-rich group would result in significant enhancement of the reactivity, and the t50 could be as short as several minutes. A similar substrate without such a neighboring group shows a much slower rate. An exo-norborne derivative reacts much faster than the corresponding endo-isomer. Alkenes with poor nucleophilicity are less favored for the ROCM process, so is less strained cyclooctene.

Determination of the Orientation of Pendants on Rigid-Rod Polymers.

Bis-norbornene and bis-cyclobutene with different kinds of linkers have been extensively used for the synthesis of double stranded ladderphanes under ruthenium- or molybdenum-catalyzed ring opening metathesis polymerization (ROMP) conditions. The key to the success relies on the selective formation of comb-like polynorbornenes or polycycloubtenes, where pendants are all aligned towards similar direction. This minireview summarizes various methods (chemical methods, spectroscopic means, and nonlinear optical measurements) for determining the comb-like conformations of pendants on these rigid-rod polymers. The approach is based on the proximal relationship between adjacent pendants. Interactions between these adjacent pendants would enable a change in chemical reactivity.

Caterpillar-shaped polysilsesquioxanes.

The preparation of linear polysilsesquioxanes having Si-OH side groups is not trivial, because direct condensation of multifunctional silanol derivatives would lead to branched or crosslinked polymers. We have successfully prepared an unsymmetrical double-decker silsesquioxane with a silyl hydride function at one end and two silanol groups at the other. Linear POSS polymers having Si-OH side groups are obtained selectively in good yields. The properties and potential applications are briefly presented.

Selective ring-opening metathesis polymerization (ROMP) of cyclobutenes. Unsymmetrical ladderphane containing polycyclobutene and polynorbornene strands.

At 0 °C in THF in the presence of Grubbs first generation catalyst, cyclobutene derivatives undergo ROMP readily, whereas norbornene derivatives remain intact. When the substrate contains both cyclobutene and norbornene moieties, the conditions using THF as the solvent at 0 °C offer a useful protocol for the selective ROMP of cyclobutene to give norbornene-appended polycyclobutene. Unsymmetrical ladderphane having polycyclobutene and polynorbornene as two strands is obtained by further ROMP of the norbornene appended polycyclobutene in the presence of Grubbs first generation catalyst in DCM at ambient temperature. Methanolysis of this unsymmetrical ladderphane gives polycyclobutene methyl ester and insoluble polynorbornene-amide-alcohol. The latter is converted into the corresponding soluble acetate. Both polymers are well characterized by spectroscopic means. No norbornene moiety is found to be incorporated into polycyclobutene strand at all. The double bonds in the polycyclobutene strand are mainly in cis configuration (ca 70%), whereas the E/Z ratio for polynorbornene strand is 8:1.

Charge and Energy Transfer Dynamics in Dimethylsilylene-Spaced Aminostyrene Stilbene Monomer Using Time-Resolved Techniques.

We used transient absorption and time-correlated single photon counting (TCSPC) techniques to investigate the charge transfer reaction in monosilylene-spaced aminostyrene stilbene monomer. With 266 nm excitation, both stilbene (sti) and aminostyrene (ast) moieties were excited. In nonpolar solvents, the transient absorption band centered at 600 nm appeared promptly and is assigned to the excited state of sti*; this state relaxes at time constant 1.2-1.4 ps and is explained to proceed energy transfer to ast 1ππ*. The second transient band at 460 nm is assigned to absorption of ast 2ππ*; this state accessed from direct excitation has a lifetime 65 ps. This agrees with the observation of 85-89 ps emission decay from the TCSPC measurements. In polar solvent, an excited absorption band centered at 530 nm appeared with a rise time constant 0.2-0.6 ps. This band is assigned to the charge transfer state. This charge transfer process occurs as the acceptor fluorophore (sti) is excited and the electron moves from the occupied π orbital of donor ast to sti* forming ast+sti-. This rise time corresponds to the combined processes of charge and energy transfers. The second rise in this charge-transfer state at time constant 0.74-1.5 ps is observed and assigned to occur from electron hopping from ast 2π* orbital to sti π*. The third time constant 18-31 ps is observed and is attributed to conversion of anti to syn form in the charge-transfer state because the syn form is more polar and further stabilized in polar environment. A rapid charge transfer process in monosilylene-spaced system although two Si-C single bonds are used as spacer is possibly because of the short distance of the ast and the sti conjugated systems, resulting in π orbital overlap between donor and acceptor.

Stereoselective Iterative Convergent Synthesis of Z-Oligodiacetylenes from Propargylic Dithioacetals.

