Amidophenoxide-to-osmium π donation is sensitive to the co-ligands; it engenders significant anisotropy in the bonding with an oxo group.
Oxobis(iminoxolene)osmium(VI) compounds ( R ap) 2 OsO ( R ap = 2-(4-RC 6 H 4 N)-4,6-t Bu 2 C 6 H 2 O) are readily deoxygenated by phosphines and phosphites to give five-coordinate ( R ap) 2 Os(PR′ 3 ) or six-coordinate ( R ap) 2 Os(PR′ 3 ) 2 . Structural data indicate that this net two-electron reduction is accompanied by apparent oxidation of the iminoxolene ligands due to their greater ability to engage in π donation to the reduced deoxy form of the osmium complex. In ( R ap) 2 Os(PR′ 3 ) 2 , the HOMO is a ligand-based combination of the iminoxolene redox-active orbitals, while the LUMO is a highly covalent metal-iminoxolene π* orbital. In the trans isomer, the HOMO is required to be ligand-localized by symmetry, while in the cis isomer, the ligands adopt a conformation that minimizes metal−ligand π* interactions in the HOMO. Kinetic studies indicate that the deoxygenations involve the ratedetermining attack of the phosphorus(III) reagent on the five-coordinate oxo complexes. Varying the substituents of the aryl groups on the iminoxolene ligands or on the triarylphosphines has little effect on the rate of oxygen atom transfer, with the best correlation shown between oxygen atom transfer rates and the HOMO−LUMO gap of the oxo complexes. This suggests that the osmium oxo group shows a balance between electrophilic and nucleophilic character in its oxygen atom transfer reactions with phosphorus(III) reagents.
The crystal and molecular structure of the cytochrome P-450 inhibitor, SKF-525A [2-(diethylamino)ethyl 2,2-diphenylpentenoate; proadifen hydrochloride] is described. Proadifen hydrochloride crystallized from an ethyl acetate and acetic acid mixture in the space group P2(1)/c with one molecule in the asymmetric unit. Cell constants are a = 18.716(4), b = 8.906(1), c = 14.201(3), beta = 109.41(1) degrees. The structure was solved using direct methods and was refined to an R value of 0.047; weighted R of 0.061 using 3757 reflections. From the crystal and molecular structure, it is seen that SKF-525A has two principal modes of complementary interactions with the enzyme available: polar and non-polar. Polar interactions that principally involve the chloride anion, the quaternary nitrogen atom and the carbonyl oxygen atom. In particular, there are two strong hydrogen bonds, one between Cl ... H-N, 3.090(1) A, the other between O2 ... H-C111 (one of the ethyl hydrogen atoms) at 3.411(2) A. The other is through non-polar interactions involving the phenyl and alkyl groups. Comparisons between proadifen hydrochloride and other inhibitors whose atomic coordinates are available reveal common features which correlate with their function. These include groups to provide necessary intermolecular contacts and bulk, such as a phenyl group and a hydrogen bond acceptor, as well as a tetrahedral atom, which allows for substrate flexibility.
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