Boronic acid

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The general structure of a boronic acid, where R is a substituent. Boronic-acid-2D.png
The general structure of a boronic acid, where R is a substituent.

A boronic acid is an organic compound related to boric acid (B(OH)3) in which one of the three hydroxyl groups (−OH) is replaced by an alkyl or aryl group (represented by R in the general formula R−B(OH)2). [1] As a compound containing a carbon–boron bond, members of this class thus belong to the larger class of organoboranes.

Contents

Boronic acids act as Lewis acids. Their unique feature is that they are capable of forming reversible covalent complexes with sugars, amino acids, hydroxamic acids, etc. (molecules with vicinal, (1,2) or occasionally (1,3) substituted Lewis base donors (alcohol, amine, carboxylate)). The pKa of a boronic acid is ~9, but they can form tetrahedral boronate complexes with pKa ~7. They are occasionally used in the area of molecular recognition to bind to saccharides for fluorescent detection or selective transport of saccharides across membranes.

Boronic acids are used extensively in organic chemistry as chemical building blocks and intermediates predominantly in the Suzuki coupling. A key concept in its chemistry is transmetallation of its organic residue to a transition metal.

The compound bortezomib with a boronic acid group is a drug used in chemotherapy. The boron atom in this molecule is a key substructure because through it certain proteasomes are blocked that would otherwise degrade proteins. Boronic acids are known to bind to active site serines and are part of inhibitors for porcine pancreatic lipase, [2] subtilisin [3] and the protease Kex2. [4] Furthermore, boronic acid derivatives constitute a class of inhibitors for human acyl-protein thioesterase 1 and 2, which are cancer drug targets within the Ras cycle. [5]

Structure and synthesis

In 1860, Edward Frankland was the first to report the preparation and isolation of a boronic acid. Ethylboronic acid was synthesized by a two-stage process. First, diethylzinc and triethyl borate reacted to produce triethylborane. This compound then oxidized in air to form ethylboronic acid. [6] [7] [8] Several synthetic routes are now in common use, and many air-stable boronic acids are commercially available.

Boronic acids typically have high melting points. They are prone to forming anhydrides by loss of water molecules, typically to give cyclic trimers.

Examples of boronic acids
Boronic acidRStructure Molar mass CAS number Melting point °C
Phenylboronic acid Phenyl Phenylboronic acid.png 121.9398-80-6216–219
2-Thienylboronic acid Thiophen 2-Thienylboronic acid.svg 127.966165-68-0138–140
Methylboronic acid Methyl Methylboronic acid.svg 59.8613061-96-691–94
cis-Propenylboronic acid propene Cis-Propenylboronic acid.svg 85.907547-96-865–70
trans-Propenylboronic acid propene Trans-Propenylboronic acid.svg 85.907547-97-9123–127

Synthesis

Boronic acids can be obtained via several methods. The most common way is reaction of organometallic compounds based on lithium or magnesium (Grignards) with borate esters. [9] [10] [11] [12] For example, phenylboronic acid is produced from phenylmagnesium bromide and trimethyl borate followed by hydrolysis [13]

PhMgBr + B(OMe)3 → PhB(OMe)2 + MeOMgBr
PhB(OMe)2 + 2 H2O → PhB(OH)2 + 2 MeOH

Another method is reaction of an arylsilane (RSiR3) with boron tribromide (BBr3) in a transmetallation to RBBr2 followed by acidic hydrolysis.

A third method is by palladium catalysed reaction of aryl halides and triflates with diboronyl esters in a coupling reaction known as the Miyaura borylation reaction. An alternative to esters in this method is the use of diboronic acid or tetrahydroxydiboron ([B(OH2)]2). [14] [15] [16]

Boronic esters (also named boronate esters)

Boronic esters are esters formed between a boronic acid and an alcohol.

