Zinc Chalcogenolates As Green Reagents

Organoselenolates and thiolates are generally weak bases and soft nucleophiles because of the high polarizability of the chalcogen atoms. They are commonly used to introduce a chalcogen functionality through substitution or addition reactions. Among all the methods developed for their preparation, the reduction of the corresponding diselenides and disulfides represents the most versatile procedure. Using these protocols selenolates and thiolates were usually prepared in situ using atom expensive reducing agents such as NaBH4, LiAlH4, or metals (Na, Li) or metal hydride (NaH). Methods: We synthetized the first bench stable selenolates, PhSeZnX (X = Cl, Br) using a completely atom economy oxidative insertion of the elemental zinc on the commercially available PhSeX. More recently, the same procedure has been used for the synthesis of the corresponding sulfur derivatives. Results: These reagents showed a characteristic nucleophilic reactivity, strongly accelerated when the reactions were performed in water suspension. In this review we will describe alkyl, aryl, vinyl and acyl substitutions, ring-opening reactions of epoxides and aziridines and Michael type addition including the role of the zinc in the control of the chemoselectivity and stereoselectivity. PhSeZnCl has been also described in the catalytic oxidation of thiols using oxygen or hydrogen peroxide. Some examples of synthetic application as well as the possibility to effect the reaction in ionic liquids will be discussed. Conclusions: The green aspect is connected not only with the use of water as reaction medium but also with the possibility to reuse the same medium for several times without loss in yields and selectivity.