A Stand-Alone Cobalt bis(dicarbollide) Photoredox Catalyst Epoxidates Alkenes in Water at Extremely Low Catalyst Load

The boron based cobaltabis(dicarbollide), Na[3,3’-Co(1,2-C2B9H11)2], has been tested as photoredox catalyst for the oxidation of alkenes in water displaying a good efficiency. Reasons to choose Na[3,3’-Co(1,2-C2B9H11)2] as photoredox catalyst were its high catalytic activity in the oxidation of alcohols in water that requires a very low load of catalyst, its high solubility there, its molecular compactness, the donor ability of the boron unit ligand, their lack of photoluminescence, the high oxidizing power for the couple Co4+/3+ and their surfactant behaviour in aqueous media. These features differentiate it from the well-known and widely used photosensitizer tris (2,2′-bipyridine) ruthenium(II) ([Ru(bpy)3]2+), which also participates in electron transfer through an outer sphere mechanism. The comparison of the [Ru(bpy)3]2+performance in the photo oxidation of alkenes in water following identical procedures as for Na[3,3’-Co(1,2-C2B9H11)2], are fully in favor of Na[3,3’-Co(1,2-C2B9H11)2], showing for [Ru(bpy)3]2+ a very low or null efficiency. With a catalyst load of 0,1 mol % conversions between 65-97% have been obtained in short reaction times, 15 minutes, with moderate selectivity values for the corresponding epoxide, due to the formation of side products as diols. But when the catalyst load is reduced to 0.01 mol %, the selectivity for the corresponding epoxide has been increased considerably, being the only compound formed after 15 minutes of reaction (selectivity >99%). High TON values have been obtained (TON=8500) for the epoxidation of aromatic and aliphatic alkenes in water. We have verified that Na[3,3’-Co(1,2-C2B9H11)2], acts as a photocatalyst in both processes, in the epoxidation of alkenes and in their hydroxylation in aqueous medium with a higher rate for epoxidation than for hydroxylation. Preliminary photooxidation tests using methyl oleate as substrate, led to the selective epoxidation of the double bond. These results represent a promising starting point for the development of practical processes for the processing of, among others, unsaturated fatty acids, such as the valorisation of animal fat waste by using this sustainable photoredox catalyst.