New chiral phosphine-phosphite ligands in the enantioselective palladium-catalyzed allylic alkylation

A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities.     

Aminosulf(ox)ides as ligands for Iridium(I)-catalyzed asymmetric transfer hydrogenation

A new class of efficient catalysts was developed for the asymmetric transfer hydrogenation of unsymmetrical ketones. A series of chiral N,S-chelates (6-22) was synthesized to serve as ligands in the iridium(I)-catalyzed reduction of ketones. Both formic acid and 2-propanol proved to be suitable as hydrogen donors. Sulfoxidation of an (R)-cysteine-based aminosulfide provided a diastereomeric ligand family containing a chiral sulfur atom. The two chiral centers of these ligands showed a clear effect of chiral cooperativity. In addition, aminosulfides containing two asymmetric carbon atoms in the backbone were synthesized. Both the sulfoxide-containing beta-amino alcohols and the aminosulfides derived from 1,2-disubstituted amino alcohols gave rise to high reaction rates and moderate to excellent enantioselectivities in the reduction of various ketones. The enantioselective outcome of the reaction was favorably affected by selecting the most appropriate hydrogen donor. Enantioselectivities of up to 97% were reached in the reduction of aryl-alkyl ketones.     

Coefficient of thermal diffusivity of ChS-139 steel in wide temperature range

Laser flash method with 2–4% accuracy was used to determine the coefficient of thermal diffusivity of ferrite-martensite steel ChS-139 in temperature range 296–1274 K. Temperatures of Curie and martensite-austenite-martensite phase changes have been determined.     

Density of liquid and solid Mg-Pb alloys

Experimental data on the thermal properties of Mg-Pb alloys in solid and liquid states are generalized and analyzed. The temperature and composition dependences of molar volume and volumetric thermal expansion coefficient of the magnesium-lead system are constructed and discussed. The phase diagram of the system is refined.     

Hypersonic multicycle gas jet with a high degree of underexpansion at the nozzle exit

Experimental research on highly underexpanded gas jets flowing from supersonic nozzles into an ambient medium has shown that for fairly large Mach and Reynolds numbers at the nozzle exit the jet consists of several cycles or barrels [1–3]. The authors have made a theoretical study of these jets, using the nonstationary analogy method (law of plane sections). An approximate model of the flow is constructed and an analytic solution is obtained for the location of the boundary of the multicycle jet. The corresponding equivalent nonstationary problem of the expansion of a cylindrical slug of gas is solved numerically. The results are found to be consistent with the experimental data and make it possible to explain a number of observations. It is shown, for example, that the experimentally observed decrease in the amplitude of the cycles (maximum radius of the barrels) as they progress downstream is due to the dissipation of energy in shock waves. It appears that the length of the cycles is more or less independent of the dissipation and almost constant. The effect of the removal (supply) of heat, for example, due to radiation, the relaxation of internal degrees of freedom, etc., on the geometry of the jet is examined. It is shown that the removal of heat leads to a decrease in the amplitude of the cycles and the supply of heat to an increase, but, like dissipation, neither affects the length of the cycles. The problem of a jet in a weakly inhomogeneous atmosphere is solved. It has shown that the jet geometry possesses an adiabatic invariant linking the length and amplitude of the cycles.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 27–35, November–December, 1984.     

A limit theorem with weights for quadratic Dirichlet L-functions

Translated from Matematicheskie Zametki, Vol. 55, No. 2, pp. 128–129, February, 1994.     

Mean value of Dirichlet L-functions in the critical strip

Vilnius University. Translated from Litovskii Matematicheskii Sbornik (Lietuvos Matematikos Rinkinys), Vol. 31, No. 4, pp. 678–686, October–December, 1991.