Borinic acid

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Borinic acid contains OH
Generic borinate ester

Borinic acid H
2
BOH
, also known as boronous acid, is an oxyacid anion of boron with formula H
2
BOH
. Borinate is the associated anion of boron with formula H
2
BO
, however being a lewis acid the form in basic solution is H
2
B(OH)
2
.

Borinic acid can be formed as the first step in the hydrolysis of diborane.[1] BH3+H2O H2BOH + H2 Borinic acid itself is unstable and only lasts for a few seconds during the hydrolysis reaction. However by using microwave spectroscopy various properties can be determined. The B-O distance is 1.352 Å, O-H distance 0.96 Å, B-H length is probably 1.2 Å. The angle between bonds at the oxygen atom BOH = 112° and the angles at boron are cis-HBO 121°, and trans-HBO = 117°. The dipole moment is 1.506 Debye.[2]

Borinic acid can form esters such as methoxyborane. This too is unstable only lasting about ten seconds. It can be formed by heating diborane and methanol gas together.[3]

By substituting organic components instead of hydrogen, more generic borinic acids (containing RR'BOH) or borinic esters (RR'BOR") can be formed. Esters will tend to be stable in acidic conditions, but in alkaline conditions the boron atom can gain a negative charge and attach two hydroxyl groups, or two ester bonds. RR'B-(OH)2 or RR'B-(OR")2. The anionic borinate ion can very easily form esters with diols such as ethylene glycol or sugars.[4]

Naming[edit]

IUPAC naming for Borinic acid is a unique name for acid.[5] The anhydrides are named diboroxanes, H2BOBH2, as the base compound and H being able to be substituted, e.g. tetraethyldiboroxane, as the anhydride for diethylborinic acid. Organic naming standard in the bluebook allows skeletal replacement naming where the name is shorter, 3-borapentan-3-ol versus diethyl borinic acid. The grouping -BH-O-BH2 is called diboroxanyl. Substituting sulfur for oxygen gives borinothioic acid (H2BSH). (dimethylboranyl)oxy is used for the group (CH3)2B-O− and methyl(hydroxy)boranyl for the grouping CH3B(OH)-.

Formation[edit]

There are several ways to produce substituted borinic acids.[6]

Firstly borinic acids can be made from oxidising trialkyl borane starting materials [R3B] with exposure to moist air, or treatment with iodine, which makes a dialkyliodoborane [R2BI]. Hydrolysis then results in the boronic acid (R2BOH).[6] Trialkylborates [(RO)3B] or trialkoxyboroxine [(ROBO)3] can be reduced to borinic acid by us of a Grignard reagent. Grignard reagents can also reduce a boronic ester [RB(OR')2] to a borinic ester.[6]

Bu3B + N2CHCOR → BuCH=C(R)OBBu2

Bu3B + CH2=CHCOCH3 → BuCH2CH=C(CH3)OBBu2

RCOC2H5 + R2BOTf → RC(OBR2)=CHCH3

(Tf = Trifluoromethanesulfonate)

[Z] enolate gives syn aldol when reacted with aldehyde, where as [E] enolate gives and anti aldol

Dialkyl boron chloride (R2BCL) with tertiary amine react with ketones to form an enol borinate.[7]

A trialkoxyborane can react with lithium containing organic molecules to eliminate lithium and one or two alkoxy groups to make boronic and borinic esters.[8]

Purification of the mixtures that result from the reactions is required, as often boronic esters will also be produced and mixed in with the borinic esters.[6] The method of Letsinger is dissolve the mixture in ether and precipitate the borinic ester by forming a complex with ammonia. Treatment with ethanolamine ends up making an aminoetylborinate.[6]

Compounds[edit]

R2BOR' borinic acid R'=H anhydride esters R'
R hydrogen -O- aminoethyl ethyl n-propyl n-butyl 3-methylpropyl 1-methylpropyl phenyl ethylene glycol methyl 8-quinolinyl
phenyl YesY [9] YesY cas 524-95-8 YesY cas 43185-52-0 YesY YesY cas 15323-04-3 YesY YesY YesY YesY cas 13471-36-8 SID 535455
o-tolyl YesY YesY
m-tolyl YesY
p-tolyl YesY[10] YesY
p-anisyl YesY YesY[11] YesY
p-biphenyl YesY YesY[12] YesY
p-chlorophenyl YesY cas 89566-59-6 YesY[13] YesY cas 61733-90-2 YesY cas 564483-61-0
3-chlorophenyl YesY cas 433338-06-8
α-naphthyl YesY YesY[14] YesY[15] cas 6962-88-5
β-naphthyl YesY YesY[11]
p-bromophenyl YesY YesY[11] YesY[16]
2-methyl-5-chlorophenyl YesY YesY[13]
2-thienyl [17] SID 3881207 SID 8142470
mesityl sid 4278417 CAS 20631-84-9[18][19][20]
methyl YesY cas 13061-97-7 YesY cas 86610-16-4 YesY cas 4443-43-0
ethyl YesY cas 4426-31-7 YesY[21] 7318-84-5 YesY cas 7397-46-8
allyl YesY
n-butyl YesY cas 1189-31-7 YesY[22] cas 19324-14-2 YesY cas 2344-21-0 YesY YesY YesY YesY YesY YesY cas 2344-21-0
4-methylbutyl YesY
2-chlorovinyl YesY YesY[23]
3,5-dimethylphenyl YesY[24]
propyl YesY cas 53678-60-7 YesY cas 2938-89-8
1-methylpropyl cas 4026-69-1
2-methylpropyl cas 4026-82-8

