Polymerization of α-methyleneindane: A cyclic analog of α-methylstyrene

α-Methyleniedane (MI), a cyclic analog of α-methylstyrene which does not undergo radical homopolymerization under standard conditions, was synthesized and subjected to radical, cationic, and anionic polymerizations. MI undergoes radical polymerization with α,α′-azobis(isobutyronitrile) in contrast to α-methylstyrene, owing to its reduced steric hindrance, though the polymerization is slow even in bulk. Cationic and anionic polymerization of MI with BF₃OEt₂ and n-butyllithium, respectively, proceed rapidly. The thermal degradation behavior of the polymer depends on the polymerization conditions. The anionic and radical polymers are heteortactic-rich. Reactivity ratios in bulk radical copolymerization on MI (M₂) with methacrylate (MMA, M₁) were determined at 60°C (r₁ = 0.129 and r₂ = 1.07). In order to clarify the copolymerization mechanism, radical copolymerization of MI with MMA was investigated in bulk at temperatures ranging from 50 to 80°C. The Mayo–Lewis equation has been found to be inadequate to describe the result due to depolymerization of MI sequences above 70°C.     

Reaction of α,β-unsaturated Fischer carbene complexes with allyl alkoxide

Optically enriched homo-binuclear Fischer chromium carbene complexes with planar chiral arene chromium complexes gave α-allyl β-arylpropionates up to 97% ee by reaction with allyl alkoxide and subsequent photo-oxidative demetalation. The chiral hetero-binuclear tungsten carbene complexes afforded anti α-allyl β-hydroxy β-arylpropionates as a major product up to 92/8 dr by the same reaction sequence. High diastereoselectivity in these reactions is contributed to the planar chirality of the arene chromium complex, even though the reaction was carried out under vigorous basic media. The reaction products, α-allyl β-arylpropionates were derived by 1,3-M(CO)₅ shift and subsequent [3,3]-sigmatropic rearrangement. Also, the corresponding chromium-uncomplexed α,β-unsaturated Fischer carbene complexes afforded α-allyl β-arylpropionates under the same conditions. Formation of β-allyl β-arylpropionates via 1,2-M(CO)₅ shift followed by [3,4]-sigmatropic rearrangement was not observed in both reactions of chromium-coordinated and the corresponding chromium-uncoordinated α,β-unsaturated Fischer carbene complexes with allyl alkoxide in the presence of base.     

Stereoselective synthesis of atropisomeric korupensamines A and B utilizing planar chiral arene chromium complex

Naphthyl tetrahydroisoquinoline alkaloids, atropisomeric korupensamines A and B and ent-korupensamine B, were synthesized by syn-selective cross-coupling of a planar chiral arene chromium complex with naphthylboronic acid and subsequent axial isomerization or tricarbonylchromium migration to the inverted arene face as a key step. Palladium(0)-catalyzed cross-coupling of planar chiral arene chromium complex 12 with naphthylboronic acid 9 gave syn-biaryl coupling product 13. syn-Biaryl chromium complex 13 was heated in 1:1 mixture of di-n-butyl ether and 1,2-dichloroethane to give a face-inverted anti-biaryl chromium complex 14 without axial isomerization. Korupensamine A was synthesized from the syn-biaryl chromium complex 13 via o-formyl syn-biaryl chromium complex 10, and ent-korupensamine B was prepared from the face-inverted anti-biaryl chromium complex 14. On the other hand, difluoro-substituted syn-biaryl chromium complex 40 with a formyl group afforded anti-biaryl chromium complex 41 containing a rotated central bond by heating in xylene. The chromium-complexed fluorine atom was easily substituted with an isopropoxy group by nucleophilic substitution. Use of these reactions allowed (+)-2-bromo-3,5-difluorobenzaldehyde chromium complex (37) as a single chiral source to be converted to atropisomeric korupensamines A and B, respectively.     

Thermochromism of metal ion complexes of semiquinone radical anions. Control of equilibria between diamagnetic and paramagnetic species by Lewis acids

Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ(.-)) produced by the electron-transfer reduction of PTQ by 1,1'-dimethylferrocene (Me(2)Fc) in the presence of metal ions (Mg(2+) and Sc(3+)) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is in equilibrium, when the concentration of the PTQ(.-)-Mg(2+) complex (lambda(max) = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is complete in the presence of Sc(3+), which is a much stronger Lewis acid than Mg(2+), to produce the PTQ(.-)-Sc(3+) complex (lambda(max) = 631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ(.-)-Sc(3+) complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q(.-)) forms a 2:2 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(2)-Q] with Q and Sc(3+) ions at 298 K (yellow color), which is converted to a 2:3 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(3)-Q] with a strong absorption band at lambda(max) = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc(3+) ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.     

Synthesis of quinoxaline derivatives through condensation of 1,2-diaminobenzenes with β-keto sulfoxides

The reaction of o-phenylenediamine with α-methylsulfinylcyclohexanone and α-methylsulfinylcyclopentanone in the presence of acetic acid afforded 1,2,3,4-tetrahydrophenazine and 2,3-dihydro-1H-cyclopenta[b]-quinoxaline, respectively. 3,4-Diaminotoluene and 3,4-diaminochlorobenzene were reacted with α-methyl-sulfinylacetophenone to give a mixture of the corresponding 6- and 7-substituted 2-phenylquinoxaline. Condensation of 3,4-diaminomethoxybenzene with α-methylsulfinylacetophenone gave 7-methoxy-2-phenylacetophenone, whereas, the same reaction between 3,4-diaminonitrobenzene and α-methylsulfinylacetophenone yielded 6-nitro-2-phenylquinoxaline.     

