Showing papers by "Curt Wentrup published in 2011"

Nitrenes, carbenes, diradicals, and ylides. Interconversions of reactive intermediates.

Abstract: Rearrangements of aromatic and heteroaromatic nitrenes and carbenes can be initiated with either heat or light. The thermal reaction is typically induced by flash vacuum thermolysis, with isolation of the products at low temperatures. Photochemical experiments are conducted either under matrix isolation conditions or in solution at ambient temperature. These rearrangements are usually initiated by ring expansion of the nitrene or carbene to a seven-membered ring ketenimine, carbodiimide, or allene (that is, a cycloheptatetraene or an azacycloheptatetraene when a nitrogen is involved). Over the last few years, we have found that two types of ring opening take place as well. Type I is an ylidic ring opening that yields nitrile ylides or diazo compounds as transient intermediates. Type II ring opening produces either dienylnitrenes (for example, from 2-pyridylnitrenes) or 1,7-(1,5)-diradicals (such as those formed from 2-quinoxalinylnitrenes), depending on which of these species is better stabilized by resonance. In this Account, we describe our achievements in elucidating the nature of the ring-opened species and unraveling the connections between the various reactive intermediates. Both of these ring-opening reactions are found, at least in some cases, to dominate the subsequent chemistry. Examples include the formation of ring-opened ketenimines and carbodiimides, as well as the ring contraction reactions that form five-membered ring nitriles (such as 2- and 3-cyanopyrroles from pyridylnitrenes, N-cyanoimidazoles from 2-pyrazinyl and 4-pyrimidinylnitrenes, N-cyanopyrazoles from 2-pyrimidinylnitrenes and 3-pyridazinylnitrenes, and so forth). The mechanisms of formation of the open-chain and ring-contraction products were unknown at the onset of this study. In the course of our investigation, several reactions with three or more consecutive reactive intermediates have been unraveled, such as nitrene, seven-membered cyclic carbodiimide, and open-chain nitrile ylide. It has been possible in some cases to observe them all and determine their interrelationships by means of a combination of matrix-isolation spectroscopy, photochemistry, flash vacuum thermolysis, and computational chemistry. These studies have led to a deeper understanding of the nature of reactive intermediates and chemical reactivity. Moreover, the results indicate new directions for further exploration: ring-opening reactions of carbenes, nitrenes, and cyclic cumulenes can be expected in many other systems.

2-quinoxalinylnitrenes and 4-quinazolinylnitrenes: rearrangement to cyclic and acyclic carbodiimides and ring-opening to nitrile ylides.

Abstract: This work was undertaken with the aim to obtain direct evidence for the interrelationships between hetarylnitrenes, their ring-expanded cyclic carbodiimide isomers, and ring-opened nitrile ylides. Tetrazolo[1,5-a]quinoxaline 11T and tetrazolo[5.1-c]quinazoline 13T undergo valence tautomerization to the corresponding azides 11A and 13A on mild flash vacuum thermolysis (FVT). Photolysis in Ar matrixes at ca. 15 K affords the triplet nitrenes 12 and 14, identified by ESR, UV, and IR spectroscopy. The nitrenes are converted photochemically to the seven-membered ring carbodiimide 15 followed by the open-chain carbodiimide 22. The 3-methoxy- and 3-chloro-2-quinoxalinylnitrenes 24 yield the ring-expanded carbodiimides 26 very cleanly on matrix photolysis, whereas FVT affords N-cyanobenzimidazoles 28. The ring-opened nitrile ylides 36 and 49 are identified as intermediates in the photolyses of 2-phenyl-4-quinazolinylnitrene 32 and 7-nitro-2-phenyl-4- quinazolinylnitrene 47. In these systems, a photochemically reversible interconversion of the seven-membered ring carbodiimides 35 and 48 and the nitrile ylides 36 and 49 is established. Recyclization of open-chain nitrile ylides is identified as an important mechanism of formation of ring contraction products (N-cyanobenzimidazoles).

