Alpine borane explained

Alpine borane is the commercial name for an organoboron compound that is used in organic synthesis. It is a colorless liquid, although it is usually encountered as a solution. A range of alkyl-substituted borane are specialty reagents in organic synthesis. Two such reagents that are closely related to Alpine borane are 9-BBN and diisopinocampheylborane.

Preparation and reactions

This reagent is generated by treating 9-BBN with α-pinene.^[1]

This sterically crowded chiral trialkylborane can stereoselectively reduce aldehydes in what is known as the Midland Alpine borane reduction, or simply the Midland reduction.^[2]

C₈H₁₂B-pinanyl + RCDO → C₈H₁₂BOCHDR + (+)-d-pinene

Hydrolysis of the resulting borinic ester affords the alcohol:

C₈H₁₂BOCHDR + H₂O → C₈H₁₂BOH + HOCHDR

It is also effective for the stereoselective reduction of certain acetylenic ketones.^[3] The reaction is proposed to involve formation of an adduct by coordination of the carbonyl oxygen to boron. Intramolecular hydride transfer from the pinane substituent to the carbonyl carbon ensues. Many substrates for the Midland reduction have a low steric group such as an alkyne^[4] or a nitrile^[5] so as to increase selectivity. Stereochemical control comes from coordination of the carbonyl bulky borane, followed by hydride transfer opposite the largest group.^{[1] [6]}

See also

• Corey-Itsuno reduction
• Asymmetric hydrogenation

Notes and References

1. Asymmetric reductions with organoborane reagents . M. M. Midland . Chemical Reviews . 1989 . 89 . 7 . 1553–1561 . 10.1021/cr00097a010.
2. M. Mark Midland "B-3-Pinanyl-9-borabicyclo[3.3.1]nonane" in Encyclopedia of Reagents for Organic Synthesis 2001 John Wiley, New York.. Article Online Posting Date: April 15, 2001
3. 1985. M. Mark. Midland. Richard S.. Graham . Asymmetric Reduction of α,β-Acetylenic Ketones with B-3-Pinanyl-9-Borabicyclo[3.3.1]nonane: (R)-(+)-1-Octyln-3-ol . 63 . 57 . 10.15227/orgsyn.063.0057. Organic Syntheses.
4. Intramolecular Arene-Alkyne Photocycloaddition M. C. Pirrung J. Org. Chem.; 1987; 52(8); pp 1635 - 1637;
5. 10.1021/jo00217a053 . Efficient Asymmetric Reduction of Acyl Cyanides with B-3-Pinanyl 9-BBN (Alpine-Borane) . 1985 . Midland . M. Mark . Lee . Penny E. . The Journal of Organic Chemistry . 50 . 17 . 3237–3239 .
6. Book: Li, J. J. . Name Reactions, A Collection of Detailed Mechanisms and Synthetic Applications . limited . Springer . 2009. 4th . New York, New York . 359–360 . 978-3-642-01052-1 .