(Hive Addict)
09-25-03 11:49
No 461039
(Rated as: excellent)
Ethyl tosylate (TsOEt) from p-Toluenesulfonic acid (TsOH)
1kg TsOH ~ $25
1L CH2(COOEt)2 $60 ?
A solution of commercial TsOH*H2O (19.0g 100mmol) in a dry solvent (30ml) such as benzene or toluene was heated under reflux with azeotropic removal of water in a Dean-Stark trap for 3h to give anhydrous TsOH. After the solution had cooled, the appropriate aliphatic or aromatic carboxylic acid ester (34mmol) was added, and then the mixture was refluxed for the time indicated in the tables. Ice-water was added to the reaction mixture after cooling. The whole was extracted with benzene. The benzene layer was washed with water, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The resulting oil could be purified by column chromatography (alumina, benzene). Conditions and yields are in the tables.
Data from table 1:
CH2(COOEt)2 :TsOH:ester 3:1 solvent: toluene time: 48h yield: 44% (38% after 24h)
AcAcOEt : TsOH:ester 3:1 solvent: toluene time: 24h yield: 50%
PhCOCH2COOEt TsOH:ester 3:1 solvent: toluene time: 24h yield: 60%
An example of the recovery of unchanged carboxylic acid esters and excess TsOH is as follows.
A solution of anhydrous TsOH (9,5g 50mmol) and ethyl cyclohexanecarboxylate (2.6g 17mmol) in toluene (15ml) was refluxed for 24h. Ice-water was added to the reactin mixture after cooling. The whole was extracted with benzene. The solvent layer was washed with 2N NaOH, dired over anh. Na2SO4 and conc'ed under red. pressure. Distillation of the resulting oil under red. pressure gave 0.98g (37%) of ethyl cyclohexanecarboxylate, bp 70-71°C (21mmHg) and then yielded 1.70g (51%) of TsOEt, bp 118-121°C (2mmHg).
Cyclohexanecarboxylic acid (1.26g, 58%) was obtained from the aqu. NaOH soln. by routine workup. The aqu. layer which remained after benzene extraction of the reaction mixture was concentrated to dryness in vacuo: the resulting oil solidified on standing. The solid was dried under red. pressure to recover 7.0g of TsOH. TsOH was also recovered as TsONa by using the salting-out procedure.
Ref: Nitta, Yoshihiro; Arakawa, Yasushi; Chem.Pharm.Bull. 33(4), 1380-1386 (1985)
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