A series of (t)Bu-substituted Z-oligodiacetylenes (Z-ODAs) are synthesized from the reactions of allenyl/propargylic zinc reagents, obtained from the corresponding propargylic dithiolanes and BuLi, with dithiolane-substituted propargylic aldehydes followed by stereospecific elimination of ß-thioalkoxy alcohols under Mitsunobu conditions. The stereochemical assignments are based on NOE experiments. The X-ray structure of the hexamer further supports the Z configuration for each of the double bonds in these ODAs. The photophysical properties of these Z-ODAs have been examined and are compared with known related E- and Z-ODAs with different substituents.

Stereoselective formation of eight-membered rings by radical cyclization of silylenedioxy-tethered bis-methacrylate derivatives.

Radical-initiated addition of CCl4, Cl3CBr, PhSH, and (TMS)3SiH to (bisisopropyl)silylenedioxy-tethered bis-methacrylate derivatives gives the corresponding eight-membered ring cyclic adducts stereoselectively. Hydrolysis of halo-substituted cyclic adducts with HCl in methanol affords the corresponding valerolactones, and the stereochemistry was determined by the X-ray crystallography on a dibromobenzoate derivative. DFT calculation on the eight-membered radical intermediate offers a plausible rationale for the stereoselectivity of the reaction.

Excimer formation in a confined space: photophysics of ladderphanes with tetraarylethylene linkers.

Communication between chromophores is vital for both natural and non-natural photophysical processes. Spatial confinements offer unique conditions to scrutinize such interactions. Polynorbornene- and polycyclobutene-based ladderphanes are ideal model compounds in which all tetraarylethylene (TAE) linkers are aligned coherently. The spans for each of the monomeric units in these ladderphanes are 4.5-5.5 Å. Monomers do not exhibit emission, because bond rotation in TAE can quench the excited-state energy. However, polymers emit at 493 nm (Φ=0.015) with large Stokes shift under ambient conditions and exhibit dual emission at 450 and 493 nm at 150 K. When the temperature is lowered, the emission intensity at 450 nm increases, whereas that at 493 nm decreases. At 100 K, both monomers and polymers emit only at 450 nm. This shorter-wavelength emission arises from the intrinsic emission of TAE chromophore, and the emission at 493 nm could be attributed to the excimer emission in the confined space of ladderphanes. The fast kinetics suggest diffusion-controlled formation of the excimer.

Oligonorbornenes with hammock-like crown ether pendants as artificial transmembrane ion channel.

Trimeric oligonorbornenes with hammock-like crown ether pendants 3b and 3c were selectively synthesized by cascade metathetical cyclopolymerization upon treatment with the first generation Grubbs catalyst. These crown-ether-containing oligonorbornenes are impregnated in egg yolk phosphatidylcholine (EYPC) liposome as an artificial ion channel. The efficiency of the sodium ion transport properties has been examined. Oligomer 3c having a polar hydroxy end group exhibits the highest transport efficiency, which is comparable with the best efficiencies reported in literature. The orientation of the crown ether moieties in these oligomers may be critical for the ion transport properties.

Ladderphanes: a new type of duplex polymers.

A polymeric ladderphane is a step-like structure comprising multiple layers of linkers covalently connected to two or more polymeric backbones. The linkers can be planar aromatic, macrocyclic metal complexes, or three-dimensional organic or organometallic moieties. Structurally, a DNA molecule is a special kind of ladderphane, where the cofacially aligned base-pair pendants are linked through hydrogen bonding. A greater understanding of this class of molecules could help researchers develop new synthetic molecules capable of a similar transfer of chemical information. In this Account, we summarize our studies of the strategy, design, synthesis, characterization, replications, chemical and photophysical properties, and assembly of a range of double-stranded ladderphanes with many fascinating structures. We employed two norbornene moieties fused with N-arylpyrrolidine to connect covalently with a range of relatively rigid linkers. Ring opening metathesis polymerizations (ROMP) of these bis-norbornenes using the first-generation Grubbs ruthenium-benzylidene catalyst produced the corresponding symmetrical double-stranded ladderphanes. The N-arylpyrrolidene moiety in the linker controls the isotactic selectivity and the trans configuration for all double bonds in both single- and double-stranded polynorbornenes. The π-π interactions between these aryl pendants may contribute to the high stereoselectivity in the ROMP of these substrates. We synthesized chiral helical ladderphanes by incorporating asymmetric center(s) in the linkers. Replication protocols and sequential polymerization of a monomer that includes two different polymerizable groups offer methods for producing unsymmetical ladderphanes. These routes furnish template synthesis of daughter polymers with well-controlled chain lengths and polydispersities. The linkers in these ladderphanes are well aligned in the center along the longitudinal axis of the polymer. Fluorescence quenching, excimer formation, or Soret band splitting experiments suggest that strong interactions take place between the linkers. The antiferromagnetism of the oxidized ferrocene-based ladderphanes further indicates strong coupling between linkers in these ladderphanes. These polynorbornene-based ladderphanes can easily aggregate to form a two-dimensional, highly ordered array on the graphite surface with areas that can reach the submicrometer range. These morphological patterns result from interactions between vinyl and styryl end groups via π-π stacking along the longitudinal axis of the polymer and van der Waals interaction between backbones of polymers. Such assembly orients planar arene moieties cofacially, and polynorbornene backbones insulate each linear array of arenes from the adjacent arrays. Dihydroxylation converts the double bonds in polynorbornene backbones of ladderphanes into more hydrophilic polyols. Hydrogen bonding between these polyol molecules leads to self-assembly and produces structures with longitudinally staggered morphologies on the graphite surface.