Comparison between boronic acids and boronic esters
CompoundGeneral formulaGeneral structure
Boronic acidRB(OH)2
Boronic-acid-2D.png
Boronic esterRB(OR)2
Boronate-ester-2D.png

The compounds can be obtained from borate esters [17] by condensation with alcohols and diols. Phenylboronic acid can be selfcondensed to the cyclic trimer called triphenyl anhydride or triphenylboroxin. [18]

Examples of boronic esters
Boronic esterDiolStructural formula Molar mass CAS number Boiling point (°C)
Allylboronic acid pinacol ester pinacol Allylboronic acid pinacol ester.svg 168.0472824-04-550–53 (5 mmHg)
Phenyl boronic acid trimethylene glycol ester trimethylene glycol Phenylboronic acid trimethylene glycol ester.svg 161.994406-77-3106 (2 mm Hg)
Diisopropoxymethylborane isopropanol Diisopropoxymethylborane.svg 144.0286595-27-9105 -107

Compounds with 5-membered cyclic structures containing the C–O–B–O–C linkage are called dioxaborolanes and those with 6-membered rings dioxaborinanes.

Organic chemistry applications

Suzuki coupling reaction

Boronic acids are used in organic chemistry in the Suzuki reaction. In this reaction the boron atom exchanges its aryl group with an alkoxy group from palladium.

Chan–Lam coupling

In the Chan–Lam coupling the alkyl, alkenyl or aryl boronic acid reacts with a N–H or O–H containing compound with Cu(II) such as copper(II) acetate and oxygen and a base such as pyridine [19] [20] forming a new carbon–nitrogen bond or carbon–oxygen bond for example in this reaction of 2-pyridone with trans-1-hexenylboronic acid:

ChanLamCoupling.png

The reaction mechanism sequence is deprotonation of the amine, coordination of the amine to the copper(II), transmetallation (transferring the alkyl boron group to copper and the copper acetate group to boron), oxidation of Cu(II) to Cu(III) by oxygen and finally reductive elimination of Cu(III) to Cu(I) with formation of the product. In catalytic systems oxygen also regenerates the Cu(II) catalyst.

Liebeskind–Srogl coupling

In the Liebeskind–Srogl coupling a thiol ester is coupled with a boronic acid to produce a ketone.

Conjugate addition

The boronic acid organic residue is a nucleophile in conjugate addition also in conjunction with a metal. In one study the pinacol ester of allylboronic acid is reacted with dibenzylidene acetone in such a conjugate addition: [21]

BoronicAcidConjugateAddition.png
The catalyst system in this reaction is tris(dibenzylideneacetone)dipalladium(0) / tricyclohexylphosphine.

Another conjugate addition is that of gramine with phenylboronic acid catalyzed by cyclooctadiene rhodium chloride dimer: [22]

BoronicAcidGramineReaction.png

Oxidation

Boronic esters are oxidized to the corresponding alcohols with base and hydrogen peroxide (for an example see: carbenoid)

Homologation

In this reaction dichloromethyllithium converts the boronic ester into a boronate. A Lewis acid then induces a rearrangement of the alkyl group with displacement of the chlorine group. Finally an organometallic reagent such as a Grignard reagent displaces the second chlorine atom effectively leading to insertion of an RCH2 group into the C-B bond. Another reaction featuring a boronate alkyl migration is the Petasis reaction.

Electrophilic allyl shifts

Allyl boronic esters engage in electrophilic allyl shifts very much like silicon pendant in the Sakurai reaction. In one study a diallylation reagent combines both [24] [note 1] :

DoubleAllylationByBoronicAcid.png

Hydrolysis

Hydrolysis of boronic esters back to the boronic acid and the alcohol can be accomplished in certain systems with thionyl chloride and pyridine. [25] Aryl boronic acids or esters may be hydrolyzed to the corresponding phenols by reaction with hydroxylamine at room temperature. [26]

C–H coupling reactions

The diboron compound bis(pinacolato)diboron [27] reacts with aromatic heterocycles [28] or simple arenes [29] to an arylboronate ester with iridium catalyst [IrCl(COD)]2 (a modification of Crabtree's catalyst) and base 4,4′-di-tert-butyl-2,2′-bipyridine in a C-H coupling reaction for example with benzene:

