Photophysics of 6-(2′-hydroxy-4′-methoxyphenyl)-s-triazine photostabilizers

The room temperature photophysical properties of several sulphonated and unsulphonated 6-(2′-hydroxy-4′-methoxyphenyl)-s-triazines were investigated in a range of solvents by means of steady state and picosecond fluorescence spectroscopy. Compounds possessing phenyl or p-tolyl groups in the s-triazinyl ring exhibit only a very weak normal Stokes-shifted fluorescence, arising from the initially excited chromophore. Substitution of phenoxy groups into the s-triazinyl ring results in the appearance of an additional longer-wavelength fluorescence which is assigned to the keto tautomer, formed following excited state intramolecular proton transfer (ESIPT). The rate constant for the (ESIPT) process that occurs in sodium 3-(3′,5′-diphenoxy-2′,4′,6′-triazinyl)-4-hydroxy-2-methoxybenzene sulphonate in water is estimated to be greater than 10¹¹ s⁻¹.     

Selective 5′-desilylation of 3′,5′-disilyi AraU derivatives under basic conditions

3′,5′-di-tert-butyldimethylsilyl-2,2′-anhydrouridine 3 was hydrolyzed under basic conditions to yield selectively 3′-tert-butyldimethylsilylarabinouridine 5 in 85 % yield. 3′,5′-disilyl-arabinouridine derivatives 4 and 6 also led selectively to 5 under the same conditions. These reactions suggest an intramolecular participation of the 2′-hydroxyl group in the desilylation and can be used to prepare rapidly and in high yield 3′-silylaraU derivatives.     

1′,3′,3′-Trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline]: its thermal enantiomerization and the equilibration with its merocyanine

The enantiomers of the title compound, the important photochromic material (RS)-1b, have been enriched semipreparatively by liquid chromatography. As a consequence, we were able to determine the barrier of the thermal interconversion (R)-1b(S)-1b via time-dependent polarimetry, amounting to ΔG^≠=85.9 kJ/mol at 22.0°C in d⁶-DMSO (Table 2). The thermal equilibration of the corresponding merocyanine 2b was monitored in d⁶-DMSO by time-dependent ¹H NMR, resulting in ΔG₁^≠=102.8 and ΔG₂^≠=92.0 kJ/mol at 22°C (Table 1). This means that, starting from (RS)-1b, the opened isomer 2b is attained by a slow reaction (ΔG₁^≠=102.8 kJ/mol, Fig. 4). Therefore, the merocyanine 2b cannot be identified with the intermediate required for the fast process of C(sp³)–O bond cleavage (ΔG^≠=85.9 kJ/mol) upon the above enantiomerization of (RS)-1b. Apparently, these two thermal isomerizations (Fig. 4) are independent of each other. The structure of the unknown intermediate of the interconversion (R)-1b(S)-1b must therefore differ from the known one of merocyanine 2b.

Table 1. Equilibration between spiro compounds (RS)-1 and merocyanines 2 at 22°C, measured by time-dependent UV absorptions[3] for (RS)-1a2a and by time-dependent ¹H NMR intensities for the other compounds

Full-size table (8K)

Magnetic susceptibility of 1,1′,2,2′-tetramethylcobaltocene and 1,1′-diethylcobaltocene: Evidence for the dynamic Jahn-Teller effect

The magnetic susceptibility of 1,1′,2,2′-tetramethylcobaltocene, Co[C₅H₃(CH₃)₂]₂, and 1,1′-diethylcobaltocene, Co(C₅H₄C₂H₅)₂, has been studied between 0.99 and 296 K. The data are well reproduced by a calculation of the dynamic Jahn-Teller effect for the ²E_1g(a_1g²e_2g⁴e_1g) ground state of D_5d symmetry. A suitable set of parameter values is given by ζ = 100 cm⁻¹, δ = 150 cm⁻¹, k_JT = 0.40, κ = 0.70. The magnetism of cobaltocene, Co(C₅H₅)₂, may be described by parameter values of comparable magnitude. The results imply a significantly larger reduction of the spin-orbit coupling parameter ζ due to covalency than of the orbital reduction factor κ.     

