Catalytic asymmetric Michael reaction of beta-keto esters: effects of the linker heteroatom in linked-BINOL

We describe the development of a general catalytic asymmetric Michael reaction of acyclic beta-keto esters to cyclic enones, in which asymmetric induction occurs at the beta-position of the acceptors. Among the various asymmetric catalyst systems examined, the newly developed La-NR-linked-BINOL complexes (R = H or Me) afforded the best results in terms of reactivity and selectivity. In general, the NMe ligand 2 was suitable for the combination of small enones and small beta-keto esters, and the NH ligand 1 was suitable for bulkier substrates (steric tuning of the catalyst). Using the La-NMe-linked-BINOL complex, the Michael reaction of methyl acetoacetate (8a) to 2-cyclohexen-1-one (7b) gave the corresponding Michael adduct 9ba in 82% yield and 92% ee. The linker heteroatom in linked-BINOL is crucial for achieving high reactivity and selectivity in the Michael reaction of beta-keto esters. The amine moiety in the NR-linked-BINOL can also tune the Lewis acidity of the central metal (electronic tuning of the catalyst), which was supported by density functional studies and experimental results. Another advantage of the NR-linked-BINOL ligand as compared with O-linked-BINOL is the ease of modifying a substituent on the amine moiety, making it possible to synthesize a variety of NR-linked-BINOL ligands for further improvement or development of new asymmetric catalyses by introducing additional functionality on the linker with the amine moiety. The efficiency of the present asymmetric catalysis was demonstrated by the synthesis of the key intermediate of (-)-tubifolidine and (-)-19,20-dihydroakuammicine in only five steps compared to the nine steps required by the original process from the Michael product of malonate. This strategy is much more atom economical. On the basis of the results of mechanistic studies, we propose that a beta-keto ester serves as a ligand as well as a substrate and at least one beta-keto ester should be included in the active catalyst complex. Further improvement of the reaction by maintaining an appropriate ratio of the La-NMe-linked-BINOL complex and beta-keto esters is also described.     

Cationic ring-opening transfer polymerization of spiro ortho esters initiated by carbon black

The ring-opening transfer polymerization of spiro ortho esters (SOE) initiated by carbon black was investigated. In the absence of carbon black, no polymerization occurred at all. In the presence of channel black containing carboxyl group, the ring-opening transfer polymerization of SOE was initiated at 50-70°C. to give polyether ester, namely alternating copolymer of epoxide and lactone. The rate of polymerization of 1,4,6-trioxaspiro[4.4]nonane and 1,4,6-trioxaspiro[4.5]decane was considerably small compared with that of 1,4,6-trioxaspiro[4.6]undecane. The activation energy of the polymerization of 2-chloromethyl-1,4,6-trioxaspiro[4.6]undecane was estimated to be 6.0 kcal/mol. The initiating activity of carbon black increased with an increase in carboxyl group content of carbon black. Furnace black that contained no carboxyl group was unable to initiate the polymerization. Furthermore, the carbon black lost the initiating ability of the polymerization upon the blocking of carboxyl group on the surface by the treatment with potassium hydroxide or diazomethane. Based on these results, it was concluded that carboxyl group on carbon black plays an important role in the initiation. During the polymerization, a part of the polymer formed was grafted onto carbon black: the grafting ratio was 10–30%. The mechanisms of initiation and grafting were discussed.     

Potential distribution across membranes with surface-charge layers. Effects of ionic solubility

A model is presented for the potential distribution across a charged membrane. The membrane-fixed charges are assumed to be distributed through a surface layer of non-zero thickness on the membrane. We treat the surface layer as a different phase from the surrounding solution phase. The potential arises from the membrane-fixed charges and from different solubilities of positive and negative electrolyte ions in the two phases. Equations are presented for the potential distribution, which involve the partition coefficients of electrolyte ions and the relative permittivity of the surface layer.     

Diffuse Double-Layer Interaction between Two Identical Spherical Colloidal Particles

A simple method is given for calculating the potential energy of the diffuse double-layer interaction between two identical spherical colloidal particles in a symmetrical electrolyte solution with the help of Derjaguin's approximation. This method uses accurate analytic expressions for the corresponding interaction energy between two parallel similar plates obtained previously (Colloids Surf. A Physicochem. Eng. Asp. 146, 213 (1999); J. Colloid Interface Sci. 212, 130 (1999)). Agreement with numerical data provided by Honig and Mul (J. Colloid Interface Sci. 36, 258 (1971)) is excellent particularly for small particle separations. Copyright 2000 Academic Press.     

Electrostatic interaction between ion-penetrable membranes in a salt-free medium

A set of coupled equations is given which determines the distributions of the electric potential and counterions in a system of two interacting identical ion-penetrable membranes of thickness d at separation h immersed in a salt-free medium containing only counterions. The solution to these coupled equations also gives the electrostatic repulsive force between the membranes. It is shown that the interaction force remains finite at h-->0, unlike the case of the interaction between two planar charged surfaces (d-->0), and that the interaction force becomes independent of the membrane fixed charge and membrane thickness d at very large h. Finally, an approximate single transcendental equation giving the solution to the coupled equations is derived.     

