3 Synthetic methods : Part (iii) Heteroatom methods

Abstract

This chapter concentrates on the organic chemistry of phosphorus, sulfur, selenium, tellurium, and silicon, particularly the formation of carbon–heteroatom bonds, and heteroatom facilitated carbon–carbon bond formation; the corresponding heterocyclic, free radical and transition metal chemistry is covered more thoroughly elsewhere. The review was compiled using a combination of on-line searching and selective reading of the literature. A particular focus this year is allylation chemistry that relies upon activation of allylsilanes, discussed in Section 4. Interesting recent developments in this strategy rely upon the capability of silicon to form a hypercoordinate species upon coordination of a Lewis base, with organophosphorus and organosulfur species, among others, being suitable. Kobayashi and co-workers have defined these Lewis bases as “neutral coordinate-organocatalysts” to distinguish them from species such as alkoxides and halides and to emphasise the fact that these reactions fall into the category of metal-free organic molecule catalysed reactions. Particularly attractive are chiral Lewis bases which turn-over in the reactions, resulting in protocols for metal-free asymmetric synthesis. This strategy is applicable to metals or semi-metals other than silicon, but there are few, if any, such examples yet reported; silicon has the major advantage over tin of lack of toxicity—thus allylstannanes are becoming side-lined in modern synthetic methodology—but allyl boron compounds may be amenable to this approach. Other modes of activation are also successful and these, and the Lewis base approach, are being applied to reactions other than allylations, see Section 4.