Facile Asymmetric Synthesis of α-Amino Acids Employing Chiral Ligand-Mediated Asymmetric Addition Reactions of Phenyllithium with Imines

A three-step methodology involving an external chiral ligand-mediated asymmetric addition of phenyllithium to an anisidine imine, oxidative removal of N-PMP group, and finally oxidative conversion of the phenyl group to a carboxyl group provides a facile synthesis of optically pure α-amino acid derivatives bearing a bulky α-substituent.     

Compartmentalization in TEMPO‐Mediated Radical Polymerization in Dispersed Systems: Effects of Macroinitiator Concentration

The influence of the initial macroinitiator concentration ([PT]₀) on compartmentalization effects (segregation effects and confined space effects) in 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO)‐mediated radical polymerization of styrene in a dispersed system at 125 °C has been investigated by simulations employing modified Smith‐Ewart equations. The modeling approach accounts for compartmentalization of both propagating radicals and nitroxide, as well as the generation of radicals by thermal initiation of styrene. The manifestation of compartmentalization effects occurs at significantly greater particle diameters (d) for low [PT]₀; at [PT]₀ = 0.002 M , the polymerization rate, control and livingness are affected by compartmentalization for d < 120 nm, whereas the system behaves as in the corresponding bulk system for d > 45 nm at [PT]₀ = 0.2 M . The results are discussed with regards to the specific effects of compartmentalization on deactivation and bimolecular termination.

     

Nitroxide‐Mediated Radical Polymerization in Miniemulsion at Stationary State: Rationale for Independence of Polymerization Rate on Nitroxide Partitioning Using Oil‐Phase Initiation

Summary: Simulations based on the kinetics and mechanism of nitroxide‐mediated free radical polymerization (NMP) have been carried out in order to understand the hitherto largely unexplained effects (or lack thereof) of nitroxide partitioning in aqueous miniemulsion NMP. The focus has been on the miniemulsion NMP of styrene mediated by TEMPO and 4‐hydroxy‐TEMPO, two nitroxides with very similar activation‐deactivation equilibria, but very different organic phase‐aqueous phase partition coefficients. The general conclusion is that the organic phase propagating radical and nitroxide concentrations are unaffected by the partition coefficient in the stationary state, but the rate of polymerization and the extent of bimolecular termination increase with increasing nitroxide water solubility in the pre‐stationary state region. Specific NMP systems are, therefore, affected differently by nitroxide partitioning depending on whether polymerization predominantly occurs in the stationary state or not, which in turn is governed mainly by the activation‐deactivation equilibrium constant and the rate of thermal initiation.

Simulated organic‐phase propagating radical concentrations in the presence of thermal initiation for TEMPO‐mediated miniemulsion free radical polymerization of styrene for different nitroxide partitioning coefficients at 125 °C.     

A New Tetracyclic Bromopyrrole-Imidazole Derivative through Direct Chemical Diversification of Substances Present in Natural Product Extract from Marine Sponge Petrosia (Strongylophora) sp.

Chemical diversification of substances present in natural product extracts can lead to a number of natural product-like compounds with a better chance of desirable bioactivities. The aim of this work was to discover unprecedented chemical conversion and produce new compounds through a one-step reaction of substances present in the extracts of marine sponges. In this report, a new unnatural tetracyclic bromopyrrole-imidazole derivative, rac-6-OEt-cylindradine A (1), was created from a chemically diversified extract of the sponge Petrosia (Strongylophora) sp. We also confirmed that 1 originated from naturally occurring (-)-cylindradine A (2) via a new reaction pattern. Moreover, (-)-dibromophakellin (3) and 4,5-dibromopyrrole-2-carboxylic acid (4), as well as 2, were reported herein for the first time in this genus. Studies on the possible reaction mechanism and bioactivities were also conducted. The results indicate that the direct chemical diversification of substances present in natural product extracts can be a speedy and useful strategy for the discovery of new compounds.     

Synthesis and Aromaticity of Benzene-Fused Doubly N-Confused Porphyrins

A bicyclo[2.2.2]octadiene(BCOD)-fused bis(dipyrromethane) derivative was reacted with methylal in the presence of trifluoroacetic acid followed by oxidation with chloranil to give a doubly N-confused phlorin derivative, which did not undergo the retro-Diels–Alder reaction extruding an ethylene molecule on heating. In contrast, a 7-tert-butoxybicyclo[2.2.1]heptadiene(7-BuOBCHD)-fused bis(dipyrromethane) yielded benzene-ring-fused doubly N-confused porphyrins by following similar reaction sequences.     