Double stranded polymeric ladderphanes with 16-π-electron antiaromatic metallocycle linkers.

Three double stranded polymeric ladderphanes with 16-π-electron antiaromatic metallocycle linkers are synthesised by ring opening metathesis polymerisation of the corresponding bisnorbornene monomers. Scanning tunnelling microscopic (STM) images indicate that these polymers can assemble nicely on a graphite surface to form a highly ordered pattern which has been observed in other ladderphanes with different kinds of aromatic linkers. Little change in (1)H NMR, absorption spectra and electrochemical oxidation potential between these polymers and the corresponding monomers suggest that there would be no interactions between adjacent antiaromatic linkers in these polymeric ladderphanes. Presumably, the distance between two antiaromatic rings in these ladderphanes (5-6 Å) is far too long in comparison with that between two rings in methylene-bridged antiaromatic superphanes (2.5 Å<), where stabilisation is predicted by theoretical calculations.

Folding of alternating dialkylsilylene-spaced donor-acceptor copolymers: the oligomer approach.

A series of oligmers with donor-acceptor pairs separated by diisopropylsilylene (iPr(2)Si) spacers, composed of monomer 4b, dimer 5, trimer 6, and tetramer 7, were synthesized to scrutinize the folding behavior. Monomer 4a with a dimethylsilylene (Me(2)Si) spacer was also prepared for comparison. The 4-aminostyrene moiety was used as the donor and the stilbene moiety as the acceptor. Both steady-state and time-resolved fluorescence spectroscopic measurement were made. Regardless of the substituents on the silicon atom, the emission spectra of 4a and 4b exhibit both local excited (LE) emission of the acceptor chromophore and emission from the charge-separated state (CT emission), which are similar to that of the corresponding Me(2)Si-spaced copolymer 2a with the same donor and acceptor chromophores, but different from that of the copolymer with the iPr(2)Si spacer 2b. Dimer 5 behaves like 4 and 2a. As the chain length of the oligomers increases, the emission properties of the higher homologues become prone to that of 2b. Thus, tetramer 7 exhibits emission from the charge-transfer complex, which is essentially same as that of 2b. Moreover, charge-transfer absorptions emerge in 6 and 7. These results suggest that the folding nature of oligomers approaches that of the corresponding polymer, as the degree of oligomerization increases, and the electronic interactions between adjacent donor-acceptor pairs are controlled by the steric effect of the substituents on the silicon atoms and concomitant amplification of the stabilizing energy by extending the distance of the folding structure.

Controlling conformations in alternating dialkylsilylene-spaced donor-acceptor copolymers by a cooperative Thorpe-Ingold effect and polymer folding.

A series of dialkylsilylene-spaced copolymers 6 and 7, which contain Me(2)Si and iPr(2)Si spacer groups, respectively, and have alternating donor and acceptor chromophores, have been designed and regioselectively synthesized by hydrosilylation. The ratio of the donor and acceptor chromophores for each repeat unit is 2:1, and the two donor chromophores are linked by a trimethylene bridge. A 4-aminostyrene moiety is used as the donor and a series of acceptor chromophores with different reduction potentials are employed. Both steady-state and kinetic measurements reveal that the photoinduced electron transfer (PET) in 6 obeyed the Marcus theory in which normal and inverted regions are observed. On the other hand, the iPr(2)Si-spaced copolymers 7 exhibit absorption and emission from the charge-transfer complexes exclusively due to ground-state interactions between the donor and acceptor chromophores. The discrepancy in photophysical behavior may have arisen from the difference in distance between the adjacent donor and acceptor chromophores. The bulkiness of the substituents on the silicon atom (i.e., Me versus iPr) may exert the Thorpe-Ingold effect on the local conformation around the silicon atom. The differences in the small energetic barriers for each of the conformational states may be amplified by extending the distance of the folding structure, which results in perturbing the conformation of the polymers. These results suggest that the electronic interactions between adjacent donor-acceptor pairs in these copolymers are controlled by the synchronization of the substitution effect and corresponding polymeric structures.

