A comparison of the DNMR behaviour of methyl 5,6-bis(2-methoxyphenyl)-1,4-dimethyl-7-oxobicyclo[2.2.1]hept-5-en-2-<i style="mso-bidi-font-style:normal">endo</i>-carboxylate and its 7-oxa analogue

735-741Methyl 5,6-Bis(2-methoxyphenyl)-1,4-dimethyl-7-oxobicyclo[2.2.1] hept-5-en-2-endo-carboxylate, a moderately crowded norbornenone ester, exhibits complex VT-DNMR behav iour. A similar behaviour is not seen in its 7-oxa analogue, showing that conformational transmission from position 7 has a crucial influence on the distance parameters that govern the dynamic processes involving the substituents on the bicycloheptene framework

The structures of ethyl 1,4,5-exo-trimethyl-7-oxo-2,3-diphenylbicyclo[2.2.1]hept-2-ene-5-endo-carboxylate (A) and ethyl 1,4-dimethyl-7-oxo-2,3-diphenylbicyclo[2.2.1]hept-2-ene-5-endo-carboxylate (B)

Crystals of C<SUB>25</SUB>H<SUB>26</SUB>O<SUB>3</SUB> are triclinic, space group Pï, with a=9.197(2), b=10.357 (2), c=11.086 (3) Å, α=94.07 (2), β=98.21 (3), γ=90.82(2) °, V=1042.2 Å<SUP>3</SUP>, Z=2, D<SUB>obs</SUB>=1.19, , D<SUB>calc</SUB>=1.198 Mg m<SUP>-3</SUP> and μ (Cu Kα)=0.661 mm<SUP>-1</SUP>; R=6.2% for 2525 significant reflections. Crystals of C<SUB>24</SUB>H<SUB>24</SUB>O<SUB>3</SUB> (B) are monoclinic, space group P21/a, with a=11.584 (2), b=11.315 (2), c=15.900 (4) Å, β=104.02 (2)°, V=2022.4 Å<SUP>3</SUP>, Z=4, D<SUB>obs</SUB>=1.199, D<SUB>calc</SUB>=1.185 Mg m<SUP>-3</SUP>and μ (Cu Kα=0.081 mm<SUP>-1</SUP>; R=4.2% for 1990 significant reflections. The steric strain introduced by the bulky sbstituents on the norbornenone system is discussd. In addition, a possible relationshp between the orienation of the ester function and the anisochrony of O-methylene protons is brought out

Selectivities in the formation of pyridines and pyrimidines by ammonia-induced cyclocondensations of vinamidiniums

Arylvinamidines (2-, 3- or 4-aryl-4-(N,N-dimethyl)amino-1-azabuta-1,3-dienes), generated from 1,1,5,5-tetramethyl-2- or -3-phenyl-1,5-diazapentadienium salts, cyclocondense orientation-specifically under two regioselections forming 1-4' + 4-3' and 1-2' + 4-1' bonds on exposure to ammonia. The initial cyclates aromatise eliminatively to give mixtures of diarylpyridines and arylpyrimidines. The 2-arylvinamidines do not participate as 2-centre reactants and their 4-aryl isomers not as 4-centre reactants in the cyclocondensations which appear to be stepwise and not concerted. Reasons for the selective participation appear to be that the required eliminations from the initial cyclates are disfavoured in the first case and that a geometric factor prevents cyclate-formation in the second

Enhancement of the leaching rate of uranium in the presence of ultrasound

In the present work, experiments have been performed for leaching (recovery) of uranium from MgF2 (byproduct from the uranium ore recovery process) under various conditions of leach acid concentration with and without the presence of ultrasound, under different energy dissipation rates (different rotational speeds of conventional stirring by impeller), and on different MgF2 particle size distributions. The enhancement of the leaching rate due to ultrasound is found to occur in two steps: (1) MgF2 particle fragmentation leading to high specific solid-liquid interfacial area and by increase in the surface diffusional rate of the reactive species; (2) enhancement in the convective diffusivity of the leach acid solvent through micropores of the MgF2 agglomerate structure due to convective motion created by the cavitation phenomena (shock wave propagation, microjet formation) at the solid-liquid interface. Thus, the overall recovery has been increased by the application of ultrasound with several additional advantages such as low leach acid concentration and decrease in the leaching operation time. The energy dissipation rate with the use of ultrasound was very high, yet, at an equivalent energy dissipation rate in the form of conventional stirring, leaching rates or the final extent of the leaching could not be matched. This indicated that the scale (time and spatial) of energy dissipation has important effect on the overall leaching rate. Kinetics shows that the leaching operation can be explained as a classical shrinking core kinetics phenomenon with pore diffusion resistance as the rate-limiting step

Sono-chemical leaching of uranium

A fundamental study on the mechanism of uranium leaching in nitric acid and sulphuric acid media has been carried out to understand the effect of the ultrasound on leaching. The use of ultrasound clearly improves the leaching rate within the studied leach acid media. The enhancement in the leaching rate in the presence of ultrasound is higher with low leach acid concentration of nitric acid (HNO3), and it is high at high leach acid concentration in the case of sulphuric acid (H2SO4) being used as a leachant, when compared to conventional mechanical agitation. The basic reason behind this observed variation is explained on the basis of the reaction mechanism involving the oxidative conversion of acid insoluble tetravalent uranium form to the soluble hexavalent form of uranium in the presence of ultrasound at a faster rate