Solvent-free Hydroxylation of C-H bonds by Fe-complex: A Green Approach for Activation of Small Molecules and Macromolecules

One approach to mitigate the crisis of plastic waste is “chemical upcycling”, in which waste plastic is converted into products with higher economic value. Towards this goal, several metal-catalyzed post-functionalization of polymers have been reported, with variable success, mostly on account of lack of selectivity, use of harsh reaction conditions, and use of environmentally unfriendly solvents. In this work, we demonstrate selective hydroxylation of the backbone 3° C-H bonds in polyolefin macromolecules using in-house developed (Et4N)2[FeIII-(Ph,Me-bTAML)] (3) complex and solid Na2CO3.1.5 H2O2 (SPC; Sodium percarbonate) under solvent-free mechanochemical conditions. The reaction condition only employs simple mechanochemical grinding at room tempera-ture. The polar functional group -OH was successfully incorporated into the polymer backbone without any chain degradation and crosslinking. The same reaction conditions were also employed to selectively hydroxylate small organic molecules including com-plex natural products. The rate and selectivity of the reaction towards 3° C-H bonds far exceed that performed under homogene-ous conditions. Mechanistic investigation indicates the formation of the well-characterized oxoiron(V) intermediate upon mechani-cal grinding of 3 and SPC. The high selectivity observed under solvent-free conditions is due to the elimination of solvent-induced side-reaction of this intermediate. This represents a very sustainable process since it uses environmentally benign reagents (iron complex, “oxygen bleach”), thus eliminating the use of hazardous solvents. The workup protocol involves simple washing with water where both the spent catalyst and the oxidant are soluble.