Triplet halocarbene chemistry: p-nitrophenylchlorocarbene and p-nitrophenylbromocarbene

Reactions of p-nitrophenylchlorocarbene with cumene and of p-nitrophenylbromocarbene with toluene afford C-H abstraction-recombination products that suggest the involvement of triplet arylhalocarbenes.     

Suzuki-Miyaura monocouplings of p-dibromobiphenyl and substituted p-dibromo(penta-p-phenylenes)

The mono/bis ratio for the Suzuki-Miyaura cross coupling of p-dibromobiphenyl and p-dibromo(penta-p-phenylenes) with arylboronic acids and esters has been studied. The coupling reaction is demonstrated to be highly selective for monoarylation when the substrate is a p-dibromooligoarene, while selective biarylation is obtained for p-diiodoterphenyl. The mono/bis coupling-ratio for these compounds was highly sensitive to the nature of the halogen involved, however steric hindrance or electronic characteristics of the boronic derivative did not affect the selectivity of the reaction. The reaction yields observed were higher at room temperature and when arylboronic pinacol esters were used. These reactions also offer a useful method for the preparation of asymmetrically substituted terphenyls and hexa-p-phenylenes, giving good yields.     

Metalations utilizing aryllithiums; ortho-functionalization of p-bromoanisole (pBrA)

An ortho-metalation protocol has been developed, which permits the survival of a bromine substituent in p-bromoanisole. Eight derivatives of the generated ortho-lithiated intermediate have been prepared. A neglected metalation concept is being explored here; one which proposes that minimizing the pK_a difference between the aryl substrate and the conjugate acid of the metalating agent will lead to a regiospecific and selective metalation process.     

Autoxidation of p-hydroxydiphenylamine

Autoxidation of p-hydroxydiphenylamine (HDPA) gives N-phenyl-p-benzoquinoneimine and hydrogen peroxide as primary products over a wide range of pH and the previously reported formation of N-(4-hydroxyphenyl)-p-benzoquinoneimine in basic media is in error. An induction period which is independent of substrate concentration but decreases with increase in pH is observed. The rate of oxidation is first order with respect to oxygen and half order with respect to HDPA. Oxygen is used in the system at the rate comparable to its rate of dissolution. On the basis of various kinetic parameters, a mechanism involving electron transfer to oxygen from HDPA anion and subsequent formation of HO₂ radical is postulated. The agreement of the experimental results with the derived rate expression is shown to support the mechanism.Singlet oxygen, produced in situ by methylene blue sensitization, leads to oxidation but with different products indicating that this species is not involved in the autoxidation.     

pp Homoconjugation in organic radicals

ESR data for various radicals of type R₂?CR₂X, when R is H or alkyl, and X is either Cl, Br or I, or PR₂ and AsR₂, are discussed in terms of pp homoconjugation. For the β-halogen radicals, the small size of the anisotropic coupling provides some evidence for this effect, but for the β-PR₂ and β-AsR₂ radicals, the greatly enhanced isotropic coupling to ³¹P or ⁷⁵As compared with those for the corresponding cation-radicals, R₂?CR₂PR⁺₃ or R₂?CR₂AsR⁺₃, is taken as good evidence for homoconjugation.     

Novelties of solid-liquid phase transfer catalysed synthesis of p-toluenesulfonyl fluoride from p-toluenesulfonyl chloride

Organic sulfonyl fluorides are of interest owing to their insecticidal, germicidal and enzyme inhibitory properties. In the current work synthesis of p-toluenesufonyl fluoride was accomplished by reacting p-toluenesulfonyl chloride with solid potassium fluoride using PEG-400 as a catalyst under solid-liquid phase transfer catalysis (S-L PTC) at 30 °C. p-Toulenesulfonyl fluoride is used as peroxygen bleach activator. It also finds use in the treatment of Alzheimer's disease. The mechanism is based on homogeneous solubilization of solid. PEG forms a complex with metal cation which associates with the nucleophile and it participates in S_N2 type reaction. The reaction is intrinsically kinetically controlled. A complete theoretical analysis is done to determine both the rate constant and equilibrium constant from the same set of data. The activation energy and Gibbs free energy are also calculated.     

Uptake of weathered p,p′-DDE by plant species effective at accumulating soil elements

