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

<P>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] </P><P>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] </P><P>DIBAL-H is commercially available pure or in solution of alkanes, ethers, dichloromethane, toluene, etc. </P><P>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).</P>

Abstracts

<TD VALIGN="TOP"> 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. </TD><TD VALIGN="TOP"> Table

</TD><TD VALIGN="TOP"> 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. </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> 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. </TD><TD VALIGN="TOP">

</TD>

References

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<A NAME="RV0350301TX-8">8</A> Wipf P. In Comprehensive Organic Synthesis   Vol. 5:  Trost BM. Pergamon Press; Oxford: 1991.  p.827 

<A NAME="RV0350301TX-9">9</A> Chakraborty A. Marek I. Chem. Commun.  1999,  2375