The preparation and coordination chemistry of 2,2′:6′,2″-terpyridine macrocycles—VI. Planar hexadentate macrocycles incorporating 2,2′: 6′,2″-terpyridine

The template condensation of 6,6″-bis(-methylhydrazino)-2,2′: 6′,2″-terpyridines L² and L³ with 2,6-pyridinedialdehyde may give a number of different products depending upon the metal ion which is used. In the presence of nickel(II) the products are either the nickel(II) complexes of the 18-membered ring macrocycles L⁴ or L⁵ or the free macrocycles. The metal ion acts as a transient template and is removed in a chloride ion specific demetallation. The use of dimethyltin(IV) as a template results in the formation of complexes of the ring contracted macrocycles L⁶ or L⁷.     

Synthesis of 1,1′-methylenedi[(1R,1′R,3R,3′R,5R,5′R)-8-oxabicyclo[3.2.1]oct-6-en-3-ol] and derivatives: precursors of long-chain polyketides

Racemic 1,1′-methylene[(1RS,1′RS,3RS,3′RS,5RS,5′RS)-8-oxabicyclo[3.2.1]oct-6-en-3-ol] ((±)-6) derived from 2,2′-methylenedifuran has been resolved kinetically with Candida cyclindracea lipase-catalysed transesterification giving 1,1′-methylenedi[(1R,1′R,3R,3′R,5R,5′R)-8-oxabicyclo[3.2.1]oct-6-en-3-ol] (−)-6 (30% yield, 98% ee) and 1,1′-methylenedi[(1S,1′S,3S,3′S,5S,5′S)-8-oxabicyclo[3.2.1]oct-6-en-3-yl] diacetate (+)-8, (40% yield, 98% ee). These compounds have been converted into 1,1′-methylenedi[(4S,4′S,6S,6′S)- and (4R,4′R,6R,6′R)-cyclohept-1-en-4,6-diyl] derivatives.     

An unexpected [2+2]-cycloaddition reaction of 4-methyldithieno-[3,4-b:3′,2′-d]pyridinium iodide with dimethyl acetylenedicarboxylate

An unexpected [2+2]-cycloaddition occured in the reaction of 4-methyldithieno-[3,4-6:3′,2′-d]pyridinium iodide (3)with two equivalents of DMAD, giving 4-(trans-1,2-dicarbomethoxy-2- iodovinyl)-5-methyl-6,7-dicarbomethoxy-4,5-dihydrothieno [23-c]quinoline (4) in 54% yield. 4 is formed via 4-methyl-5-(trans-1,2-dicarbomethoxy-2-iodo-4,5-dihydrothieno [3,4-b:3′,2′-d]pyridine (16), followed by [2+2]-cycloaddition. The primary adduct rearranges via a thiepin to an episulfide which eliminates sulfur to give 4.     

Approaches to wired terpyridine: Bithienyl alkynyl derivatives of 2,2′:6′,2″-terpyridine and their ruthenium(II) complexes

Two approaches to the formation of ruthenium(II) complexes containing ligands with conjugated 2,2′:6′,2″-terpyridine (tpy), alkynyl and bithienyl units have been investigated. Bromination of 4′-(2,2′-bithien-5′-yl)-2,2′:6′,2″-terpyridine leads to 4′-(5-bromo-2,2’-bithien-5′-yl)-2,2′:6′,2″-terpyridine (1), the single crystal structure of which has been determined. The complexes [Ru(1)₂][PF₆]₂ and [Ru(tpy)(1)][PF₆]₂ have been prepared and characterized. Sonogashira coupling of the bromo-substituent with (TIPS)CCH did not prove to be an efficient method of preparing the corresponding complexes with pendant alkynyl units. The reaction of 4′-ethynyl-2,2′:6’,2″-terpyridine with 5-bromo-2,2′-bithiophene under Sonogashira conditions yielded ligand 2, and the heteroleptic ruthenium(II) complex [Ru(2)(tpy)][PF₆]₂ has been prepared and characterized.     

Investigation on lanthanide binuclear complexes of 1,5-bis-(1′-phenyl-3′-methyl-5′-pyrazolone-4′)-1,5-pentanedione and 2,2′-bipyridine

The coordination of 1,5-bis-(1′-phenyl-3′-methyl-5′-pyrazolone-4′)-1,5-pentanedione (BPMPPD) and 2,2′-bipyridine (bipy) with lanthanide ions in water-alcohol solution has been studied. Binuclear complexes of the types : Ln₂(BPMPPD)₃(bipy)₂·nH₂O (n = 2 for Y, n = 4 for Eu, Gd, Dy, Ho, Er, Tm and Yb); Ln₂(BPMPPD)₃bipy·nH₂O (n = 10 for La, n = 3 for Pr, Nd, Sm and Tb) were formed. The compounds were characterized by elemental analysis, molar conductance, IR, UV, ¹H NMR spectroscopy, thermogravimetric analysis and fluorescence spectra.     

