Supramolecular assembly based on π-π stacking and π-cation interactions between thiacalix[6]arene and DMF

The molecular assembly of thiacalix[6]arene was formed by π-π stacking and π-DMF interactions between thiacalix[6]arene and adjoining thiacalix[6]arene and between thiacalix[6]arene and DMF, respectively. The X-ray crystal analysis also revealed that hydroxyl groups of thiacalix[6]arene adopted novel two sets of hydrogen bond with two DMF and intramolecular hydrogen bond between phenolic units, which cause to make 1,2,3-alternate configuration structure of thiacalix[6]arene.     

Electronic structures of exohedral lanthanide-C60 clusters

We have studied the electronic structures of several gas phase exohedral lanthanide (Ln)-C(60) clusters, Ln(n)C(60) (Ln=Pr, Ho, Tb, Tm, Eu, and Yb) with n=1-4, by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of their anions. Both of the spectroscopic analyses reveal that most of the Ln atoms preferably take +3 oxidation states, while Eu atoms alone assume +2 oxidation states, and that C(60) accepts up to twelve donated electrons in Ln(n)C(60). An additional photoionization examination of the oxygen atom mixing into the Ln(n)C(60) clusters demonstrated that each oxygen atom reduces two electrons from C(60). This result implies that the number of accepted electrons in C(60) can be varied by a suitable choice of the number of Ln atoms and O atoms.     

Enantioselective Synthesis of 3,4‐Dihydropyran‐2‐ones by Domino Michael Addition and Lactonization with New Asymmetric Organocatalysts: Cinchona‐Alkaloid‐Derived Chiral Quaternary Ammonium Phenoxides

Chiral quaternary ammonium phenoxides were readily prepared from commercially available cinchona alkaloids and proved to be useful new asymmetric organocatalysts. Among various chiral quaternary ammonium phenoxides, a cinchonidine‐derived catalyst that bears both a sterically hindered N1‐9‐anthracenylmethyl group and a strongly electron withdrawing 9‐O‐3,5‐bis(trifluoromethyl)benzyl group were found to be highly effective for the Michael addition of ketene silyl acetals (derived from phenyl carboxylates) and α,β‐unsaturated ketones followed by lactonization. Optically active 3,4‐dihydropyran‐2‐one derivatives were obtained in high yields with excellent control of enantio‐ and diastereoselectivity. This catalyst can be handled in air and stored at room temperature in a sealed bottle without decomposition for at least one month.     

Electronic structures of zinc and palladium tetraazaporphyrin derivatives controlled by fused benzo rings

Zinc and palladium tetracyclic aromatic complexes lying structurally between tetraazaporphyrin (TAP) and phthalocyanine (Pc), that is, monobenzo-, adjacently dibenzo-, oppositely dibenzo-, and tribenzo-fused TAPs, have been prepared, and their electronic structures investigated by electronic absorption, magnetic circular dichroism (MCD), fluorescence, phosphorescence, and time-resolved electron paramagnetic resonance (TREPR) spectroscopy, as well as cyclic voltammetry. The last-named indicated that the first oxidation potentials shift to more negative values with increasing number of the fused benzo rings, but also suggested that the first reduction potential apparently has no correlation with the size and symmetry of the pi-conjugated systems. However, this latter behavior is reasonably interpreted by the finding that the effect of the fused benzo rings on destabilization of the LUMO depends on the orbital to which they are fused (i.e., whether it is an egx or egy orbital), since the LUMOs of TAP complexes are degenerate with D4h symmetry. The energy splitting of the LUMOs, that is, DeltaLUMO, was evaluated experimentally for the first time by analyzing the relationship between the first reduction potential and the size and shape of the pi-conjugated system. Electronic absorption and MCD measurements indicate that the lowest excited singlet states are split in the case of the low-symmetry TAP derivatives, although these excited states are degenerate for Pc and TAP with D4h symmetry. These energy splittings DeltaE(SS) correlate well with the DeltaLUMO values. To investigate the electronic structures in the lowest excited triplet state, zero-field splitting (zfs) was analyzed by time-resolved EPR (TREPR) spectroscopy. The energy splitting in the lowest excited triplet state, DeltaE(TT) was quantitatively evaluated from the temperature dependence of the zfs or spin-orbit coupling of the Pd complexes. Consequently, it is demonstrated that DeltaLUMO, DeltaE(SS), and DeltaE(TT) values exhibiting a mutually good relationship can be determined experimentally.     

