Regioselective coupling of pentafluorophenyl substituted alkynes: mechanistic insight into the zirconocene coupling of alkynes and a facile route to conjugated polymers bearing electron-withdrawing pentafluorophenyl substituents

The reaction of Cp(2)ZrCl(2) with 2 equiv of BuLi at -78 degrees C, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R(1)C[triple bond]CAr(f) (R(1), Ar(f) = (CH(2))(4)Me, p-C(6)F(4)H; Me, p-C(6)F(4)H; Ph, C(6)F(5)), resulted in regioselective couplings of these alkynes to zirconacyclopentadienes in which the Ar(f) substituents preferentially adopt the 3,4-positions (beta beta) of the zirconacyclopentadiene ring. With Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) as the zirconocene reagent, the couplings could be carried out at room temperature; however, at higher temperatures significant quantities of the 2,4-fluoroaryl substituted (alpha beta) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (alpha alpha) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentadiene Cp(2)ZrC(4)-2,5-Ph(2)-3,4-(C(6)F(5))(2), which resulted from the coupling of PhC[triple bond]C(C(6)F(5)), was converted to the corresponding thiophene by reaction with S(2)Cl(2), and to an arene by reaction with MeO(2)CC[triple bond]CCO(2)Me/CuCl. Mechanistic studies on zirconocene couplings of (p-CF(3)C(6)H(4))C[triple bond]C(p-MeC(6)H(4)) indicate that the observed regioselectivities are determined by an electronic factor that controls the orientation of at least one of the two alkynes as they are coupled. Additionally, these studies suggest an unsymmetrical transition state for the zirconocene coupling of alkynes, and this is supported by DFT calculations. The reaction of [(C(6)F(5))C[triple bond]CCH(2)](2)CH(2) with Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) resulted in a zirconacyclopentadiene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (alpha alpha). Zirconocene coupling of the diyne (C(6)F(5))C[triple bond]C-1,4-C(6)H(4)-C[triple bond]C(C(6)F(5)) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups.     

Use of Dimethylsilyl Ethers for Characterizing Primary Aliphatic Alcohols: A comparison of mass spectrometric fragmentation of di- and trimethylsilyl derivatives

Mass spectral fragmentation patterns of dimethylsilyl (DMS) ethers of primary unbranched, branched, and secondary unbranched aliphatic alcohols in the C₅ to C₁₀ range are compared with those of the corresponding trimethylsilyl (TMS) derivatives. Unlike their TMS analogues, DMS ethers of primary alcohols exhibit pronounced rupture of the C? C bond adjacent to the oxygen atom within the alkyl moiety (loss of an alkyl radical R) in marked preference to cleavage within the silyl substituent (loss of CH₃). Within this class of compounds, complementary preparation of DMS derivatives can therefore be used to establish or to confirm the site, and thus the primary nature of the hydroxyl group, whereas preparation of TMS ethers may be of advantage in deducing molecular size. For the derivatives of secondary alcohols this diagnostically useful difference in fragmentation behaviour is not observed.     

INTERACTIONS AMONG PHOTOSYNTHETIC ANTENNA EXCITED STATES

Abstract. The data of Kung and DeVault (1978) showing high-order fluorescence from chromatophores of photosynthetic bacteria are analyzed in relation to other data on first-order fluorescence of photosynthetic systems, particularly that of Monger and Parson (1977). The wavelengths of emission observed (down to 445 nm) require energy equivalent to two lowest singlet-excited states. The dependence on excitation intensity is best explained by any of the following third-order processes: (a) 3 S ₁→3 S ₀; (b) 2 S ₁+ T , → 2 S ₀+ T ₁; (c) S ₁+ 2 T ₁→ 3 S ₀. However, (c) is ruled out because it predicts heavy T ₁-destruction which is not observed. Contribution from the second order process: 2 S ₁→ S ₀ is probable, but even the data of Monger and Parson show that it is insufficient by itself. Two-photon absorption: S ₀+ hv ₁→ S ₁; S ₁+ hv ₁→ S _n; S _n S ₀+ hv ₂ could also account for the high-order fluorescence and its dependence on excitation intensity. [ S ₀, S ₁ S _n are ground, first excited and a higher excited singlet states, respectively, of antenna bacteriochlorophyll, T _t is the lowest triplet state, c/v , is the exciting wavelength (694 or 868 nm) and c/v ₂ the wavelength of the high-order fluorescence (445, 535. or 600 nm), where c = velocity of light.] Maximum values are estimated for some of the rate constants.     

