Ylides are powerful intermediates or reactants leading to complex molecular structures from rather simpler starting material. The aim of this PhD was to develop new synthetic methodologies involving specifically nitrogen ylides. In a first topic, the dearomatization of various azaaromatic compounds was achieved through the double addition of metal-carbenes generated from α-diazo-β-ketoesters under [CpRu] catalysis. It was indubitably proven that nitrogen ylide intermediates were involved in the reaction pathway. Moreover, starting from various nitrogen ylide intermediates, the successful introduction of symmetrical malonate-based carbenes was possible. Several dearomatized products were thus obtained for both processes (one-pot and stepwise) in high yields. In a second topic, an unexpected Ru(II)-catalyzed cascade reaction involving diazo malonates and bicyclic alkenes was discovered leading to the formation of bridged polycyclic diaza heterocycles. The three successive steps of the reaction were clearly identified by isolation in high yields of two intermediates, including an azomethine imine.