Boronic acid

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The general structure of a boronic acid, where R is a substituent.
The general structure of a boronic acid, where R is a substituent.

A boronic acid is an alkyl or aryl substituted boric acid containing a carbon to boron chemical bond belonging to the larger class of organoboranes. Boronic acids act as Lewis acids. Their unique feature are 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 upon complexion in aqueous solutions, they 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


Contents

[edit] Boronic acids

Many air-stable boronic acids are commercially available. They are characterised by high melting points.

Boronic acid R Molar mass CAS number Melting point °C
Phenylboronic acid Phenyl Phenylboronic acid 121.93 98-80-6 216-219
2-Thienylboronic acid Thiophene 2-thienylboronic acid 127.96 6165-68-0 138 -140
Methylboronic acid Methyl methylboronic acid 59.86 13061-96-6 59.86
cis-Propenylboronic acid propene cis-propenylboronic acid 85.90 7547-96-8 65-70
trans-Propenylboronic acid propene trans-propenylboronic acid 85.90 7547-97-9 123-127
Representative boronic acids [1]


[edit] Borinic acids & esters

Borinic acids and borinic esters have the general structure R2BOR.

[edit] Boronic esters

When hydrogen is replaced by any organic residue the resulting compound is called a boronic ester or boronate ester. The compounds can be obtained from boric esters [2] Phenylboronic acid can be condensed to the cyclic trimer called triphenyl anhydride or triphenylboroxin [3]

[edit] Boronate or borate salts

Boronate salts or borate salts (not encouraged) have the general structure R4B-M+ for example potassium tetraphenylborate.


[edit] Boronic acids in organic chemistry

  • 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 a metal.
  • 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 a such a conjugate addition [4]:
Boronic acids in conjugate addition
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 [5]:
Gramine reaction with phenylboronic acid
Boronic ester homologization Homologization application
Boronic ester homologization mechanism Homologization application
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 a RCH2 group into the C-B bond.
Double allylation reagent based on boronic ester

[edit] See also

[edit] References

  1. ^ www.sigmaaldrich.com
  2. ^ Organic Syntheses, Coll. Vol. 5, p.918 (1973); Vol. 49, p.90 (1969). Link
  3. ^ Organic Syntheses, Coll. Vol. 4, p.68 (1963); Vol. 39, p.3 (1959). Link
  4. ^ Catalytic Conjugate Addition of Allyl Groups to Styryl-Activated Enones Joshua D. Sieber, Shubin Liu, and James P. Morken J. Am. Chem. Soc.; 2007; 129(8) pp 2214 - 2215; (Communication) DOI:10.1021/ja067878w
  5. ^ Benzylic Substitution of Gramines with Boronic Acids and Rhodium or Iridium Catalysts Gabriela de la Herrán, Amaya Segura, and Aurelio G. Csák Org. Lett.; 2007; 9(6) pp 961 - 964; (Letter) DOI:10.1021/ol063042m
  6. ^ 99% Chirally selective synthesis via pinanediol boronic esters: insect pheromones, diols, and an amino alcohol Donald S. Matteson, Kizhakethil Mathew Sadhu, and Mark L. Peterson J. Am. Chem. Soc.; 1986; 108(4); pp 810 - 819; DOI:10.1021/ja00264a039
  7. ^ Simple, Stable, and Versatile Double-Allylation Reagents for the Stereoselective Preparation of Skeletally Diverse Compounds Feng Peng and Dennis G. Hall J. Am. Chem. Soc.; 2007; 129(11) pp 3070 - 3071; (Communication) DOI:10.1021/ja068985t
  8. ^ 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
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