Aromatic compounds with planar tricoordinate phosphorus

Tricoordinate phosphorus can be planarized, and used as an excellent buildi ng block of aromatic compounds. The pyramidality can be reduced by (i) sigm a donor and pi acceptor groups; (ii) steric strain, which increases the bon ding angle about phosphorus; (iii) bulky substituent groups; and (iv) deloc alization or aromatic stabilization effects. The aromaticity of pi systems with planar tricoordinate phosphorus is largest among those rings containin g nitrogen or chalcogen heteroatoms. For the flat systems the aromatic stab ilization is just compensated by the energy cost of the planarization at ph osphorus. Due to this counterbalancing effect a very rich chemistry can be expected for these compounds. (C) 1999 Elsevier Science Ltd. All rights res erved.