With increasing people’s interest in renewable energy and global warming, it is important to minimize the use of harmful or toxic chemicals in the processes of chemical synthesis. Also, trimming down chemical waste and developing recyclable approaches might be helpful to do the reactions in greener fashions.
In this regard, dioxygen activation is one of the most popular research topics in organic synthesis due to its inexpensive, abundant, and environmentally friendly nature. Transient metal catalyst, photoredox catalyst, etc. are commonly used towards the aerial oxygen activation methods. These transition metals are Pd, Ni, Fe, Cu, etc.[1-3] Besides, transition metal peroxide-mediated radical reactions also generates unwanted products and thus have limited uses in materials and pharmaceuticals industries.
Organosulfur compounds containing sulfonyl group are well known in natural and non-natural products. Numerous β-hydroxysulfones derivatives are recognized for their biological activities and generally serve as potential building blocks in organic synthesis. The β-hydroxysulfones are synthesized by ring opening of epoxides, via multistep chemical or bioreduction of β-keto-sulfones, via hydroxylation of α,β-unsaturated sulfones, etc. These transformations are well established but they have major limitations e.g., a tedious synthetic procedure for precursors, no control to stop byproducts, non-user friendly reaction conditions, etc. Many oxidative cross-coupling reactions are also explored for the synthesis of organosulfur compounds.
Recently, Mal and co-workers reported an iodine catalyzed dioxygen activation in the oxysulfonylation reaction of unactivated olefins (styrenes) using sulfonyl hydrazides. Iodine and hypervalent iodine reagents gained significant attention in organic synthesis due to their low costs and non-hazardous nature. However, in this oxysulfonylation report, near quantitative synthesis of β-hydroxysulfones were reported to be achieved at 70 ºC, within 7 h of reaction time and under aerobic condition in acetonitrile (Figure 1).
Mechanistically it is shown that the reaction might have proceeded via radical pathway (Figure 1). The sulfonyl radical was produced upon reaction of sulfonyl hydrazides and iodine in presence of triplet dioxygen. Subsequently, the sulfonyl radical could react with the olefins styrene to produce a benzylic radical which trapped the dioxygen dissolved in the working solvent (acetonitrile).
1,2-Defunctionalization of olefins is not easy and straightforward because they often experience regioselectivity and stereoselectivity issues. However it is shown here (Figure 2) that under aerobic condition and using 10 mol % of iodine as a catalyst, 10 mol % of base pyridine, the unactivated olefins like styrenes reacted with sulfonyl hydrazides to obtain β-hydroxysulfones in good to excellent yields (> 99%). The styrenes with electron donating groups resulted in the best yield of β-hydroxysulfones compared to styrenes with electron withdrawing groups. Aliphatic and aromatic sulfonyl hydrazide were also found to be compatible with this reported methodology.
Overall, the developed methodology described here is simple and one of the efficient methods for aerial dioxygen activation reaction in the synthesis of regioselective β-hydroxysulfones from unactivated olefins and sulfonyl hydrazides. In one pot new C–O and C–S bonds were efficiently constructed and thus anticipated that this oxysulfonylation approach might offer a convenient access towards the synthesis of organosulfur compounds which are bioactive and pharmaceutically important.
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The study, Iodine-Triggered Aerobic Oxysulfonylation of Styrenes was recently published in the journal Advanced Synthesis & Catalysis.