Constituents of Casimiroa edulis Llave et Lex.—VI 2′,5,6-Trimethoxyflavone, 2′,5,6,7-tetramethoxyflavone (zapotin) and 5-hydroxy-2′,6,7-trimethoxyflavone (zapotinin)

Experiments are described which show that a substance isolated from the root bark of Casimiroa edulis Llave et Lex. is 2′,5,6-trimethoxyflavone (IIa). It is demonstrated that zapotin and zapotinin, isolated previously from the same plant, are very probably 2′,5,6,7-tetramethoxyflavone (IIIa) and 5-hydroxy-2′,5,6-trimethoxyflavone (IIIb), respectively. The occurrence in nature of flavonoids of type IIa, IIIa and IIIb bearing a single oxygen substituent in ring B at the 2′-position is unusual.     

Kinetics and selectivity of the copper-catalysed oxidative coupling of 4-(2′,6′-dimethylphenoxy)-2,6-dimethylphenol

The kinetics of the copper/N-methylimidazole catalysed oxidative coupling reaction with the C–O coupled dimer of 2,6-dimethylphenol (DMP or monomer), viz. 4-(2′,6′-dimethylphenoxy)-2,6-dimethylphenol (dimer), as the substrate have been studied. The reaction was found to obey Michaelis–Menten kinetics. The dimer is more easily oxidised than the monomer, but the formation of a copper–substrate complex is more difficult. The reaction rates are higher than in the case of the monomer, and the amounts of diphenoquinone (DPQ) formed are much lower. With the dimer as the substrate, the order of the reaction in copper is 2, confirming that the formation of a dinuclear copper complex is an important step in the reaction mechanism. The amount of DPQ formed is proportional to the initial amount of the dimer. A slight, but clear preference for the dimer over the monomer as the substrate has been observed from experiments with mixtures of monomer and dimer. The amount of DPQ formed decreases exponentially with an increase in the fraction of dimer in the mixture, which can be ascribed mainly to a statistical effect.     

The effect of 2,2′-bipyridine, 2,2′:6′,2″-terpyridine and 1,10-phenantroline on radiation reduction of Rh(II) to Rh(I) complexes

Rh(II) acetate binuclear complexes have been reduced by gamma rays to Rh(I) complexes when 2,2′-bipyridine, 2,2′:6′,2″-terpyridine or 1,10-phenantroline ligands are present in aqueous methanol systems. These complexes exist in several forms possessing different absorption spectra. Their concentration depends on the ratio of the initial concentration of the ligands to Rh(II).     

Micromodulation of electron-transfer rates in mixed-valence 1′,1″′-dibenzylbiferrocenium cations

X-Ray structures of 1′,1′″-disubstituted biferrocenium triiodide salts have been studied and the dramatic effects of substituents on the intramolecular electron-transfer rates are described.     

Structural consequences of oxidation of ferrocene derivatives : II. 1,1′,2,2′,4,4′-tris(trimethylene)ferrocenium perchlorate

The crystal and molecular structures of 1,1′,2,2′,4,4′-tris(trimethylene)ferrocenium perchlorate (I) were determined by X-ray crystallography. Despite the rigidity imparted to the molecule by the three non-adjacent bridges, the iron-to-ring distance of the cation was 4.4(4) pm longer than in the neutral compound, in agreement with what was reported for non-bridged ferrocene derivatives. The increased separation of the rings was accommodated by an increase in angle between the -carbon atoms and the ring-planes and by an increase in the ring-ring tilt angle.     

Formation of hydrogen-bonded complexes of 3,3′,5,5′-tetrabromo-2,2′-biphenol with MTBD and triethylamine

The complexes of 3,3′,5,5′-tetrabromo-2,2′-biphenol (TBBPh) with 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) and triethylamine (TEA) were studied by FTIR spectroscopy. In chloroform and in acetonitrile a proton transfer from TBBPh to N-bases (MTBD, TEA) occurs. In chloroform solution the protonated N-base molecules are hydrogen-bonded to the deprotonated TBBPh molecules, whereas in acetonitrile the complexes dissociate. The intra- as well as intermolecular hydrogen bonds within the chains show large proton polarizability.     

Total, asymmetric synthesis of (1r-1-c-(6′-amino-7′h-purin-8′-yl)-1,4-anhydro-3-azido-2,3-dideoxy-d-erythro-pentitol.

