Way Zen Lee

Characterization of a paramagnetic mononuclear nonheme iron-superoxo complex

O2 bubbling into a THF solution of FeII(BDPP) (1) at −80 °C generates a reversible bright yellow adduct 2. Characterization by resonance Raman and Mössbauer spectroscopy provides complementary insights into the nature of 2. The former shows a resonance-enhanced vibration at 1125 cm–1, which can be assigned to the ν(O–O) of a bound superoxide, while the latter reveals the presence of a high-spin iron(III) center that is exchange-coupled to the superoxo ligand, like the FeIII–O2– pair found for the O2 adduct of 4-nitrocatechol-bound homoprotocatechuate 2,3-dioxygenase. Lastly, 2 oxidizes dihydroanthracene to anthracene, supporting the notion that FeIII–O2– species can carry out H atom abstraction from a C–H bond to initiate the 4-electron oxidation of substrates proposed for some nonheme iron enzymes.

Highly enantioselective Rh-catalyzed alkenylation of imines: synthesis of chiral allylic amines via asymmetric addition of potassium alkenyltrifluoroborates to N-tosyl imines.

For the first time, simple N-tosyl aryl aldimines, prepared from the condensation of tosyl amide and aromatic aldehydes, can be used as substrates in the rhodium catalyzed 1,2-addition reaction using alkenylboron nucleophiles. In the presence of 1.5 mol % of [RhCl(1e)]2, enantioselective addition of various potassium alkenyltrifluoroborates to aryl aldimines furnished the corresponding chiral allylic amines in 73-96% yield and 72->99.5% ee. Notably, this method efficiently provides the di-, tri-, and tetrasubstituted allylic N-tosyl amines with high asymmetric induction.

A Discrete Five‐Coordinate NiIII Complex Resembling the Active Site of the Oxidized Form of Nickel Superoxide Dismutase

Superoxide anion (O2 ), an unavoidable byproduct in aerobic organisms, can cause irreversible cellular damage. To suppress such oxidative stress three classes of superoxide dismutase (SOD) have evolved in various organisms: 1) CuZnSOD, 2) FeSOD or MnSOD, and 3) NiSOD. The Ni-containing superoxide dismutase (NiSOD) discovered recently from Streptomyces species and marine cyanobacteria can catalyze the disproportionation of O2 into O2 and H2O2 through a cycle of Ni II and Ni oxidation states (Scheme 1). The X-ray structure of the reduced NiSOD re-

Phosphorus complexes of meso-triaryl-25-oxasmaragdyrins.

The aromatic PO2 complexes of meso-triaryl-25-oxasmaragdyrins were synthesized by treating the free base 25-oxasmaragdyrins with POCl3 in toluene/triethylamine at refluxing temperature. The complexes are stable and characterized by X-ray and different spectroscopic techniques. In these complexes, the phosphorus(V) ion was bound to two pyrrolic nitrogen atoms of the smaragdyrin macrocycle and two oxygen atoms in tetrahedral geometry. The X-ray structure revealed that the smaragdyrin macrocycle showed significant distortion upon insertion of a PO2 unit, and the phosphorus atom lies 1.339 Å above the mean plane defined by three meso-carbon atoms of the macrocycle. These complexes absorb strongly in the visible region and are 2.5 times more strongly fluorescent than free base 25-oxasmaragdyrins. The smaragdyrin macrocycle becomes electron-deficient upon complexation with a PO2 unit because these complexes are easier to reduce but difficult to oxidize compared to free base smaragdyrins. We designed and synthesized a covalently linked BODIPY-PO2-smaragdyrin dyad and demonstrated efficient energy transfer from the BODIPY unit to the PO2-smaragdyrin unit.

Synthesis and Functionalization of BF2-Complexes of meso-Free 25-Oxasmaragdyrin

BF2-complex of meso-free 25-oxasmaragdyrin is synthesized under simple reaction conditions in high yield, and the reactivity of meso-free carbon atom was demonstrated by carrying out functionalization followed by coupling reactions.

