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A silver-catalyzed tandem carbon-carbon triple bond hydroazidation, radical addition, and cyclization of biphenyl acetylene is described under mild conditions, leading to the formation of 6-methyl sulfonylated phenanthridines in good yields. In this novel cascade reaction, most of the atoms are incorporated into the product without cleavage of the C·C bond. Mechanistic studies suggest the reaction should proceed through an iminyl radical reactive intermediate. [ABSTRACT FROM AUTHOR]

Here we report a silver-catalyzed alkynyl carbene insertion into β-ketocarbonyls using alkynyl N -nosylhydrazones as alkynyl carbene precursors, which provides access to trisubstituted allenyl ketones. This reaction represents the first example of an alkynyl carbene insertion into a C-C σ bond, affording products homologated with an sp 2 carbon center. The products are useful substrates for further transformations. Experimental investigations and theoretical calculations suggest the reaction proceeds through a stepwise enol cyclopropanation/retro-aldol pathway.