Quantum yields of photosubstitution of the nitro group in nitrobenzene-tetrachloromethane mixtures

The quantum yield of nitrobenzene disappearance has been determined. The mechanism of the photosubstitution of nitro group by the photolysis of nitrobenzene-tetrachloromethane mixture is discused.     

Photochemical reactions of chromium hexacarbonyl with tetraalkyldiphosphine disulfides

New complexes [Cr(CO)₄(R₂P(S)P(S)R₂)] and [Cr₂(CO)₁₀(-R₂P(S)P(S)R₂)] (R = Me, Et, Pr ⁿ , Bu ⁿ ), (1a)–(1d) and (2a)–(2d) [(1a), R = Me; (1b), R = Et; (1c), R = Pr ⁿ ; (1d), R = Bu ⁿ ; (2a), R = Me; (2b), R = Et; (2c), R = Pr ⁿ ; (2d), R = Bu ⁿ ] have been prepared by the photochemical reaction of Cr(CO)₆ with R₂P(S)P(S)R₂ (R = Me, Et, Pr ⁿ and Bu ⁿ ) and characterized by elemental analyses, FT-i.r., ³¹P-[¹H]-n.m.r. spectroscopy and FAB-mass spectrometry. The spectroscopic data suggest cis-chelate bidentate coordination of the ligand in [Cr(CO)₄(R₂P(S)P(S)R₂)] and cis-bridging bidentate coordination of the ligand between two metals in [Cr₂(CO)₁₀(-R₂P(S)P(S)R₂)] (R = Me, Et, Pr ⁿ and Bu ⁿ ).     

Recovery of chromium hexacarbonyl after nucleophilic reactions of arenechromium tricarbonyl complexes

The arenechromium tricarbonyl complexes C₆H₄R₂Cr(CO)₃, (R = H, CHMe₂), obtained from arenes C₆H₄R₂ and Cr(CO)₆, react with nucleophiles (Nu = CH(CH₃)CN, CH(CH₃)CO₂-t-Bu), to form adducts which give, after treatment with CF₃CO₂H under carbon monoxide, the substituted cyclohexadienes C₆H₅R₂Nu (R = H, CHMe₂) and Cr(CO)₆ in good yield.     

An application of borane as a protecting group for pyridine

Efforts to couple 4 with 12 employing base mediation are problematic due to the formation of 6. To circumvent this issue, 12 was converted to the pyridine borane complex (13). Alkylation of 4 with 13 provided 3 after removal of the borane under acidic conditions and saponification of the ester.     

New nitronate sigma complexes and the mechanism of nucleophilic aromatic photosubstitution para to a nitro group

[reaction: see text] Photolysis of 4-nitroanisole with aliphatic amines gives mainly N-substituted 4-nitroanilines. Reactions of this type have been widely attributed to a geminate radical mechanism. We questioned this interpretation and have searched for and found by NMR spectroscopy a new class of stable nitronate adducts generated under the reaction conditions. The adducts imply that photosubstitution by amines para to the nitro group occurs by meta sigma complex formation followed by an unprecedented sigmatropic rearrangement.     

Hydrosilylation of alkenes catalysed by rhodium complexes immobilized on silica via a pyridine group

2-(2-Trimethoxysilylethyl)pyridine, together with 3-methcryloxypropyltrimethoxysilane, was used to prepare a series of rhodium carbonyl complexes bound to silica via a pyridine group. The rhodium complex Rh₂(CO)₄Cl₂ (Rh₂) was used as the starting compound, and the immobilized complexes were prepared by four different routes which yielded both surface-bonded complexes and complexes bonded within the silicate matrix. These complexes were efficient catalysts of hydrosilylation of octene by triethxysilane. All the immobilized complexes were more than their homogeneous analogues and some could be re-used.     

Organometallic polymers. XVI. Copolymerization of styrenetricarbonylchromium and the reaction of polystyrene with chromium hexacarbonyl

Styrenetricarbonylchromium (IV) has been synthesized. Monomer IV did not homopolymerize with free-radical initiation but copolymerized with styrene, methyl acrylate, and vinylcymantrene. The copolymerizations were carried out in benzene solutions at 70°C with azobisisobutyronitrile as the initiator. The relative reactivity ratios were determined for the styrene and methyl acrylate copolymerizations. They were (defining M₁ as monomer IV) r₁ ? 0, r₂ ? 1.39 for styrene copolymerizations and r₁ ? 0, r₂ ? 0.75 for methyl acrylate copolymerization. Polystyrene reacted with chromium-hexacarbonyl in refluxing DME to produce a polymer in which about 32% of the benzene rings were complexed with ? Cr(CO)₃ units. The use of a polystyrene of narrow molecular weight distribution in this reaction demonstrated that no decomposition of the polystyrene chains occurred.     

Photochromism of new unsymmetrical isomeric diarylethenes bearing a pyridine group

A new class of unsymmetrical isomeric diarylethenes bearing a terminal pyridine group was synthesized and their structures were determined by single-crystal X-ray diffraction analysis. The effects of the nitrogen atom position in the pyridine group on their properties including photochromism, acidchromism, and fluorescence were investigated systematically. Among the three isomeric derivatives, the cyclization quantum yield and the absorption maxima of the ortho-substituted diarylethene were the biggest, while the fluorescence quantum yield of the para-substituted diarylethene was the biggest. Furthermore, the three isomeric diarylethenes exhibited multi-addressable switching behaviors by the stimulation of acid/base and light. Addition of trifluoroacetic acid to solutions of these diarylethenes produced the protonated derivatives, which also showed excellent photochromic behaviors, but only the para-substituted diarylethene exhibited a notable color change of solution from crimson to violet. The results revealed that the effects of the nitrogen atom position in the terminal pyridine group played a very important role during the process of photoisomerization for these isomeric diarylethene derivatives.     

A novel photosubstitution of dicyanobenzenes by allylic and benzylic silanes

The photosubstitution of o- and p-dicyanobenzenes by allylic and benzylic silanes in acetonitrile occurred efficiently, giving the corresponding mono-allylated and benzylated cyanobenzenes.     

Liquid crystal properties of novel diacetylenes having a pyridine end group

Asymmetrical diacetylenes with a pyridine ring bonded directly to one end of the diacetylene unit and an aryl system containing a flexible alkoxy chain at the opposite end, have been synthesized and show liquid crystalline behaviour. The mesophase behaviour is shown to be dependent on the length of the flexible alkoxy chain. Incorporation of one of these diacetylenes in a polymer 'guest-host' system at a 15 wt% concentration gave a high X⁽²⁾ non-linear optical susceptibility of 4.27 × 10^-9e.s.u.     

Quantum yield of photoenhanced currents in crystalline tetracene

A study of photoenhanced currents (PEC) in crystalline tetracene has been made. Using the well-established theory of space-charge controlled currents in insulators, it is shown that the quantum yield for the release of trapped carriers by triplet excitons is less than unity.     

Nucleophilic photosubstitution of halogen in the heterocyclic series

A new method is described for the introduction of a cyano group into pyridine and pyrimidine rings and for the introduction of nitro and thiocyano groups into the pyridine ring by UV irradiation of halo-substituted pyridines and pyrimidines in the presence of the appropriate nucleophiles.     

Expansion of a pyridine ring by a nitrene

Pyrolysis of 2-(o-azidobenzyl)pyridine (3) gives 3H-1,2-diazepino[1,7-a]indole (5), in which the intermediate nitrene has expanded the pyridine ring.