Ureas as a New Nucleophilic Reagents for SNAr Amination and Carbamoyl Amination Reactions in 1,3,7‐Triazapyrene Series

The ability of urea anions to react as nucleophiles with alkoxy derivatives of 1,3,7‐triazapyrenes has been investigated. It was found that against all expectations, the products of the substitution of an alkoxy groups (S_N^ipso ) by amino group were isolated in good yields. The reactions proceed in anhydrous dimethyl sulfoxide solution at room temperature. But when anions of the mono‐substituted ureas containing bulky substituents were used, the first products of the earlier unknown S_NAr reactions of alkyl carbamoyl amination were obtained.     

Strategy for Molecular Design of Photochromic Diarylethenes Having Thermal Functionality

Thermal reactivities of photochromic diarylethene closed‐ring isomers can be controlled by the introduction of substituents at the reactive positions. Diarylethenes having bulky alkyl groups undergo thermal cycloreversion reactions. When bulky alkoxy groups are introduced, the diarylethenes have both thermal cycloreversion reactivities and low photocycloreversion quantum yields. Such photochromic compounds can be applied to thermally reusable photoresponsive‐image recordings. The thermal cycloreversion reactivity of the closed‐ring isomers can be evaluated using specific steric substituent constants and be correlated with the parameters. By introduction of trimethylsilyl or methoxymethyl groups at the reactive positions, the diarylethene closed‐ring isomers undergo thermal irreversible reactions to produce by‐products at high temperatures. These diarylethenes may be useful for secret‐image recordings. Furthermore, thiophene‐S,S‐dioxidized diarylethenes having secondary alkyl groups at the reactive positions undergo thermal by‐product formation reactions, in addition to the photostability of the colored closed‐ring isomers. Such materials may be used for light‐starting thermosensors. The thermal by‐product formation reactivity can be evaluated by the specific substituent constants and theoretical calculations of quantum chemistry. These results supply the strategy for the molecular design of the photochromic diarylethenes having thermal functionality.

     

Boron-bridged π-conjugated ladders as efficient electron-transporting emitters

Four diboron-bridged ladder molecules 1-4 have been designed and synthesized. X-ray diffraction analysis revealed that the bulky phenyl substituents on boron centers efficiently prevented π stacking of the luminescent ladder unit. Characterizations of these complexes demonstrated that the construction of diboron-containing ladder-type skeletons endowed these materials with good thermal stability, high fluorescence quantum yields, and strong electron affinity. The highly efficient nondoped organic light-emitting diodes using complexes 1 and 2 as electron-transporting emitters exhibited maximum luminance values of 16,930 and 18,060 cd/m(2) with turn-on voltages of 3.5 and 2.5 V as well as maximum luminous efficiencies of 6.4 and 5.4 cd/A, respectively.     

Bright blue phosphorescence from cationic bis-cyclometalated iridium(III) isocyanide complexes

We report new bis-cyclometalated cationic iridium(III) complexes [(C(^)N)(2)Ir(CN-tert-Bu)(2)](CF(3)SO(3)) that have tert-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4'-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-tert-butyl, or5-trifluoromethyl) as C(^)N ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58-3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440-459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46-82 μs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky tert-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid.     

Theoretical study of catalytic dinitrogen reduction under mild conditions

Density functional theory results on key steps of Schrock's catalytic cycle are presented. The quantum chemical modeling of the dinitrogen-reducing reaction steps is conducted without simplifying the bulky HIPT [HIPT = 3,5-(2,4,6-iPr(3)C(6)H(2))(2)C(6)H(3)] substituents at the triamidoamine ligand.     

