[7646-93-7] · HKO4S · Potassium Hydrogen Sulfate · (MW 136.18)
Alternate Name: potassium bisulfate.
Physical Data: mp 214 °C; d 2.322 g cm-3.
Solubility: sol water, isopropyl alcohol, acetone.
Form Supplied in: white powder or granules; fused solid.
Purification: crystallized from ethanol.
Handling, Storage, and Precautions: store in a cool dry place; hygroscopic, corrosive; wash thoroughly after repeated handling; causes burns.
Isobutyraldehyde is efficiently protected as its acetal with 2-ethylhexanol in the presence of KHSO4 in 93% yield (eq 1).1 KHSO4 is comparable as a catalyst to benzenesulfonic acid in yield and crude product purity and gives a better colored product as compared to the same reaction catalyzed by Sulfuric Acid (see also p-Toluenesulfonic Acid).
Dienones can be generated using KHSO4 under acidic conditions. For example, a steroidal tosylate was refluxed for 45 min with KHSO4 in acetic acid to produce the dienone as the major product (eq 2).3 A carotenoid aldehyde has been prepared by dehydrating a monoprotected triol with KHSO4 at 150 °C (eq 3).4 The unsaturated aldehyde was produced in quantitative yield.
Treating aryl alcohols with fused KHSO4 affords the corresponding alkene. For example, m-chlorophenylmethylcarbinol, KHSO4, and a catalytic amount of p-t-butylcatechol under partial vacuum for 6 h gave m-chlorostyrene in 80-83% yield (eq 4).2 Aryl alcohols are also dehydrated when treated with KHSO4 in the presence of Copper powder. For example, treatment of 1-(2-fluorenyl)ethanol with a mixture of KHSO4/Cu followed by sublimation at 180-200 °C for 30 min afforded the corresponding terminal aryl alkene in 31% yield and a coupled product in 17% yield.5 KHSO4 has also been used to prepare butadienes. Dehydration of 4-phenylbut-3-en-2-ol with KHSO4 gave trans-1-phenylbuta-1,3-diene (see also Alumina).6
Pyruvic acid has been prepared by dehydrating tartaric acid with KHSO4 at 210-220 °C (eq 5).7 Key intermediates in the synthesis of branched long chain fatty acids involve the use of KHSO4. For example, treatment of the diol produced from the reaction of methyl n-pentyl ketone with the Grignard complex of but-4-yn-1-ol with KHSO4 followed by catalytic hydrogenation and oxidation afforded 4-methylnonane-1-carboxylic acid (eq 6).8
Dehydrative coupling using KHSO4 has been used in the synthesis of vitamin K1. Thus treatment of 2-methyl-1,4-naphthohydroquinone-1-acetate and phytol with KHSO4 in dioxane for 90 min at 76 °C gave the corresponding allene (eq 7).9 Hydrolysis of this product gave vitamin K1 in 50% overall yield.
KHSO4 can be used to generate ethers. For example, naphthalan has been prepared in 81% yield by heating fused KHSO4 and 1,8-naphthalyl alcohol for 10 min at 120 °C (eq 8).10 Highly selective monoetherification of symmetrical diols has been accomplished by using KHSO4 supported on silica gel. Treating 1,4-butanediol and dihydropyran with a catalytic amount of KHSO4-SiO2 in hexane afforded, after 50 min, 91% yield of monoether and 6% diether.11 This catalyst system also efficiently dehydrates alcohols.12
Mark R. Sivik
Procter & Gamble, Cincinnati, OH, USA