N-Glycosylation of 2,3-dideoxyfuranose derivatives having a (diethoxyphosphorothioyl)difluoromethyl group at the 3α-position

Lewis acid-mediated N-glycosylation of 2,3-dideoxyribofunanosides having a (diethoxyphosphorothioyl)difluoromethyl group at the 3α-position with silylated nucleobases was examined. The phosphorothioyldifluoromethyl was found to be an effective functional group for the diastereoselective synthesis of β-N¹-pyrimidine-nucleotide analogues 26 and 28-30. However, the method was not useful for the diastereoselective synthesis of adenine nucleotide analogues. The nucleotide analogue 26 was transformed to the difluoromethylenephosphonate analogue 31 of thymidine-3′-phosphate by oxidation with m-CPBA, followed by aqueous work-up.     

Heteroleptic [Bis(oxazoline)](dipyrrinato)zinc(II) Complexes: Bright and Circularly Polarized Luminescence from an Originally Achiral Dipyrrinato Ligand

Heteroleptic zinc(II) complexes synthesized using achiral dipyrrinato and chiral bis(oxazoline) ligands show bright fluorescence with quantum efficiencies of up to 0.70. The fluorescence originates from the ¹π–π^* photoexcited state localized exclusively on the dipyrrinato ligand. Furthermore, the luminescence is circularly polarized despite the achirality of the dipyrrinato ligand. Single‐crystal X‐ray structure analysis discloses that the chiral bis(oxazoline) ligand undergoes intramolecular π–π stacking with the dipyrrinato ligand, inducing axial chirality in the dipyrrinato moiety.     

OFF-OFF-ON switching of fluorescence and electron transfer depending on stepwise complex formation of a host ligand with guest metal ions

Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and a series of metal ions (M(n+) = Sc3+, Y3+, Ho3+, Eu3+, Lu3+, Nd3+, Zn2+, Mg2+, Ca2+, Ba2+, Sr2+, Li+), where TPPZ forms a 2:1 complex [(TPPZ)2-M(n+)] and a 1:1 complex [TPPZ-M(n+)] with Mn+ at low and high concentrations of metal ions, respectively. The fluorescence intensity of TPPZ begins to increase at high concentrations of metal ions, when the 2:1 (TPPZ)2-M(n+) complex is converted to the fluorescent 1:1 TPPZ-M(n+) complex. This is regarded as an "OFF-OFF-ON" fluorescence sensor for metal ions depending on the stepwise complex formation between TPPZ and metal ions. The fluorescence quantum yields of the TPPZ-M(n+) complex vary depending on the metal valence state, in which the fluorescence quantum yields of the divalent metal complexes (TPPZ-M2+) are much larger than those of the trivalent metal complexes (TPPZ-M3+). On the other hand, the binding constants of (TPPZ)2-M(n+) (K1) and TPPZ-M(n+) (K2) vary depending on the Lewis acidity of metal ions (i.e., both K1 and K2 values increase with increasing Lewis acidity of metal ions). Sc3+, which acts as the strongest Lewis acid, forms the (TPPZ)2-Sc3+ and TPPZ-Sc3+ complexes stoichiometrically with TPPZ. In such a case, "OFF-OFF-ON" switching of electron transfer from cobalt(II) tetraphenylporphyrin (CoTPP) to O2 is observed in the presence of Sc3+ and TPPZ depending on the ratio of Sc3+ to TPPZ. Electron transfer from CoTPP to O2 occurs at Sc3+ concentrations above the 1:2 ratio ([Sc3+]/[TPPZ]0 > 0.5), when the (TPPZ)2-Sc3+ complex is converted to the TPPZ-Sc3+ complex and TPPZ-(Sc3+)2, which act as promoters of electron transfer (ON) by the strong binding of O2*- with Sc3+. In sharp contrast, no electron transfer occurs without metal ion (OFF) or in the presence at Sc3+ concentrations below the 1:2 ratio (OFF), when the (TPPZ)2-Sc3+ complex has no binding site available for O2*-.     

Origin of the tunnel anisotropic magnetoresistance in Ga(1-x)Mn(x)As/ZnSe/Ga(1-x)Mn(x)As magnetic tunnel junctions of II-VI/III-V heterostructures

We investigated spin-dependent transport in magnetic tunnel junctions made of III-V Ga(1-x)Mn(x)As electrodes and II-VI ZnSe tunnel barriers. The high tunnel magnetoresistance (TMR) ratio up to 100% we observed indicates high spin polarization at the barrier/electrodes interfaces. We found anisotropic tunneling conductance having a magnitude of 10% with respect to the direction of magnetization to linearly depend on the magnetic anisotropy energy of Ga(1-x)Mn(x)As. This proves that the spin-orbit interactions in the valence band of Ga(1-x)M(x)As are responsible for the tunnel anisotropic magnetoresistance (TAMR) effect.