Novel pyridinium salts as cationic thermal and photoinitiators and their photosensitization properties

Novel pyridinium salts [N‐(α‐phenylbenzyl)‐, N‐(1‐naphthylmethyl)‐, or N‐cinnamyl p‐ or o‐cyanopyridinium hexafluoroantimonates] were synthesized by the reaction of p‐ or o‐cyanopyridine and the corresponding bromides followed by anion exchange with KSbF₆. These pyridinium salts polymerized epoxy monomers at lower temperatures than previously reported for N‐benzyl‐2‐cyanopyridinium hexafluoroantimonate. The o‐substituted pyridinium salts showed higher activity than the p‐substituted ones, and the crosslinked epoxy polymers cured with these initiators showed higher glass‐transition temperatures. These pyridinium salts photoinitiated radical polymerization as well as cationic polymerization. The photopolymerization was accelerated by the addition of aromatic ketones as photosensitizers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1037–1046, 2002     

PHOTOCHEMICAL REACTION OF 13-CIS-BACTERIORHODOPSIN STUDIED BY LOW TEMPERATURE SPECTROPHOTOMETRY

Abstract— The photoreaction cycle of 13- cis -bacteriorhodopsin (13- cis -bR) was investigated by low temperature spectrophotometry using two different preparations; 13- cis -bR constituted from bacterioopsin and 13- cis -retinal, and dark-adapted bacteriorhodopsin (bR^D), which is an equi-molar mixture of 13- cis -bR and trans -bR.
By irradiation with 500 nm light at — 190°C, 13- cis -bR was converted to its batho-product, batho-13- cis -bR (batho-bR¹³), which is different from batho-product from trans -bR, batho-bR^t. On warming batho-bR¹³ to -5°C in the dark, it completely changed to trans -bR. We estimated the composition of 13- cis -bR and trans -bR in the warmed sample spectrophotometrically and then the absorption spectrum of batho-bR¹³ was calculated. The absorption maximum lies at 608 nm, 1250 cm⁻¹ longer than that of 13- cis -bR; the molar extinction coefficient (ε) is about 74000 M ⁻¹ cm⁻¹, larger than that of 13- cis -bR (52000 M ⁻¹ cm⁻¹).
On the warming the sample containing batho-bR¹³ formed by irradiating 13- cis -bR or bR^D at — 190°C, we could not detect other intermediates such as the lumi- or meta-intermediates seen in trans-bR system.     

Spectroscopy, electrochemistry, and molecular orbital calculations of metal-free tetraazaporphyrin, -chlorin, -bacteriochlorin, and -isobacteriochlorin

Metal-free tetraazachlorin (TAC), -bacteriochlorin (TAB), and -isobacteriochlorin (TAiB) were characterized by electronic absorption, magnetic circular dichroism (MCD), fluorescence, and time-resolved ESR (TR-ESR) spectroscopy, and by cyclic voltammetry. The results are compared with those of metal-free tetraazaporphyrin (TAP). The potential difference DeltaE between the first oxidation and reduction couples decreases in the order TAP>TAiB>TAC>TAB. The splitting of both the Q and Soret bands decreases in the order TAB>TAC>TAP>TAiB. Corresponding to the split absorption bands, MCD spectra show a minus-to-plus pattern with increasing energy in both the Q and Soret regions, which suggests that the energy difference between the HOMO and second HOMO is larger than that between the LUMO and second LUMO. These spectroscopic properties and redox potentials were reproduced by molecular orbital calculations using the ZINDO/S Hamiltonian. The fluorescence quantum yields of the reduced species are much smaller than that of TAP. The zero-field splitting (ZFS) parameters D and E of the excited triplet states (T1) of these species decrease and increase, respectively, on going from TAP to TAC and further to TAB. The D and E values of TAiB are larger than those of the other species. The results are supported by the absence of interaction between the spin over reduced pyrrole moieties of the HOMO and over the LUMO, and by calculations of ZFS under a half-point-charge approximation.     