Supramolecular porphyrin-DABCO array in single- and double-stranded polynorbornenes.

Zinc-porphyrin-appended norbornene derivative 6, the corresponding dimer 7, coherently aligned single-stranded polynorbornene 5, and zinc-porphyrin-linked double-stranded polymeric ladderphane 3 have been used for complexation with 1,4-diazabicyclo[2.2.2]octane (DABCO) to generate an array of porphyrin-DABCO supramolecular scaffolds. The stoichiometries of the complex formation are analyzed in detail by (1)H NMR spectroscopy as well as absorption and emission spectroscopic methods. Bidentate ligand DABCO is found to form with 7, 5, and 3 to give 2-to-1 porphyrin-DABCO sandwich structures. Monomer 6, on the other hand, forms a 2-to-1 porphyrin-DABCO sandwich structure at high concentration (20 mM), but a 1-to-1 porphyrin-DABCO complex at low concentration (10(-6) M). Time-resolved fluorescence decays revealed a similar behavior for the 2-to-1 porphyrin-DABCO sandwich structures that are formed from 3, 5, and 7.

Phenanthrene-tethered Furan-containing cyclophenes: synthesis and photophysical properties.

Two phenanthrene-fused furan-containing teraryl cyclophenes 5 and 6 are synthesized. These cyclophenes exhibit charge transfer band in the absorption spectra, unusually large Stokes shifts in the emission spectra, and exceptionally high mubeta values in the electric-field-induced second-harmonic generation (EFISH) experiments. The mubeta(1.91) values for 5 and 6 are 438 and 777 x 10(-48) esu, respectively. The bridging double bond in 5 and 6 can serve as either an electron donor or acceptor depending on the nature of the substituent on furan rings. DFT calculations at the B3LYP/6-31G** level indicate that the electron density distributions in HOMO and LUMO are very different. Interaction between the oligoaryl systems and the double bond may account for the significant enhancement in hyperpolarizability.

Thorpe-Ingold effect in organosilicon chemistry.

Recent advances in the use of the concept of the Thorpe-Ingold effect to rationalise some synthetic chemistry involving organosilicon compounds and, in particular, the photophysical properties of certain dialkylsilylene spaced conjugated chromophores are discussed. The silicon moieties are considered to be insulating tetrahedral spacers in these silicon-containing copolymers. The substituents on silicon can readily be tuned and the steric environment of these substituents around the silicon may dictate the conformation (or helicity) of the copolymers and hence their photophysical properties. This substituent effect on silicon can be understood by the concept of the Thorpe-Ingold effect.

Hydrogen-bonding induced cooperative effect on the energy transfer in helical polynorbornenes appended with porphyrin-containing amidic alanine linkers.

Polynorbornenes appended with porphyrins containing a range of different linkers are synthesized. The use of bisamidic chiral alanine linkers between the pending porphyrins and the polymeric backbone has been shown to bring the adjacent porphyrin chromophores to more suitable orientation for exciton coupling owing to hydrogen bonding between the adjacent linkers. The hydrogen bonding between the adjacent pendants in these polymers may induce a cooperative effect and therefore render single-handed helical structures for these polymers. Such a cooperative effect is reflected in the enhancement of FRET efficiencies between zinc-porphyrin and free base porphyrin in random copolymers.

Observing second-order nonlinear optical properties by symmetry breaking in centrosymmetric furan-containing oligoaryl cyclophandienes.

Centrosymmetric furan-containing cyclophandienes 3 and 4, synthesized by our furan annulation protocol, have been shown to exhibit extraordinarily large Stokes shifts and second-order nonlinear optical beta values. The beta values for 3 and 4 measured at 1.32 mum are 208 and 530x10(-30) esu, respectively. The beta values of 3 and 4 are similar to those of respective cyclophenes 1 a and 7 in which strong hyperpoarizable interactions between two twisted pi-systems (oligoaryl and bridging double bond) might take place. Symmetry breaking due to the resonance contribution (cf. 2) and the unique structural features of 3 and 4 has been used to account for this unusual photophysical behavior.