Substituent effects of iridium complexes for highly efficient red OLEDs

This study reports substituent effects of iridium complexes with 1-phenylisoquinoline ligands. The emission spectra and phosphorescence quantum yields of the complexes differ from that of tris(1-phenylisoquinolinato-C2,N)iridium(iii)(Irpiq) depending on the substituents. The maximum emission peak, quantum yield and lifetime of those complexes ranged from 598-635 nm, 0.17-0.32 and 1.07-2.34 micros, respectively. This indicates the nature of the substituents has a significant influence on the kinetics of the excited-state decay. The substituents attached to phenyl ring have an influence on a stability of the HOMO. Furthermore, those substituents have effect on the contribution to a mixing between 3pi-pi* and (3)MLCT for the lowest excited states. Some of the complexes display the larger quantum yield than Irpiq, which has the quantum yield of 0.22. The organic light emitting diode (OLED) device based on tris [1-(4-fluoro-5-methylphenyl)isoquinolinato-C2,N]iridium(iii)(Ir4F5Mpiq) yielded high external quantum efficiency of 15.5% and a power efficiency of 12.4 lm W(-1) at a luminance of 218 cd m(-2). An emission color of the device was close to an NTSC specification with CIE chromaticity characteristics of (0.66, 0.34).     

Di(alkoxy)- and di(alkylthio)-substituted perylene-3,4;9,10-tetracarboxy diimides with tunable electrochemical and photophysical properties

Nucleophilic substitution of N,N'-dicyclohexyl-1,7-dibromoperylene-3,4:9,10-tetracarboxydiimide (PTCDI) with an excess of corresponding alkanol in the presence of sodium hydride or anhydrous potassium carbonate at 85-100 degrees C provided both di(alkoxy)- and mono(alkoxy)-substituted PTCDI compounds, namely, N,N'-dicyclohexyl-1,7-di(alkoxy)perylene-3,4:9,10-tetracarboxydiimide (3) and N,N'-dicyclohexyl-1-bromo-7-alkoxyperylene-3,4:9,10-tetracarboxydiimide (2). Starting from mono(alkoxy)-substituted PTCDI, nucleophilic substitution with thiol, thiophenol, or alkylamine led to the formation of unsymmetrical 1,7-di(substituted) PTCDI compounds (7-10). For the purpose of comparative studies, symmetrical di(substituted) N,N'-dicyclohexyl-1,7-di(alkylthio)perylene-3,4:9,10-tetracarboxydiimide (4), N,N'-dicyclohexyl-1,7-di(thiophenyl)perylene-3,4:9,10-tetracarboxydiimide (5), and N,N'-dicyclohexyl-1,7-di(alkylamine)perylene-3,4:9,10-tetracarboxydiimide (6) have also been prepared by a similar nucleophilic substitution. These newly prepared PTCDI compounds have been characterized by a wide range of spectroscopic methods in addition to elemental analysis. Electronic absorption and fluorescence studies revealed that the absorption and emission bands as well as the fluorescence quantum yield can be tuned continuously over a large range by incorporating substituents with different electron-donating abilities.     

Effects of water stress and high temperature on photosynthetic rates of two species of Prosopis

The main aim of this research was to compare the photosynthetic responses of two species of Prosopis, Prosopis chilensis (algarrobo) and Prosopis tamarugo (tamarugo) subjected to heat and water stress, to determine how heat shock or water deficit, either individually or combined, affect the photosynthesis of these two species. The photosynthetic rates expressed as a function of photon flow density (PFD) were determined by the O(2) liberated, in seedlings of tamarugo and algarrobo subjected to two water potentials: -0.3MPa and -2.5MPa and to three temperatures: 25 degrees C, 35 degrees C and 40 degrees C. Light response curves were constructed to obtain light compensation and light saturation points, maximum photosynthetic rates, quantum yields and dark respiration rates. The photochemical efficiency as the F(v)/F(m) ratio and the amount of RUBISCO were also determined under heat shock, water deficit, and under the combined action of both stress. Photosynthetic rates at a light intensity higher than 500mumole photons m(-2)s(-1) were not significantly different (P>0.05) between species when measured at 25 degrees C under the same water potential. The maximum photosynthetic rates decreased with temperature in both species and with water deficit in algarrobo. At 40 degrees C and -2.5MPa, the photosynthetic rate of algarrobo fell to 72% of that of tamarugo. The quantum yield decreased in algarrobo with temperature and water deficit and it was reduced by 50% when the conditions were 40 degrees C and -2.5MPa. Dark respiration increased by 62% respect to the control at 40 degrees C in tamarugo while remained unchanged in algarrobo. The photochemical efficiency decreased with both, high temperature and water deficit, without differences between species. RUBISCO content increased in algarrobo 35 degrees C. Water deficit reduced the amount of RUBISCO in both species. The results of this work support the conclusion that in both Prosopis species, the interaction between high temperature and water deficit affects photosynthesis responses greater than each individual stress, and that the interactive effect is more pronounce in algarrobo than in tamarugo.     

