Sodium Triacetoxyborohydride

Biographical Sketches

Lakhinath Saikia was born in 1983 in Jorhat, Assam, India. He received his B.Sc. degree in Chemistry from the Dibrugarh University, Assam in 2004, and his M.Sc. degree in Organic Chemistry from the same university in 2006. Presently, he is working towards his Ph. D. degree under the supervision of Dr. A. J. Thakur, Associate Professor, Department of Chemical Sciences, Tezpur University. His current research interests focus on radical cyclizations for the synthesis of pyrimidine derivatives.

Introduction

Sodium triacetoxyborohydride [Na(OAc)₃BH, abbreviated as STAB-H] is a versatile reagent in organic synthesis. In addition to its superior ability in effecting reductive amination of aldehydes and ketones, it can reduce N-heterocycles (indoles, quinolines, and isoquinolines), imines, enamines, oximes, amides, aryl ketones, acetals, and other substrates. [¹]

STAB-H is a milder and more selective reducing agent than NaBH₄. The mild nature of STAB-H may be attributed both to the bulky nature of the reagent and to the inductive electron-withdrawing ability of the three acetoxy groups which stabilize the boron-hydrogen bond. [²] It has more advantages over Na(CN)BH₃ also due to the lack of toxicity.

The preparation of triacetoxyborohydride was first performed by Wartik and Pearson through the reaction of NaBH₄ and CO₂ (Scheme  [¹] ). [³] Furthermore, it can be also generated in situ from NaBH₄ and acetic acid. Aldehydes, but not ketones, are smoothly reduced to alcohols with STAB-H, prepared from sodium borohydride and acetic acid in benzene [4] or in N,N-dimethylacetamide. [5]

Scheme 1

Abstracts

(A) Boros et al. recently reported the synthesis of diazaindoline 3, where the key step involved rapid reductive amination of aldehyde 1 with aniline 2 by sodium triacetoxyborohydride (STAB-H) and TFA. [6]
(B) A tandem reductive amination-lactamization strategy using STAB-H, 1-benzyl-4-piperidone (4) and γ- or δ-amino esters or ­acids resulted in lactam 5 [7]
(C) Dialdehyde 6 can be converted into amine 7 with STAB-H and benzylamine, whereas lactol 8 likewise affords lactam 9 under similar conditions. [8] [9]
(D) As mentioned already, STAB-H is a milder reducing agent than NaBH4 and hence selective. It can reduce vinylogus carbamate 10 selectively without affecting other functionalities. [¹0]
(E) The reduction of an imine can be effected by STAB-H. The reduced form of the quarter pyrroles 13 can be obtained by using STAB-H from the stable oxidized form 12. [¹¹]
(F) It is shown that STAB-H effectively reduced amide 14 to 15 in the total synthesis of a selective D1 antagonist useful in the treatment of psychoses, depression, and D1-dependent neurological disorders. [¹²]
(G) Cleavage of oxazolidines 16 can easily be carried out with STAB-H [¹³] to provide 17 and this tactic was featured in the first enantiospecific synthesis of salinosporamide A. [¹4]

References

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