Recently, combinations of cyclopentadienyl ruthenium(II) complexes and diimine ligands were found to catalyze several types of transformations such as allylic substitutions, Carroll rearrangements and diazo decompositions. The project of this PhD thesis was to explore new methodologies involving this catalytic combination to decompose α-diazo-β-ketoesters in presence of oxygenated Lewis bases. Formation of carbonyl or oxonium ylides and subsequent insertion reactions were studied, forming valuable heterocyclic compounds. In presence of cyclic ketones, lactones and carbonates spiro bicyclic ketals, orthoesters and orthocarbonates are afforded respectively. Moreover, in presence of γ-lactams, a Brook-like rearrangement is observed. The favored 1,2-acyl migration yields the olefination product as single regioisomer. Finally, in the case of oxetanes, the direct formation of dioxepine motif is observed with retention of configuration. This process is only possible with ruthenium(II) complexes as under rhodium(II) catalysis macrocyclisation reactions occur.