Lithium triethylborohydride is the organoboron compound with the formula LiEt<sub>3</sub>BH. Commonly referred to as LiTEBH or Superhydride, it is a powerful reducing agent used in organometallic and organic chemistry. It is a colorless or white liquid but is typically marketed and used as a THF solution. The related reducing agent sodium triethylborohydride is commercially available as toluene solutions.
LiBHEt<sub>3</sub> is a stronger reducing agent than lithium borohydride and lithium aluminium hydride.
LiBHEt<sub>3</sub> is prepared by the reaction of lithium hydride (LiH) and triethylborane (Et<sub>3</sub>B) in tetrahydrofuran (THF):
The resulting THF complex is stable indefinitely in the absence of moisture and air.
Alkyl halides are reduced to the alkanes by LiBHEt<sub>3</sub>.
LiBHEt<sub>3</sub> reduces a wide range of functional groups, but so do many other hydride reagents. Instead, LiBHEt<sub>3</sub> is reserved for difficult substrates, such as sterically hindered carbonyls, as illustrated by reduction of 2,2,4,4-tetramethyl-3-pentanone. Otherwise, it reduces acid anhydrides to alcohols and the carboxylic acid, not to the diol. Similarly lactones reduce to diols. ñ,ò-Enones undergo 1,4-addition to give lithium enolates. Disulfides reduce to thiols (via thiolates). LiBHEt<sub>3</sub> deprotonates carboxylic acids, but does not reduce the resulting lithium carboxylates. For similar reasons, epoxides undergo ring-opening upon treatment with LiBHEt<sub>3</sub> to give the alcohol. With unsymmetrical epoxides, the reaction can proceed with high regio- and stereo- selectivity, favoring attack at the least hindered position:
Acetals and ketals are not reduced by LiBHEt<sub>3</sub>. It can be used in the reductive cleavage of mesylates and tosylates. LiBHEt<sub>3</sub> can selectively deprotect tertiary N-acyl groups without affecting secondary amide functionality. It has also been shown to reduce aromatic esters to the corresponding alcohols as shown in eq 6 and 7.
LiBHEt<sub>3</sub> also reduces pyridine and isoquinolines to piperidines and tetrahydroisoquinolines respectively. <br />The reduction of ò-hydroxysulfinyl imines with catecholborane and LiBHEt<sub>3</sub> produces anti-1,3-amino alcohols shown in (8).
LiBHEt<sub>3</sub> reacts exothermically, potentially violently, with water, alcohols, and acids, releasing hydrogen and the pyrophoric triethylborane.