The temperature dependence of the magnetic susceptibility, optical reflectivity and electrical conductivity of [Fe(HB(pz) 3 ) 2 ] (pz = pyrazolyl) revealed irreversible changes in the material during the low-spin to high-spin transition when the ''as-prepared'' sample was heated above B400 K for the first time. During this first heating sequence, the initially fine powder sample became coarse, and its crystal structure changed from tetragonal to monoclinic. Single-crystals of the monoclinic form suitable for X-ray analysis could be isolated after the first thermal cycle, and their structure was resolved in the P2 1 /n (Z = 4) space group. Successive cooling and heating cycles did not lead to further modification of the crystal structure, and the temperature dependence of the physical properties remained invariable. Remarkably, the electrical conductivity of the sample measured at 293 K dropped from 6.1 Â 10 À8 to 2.1 Â 10 À11 S m À1 following the first thermal cycle-suggesting possible applications of this material in read-only memory devices (ROM).

Nitrene-transfer reactions have been a powerful synthetic method for direct incorporation of nitrogen atoms into organic molecules. Discovery of novel nitrene-transfer reactions has been dominantly supported by not only the improvement in transition-metal catalysts but also by the employment of novel precursors of nitrenoids. Since the pioneering works utilizing organic azides or iminoiodinanes as practical synthetic tools for nitrogen-containing compounds were reported, a new approach using various N-heterocycles containing strain energy or a weak bond has emerged. In this review, we briefly summarize the history of nitrene-transfer chemistry from the viewpoint of its precursors. In particular, the use of N-heterocycles such as 2H-azirines, 1,4,2-dioxazol-5-ones, 1,2,4-oxadiazol-5-ones, isoxazol-5(4H)-ones, and isoxazoles are comprehensively described, showing the recent remarkable progress in this chemistry.

Highly strained 2H-azirines with a tetrasubstituted stereocenter were synthesized by the enantioselective isomerization of isoxazoles with a chiral diene-rhodium catalyst system. The effect of ligands and the coordination behavior support the proposed catalytic cycle in which the coordination site is fixed in favor of efficient enantiodiscrimination by a bulky substituent of the ligand. In silico studies also support the existence of a rhodium-imido complex as a key intermediate for enantiodiscrimination.

We report the divergent catalytic transformation of alkene-tethered isoxazol-5­(4H)-ones by using rhodium and cobalt catalysts to afford 2H-pyrroles (with Rh catalyst) and azabicyclic cyclopropanes (with Co catalyst). The rhodium-catalyzed 2H-pyrrole formation involving hydrogen shift is supported by deuterium-labeling experiments. The control experiments in the cobalt-catalyzed reaction indicate that the bicyclic aziridines as the primary product undergo a skeletal rearrangement assisted by metal iodide salts.

Highly strained 2H-azirines with at etrasubstituted stereocenter were synthesized by the enantioselective isomerization of isoxazoles with ac hiral diene-rhodium catalyst system. The effect of ligands and the coordination behavior support the proposed catalytic cycle in which the coordination site is fixedi nf avor of efficient enantiodiscrimination by abulky substituent of the ligand. In silico studies also support the existence of arhodium-imido complex as akey intermediate for enantiodiscrimination. Scheme 1. Classification of synthetic methods for 2H-azirines. Ts = p-toluenesulfonyl.Scheme 2. Transition-metal-catalyzed transformation of isoxazole derivatives.