Synthesis of meso‐Pyrrole‐Substituted 22‐Oxacorroles by a “3+2” Approach

Unsymmetrical 22-oxacorrole containing two aryl groups and one pyrrole group at the meso position was synthesized by condensing one equivalent of 16-oxatripyrrane with one equivalent of meso aryl dipyromethane under mild acid-catalyzed conditions followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). This [3+2] condensation approach was expected to yield meso-free 25-oxasmaragdyrin but unexpectedly afforded unsymmetrical meso-pyrrole-substituted 22-oxacorrole. We demonstrated the versatility of the reaction by synthesizing four new meso-pyrrole-substituted 22-oxacorroles. The reactivity of α-position of meso-pyrrole was tested by carrying out various functionalization reactions such as bromination, formylation, and nitration and obtained the functionalized meso-pyrrole-substituted 22-oxacorroles in decent yields. The X-ray structure obtained for one of the functionalized meso-pyrrole substituted 22-oxacorrole revealed that the macrocycle was nearly planar and the meso-pyrrole was in the perpendicular orientation with respect to the macrocyclic plane. The meso-pyrrole-substituted 22-oxacorroles absorb strongly in 400-700 nm region with one strong Soret band and four weak Q bands. The 22-oxacorroles are strongly fluorescent and showed emission maxima at ≈650 nm with decent quantum yields and singlet-state lifetimes. The 22-oxacorroles are redox-active and exhibited three irreversible oxidations and one or two reversible reduction(s). A preliminary biological study indicated that meso-pyrrole corroles are biocompatible.

Unusual formation of 21-oxacorrole from 21-oxaporphyrin induced by phosphoryl chloride.

An unusual formation of 21-oxacorrole from 21-oxaporphyrin by concomitant elimination of a meso-aryl group and ring contraction is reported.

Synthesis, structure, spectral and electrochemical properties of B(OR)2-smaragdyrin complexes.

A series of B(OR)2-smaragdyrin complexes were synthesized by treating BF2-smaragdyrin complex with various aliphatic and aromatic alcohols in the presence of AlCl3 in CH2Cl2 at room temperature. The compounds are stable and characterized by various spectroscopic and electrochemical techniques. The X-ray structure solved for one of the B(OR)2-smaragdyrin complex showed that the macrocycle is planar and the boron atom is in tetrahedral geometry coordinated to two pyrrole nitrogens of the macrocycle and two oxygen atoms of the axial -OR groups. (1)H NMR studies indicated that the alkoxy or aryloxy groups present on the boron atom experience the macrocyclic ring current effect and appear in the up-field region. The absorption properties of B(OR)2-smaragdyrin complexes showed a strong absorption band at ~700 nm along with other absorption bands in the 450-650 nm region like BF2-smaragdyrin complex. Since B(OR)2-smaragdyrin complexes are electron rich due to the presence of electron rich alkoxy and aryloxy groups, the B(OR)2-smaragdyrin complexes are easier to oxidize but difficult to reduce compared to BF2-smaragdyrin. The B(OR)2-smaragdyrin complexes are found to be weakly fluorescent than BF2-smaragdyrin.

Ambient Stable Trigonal Bipyramidal Copper(III) Complexes Equipped with an Exchangeable Axial Ligand

A stable trigonal bipyramidal copper(III) complex, [PPN][Cu((TMS)PS3)Cl] (1, wherein PPN represents bis(triphenylphosphine)iminium), was synthesized from CuCl2/PPNCl via intramolecular copper(II) disproportionation. Under ambient conditions, the axial chloride of 1 is exchangeable in solution thus making 1 serve as an intermediate to prepare trigonal bipyramidal copper(III) derivatives, e.g., [PPN][Cu((TMS)PS3)(N3)] (2) and [Cu((TMS)PS3)(DABCO)] (3). Diamagnetic complexes 1-3 were fully characterized by X-ray crystallography, NMR, UV-vis, and Cu K-edge absorption spectroscopy. A series of UV-vis titrations were performed to investigate the relative ligand affinity toward the [Cu((TMS)PS3)] moiety, verifying the 1:1 binding equilibrium between various ligands. Compared to known copper(III) compounds, Cu K-edge absorptions of 1-3 possess lower pre-edge energy and higher shakedown transition energy, which, respectively, attribute to the electron donation from (TMS)PS3(3-) ligand and their trigonal ligand field.

A Stable Seven-Membered Heterocycle, Containing B, C, N, O, and P Atoms, inside a Smaragdyrin Macrocycle

Unprecedented examples of smaragdyrin macrocycles containing seven membered heterocyclic rings were synthesized under simple reaction conditions in high yields. The heterocycle formed inside smaragdyrin macrocycle is rare example of heterocycle containing five different atoms, such as B, C, N, O, and P atoms. The mixed B(III) and P(V) complexes of smaragdyrin macrocycles showed new structural, spectral, and electrochemical properties.