Stereochimie de la cycloadditton des diarylnitrilimines sur les n-alkyl et les dialkyl-1,2 dihydro-1,2 isoquinoleines

The PMR spectroscopic properties (60 MHz) of cycloadducts of diarylnitrilimins towards 1,2-dihydroisoquinolines mono alkylated at the N-atom or dialkylated at the C¹ -carbor and N-atoms have been studied.Date allow us to determine the stereochemistry of the cycloaddition reaction which is regio and diastereospecific. Configuration and conformation of the cycloadducts are specified.     

Scanning near field thermal microscopy on a micromachined thin membrane

A resistive probe based scanning thermal microscope (SThM) has been used to perform complementary near field thermal measurements on the surface of a thin semiconductor membrane. This thin structure is part of a micromachined thermal rf power sensor and includes an integrated resistive heater used as an absorbing element for the input power. The resulting 2D surface temperature distribution and the 2D thermal wave propagation characteristic were determined. Considering the thermal wave behaviour at near field conditions, the local thermal conductivity of the thin membrane and the surrounding bulk material was investigated by usage of the 3ω-method.     

Regioselectivity and specificity in cycloaddition of diphenylnitrilimine

Diphenylnitrilimine cycloaddition reactions on 1,2-dihydronaphtalenes are only regioselective. Use of 1- or 2-tetralones enamines as dipolarophiles allows to attain regiospecificity.     

Technology aspects of GaN-based diodes for high-field operation

This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.     

Field emission from quantum size GaN structures

Whisker structures and quantum dots fabricated by photoelectrochemical (PEC) etching of undoped and doped metalorganic chemical vapor deposition (MOCVD)-grown GaN (2×10¹⁷ or 3×10¹⁸ cm⁻³) are investigated in relation with their field-emission characteristics. Different surface morphologies, corresponding to different etching time and photocurrent, results in different field-emission characteristics with low turn-on voltage down to 4 V/μm and the appearance of quantum-size effect in the I–V curves.     

1,3‐Dipolar Cycloaddition Reactions of Indan‐1‐one Enamines across Arylnitrile Oxides Leading to Novel Cyclic Isoxazoline Derivatives

Synthesis of a series of cyclic fused‐isoxazolines has been accomplished by regioselective and diastereoselective 1,3‐dipolar cycloaddition of 3‐methylindan‐1‐one enamines ( 1a , 1b , 1c ) and 3‐phenylindan‐1‐one enamines ( 2a , 2b , 2c ) to arylnitrile oxides ( 3d , 3e , 3f , 3g , 3h ). The structure of the cycloadducts was elucidated by ¹H and ¹³C NMR spectroscopy. The proposed regio‐ and stereochemistry of fused‐compounds ( 4 ) and ( 5 ) has also been corroborated by two single‐crystal X‐ray diffraction studies carried out on 4‐methyl‐8b‐morpholinyl‐3‐(p‐tolyl)‐4H‐3a,8b‐dihydroindeno[2,3‐d]isoxazoline ( 4be ) and 3‐(p‐anisyl)‐4‐phenyl‐8b‐pyrrolidinyl‐4H‐3a,8b‐dihydroindeno[2,3‐d]isoxazoline ( 5af ) and by means of density functional theory calculations.     

Synthesis of diversely functionalized pyrrolizidines and indolizidines using olefin ring-closing metathesis

Various nitrogen-fused tricyclic compounds, having benzoindolizidine and benzopyrrolizidines ring systems were synthesized via ene-ene metathesis using the first and second-generation Grubbs catalyst. The ene-ene metathesis proceeded smoothly in refluxing CH₂Cl₂ with 3.0 mol % of G₁, giving good yields (78-86%) of the benzoindolizidine products 12a,b. The benzopyrrolizidine 6 was prepared after optimization in 64% yield by using 5.0+5.0 mol % of G₂. The resulting olefin moiety of the indolizidine framework is a suitable precursor for polyhydroxy structures via the Sharpless process. The structures of the polyhydroxylated adducts were determined by ¹H NMR spectra and single-crystal X-ray analysis.     

Spiroheterocycles from reaction of nitrones with methylene-gamma-butyrolactones and some of their rearrangements

Cycloaddition of nitrones 1 with methylene-gamma-butyrolactones 2, 3 and 4 afforded spiroadducts 5, 6 and 7 with high selectivity. Mixtures of diastereoisomers were usually obtained, whose structures were established by 1H and 13C NMR spectroscopies or X-ray crystallography. Treatment of spiroadducts in acidic and alkaline media led to different, unexpected and novel rearrangements.     

Silver-catalyzed spirolactonization: first synthesis of spiroisoindole-γ-methylene-γ-butyrolactones

γ-Acetylenic carboxylic acids are cyclized to spirolactones under mild conditions, in the presence of Ag₂CO₃ catalyst. The corresponding spiro-5-alkylidene-γ-butyrolactones were isolated in high yields, and this process constitutes an easy and efficient route to analogous structures of natural products of biological interest.     

GaN micromachined FBAR structures for microwave applications

This paper presents the manufacturing of GaN membrane supported F-BAR structures. The 2.2 μm thick GaN layer was grown using MOCVD techniques on a high-resistivity 111-oriented silicon substrate. Conventional contact lithography, electron-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachining techniques were used for the release of the GaN membrane. The bottom-side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a pronounced resonance around 1.2 GHz. The extracted value of acoustic velocity is in good agreement with those reported by other authors on materials fabricated by other methods. The demonstrated FBAR function in epitaxially grown GaN layers opens new avenues for a low-cost monolithic integration with GaN-based electronics and sensing devices.