Photochemistry of dicyanopyridines

The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient (τ ≈ 10–12 μs, λ_max ≈ 360–380 nm) was observed, suggesting the formation of a protonated excited state.

Irradiation in the presence of amines resulted in the production of the pyridyl radical anion (τ ≈ 40–80 μs, air sensitive, λ_max ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10⁻⁸ M⁻¹ s⁻¹.

In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical (τ ≈ 30–200 μs, λ_max ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.     

Synthesis of Enantiomerically Pure Ferrocenes from Glycofuranosyl-cyclopentadienes,synthetic equivalents of (alkoxyalkyl)fulvenes

Cyclopentadienyl C-glycosides (= glycosyl-cyclopentadienes) have been prepared as latent fulvenes. Their reaction with nucleophiles leads to cyclopentadienes substituted with (protected) alditol moieties and, hence, to enantiomerically pure metallocenes. Treatment of 1 with cyclopentadienyl anion gave the epimeric glycosyl-cyclopentadienes 6 / 7 (Scheme 1). Each epimer consisted of a ca. 1:1 mixture of the 1, 3-and 1, 4-cyclopentadienes a and b , respectively, which were separated by prep. HPLC. Slow regioisomerisation occurred at room temperature. Diels-Alder addition of N-phenylmaleimide to 6a / b ca. 3:7 at room temperature yielded three ‘endo’-adducts, i.e., a disubstituted alkene ( 8 or 9 , 25%) and the trisubstituted alkenes 10 (45%) and 11 (13%). The structure of 10 was established by X-ray analysis. Reduction of 6 / 7 (after isolation or in situ) with LiAlH₄ gave the cyclopentadienylmannitols 12a / b (80%) which were converted to the silyl ethers 13a / b (Scheme 2). Lithiation of 13a / b and reaction with FeCl₂ or TiCl₄ led to the symmetric ferrocene 14 (76%) and the titanocene 15 (34%), respectively. The mixed ferrocene 16 (63%) was prepared from 13a / b and pentamethylcyclopentadiene. Treatment of 6 / 7 with PhLi at ?78° gave a 5:3 mixture of the 1-C-phenylated alcohols 17a / b and 18a / b (71%) which were silylated to 19a / b and 20a / b , respectively. Lithiation of 19 / 20 and reaction with FeCl₂ afforded the symmetric ferrocenes 21 and 22 and the mixed ferrocene 23 (54:15:31, 79%) which were partially separated by MPLC. The configuration at C(1) of 17–22 was assigned on the basis of a conformational analysis. The reaction of the ribofuranose 24 with cyclopentadienylsodium led to the epimeric C-glycosides 27a / b and 28a (57%, ca. 1:1, Scheme 3). The in-situ reduction of 27 / 28 with LiAlH₄ followed by isopropylidenation gave 25a / b (65%) which were transformed into the ferrocene 26 (79%) using the standard method. Phenylation of 27 / 28 , desilylation, and isopropylidenation gave a 20:1 mixture of 33a / b and 34a / b (86%) which was separated by prep. HPLC. The same mixture was obtained upon phenylation of the fulvene 32 which was obtained in 36% yield from the reaction of the aldehydo-ribose 30 with cyclopentadienylsodium at ?100°. Lithiation of 33 / 34 and reaction with FeCl₂ gave the symmetric ferrocene 35 (88%). Similarly, the aldehydo-arabinose 36 was transformed via the fulvene 37 (32%) into a 18:1 mixture of 38a / b and 39a / b (78%) and, hence, into the ferrocene 40 (83%). Conformational analysis allowed to assign the configuration of 33–35 , whereas an X-ray analysis of 40 established the (1S)-configuration of 38a / b and 40 . The opposite configuration at C(1) of 38a / b and 33a / b was established by chemical degradation (Scheme 4). Hydrogenation (→ 41 and 44 , resp.), deprotection (→ 42 and 45 , resp.), NaIO₄ oxidation, and NaBH₄ reduction yielded (+)-(S)- 43 and (?)-(R)- 43 , respectively.     

Tuning the anion binding properties of calixpyrroles by means of p-nitrophenyl substituents at their meso-positions

Extended cavity calix[4]pyrroles and a calix[6]pyrrole were synthesized by cyclization of 5-methyl-5-(4-nitrophenyl)dipyrromethane with acetone in the presence of acid. The solid-state structures of the novel macrocycles were determined by X-ray crystallography. The host-guest chemistry of these receptors towards halide ions was investigated in solution by ¹H NMR titration techniques and compared with those of the meso-octamethylcalix[4]pyrrole and meso-dodecamethylcalix[6]pyrrole. The binding of chloride anions was observed to occur with different affinities on the two faces of the novel calix[6]pyrrole derivative described here.     

Synthesis of 2-oxazolines mediated by N,N′-diisopropylcarbodiimide

N-(β-Hydroxy)amides can be cyclised by reaction with diisopropylcarbodiimide (DIC) to give the corresponding 2-oxazolines in high yields. The reaction requires only very mild Lewis-acid catalysis (5 mol % Cu(OTf)₂) and can be accomplished with simple heating, or in very short reaction times under microwave irradiation.     

A stabilized formulation of IBX (SIBX) for safe oxidation reactions including a new oxidative demethylation of phenolic methyl aryl ethers

[reaction: see text] SIBX is a nonexplosive formulation of IBX that can be used as a suspension in a variety of standard organic solvents such as refluxing EtOAc and THF to oxidize safely alcohols into aldehydes and ketones. The use of hot THF is limited to the oxidation of allylic and benzylic alcohols. Most yields are comparable to those obtained with IBX or DMP. SIBX can also be used to perform oxygenative demethylation of 2-methoxyarenols into orthoquinones and catechols.     

Effect of gravitational radiation upon electromagnetic waves in a dispersive medium

We show that linearized gravitational radiation produces fluctuations in intensity and position of a distant source if the ray travels in a dispersive medium. The effect, however, depends upon the nongeodesic character of the ray and does not occur in an electrostatic plasma. When the index of refraction n is greater than unity a Cerenkov type resonance produces scintillation proportional to D _O ^3/2 (D _O being the distance of the source) and a dancing proportional to D _O ^1/2 if, instead, n<1 the scintillation behaves like D _O and the dancing does not diverge as D _O . The calculation is performed in detail for a random and isotropic spectrum of gravitational waves W(). This effect allows one to set an upper limit to W() at the frequency at which the fluctuations are observed, but for the rarified interstellar and intergalactic plasmas these limits are not very interesting.     

A fast-neutron coincidence collar using liquid scintillators for fresh fuel verification

Journal of Radioanalytical and Nuclear Chemistry - The IAEA has developed a liquid scintillator-based system for the non-destructive assay of 235U in fresh fuel assemblies. The fast neutron...     

Scalar conservation laws with fractional stochastic forcing: Existence,uniqueness and invariant measure

We study a fractional stochastic perturbation of a first-order hyperbolic equation of nonlinear type. The existence and uniqueness of the solution are investigated via a Lax–Ole?nik formula. To construct the invariant measure we use two main ingredients. The first one is the notion of a generalized characteristic in the sense of Dafermos. The second one is the fact that the oscillations of the fractional Brownian motion are arbitrarily small for an infinite number of intervals of arbitrary length.