Vinca alkaloids. XXXII. Microbiological conversions of vindoline, a major alkaloid from Vinca rosea L

Vindoline, a major alkaloid from Vinca rosea _L was subjected to microbiological conversion using Streptomyces cultures. Several new metabolites were isolated and their structures elucidated.     

Mechanistic and computational studies of oxidatively-induced aryl-CF3 bond-formation at Pd: rational design of room temperature aryl trifluoromethylation

This article describes the rational design of first generation systems for oxidatively induced Aryl-CF(3) bond-forming reductive elimination from Pd(II). Treatment of (dtbpy)Pd(II)(Aryl)(CF(3)) (dtbpy = di-tert-butylbipyridine) with NFTPT (N-fluoro-1,3,5-trimethylpyridinium triflate) afforded the isolable Pd(IV) intermediate (dtbpy)Pd(IV)(Aryl)(CF(3))(F)(OTf). Thermolysis of this complex at 80 °C resulted in Aryl-CF(3) bond-formation. Detailed experimental and computational mechanistic studies have been conducted to gain insights into the key reductive elimination step. Reductive elimination from this Pd(IV) species proceeds via pre-equilibrium dissociation of TfO(-) followed by Aryl-CF(3) coupling. DFT calculations reveal that the transition state for Aryl-CF(3) bond formation involves the CF(3) acting as an electrophile with the Aryl ligand serving as a nucleophilic coupling partner. These mechanistic considerations along with DFT calculations have facilitated the design of a second generation system utilizing the tmeda (N,N,N',N'-tetramethylethylenediamine) ligand in place of dtbpy. The tmeda complexes undergo oxidative trifluoromethylation at room temperature.     

Vince alkaloids XXXIV). Preparation of des-Na-methyl-vindoline by microbiological conversion of vindoline, a major alkaloid from Vinca rosea L

The conversion of vindoline (II) using S. albogriseolus led to the isolation of several compounds including des-N_a-methyl vindoline (III) in 8–10% yield.     

Chemical etching of silicon by CO2-laser-induced dissociation of NF3

The pulsed infrared laser dissociation of NF₃ is reported for the first time, and is used to investigate silicon etching. The role played by collision-enhanced multiple-photon absorption and dissociation is considered, with data on the nonlinear decrease of the absorption cross-section with increasing pulse energy and increasing pressure presented. Using an experimental arrangement in which the laser beam is focussed parallel to the surface, the dissociation process induces spontaneous etching of silicon. Fluorinecontaining radicals diffuse from the focal volume to the surface where a heterogeneous chemical reaction occurs. Etching was monitored by use of a quartz-crystal microbalance upon which a thin film of amorphous silicon was deposited. For a surface with no previous exposure to the photolysis products, dissociation causes the formation of a surface layer prior to the onset of etching. X-ray photoelectron spectroscopy demonstrates this to be a fluorosilyl layer possessing a significant concentration of SiF₃ and SiF₄. In contrast, a surface already thickly fluorinated does not form a thicker layer once laser pulsing commences again. In this case, etching starts immediately with the first pulse. The etch yield dependencies on several parameters were obtained using silicon samples possessing a thick fluorosilyl surface layer. These parameters are NF₃ pressure, laser wavenumber, pulse energy, buffer gas pressure, and perpendicular distance from focal volume to surface. Modeling of the etch yield variation with perpendicular distance shows the time-integrated flux of radicals impinging on the surface to be inversely proportional to the distance. Attempts at etching SiO₂ under identical conditions were unsuccessful despite the evidence that thin native oxide films are removed during silicon etching.     

Carbon(sp(3) )Fluorine Bond-Forming Reductive Elimination from Palladium(IV) Complexes

Pd(IV) -fluoride complexes, some of which are remarkably insensitive to water, have been synthesized and used in the title reaction, which proceeds with high selectivity to give the product of the C(sp(3) )?F coupling (see scheme, TfO=trifluoromethanesulfonate). Preliminary mechanistic studies implicate a pathway involving dissociation of pyridine followed by direct C?F coupling at the Pd center.     

Pulsed 532 nm laser wirestripping: Removal of dye-doped polyurethane insulation

Removal of rhodamine 6G doped polyurethane insulation coated onto 50 m diameter wire is shown to proceed efficiently and cleanly by irradiation with 532 nm Q-switched pulses from a Nd:YAG laser. The stripping action produced by this method is similar in quality to excimer laser wirestripping. Several experimental parameters were explored including fluence, pulse duration, dye concentration, and the number of incident pulses. Acceptable stripping conditions were obtained for a 3–5 s exposure at 10 Hz, using a dye concentration of 10% by weight, and 12 n pulses at 650 mJ/cm². Nearly 0.5 m/pulse is removed at this fluence, which exceeds the threshold fluence of 600 mJ/cm² by only 50 mJ/cm². The measured 532 nm absorption coefficient of the 10% dye-doped polyurethane was 4×10⁴ cm^–1. Lower fluences and/or dye concentrations produced inadequate stripping, while shorter duration pulses caused unacceptable melting of the thin gold layer which covered the copper core of the wire. Pulse-by-pulse photographs of the stripping action clearly show melting of the dye/polymer insulation, and thermal rollback of the insulation near the stripped end. Regardless, excellent edge definition is obtained by this method.     

