Chemistry of diazo compounds is dominated by transition metal catalysis but recently, photoinitiated reactions of diazo compounds have attracted a lot of attention. This mini-review describes recent discoveries on the reactivity of diazo compounds under light irradiation.

This article describes direct photoalkylation of electron-rich aromatic compounds with diazo esters. C-2 alkylated indoles and pyrroles are obtained with good yields even though the photocatalyst (Ru(bpy)3Cl2) loading is as low as 0.075 mol %. For substrates bearing electronwithdrawing substituents the addition of a catalytic amount of N,N-dimethyl-4-methoxyaniline is required. Both EWG-EWG and EWG-EDG substituted diazo esters are suitable as alkylating agents. The reaction selectivity and mechanistic experiments suggest that carbenes/carbenoid intermediates are not involved in the reaction pathway, instead radical formation is proposed Five membered heteroaromatic rings are common structural motifs in pharmaceuticals, agrochemicals, and functional dyes. 1 Indoles and pyrroles exhibiting biological activity often possess at least one alkyl substituent attached to the aromatic ring either at 2 or 3 position. Consequently, mild and regioselective methods for C(sp 2 )-C(sp 3 ) bond formation are of high importance. Classic methods for C-H alkylation of electron-rich heteroaromatic compounds such as Friedel-Crafts type reactions or metallation followed by reaction with electrophiles are suitable for functionalization of simple derivatives, as they often require conditions that are not compatible with many functional groups. Current procedures employing transition metal catalysis broaden the scope, yet due to the character of a catalyst, anhydrous conditions, and elevated temperatures are often required. 2 For indole derivatives usually reactions lead to less challenging C3 alkylation products, if no directing group is present. 3 a Reaction conditions: 1 (1.25 mmol, 5 equiv), 2 (0.25 mmol, 1 equiv), Ru(bpy)3Cl2 (2.5 mol %), CH3CN (2.5 mL), blue LED irradiation, 8 h. b No light irradiation. c Reaction mixture was not degassed.The proposed mechanism for the alkylation of indoles is shown in Scheme 2. We assumed that the crucial step of this photochemical alkylation involves single electron reduction of diazoethyl acetate. But a detailed mechanistic overview needed further exploration. The addition of TEMPOa free radical scavenger halted the reaction completely proving that radical species are indeed involved. The Stern-Volmer analysis indicated that only EDA (1) quenches luminescence of the Ru-catalyst.

Photocatalytic reactions of diazoacetates with aldehydes led to a-alkylated carbonyl compounds instead of the expected cyclopropane derivatives.T he reactionr equiresadual catalytic systemphotocatalysis merged with enamine-iminiumc atalysis.N MR,E PR,U V/Vis,a nd ESI-MS analyses provided sufficient data to corroborate the proposed radical mechanism -e namine catalysism erged with photocatalysis.