Ten plant species previously shown to accumulate inorganic elements effectively from natural solids were grown under field conditions in p,p′-dichlorodiphenyldichloroethane (p,p′-DDE) contaminated soil. The plant species, which included rye, mustard, canola, vetch, pigeonpea, clover, peanut, and 3 cultivars of white lupin, represented both monocots and dicots, as well as two major families within the dicots: the Brassicaceae and the Fabaceae. The plants varied widely in their ability to phytoextract and translocate weathered p,p′-DDE. The percentage of contaminant phytoextracted ranged from 0.06% (white lupin) to 0.22% (clover, vetch), and the translocation factors (TF; contaminant concentration ratio of stems to roots) ranged from 0.04 (clover, white lupin) to 0.37 (canola). An inverse relationship exists between the amount of contaminant in the roots as measured by the root BCF (bioconcentration factor; dry weight contaminant concentration ratio of root to soil) and the TF. Duplicate mounds of each species were periodically amended with nitrogen (N), phosphorus (P), nitrogen and phosphorus together (N/P); a minus phosphorus treatment involved the addition of AlSO₄ to the soil prior to planting. The effect of nutrient regime on plant biomass, p,p′-DDE uptake and translocation, and inorganic element content varied greatly among the 10 plant species. For some species (rye, vetch, pigeonpea, clover, white lupin), reductions or non-significant changes in p,p′-DDE uptake were observed under the nutrient treatments and were not correlated with plant biomass effects. For mustard, canola, and peanut, the percentage of p,p′-DDE phytoextracted in the various treatments was more than doubled and was directly correlated with a two-fold increase in total plant biomass. Although it is generally assumed that fertilizer amendments will enhance the phytoremediation of organic and inorganic pollutants, the data here suggest that such effects are highly species specific and in some cases may actually decrease remediation potential.     

Herstellung und polymerisation isomerer p-methylnaphthylstyrole

Two different pathways were used to synthesize p-(α-and p-(β-methylnaphthyl)styrene; polymerizations by radical as well as anionic initiation were investigated. With sodium metal in THF solution, the polymers yield the stoichiometric charge transfer complexes. These reactions proceed with crosslinking of the polymers even under extremely dry conditions.     

Novelties of solid-liquid phase transfer catalysed synthesis of p-toluenesulfonyl fluoride from p-toluenesulfonyl chloride

Organic sulfonyl fluorides are of interest owing to their insecticidal, germicidal and enzyme inhibitory properties. In the current work, synthesis of p-toluenesufonyl fluoride was accomplished by reacting p-toluenesulfonyl chloride with solid potassium fluoride using PEG-400 as a catalyst under solid-liquid phase transfer catalysis (S-L PTC) at 30 °C. p-Toluenesulfonyl fluoride is used as peroxygen bleach activator. It also finds use in the treatment of Alzheimer's disease. The mechanism is based on homogeneous solubilization of solid. PEG forms a complex with metal cation which associates with the nucleophile and it participates in SN₂ type reactions. The reaction is intrinsically kinetically controlled. A complete theoretical analysis is done to determine both the rate constant and equilibrium constant from the same set of data. The activation energy and Gibbs free energy are also calculated.     

13C shielding tensors in p-alkoxybenzenes

All ¹³C shielding tensors in p-dimethoxybenzene and p-diethoxybenzene were determined from powder patterns by means of proton enhanced nuclear induction spectroscopy. In contrast to alkyl-substituted benzenes the tensor sets differ considerably at the various aromatic sites and all tensor elements at the two nonequivalent ortho positions are influenced by the asymmetric substituent. In the solid state p-diethoxybenzene was found to have a planar or nearly planar conformation, like p-dimethoxybenzene.     

Solid state reaction between p-benzoquinone and dihydroxybenzene

Reaction kinetics of p-benzoquinone-p-dihydroxybenzene; p-benzoquinone-m-dihydroxybenzene and p-benzoquinone-o-dihydroxybenzene systems have been studied in the solid state using capillary, gravimetric, and dilatometric techniques. It is found that p-benzoquinone is the diffusing species. The diffusion occurs through surface migration and depends very much on the symmetry of the dihydroxybenzene molecules. Gravimetric studies indicate that cracks are formed in the product during reaction and that the reaction reaches completion. Dilatometric studies indicate that the reaction products are more compact and contraction in volume takes place during the course of reaction.     

Photolysis of copolymers of p-methoxyacrylophenone with p-biphenyl methacrylate and 2-naphthyl methacrylate

Copolymers of p-methoxyacrylophenone (PMeOAP) with 2-naphthyl methacrylate (2NMA) and p-biphenyl methacrylate (BMA) containing 0–4·2 wt % of 2NMA and 0–4·9 wt % of BMA were prepared. The photolysis of these copolymers under 366 nm irradiation in dilute ethyl benzoate solution at ambient temperature was studied viscometrically. Increasing content of triplet energy acceptors (2NMA and BMA) in copolymers caused decrease of quantum yields for main chain scissions. The same efficiency was observed for bound 2NMA and BMA, though naphthalene was about twice more efficient than biphenyl in intermolecular triplet energy transfer. The 25-times longer life-time of the excited triplet state of poly-p-methoxyacrylophenone (PPMeOAP) compared to polyacrylophenone does not lead to increase of intramolecular triplet energy transfer. According to Perrin's model of static quenching. the calculated radius of the quenching sphere in both copolymers of PMeOAP was about 16 A and in copolymers 1- and 2-vinylnaphthalene-acrylophenone about 13 A. The value obtained for low-molecular model compounds was about 10 A. Results indicate that there is probably no isothermic triplet energy transfer between identical p-methoxyacrylophenone chromophores.     

