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The synthesis of pyridazinium salts was achieved from readily available phenylazosulfonates in a single reaction step. The reaction proceeds via the formation of short-lived phenyldiazenes, which-owing to the strongly acidic conditions-are partially protonated. The phenyldiazenes then undergo a rapid cycloaddition to furans to give pyridazinium salts via elimination of water. The fact that the pyridazinium synthesis shows a low sensitivity toward oxygen, although phenyldiazenes occur as intermediates, can be explained by the very fast cycloaddition step and the partial protonation of the phenyldiazene.

Taking advantage of the "differentiating effect" of the solvent methanol, deuterations of electron-rich aromatic systems can be carried out under mild acid catalysis and thus under far milder conditions than known so far. The exceptional functional group tolerance observed under the optimized conditions, which even includes highly acid-labile groups, results from a hitherto unexploited shifted selectivity in protonation, and enabled simple and straightforward access to complex deuterium-labeled compounds.

The oxidation of alkyl thiols to disulfides has been achieved under mild conditions using a chemiluminescent 1,2-dioxetane as a stoichiometric oxidant. Besides the mild and biocompatible reaction conditions, this approach offers the possibility to monitor the presence of thiols through oxidation and chemiluminescence of the remaining dioxetane.

A new type of waste recycling strategy is described in which nitrogen oxides or nitric acid are directly employed in photocatalyzed hydroxylations and nitrohydroxylations of benzenes. Through these transformations, otherwise costly denitrification can be combined with the synthesis of valuable compounds for various applications.

Although general interest in radical arylation reactions has grown rapidly in recent years, poor regioselectivities and the need to use a large excess of the radical-accepting arene have hindered their application to substituted benzenes. We now describe experimental and computational investigations into the substituent effects that lead to regioselective addition based on the recent discovery of anilines as outstanding substrates for radical arylations.

The titanium(III)-mediated radical arylation of 3-hydroxypyridines was found to proceed with high regioselectivity for the 2-position. Using aryldiazonium chlorides, which were prepared from the corresponding anilines, as aryl radical sources, a range of 3-hydroxy-2-phenylpyridines were obtained in moderate to good yields under simple reaction conditions. Reactions of ortho-carboxylic ester substituted phenyldiazonium salts directly provided tricyclic benzopyranopyridinones.

Allylation and benzylation of phenylazocarboxylic tert-butyl esters have been achieved under Barbier-type reaction conditions and in very short reactions times using the corresponding allyl and benzyl bromides or iodides in combination with zinc powder. Whereas all reactions occurred exclusively at the β-nitrogen atom of the azocarboxylic esters, the linkage of allyl units was shown to depend on the substitution pattern at the double bond of the allyl halide. The hydrazines obtained are useful precursors for indoles and indazoles.

Substituted 2-aminobiphenyls have been prepared from arylhydrazine hydrochlorides and anilines in biphasic radical arylation reactions with dioxygen from air as a most simple and readily available oxidant. Under optimized conditions, the free amino functionality of the aniline leads to high ortho:meta regioselectivities, now even for anilines bearing a donor substituent in the para position. Finally, the mild and metal-free new access to aminobiphenyls was shown to be applicable on a gram scale.

Substituted 2-aminobiphenyls have been prepared from arylhydrazines and anilines via radical arylation reactions under simple oxidative conditions. The strong directing effect of the free and unprotonated amino functionality leads to high regioselectivities, and anilines have been shown to be significantly better aryl radical acceptors than nitrobenzenes or phenyl ethers. The methodology is also applicable to phenols, which react best as phenolates under strongly basic conditions. Finally, radical arylation reactions of anilines and anilinium salts under various conditions have for the first time demonstrated that regioselectivity can also be controlled through the rearomatization step and that the addition of an aryl radical to a substituted benzene might even be reversible.

tert-Butyl phenylazocarboxylates are versatile building blocks for synthetic organic chemistry. Nucleophilic substitutions of the benzene ring proceed with aromatic amines and alcohols under mild conditions. The attack of aliphatic amines may be directed to the aromatic core as well as to the carbonyl unit leading to azocarboxamides. The benzene ring can further be modified through radical reactions, in which the tert-butyloxycarbonylazo group enables the generation of aryl radicals at either elevated temperatures or under acidic conditions. Radical reactions include oxygenation, halogenation, carbohalogenation, carbohydroxylation, and aryl-aryl coupling.

Allylation and vinylation of aryl radicals generated from aryl diazonium salts provides rapid and efficient access to chlorinated and brominated derivatives of styrene and allylbenzene. Allyl chlorides were found to be better substrates than bromides due to decreased halogen transfer. Donor- and acceptor-substituted diazonium salts are well tolerated. The products represent important precursors for numerous further transformations.

Procedures for the carbodiazenylation of nonactivated olefins with a wide range of aryldiazonium salts have been developed. The azo compounds obtained can serve as valuable precursors for beta-arylamines (carboamination products), beta-amino acids, ketones, and various heterocyclic structures.