Representative Examples Main Group Chemistry

Owing to the pronounced polarity of the E14—E15 multiple bonds, the compounds containing Si—P multiple bonds represent a flourishing class of main-group chemistry. For example, since the first report on a phosphasilene (compound containing Si=P double bond) by Bickelhaupt in 1984 (1), a number of phosphasilenes have been reported generally by salt metathesis reactions. 

The most frequently encountered reactions in organic sulfur chemistry are nucleophilic displacement reactions. The mechanism and steric course of reactions have been the main points of interest of research groups all over the world, in particular, Andersen, Cram, Johnson, and Mislow in the United States Kobayashi and Oae in Japan Kjaer in Denmark and Fava and Montanari in Italy. The results of these investigators have been discussed exhaustively in many reviews on sulfur stereochemistry. In a recent report on nucleophilic substitution at tricoordinate sulfur, the literature was covered by Tillett (10) to the end of 1975. Therefore only some representative examples of nucleophilic substitution reactions at chiral sulfur are discussed here. However, recent results obtained in the authors laboratory are included. 

In recent reviews of the chemistry of organic azido compounds attention has been focused mainly on the reactions rather than the synthesis of azides ". The last comprehensive account of the introduction of the azido group was published by Boyer and Canter in 1954 and for the purposes of this chapter we have concerned ourselves primarily with developments subsequent to this review. Reference to work cited in the review of Boyer and Canter is made only where it is pertinent to the discussion. Because of the very large number of azide syntheses described in the chemical literature no attempt has been made at an encyclopaedic coverage. Instead, each major synthetic method has been discussed from a mechanistic viewpoint with representative examples being used as illustration. Only reactions in which a carbon-azide bond is formed are considered reactions in which organic azides are implicated as intermediates but are not isolated, e.g. the Schmidt reaction, were deemed to be outside the scope of this chapter. 

The main significance of the examined polycyclic ligands is in the creation of the basis of coordination chemistry for the -elements of Groups IA and IIA of the Periodic Table. Not long ago the above complexes of these elements were only represented by a few single examples at present, hundreds of them have been described. Another important aspect to be mentioned is the use of crown-ether complexes as biomimetic models of ionophores [579 581]. 

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