Introduction
Tetrabutylammonium hydrogen sulfate (TBAHS, 1), is a stable, hygroscopic, white solid (mp 169–171 °C). It is widely used in various fields of chemistry as phase-transfer catalyst (PTC) and ion-pairing reagent as mobile phase additive in HPLC. It has been also used in a variety of organic transformations some of which include the syntheses of triarylpyridines,[
1
] N-monosubstituted α-keto amides,[
2
] cyclic and acyclic β-disubstituted, α,β-unsaturated ketones,[
3
] 3-alkylated indoles,[
4
] benzopyran-annulated pyrano[2,3-c]pyrazoles,[
5
] N1-alkylated 3,4-dihydropyrimidine-2(1H)-ones,[
6
] 2-O-deacetylated glucosyl hydroxamates,[
7
] 1,2,3-triazoles,[
8
] 1,8-dioxooctahydroxanthenes,[
9
] β- and γ-amino ethers, morpholines and their higher homologues,[
10
] etc.
Although TBAHS is commercially available and inexpensive, it can be prepared from tetrabutylammonium azide[
11
] by treating it with potassium hydrogen sulfate and 10% aqueous sulfuric acid in aqueous medium (Scheme [1], eq. 1) or from tetrabutylammonium thiocyanate[
12
] by treating it with relatively concentrated sulfuric acid (70%) at 75 °C (Scheme [1], eq. 2).
Scheme 1 Synthesis of tetrabutylammonium hydrogen sulfate (TBAHS, 1)