Carbenes are very important reactive intermediates to access a variety of complex molecules and are applied widely in organic synthesis and drug discovery. Typically, their chemistry is accessed by the use of transition metal catalysts. Herein, we describe the application of low‐energy blue light for the photochemical generation of carbenes from donor–acceptor diazoalkanes. This catalyst‐free and operationally simple approach enables highly efficient cyclopropenation reactions with alkynes and the rearrangement of sulfides under mild reaction conditions, which can be utilized for both batch and continuous‐flow processes.

Novel iodine-induced sulfonylation and sulfenylation of imidazopyridines have been described using sodium sulfinates as the sulfur source. This strategy enables highly selective difunctionalization of imidazo[1,2-a]pyridine to access sulfones and sulfides in good yields. A wide range of substrates and functional groups were well-tolerated under optimized conditions. Moreover, control experiments have been conducted, indicating a radical pathway involved in the reaction mechanisms.

Among the available methods to increaset he molecular complexity,s igmatropic rearrangementso ccupy a distinct positioni no rganic synthesis. Despite being known for over ac entury sigmatropic rearrangement reactions of ylides via carbene transfer reaction have only recently come of age. Most of the ylide mediated rearrangement processes involve rupture of a s-bond and formation of an ew bond between p-bond and negatively charged atom followed by simultaneous redistribution of p-electrons. This minireview describes the advances in this research area made in recent years, which now opens up metal-catalyzed enantioselective sigmatropic rearrangement reactions, metal-free photochemicalr earrangement reactions and novel reaction pathways that can be accessed via ylide intermediates. Scheme1.Rearrangement reactions.

Depending on the solvent used, benzyl sulfides undergo in the presence of a rhodium(ii) catalyst either [1,2]- or [2,3]-sigmatropic rearrangement.

Cyclopropanation reactions of carbenes with arenes provide a straightforward pathway to norcaradienes or cycloheptatrienes. This reaction normally requires harsh reaction conditions or transition-metal catalysts. In this report, we describe the metal-free visible-light photolysis of aryl diazoacetates in aromatic solvents, which provides access to the norcaradiene ring system in a highly regio- and stereoselective manner. The mild reaction conditions of this approach also allow chemoselective cyclopropanation of substituted arenes without competing C–H functionalization reactions.

Sigmatropicr earrangement reactions constitute one of the most fundamental reactionso fc arbenes. While state-of-the-arts ynthetic methods require the use of expensivep recious metalc atalysts, the application of visible light for the photolysiso fa-aryldiazoacetates is much less investigated and provides an operationally simple entry to carbenes under mild reaction conditions. Herein,w e report on blue-light induced sigmatropic rearrangement reactions of sulfur compounds with a-aryldiazoacetates. This process, depending on the substitutionp attern of the sulfide, opens up formali nsertion reactions of carbenes into SÀN, SÀC, or CÀHb onds.Scheme1.Photochemical sigmatropicrearrangement reactions.

Nitrene transfer reactions represent one of the key reactions to rapidly construct new carbon-nitrogen bonds and typically require transition metal catalysts to control the reactivity of the pivotal nitrene intermediate. Herein, we report on the application of iminoiodinanes in amination reactions under visible light photochemical conditions. While a triplet nitrene can be accessed under catalyst-free conditions, the use of a suitable photosensitizer allows the access of a nitrene radical anion. Computational and mechanistic studies rationalize the access and reactivity of triplet nitrene and nitrene radical anion and allow the direct comparison of both amination reagents. We conclude with applications of both reagents in organic synthesis and showcase their reactivity in the reaction with olefins, which underline their markedly distinct reactivity. Both reagents can be accessed under mild reaction conditions at room temperature without the necessity to exclude moisture or air, which renders these metal-free, photochemical amination reactions highly practical.

We report on the use of visible light to conduct carbene-transfer reactions of donor/acceptor diazoalkanes with cyclooctatetraene and polyunsaturated carbocycles to give the corresponding cyclopropanes in excellent yields with excellent stereoselectivities. This photochemical protocol proved to be superior to conventional metal-catalyzed cyclopropanation reactions and now provides platform chemicals containing a cyclic conjugated all-cis triene. The cyclopropane is an important structural feature to prevent 6π electrocyclization. Instead, the triene moiety can now be selectively functionalized in cycloaddition reactions.