(+)-Limonene Functionalization: Syntheses, Optimization, and Scale-up Procedures for Sustainable Polymer Building Blocks

(+)-Limonene has been used as a starting material for the synthesis of eco-friendly building blocks for new biobased polymers. A very effective protocol has been developed for the solvent-free Rh-catalyzed hydroformylation, which led to the isolation of Lim-CHO in high quantities (>150 mL). (+)-Limonene and Lim-CHO have been tested as substrates for hydroboration-oxidation, which has been optimized to yield the short and long diols in >100 g quantities. Both the long and short diols have been used as precursors for the synthesis of their respective amines, considering two alternate routes. The first consisted of the derivatization of the diols into mesylates, which have been converted to the diazides with NaN3 via SN2; a final catalytic hydrogenation led to the diamines. The second route involved the Swern oxidation of the diols to obtain the respective ketoaldehydes, which have been turned into the dihydrazones and subsequently reduced with H2 on Pd/C to afford the amines. All steps were optimized in order to result in quantitative yields, requiring very easy work-up steps and avoiding any use of flash chromatography, thus drastically reducing solvent waste and byproduct formation.