Concentration dependence of the distribution coefficient for macromolecules in porous media

Partitioning of macromolecules between pore and bulk solutions directly affects both equilibrium and transport processes such as exclusion chromatography and movement of solutes through porous media. Because of interactions between macromolecules and the pore wall, the variation of the macromolecule activity with concentration is different inside the pore than in bulk solution. This difference causes a concentration dependence of the distribution coefficient, as reported in experiments involving exclusion chromatography. In order to explain this effect, we develop a model for a concentration-dependent distribution which explicitly accounts for a coupling between pore–macromolecule and macromolecule–macromolecule interactions. Predictions using this model are reported for the case of rigid spherical macromolecules in both cylindrical and slit pores, including both steric (hard sphere–hard wall) and long-range (screened electrostatic) interactions. An important result is the existence of a general correlation between the first order concentration effect and measurable properties of the macromolecule and porous medium.     

Synthetic, mechanistic, and computational investigations of nitrile-alkyne cross-metathesis

The terminal nitride complexes NW(OC(CF 3) 2Me) 3(DME) ( 1-DME), [Li(DME) 2][NW(OC(CF 3) 2Me) 4] ( 2), and [NW(OCMe 2CF 3) 3] 3 ( 3) were prepared in good yield by salt elimination from [NWCl 3] 4. X-ray structures revealed that 1-DME and 2 are monomeric in the solid state. All three complexes catalyze the cross-metathesis of 3-hexyne with assorted nitriles to form propionitrile and the corresponding alkyne. Propylidyne and substituted benzylidyne complexes RCW(OC(CF 3) 2Me) 3 were isolated in good yield upon reaction of 1-DME with 3-hexyne or 1-aryl-1-butyne. The corresponding reactions failed for 3. Instead, EtCW(OC(CF 3)Me 2) 3 ( 6) was prepared via the reaction of W 2(OC(CF 3)Me 2) 6 with 3-hexyne at 95 degrees C. Benzylidyne complexes of the form ArCW(OC(CF 3)Me 2) 3 (Ar = aryl) then were prepared by treatment of 6 with the appropriate symmetrical alkyne ArCCAr. Three coupled cycles for the interconversion of 1-DME with the corresponding propylidyne and benzylidyne complexes via [2 + 2] cycloaddition-cycloreversion were examined for reversibility. Stoichiometric reactions revealed that both nitrile-alkyne cross-metathesis (NACM) cycles as well as the alkyne cross-metathesis (ACM) cycle operated reversibly in this system. With catalyst 3, depending on the aryl group used, at least one step in one of the NACM cycles was irreversible. In general, catalyst 1-DME afforded more rapid reaction than did 3 under comparable conditions. However, 3 displayed a slightly improved tolerance of polar functional groups than did 1-DME. For both 1-DME and 3, ACM is more rapid than NACM under typical conditions. Alkyne polymerization (AP) is a competing reaction with both 1-DME and 3. It can be suppressed but not entirely eliminated via manipulation of the catalyst concentration. As AP selectively removes 3-hexyne from the system, tandem NACM-ACM-AP can be used to prepare symmetrically substituted alkynes with good selectivity, including an arylene-ethynylene macrocycle. Alternatively, unsymmetrical alkynes of the form EtCCR (R variable) can be prepared with good selectivity via the reaction of RCN with excess 3-hexyne under conditions that suppress AP. DFT calculations support a [2 + 2] cycloaddition-cycloreversion mechanism analogous to that of alkyne metathesis. The barrier to azametalacyclobutadiene ring formation/breakup is greater than that for the corresponding metalacyclobutadiene. Two distinct high-energy azametalacyclobutadiene intermediates were found. These adopted a distorted square pyramidal geometry with significant bond localization.     

Hydroaminomethylation Beyond Carbonylation: Allene–Imine Reductive Coupling by Ruthenium‐Catalyzed Transfer Hydrogenation

Ruthenium(II)‐catalyzed hydrogen transfer from 2‐propanol mediates reductive coupling of 1,1‐disubstituted allenes with formaldimines with complete branch‐regioselectivity, thus representing a new method for hydroaminomethylation beyond classical hydroformylation/reductive amination.