Efficient synthesis of p-bis-(chlorodifluoromethyl)benzene

Selective, high yield partial fluorination of p-bis-(trichloromethyl)benzene to p-bis-(chlorodifluoromethyl)benzene has been accomplished by warming a slurry of the p-bis-(trichloromethyl)benzene in anhydrous HF which also contains a small quantity of inert solvent, such as 1,2-dichloroethane.     

Unprecedented N(H)-bridged tetraaza[1.1.1.1]m,p,m,p-cyclophanes

The first uncatalyzed preparation of tetraaza[1.1.1.1]m,p,m,p-cyclophanes (symmetrical and unsymmetrical) gave access to previously unknown N(H)-bridged derivatives that could be further substituted. NMR studies and theoretical calculations show that these macrocycles adopt an 1,3-alternated conformation.     

Photolysis of poly(p-isopropyl styrene)

The photodegradation of films of poly(p-isopropyl styrene) with 254 nm radiation at 10^?6mbar and 25±1° has been studied. The principal product is hydrogen but smaller quantities of methane and ethane and a trace of propane are also formed, indicating that the p-substituent also undergoes decomposition. Ultraviolet spectra show rapid changes, particularly in the 230–240 nm region, and a long wave absorption system extending into the visible region also appears. Solubility data indicate the simultaneous occurrence of cross-linking and chain scission, rates of both processes being significantly greater than those for poly(styrene); these results are explicable in terms of the inductive (+I) stabilization (due to the p-C₃H₇ group) of radicals formed on chain scission and of the participation of the p-C₃H₇ group in addition reactions. Quantum yields for the evolution of the gaseous products have been estimated.     

Synthesis and properties of p-benzoquinone-fused hexadehydro[18]annulenes

A series of hexadehydro[18]annulenes fused with different numbers of p-benzoquinone, 4-6, were synthesized by stepwise transformation of the p-dimethoxybenzene moiety of the precursor dehydroannulene 3 fused with three 3,6-dimethoxy-4,5-dimethylbenzene units at 1,2-positions into p-benzoquinone using ceric ammonium nitrate. The UV-vis spectra of compounds 4 and 5, which have both electron-donating p-dimethoxybenzene unit(s) and electron-accepting p-benzoquinone unit(s) in the π-systems, showed the maximum absorption bands bathochromically shifted in comparison with 3 having only p-dimethoxybenzene units and 6 having only p-benzoquinone units. However, the solvatochromism expected for 4 and 5 was found to be quite weak possibly because the HOMO and LUMO (B3LYP/6-31G(d)) are not localized but rather delocalized over the whole π-systems.     

Synthesis of cyclic compounds consisting of face-to-face p-oligophenyls

Facile synthesis of xanthene-based π-stacked compounds consisting of face-to-face p-oligophenyls has been described. Just by mixing xanthene-4,5-diboronic acid and p-oligophenyls containing a benzenetetraol unit yielded cyclic compounds selectively due to the reversibility of the boronate esterification. In the ground state, weak π–π interactions between two p-oligophenyl moieties were observed, whereas their π–π interactions were clearly shown in the excited state.     

Semipermanent p-nitrobenzyloxycarbonyl (pNZ) protection of Orn and Lys side chains: prevention of undesired α-Fmoc removal and application to the synthesis of cyclic peptides

Semipermanent side-chain protection of Orn and Lys with p-nitrobenzyloxycarbonyl (pNZ) for Fmoc/^tBu chemistry does not result in the unwanted removal of α-Fmoc that occurs when groups such as Alloc are used for the same application. Furthermore, pNZ can be used in conjuction with p-nitrobenzyl ester (pNB) to prepare cyclic peptides.     

Synthesis of functionalized p-dihydrobenzoquinones and p-benzoquinones based on [3+3] cyclizations of 1,3-bis-silyl enol ethers

Functionalized mono-protected p-dihydrobenzoquinones were prepared by [3+3] cyclization of 1,3-bis-silyl enol ethers with 2-acyloxy-3-(silyloxy)alk-2-en-1-ones. Deprotection and oxidation of the products afforded the corresponding p-benzoquinones.     

Synthesis of oligo(p-phenylene vinylene)-porphyrin-oligo(p-phenylene vinylene) triads as antenna molecules for energy transfer

The oligo(p-phenylene vinylene)-porphyrin-oligo(p-phenylene vinylene) (P-OPVn, n=2, 4, where n is the number of phenyl rings) and the complex with Zn²⁺ based on P-OPVn were synthesized for investigating their photophysical properties via UV-vis, voltammetry, steady-state and time-resolved fluorescence spectra. In these molecules two OPV moieties as energy donors were linked to porphyrin center by virtue of Wittig reaction. The detailed studies of photophysical properties indicate that OPV group can act as an antenna unit for effective intramolecular energy transfer.