Free-probe fluorescence of light-up probes.

The fluorescence enhancement of light-up probes (thiazole orange (TO) conjugated peptide nucleic acids (PNAs)) upon hybridization to target nucleic acid depends on the probe sequence, mainly due to large variations in free-probe fluorescence. Here we study three probes where the fluorescence in free state varies more than 50-fold. We find that this variation is due to a fraction that has TO intramolecularly "back-bound" to the PNA bases. The intramolecular affinity constant for this unimolecular interaction was determined by temperature titrations using absorption spectroscopy, and the fluorescence quantum yields of the probes in back-bound conformation were calculated. The molar ratio of probes in back-bound conformation was 0.70-0.96 at 30 degrees C and 0.40-0.73 at 60 degrees C, and the fluorescence quantum yield in back-bound conformation varied between 0.0020 and 0.077 at 30 degrees C, and 0.00065-0.029 at 60 degrees C. These data show that the variation in free-probe fluorescence depends mainly on the fluorescence quantum yield of the probe in back-bound conformation and to a much lesser extent on the tendency of the probe to adopt the back-bound conformation. With increasing temperature the free-probe fluorescence decreases owing to both reduced degree of back-binding and a decrease of the fluorescence quantum yield in back-bound conformation.     

Crystallization behaviors of nanosized MgO particles from magnesium alkoxides

The effects of the size of the alkoxy group on the thermal decomposition behavior of magnesium alkoxides (magnesium methoxide and ethoxide) and the crystallization behavior of MgO was investigated using thermogravimetry, Fourier-transformed infrared spectroscopy, X-ray powder diffraction, and transmission electron microscopy. As the size of the alkyl group increased, the decomposition temperature decreased and resultant MgO crystallization of the alkoxide precursor was enhanced. In an inert N(2) atmosphere, the decomposition temperature of magnesium ethoxide was about 260 degrees C, which was lower than that of magnesium methoxide by approximately 70 degrees C. The degree of the crystallization of MgO particles from the ethoxide was also significantly higher than that of the methoxide. This result is explained in terms of the O-R bonding strength of the alkoxide. With use of the Kissinger method, the activation energy for the thermal decomposition of magnesium alkoxide was found to be dependent on the size of the alkyl group. The activation energies were 161+/-23 and 130+/-24 kJ/mol for the magnesium methoxide and the magnesium ethoxide, respectively.     

C-H bond activation by unsymmetrical 2-(N-arylimino)pyrrolide Pt complexes: geometric effects on reactivity

Reactions of chloroplatinum methyl complexes with N-(arylimino)pyrrolide anions afford cis and trans neutral platinum methyl complexes. Isomers with methyl trans to the pyrrolide nitrogen activate benzene C-H bonds at 85 degrees C more than 80 times faster than the corresponding cis isomer. In addition, reactions of platinum dimethyl complexes with N-(arylimino)pyrroles (Ar = 4-substituted phenyl) in C6D6 at ambient temperature give unlabeled methane and cis methyl complex containing heavily deuterated Pt-Me. In contrast, bulky aryl substituents give methane isotopomers and trans-Pt-Ph product. The origins of these observations are discussed.     

Regiospecifity of acetylene photocycloadditions to fluorosubstituted benzenes

Irradiation of a cyclohexane solution of hexafluorobenzene and 1-phenyl-2-alkylsubstituted acetylenes resulted in the formation of bicyclo ( 4.2.0) octatriene derivatives, the quantum yield of (2+2) photoaddition depending on the structure of the acetylene, the con- centration of hexafluorobenzene, and the solvent polarity. Reaction with phenylacetylene resulted in the formation of two isomeric cyclo- octatetraene derivatives. Regiospecific (2+2) photoaddition of 1- phenyl-2-tert-butylacetylene to position C-3 and C-4 in alkoxysubsti- tuted pentafluorobenzene derivatives was observed. (2+2) photoaddition of 1-phenylpropyne-1 to alkoxysubstituted pentafluorobenzenes resulted in 3-phenyl-4-methyl-7-alkoxy-1,2,5,6,8-pentafluorobicyclo (4.2.0) octa- triene derivatives. Replacement of the fluorine atom in hexafluoro- benzene with alkoxy substituents diminished quantum yields of (2+2) photocycloadditions.     

