Laboratory Experiments on Phase-Transfer-Catalyzed Reactions of Neutral Molecules Nawal K. Mathur and Chander K. Narang University of Jodhpur, Jodhpur-342001, India In heterophase reactions, a systematic and planned selection (1)of phase transfer catalyst (PTC) i s only possible when the reactants are either anionic nucleophiles (PTC'souaternarv ammonium or nhos~honiumsalts. or crown ;them) (2: 3) or cationic elekrophiles (PTC's-stable and lino~hilicanions. e.e.. tetra~heuvlhorateand tetrakis-3.5b~r(~rifluorornethylp?lenylb~rate;(4~. However, there is no eeneral rule for selection of a W C for a neutral reactant. Fluoride ion, in conjunction with a quaternary ammonium salt, has been suggested as a PTC (5) for certain hydrogen bonding molecular species, e.g., H202, catboxylic acids, phenols, thiols, some enols, etc. A large number of carhonyl derivatizing reagents, e.g., NHzOH. HC1, NzHa, NHzNHCONHz, and NHzNHCSNHz are uncharged molecules but weakly basic nucleophiles (pK. 12). It is customary to use these reagents for derivatization of water-insoluble aldehvdes and ketones. in a mutual solvent, e.g., aqueous ethanol (6).Most of s"ch reactions are weak acid catalvzed, . . and it is usual to add acetic acid to the reaction mixture. In order to illustrate the application of PTC, the preparation of henzophenone oximiwas attempted under diffkrent conditions.
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Experlment 1 In this experiment, benzyltriethylammoniumchloride (BTEAC) was used as PTC, in conjunction with NaF. The aqueous phase consisted of 0.70 g (0.01 mol) NHzOH. HC1 neutralized with 0.4 g (0.01 mol) NaOH, and 0.42 g (0.01 mol) NaF 0.26 g (0.001 mol) BTEAC in 10 mL water. The organic phase contained 1.82 g (0.01 mol) benzophenone in 10 mL dichloromethane or toluene. On mixing and stirring the two solutions at 55-60 'C for 4 h, it was found that most of the ketone remained unchanged.
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Experlment 2 In an alternative strategy, the weakly basic derivatization reagent was converted into a Lipophilic salt of a medium-sized (CsCs) alkanoic acid, i.e., a weak acid-weak base conjugate species. The reagent used was 2-ethylheaanoic acid WEHA), which acted as a bifunctional catalyst, viz. e weak acid catalyst as well as a PTC. We describe below two preparations that prove the efficacy of this strategy.
used. To the mixture, 1.44 g (0.01 mol) of 2-EHA was added. The mixture was stirred for 1.5 h at 5540 OC. The organic phase was washed with 1%NaHC03,1%HC1 and water (2 X 10 mL each) and then dried over anhydrous NazSOa. On evaporation of the solvent, almost pure benzophenone-oxime (mp 139-141 'C; lit. value: 141 'C; yield: 85%)was obtained. Synthesis of p-Toluenesulfonyl Hycfrazide In an earlier reported method (7)for the preparation of this compound, the benzene solution of tosyl chloride and aqueous solution of hydrazine are shaken for several hours. Organic Synthesis Coll. Vol. 5 describes this synthesis (8) using a water-miscible solvent, THF. We wished to see if this slow reactioseonfined to immiscible phases, could be acclerated by a suitable PTC. The aqueous phase consisted of 1.28 g (50%aqueous) hydrazine containing an equivalent of 0.022 mol of NzHn, and 0.142 g (0.001 mol) of 2-EHA. The organic phase contained 1.91g (0.01 mol) tosyl chloride in 10 mL benzene. The two solutions were mined and stirred at room temperature. Since the reaction is only mildly exothermic, no external cooling was required (differencefrom the OS method where the temperature must be maintained at 10-12 OC in order to control the exothermic reaction). After separation, the organic phase was washed with 1%NaHCOa,l%HC1, and water (2 X 10 mL each),and dried over anhydrous sodimsulfate. Onevaporation of the solvent, 1.71 g tosyl hydazide (mp 109-111 O C ; lit. value: 109-110 "C; yield: 90%)was obtained. In these experiments, the choice of 2-EHA as PTC was based on the fact that being a branched chain acid, it is not likely to give rise to micelle formation. PTC activity of F--QtX- pair arises due to Hbonding with the weakly acidic reactant, HY(Qt-F ..H-Y (5).The ineffectivenessof NHzOH and N2Hl in the presence of fluoride is probably due to that these donot form strong H bond with fluoride ion. Acknowledgment T h e authors acknowledge with thanks the help rendered by R. Mathur and S. Mathur, in carrying out these experiments.
Literature Cited
Preparation of Benzophenone Oxime The two immiscible reactant solutions were the same as described earlier except that in the aqueous solutionno NaF and BTEAC were
Volume 67
Number 3
March 1990
273
