[22560-21-0] · C6H16BK · Potassium Triethylborohydride · (MW 138.13)
(reducing agent in organic and organometallic synthesis)
Physical Data: mp 95 °C,1 d
Solubility: sol THF, diethyl ether, benzene, toluene.
Form Supplied in: 1 M solution in THF.
Analysis of Reagent Purity: 11B NMR (THF, standard BF3.Et2O), d -12.83 ppm (d, JB-H 70 Hz).2 Active hydride is determined by hydrolysis and measuring the hydrogen evolved according to the standard procedure;4 potassium, by hydrolysis and titration against standard acid;2 boron, by oxidation and GC analysis of alcohol produced after oxidation.2
Preparative Methods: by the reaction of Potassium Hydride with Triethylborane in THF,2 or benzene.1 From Potassium Triisopropoxyborohydride and triethylborane in THF3 and from sodium triethylborohydride and potassium amalgam in benzene.1
Handling, Storage, and Precautions: the solution in THF is moisture- and air-sensitive, corrosive, and flammable. Reacts violently with water. Handle and store under nitrogen or argon in a cool place. Stable over prolonged periods of time. Use in a fume hood. Avoid contact with eyes, skin, and clothing.
Potassium triethylborohydride belongs to the family of trialkylborohydrides. It is a powerful reducing agent, reacting with many functional groups. Lower O-chelating ability of potassium as compared to lithium and sodium cations, bulkiness, and stability to disproportionation of the anion are its features. The reducing characteristics of potassium triethylborohydride are shown in Table 1.5
2-Methylcyclohexanone is reduced to 2-methylcyclohexanol (cis:trans = 79:21), 4-t-butylcyclohexanone to 4-t-butylcyclohexanol (cis:trans = 20:80), norcamphor to norborneol (endo:exo = 97:3), and anthraquinone to 9,10-dihydro-9,10-dihydroxyanthracene.5 Ethyl caproate, ethyl cyclohexylcarboxylate, and ethyl and isopropyl benzoate are reduced in the presence of cyclohexanone oxime, N,N-dimethylcaproamide, and capronitrile. Ethyl esters are reduced preferentially to t-butyl esters.6
2´-Alkyl-3´-oxo amides are reduced to the corresponding 3-hydroxy amides with high anti diastereoselectivity (20:1 to 80:1). Optically active 2´-alkyl-3´-oxo amides give even higher stereoselectivity (>99:1). The reaction provides an alternative to asymmetric aldol condensation (eq 1).7 Zinc Borohydride, having a cation of higher chelating ability than potassium, reacts with syn diastereoselectivity, presumably via an intermediate six-membered chelate. 2-Aryl-2-methyl-1-indanones are stereoselectively reduced to 2-aryl-2-methyl-1-indanols.8
In the presence of triethylborane, potassium triethylborohydride reacts with carbon monoxide. The organoborane intermediate produced yields 1-propanol upon oxidation (eq 2).9 The reaction is an example of a hydride-induced carbonylation of organoboranes leading to alcohols and aldehydes.10 Lithium trimethoxyaluminum hydride (LTMA)11 and Potassium Triisopropoxyborohydride (KIPBH)12 are the hydrides most often used in these reactions.
Potassium triethylborohydride finds application in the reduction of metal-metal bonds in transition metal dinuclear carbonyl complexes, e.g. Bi-Mo,13 and Fe-Fe.14,15 Metal carbonyl monoanions and dianions are generated and can be used for further transformations (eq 3).
Mononuclear anionic transition metal formyl complexes are prepared by the reduction of neutral carbonyl complexes with potassium triethylborohydride and other dialkylborohydrides (eq 4).16 The method is of considerable generality. Formyl-transition metal complexes appear to be the intermediates in the Fischer-Tropsch industrial process. The reagent is also used in other organometallic syntheses.17
Nicolaus Copernicus University, Torun, Poland
Herbert C. Brown
Purdue University, West Lafayette, IN, USA