Triethylborane (Et₃B)

Introduction

Triethylborane is a colourless liquid that is spontaneously flammable in air, burning with a characteristic green flame. It is readily autoxidised by molecular oxygen via a radical mechanism (Scheme 1).

Scheme 1

This reactivity of triethylborane leads to one of its most common applications, i.e. as an initiator in radical reactions.

The first use of triethylborane as a radical initiator was reported in 1989 by Oshima. [1] Its main advantage is its effectiveness at low temperature (-78 °C), which is useful in the case of stereoselective radical reactions [2] [3] or with thermally unstable reaction products. The use of AIBN, which forms radicals by thermal decomposition, is clearly not feasible in these cases, making triethylborane an attractive alternative. The use of organoboranes as a source of radicals has been recently reviewed. [4] It has been extensively used in the development of tin-free radical chemistry, bypassing the tedious purification procedures often associated with the use of tin reagents. [5-7] It has also been used in solid-supported radical C-C bond formation. [8] Triethylbor­ane has also been used in a novel one-pot cross-coupling of alkynes with aryl iodides to synthesise functionalised (Z)-arylalkenes from unprotected alkynes via a (Z)-alkenylindium species. [9] Radical initiator-dependent reactivity has been noted in the three-component reaction of aldehydes, aryl amines and THF. [10] Triethylborane can also be used as a radical initiator in both ionic liquids [11] and in aqueous systems. [8] [12]

Triethylborane has also found application in non-radical processes, such as promoting the palladium-catalysed allylation of active methylene compounds [13] and amines [14] with unactivated allylic alcohols and the triethylborane-triggered intermolecular domino three-component ­Michael-aldol reaction. [15]

It has been noted, especially for radical reactions, that the quality of the triethylborane used is critical. In general, freshly opened bottles of commercial solutions or solutions freshly prepared from pure material give the best ­results. [4] [16]

References

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• 13 Kimura M. Mukai R. Tanigawa N. Tanaka S. Tamaru Y. Tetrahedron  2003,  59:  7767 
• 14 Kimura M. Futamata M. Shibata K. Tamaru Y. Chem. Commun.  2003,  234 
• 15 Chandrasekhar S. Narsihmulu C. Reddy NR. Reddy MS. Tetrahedron Lett.  2003,  44:  2583 
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