A convenient multigram synthesis of highly enantioenriched methyl 3-silylglycidates

A multigram scale synthesis of the four stereoisomers of methyl 3-silylglycidates (epoxysilanes) with high enantiopurity is described. Key reactions include a Sharpless asymmetric epoxidation (SAE) of a trans-vinylsilane and an enzymatic resolution of a racemic cis-epoxysilane to establish the desired configurations. Few chromatographic separations (5 columns out of 13 steps) are required for purification, establishing a convenient reaction sequence for both the trans- and cis-isomers.     

Drilling rate of five metals with picosecond laser pulses at 355, 532, and 1064 nm

Experimental results on picosecond laser processing of aluminum, nickel, stainless steel, molybdenum, and tungsten are described. Hole drilling is employed for comparative analysis of processing rates in an air environment. Drilling rates are measured over a wide range of laser fluences (0.05–20?J/cm²). Experiments with picosecond pulses at 355?nm are carried out for all five metals and in addition at 532?nm, and 1064?nm for nickel. A comparison of drilling rate with 6-ps and 6-ns pulses at 355?nm is performed. The dependence of drilling rate on laser fluence measured with picosecond pulses demonstrates two logarithmic regimes for all five metals. To determine the transition from one regime to another, a critical fluence is measured and correlated with the thermal properties of the metals. The logarithmic regime at high-fluence range with UV picosecond pulses is reported for the first time. The energy efficiency of material removal for the different regimes is evaluated. The results demonstrate that UV picosecond pulses can provide comparable quality and higher processing rate compared with literature data on ablation with near-IR femtosecond lasers. A significant contribution of two-photon absorption to the ablation process is suggested to explain high processing rate with powerful UV picosecond pulses.     

Electrochemical and spectroscopic characterisation of structural reorganisation in N,N′–dialkylpyridinium cation radical deposits

The electrochemical reduction to form a deposit of the radical cation salt for a series of n-alkyl asymmetrically substituted viologens and the corresponding anodic oxidation processes have been investigated using electrochemical methods in in situ uv-visible reflectance spectroscopy. Particular attention was focussed on the nature of the restructuring involved in the ageing of the deposit. The use of both p- and s-polarised radiation clearly showed that initially the viologen units were deposited with the major molecular axis parallel to the electrode and on ageing they reorganised into a preferred orientation perpendicular to the electrode. The viologens with the larger substituents were initially deposited as aggregates of mixed micelles containing both cation radical and dication units. The latter were reduced at a well defined potential, the process being seen as a highly-reversible sharp current spike on the linear sweep voltammograms.