Rare earth alkyl and hydride complexes bearing silylene-linked cyclopentadienyl-phosphido ligands. Synthesis, structures, and catalysis in olefin hydrosilylation and ethylene polymerization

A series of silylene-linked cyclopentadienyl-phosphido rare earth alkyl and hydride complexes of type Me₂Si(C₅Me₄)(PR′)LnR (Ln=Y, Yb, Lu; R′=Ph, Cy, C₆H₂^tBu₃-2,4,6; R=CH₂SiMe₃, H) have been synthesized and structurally characterized, and their activity in ethylene polymerization and olefin hydrosilylation has been studied. These complexes represent the first examples of rare earth alkyl and hydride complexes bearing cyclopentadienyl-phosphido ligands, which are in sharp contrast both structurally and chemically with the analogous cyclopentadienyl-amido and metallocene complexes.     

Exciton migration dynamics in a dendritic molecule: quantum master equation approach using ab initio molecular orbital configuration interaction method

We investigate the exciton migration dynamics in a dendritic molecular model composed of pi-conjugation linear-leg units (acetylenes and diacetylene) and a benzene ring (branching point) using the quantum master equation approach with the ab initio molecular orbital (MO) configuration interaction (CI) method. The efficient migration of exciton from short-length linear legs (acetylenes) to long-length linear leg (diacetylene) via a benzene ring is observed. As predicted in previous studies, the exciton (electron and hole) distributions are relatively well localized in each generation segmented by the meta-branching point (meta-substituted benzene ring) though the electron and hole distributions are delocalized and are somewhat spatially different from each other within each generation. It is found that the excitons localized in the generation composed of short linear legs occupy in higher-lying exciton states, while those in the generation composed of long linear legs do in lower-lying ones. These features suggest the decoupling of pi-conjugation at the meta-branching point. On the other hand, the relaxation effect between exciton states is found to be caused by the exciton-phonon coupling, in which the existence of common configurations (electron-hole pairs) in CI wave functions between adjacent exciton states (having primary distributions on short and long linear-leg regions, respectively) is important for the relaxation between their exciton states. This feature indicates the importance of partial penetration of pi-conjugation through the meta-substituted benzene ring in excited states for such exciton migration.     

Preparation of a copper ion complex of sterically congested diphenyldiazomethanes having a pyridine ligand and characterization of their photoproducts

To show that persistent high-spin polycarbenes can be realized by utilizing hetero spin systems, two diphenyldiazomethanes having pyridyl groups, i.e., bis{4-(4-pyridyl)-2,6-dimethylphenyl}diazomethane (4,4'-DPy-1-N(2)) and {2,4-di(4-pyridyl)-6-bromophenyl}(2,6-dimethyl-4-tert-butylphenyl)diazomethane (2,4-DPy-1-N(2)), were prepared. Triplet carbenes, 4,4'-DPy-1 and 2,4-DPy-1, generated by photolysis of the corresponding diazomethanes were characterized by spectroscopic means (ESR and UV/vis in matrix at low temperatures and laser flash photolysis in solution at room temperature). The results showed that they were fairly persistent. Magnetic properties of the photoproducts from a 1:1 complex between DPy-1-N(2) and Cu(hfac)(2) (hfac = hexafluoroacetylacetonate) were characterized by ESR and a superconducting quantum interference device (SQUID) magneto/susceptometer. The field dependences of magnetization for the complexes, expressed by using M versus H/T plots, were analyzed in terms of the Brillouin function to be S = 6.80 (F = 0.60) for the 1:1 complex of 4,4'-DPy-1 and Cu(hfac)(2) and S = 3.71 (F = 0.73) for the 1:1 complex of 2,4-DPy-1 and Cu(hfac)(2) at 2.0 K. Thus, it has been demonstrated that a high-spin species is actually generated in the photoproducts and that the complexed carbenes showed significant stability.     

Preparation of cationic nanoparticles by the particle dissolution method from submicron-sized,styrene-butyl acrylate-dimethylaminoethyl methacrylate terpolymer particles

Cationic nanoparticles were prepared from submicron-sized styrene-butyl acrylate-dimethylaminoethyl methacrylate terpolymer (59.2/20.8/20.0, molar ratio) particles in a polyoxyethylene nonylphenylether nonionic emulsifier aqueous solution at pH 2.0 above 150 °C.