Five-membered Rings with Two Nonadjacent Heteroatoms with at least One Boron

Abstract

Simple five-membered ring cyclic compounds containing two nonadjacent heteroatoms with at least one boron atom are electron-poor, an exception being the 1,3-thiaborolide anion which is aromatic. There are five different ring types, with two boron atoms, a boron and a nitrogen atom, a boron and a sulfur atom, a boron and a silicon atom, or a boron and a tin atom. The abstraction of a hydrogen atom from 2,3-dihydro-1H-1,3-diboroles forms 1,3-diborolyl radicals. These three-electron species can π-coordinate to one or two transition metal atoms to form sandwich or polydecker complexes. Without H-2 abstraction, 2,3-dihydro-1H-1,3-diboroles can act as four-electron donor ligands to metals through coordination of the H-2 atom and the CC bond. Carbon monoxide reacts with 1,3-diboroles by insertion into the B–C bonds. Iodine atoms attached to the ring heteroatoms of 1,3-diboroles are easily exchanged for methyl, ethoxy, or trimethylsilylmethylene groups. 1,3-Diethoxy-2,3-dihydro-1H-1,3-diboroles are converted into 1,3-di-t-butyl-2,3-dihydro-1H-1,3-diboroles. For the other ring types, no reactions, of either their ring atoms or of their ring substituents, have appeared in the literature in the period under review. The synthesis of these five-membered rings can be grouped according to the number of bonds formed at the cyclization step. There are two reports involving syntheses of 2,3-dihydro-1H-1,3-stannaboroles that occur by formation of one bond while all the other rings have been synthesized by methods involving the formation of two bonds.