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Synlett 2007(6): 0995-0996  
DOI: 10.1055/s-2007-973864
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© Georg Thieme Verlag Stuttgart · New York

Lithium Amino Borohydrides (LABs)

Partha Pratim Saikia*
Natural Products Chemistry Division, Regional Research ­Laboratory, Jorhat 785006, India
e-Mail: ppsaikia@yahoo.co.in;

Further Information

Publication History

Publication Date:
26 March 2007 (online)

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Biographical Sketches

Partha Pratim Saikia was born in Sivasagar, Asom (India) in 1980. He received his B.Sc. (2002) from Dibrugarh University and M.Sc. (2004) from Gauhati University, Asom (India) with specialization in Organic Chemistry. At present he is working towards his Ph.D. at the Natural Products Chemistry Division, Regional Research Laboratory (CSIR), Jorhat (India) under the guidance of Dr. Nabin C. Barua. His research interests include stereoselective total synthesis of natural products of biological significance and development of new synthetic methodologies for target-oriented synthesis.

Introduction

Lithium amino borohydride reagents (LABs) are a new class of powerful and selective reagents that were first ­introduced by Singaram et al. in 1992. [1a] The reactivity of these reagents is comparable to lithium aluminium ­hydride. However, they have several advantages over ­lithium aluminium hydride, e.g., they are air-stable, non-pyrophoric, thermally stable and hydrolyse only slowly in protic solvents above pH 4. Thus LABs can perform in air virtually all of the transformations for which LAH is commonly used and offer significant advantages in safety, ­selectivity, and ease of handling and simple work-up ­procedures. In short, LAB is an attractive alternative to LAH or super hydride reduction.

LABs can be prepared as solids or 1-2 M THF solutions, or they can be generated in situ for immediate use by the reaction of n-BuLi or MeLi with amine-borane complexes to the corresponding LABs. [1] LABs can be prepared from any primary or secondary amine, thus allowing ­precise steric and electronic control of their reactivity by modulation of the substituents on the nitrogen atom.

In 1995, Kagan and co-workers reported the only chiral LAB reagent. However, the reduction of ketones using this reagent afforded the corresponding alcohols in low ee (5-9%). [2]

Abstracts

(A) The LAB reagents are capable of reducing a wide range of functional groups, which are summarized in the following scheme. [1d]

(B) In addition to hydride transfer, LABs can transfer the amine moiety. Under mild reaction conditions, they promote the amination of 2-halopyridine (X = F, Cl, Br), providing the 2-(dialkyl­amino)pyridines in excellent yield and purity. [3]

(C) The ability of LAB reagents to function as both reducing agent and amination reagent allows the performance of tandem amination-reduction reactions. [4]

(D) Braslau et al. reported chemoselective reduction of esters using LAB to afford the corresponding N-alkoxyamine alcohols, which are used as initiators in ‘living’ free radical polymerizations. [5]

(E) LAB is now widely used for the removal of Evan’s chiral ­auxiliary and reduction of amides to the corresponding alcohols. Theodorakis and co-workers employed this reaction to reduce an amide to the corresponding alcohol in the total synthesis of ­borelledin. [6]

(F) Sessler et al. reported the synthesis of a new set of dipyr­rolylpyrazines utilizing lithium pyrrolidino borohydride. Pyrazine oligomers are anion receptors for various biologically important anions. [7]

(G) At low temperature, LABs react with halides to afford the ­corresponding amines. [8] Lodeiro and co-workers extended this chemistry to the synthesis of a new photoinduced electron-transfer (PET) system. [9]

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