[43064-96-6] · C17H39BN2 · Tetra-n-butylammonium Cyanoborohydride · (MW 282.39)
(selective, mild reducing reagent for aldehydes, conjugated aldehydes and ketones, alkyl iodides and bromides; useful for reductive aminations of aldehydes and ketones in nonpolar solvents)
Alternate Name: TBAC.
Physical Data: mp 144-145 °C.
Solubility: sol most polar (e.g. alcohols, carboxylic acids), polar aprotic (e.g. HMPA, DMSO, DMF), and nonpolar (e.g. CH2Cl2, benzene, hexane) solvents; sparingly sol H2O, ether.
Form Supplied in: white, nonhydroscopic solid; commercially available.
Preparative Methods: from n-Bu4NHSO4 suspended in H2O, treated with Sodium Hydroxide and Sodium Cyanoborohydride, and extracted into CH2Cl2; recrystallized from ethyl acetate. Other tetraalkylammonium derivatives are available3a (including polymer-supported)3b or can be generated in situ from NaBH3CN and Aliquat 336.3c
Under neutral conditions in HMPA at ambient temperature, only primary alkyl iodides and allylic bromides are efficiently reduced with TBAC. Most other functional groups, including aldehydes, ketones, esters, amides, nitro groups, and epoxides, are inert. At higher temperatures (e.g. 70-100 °C), benzylic halides, primary and secondary bromides, and naphthalenic iodides and bromides are also reduced (Table 1).4
In acidic media, TBAC reduces aldehydes and ketones to alcohols (eqs 1 and 2). However, the greater susceptibility of aldehydes allows selective reductions in the presence of ketones (and halides) at low acid concentrations (eq 3).1a,5 Alicyclic a,b-unsaturated and a,b-alkynic carbonyls are reduced to allylic and propargylic alcohols, respectively, in acidic media (eq 4).2
TBAC is effective for reductive aminations of aldehydes and ketones in nonpolar media including CH2Cl2, hexane, THF, and benzene (eq 5).3c Alternatively, the combination of Sodium Cyanoborohydride and Aliquat 336 (methyltrialkylammonium chloride) may be used to generate tetraalkylammonium cyanoborohydride in situ for reductive aminations (eq 6).3c
The excellent chemoselectivity available recommends this reagent for reduction of primary alkyl iodides and benzylic bromides at ambient temperature in HMPA since virtually no other functional groups are affected. At higher temperatures, other halides are also reduced chemoselectively. Comparison between TBAC and other cyanoborohydrides (particularly NaBH3CN) for halide reductions is provided by Hutchins.4 In acidic media, effective chemoselective reductions of aldehydes in the presence of ketones may be obtained.
TBAC allows reductive aminations in nonpolar solvents as an alternative to NaBH3CN, which is used in polar solvents.
Robert O. Hutchins
Drexel University, Philadelphia, PA, USA
MaryGail K. Hutchins
LNP Engineering Plastics, Exton, PA, USA