Diisobutylaluminun Hydride (DIBAL-H)

Biographical Sketches

Mª Isabel Fernández-Bachiller studied chemistry at the Universidad Complutense of Madrid (1995-2000), where she was involved in undergraduate research on molecular modelling with Dr. M. J. R. Yunta. She is currently working on her Ph.D. thesis about Alz-heimer"s disease in the Medicinal Chemistry Institute (CSIC), with Dr. M. I. Rodríguez-Franco.

Introduction

Among reducing agents, diisobutylaluminun hydride (DIBAL-H) stands out, having been extensively used for a broad variety of reductive transformations in organic chemistry. It is easy to use and reduces many functional groups under mild reaction conditions. [1] Aldehydes, ketones, acids, esters, acid chlorides, amides, nitriles, isocyanates, nitro compounds, and disulfides are examples of suitable substrates, whereas alkyl halides are unreactive towards DIBAL-H. Usually, reactions are strongly solvent and/or temperature dependent and as an example, sulfides, sulfones, and sulfonic acids are unreactive in toluene at 0 ºC. [2]

Along with the reductive capability of DIBAL-H, its Lewis acid properties extend the scope of transformations, and the combination of these two properties has been used in the stereoselective synthesis of valuable products in chemistry, like pharmaceuticals. [3]

DIBAL-H is commercially available pure or in solution of alkanes, ethers, dichloromethane, toluene, etc.

Precautions: Neat DIBAL-H is a pyrophoric liquid and its solutions react violently with water, oxygen and related compounds. It is necessary to work in a fume hood, using anhydrous solvents, under inert atmosphere (argon or nitrogen).

Abstracts

A) Considerable attention has been paid to the application of DIBAL-H, alone or in the presence of ZnCl2 in asymmetric synthesis for the steroeselective reduction of hydroxyketones, [4] aminoketones, [5] and β-ketosulfoxides. [6] These reactions yield preferably 1,3-syn diols (Table [1] ), through a six-membered ring complex between the Al or Zn atom and the 1,3-difunctional substrate, whose keto group then undergoes hydride attack at the less sterically hindered face, due to the large volume occupied by DIBAL-H. In general, these reactions are solvent dependent, and the best results have been found in THF.	Table
B) Sharma et al [7] have used the Lewis acid character of DIBAL-H to effect Claisen rearrangements, a valuable synthetic tool for the formation of carbon-carbon bonds, [8] in allyl aryl ethers (4). DIBAL-H was found to be very effective for exclusive migration to give ortho Claisen product (5) under mild reaction conditions, although the formation of compound 6 is also observed in small quantities (13:1), through a hydro-alumination and hydrolysis reactions.
C) The combination of DIBAL-H/THF in a 1:1 ratio provides an efficient method for iodine transfer cyclizations for the formation of five-membered rings. [9] Different iodoalkynylacetals 7 (X = O, R2 = OBu) and iodoalkynes 7 (X = CH2 R2 = H) are employed, to obtain the corresponding tetrahydrofuran (X = O) or carbocycle (X = CH2) 8 in good yield (60-90%). The cyclization proceeds by a radical pathway and no trace of the fragmentation of the starting material into the propargylic alcohol or enol ether is observed. In all cases, the two isomers Z, E for the vinyl iodides were obtained.

References

• 1 Yoon NM. Gyoung YS. J. Org. Chem.  1985,  50:  2443 
• 2 Galatsis P. In Handbook of Reagents for Organic Synthesis. Oxidizing and Reducing Agents   Burke SD. Danheiser RL. Wiley; Chichester UK: 1999.  p.143 
• 3 Heitsch H. König WA. Decker H. Bormann C. Fiedler H.-P. Zähner H. J. Antibiotics  1989,  42:  711 
• 4 Kiyooka S.-I. Kuroda H. Shimasaki Y. Tetrahedron Lett.  1986,  27:  3009 
• 5 Barluenga J. Aguilar E. Fustero S. Olano B. Viado AL. J. Org. Chem.  1992,  57:  1219 
• 6 Solladié G. Demailly G. Greck C. Tetrahedron Lett.  1985,  26:  435 
• 7 Sharma GVM. Ilangovan A. Sreenivas P. Mahalingam AK. Synlett  2000,  5:  615 
• 8 Wipf P. In Comprehensive Organic Synthesis   Vol. 5:  Trost BM. Pergamon Press; Oxford: 1991.  p.827 
• 9 Chakraborty A. Marek I. Chem. Commun.  1999,  2375