Amino-, Alkoxy-, and Alkylthio-Isocyanates and -Isothiocyanates, RX-NCY, their Isomers RX-YCN and RX-CNY, and their Rearrangements

Abstract: Hetero-substituted isocyanates and isothiocyanates RX-NCY (X = R2N, RO, or RS; Y = O or S) and the isomeric cyanates RX-OCN, thiocyanates RX-SCN, nitrile oxides RX-CNO, and nitrile sulfides RX-CNS are highly reactive compounds, often transient at room temperature. The chemistry of these compounds is reviewed. A number of rearrangement reactions is described, particularly [3,3]- sigmatropic shifts, e.g. PhX-NCY → o-HX-C6H4-YCN (X = NR, O, S; Y = O, S); and retro-ene type reactions, RCH2X-NCY → RCH=X + HYCN (X = NR, O, S; Y = O, S). In addition, potential 1,4-shifts of substituent groups of the type R-Y-CNX → R-X-N=C=Y; 1,3- shifts R-C(=Y)-N=X → R-X-N=C=Y; and 1,2-shifts R-C(=Y)-N=X → R-Y-CNX were identified computationally.

Rearrangements and interconversions of heteroatom-substituted isocyanates, isothiocyanates, nitrile oxides, and nitrile sulfides, RX-NCY and RY-CNX.

Abstract: Isocyanates and isothiocyanates of the type RX-NCY (X and Y = O or S) and the isomeric nitrile oxides and nitrile sulfides RY-CNX are highly reactive compounds. A number of potential 1,4-shifts of substituent groups of the type R-Y-CNX → R-X-N═C═Y, 1,3-shifts R-C(═Y)-N═X → R-X-N═C═Y, and 1,2-shifts R-C(═Y)-N═X → R-Y-CNX have been evaluated computationally. The results obtained for the relatively new functional MPW1K and the well-established B3LYP, together with a triple-ζ quality basis set, are very similar. The 1,3- and 1,4-halogen shifts in the title compounds are usually highly exothermic and possess low activation barriers. 1,3-Aryl shifts are feasible for for 5e → 6e (Ar-CO-NSO(2) → Ar-SO(2)-NCO) with activation barriers of less than 40 kcal/mol. Additionally, several 1,3- and 1,4-hydrogen shifts and the 1,4-methyl-shift in methoxynitrile sulfide MeO-CNS to methylsulfenyl isocyanate MeS-NCO (4c → 6c) are potentially feasible. The 1,2-shift reactions 4b → 5b (HO-NCS → H-CS-NO) and 4c → 5c (Ar-O-CNS→ Ar-CO-NS) are good candidates for experimental observation with activation energies around 30 kcal/mol.

Cumulene Rearrangements: Ketene—Ketene, Isocyanate—Isocyanate, Thioketene—Ketene, Imidoylketene—Ketenimine, and Ketene—Allene Rearrangements

Abstract: Interconversions between alpha-oxoketenes, imidoylketenes and alpha-oxoketenimines, thioacylketenes and acylthioketenes, vinylketenes and acylallenes, isocyanates, and thioacylisocyanates and acylisothiocyanates take place by means of 1,3-shifts of substituents, which are facilitated by electron-rich migrating groups, especially those containing lone pairs on the migrating atoms. A bonding interaction between the lone pair orbital and the LUMO of the cumulene moiety stabilizes the transition state and can even make it become an intermediate. The 1,3-shifts can also be said to be pseudopericyclic reactions. The 1,3-migration of aryl groups is accelerated by electron-donating substituents in the phenyl ring. In general, for like substituents, migratory aptitudes decrease in the series alpha-oxoketene > imidoylketenes > acylallene > vinylketene.

Amino‐, Alkoxy‐, and Alkylthio‐isocyanates and ‐isothiocyanates, RX—NCY, Their Isomers RX—YCN and RX—CNY, and Their Rearrangements

Abstract: Hetero-substituted isocyanates and isothiocyanates RX-NCY (X = R2N, RO, or RS; Y = O or S) and the isomeric cyanates RX-OCN, thiocyanates RX-SCN, nitrile oxides RX-CNO, and nitrile sulfides RX-CNS are highly reactive compounds, often transient at room temperature. The chemistry of these compounds is reviewed. A number of rearrangement reactions is described, particularly [3,3]- sigmatropic shifts, e.g. PhX-NCY → o-HX-C6H4-YCN (X = NR, O, S; Y = O, S); and retro-ene type reactions, RCH2X-NCY → RCH=X + HYCN (X = NR, O, S; Y = O, S). In addition, potential 1,4-shifts of substituent groups of the type R-Y-CNX → R-X-N=C=Y; 1,3- shifts R-C(=Y)-N=X → R-X-N=C=Y; and 1,2-shifts R-C(=Y)-N=X → R-Y-CNX were identified computationally.

