Boron-containing small rings: synthesis, properties, and application prospects

The studies published over the last 15 years on the synthesis, physicochemical properties, and application prospects of saturated and unsaturated three- and four-membered boron-containing carbocycles with one boron atom — boriranes, borirenes, boretanes, and 1,2-dihydroboretanes — are summarized and systematized. Original methods for the synthesis of boriran(en)es based on photochemical isomerization of organoboranes, double hydroboration of acetylenes with imidazol-2-ylideneboranes, and [2+1]-cycloaddition of borylenes (:B–R) to unsaturated compounds are considered. A new method for the synthesis of substituted boriranes by Cp2TiCl2-catalyzed cycloboration of olefins with boron halides in the presence of metallic Mg is presented. The not numerous data on the synthesis of four-membered boracyclanes (boretanes and 1,2-dihydroboretes) based on thermal isomerization of cyclopropylboranes, [2+2]-cycloaddition of methyleneboranes to nitriles, 1,1-diethoxyethylene, or alkynes, as well as transmetallation of 1,8-dilithium naphthalene and titanacyclobutenes with boron halides, are summarized.