Factors influencing the thermal decomposition of transition metal complexes with 2-OH-aryloximes under nitrogen

Summary The thermal behavior of copper(II), nickel(II) and palladium(II) complexes with two anionic varieties of 2-OH-aryloximes (ox), [M(ox)₂] (2-hydroxypropiophenonoxime and 2-hydroxy-4-methoxy-benzophenonoxime) was studied by using simultaneous TG/DTG-DTA technique under nitrogen in the temperature range 40-700°C. The behavior was compared with that in static air, which had been previously studied. It was found that the metal, the substituents on the ligand and the heating rate influenced their thermal decomposition. The thermal stability of the complexes with the same ligand depended on the metallic cation, following the order Pd(II)>Ni(II)>Cu(II). It also depended on the type of ligand, increasing with bulky substituents on the oximic carbon and the benzene ring. The sample mass almost did not affect their decomposition mode. The residues at 700°C of all complexes consisted of a carbonaceous oxide, determined by energy dispersive spectrometry (EDS) and IR spectroscopy     

Effective synthesis of aryl ethers and coumaranones employing the palladium-catalyzed enyne-diyne

Alkyl-, alkoxy-, and aryloxy-substituted conjugated enynes 1 in the presence of Pd(PPh(3))(4) catalyst smoothly underwent the regiospecific [4 + 2] cycloaddition reaction with conjugated alkyl- and alkoxy-substituted symmetric diynes 2 to give multisubstituted aryl ethers 3 in good to high yields. Benzannulation of enynes 1d-g with unsymmetric diyne 6, possessing alkyl and alkoxy groups at acetylenic terminii, in most cases produced an aromatic product 8 with an alkoxy group of diyne attached to the ethynyl moiety of the aromatic product. Remarkably, alkoxy-substituted diynes 2c and 6 underwent the benzannulation reaction with 1 at an unusually low temperature of 0 degrees C! One-pot consecutive benzannulation of alkyl-substituted enynes 1d,e and alkoxy-substituted enynes 1f,g with alkoxy-substituted diynes 2c and 6 followed by protonolysis with TsOH afforded coumaranones 9a-c and 10 in reasonable to high overall yields.     

Polymeric siloxanes with a substituent and the spirobenzopyran moiety: Effect of polar substituent on the photochromic properties

Oligomeric polysiloxanes having the spirobenzopyran moiety and various substituents were prepared. The polymers were photochromic; the color changed from colorless to red on UV irradiation and the color faded on visible irradiation or on standing in the dark. The thermal decoloration was depressed by the polar substituents. However, the effect of the bulky substituent, which had been observed in the rigid siloxane resins, was not detected. In the inorganic/organic hybrid siloxane gels, the silica/(spirobenzopyran-bonding polysiloxane) system and the (spirobenzopyran-bonding silica)/polysiloxane system, the thermal decoloration was also affected by the polar substituent introduced into the polysiloxane portion. The effect was larger in the first case than in the second. The result can be interpreted by the term of the preferential diffusion of polar spirobenzopyran to the vicinity of the polar substituents during the gelation.     

Stereoselective one-pot three-component coupling approach towards the synthesis of the AC ring system of taxanes

The stereoselective one-pot three-component coupling reaction was accomplished by 1,4-addition of the protected cyanohydrin ether 9f to cyclohexenone 10g and subsequent addition of the resulting enolate to formaldehyde in high yield for the formation of the AC ring system of taxanes. We found that the bulky substituents at the 10-position in the A ring prevent the desired 1,4-addition. Similarly, the bulky trialkylsiloxy groups at the 4-position in the C ring prevent the 1,4-addition and electron-donating alkoxy groups at the same position induce the undesired retro-Michael reaction.     

