Organic unsymmetrical disulfides have been extensively applied in various academic and industrial fields including intermediates in organic synthesis, agriculture, and food science, natural materials, biochemistry, pharmaceutical and medicine chemistry, polymers, material engineering, etc. They play a crucial role in the fabrication of various biological active sulfur heterocycles. Due to broad and extensive applications, many methods have been developed for the synthesis of unsymmetrical S−S and efforts have been made to improve some issues such as cost, energy efficiency, green chemistry, avoid or minimizing waste generation. Several outstanding review articles have been previously published highlighting the advances of S−S bond formation, in general, using various reagents under different conditions in the absence or presence of oxidants/catalysts. In 2020, a review paper was published by our group focusing on recent developments since 2014 in the synthesis of organic symmetrical disulfides. However, investigations on new catalytic methods are being regularly reported and new types of unsymmetrical disulfides are synthesized. The present overview has attempted to systematically summarize recent advances in the process of unsymmetrical S−S bond formation with a major focus since 2010, highlighting mechanistic considerations, substrate scope, advantages, and limitations. The patents are not studied in this overview.

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Sulfur-based additives have excellent extreme pressure (EP) performance. However, a high amount of sulfur causes metal corrosion. The non-corrosive and ashless organo-disulfides have been synthesized via Bunte salt method. The unsymmetrical dodecyl octyl disulfide (DDODS) with good oil-solubility provides better EP and anti-wear (AW) properties. The DDODS shows outstanding EP value (1961 N) and small WSA (9 mm²) in naphthenic base oil (NBO), respectively. This performance is superior compared to symmetrical didodecyl disulfide (DDDS). The outstanding performance of DDODS is due to the good solubility in NBO and the formation of a sulfur film on the metal surface. The formation of iron sulfide (FeS) under EP conditions provides excellent tribofilm compared to iron (III) oxide (Fe2O3) and iron sulfate (FeSO4).

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