Catecholborane

Catecholborane (abbreviated HBcat) is an organoboron compound that is useful in organic synthesis. This colourless liquid is a derivative of catechol and a borane, having the formula C 6 H 4 O 2 BH.

Catecholborane (abbreviated HBcat) is an organoboron compound that is useful in organic synthesis. This colourless liquid is a derivative of catechol and a borane, having the formula C 6 H 4 O 2 BH.

Synthesis and structure

Traditionally catecholborane is produced by treating catechol with borane (BH 3 ) in a cooled solution of THF. However, this method results in a loss of 2 mole equivalents of the hydride. Nöth and Männig described the reaction of alkali-metal boron hydride (LiBH 4 , NaBH 4 , KBH 4 ) with tris(catecholato)bisborane in an ethereal solvent such as diethyl ether. In 2001, Herbert Brown and coworkers prepared catecholborane by treatment of tri-o-phenylene bis-borate with diborane. Unlike borane itself or alkylboranes, catechol borane exists as a monomer. This behavior is a consequence of the electronic influence of the aryloxy groups that diminish the Lewis acidity of the boron centre. Pinacolborane adopts a similar structure.

Reactions

Catecholborane is less reactive in hydroborations than borane-THF or borane-dimethylsulfide. When catecholborane is treated with a terminal alkyne, a trans vinylborane is formed: C 6 H 4 O 2 BH + HC 2 R → C 6 H 4 O 2 B-CHCHR The product is a precursor to the Suzuki reaction and is the only borane which stops at the alkene instead of reacting further to the alkane. Catecholborane may be used as a stereoselective reducing agent when converting β-hydroxy ketones to syn 1,3-diols. Catecholborane oxidatively adds to low valent metal complexes, affording boryl complexes. C 6 H 4 O 2 BH + Pt(PR 3 ) 2 → (C 6 H 4 O 2 B)Pt(PR 3 ) 2 H

References

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