Sharpless reaction characteristics

Sharpless and Flood682 observed reactions characteristic of monooxygenases, such as stereospecific epoxidation of olefins and arene hydroxylation, with oxotransition metal (M=0) oxidants. For example, Cr02X2 (X=C1, OAc) oxidized tritium labeled naphthalene to naphthoquinone accompanied by tritium migration. Rearrangement is suggestive of arene oxide transients. 

Among the available methods for introducing an unsaturated carbon-carbon bond into organic molecules, selenoxide elimination reaction has been shown to be quite useful because of its simple procedure and its characteristic regioselec-tivity. Jones et al., who discovered the first selenoxide elimination, proposed an intramolecular mechanism entailing a five-membered ring structure to explain its syn nature [11]. This proposition was shown to be correct by Sharpless et al. who applied the method that was utilized by Cram to determine the stereochemistry of elimination in amine oxides [12]. Thus, the oxidation of erythro-selenide afforded only Z-olefin and that of f/zreo-selenide gave only -olefin (Scheme 4). 

The outstanding characteristics mentioned for both the imcatalyzed and the catalyzed reactions have led to the 1,3-dipolar cycloadditions of azide and alkynes being considered the most efficient reaction under the new concept of click chemistry [ 12], The term coined by Sharpless and coworkers refers to a modular synthetic approach based on a set of the most practical end rehable chemical reactions. These are wide in scope, highly efficient in terms of both conversion and selectivity under very mild and simple reaction conditions, and use simple product isolation procedures. An indicative parameter of the importance of the 1,3-dipolar cycloaddition of azide and alkyne as a prototype of a click chemistry reaction is the exponential number of applications based on this reaction that have appeared in the literature since the description in 2002 of the catalyzed version. Currently, it can be asserted that click chemistry and the Huisgen 1,3-dipolar cycloaddition of azides and alkynes are synonymous. 

S.2.2.4. Triazoles from Azides. Cycloadditions of this type constitute a valuable synthetic route to the triazole ring system. This is shown in Scheme 5.30. This combination dates back to the early work of Huis-gen, but in more recent times it was discovered to be subject to catalysis by Cu(I) compounds. The reactions are fast under mild conditions, have high regiospecificity, and occur in a variety of solvents including water. In addition, reaction products are easily isolated. Reactions with these characteristics have become known as comprising click chemistry this term was coined by K. B. Sharpless. The first and most commonly used reaction referred to by this name is indeed the azide-alkyne cycloaddition, and new interest has developed in triazole hemistry since the discoveiy of the copper catalysis. In addition to its use in organic... 

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