We report an approach, named chemTEM, to follow chemical transformations at the single-molecule level with the electron beam of a transmission electron microscope (TEM) applied as both a tunable source of energy and a sub-angstrom imaging probe. Deposited on graphene, disk-shaped perchlorocoronene molecules are precluded from intermolecular interactions. This allows monomolecular transformations to be studied at the single-molecule level in real time and reveals chlorine elimination and reactive aryne formation as a key initial stage of multistep reactions initiated by the 80 keV e-beam. Under the same conditions, perchlorocoronene confined within a nanotube cavity, where the molecules are situated in very close proximity to each other, enables imaging of intermolecular reactions, starting with the Diels–Alder cycloaddition of a generated aryne, followed by rearrangement of the angular adduct to a planar polyaromatic structure and the formation of a perchlorinated zigzag nanoribbon of graphene as the final product. ChemTEM enables the entire process of polycondensation, including the formation of metastable intermediates, to be captured in a one-shot “movie”. A molecule with a similar size and shape but with a different chemical composition, octathio[8]circulene, under the same conditions undergoes another type of polycondensation via thiyl biradical generation and subsequent reaction leading to polythiophene nanoribbons with irregular edges incorporating bridging sulfur atoms. Graphene or carbon nanotubes supporting the individual molecules during chemTEM studies ensure that the elastic interactions of the molecules with the e-beam are the dominant forces that initiate and drive the reactions we image. Our ab initio DFT calculations explicitly incorporating the e-beam in the theoretical model correlate with the chemTEM observations and give a mechanism for direct control not only of the type of the reaction but also of the reaction rate. Selection of the appropriate e-beam energy and control of the dose rate in chemTEM enabled imaging of reactions on a time frame commensurate with TEM image capture rates, revealing atomistic mechanisms of previously unknown processes.

Stable negative ions containing up to sixteen silicon atoms have been measured by mass spectromettry in RF power-modulated silane plasmas for amorphous silicon deposition. These hydrogenated silicon cluster ions reach much higher masses than the positive ions, which have no more than six silicon atoms. This supports the view that negative ions are the precursors to particulate formation in silane plasmas. The time-dependent fluxes d positive and negative ions from the plasma are shown with a 5 p s time resolution. Possible cluster reaction sequences are discussed and the effect of visible light on the negative ion signal i s commented upon.

No abstract

The review integrates and analyzes the published data on the chemical reactivity of α-cyanothioacetamide, a convenient starting compound for the preparation of activated alkenes, functionally substituted pyrans, thiopyrans, pyridines, thiophenes, pyrroles, quinolines, isoquinolines, pyrimidines, thienopyrroles, pyrazolopyridines, pyridothienopyrimidines and pyrimidothienodiazines. The relatively small α-cyanothioacetamide molecule has several reaction centres. Nucleophilic reactions of the methylene group are covered most extensively, including the Thorpe reaction; reactions with diazonium salts, azides, iso(thio)cyanates and nitroso compounds; nucleopilic substitution; Knoevenagel condensation; Michael reaction; and multicomponent syntheses. Separate parts of the review are devoted to nucleophilic reactions involving the thiocarbonyl and amino groups and electrophilic reactions involving thiocarbonyl and nitrile groups. The focus is on the publications of the last 15 years. The bibliography includes 350 references.

The addition of diethyl phosphite to cyclic imines bearing alkyl, aryl, or heteroaryl substituents at the a-position in diethyl ether at room temperature presents an efficient route to substituted cyclic a-aminophosphonates. The application of boron trifluoridediethyl ether complex as a catalyst significantly accelerates the reaction.a-Aminophosphonates have attracted constant attention over several decades because of the high and diverse biological activity determining their practical applications, and there have been ongoing searches for and investigations of novel structures of this type. 1 Being analogous to amino acids, they have been found to be useful for a number of applications, e.g. as antibiotics, 2 enzyme inhibitors, 3 anticancer agents, 4 and herbicides. 5 These aaminophosphonate compounds are also of undoubted interest as ligands in homogeneous 6 or organic 7 catalysis.Cyclic or heterocyclic rings introduced into the molecular skeleton increase the rigidity and modify the electronic effects of these compounds. Thus, many cyclic a-aminophosphonic acids bearing an exocyclic amino group have been prepared, mostly in racemic series. 8 However, among the a-aminophosphonates synthesized, those containing nitrogen as a ring heteroatom are scarce. The synthetic routes to such products are limited mostly to the addition of hydrophosphoryl compounds to triazine derivatives, 9 lactam alkylation using dialkyl phosphite sodium salts 10 or multistep asymmetric synthesis 11 for unsubstituted five-and six-membered compounds, and some procedures developed for aziridinylphosphonates. 12 The synthesis of diethyl 2-methyl-and 2-phenylpyrrolidin-2-ylphosphonates, used as precursors for phosphorylated nitrones, has also been reported. 13 Cyclic imines with various aliphatic or aromatic substituents at the a-position can be easily prepared according to literature procedures 14 from cheap, commercially available starting materials, and these imines open up broad synthetic possibilities. We have demonstrated their application in the synthesis of biologically and synthetically attractive molecules, such as derivatives of indole, pyrazole, isoxazole, amino acids, and seminatural peptides. 15 Taking into account that the most convenient approach to build phosphonate P-C-N systems generally comprises the addition of dialkyl phosphites 16-20 or alkali metal phosphides 21 to the carbon-nitrogen double bond of Schiff bases, we believe that the phosphorylation of substituted cyclic imines may be an advantageous strategy to obtain a-substituted cyclic a-aminophosphonates.In this paper, we report the effective synthesis of cyclic aaminophosphonates with different ring sizes on the basis of the above-mentioned methodology.Depending on the structure and electrophilicity of a Schiff base, dialkyl phosphites are known to add to its carbonnitrogen double bond under thermal, 16a,b ultrasonic, 16c or microwave 18 initiation and in the presence of strong bases 17 or Lewis acids. 18,20 We found, however, that the addition of diethyl phosp...