Micellar catalysis of organic reactions. Part 37. A comparison of the catalysis of ester and amide hydrolysis by copper-containing micelles

Abstract

The hydrolysis of a number of nitroactivated esters and amides has been studied in the presence of copper-containing metallomicelles at neutral pH. The relative rates of hydrolysis in the pure metallomicelle and in co-micelles with either cetyltrimethylammonium bromide (ctab) or Triton X-100 depends on the hydrophobicity of the substrate and whether it is completely solubilized by the copper micelle. Thus it depends on the concentration of the copper micelle. At low concentrations of copper micelle (0.2 mM) where the substrate is incompletely solubilized, the reaction is faster when 2 mM ctab is added. At higher concentrations (>0.6 mM) where the substrate may be almost completely solubilized by the copper micelle, the reaction is slower when 2 mM ctab is added. For ester hydrolysis the presence of either a carboxylic acid group or a heterocyclic nitrogen atom close to the reaction centre resulted in much larger catalysis by the metallomicelle than for model compounds without these additional groups. It is postulated that these groups coordinate with the metal ion and thus present the reaction centre close to a metal-bound hydroxyl resulting in a significant increase in the rate of bond formation, which is the rate-determining step for ester hydrolysis. For amide hydrolysis the presence of a carboxyl group ortho to the reaction centre did not lead to larger catalysis by the copper micelle than for the compound without this group. This difference is attributed to the different rate-determining steps for amide and for ester hydrolysis. Keywords: metallomicelles, ester and amide hydrolysis.

Bicarbonate Surfoxidants: Micellar Oxidations of Aryl Sulfides with Bicarbonate-Activated Hydrogen Peroxide

“…Many studies have shown how ionic surfactants with reactive counterions ( reactive ion surfactants ) can accelerate chemical reactions because of high local concentrations of reactant ions and substrates in the micellar pseudophase. − Recent reports illustrate the promise of approaches in which a catalyst such as a metal ion is used as the counterion. − For example, Kobayashi and co-workers 14 investigated anionic surfactants with Lewis acid counterions (scandium(III)) and showed that aldol, allylation, and Mannich-type reactions could be carried out efficiently in water. In another example, Engberts and co-workers have shown that Diels−Alder reactions can be promoted significantly (10 6 -fold) in water by anionic surfactants with copper(II) counterions …”

Catalysis of Aryl Ester Hydrolysis in the Presence of Metallomicelles Containing a Copper(II) Diethylenetriamine Derivative

“…Copper(II) chloride (BDH) and copper(II) acetate (BDH) were of analytical grade and used without further purification. 4-Acetoxy-3-nitrobenzoic acid ( 2 ) and 2-nitrophenyl acetate ( 4 ) were available from previous studies …”

“…Many studies of ester hydrolyses in metallomicelles have examined the hydrolysis of activated carboxylic esters such as 4-nitrophenylpicolinate (PNPP). ,,,− This substrate and its isomer, 4-nitrophenylisonicotinate (PNPIN), are particularly suitable for studying these effects. The high binding affinity of the pyridine nitrogen for the transition metal ensures strong binding of these substrates to the metallomicelles, and known and contrasting presentation geometry. , In our previous studies, we have investigated substrates which coordinate to metallosurfactants through a carboxylate anion. These included the isomeric pair 2-acetoxy-5-nitrobenzoic acid ( 1 ) and 4-acetoxy-3-nitrobenzoic acid ( 2 ) (Chart ).…”

“…The tridentate ligation was expected to provide superior pH and thermal stability over the bidentate coordination in the first-generation metallosurfactant. Furthermore, it was of interest to compare the catalytic efficiency of the bidentate coordinated Cu(II)TTMED with the tridentate Cu(II)TDET, since it is well-established that the structure of the lipophilic surfactant ligand in metallomicelles is known to influence the rate and mechanism of ester hydrolysis. ,,,,− Structural variation in the ligand is known to affect the critical micelle concentration (cmc), , the p K a of metal-bound water or alcohol, and the coordination geometry of the metal ion …”

Micellar Catalysis as a Clean Alternative for Selective Epoxidation Reactions

“…A preliminary process design is shown in Figure 8 For the process design several key issues can be identified. First of all, the micelle is an organized medium, which is known to alter reaction rates and selectivities (1)(2)(3)(4)(5)(6). Furthermore, the micellar solutions are micro-heterogeneous two-phase media, where the components are distributed between the micelle and the water phase.…”