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.     

Detection of formate, rather than carbon monoxide, as the stoichiometric coproduct in conversion of fatty aldehydes to alkanes by a cyanobacterial aldehyde decarbonylase

The second of two reactions in a recently discovered pathway through which saturated fatty acids are converted to alkanes (and unsaturated fatty acids to alkenes) in cyanobacteria entails scission of the C1-C2 bond of a fatty aldehyde intermediate by the enzyme aldehyde decarbonylase (AD), a ferritin-like protein with a dinuclear metal cofactor of unknown composition. We tested for and failed to detect carbon monoxide (CO), the proposed C1-derived coproduct of alkane synthesis, following the in vitro conversion of octadecanal (R-CHO, where R = n-C(17)H(35)) to heptadecane (R-H) by the Nostoc punctiforme AD isolated following its overproduction in Escherichia coli. Instead, we identified formate (HCO(2)(-)) as the stoichiometric coproduct of the reaction. Results of isotope-tracer experiments indicate that the aldehyde hydrogen is retained in the HCO(2)(-) and the hydrogen in the nascent methyl group of the alkane originates, at least in part, from solvent. With these characteristics, the reaction appears to be formally hydrolytic, but the improbability of a hydrolytic mechanism having the primary carbanion as the leaving group, the structural similarity of the ADs to other O(2)-activating nonheme di-iron proteins, and the dependence of in vitro AD activity on the presence of a reducing system implicate some type of redox mechanism. Two possible resolutions to this conundrum are suggested.     

The activity of Escherichia coli cyclopropane fatty acid synthase depends on the presence of bicarbonate

Cyclopropane fatty acid (CFA) synthases catalyze the formation of cyclopropane rings on isolated and unactivated olefinic bonds within various fatty acids; the methylene carbon is derived from the activated methyl group of (S)-adenosylmethionine. The E. coli enzyme is the prototype for this class of enzymes, which include the cyclopropane mycolic acid (CMA) synthases, which are potential targets for the design of antituberculosis agents. Crystal structures of several CMA synthases have recently been solved, and electron density attributed to a bicarbonate ion was found in or near the active site. Because a functional assay for CMA synthases has not been developed, the relevance of the bicarbonate ion has not been established. CFA synthase is 30-35% identical to the CMA synthases that have been analyzed structurally, suggesting that the mechanisms of these enzymes are conserved. In this work, we show that indeed the activity of CFA synthase requires bicarbonate, and that it is inhibited by borate, a planar trigonal molecule that mimics the structure of bicarbonate. We also show that substitutions of the conserved amino acids that act as ligands to the bicarbonate ion based on the structure of CMA synthases result in drastic losses in the activity of the protein.     

Mechanism for CuPt-type ordering in mixed III–V epitaxial layers

A model is proposed to rationalize the occurence of CuPt-type ordering in mixed III–V epitaxial layers grown on (001) substrates. It is invoked that 2× surface reconstruction occuring on group V terminated (001) surfaces produces dilated and compressed regions in sub-surface layers. The presence of these regions biases the occupation of certain sites during the growth of a layer consisting of atoms differing in their tetrahedral radii. By assuming that the reconstruction always occurs at the growing surface, it is possible to explain the evolution of CuPt-type ordering in epitaxial layers. Also, based on the model, experimental observations pertaining to the influence of growth conditions, surface misorientation and steps on ordering can be rationalized.     

Nature,origin and effect of dislocations in epitaxial semiconductor layers

The paper is concerned with the manner in which dislocations in Group 3–5 compound epitaxial layer structures are generated, propagate and interact with one another. The different types of epitaxial layer and dislocation behaviour are initially reviewed. The examination methods used are based on electron microscopy. The TEM is used in conjunction with plan-view, cross-section, and angle-lap specimens to determine the detailed nature and three-dimensional distribution of the dislocations. The SEM EBIC and CL methods are used with bulk specimens to obtain electrical and luminescent information. In particular, the latter methods give micrograph-type images showing dark spots and lines corresponding to individual dislocations, the contrast arising because of electrical carrier recombination taking place at the dislocations. These methods are used to investigate the dislocation behaviours occurring in a wide range of specimens including homo- and hetero-epitaxial layers, and embracing small, medium and large mismatches. An attempt is made to obtain quantitative data concerning the processes occurring. The work has provided a better understanding in many instances, and sometimes allowed the occurrence of the dislocations to be better controlled. An example is given of the application of the results to the improvement in the quality of a GaAs transmission photocathode device-type structure.     

Photoelastic Investigation of Slippage in Shrink-Fit Assemblies

The development of torque-induced shear stresses in the presence of slippage, and the residual stresses remaining after torque-induced slippage, are analysed using frozen-stress photoelasticity. Shaft/ring specimens were manufactured from epoxy photoelastic material and were assembled by shrink-fitting prior to being loaded under various regimes, notably the application and release of a torque load. The interface pressure was predicted from Lamé thick cylinder theory, and was also estimated by fitting the Lamé model to the measured stress distributions. The distributions of interface shear stress were calculated from averaged photoelastic data, and compared with the results of a dislocation-based model and with a nonlinear finite element model. For a torque loaded specimen there was good agreement between experimental, theoretical and FE data. Another specimen was loaded in torque then unloaded, with results showing the expected features of slippage and residual stress.