Aromatic phosphorus heterocycles

Diazaphospholes constitute the most widely investigated class of heterophospholes, the 67t-aromatic phosphorus heterocycles [1,2]. Diazaphospholes are unique in the manner that the five-membered ring incorporates one phosphorus atom. First diazaphosphole representative, i.e. 2//-[l,2,3 diazaphospholc was obtained as early as 1967 [3] and until 1980s the interest of organophosphorus chemists remained in the development of different synthetic routes and in investigating their varied reactivity due to the structural diversity within the class [4], On the basis of the relative positions of the three heteroatoms in the five-membered ring, six monocyclic diazaphosphole systems (A-F) are possible and all of them have been reported (Structure 1). 

Keywords Aromatic phosphorus heterocycles Phospholides P ligands Zwitterions CT-Donor/ r-acceptor properties... 

Chapter 2 by Laszlo Nyulaszi and Zoltan Benko deals with the chemistry and physical organic chemistry of aromatic phosphorus heterocycles and is divided into four subchapters dealing with three-, four-, five-, and six-membered rings, in which there may be more than one phosphorus atom. The chapter begins with a clear presentation of the electronic structure that phosphorus may achieve in molecules with CP bonds. For cyclopentadiene and phosphole eight aromaticity indices are collected. Almost all of them indicate that phosphole is more aromatic than cyclopentadiene. It is also shown that even small structural effects (substituent, bonding modes) can have a substantial impact on the chemistry of the reported systems. 

Comprehensive reviews of aromatic phosphorus heterocycles including heterophospholes were recently published (01CRV1229, 09M11). 

Table 19. C- P coupling constants in some potentially aromatic phosphorus heterocycles ...

For several reasons the development of the chemistry of 6n heteroaromatic arsenic systems, for example arsinine (3), was much less vigorous than that of the phosphinines. While t-butylphos-phaethyne (4) and several other phosphaethynes are important building blocks for the synthesis of six-membered aromatic phosphorus heterocycles with more than one heteroatom, only one compound with a C=As triple bond, the 2,4,6-tri- -butylphenyl derivative (5), has been described <86AG(E)264>, and this has not been thoroughly studied. The only six-membered As-heteroaromatic system with more than one heteroatom is the l,3A -azarsinine (6) <88TL535>. Since pentavalent... 

Modelli A, Hajgato B, Nixon JF, Nyuldszi L (2004) Anionic states of six-membered aromatic phosphorus heterocycles as studied by electron transmission spectroscopy and ab initio methods. J Phys Chem A 108 7440-7447... 

---