1,3-Diphenylindenyl zirconium complexes. Synthesis,structure, and reduction reactions

New zirconium sandwich complexes (η⁵-C₉H₅Ph₂)₂ZrCl₂ (1) and (η⁵-C₉H₅Ph₂)-(η⁵-C₅(CH₃)₅)ZrCl₂ (2) were synthesized and characterized. The molecular structures of complexes 1 and 2 were established by X-ray diffraction. The reduction of 1 and 2 does not stop at the formation of the Zr^II derivatives with the η⁹-coordinated indenyl ligand; instead the reaction results in the destruction of the metallocene structure. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1626–1629, August, 2008.     

Enamine Chemistry—XXIV.: Synthesis,thiation, and reduction of lactams

Enamines, 1, prepared from cyclohexanones or cyclopentanones are reacted with acrylamide to give lactams, the condensed 2-piperidones, 2. Ethyl 2-(1-pyrrolidinyl)-2-cyclohexene-1-propanoate, 3, when treated with primary amines, produces the corresponding N-substituted 2-piperidones, 4. Ethyl 2-(1-pyrrolidinyl)-2-cyclohexene-1-ethanoates and ethyl 2-(1-pyrrolidinyl)-2-cyclopentene-1-ethanoate, 5, react with primary amines to give condensed N-substituted 2-pyrrolidones, 6, and non-cyclic imines, 7. The starting enamines, 1, treated with 2-bromo acetamides only afford the N-alkylated compounds 8 (2-pyrrolidino acetamides), and the regioselectivity of this reaction is rationalized in terms of the HSAB-principle. Compound 1 undergoes an exchange reaction (aminolysis) when reacted with primary amines to give the imines 9. Thiation of the lactams 2 and 6 with the Lawesson reagent (LR), affords the corresponding thiolactams, 10. Reduction of the lactams and thiolactams, 2, 6, and 10 by LAH gives the imines, 11, and the enamines, 16. Further reduction of 11 (LAH) affords the saturated amines, 15. The stereochemistry for the formation of 15 is discussed using the torsion angle notation and the principal of least torsional distortion. In a one-pot reaction using LAH-acetic anhydride the lactams, 2, and the thiolactams, 10, are transformed into the enamides, 14. Compound 14 was also obtained from 11 by direct acetylation with acetic anhydride in the presence of triethylamine.     

Effect of temperature on the enantioselectivity in the oxazaborolidine-catalyzed asymmetric reduction of ketones. Noncatalytic borane reduction, a nonneglectable factor in the reduction system

The effect of temperature on the enantioselectivity of the oxazaborolidine-catalyzed asymmetric borane reduction of ketones has been investigated carefully using alkyl aryl ketones with a variety of functional groups and a B-methoxyoxazaborolidine derived from trimethyl borate and (S)-alpha,alpha-diphenylprolinol as a catalyst. The reductions were carried out over a range of temperatures in THF and toluene with or without the catalyst. The reductive rates increase along with increasing reaction temperature with or without the catalyst by determining the conversion of the ketones to alcohols by GC analysis. However, the rates of the catalytic reductions increase faster than those without the catalyst. The results indicate that the noncatalytic borane reduction is an important factor to the enantioselectivity in the reduction. The highest enantioselectivities were usually obtained between 20 and 30 degrees C in the asymmetric reduction.     

The birch reduction of heterocyclic compounds. 11. Birch reduction and reductive alkylation of furamides

The Birch reduction-alkylation of some N,N-dialkylfuramides and its application to the useful intermediates for the natural product synthesis are described.     

Furopyridines. XXXI. Birch reduction of furopyridines

Birch reduction of four furopyridines 1a‐d effected the characteristic cleavage of the furan ring, giving ethnylpyridinols 2a‐d , vinylpyridinols 3b,d , and ethylpyridinols 4a‐d , and the reduction of the furan ring, giving dihydrofuropyridine 5c,d.     

Low-valent niobium-catalyzed reduction of alpha,alpha,alpha-trifluorotoluenes

[reaction: see text] In the presence of 5 mol % of niobium(V) chloride, alpha,alpha,alpha-trifluorotoluene derivatives were reduced with lithium aluminum hydride to give toluene derivatives in good yields. Stepwise, partial reduction of bis(trifluoromethyl)benzene derivative was also demonstrated.     

