Solvent-Free Hydroxylation of Unactivated C–H Bonds in Small Molecules and Macromolecules by a Fe Complex

One approach to mitigate the crisis of plastic waste is "chemical upcycling", in which waste plastic is either converted into products with higher economic value or depolymerized to its constituent monomer(s). Toward this goal, several metal-catalyzed postfunctionalizations of polymers have been reported, with variable success, mostly on account of a lack of selectivity, the use of harsh reaction conditions, and the use of environmentally unfriendly solvents. We herein demonstrate the selective hydroxylation of the backbone 3° C–H bonds in synthetic macromolecules (polyolefin and polystyrene) using the in-house developed (Et4N)2[FeIII-(Ph,Me-bTAML)] (3) complex and solid Na2CO3·1.5H2O2 (SPC; sodium percarbonate) under solvent-free mechanochemical conditions. The reaction only employs simple mechanochemical grinding or ball milling at room temperature. The polar functional group –OH was successfully incorporated into the polymer backbone without any chain degradation and cross-linking. The same reaction conditions were also employed to selectively hydroxylate small organic molecules including complex natural products. The rate and selectivity of the reaction toward 3° C–H bonds far exceed that performed under homogeneous conditions. Mechanistic investigation indicates the formation of the well-characterized oxoiron(V) intermediate upon mechanical grinding of 3 and SPC. The high selectivity observed under solvent-free conditions is due to the elimination of the solvent-induced side reaction of this intermediate. This reaction represents an environment-friendly process since it uses environmentally benign reagents (iron complex, "oxygen bleach") and eliminates the use of hazardous solvents. The workup protocol involves simple washing with water, where both the spent catalyst and the oxidant are soluble. Selective mechanochemical oxidation of alkyl and benzylic 3° C–H bonds often found in commercial polymers, such as polyolefin and polystyrene, may offer a potentially useful method to generate oxyfunctionalized material and also provide routes for the deconstruction of macromolecules with strong C–C bonds under mild conditions.