Biomedical subjects

Lee A Friedman

Publications and source records attributed to Lee A Friedman.

4 recordsLinked to original sources

PubMed2003

Rhenium-promoted diastereo- and enantioselective cyclopentannulation reactions: furans as 1,3-propene dipoles.

The rhenium furan complexes TpRe(CO)(MeIm)(eta2-2-methylfuran) (1) and TpRe(CO)(MeIm)(eta2-2,5-dmethylfuran) (2) undergo Lewis acid-promoted cyclopentannulation reactions with enones and enals to generate 3-acetylcyclopentene complexes. During the reaction, a rearrangement occurs such that the alpha and beta carbons of the enone are incorporated into the new carbocycle. Treatment of these complexes with an oxidant (H2O2 or silver triflate) liberates the acetylcyclopentene. When a resolved form of the rhenium complex is used, the acetylcyclopentenes can be obtained enantioselectively.

Aldehydes↗

PubMed2002

Dihapto coordination of carboxylic acid derivatives with an asymmetric rhenium pi-base: a new mechanism for amide isomerization?

The asymmetric pi basic metal fragment [TpRe(CO)(MeIm)] (Tp = hydridotris(pyrazolyl)borate, MeIm = 1-methylimidazole) forms thermally stable complexes with ethyl acetate, acetic anhydride, N-methylsuccinimide, N-acetylpyrrole, and N-methylmaleimide in which the metal binds a carbonyl group in a pi fashion. In all cases a single diastereomer is observed, indicating that one enantioface of the carbonyl is selectively coordinated. X-ray and NMR data for the compound TpRe(CO)(MeIm)(eta(2)-N-methylsuccinimide) indicate that metal coordination effectively removes the pi interaction between the bound carbonyl and the nitrogen of the succinimide ring.

Amides↗

PubMed2002

Ligand-localized electron trapping at sensitized semiconductor interfaces.

Nanocrystalline (anatase), mesoporous TiO2 thin films were derivatized with [Ru(bpy)2(deebq)](PF6)2 or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine and deebq is 4,4'-diethylester-2,2'-biquinoline. Both compounds bind to the nanocrystalline TiO2 films with typical limiting surface coverages of 7 (+/-2) x 10-8 mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.60 V vs SCE) occurs prior to TiO2 reduction. Steady-state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced compound, MII(deebq-)(bpy)2+/TiO2. Neither the photoluminescent excited states or the reduced forms of these compounds inject electrons efficiently into TiO2. Transient absorption measurements after a approximately 10-ns laser pulse, reveal greater than 80% MLCT excited states and a smaller fraction of extremely long-lived charge-separated state intermediates assigned to equal concentrations of MII(deebq-)(bpy)2+/TiO2 and MIII(deebq)(bpy)23+/TiO2. The results are consistent with a mechanism of ultrafast electron injection followed by ligand-localized trapping on a second compound. The quantum yield for formation of the charge-separated states (phiCSS) is excitation wavelength dependent. With 417 nm excitation, phiCSS(417) = 0.14 +/- 0.03, and this decreases with 532.5 nm excitation, phiCSS(532.5) = 0.08 +/- 0.03, and 683 nm excitation for M = Os, phiCSS(683) = 0.05 +/- 0.01. Electron transfer to yield ground-state products, MII(deebq-)(bpy)2+/TiO2 + MIII(deebq)(bpy)23+/TiO2 --> 2 MII(deebq)(bpy)22+/TiO2, occurs with a driving force of 2.05 eV for Ru/TiO2 and 1.64 eV for Os/TiO2. The dynamics of this process were quantified on a millisecond time scale and were found to follow second-order kinetics. The intermediates are sufficiently long-lived that continued pulsed excitation at 10 Hz leads to high concentrations and the formation of transient images on the semiconductor surface that are easily observed by the naked eye.

Journal Article↗

PubMed2002

Cycloaddition reactions of dihapto-coordinated furans.

Complexes of the type [TpRe(CO)(L)(eta(2)-furan)], where Tp = hydridotris(pyrazolyl)borate and L = PMe(3) (1) or (t)BuNC (2), undergo dipolar cycloadditions with TCNE (tetracyanoethylene) to afford 7-oxabicycloheptene complexes 3 and 4, respectively. The corresponding 2-methylfuran complexes (5 and 7) for these L ligands give similar methyloxabicycloheptene complexes (6 and 8), with a diastereomer ratio >20:1 for 8. For L = N-methylimidazole (MeIm, 9), TCNE oxidizes the complex, but cycloadditions can be achieved with DMAD (dimethyl acetylenedicarboxylate) as the electrophile. Three complexes are observed: one in which DMAD undergoes a cycloaddition with the carbonyl ylide form of the complex (10C), and two complexes that are coordination diastereomers where DMAD has undergone a formal [2+2] cycloaddition with the uncoordinated double bond of the 4,5-eta(2)-furan ligand (10B and 10A). With the imidazole complex of 2-methylfuran (11), only the [2+2] products (12B and 12A) are observed. In the case of the 2,5-dimethylfuran complex (L = MeIm, 13), which is formed as a single coordination diastereomer, only one [2+2] product is observed (14), the structure of which was confirmed by X-ray crystallography. Oxidative decomplexation of 14 results in liberation of the free oxabicyclo[3.2.0]heptadiene 15, which can be thermally converted to the corresponding oxepin 16 in 70% yield.

Bridged Bicyclo Compounds, Heterocyclic↗