Organoboron chemistry

Organoboron

Organoborane or organoboron compounds are chemical compounds that are organic derivatives of BH₃, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compounds.^[1][2] Organoboron compounds are important reagents in organic chemistry enabling many chemical transformations, the most important one called hydroboration.

Properties of the B-C bond

The C-B bond has low polarity (the difference in electronegativity 2.55 for carbon and 2.04 for boron), and therefore alkyl boron compounds are in general stable though easily oxidized.

In part because its lower electronegativity, boron often forms electron-deficient compounds, such as the triorganoboranes. Vinyl groups and aryl groups donate electrons and make boron less electrophilic and the C-B bond gains some double bond character. Like the parent borane, diborane, organoboranes are classified in organic chemistry as strong electrophiles because boron is unable to gain a full octet of electrons. Unlike diborane however, most organoboranes do not form dimers.

Synthesis

From Grignard reagents

Simple organoboranes such as triethylborane or tris(pentafluorophenyl)boron can be prepared from trifluoroborane (as the ether complex) and the ethyl or pentafluorophenyl Grignard reagent.

From alkenes

Boranes react rapidly to alkenes in a process called hydroboration. This concept was discovered by Dr. Herbert Charles Brown, work for which he eventually received the Nobel Prize (jointly with Georg Wittig for his discovery of the Wittig reaction). Although diborane as a pure compound is a dimer, BH₃ forms 1:1 complexes with basic solvents, for instance THF. In an ordinary electrophilic addition reaction of HX (X = Cl, Br, I, etc.), Markovnikov's rule, which states that the less electronegative atom, usually hydrogen, adds to the least substituted carbon of the double bond, this determines regioselectivity. With boranes the mode of action is the same, the hydrogen adds to the most substituted carbon because boron is less electronegative than hydrogen. When a positive charge develops in the alkene on the most substituted carbon atom, that is where the partially negatively charged hydrogen atom adds, leaving the least substituted carbon atom for the boron atom. The so-called anti-Markovnikov addition because when the boron is replaced with a hydroxyl group the overall reaction is addition of water over the double bond in what appears to be an anti-Makovnikov addition.

This is most pronounced when the boron compound has very bulky substituents. One organoboron reagent that is often employed in synthesis is 9-BBN which is generated from the reaction of cyclooctadiene and diborane.^[3] Hydroborations take place stereospecifically in a syn mode, that is on the same face of the alkene. In this concerted reaction the transition state is represented as a square with the corners occupied by carbon, carbon, hydrogen and boron with maximum overlap between the two olefin p-orbitals and the empty boron orbital.

By borylation

Metal-catalyzed C-H Borylation reactions are transition metal catalyzed organic reactions that produce an organoboron compound through functionalization of aliphatic and aromatic C-H bonds. A common reagent in this type of reaction is bis(pinacolato)diboron.

Reactions

Hydroboration-oxidation

In organic synthesis the hydroboration reaction is taken further to generate other functional groups in the place of the boron group. The hydroboration-oxidation reaction offers a route to alcohols by oxidation of the borane with hydrogen peroxide or to the carbonyl group with the stronger oxidizing agent chromium oxide.

Rearrangements

A second group of reactions that organoboron compounds are involved in create new carbon carbon bonds. Carbon monoxide is found to react very easily with a trialkylborane. What follows is a 1,2-rearrangement when an alkyl substituent on the anionic boron migrates to the adjacent electrophilic carbon of the carbonyl group. The carbonyl group can then be reduced to a hydroxyl group .

Allylboration

Asymmetric allylboration demonstrates another useful application of organoboranes in carbon–carbon bond formation.^[4] In this example from Nicolaou's synthesis of the epothilones,^[5] asymmetric allylboration (using an allylborane derived from chiral alpha-pinene) is used in conjunction with TBS protection and ozonolysis. Overall, this provides a two-carbon homologation sequence that delivers the required acetogenin sequence.

As reducing agent

Borane hydrides such as 9-BBN and L-selectride (lithium tri-sec-butylborohydride) are reducing agents. An example of an asymmetric catalyst for carbonyl reductions is the CBS catalyst. This catalyst is also based on boron, the purpose of which is coordination to the carbonyl oxygen atom.

Borates

Trialkylboranes, BR₃, can be oxidized to the corresponding borates, B(OR)₃. One method for the determination of the amount of C-B bonds in a compound is by oxidation of R₃B with trimethylamine oxide (Me₃NO) to B(OR)₃. The trimethylamine (Me₃N) formed can then be titrated.

