Polymer immobilised TEMPO (PIPO): an efficient catalyst for the chlorinated hydrocarbon solvent-free and bromide-free oxidation of alcohols with hypochlorite Arn´ e Dijksman, Isabel W. C. E. Arends and Roger A. Sheldon* Laboratory for Organic Chemistry and Catalysis, Department of Biotechnology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands. E-mail: secretariat-ock@stm.tudelft.nl Received (in Cambridge, UK) 9th December 1999, Accepted 7th January 2000 PIPO, a readily prepared polymer immobilised TEMPO, can be employed as an efficient recyclable heterogeneous cata- lyst for the chlorinated hydrocarbon solvent-free and bromide-free bleach oxidation of a variety of alcohols and polyols. The use of stable nitroxyl radicals, such as TEMPO, as catalysts for the oxidation of alcohols to aldehydes, ketones and carboxylic acids is well documented. 1 Typically, these trans- formations employ 1 mol% of the nitroxyl radical and a stoichiometric amount of a terminal oxidant, e.g. sodium hypochlorite, 2 MCPBA (m-chloroperbenzoicacid), 3 sodium bromite, 4 trichloroisocyanuric acid 5 and oxygen in combination with CuCl 6 or RuCl 2 (PPh 3 ) 3 . 7 In particular, the TEMPO-bleach protocol using bromide as co-catalyst introduced by Anelli et al. 2 is finding wide application in organic synthesis. Although only a small amount of catalyst is used, recyclability is an issue and several heterogeneous TEMPO systems have been re- ported. 8 For example, MCM-41 8g and silica-supported TEM- PO 8h,i have been applied in oxidation reactions using hypo- chlorite as the oxidant. The preparation of these catalysts involves initial functionalisation of the support followed by covalent attachment of a 4-substituted TEMPO. Here, we report the use of a readily prepared polymer immobilised TEMPO as a catalyst for alcohol oxidations. It was derived from a commercially available oligomeric, sterically hindered amine, poly[[6-[(1,1,3,3-tetramethylbutyl)amino]- 1,3,5-triazine-2,4-diyl][2,2,6,6-tetramethyl-4-piperidinyl)- imino]-1,6-hexanediyl[(2,2,6,6-tetramethyl-4-piperidinyl- imino]], better known as Chimassorb 944 (MW ≈ 3000; see Scheme 1 for structure). This compound is used as an antioxidant and a light stabiliser for plastics. It contributes significantly to the long term heat stability of polyolefins and has broad approval for use in polyolefin food packaging. 9 Nitroxyl radicals are normally prepared by treating the analogous secondary amine with Na 2 WO 4 ·2H 2 O and hydrogen peroxide. 10 In the case of Chimassorb 944, the same procedure was applied resulting in the formation of an oligomeric TEMPO (Scheme 1). Probe-MS data revealed that the mass of each segment increased by 30 owing to transformation of two secondary amine moieties into the corresponding nitroxyl radicals. This new polymer immobilised TEMPO, further referred to as PIPO (polyamine immobilised piperidinyl oxyl), proved to be an effective catalyst for oxidations of alcohols with hypochlorite using the Anelli protocol. 2 † Primary and secondary aliphatic and benzylic alcohols were smoothly converted to the corresponding aldehydes and ketones in CH 2 Cl 2 (Table 1). Under these conditions the system was homogeneous as PIPO is soluble in dichloromethane. In contrast, in the absence of solvent (entry 3) PIPO was an active heterogeneous catalyst. The heterogeneous nature of the catalyst was confirmed in a filtration experiment, in which the reaction mixture was filtered after 10 min. The filtrate showed no activity at all during 1 h after filtration. The residue, however, could be reused at least twice as a catalyst. The minor loss of activity ( < 5%) observed is probably due to mechanical losses occurring during filtration of the small amount of catalyst. Further investigation revealed that the use of bromide was not necessary. Thus, in contrast to the conventional TEMPO-bleach oxidations, which use dichloromethane as solvent and bromide as a co-catalyst, 2 PIPO catalyses the oxidation of a variety of alcohols in the absence of organic solvent and using only a hypochlorite solution (0.35 M, pH 9.1) as the oxidant (Table 2). However, under these conditions primary aliphatic alcohols such as octan-1-ol, gave low selectivities to aldehydes owing to over-oxidation of octanal to octanoic acid (entry 1). This problem was circumvented by using MTBE as the organic solvent, in which PIPO is not soluble, affording an increase in selectivity to 94% (entry 2). Here again, filtration experiments confirmed that this system was heterogeneous, analogous to the solvent-free conditions. In addition to primary and secondary aliphatic alcohols (entries 2–7), benzylic alcohols were also efficiently oxidised (entries 9 and 10), complete conversion being observed within 30 min. In competition experiments, the catalyst showed a marked preference for primary alcohols (entries 8 and 11). This is analogous to the already reported homogeneous 2 and heterogeneous 8h TEMPO systems. Chirality on the a-position is not affected during oxidation as shown by the selective oxidation of (S)-2-methylbutan-1-ol to (S)-2-methylbutanal (entry 12). 11 Scheme 1 Synthesis of PIPO. Table 1 PIPO-catalysed oxidation of alcohols with bromide/hypo- chlorite a Entry Substrate Product t/min Conv.(%) b Sel.(%) b 1 Octan-1-ol Octanal 20 > 99 > 99 2 Octan-2-ol Octan-2-one 20 > 99 > 99 3 c 45 95 > 99 4 Benzyl alcohol Benzaldehyde 20 > 99 > 99 5 1-Phenylethanol Acetophenone 20 > 99 > 99 a 0.8 mmol substrate, 2.5 mg PIPO (1 mol% nitroxyl), 2 ml CH 2 Cl 2 , 0.16 ml 0.5 M KBr solution (10 mol%), 0.14 g KHCO 3 , 2.86 ml 0.35 M hypochlorite solution (1.25 equiv.), 0 °C. b Conversion and selectivity determined by GC using n-hexadecane as internal standard. c No CH 2 Cl 2 (solvent-free). This journal is © The Royal Society of Chemistry 2000 Chem. Commun., 2000, 271–272 271