Regulation of geometry around the ruthenium center of bis(2-pyridinecarboxylato) complexes by the nitrosyl moiety: syntheses, structures, and theoretical studies

cis-[Ru(NO)Cl(pyca)(2)] (pyca = 2-pyridinecarboxylato), in which the two pyridyl nitrogen atoms of the two pyca ligands coordinate at the trans position to each other and the two carboxylic oxygen atoms at the trans position to the nitrosyl ligand and the chloro ligand, respectively (type I shown as in Chart 1), reacted with NaOCH(3) to generate cis-[Ru(NO)(OCH(3))(pyca)(2)] (type I). The geometry of this complex was confirmed to be the same as the starting complex by X-ray crystallography: C(13.5)H(13)N(3)O(6.5)Ru; monoclinic, P2(1)/n; a = 8.120(1), b = 16.650(1), c = 11.510(1) A; beta = 99.07(1) degrees; V = 1536.7(2) A(3); Z = 4. The cis-trans geometrical change reaction occurred in the reactions of cis-[Ru(NO)(OCH(3))(pyca)(2)] (type I) in water and alcohol (ROH, R = CH(3), C(2)H(5)) to form [[trans-Ru(NO)(pyca)(2)](2)(H(3)O(2))](+) (type V) and trans-[Ru(NO)(OR)(pyca)(2)] (type V). The reactions of the trans-form complexes, trans-[Ru(NO)(H(2)O)(pyca)(2)](+) (type V) and trans-[Ru(NO)(OCH(3))(pyca)(2)] (type V), with Cl(-) in hydrochloric acid solution afforded the cis-form complex, cis-[Ru(NO)Cl(pyca)(2)] (type I). The favorable geometry of [Ru(NO)X(pyca)(2)](n)(+) depended on the nature of the coexisting ligand X. This conclusion was confirmed by theoretical, synthetic, and structural studies. The mono-pyca-containing nitrosylruthenium complex (C(2)H(5))(4)N[Ru(NO)Cl(3)(pyca)] was synthesized by the reaction of [Ru(NO)Cl(5)](2)(-) with Hpyca and characterized by X-ray structural analysis: C(14)H(24)N(3)O(3)Cl(3)Ru; triclinic, Ponemacr;, a = 7.631(1), b = 9.669(1), c = 13.627(1) A; alpha = 83.05(2), beta = 82.23(1), gamma = 81.94(1) degrees; V = 981.1(1) A(3); Z = 2. The type II complex of cis-[Ru(NO)Cl(pyca)(2)] was synthesized by the reaction of [Ru(NO)Cl(3)(pyca)](-) or [Ru(NO)Cl(5)](2)(-) with Hpyca and isolated by column chromatography. The structure was determined by X-ray structural analysis: C(12)H(8)N(3)O(5)ClRu; monoclinic, P2(1)/n; a = 10.010(1), b = 13.280(1), c = 11.335(1) A; beta = 113.45(1) degrees; V = 1382.4(2) A(3); Z = 4.     

Dynamic coordination polymers with 4,4'-oxybis(benzoate): reversible transformations of nano- and nonporous coordination frameworks responding to present solvents

Reversible construction of a nanoporous framework from a nonporous framework has been found in the zinc(II) coordination polymer with 4,4'-oxybis(benzoate) (oba). [Zn(2)(oba)(2)(dmf)(2)].2DMF (1), which has 1 nm scale channels, transforms to the nonporous coordination polymer [Zn(oba)(H(2)O)] (2) with the loss of the open framework. Compound 2 on treatment with DMF reversibly yields nanoporous compound 1.     

Facile Synthesis and Palladium‐Catalyzed Cross‐Coupling Reactions of 2,3‐Bis(pinacolatoboryl)‐1,3‐butadiene

Treatment of 1,1‐bis(pinacolatoboryl)ethene with an excess of 1‐bromo‐1‐lithioethene gave 2,3‐bis(pinacolatoboryl)‐1,3‐butadiene in high yield. Palladium‐catalyzed cross‐coupling of the resulting diborylbutadiene with aryl iodides took place smoothly in the presence of a catalytic amount of Pd(OAc)₂/PPh₃ and aqueous KOH to give 2,3‐diaryl‐1,3‐butadienes in good yields. The coupling reaction with commercially available 4‐acetoxyphenylmethyl chloride under the same conditions followed by hydrolysis of the acetyl groups gave anolignan B in a one‐pot manner. A variety of [3]‐ to [6]dendralenes were synthesized by palladium‐catalyzed coupling of the diene or 1,1‐bis(pinacolato)borylethene with alkenyl or dienyl halides, respectively, in good yields.