Boronic acids RB(OH)₂ react with potassium bifluoride K[HF₂] to form trifluoroborate salts K[RBF₃]^[6] which are precursors to nucleophilic alkyl and aryl boron difluorides, ArBF₂.^[7] The salts are more stable than the boronic acids themselves and used for instance in alkylation of certain aldehydes:^{[8][note 1]}

Suzuki reaction and related reactions

Organoboron compounds also lend themselves to transmetalation reactions, especially with organopalladium compounds. This reaction type is exemplified in the Suzuki reaction, which involves coupling of aryl- or vinyl-boronic acid with an aryl- or vinyl-halide catalyzed by a palladium(0) complex,^[9]

This reaction is an important method for making carbon-carbon bonds.

Classes of organoboron compounds

Triorganoboranes and hydrides

Among the most studied classes of organoboron compounds have the formula BR_nH_3-n. As discussed above, these compounds are used as catalysts, reagents, and synthetic intermediates. The trialkyl and triaryl derivatives feature trigonal planar boron center that is typically only weakly Lewis acidic. Except for very bulky derivatives, the hydrides exist as dimers, reminiscent of the structure of diborane itself.^[10]

Borinic and boronic acids and esters (BR_n(OR)_3-n)

Compounds of the type BR_n(OR)_3-n are called borinic esters (n = 2), boronic esters (n = 1), and borates (n = 0). Boronic acids are used in Suzuki reaction. Trimethyl borate, which is debatably not an organoboron compound, is an intermediate in the production of sodium borohydride.

Carboranes

Carboranes are cluster compounds with carbon and boron vertices. The best known is orthocarborane, with the formula C₂B₁₀H₁₂. Although they have few commercial applications, carboranes have attracted much attention as precursors to reagents and new materials. Anionic derivatives, dicarbollides, e.g., [C₂B₉H₁₁]²⁻ are ligands that behave like cyclopentadienide.

Bora-substituted aromatic compounds

In borabenzene, one CH center in benzene is replaced by boron. These compounds are invariably isolated as adducts, e.g., C₅H₅B-pyridine. The cyclic compound borole, a structural analog of pyrrole, has not been isolated, but substituted derivatives known as boroles are known. The cyclic compound borepin is aromatic.

Boryl compounds

Boryl anions have the formula R₂B⁻. Nucleophilic anionic boryl compounds have long been elusive but a 2006 study described a boryllithium compound, which reacts as a nucleophile:^[11][12] Organometallic compounds with metal to boron bonds, i.e., M–BR₂), are known as boryl complexes. Related ligands are borylenes (M–B(R)–M).

The absence of lithium boryl compounds is notable because in other period 2 elements lithium salts are common e.g. lithium fluoride, lithium hydroxide lithium amide and methyllithium. The gap highlights the very low electronegativity of boron. Reaction of base with a borohydride R₂BH does not result in deprotonation to the boryl anion R₂B⁻ but to formation of the boryl anion R₂B⁻H(base)⁺. This reaction product has a complete octet.^[13] Instead the boryl compound is prepared by reductive heterolysis of a boron-bromide bond by lithium metal. The new boryl lithium compound is very similar to and isoelectronic with N-heterocyclic carbenes. It is designed to benefit from aromatic stabilization (6-electron system counting the nitrogen lone pairs and an empty boron p-orbital, see structure A) and from kinetic stabilization from the bulky 2,6-diisopropylphenyl groups. X-ray diffraction confirms sp2 hybridization at boron and its nucleophilic addition reaction with benzaldehyde gives further proof of the proposed structure.

Alkylideneboranes

Alkylideneboranes of the type RB=CRR with a boron – carbon double bond are rarely encountered. An example is borabenzene. The parent compound is HB=CH₂ which can be detected at low temperatures. A fairly stable derivative is CH₃B=C(SiMe₃)₂ but is prone to cyclodimerisation.^[14]

NHC adducts of boron

NHCs and boranes form stable NHC borane adducts.^[15] Triethylborane adducts can be synthesised directly from the imidazolium salt and lithium triethylborohydride. Members of this compound class are investigated for use as reagent or catalyst.

Diborenes

Chemical compounds with boron to boron double bonds are rare. In 2007 the first neutral diborene (RHB=BHR) was presented.^[16][17][18] Each boron atom has a proton attached to it and each boron atom 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.

Other uses

TEB – Triethylborane was used to ignite the JP-7 fuel of the Pratt & Whitney J58 variable cycle engines powering the Lockheed SR-71 Blackbird.

Notes

See also

• Compounds of carbon with other elements in the periodic table:

References