Nitrogen-arsenic rings

In this section, syntheses of a variety of inorganic nitrogen compounds are described. The syntheses include those of triazanium chlorides alkylamino derivatives of boron, arsenic, and tin and trimeric and tetrameric boron-nitrogen ring structures as exemplified by borazine (HNBH)3. 

ARSENIC-, ANTIMONY- AND BISMUTH-NITROGEN RINGS 11.5.1 Cyclopnictazanes... 

The molecular structures of diazaarsoles and thiazaarsoles known from x-ray diffraction are given in Table 1. The five-membered rings are planar and their bond lengths are in accord with a delocalized cyclic 7r-system. A peculiar feature of the 4-triazenyl-l,2,3-diazaarsole structure is a short intramolecular contact of 250(1) pm between the terminal nitrogen atom of the side chain and the arsenic atom. 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

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