Hydrazines and diazenes are structurally and electronically unique molecules which have two adjacent nitrogen atoms either connected by single or double bond. The Lambert group has demonstrated the underexplored features of these molecules and revealed that these compounds are competent catalysts or mediators in various organic transformations. This dissertation will discuss two reaction platforms each enabled by hydrazine and diazene: (1) hydrazine-catalyzed or mediated carbonyl-olefin metathesis and (2) cyclopropanation via photocatalytic denitrogenation of diazene. Chapter 1 introduces general methodologies of carbonyl-olefin metathesis and provides background on their developments. This chapter will discuss the advantages and limitations of methods and how hydrazine has arisen as a new class of organocatalyst in the field of carbonyl-olefin metathesis. Chapter 2 presents the development of hydrazine-catalyzed ring-closing carbonyl-olefin metathesis (RCCOM) to synthesize polycyclic heteroaromatic compounds (PHAs). This work demonstrates how hydrazine catalyst can overcome the limitation of Lewis acid catalyzed methodology and addresses broad functionality tolerance. Chapter 3 describes the development of a new design of hydrazine catalyst and potential reactivity towards ring-opening carbonyl-olefin metathesis (ROCOM). Synthetic pathways to five-membered ring hydrazine-pyrazolidine motifs- have been developed; notably enantiomerically pure pyrazolidines have been generated. The optically active pyrazolidine-mediated asymmetric ROCOM was evaluated. Based on the results, the chapter suggests future direction of the next generation catalyst design. Finally, Chapter 4 introduces the chemistry of diazene by describing its synthetic utility for highly substituted cyclopropanes. Photochemical reaction allows tosylhydrazone and unactivated olefins to generate cyclic diazenes in situ, and under mild conditions, energy transfer photocatalytic platform enables denitrogenation of diazenes. This one-pot, two-step process generated cyclopropanes accommodating a broad range of functionality, highlighting the novelty of photocatalytic methodologies.