Organoboron chemistry

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

The most-studied class of organoboron compounds has the formula BRnH3−n. These compounds are catalysts, reagents, and synthetic intermediates. Except a few bulky derivatives, the primary and secondary hydrides (n = 1 or 2) are, like diborane itself, strongly Lewis acidic and dimerize in condensed phases. The trialkyl and triaryl derivatives, e.g. triethylboron, are typically only weakly Lewis acidic, and form monomers with a trigonal, planar boron center. Monoalkyl boranes are relatively rare. When the alkyl group is small, such as methyl, monoalkylboranes often redistribute to mixtures of diborane and di- and trialkylboranes. One example of an isolable (bulky) primary borane is thexylborane (ThxBH2), produced by the hydroboration of tetramethylethylene: A chiral example is monoisopinocampheylborane, obtained by hydroboration of (−)‐α‐pinene with borane dimethyl sulfide. Although often written as IpcBH2, it is a dimer, [IpcBH2]2. Dialkylboranes are also rare with small alkyls. One common preparation reduces dialkylhalogenoboranes with metal hydrides. An important application in organic synthesis is transmetallation to form organozinc compounds. Nevertheless, some diaryl and dialkylboranes are well known. Dimesitylborane is a dimer (C6H2Me3)4B2H2) that reacts only slowly with simple terminal alkenes. It adds to alkynes to give alkenylboranes. A hindered dialkylborane is disiamylborane, abbreviated Sia2BH, also a dimer. Owing to its steric bulk, it selectively hydroborates less hindered, usually terminal alkenes in the presence of more substituted alkenes. Disiamylborane must be freshly prepared as its solutions can only be stored at 0 °C for a few hours. Dicyclohexylborane Chx2BH exhibits improved thermal stability than Sia2BH. A versatile dialkylborane is 9-BBN. Also called "banana borane", it exists as a dimer. It can be distilled without decomposition at 195 °C (12mm Hg). Reactions with 9-BBN typically occur at 60–80 °C, with most alkenes reacting within one hour. Tetrasubstituted alkenes add 9-BBN at elevated temperature. Hydroboration of alkenes with 9-BBN proceeds with excellent regioselectivity. It is more sensitive to steric differences than Sia2BH, perhaps because of it rigid C8 backbone. 9-BBN is more reactive towards alkenes than alkynes.

Oxyacids and esters 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.

Adducts Boranes and borinic, boronic, and borate esters all form adducts with appropriate Lewis bases. Strong bases do not deprotonate boranes of the form R2BH. Instead these reactions afford the octet-complete adduct R2HB-base. NHCs and boranes form stable NHC-borane adducts. Triethylborane adducts can be synthesised directly from the imidazolium salt and lithium triethylborohydride.

Polyhedral 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. Borane cluster structures are built from the triangular (BR)3 unit, which is almost unknown in isolation. However, the corresponding aromatic dianion, (BR)2−3, forms from careful dehalogenation of a RNBCl2 species.

Boryl complexes and radicals

Organometallic compounds with metal-boron bonds (M–BR2) are boryl complexes, corresponding to the notional boryl anion R2B−, although the latter cannot be produced through deprotonation (see § Adducts). In one synthesis, the boryl anion moiety arose through lithium-halogen exchange: As shown, the product is isoelectronic to an N-heterocyclic carbene. Related ligands are borylenes (M–B(R)–M).

Unsaturated compounds Alkylideneboranes (RB=CRR) with a boron–carbon double bond are rare. One example, HB=CH2, can be detected at low temperature. The derivative CH3B=C(SiMe3)2 is fairly stable, but prone to cyclodimerisation.

Some boron-substituted heterocycles are aromatic, but very few such arenes are stable. 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 has been isolated and is aromatic. Boron-boron multiple bonds are rare, although doubly-bonded dianions have been known since the 1990s. Neutral analogues use NHC adducts, such as the following diborane(2) derivative:

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