Using a system that accelerates the serendipitous discovery of new reactions by evaluating hundreds of DNA-encoded substrate combinations in a single experiment,we explored a broad range of reaction conditions for new bond-forming reactions.We discovered reactivity that led to a biomolecule-compatible,Ru(Ⅱ)-catalyzed,visible light-induced azide reduction reaction.In contrast with current azide reduction methods,this reaction is highly chemoselective and is compatible with alcohols,phenols,acids,alkenes,alkynes,aldehydes,alkyl halides,alkyl mesylates,and disulfides.The remarkable functional group compatibility and mild conditions of this reaction enabled azide reduction to be performed on nucleic acid and oligosaccharide substrates without the detectable occurrence of side reactions.The reaction was also performed in the presence of a protein enzyme without loss of enzymatic activity,in contrast with two commonly used azide reduction methods.The visible light dependence of this reaction provides a means of photouncaging functional groups such as amines and carboxylates on biological macromolecules without using UV irradiation.