Results 1 - 10 of 29993
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[en] Second-order rate constants (kN) have been determined spectrophotometrically for reactions of 2,4- dintrophenyl 2-furoate (2) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 .deg. C. The furoate 2 is more reactive than 2,4-dintrophenyl benzoate (1) toward all the amines studied. The higher acidity of 2-furoic acid (pKa = 3.16) compared with benzoic acid (pKa = 4.20) has been suggested to be responsible for the reactivity order, at least in part. The Brφnsted-type plots for the reactions of 1 and 2 are curved downwardly, indicating that the aminolyses of both 1 and 2 proceed through a zwitterionic tetrahedral intermediate (T±) with a change in the rate-determining step on changing the amine basicity. Dissection of the kN values into their microscopic rate constants has revealed that the pKa .deg. and k2/k-1 ratios for the reactions of 1 and 2 are identical, indicating that the nature of the nonleaving group (i.e., benzoyl and 2-furoyl) does not affect the reaction mechanism. The k1 values have been found to be larger for the reactions of 2 than for those of 1, which is fully responsible for the fact that the former is more reactive than the latter
[en] The mechanism of nucleophilic displacement was studied by using three variable systems of ρX,ρY and ρZ obtained from the change of substituent X, Y, and Z for the reaction of (Z)-substituted benzyl (X)-benzenesulfonates with (Y)-substituted thiobenzamides in acetone at 45.deg.C. The results ρZ<0 and ρYZ>ρXZ indicate that this reaction series proceeded via a dissociative SN2 mechanism. The prediction of the movement of TS by using the sign of ρXY·ρYZ accorded with the Hammond postulate
[en] Oxindole and its derivatives exist in a large number of natural and biologically active molecules. Particularly, 3,3- disubstituted oxindoles make up the core of various medicinals and natural products. The synthetic application and potential biological significance of chiral 3,3-disubstituted oxindoles have led to a demand for the efficient asymmetric synthetic methods. We have developed a highly efficient catalytic enantioselective conjugate addition reactions of 3-fluorooxindoles to activated vinyl sulfone using 1.5 mol% of binaphthyl-modified bifunctional catalyst I. The desired Michael products were obtained in high yields and excellent enantioselectivities (91–95% ee) were observed for 3-fluorooxindoles examined in this work. We believe that this method provides a practical entry for the preparation of synthesis of medicinally useful chiral 3-fluoro 3-substituted oxindoles. Further study of these bifunctional organocatalysts in other asymmetric reactions is being under conducted
[en] Scanning tunneling microscopy (STM) is used for replacement patterning of self-assembled monolayers (SAMs) of thiols on a sub-10 nm scale. Contrasting other schemes of scanning probe patterning of SAMs, the exchange of molecules relies on differences in conductance and, thus, occurs under tunneling conditions where the resolution of the tip is maintained. Exchange takes place at the boundary between different thiols but only when the tip moves from areas of lower to higher conductance. In combination with SAMs which exhibit excellent structural quality, patterns with a contour definition of ± 1 molecule, lines as thin as 2.5 nm and islands with an area of less than 20 nm2 are straightforwardly produced. It is suggested that the shear force exerted onto the molecules with the lower conductance triggers displacement of the one with higher conductance.
[en] A new synthetic approach to α-keto cyano-phosphorane ylides from olefins utilizing a novel xanthate reagent 6 has been developed. There are several advantages expected from this new approach e. g., easy preparation of 6 from commercial reagents in excellent yield, mild reaction conditions, good to excellent overall yields. Currently we are doing additional experiments to determine the scope and limitation of this new approach, and those results will be reported in due course. We have developed new synthetic approaches to α-keto cyanophosphorane ylides from readily available chemicals using specially designed new reagents to overcome such limitation. For example, α-keto cyanophosphorane ylides have been prepared from carbonyl compounds using a new Horner-Wadsworth-Emmons (HWE) reagent, from alkyl bromides using a new phenylsulfinyl reagent through alkylation-sulfoxide elimination sequence, and from alkyl halides using a new phenylsulfonyl reagent via sequential alkylation-reductive desulfonylation protocol