In vitro enzymatic oxidation of a fluorine-tagged sulfido substrate analogue: a 19F NMR investigation

1H-decoupled 19F NMR has been used to monitor the highly regioselective oxidation of a fluorine-tagged thia-fatty acid derivative by castor stearoyl-ACP delta9 desaturase. The major enzymatic product, after reductive work-up, was identified as 9-fluoro-1-nonanol. This compound could be easily distinguished from substrate and a 9-sulfoxy by-product on the basis of its 19F NMR chemical shift and spiking experiments using authentic standards. Structural assignment of the cleavage product was confirmed by GC-MS analysis of the enzymatic products.     

Intramolecular cyclopropanation of unsaturated terminal aziridines

[STRUCTURE: SEE TEXT] Regio- and stereoselective deprotonation of bishomoallylic terminal N-Bus (Bus=tert-butylsulfonyl)-protected aziridines generate aziridinyl anions that undergo diastereoselective intramolecular cyclopropanation giving trans-2-aminobicyclo[3.1.0]hexanes in good to excellent yields.     

X-ray absorption edge spectroscopy and computational studies on LCuO2 species: Superoxide-Cu(II) versus peroxide-Cu(III) bonding

The geometric and electronic structures of two mononuclear CuO2 complexes, [Cu(O2){HB(3-Ad-5-(i)Prpz)3}] (1) and [Cu(O2)(beta-diketiminate)] (2), have been evaluated using Cu K- and L-edge X-ray absorption spectroscopy (XAS) studies in combination with valence bond configuration interaction (VBCI) simulations and spin-unrestricted broken symmetry density functional theory (DFT) calculations. Cu K- and L-edge XAS data indicate the Cu(II) and Cu(III) nature of 1 and 2, respectively. The total integrated intensity under the L-edges shows that the 's in 1 and 2 contain 20% and 28% Cu character, respectively, indicative of very covalent ground states in both complexes, although more so in 1. Two-state VBCI simulations also indicate that the ground state in 2 has more Cu (/3d8) character. DFT calculations show that the in both complexes is dominated by O2(n-) character, although the O2(n-) character is higher in 1. It is shown that the ligand L plays an important role in modulating Cu-O2 bonding in these LCuO2 systems and tunes the ground states of 1 and 2 to have dominant Cu(II)-superoxide-like and Cu(III)-peroxide-like character, respectively. The contributions of ligand field (LF) and the charge on the absorbing atom in the molecule (Q(mol)M) to L- and K-edge energy shifts are evaluated using DFT and time-dependent DFT calculations. It is found that LF makes a dominant contribution to the edge energy shift, while the effect of Q(mol)M is minor. The charge on the Cu in the Cu(III) complex is found to be similar to that in Cu(II) complexes, which indicates a much stronger interaction with the ligand, leading to extensive charge transfer.     

The influence of RAFT on the rates and molecular weight distributions of styrene in seeded emulsion polymerizations

Seeded emulsion polymerizations of styrene in the presence of two reversible addition–fragmentation chain‐transfer (RAFT) agents were studied. We designed the seed to be small to observe the effects of exit and, we made the seed of poly(methyl methacrylate) so that the molecular weight distributions of poly(styrene) by gel permeation chromatography could be obtained independently by UV detection. The rates were significantly retarded by the presence of a RAFT agent, with the retardation being greater with an EMA RAFT agent [2‐(ethoxycarbonyl)propyl‐2‐yl dithiobenzoate] than with a cumyl RAFT agent (2‐phenylprop‐2‐yl dithiobenzoate). We propose that exit from the particles after fragmentation was the main cause of retardation. In addition, the number‐average molecular weight and polydispersities (broad) did not resemble the characteristic living behavior found in bulk or solution. This was a result of the continuous transportation of RAFT agent into the particles during interval II and the transportation of a small amount during interval III. A conspicuous red layer was also observed at the beginning of the polymerization. The red layer consisted of low molecular weight dormant species swollen with monomer. Once the switch from interval II to interval III occurred, the low molecular weight species coalesced to form a red coagulant. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3864–3874, 2000