A Palladium(II)‐Catalyzed C?H Activation Cascade Sequence for Polyheterocycle Formation

Polyheterocycles are found in many natural products and are useful moieties in functional materials and drug design. As part of a program towards the synthesis of Stemona alkaloids, a novel palladium(II)‐catalyzed C H activation strategy for the construction of such systems has been developed. Starting from simple 1,3‐dienyl‐substituted heterocycles, a large range of polycyclic systems containing pyrrole, indole, furan and thiophene moieties can be synthesized in a single step.     

Experimental visualization of bubble formation from an orifice in microgravity in the presence of electric fields

 The formation of air bubbles injected into a stagnant, isothermal liquid in microgravity through an orifice was studied. The bubbles grew very large in microgravity. They attained a nearly spherical shape and showed pronounced affinity towards coalescence in the absence of electric fields and other perturbations. Under the influence of electric fields, periodic detachment was observed, with bubble sizes larger than in terrestrial conditions. The bubble shape was elongated. After detachment, the bubbles moved away from the electrode at which they formed without coalescing with other bubbles. Experimental data on bubble shape and size at detachment showed good agreement with models. Received: 26 March 2001 / Accepted: 16 August 2001     

Unusually Facile Thermal Homodienyl‐[1,5]‐Hydrogen Shift Reactions in Photochemically Generated Vinyl Aziridines

A range of photochemically generated tri‐ and tetracyclic vinyl aziridines have been found to undergo a general and surprisingly low temperature ring opening through a [1,5]‐hydrogen shift reaction. The rate of the process was found to be highly dependent on the structure and substitution around the azirdine ring and the alkene terminus, with some substrates being observed to undergo ring opening at temperatures as low as 25 °C. The rigid nature of these polycyclic systems precludes a conformational explanation of these rate differences, and an Eyring study confirmed a negligible entropic barrier to the reaction. However, the Eyring plots for two different aziridines systems showed a significant difference in their enthalpies of activation. It is therefore believed that the levels of aziridine ring strain, as well as electronic effects, are the dominant factors in this sequence.     

Batch versus Flow Photochemistry: A Revealing Comparison of Yield and Productivity

The use of flow photochemistry and its apparent superiority over batch has been reported by a number of groups in recent years. To rigorously determine whether flow does indeed have an advantage over batch, a broad range of synthetic photochemical transformations were optimized in both reactor modes and their yields and productivities compared. Surprisingly, yields were essentially identical in all comparative cases. Even more revealing was the observation that the productivity of flow reactors varied very little to that of their batch counterparts when the key reaction parameters were matched. Those with a single layer of fluorinated ethylene propylene (FEP) had an average productivity 20 % lower than that of batch, whereas three‐layer reactors were 20 % more productive. Finally, the utility of flow chemistry was demonstrated in the scale‐up of the ring‐opening reaction of a potentially explosive [1.1.1] propellane with butane‐2,3‐dione.     

Experimental and theoretical studies of the basicity and proton affinity of SiF4 and the structure of SiF4H+

A combined experimental and theoretical approach has been employed to establish the basicity and proton affinity of SiF₄ and the structure of SiF₄H⁺. The kinetics and energetics for the transfer of a proton between SiF₄, N₂, and Xe have been explored experimentally in helium at 0.35±0.02 torr and 297±3 K with a selected-ion flow tube apparatus. The results of equilibrium constant measurements are reported that provide a basicity and proton affinity for SiF₄ at 297±3 K of 111.4±1.0 and 117.7±1.2 kcal mol^?1, respectively. These values are more than 2.5 kcal mol^?1 lower than currently recommended values. The basicity order was determined to be GB(Xe)>GB(SiF₄)>GB(N₂), while the proton-affinity order was shown to be PA(Xe)>PA(N₂)>PA (SiF₄). Ab initio molecular orbital computations at MP4SDTQ(fc)/6-311++G(3df,3pd) using geometries from B3LYP/6-31+G(d,p) indicate a value for PA(SiF₄)=118.7 kcal mol^?1 that is in good agreement with experiment. Also, the most stable structure of SiF₄H⁺ is shown to correspond to a core SiF ₃ ⁺ cation solvated by HF with a binding energy of 43. 9 kcal mol^?1. Support for this structure is found in separate SIFT collision induced dissociation (CID) measurements that indicate exclusive loss of HF.     

Thermal and optical studies on hexa-2,4-diyne-1,6-diyl-bis(4-n-hexoxybenzoate)

As part of a programme investigating mesogenic diacetylenes a symmetrically disubstituted diacetylene has been synthesized and polymerized. Thermal, thermo-optical and IR studies have been carried out to investigate the different crystalline forms of the diacetylene.     

The Synthesis, Polymerization and Second Order Non-Linear Optical Properties of Novel, Conjugated, Unsymmetrical Diacetylenes Showing Liquid Crystal Behaviour

A number of novel, conjugated, unsymmetrically disubstituted diphenyl-diacetylenes have been synthesized and polymerized in the liquid-crystalline phase to yield liquid-crystalline polymers. The efficiency of second harmonic generation is reported for both monomers and polymers.