Detection of norbolethone,an anabolic steroid never marketed,in athletes' urine

Norbolethone (13-ethyl-17-hydroxy-18,19-dinor-17alpha-pregn-4-en-3-one) is a 19-nor anabolic steroid first synthesized in 1966. During the 1960s it was administered to humans in efficacy studies concerned with short stature and underweight conditions. It has never been reported by doping control laboratories. Norbolethone was identified in two urine samples from one athlete by matching the mass spectra and chromatographic retention times with those of a reference standard. The samples also contained at least one likely metabolite. The samples were also unusual because the concentrations of endogenous steroids were exceptionally low. Since norbolethone is not known to be marketed by any pharmaceutical company, a clandestine source of norbolethone may exist.     

Synthesis and characterization of [(diene)Rh[mu-OSi(OtBu)3]]2, a precursor to silica-bound rhodium

Reaction of [(diene)RhCl]2 with 2 equiv of KOSi(OtBu)3 afforded [(diene)Rh[mu-OSi(OtBu)3]]2, where diene=cod (1) and nbd (2). Multinuclear NMR studies reveal that 1 and 2 have a dimeric structure with bridging tris(tert-butoxy)siloxy ligands. These dimers are folded along the O...O axis. Complexes and reacted with PR3(R=Me, Ph) to give monomeric products, the formulae of which depend on the amount of PR3 added ((diene)Rh[OSi(OtBu)3](PR3) and Rh[OSi(OtBu)3](PMe3)3). The behavior of 1 towards water and methanol is discussed. Thermogravimetric analyses (TGAs) of 1 and 2 reveal rather sharp conversions to rhodium-containing materials. Thermolysis of 1 in toluene at 180 degrees C resulted in formation of a black precipitate, which contained rhodium nanoparticles with an average diameter of 22 nm, as determined by powder X-ray diffraction (PXRD), after calcination at 300 degrees C for 1 h.     

Catalytic hydrosilylation of alkenes by a ruthenium silylene complex. Evidence for a new hydrosilylation mechanism

Novel ruthenium and osmium silylene species containing Si-H bonds have been synthesized and characterized by NMR spectroscopy. The ruthenium complex [Cp*(iPr3P)(H)2Ru=Si(H)Ph.Et2O][B(C6F5)4] catalyzes the hydrosilyation of alkenes with excellent substrate selectivity for primary silanes and exclusive anti-Markovnikov regiochemistry. Evidence for a novel mechanism involving direct addition of an alkene to the silylene Si-H bond is presented.     

Synthetic enantiopure aziridinomitosenes: preparation, reactivity, and DNA alkylation studies

An enantiocontrolled route to aziridinomitosenes had been developed from l-serine methyl ester hydrochloride. The tetracyclic target ring system was assembled by an internal azomethine ylide cycloaddition reaction based on silver ion-assisted intramolecular oxazole alkylation and cyanide-induced ylide generation via a labile oxazoline intermediate (62 to 66). Other key steps include reductive detritylation of 26, methylation of the N-H aziridine 56, oxidation of the sensitive cyclohexenedione 68 to quinone 70, and carbamoylation using Fmoc-NCO. Although the aziridinomitosene tetracycle is sensitive, a range of protecting group manipulations and redox chemistry can be performed if suitable precautions are taken. A study of DNA alkylation by the first C-6,C-7-unsubstituted aziridinomitosene 11a has been carried out, and evidence for DNA cross-link formation involving nucleophilic addition to the quinone subunit is described.     

Comparison of capillary electrophoresis with traditional methods to analyse bovine whey proteins

The separation of the four major whey proteins by sodium dodecyl sulphate (SDS)-capillary gel electrophoresis (CGE) is described. Whilst commercially purified whey proteins could be analysed using the recommended protocol, the more complex nature of an acid whey and a reconstituted whey protein concentrate (WPC) powder necessitated considerable refinement of the CGE sample buffer. Individual whey proteins in the acid whey and WPC samples were then also separated and quantitated using capillary zone electrophoresis, polyacrylamide gel electrophoresis (PAGE) and HPLC methods and the results were compared. The values obtained for -lactalbumin (-Lac) and β-lactoglobulin (β-Lg) were consistent throughout the various methods, although size-exclusion HPLC, SDS-PAGE and SDS-CGE could not separate the two β-Lg variants or the glycosylated form of -Lac from the β-Lg. There was considerable variation in the values for the bovine serum albumin and immunoglobulin determined by the different methods and it was concluded that none of the methods could satisfactorily quantitate all four whey proteins.     

Trajectory studies of energy transfer: CH3NC with He,Xe, H2 and N2

A trajectory program was used to simulate the collisions of CH₃NC with He, Xe, H₂ and N₂. Calculated energy transfer is in accord with experiment. The pattern of CH₃NC vibrational mode energization is found to be noticeably non-random. The approximate sampling methods used in thermal unimolecular trajectory studies produce a more uniform state distribution.