IridiumCHactivationMiyauraHartwig.svg

In one modification the arene reacts using only a stoichiometric equivalent rather than a large excess using the cheaper pinacolborane: [30]

IridiumAreneBorylation.svg

Unlike in ordinary electrophilic aromatic substitution (EAS) where electronic effects dominate, the regioselectivity in this reaction type is solely determined by the steric bulk of the iridium complex. This is exploited in a meta-bromination of m-xylene which by standard AES would give the ortho product: [31] [note 2]

MetahalogenationArylborylationMurphy2007.svg

Protonolysis

Protodeboronation is a chemical reaction involving the protonolysis of a boronic acid (or other organoborane compound) in which a carbon-boron bond is broken and replaced with a carbon-hydrogen bond. Protodeboronation is a well-known undesired side reaction, and frequently associated with metal-catalysed coupling reactions that utilise boronic acids (see Suzuki reaction). For a given boronic acid, the propensity to undergo protodeboronation is highly variable and dependent on various factors, such as the reaction conditions employed and the organic substituent of the boronic acid:

A simple protodeboronation in acidic medium Protodeboronation Scheme.png
A simple protodeboronation in acidic medium

Supramolecular chemistry

Saccharide recognition

An example of a fluorescent complex of a diboronic acid and tartaric acid Tony D James enantioselective diboronic acid fluorescent sensor.png
An example of a fluorescent complex of a diboronic acid and tartaric acid

The covalent pair-wise interaction between boronic acids and hydroxy groups as found in alcohols and acids is rapid and reversible in aqueous solutions. The equilibrium established between boronic acids and the hydroxyl groups present on saccharides has been successfully employed to develop a range of sensors for saccharides. [33] One of the key advantages with this dynamic covalent strategy [34] lies in the ability of boronic acids to overcome the challenge of binding neutral species in aqueous media. If arranged correctly, the introduction of a tertiary amine within these supramolecular systems will permit binding to occur at physiological pH and allow signalling mechanisms such as photoinduced electron transfer mediated fluorescence emission to report the binding event.

Potential applications for this research include blood glucose monitoring systems to help manage diabetes mellitus. As the sensors employ an optical response, monitoring could be achieved using minimally invasive methods, one such example is the investigation of a contact lens that contains a boronic acid based sensor molecule to detect glucose levels within ocular fluids. [35]

Safety

Some commonly used boronic acids and their derivatives give a positive Ames test and act as chemical mutagens. The mechanism of mutagenicity is thought to involve the generation of organic radicals via oxidation of the boronic acid by atmospheric oxygen. [36]


Notes

  1. ↑ In this sequence the boronic ester allyl shift is catalyzed by boron trifluoride. In the second step the hydroxyl group is activated as a leaving group by conversion to a triflate by triflic anhydride aided by 2,6-lutidine. The final product is a vinyl cyclopropane. Note: ee stands for enantiomeric excess
  2. ↑ In situ second step reaction of boronate ester with copper(II) bromide

Related Research Articles

The Suzuki reaction or Suzuki coupling is an organic reaction that uses a palladium complex catalyst to cross-couple a boronic acid to an organohalide. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemistry with Richard F. Heck and Ei-ichi Negishi for their contribution to the discovery and development of noble metal catalysis in organic synthesis. This reaction is sometimes telescoped with the related Miyaura borylation; the combination is the Suzuki–Miyaura reaction. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls.

<span class="mw-page-title-main">Organoboron chemistry</span> Study of compounds containing a boron-carbon bond

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.

<span class="mw-page-title-main">Bamford–Stevens reaction</span> Synthesis of alkenes by base-catalysed decomposition of tosylhydrazones

The Bamford–Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. It is named for the British chemist William Randall Bamford and the Scottish chemist Thomas Stevens Stevens (1900–2000). The usage of aprotic solvents gives predominantly Z-alkenes, while protic solvent gives a mixture of E- and Z-alkenes. As an alkene-generating transformation, the Bamford–Stevens reaction has broad utility in synthetic methodology and complex molecule synthesis.