Catalytic asymmetric synthesis of quaternary carbons bearing two aryl substituents. Enantioselective synthesis of 3-alkyl-3-aryl oxindoles by catalytic asymmetric intramolecular heck reactions

A practical sequence involving three consecutive palladium(0)-catalyzed reactions has been developed for synthesizing 3-alkyl-3-aryloxindoles in high enantiopurity. The Heck cyclization precursors 10 and 11a-k are generated in one step by chemoselective Stille cross-coupling of 2'-triflato-(Z)-2-stannyl-2-butenanilide 9 with aryl or heteroaryl iodides. The pivotal catalytic asymmetric Heck cyclization step of this sequence takes place in high yield and with high enantioselectivity (71-98% ee) with the Pd-BINAP catalyst derived from Pd(OAc)(2) to construct oxindoles containing a diaryl-substituted all-carbon quaternary carbon center. A wide variety of aryl and heteroaryl substituents, including ones of considerable steric bulk, can be introduced at C3 of oxindoles in this way (Table 4). The only limitations encountered to date are aryl substituents containing ortho nitro or basic amine functionalities and the bulky N-alkyl-7-oxindolyl group. Asymmetric Heck cyclization of butenalide 22 having an o-(N-acetyl-N-benzylamino)phenyl substituent at C2 provided a approximately 1:1 mixture of amide atropisomers 23 and 24 in high yield and high enantioselectivity. These atropisomers are formed directly upon Heck cyclization of 22 at 80 degrees C, as they interconvert thermally to only a small extent at this temperature.     

Two types of intramolecular addition of an Al-N multiple-bonded monomer LAlNAr' arising from the reaction of LAl with N3Ar' (L = HC[(CMe)(NAr)]2, Ar' = 2,6-Ar2C6H3, Ar = 2,6-iPr2C6H3)

The reaction of beta-diketiminated aluminum(I) monomer LAl with a large bulky azide N3Ar' (L = HC(CMeNAr)2, Ar' = 2,6-Ar2C6H3, Ar = 2,6-iPr2C6H3) in the temperature range from -78 degrees C to room temperature affords two different isomers 2 and 3, which have been characterized by spectroscopic and X-ray structural analyses, as well as elemental analysis. The variable-temperature 1H NMR kinetic studies of this reaction indicate the existence of the monomer LAlNAr' (1) at low temperature and the thermal stability of the compounds increases in the order of 1 < 2 < 3.     

Gold(I)-catalyzed intermolecular hydroarylation of alkenes with indoles under thermal and microwave-assisted conditions

An efficient method for intermolecular hydroarylation of aryl and aliphatic alkenes with indoles using a combination of [(PR(3))AuCl]/AgOTf as catalyst under thermal and microwave-assisted conditions has been developed. The gold(I)-catalyzed reactions of indoles with aryl alkenes were achieved in toluene at 85 degrees C over a reaction time of 1-3 h with 2 mol% of [(PR(3))AuCl]/AgOTf as catalyst. This method works for a variety of styrenes bearing electron-deficient, electron-rich, and sterically bulky substituents to give the corresponding products in good to high yields (60-95%). Under microwave irradiation, coupling of unactivated aliphatic alkenes with indoles gave the corresponding adducts in up to 90% yield. Selective hydroarylation of terminal C=C bond of conjugated dienes with indoles gave good product yields (62-81%). On the basis of deuterium-labeling experiments, a reaction mechanism involving nucleophilic attack of Au(I)-coordinated alkenes by indoles is proposed.     

Intramolecular Diels-Alder reaction of N-allyl 2-furoyl amides: effect of steric strain and amide rotational isomerism

Intramolecular Diels-Alder reactions of various N-allyl 2-furoyl amides with different substituents on the nitrogen atom were investigated. The reaction of amides having bulky substituents proceeded at a faster rate than the analogs whose substituents were of less bulkiness. From the systematic experimental survey of the substituent effects and the energetic evaluation based on the DFT calculations at the B3LYP/6-31G(d) level, the enhanced reactivity was ascribed to the relief of steric strain upon cyclization rather than the amide rotational isomerism governed by the bulky substituents.