Choon Sup Ra

SYNTHESES OF POLYMETHYLENE BRIDGED DINUCLEAR ZIRCONOCENES AND INVESTIGATION OF THEIR POLYMERISATION ACTIVITIES

Abstract The polymethylene bridged dinuclear zirconocenes [(CH2)n (C5H4)2][(C9H7)ZrCl2]2 (n=3 (10), 5 (11), 7 (12), 9 (13)) have been synthesised by treating not only the respective disodium salts of the ligands with two equivalents of (C9H7)ZrCl3 in THF, but also the distannylated derivatives of the ligands with two equivalents of (C9H7)ZrCl3 in toluene. All complexes are characterised by 1H- and 13C-NMR and mass spectrometry and elemental analysis. It turned out that the values of Δδ=[δd−δp], the chemical shift difference between the distal (δd) and proximal (δp) protons, for the produced dinuclear compounds (0.1 for 10 and 11, and 0.11 for 12 and 13) were smaller than the Δδ values of the known polysiloxane bridged dinuclear compounds. The chemical shifts of the bridgehead carbons in these complexes are about 135 ppm shifted downfield with respect to the other two resonances at cyclopentadienyl ring (113.7 and 113.7 ppm, respectively). In order to investigate the catalytic properties of the dinuclear complexes and mononuclear metallocenes, ethylene polymerisation has been conducted in the presence of MMAO. The most important feature is that the polymethylene bridged dinuclear metallocenes represent enormously improved activities compared with the activities from the corresponding mononuclear metallocene as well as the polysiloxane bridged dinuclear zirconocenes. In addition, the influence of both the nature and length of the bridging ligand upon the reactivities of the dinuclear metallocenes has also been observed.

Effects of alkoxy groups on arene rings of lignin β-O-4 model compounds on the efficiencies of single electron transfer-promoted photochemical and enzymatic C-C Bond Cleavage Reactions.

To gain information about how alkoxy substitution in arene rings of β-O-4 structural units within lignin governs the efficiencies/rates of radical cation C1-C2 bond cleavage reactions, single electron transfer (SET) photochemical and lignin peroxidase-catalyzed oxidation reactions of dimeric/tetrameric model compounds have been explored. The results show that the radical cations derived from less alkoxy-substituted dimeric β-O-4 models undergo more rapid C1-C2 bond cleavage than those of more alkoxy-substituted analogues. These findings gained support from the results of DFT calculations, which demonstrate that C1-C2 bond dissociation energies of β-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two β-O-4 units, containing different degrees of alkoxy substitution, regioselective radical cation C-C bond cleavage was observed to occur in one case at the C1-C2 bond in the less alkoxy-substituted β-O-4 moiety. However, regioselective C1-C2 cleavage in the more alkoxy-substituted β-O-4 moiety was observed in another case, suggesting that other factors might participate in controlling this process. These observations show that lignins containing greater proportions of less rather than more alkoxylated rings as part of β-O-4 units would be more efficiently cleaved by SET mechanisms.

Ab initio studies of the allylic hydroxylation: DFT calculation on the reaction of 2-methyl-2-butene with selenium dioxide

The mechanism of the allylic oxidation of 2-methyl-2-butene with selenium dioxide has been investigated by ab initio quantum mechanics. Transition states for two major steps (an ene reaction and a [2,3]-sigmatropic rearrangement) of this reaction have been optimized by the B3LYP/6-311+G(d,p) method. A comparison of the energies of the transition states shows that the anti-endo and syn-endo approaches are the efficient routes in the ene reaction and the methyl (C4) group is sited in a pseudo-equatorial environment in cyclic transition states during the [2,3]-rearrangement. Calculations also show the kind of the terminal alkyl (C4) substituents may control (E)-selectivity in the formation of the allylic alcohols.

Method for the Synthesis of Amine-Functionalized Fullerenes Involving SET-Promoted Photoaddition Reactions of α-Silylamines

A novel method for the preparation of structurally diverse fullerene derivatives, which relies on the use of single electron transfer (SET)-promoted photochemical reactions between fullerene C60 and α-trimethylsilylamines, has been developed. Photoirradiation of 10% EtOH-toluene solutions containing C60 and α-silylamines leads to high-yielding, regioselective formation of 1,2-adducts that arise through a pathway in which sequential SET-desilylation occurs to generate α-amino and C60 anion radical pair intermediates, which undergo C-C bond formation. Protonation of generated α-aminofullerene anions gives rise to formation of monoaddition products that possess functionalized α-aminomethyl-substituted 1,2-dihydrofullerene structures. Observations made in this effort show that the use of EtOH in the solvent mixture is critical for efficient photoproduct formation. In contrast to typical thermal and photochemical strategies devised previously for the preparation of fullerene derivatives, the new photochemical approach takes place under mild conditions and does not require the use of excess amounts of substrates. Thus, the method developed in this study could broaden the scope of fullerene chemistry by providing a simple photochemical strategy for large-scale preparation of highly substituted fullerene derivatives. Finally, the α-aminomethyl-substituted 1,2-dihydrofullerene photoadducts are observed to undergo photoinduced fragmentation reactions to produce C60 and the corresponding N-methylamines.

Controlled radical polymerization of vinyl acetate in supercritical CO2 catalyzed by CuBr/terpyridine

Poly(vinyl acetate) (PVAc) was synthesized by the atom transfer radical polymerization of vinyl acetate (VAc) in supercritical carbon dioxide using CuBr/2,2′:6′,2″-terpyridine complex as a catalyst and ethyl 2-bromoisobutyrate as an initiator. Polymerization proceeded in a controlled manner, and low to moderate conversion was achieved within a reasonable time. The effects of the monomer amount, temperature, pressure, initiator, and ligand loading on monomer conversion, molecular weight and molecular weight distribution of the polymer were examined. The reaction kinetics was also investigated. The polymerization reaction was found to be first-order with respect to the monomer concentration. The molecular weights of the resulting PVAc increased linearly with increasing VAc conversion.

Synthesis and biological evaluation of kresoxim-methyl analogues as novel inhibitors of hypoxia-inducible factor (HIF)-1 accumulation in cancer cells

We designed and synthesized strobilurin analogues as hypoxia-inducible factor (HIF) inhibitors based on the molecular structure of kresoxim-methyl. Biological evaluation in human colorectal cancer HCT116 cells showed that most of the synthesized kresoxim-methyl analogues possessed moderate to potent inhibitory activity against hypoxia-induced HIF-1 transcriptional activation. Three candidates, compounds 11b, 11c, and 11d were identified as potent inhibitors against HIF-1 activation with IC50 values of 0.60-0.94µM. Under hypoxic condition, compounds 11b, 11c, and 11d increased the intracellular oxygen contents, thereby attenuating the hypoxia-induced accumulation of HIF-1α protein.