The “naked sugar” (+)-(1R,2R,4R)-2-cyano-7-oxabicyclo[2.2.1]hept-5-en-2-exo-yl acetate ((+)-3) was converted in ten synthetic steps into the new C-nucleoside (1R)-1-C-(6′-amino-7′H-purin-8′-yl)-1,4-anhydro-3-azido-2,3-dideoxy- D-erythro-pentitol ((+)-2) in 19% overall yield.     

Structural, spectral and thermal studies of N-2-(4-picolyl)- and N-2-(6-picolyl)-N′-(2-chlorophenyl)thioureas and N-2-(6-picolyl)-N′-(2-bromophenyl)thiourea

N-2-(4-picolyl)-N′-2-chlorophenylthiourea, 4PicTu2Cl, monoclinic, P2₁/c, a=10.068(5), b=11.715(2), β=96.88(4)°, and Z=4; N-2-(6-picolyl)-N′-2-chlorophenylthiourea, 6PicTu2Cl, triclinic, P-1, a=7.4250(8), b=7.5690(16), c=12.664(3) Å, =105.706(17), β=103.181(13), γ=90.063(13)°, V=665.6(2) ų and Z=2 and N-2-(6-picolyl)-N′-2-bromophenylthiourea, 6PicTu2Br, triclinic, P-1, a=7.512(4), b=7.535(6), c=12.575(4) Å, a=103.14(3), β=105.67(3), γ=90.28(4)°, V=665.7(2) ų and Z=2. The intramolecular hydrogen bonding between N′H and the pyridine nitrogen and intermolecular hydrogen bonding involving the thione sulfur and the NH hydrogen, as well as the planarity of the molecules, are affected by the position of the methyl substituent on the pyridine ring. The enthalpies of fusion and melting points of these thioureas are also affected. ¹H NMR studies in CDCl₃ show the NH′ hydrogen resonance considerably downfield from other resonances in their spectra.     

H, C, N NMR, ESI mass spectral and single crystal X-ray structural characterization of three spiro[pyrrolidine-2,3′-oxindoles]

Three spiro[pyrrolidine-2,3′-oxindoles], 1,1′,2,2′,5′,6′,7′,7′a-octahydro-2-oxo-1′-phenyl-spiro[3H-indole-3,3′-[3H]-pyrrolizine]-2′-carboxylic acid methyl ester (1), 1,1′,2,2′,5′,6′,7′,7′a-octahydro-2-oxo-1′-nitro-2′-phenyl-spiro[3H-indole-3, 3′-[3H]-pyrrolizine] (2) and 1,1′,2,2′,5′,6′,7′,7′a-octahydro-2-oxo-1′-nitro-2′-(4″-chlorophenyl)-spiro[3H-indole-3,3′-[3H]-pyrrolizine] (3) have been synthesized and their ¹H, ¹³C and ¹⁵N spectra assigned. The chemical shift assignments are based on Pulsed Field Gradient (PFG) Double Quantum Filter (DQF) ¹H, ¹H correlation spectroscopy (COSY), PFG ¹H, ¹³C Heteronuclear Multiple Quantum Coherence (HMQC) and PFG ¹H,X (X = ¹³C and ¹⁵N) Heteronuclear Multiple Bond Correlation (HMBC) experiments. The single crystal X-ray structures of 1–3 have been determined. Compounds 1 and 2 crystallized in monoclinic space group C2/c and compound 3 in monoclinic space group P2₁/c, respectively. Also the ESI-TOF MS data of 1–3 are given.     

Homoleptic carbene complexes VI. Bis{1,1′-methylene-3,3′-dialkyl-diimidazolin-2,2′-diylidene}palladium chelate complexes by the free carbene route

1,1′-Methylene-3,3′-dialkyldiimidazolium salts have been deprotonated with n-butylithium in the presence of palladium(II) iodide to give the percarbene complexes 1 (alkyl=Me) and 2 (alkyl=Et), each containing two bidentate 1,1′-methylene-3,3′-dialkyldiimidazolin-2,2′-diylidene chelate ligands. The X-ray structure analysis of 1 reveals a stereochemistry in which the two spiro-linked six-membered metallacycles adopt boat-like conformations strongly bending out of the PdC₄ coordination plane in opposite directions. The carbenoid imidazole rings, which are rotated by +42 and −43°, respectively, relative to this plane, break down into two tightly bound π-systems (N=4C=4N,= C=C) connected by long C---N bonds.