The Hiyama coupling is a palladium-catalyzed cross-coupling reaction of organosilanes with organic halides used in organic chemistry to form carbon–carbon bonds. This reaction was discovered in 1988 by Tamejiro Hiyama and Yasuo Hatanaka as a method to form carbon-carbon bonds synthetically with chemo- and regioselectivity. The Hiyama coupling has been applied to the synthesis of various natural products.

<span class="mw-page-title-main">Petasis reaction</span>

The Petasis reaction is the multi-component reaction of an amine, a carbonyl, and a vinyl- or aryl-boronic acid to form substituted amines.

Boron trichloride is the inorganic compound with the formula BCl3. This colorless gas is a reagent in organic synthesis. It is highly reactive towards water.

The Negishi coupling is a widely employed transition metal catalyzed cross-coupling reaction. The reaction couples organic halides or triflates with organozinc compounds, forming carbon-carbon bonds (C-C) in the process. A palladium (0) species is generally utilized as the catalyst, though nickel is sometimes used. A variety of nickel catalysts in either Ni0 or NiII oxidation state can be employed in Negishi cross couplings such as Ni(PPh3)4, Ni(acac)2, Ni(COD)2 etc.

<span class="mw-page-title-main">Organozinc chemistry</span>

Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds.

<span class="mw-page-title-main">Organocopper chemistry</span> Compound with carbon to copper bonds

Organocopper chemistry is the study of the physical properties, reactions, and synthesis of organocopper compounds, which are organometallic compounds containing a carbon to copper chemical bond. They are reagents in organic chemistry.

<span class="mw-page-title-main">Phenylboronic acid</span> Chemical compound

Phenylboronic acid or benzeneboronic acid, abbreviated as PhB(OH)2 where Ph is the phenyl group C6H5- and B(OH)2 is a boronic acid containing a phenyl substituent and two hydroxyl groups attached to boron. Phenylboronic acid is a white powder and is commonly used in organic synthesis. Boronic acids are mild Lewis acids which are generally stable and easy to handle, making them important to organic synthesis.

<span class="mw-page-title-main">Liebeskind–Srogl coupling</span>

The Liebeskind–Srogl coupling reaction is an organic reaction forming a new carbon–carbon bond from a thioester and a boronic acid using a metal catalyst. It is a cross-coupling reaction. This reaction was invented by and named after Jiri Srogl from the Academy of Sciences, Czech Republic, and Lanny S. Liebeskind from Emory University, Atlanta, Georgia, USA. There are three generations of this reaction, with the first generation shown below. The original transformation used catalytic Pd(0), TFP = tris(2-furyl)phosphine as an additional ligand and stoichiometric CuTC = copper(I) thiophene-2-carboxylate as a co-metal catalyst. The overall reaction scheme is shown below.

In organic chemistry, a cross-coupling reaction is a reaction where two different fragments are joined. Cross-couplings are a subset of the more general coupling reactions. Often cross-coupling reactions require metal catalysts. One important reaction type is this:

Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands. Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations of CO2. This research is relevant both to organic synthesis and to the production of "solar fuels" that would avoid the use of petroleum-based fuels.

An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

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 and are therefore useful reactions for carbon–hydrogen bond activation. Metal-catalyzed C–H borylation reactions utilize transition metals to directly convert a C–H bond into a C–B bond. This route can be advantageous compared to traditional borylation reactions by making use of cheap and abundant hydrocarbon starting material, limiting prefunctionalized organic compounds, reducing toxic byproducts, and streamlining the synthesis of biologically important molecules. Boronic acids, and boronic esters are common boryl groups incorporated into organic molecules through borylation reactions. Boronic acids are trivalent boron-containing organic compounds that possess one alkyl substituent and two hydroxyl groups. Similarly, boronic esters possess one alkyl substituent and two ester groups. Boronic acids and esters are classified depending on the type of carbon group (R) directly bonded to boron, for example alkyl-, alkenyl-, alkynyl-, and aryl-boronic esters. The most common type of starting materials that incorporate boronic esters into organic compounds for transition metal catalyzed borylation reactions have the general formula (RO)2B-B(OR)2. For example, bis(pinacolato)diboron (B2Pin2), and bis(catecholato)diborane (B2Cat2) are common boron sources of this general formula.

