Dynamic High Pressure Conversion

For the dynamic high pressure conversion, also known as shock wave synthesis, an explosive shock wave is used for the conversion of h-BN into c-BN. High pressure and high temperature are reached for a short period of time (milliseconds). To guarantee a fast temperature decrease a metal powder, like copper, is added to h-BN in an amount of 5% [181]. 

This method was first applied by Du Pont to prepare ultrafine diamond powders on an industrial scale. 

Today BN synthesis by shock-wave methods is mainly used to produce superhard boron-carbon-nitrogen mixtures called heterodiamond [182, 183]. 

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A quite different set of dynamic high-pressure techniques are based on the use of chemical or nuclear explosions to produce transient shock waves of high peak pressure but short duration. With such methods, one can often penetrate the high-T, P regions where kinetic barriers become unimportant and a catalyst is unnecessary. However, the same kinetics that allows facile conversion of graphite to diamonds as the shock front arrives also allows the facile back-conversion as the shock wave passes. As a pioneer of shock-wave diamond synthesis remarked ruefully, We were millionaires for one microsecond [B. J. Alder and C. S. Christian. Phys. Rev. Lett. 7, 367 (1961) B. J. Alder, in W. Paul and D. M. Warschauer (eds). Solids under Pressure (McGraw-Hill, New York, 1963), p. 385]. 

From the theoretical analysis it follows that acting on the strength characteristics of the solid matrix of a frozen reactant mixture may be an effective means for testing the concepts developed. This was an impetus for a study of the effect of high pressures on the dynamic characteristics of the autowave regimes of chemical conversion,19 since it is known that uniform compression of solid materials results in significant strengthening. 

For instance, at room temperature when two moles of hydrogen gas (Ha) react with one mole of graphite (C), there is a complete conversion of the reactants into one mole of methane gas (CH4). However, if the reaction is carried out at high temperatures and constant pressure, it is foimd that the reaction does not proceed to completion and even after a prolonged time at that temperature and pressure, some hydrogen gas and graphite remain. The reaction thus reaches a state of chemical equilibrium where the rates of forward and reverse reactions are equal and a dynamic equilibrium is reached. 

The synthesis of jS-hydoxy-a-amino acids is important since these compounds are incorporated into the backbone of a wide range of antibiotics and cyclopeptides such as vancomycins. These highly functional compounds are also subject to dynamic kinetic resolution (DKR) processes, as the stereocenter already present in the substrate epimerizes under the reaction conditions and hence total conversions into single enantiomers are possible. These transformations can be iy -selective ° for N-protected derivatives as shown in Figure 1.27 when using a mthenium-BlNAP catalyzed system and anfi-selective when the jS-keto-a-amino acid hydrochloride salts are reduced by the iridium-MeOBlPHEP catalyst as shown in Figure 1.28. One drawback is that both these reductions use 100 atm hydrogen pressure. 

The experiment was performed with one laser used to excite the conversion to the high-spin state and a second to monitor the relaxation back to the low-spin state. This approach allowed measurements over the wide dynamic range from nanoseconds to seconds. Pressures up to 1 kbar were... 

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