Alkylated Polyethyleneimine/Polyoxometalate Synzymes as Catalysts for the Oxidation of Hydrophobic Substrates in Water with Hydrogen Peroxide Adina Haimov, † Hagai Cohen, ‡ and Ronny Neumann* ,† Department of Organic Chemistry and Chemical Research Support, Weizmann Institute of Science, RehoVot 76100, Israel Received June 21, 2004; E-mail: Ronny.Neumann@weizmann.ac.il Aqueous biphasic catalysis in the absence of organic solvent has potential for environmentally benign synthesis. 1 The technique, while attractive, has limitations such as reduced reaction rates because of slow mass transfer of the hydrophobic organic substrate to the aqueous catalyst phase and the requisite synthesis of water- soluble ligands for the organometallic catalysts. Mass transfer to the aqueous phase may be facilitated by addition of amphiphiles. Thus, reaction rates may be increased by formation of micelles, 2 (micro)emulsions, 3 and other stabilized biphasic systems. The field of biomimetic catalysis using water-soluble catalysts also presents intriguing possibilities for aqueous biphasic catalysis. One example is the use of cyclodextrins as extracting agents for organic substrates into an aqueous phase. 4 Recently, we have shown that certain water- soluble polyanionic polyoxometalates (POMs), with known catalytic properties in the activation of hydrogen peroxide, 5 can be used in aqueous biphasic catalysis for oxidation of alcohols, diols, and amines. 6 The systems were largely inactive for aqueous biphasic oxidation of hydrophobic substrates such as alkenes. 7 In the context of this research, water-soluble randomly alkylated polyethylenimines have been shown to have properties that are enzyme-like (synzymes) for general acid-base catalytic reactions. 8 Now, we show that POMs can be electrostatically bonded to quaternary ammonium sites of the alkylated polyethylenimine (Alk-PEI). These new alkylated polyethyleneimine/polyoxometalate (Alk-PEI/POM) syn- zymes (Scheme 1) also have hydrophobic regions enabling the solubilization or binding of hydrophobic substrates and therefore are active catalysts for the selective oxidation of such substrates in water. Polyethylenimine (M w ≈ 10 000) was alkylated with a mixture of 1-iodododecane and iodomethane at a ratio CH 2 CH 2 NH/C 12 H 25 I/ CH 3 I ) 10.5:1:7.4 in a literature-adapted procedure. 8e,9 From the reaction yield and XPS measurements (see below), an ∼95% efficiency in the alkylation reaction and ∼50% formation of quaternary ammonium centers can be determined; Alk-PEI has an M w ≈ 20 000. Under these random alkylation conditions, the purified Alk-PEI was first analyzed by a 15 N- 1 H HMBC experi- ment (Figure 1). One can see that for PEI (Figure 1, left) the methylene protons correlate with two different types of nitrogen atoms. Logically, primary amines are observed at 9-11 ppm ( 15 N), and secondary amines are observed at 18-20 ppm ( 15 N). From the spectrum for Alk-PEI (Figure 1, right), it is clear that 1° and 2° amines in the original PEI have been alkylated. The peaks at 25- 30 ppm ( 15 N) may be assigned to tertiary amines, and the peak at ∼40 ppm ( 15 N) may be assigned to quaternary ammonium moieties. 10 Further analysis of Alk-PEI by the XPS (Figure 2) showed a nitrogen 1s line for the tertiary amine moieties at a binding energy of 398.7 eV and nitrogen 1s lines for the quaternary ammonium moieties at 401.0 and 401.8 eV. 9,11 Measurement of the peak areas reveals that approximately 50% of the nitrogen atoms are at quaternary ammonium centers. The hydrophobicity of Alk-PEI versus the unmodified PEI polymer was estimated by measuring the contact angle on a goniometer. For unmodified PEI, the contact angle was 11 ( 4°, indicating a very hydrophilic compound. On the other hand, the contact angle for alkylated PEI was 30 ( 5°, indicating a more hydrophobic material. Since Alk-PEI remained water-soluble, one may assume that there are indeed hydrophobic regions in the synzyme. This hypothesis was further tested by measurement of the fluorescence spectra of 3-aminopyrene, a hydrophobic probe, † Department of Organic Chemistry. ‡ Chemical Research Support. Figure 1. 15 N- 1 H HMBC NMR experiment for PEI (left) and Alk-PEI (right). Figure 2. XPS N (1s) line for Alk-PEI. Solid lines are curve fits of data. Scheme 1. Components of the Alk-PEI/POM Synzyme Published on Web 09/03/2004 11762 9 J. AM. CHEM. SOC. 2004, 126, 11762-11763 10.1021/ja046349u CCC: $27.50 © 2004 American Chemical Society