<span class="mw-page-title-main">Catellani reaction</span> Chemical reaction

The Catellani reaction was discovered by Marta Catellani and co-workers in 1997. The reaction uses aryl iodides to perform bi- or tri-functionalization, including C-H functionalization of the unsubstituted ortho position(s), followed a terminating cross-coupling reaction at the ipso position. This cross-coupling cascade reaction depends on the ortho-directing transient mediator, norbornene.

<span class="mw-page-title-main">Protodeboronation</span>

Protodeboronation, or protodeborylation is a chemical reaction involving the protonolysis of a boronic acid in which a carbon-boron bond is broken and replaced with a carbon-hydrogen bond. Protodeboronation is a well-known undesired side reaction, and frequently associated with metal-catalysed coupling reactions that utilise boronic acids. For a given boronic acid, the propensity to undergo protodeboronation is highly variable and dependent on various factors, such as the reaction conditions employed and the organic substituent of the boronic acid.

Miyaura borylation, also known as the Miyaura borylation reaction, is a named reaction in organic chemistry that allows for the generation of boronates from vinyl or aryl halides with the cross-coupling of bis(pinacolato)diboron in basic conditions with a catalyst such as PdCl2(dppf). The resulting borylated products can be used as coupling partners for the Suzuki reaction.

Norio Miyaura was a Japanese organic chemist. He was a professor of graduate chemical engineering at Hokkaido University. His major accomplishments surrounded his work in cross-coupling reactions / conjugate addition reactions of organoboronic acids and addition / coupling reactions of diborons and boranes. He is also the co-author of Cross-Coupling Reactions: A Practical Guide with M. Nomura E. S.. Miyaura was a world-known and accomplished researcher by the time he retired and so, in 2007, he won the Japan Chemical Society Award.

Organogermanium compounds in cross-coupling reactions refers to a type of cross-coupling reaction where one of the coupling partners is an organogermanium compound. Usually these reactions are catalyzed by transition metal complexes.

References

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  4. ↑ Holyoak, Todd; Wilson, Mark A.; Fenn, Timothy D.; Kettner, Charles A.; Petsko, Gregory A.; Fuller, Robert S.; Ringe, Dagmar (10 June 2003). "2.4 A resolution crystal structure of the prototypical hormone-processing protease Kex2 in complex with an Ala-Lys-Arg boronic acid inhibitor". Biochemistry. 42 (22): 6709–6718. doi:10.1021/bi034434t. ISSN   0006-2960. PMID   12779325.
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  15. ↑ Molander, Gary A.; Trice, Sarah L. J.; Dreher, Spencer D. (2010). "Palladium-Catalyzed, Direct Boronic Acid Synthesis from Aryl Chlorides: A Simplified Route to Diverse Boronate Ester Derivatives". Journal of the American Chemical Society. 132 (50): 17701–17703. doi:10.1021/ja1089759. PMC   3075417 . PMID   21105666.
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  34. ↑ Rowan, Stuart J.; Cantrill, Stuart J.; Cousins, Graham R. L.; Sanders, Jeremy K. M.; Stoddart, J. Fraser (2002). "Dynamic Covalent Chemistry". Angewandte Chemie International Edition. 41 (6): 898–952. doi:10.1002/1521-3773(20020315)41:6<898::AID-ANIE898>3.0.CO;2-E. PMID   12491278.
  35. ↑ US 6850786,Wayne Front March,"Ocular analyte sensor",issued 2005-02-01
  36. ↑ Hansen, Marvin M.; Jolly, Robert A.; Linder, Ryan J. (29 July 2015). "Boronic Acids and Derivatives—Probing the Structure–Activity Relationships for Mutagenicity". Organic Process Research & Development. 19 (11): 1507–1516. doi:10.1021/acs.oprd.5b00150. ISSN   1083-6160.
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