2-APB[edit]

2-aminoethyl-diphenylborinate

2-aminoethyl-diphenylborinate also known as 2-APB, inhibits transient receptor potential channels.[25] This kind of inhibition is being researched to find treatments for prostate cancer. In particular TRPM7. 2-APB can work as a catalyst to add an alkyl group from an alkyl halide to a polyol or carbohydrate that contains a cis-vicinal diol to a precise position. It does this by first combining with the two hydroxy groups to make a ring containing OCCOB-.[26] It can also calatlyse acid chloride or chloroformate reaction a specific region of the diol.[27]

Diphenylborinic acid[edit]

Diphenylborinic acid was discovered in 1894 by Michaelis who produced it by hydrolysing the chloride. Letsinger determined its properties in 1955.[6]

Diphenylborinic acid has an extra high affinity for catechols compared with carbohydrates[28]

Diphenylborinic acid can catalyse the condensation of pyruvic acids with aldehydes to yield substituted isotetronic acid.[29]

Diphenylborinic acid is an inhibitor of several enzymes such as α-chymotrypsin, subtilisin BPN' and trypsin.[30]

Borinate radical[edit]

Borinate radicals (RR'BO·) can be formed formed from peroxyborinate decomposition.[31]

Other compounds[edit]

Other compounds include methoxy(dimethyl)borane, methoxy(methyl)boron, methoxy(methylidene)borane (with a C=B double bond).[32]

[C5H5BR]- uses a B- to be equivalent to carbon in an aromatic benzene like ring. This too is called borinate. The 1-methyl and 1-phenyl borinates can form some of the few organo-thallium(I) compounds.[33]

HB(C6F5)2 + phosphino alcohol → tBu2P+HCH2C(CH3)2OB-H(C6F5)2 → H2 + tBu2PCH2C(CH3)2OB(C6F5)2 and same for tBu2PCH2C(CF3)2OB(C6F5)2[34]

di-Tris(tert-butoxy)siloxy borinic acid HOB[OSi(O(t)Bu)3]2 can be made from tributoxyborate and tributoxysiloxane. It can form a very complex crystal with Cp2Zr(Me)[OB[OSi(O(t)Bu)3]2]2.[35]

Diborinic acids have two RBOH groups linked together by an organic connection such as diphenyl or phenyl.[36]

Applications[edit]

1,1,1,3,3,3-Hexafluoroisopropylbis(pentafluorophenyl)borinate can greatly increase solubility of LiF by complexing the F- anion.[37] This has potential to improve lithium batteries.

Borinic esters are being researched as bacterial growth inhibitors[38] due to their ability to disable some bacterial enzymes such as menaquinone methyltransferase and CcrM.[39] This may result in development of treatments for topical application on skin.[40]

References[edit]

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  39. ^ Benkovic, Stephen J.; Baker, Stephen J.; Alley, M. R. K.; Woo, Youn-Hi; Zhang, Yong-Kang; Akama, Tsutomu; Mao, Weimin; Baboval, Justin; Rajagopalan, P. T. Ravi; Wall, Mark; Kahng, Lyn Sue; Tavassoli, Ali; Shapiro, Lucy (1 November 2005). "Identification of Borinic Esters as Inhibitors of Bacterial Cell Growth and Bacterial Methyltransferases, CcrM and MenH". Journal of Medicinal Chemistry 48 (23): 7468–7476. doi:10.1021/jm050676a. 
  40. ^ Baker, SJ; Akama, T; Zhang, YK; Sauro, V; Pandit, C; Singh, R; Kully, M; Khan, J; Plattner, JJ; Benkovic, SJ; Lee, V; Maples, KR (December 1, 2006). "Identification of a novel boron-containing antibacterial agent (AN0128) with anti-inflammatory activity, for the potential treatment of cutaneous diseases.". Bioorganic & Medicinal Chemistry Letters 16 (23): 5963–7. doi:10.1016/j.bmcl.2006.08.130. PMID 16997550. 

Extra reading[edit]

  • Dimitrijević, Elena; Taylor, Mark S. (3 May 2013). "Organoboron Acids and Their Derivatives as Catalysts for Organic Synthesis". ACS Catalysis 3 (5): 945–962. doi:10.1021/cs4000848.  Review of use borinic acids as a catalyst
  • Roth, HJ; Miller, B (September 1964). "Synthesis of Phenyl- and Thienyl-Substituted Boronic and Borinic Acids.". Archiv der Pharmazie 297: 513–523. PMID 14341914.