When hearing someone describe the typical preparation of organosulfur compounds, it’s clear the approach is not a very sustainable one. Mixtures of sulfur compounds naturally in crude oil are first treated with hydrogen in a catalytic refinery process to remove the sulfur—the goal is to pull out as much sulfur as possible to create cleaner transportation fuels. The hydrogen sulfide that forms in this hydrodesulfurization step is converted to elemental sulfur. Chemists then pair this sulfur with selected reactants to reconstruct desired organosulfur compounds one product at a time. Overall, the energy-intensive multistep process is costly and generates a significant amount of waste.
A new strategy reported by Valentine P. Ananikov of the Russian Academy of Sciences’ Zelinsky Institute of Organic Chemistry and coworkers enables unprecedented direct C–H functionalization and separation of only one sulfur component at a time from mixtures of organosulfur compounds (ACS Omega 2017, DOI: 10.1021/acsomega.7b00137).
“Ananikov and coworkers describe an unexpected and striking selectivity for organosulfur compounds,” says Lanny S. Liebeskind, an organosulfur catalytic functionalization expert at Emory University. “While not yet directly applicable to the processing of fossil fuels, this very interesting study suggests at least conceptually a possible future approach to the value-added desulfurization of petroleum.”
The Russian researchers use palladium acetate along with oxygen and silver trifluoroacetate as a combination oxidant to couple an olefin, such as ethyl acrylate, to an aryl or heteroaryl sulfur compound. The reaction’s selectivity centers on the sulfur acting as a directing group to guide the C–H activation and alkenylation. Benzyl compounds provide the most favorable geometry for palladium binding, followed by phenylethyl compounds and then longer alkyl chains, which leads to exclusive functionalization of one component at a time in mixtures concocted to simulate refinery streams.
“Ananikov’s team has discovered a straightforward route to organosulfur compounds by using the cheapest palladium catalyst to mediate alkenylation of multicomponent mixtures of the sulfur compounds abundant in crude oil,” says Mario Pagliaro of Italy’s National Research Council, who specializes in green chemical processing of natural resources. “This is groundbreaking work that will change the way organosulfur compounds are produced from the current, poorly sustainable approach.”