Electrophoretic mobility of a cylindrical colloidal particle in a salt-free medium

Approximate expressions are derived for the electrophoretic mobility of dilute cylindrical colloidal particles in a salt-free medium containing only counterions. The cylinder is assumed to be infinitely long. It is shown that as in the case of a spherical particle, there is a certain critical value of the particle surface charge separating two cases. When the particle surface charge is lower than the critical value (case 1), the electrophoretic mobility increases with increasing particle surface charge per unit length. When the particle surface charge is higher than the critical value (case 2), the mobility becomes constant (for a cylinder in a transverse field) or the increase in the electrophoretic mobility with the particle surface charge becomes suppressed (for a cylinder in a tangential field). These phenomena are caused by the effect of counterion condensation in the vicinity of the particle surface. The critical value of the particle charge is essentially independent of the particle volume fraction phi for the dilute case, unlike the case of a sphere, in which case the critical charge value is proportional to ln(1/phi).     

Electrophoretic mobility of a highly charged colloidal particle in a solution of general electrolytes.

For a highly charged particle in an electrolyte solution, counterions are condensed very near the particle surface. The electrochemical potential of counterions accumulated near the particle surface is thus not affected by the applied electric field, so that the condensed counterions do not contribute to the particle electrophoretic mobility. In the present paper we derive an expression for the electrophoretic mobility mu(infinity) of a highly charged spherical particle of radius a and zeta potential zeta in the limit of very high zeta in a solution of general electrolytes with large ka (where k is the Debye-Hückel parameter) on the basis of our previous theory for the case of symmetrical electrolytes (H. Ohshima, J. Colloid Interface Sci. 263 (2003) 337). It is shown that zeta can formally be expressed as the sum of two components: the co-ion component, zetaco-ion, and the counterion component, zetacounterion (where zeta = zetaco-ion + zetacounterion) and that the limiting electrophoretic mobility mu(infinity) is given by mu(infinity) = epsilonr epsilon0 zetaco-ion(infinity)/eta + 0(1/ka), where zetaco-ion(infinity) is the high zeta-limiting form of zetaco-ion, epsilonr and eta are, respectively, the relative permittivity and viscosity of the solution, and epsilon0 is the permittivity of a vacuum. That is, the particle behaves as if its zeta potential were zetaco-ion(infinity), independent of zeta. For the case of a positively charged particle in an aqueous electrolyte solution at 25 degrees C, the value of zetaco-ion(infinity) is 35.6 mV for 1-1 electrolytes, 46.0 mV for 2-1 electrolytes, and 12.2 mV for 1-2 electrolytes. It is also found that the magnitude of mu(infinity) increases as the valence of co-ions increases, whereas the magnitude of mu(infinity) decreases as the valence of counterions increases.     

Colloid vibration potential and ion vibration potential in a dilute suspension of spherical colloidal particles

A general electroacoustic theory is presented for the macroscopic electric field in a dilute suspension of spherical colloidal particles in an electrolyte solution, which consists of the colloid vibration potential (CVP) and the ion vibration potential (IVP), induced by an oscillating pressure gradient field due to an applied sound wave. This is a unified theory that unites previous theories for CVP and those for IVP. Approximate analytic expressions are derived for CVP and IVP. The obtained IVP expression agrees with Debye's formula that is corrected by taking into account the force acting on the electrolyte ions as a result of the pressure gradient in the sound wave. The obtained CVP expression is correct to the first order of the particle zeta potential and applicable for arbitrary kappaalpha, where kappa is the Debye-Hückel parameter and alpha is the particle radius. It is found that an Onsager relation holds between CVP and dynamic electrophoretic mobility. It is also shown that the CVP from particles with very small kappaalpha approaches IVP; that is, in the limit of very small kappaalpha a particle behaves like an ion.     

Surface charge density/surface potential relationship for a spherical colloidal particle in a salt-free medium

On the basis of a theory of Imai and Oosawa (Busseiron Kenkyu52, 42 (1952); 59, 99 (1953)), approximate analytic expressions for the surface charge density/surface potential relationship for a spherical colloidal particle in a salt-free (aqueous or nonaqueous) medium containing only counterions are derived. There is a certain critical value of the surface charge density (or the total surface charge) separating two distinct cases: low surface charge density case and high surface charge density case. In the latter case counterion condensation occurs in the vicinity of the particle surface. The results are in excellent agreement with numerical calculations for the case of dilute suspensions.     

Electrophoretic mobility of a soft particle in a salt-free medium

A theory is presented for the electrophoretic mobility mu of dilute spherical soft particles (i.e., polyelectrolyte-coated particles) in salt-free media containing only counterions. As in the case of other types of particles (rigid particles and liquid drops) in salt-free media, there is a certain critical value of the particle charge separating two cases, the low-surface-charge case and the high-surface-charge case. For the low-charge case, the mobility is proportional to the particle charge and coincides with that of a soft particle in an electrolyte solution in the limit of very low electrolyte concentrations kappa-->0 (Hückel's limit), where kappa is the Debye-Hückel parameter. For the high-charge case, however, mu becomes essentially constant, independent of the particle charge, due to the counterion condensation effect.