Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.
Organoboranes and -borates enable many chemical transformations in organic chemistry — most importantly, hydroboration and carboboration. Most reactions transfer a nucleophilic boron substituent to an electrophilic center either inter- or intramolecularly. In particular, α,β-unsaturated borates and borates with an α leaving group are highly susceptible to intramolecular 1,2-migration of a group from boron to the electrophilic α position. Oxidation or protonolysis of the resulting organoboranes generates many organic products, including alcohols, carbonyl compounds, alkenes, and halides.
Properties of the B-C bond
The C-B bond has low polarity (electronegativity 2.55 for carbon and 2.04 for boron). Alkyl boron compounds are in general stable, though easily oxidized.
Boron often forms electron-deficient compounds without a full octet, such as the triorganoboranes. These compounds are strong electrophiles, but typically too sterically hindered to dimerize. Electron donation from vinyl and aryl groups can lend the C-B bond some double bond character.
Classes of organoboron compounds
Organoboranes and hydrides
The most-studied class of organoboron compounds has the formula BRnH3−n. These compounds are catalysts, reagents, and synthetic intermediates. The trialkyl and triaryl derivatives feature a trigonal-planar boron center that is typically only weakly Lewis acidic. Except a few bulky derivatives, the hydrides (n = 1 or 2) dimerize, like diborane itself. Trisubstituted derivatives, e.g. triethylboron, are monomers.
Borinic and boronic acids and esters (BRn(OR)3-n) Compounds of the type BRn(OR)3-n are called borinic esters (n = 2), boronic esters (n = 1), and borates (n = 0). Boronic acids are key to the Suzuki reaction. Trimethyl borate, debatably not an organoboron compound, is an intermediate in sodium borohydride production.
Boron clusters
Boron is renowned for cluster species, e.g. dodecaborate [B12H12]2-. Such clusters have many organic derivatives. One example is [B12(CH3)12]2- and its radical derivative [B12(CH3)12]−. Related cluster compounds with carbon vertices are carboranes; the best known is orthocarborane, C2B10H12. Carboranes have few commercial applications. Anionic derivatives such as [C2B9H11]2−, called dicarbollides, ligate similarly to cyclopentadienide.
Bora-substituted aromatic compounds
In borabenzene, boron replaces one CH center in benzene. Borabenzene and derivatives invariably appear as adducts, e.g., C5H5B-pyridine.
The cyclic compound borole, a structural analog of pyrrole, has not been isolated, but substituted derivatives (boroles) are known.
The cyclic compound borepin is aromatic.
Boryl compounds
Organometallic compounds with metal-boron bonds (M–BR2) are boryl complexes, corresponding to the notional boryl anion R2B−. Related ligands are borylenes (M–B(R)–M).
Strong bases do not deprotonate boranes R2BH. Instead these reactions afford the octet-complete adduct R2HB-base.
Compounds isoelectronic with the N-heterocyclic carbenes are known The unusual compound was prepared by reduction of a boron-bromide precursor:
Alkylideneboranes
Alkylideneboranes (RB=CRR) with a boron–carbon double bond are rare. One example is borabenzene. The parent compound, HB=CH2, can be detected at low temperature. The derivative CH3B=C(SiMe3)2 is fairly stable, but prone to cyclodimerisation.
NHC adducts of boron
NHCs and boranes form stable NHC-borane adducts. Triethylborane adducts can be synthesised directly from the imidazolium salt and lithium triethylborohydride.
Diborenes
Boron-boron double bonds are extraordinarily rare. In 2007, the University of Georgia's Gregory Robinson presented the first neutral diborene (RHB=BHR):
Each boron atom has an attached proton and is coordinated to a NHC carbene. The parent structure with the additional carbene ligands is diborane(2).
A reported diboryne is based on similar chemistry.
Synthesis
From Grignard reagents
Simple organoboranes such as triethylborane or tris(pentafluorophenyl)boron can be prepared from trifluoroborane (in ether) and the ethyl or pentafluorophenyl Grignard reagent. Further carbanion addition will effect a borate (R4B−).