Marie Curie, Radioactivity, the Atom, the Neutron, and the Positron

Abstract: The Australian Journal of Chemistry is pleased to publish a collection of papers commemorating the 100th anniversary of the Nobel Prize in chemistry awarded to Marie Curie (nee Marya (Maria) Sk"odowska) for the discovery of two new radioactive elements, polonium and radium. Her first Nobel Prize, in physics in 1903, was shared with Henri Bequerel and her husband Pierre Curie for their work on the radiation phenomena discovered by Becquerel – the so-called Becquerel rays, which were later determined by Rutherford and Royds in 1907 to be a-particles (He2þ nuclei). It wasMarie Curie who in 1898 coined the term radioactivity for this new property of matter. Although Marie and Pierre had discovered and isolated two new highly radioactive elements, Po and Ra from the uranium mineral pitchblende in 1898, the Nobel physics committee appeared to have reservations about this and made no mention of it, thereby making it possible for the chemistry Nobel committee to award her the second prize in 1911. The Curie story is reviewed herein by Clarence Hardy. It is important to realize that these discoveries were made before the structure of the atom was understood. Neither the proton nor the neutron nor indeed the atomic nucleus were known, and the electron had only just been discovered by J. J. Thomson in 1897. The bestmodel of the daywas J. J. Thomson’s ‘plum pudding model’, in which the corpuscular electrons were sitting like the plums in an ill-defined nuclear fluid. Rutherford’s ‘solar system’ model came in 1911, and Bohr’s now familiar quantummodel of the atom in 1913. The Becquerel rays were at first thought to be similar to Rontgen’s X-rays, i.e. electromagnetic in nature. The a-particles emitted by polonium became crucially important in the discovery of the neutron, which earned Chadwick the Nobel Prize in Physics in 1935, and Irene Curie and her husband Frederic Joliot the Chemistry Nobel, also in 1935, for the discovery of the transmutation of elements by a-particle bombardment of lighter elements, e.g. Alþ a(He)Pþ n. The Joliot-Curies first thought the neutron rays emanating from light elements such as beryllium and aluminium on a-bombardment were a powerful variant of g-rays (high energy electromagnetic radiation), but Chadwick realized their particulate nature. The P isotope formed in the Joliot-Curie experiment is radioactive, emitting positrons with a half-life of 3min. Thus, they discovered artificial radioactivity. Similarly, radioactive isotopes of nitrogen and silicon were produced by a-bombardment of boron and magnesium, respectively. Positrons are nowadays immensely important in positron emission tomography (see paper by Greguric et al. below). The fascinating story of the neutron – the Curie family’s legacy – is reviewed expertly by John and Alsia White. The discoveries of Po, Ra, radioactivity, the atomic nucleus, and the neutron paved the way to our current understanding of the atom and the associated advances in chemistry, physics, and medicine. The development of radiation chemistry since the Curies and till the present day is reviewed briefly by Ronald Cooper. Alison Edwards highlights modern uses of neutron diffraction in structure determination in cases where X-ray diffraction is unable to provide definitive answers. This takes advantage of the different scattering properties of X-rays and neutrons: because the neutrons are electrically neutral, they are diffracted more readily by the nuclei rather than the electrons in a molecule, thereby enabling them to reveal the precise locations of the nuclei of hydrogen and other light elements or to ascertain their absence. While neutron diffraction used to be a technique firmly in the realm of physics because forbiddingly large crystals were needed in order to obtain chemical structures, technical advances using Laue neutron diffractometers have made it possible to undertake such studies on crystals of manageable sizes (fractions of mm). Radioactive elements and isotope labelling are of practical importance in several areas of medicine. Radium and other radionuclides were introduced very early byMarie Curie herself and are still used in radiotherapy to combat malignant tumours. More recently, g-rays from e.g. Co, electron (b-rays), and proton beams from particle accelerators have come into use. The synthesis of radioactively labelled drugs (radiosynthesis) is interesting because it permits the location of the molecule on the target and elsewhere by means of 3D positron emission tomography. Ivan Greguric and coworkers describe a synthesis of positron-emitting F-labelled nicotinamide (half-life of F 110min) designed to target random metastasis of melanoma tumours. Because the ingestion of radionuclides is harmful and potentially lethal, it is of crucial importance to be able to determine their presence in drinking water – not least in Australia with its large deposits of uraniumore.MeganCook andRossKleinsmith describe a method for the simultaneous determination of a-emitting Ra-226 and b-emitting Ra-228 in water by liquid scintillation spectrometry. CSIRO PUBLISHING