The formation of carbon-carbon bonds is a fundamental transformation in organic synthesis. In spite of the myriad methods available, advantageous methodologies in terms of selectivity, availability of starting materials, operational simplicity, functional-group tolerance, environmental sustainability and economy are in constant demand. In this context, the development of new cross-coupling reactions that use catalysts based on inexpensive and non-toxic metals is attracting increasing attention. Similarly, efficient processes that do not require a metal catalyst are of extraordinary interest. Here, we report a new and efficient metal-free carbon-carbon bond-forming coupling between tosylhydrazones and boronic acids. This reaction is very general and functional-group tolerant. As the required tosylhydrazones are easily generated from carbonyl compounds, it can be seen as a reductive coupling of carbonyls, a process of high synthetic relevance that requires several steps using other methodologies.

Tosylhydrazones are useful synthetic intermediates that have been used in organic chemistry for almost 60 years. The recent discovery of a palladium-catalyzed cross-coupling reaction involving a tosylhydrazone coupling partner has triggered renewed interest in these reagents. This reaction shows nearly universal generality with regard to the hydrazone and can be employed for the preparation of polysubstituted alkenes. In the course of this research, novel metal-free C-C and C-O bond-forming reactions have been discovered. Since tosylhydrazones are readily prepared from carbonyl compounds, these transformations offer new synthetic opportunities for the unconventional modification of carbonyl compounds. This Minireview discusses all of these new reactions of a classic reagent.

Metal‐free partner: No organometallic coupling partner is required for a Pd‐catalyzed cross‐coupling reaction that employs N‐tosylhydrazones as the nucleophilic component (see scheme; Ts=4‐toluenesulfonyl).

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Ethers made easy: Heating a solution containing a tosylhydrazone and either a phenol or an alcohol in the presence of K2CO3 leads to the corresponding ethers (see scheme; MW=microwave, Ts=tosyl). The reaction is fairly general for the preparation of aryl alkyl and alkyl alkyl ethers, and represents a new method for the reductive etherification of carbonyl compounds.

A detailed study of the scope of a new Pd-catalyzed synthesis of indoles from 1,2-dihaloarenes and o-halobenzene sulfonates and imines is described. The cascade reaction comprises an imine alpha-arylation followed by an intramolecular C-N bond-forming reaction promoted by the same Pd catalyst. The reaction with 1,2-dibromobenzene shows wide scope and allows the introduction of aryl, alkyl, and vinyl substituents at different positions of the five-membered ring of the indole. The regioselective synthesis of indoles substituted in the six-membered ring can be carried out by employing o-dihalobenzene derivatives with two different halogens, taking advantage of the different reactivities of I, Br, and Cl in oxidative addition reactions. This paper also introduces a method for the efficient cleavage of the N-t-butyl group, thus allowing for the preparation of N-H indoles through the same methodology. Finally, the reaction with o-halosulfonates has been studied. The best substrates are o-chlorononaflates, which lead to indoles in very high yield. The reaction is particularly appropriate for the synthesis of the challenging 6-substituted indoles. In view of the availability of o-chlorophenols, which are direct precursors of the chlorononaflates, this reaction represents an efficient entry into indoles substituted in the six-membered ring. The concept is illustrated by the preparation of a 4,6-disubstituted indole from naturally occurring anethole.

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Simple and direct: Aldehydes and ketones can be transformed into alkyl azides through a reductive coupling of the corresponding tosylhydrazones in a process that takes place simply in the presence of K2CO3, tetrabutylammonium bromide (TBAB), and NaN3 (top of scheme). The application of this methodology followed by the Cu‐catalyzed azide–alkyne cycloaddition allows the direct transformation of carbonyl compounds into triazoles (bottom of scheme).