A novel reducing agent. The reduction of dicarbonyls,acyloins, alkynes,and alkenes with active uranium

Active uranium, prepared in hydrocarbon solvents from UCl₄ and [(TMEDA)Li]₂[naphthalene] is a novel reducing agent. The unprecedented stepwise reduction of benzil and benzoin to bibenzyl has been shwon to proceed via the unsaturated species diphenylacetylene, cis-stilbene, and trans-stilbene. Active uranium also effects oligomerizations of diphenylacetylene and trans-stilbene.     

A readily-prepared, convergent, oxygen reduction electrocatalyst

Monomeric cobalt(II) tetraphenylporphyrin immobilized in high concentrations within vapour-phase polymerized polypyrrole deposited on an ITO electrode catalyzes the 4-electron reduction of dioxygen to water, a reaction requiring concerted action by two separate catalytic groups.     

Theoretical studies on pteridines. 2. Geometries,tautomer, ionization and reduction energies of substrates and inhibitors of dihydrofolate reductase

Ab initio SCF calculations with STO-3G and 3-21G basis sets are reported for approximately 85 pteridines of interest to the study of the reaction and inhibition mechanisms of dihydrofolate reductase. These include tautomeric, protonated, deprotonated, reduced and 6-substituted forms of the 2-amino-4-oxo- and 2,4-diamino-pteridines, many of which are not easily amenable to experimental investigation. Full geometry optimizations at the SCF/STO-3G level for 30 such pteridines have been performed. A step-wise computational protocol designed to identify the minimum level of theory necessary for reliable prediction of relative tautomer, reduction and ionization energies has been developed in an effort to minimize the cost of calculations for this reasonably large N-heterobicylic system. In general, SCF/STO-3G results were found to be inadequate while SCF/3-21G results obtained with STO-3G optimized geometries agreed with all available experimental evidence with the exception of the relative acidity of 6-methyl-7,8-dihydropterin. Correlation graphs relating experimental pK_a's to the calculated ionization energies are presented: these are of potential predictive value. An analysis is given of the importance of resonance substructures, such as the guanidinium and extended-guanidinium groups, in stabilizing some preferred tautomeric and ionized forms, and in explaining the observed geometry changes.     

Heterocycles. XXV Sodium borohydride reduction of flavanonols

Solvent effects on the stereochemistry in the sodium borohydride reduction of (±)-flavanonols have been examined. The effects observed for the (±)-flavanonols with 5-OMe in 2-propanol, dioxane and methanol are explainable by the differences between the steric interactions inherent in the product-like transition states A and B. It has been also found that 5-OAc peculiarly affects the stereochemistry in the reduction to produce the (±)-catechin-type compounds in a one-pot process. The solvent and temperature effects are examined using a model analogous to the above.     

Stereospecificity of acetylene reduction catalyzed by nitrogenase.

In addition to catalyzing the reduction of dinitrogen to ammonia, the metalloenzyme nitrogenase catalyzes the reduction of a number of alternative substrates, including acetylene (C(2)H(2)) to ethylene (C(2)H(4)) and, in certain cases, to ethane (C(2)H(6)). The stereochemistry of proton addition for C(2)D(2) reduction to C(2)D(2)H(2) catalyzed by the Mo-dependent nitrogenase has been used to probe substrate binding and proton addition mechanisms. In the present work, the C(2)D(2) reduction stereospecificity of altered MoFe proteins having amino acid substitutions within the active site FeMo-cofactor environment was examined by Fourier transform infrared (FTIR) spectroscopy. Altered MoFe proteins examined included those having the alpha-subunit 96(Arg) residue substituted by Gln, Leu, or Ala, the alpha-subunit 69(Gly) residue substituted by Ser, and the alpha-subunit 195(His) residue substituted by Asn. The stereochemistry of proton addition to C(2)D(2) does not correlate with the measured K(m) values for C(2)H(2) reduction, or with the ability of the enzyme to reduce C(2)H(2) by four electrons to yield C(2)H(6). Instead, the electron flux through nitrogenase was observed to significantly influence the ratio of cis- to trans-1,2-C(2)H(2)D(2) formed. Finally, the product distribution observed for reduction of C(2)H(2) in D(2)O is not consistent with an earlier proposed enzyme-bound intermediate. An alternative model that accounts for the stereochemistry of C(2)H(2) reduction by nitrogenase based on a branched reaction pathway and an enzyme-bound eta(2)-vinyl intermediate is proposed.