Five-membered rings with two nonadjacent heteroatoms with at least one boron

Developments in the chemistry of five-membered ring heterocycles containing two non-adjacent heteroatoms, with at least one boron, in the period 2008–20 are reviewed. Application of theoretical and structural methods to these compounds is described including X-ray structure determination and both ¹¹B and ³¹P NMR. The major reactions of azaboroline and phosphaborolane systems as well as their benzo analogues are discussed, with an emphasis on the ability of these “frustrated Lewis pairs” to undergo addition and cycloaddition. Methods for the synthesis of such systems are systematically covered. The systems containing boron and phosphorus have only been reported recently and were not included in previous editions of Comprehensive Heterocyclic Chemistry.