Reagents containing phosphorus, sulfur, silicon, or boron
Compared with most of the synthetic methods, those based on reagents which contain phosphorus, sulfur, silicon, or boron have been introduced relatively recently, almost all since 1950. Phosphorus-containing reagents owe their usefulness to three characteristics of phosphorus chemistry: the ease with which phosphorus is converted into phosphorus; the relatively strong bonds formed by phosphorus to oxygen and to sulfur; and the availability of 3d orbitals for bonding. In polar solvents, the tendency for the opposite poles in the betaine to be close together would be less, consistent with the reduced stereoselectivity. Sulfur-containing reagents also owe their usefulness in part to the capacity of sulfur to utilise 3d orbitals for bonding and to occur in valence states higher than 2. Silicon-containing reagents differ from those containing phosphorus or sulfur in that silicon displays only one valency and that, although the Si—O bond is readily formed, it is also readily cleaved.