The major goal of this Thesis was to discover conceptually-novel carbon-based reactive species for the development of new reactivity rules at carbon and disconnection approaches that have been under-appreciated and missing in chemical synthesis. Firstly, we developed a general platform for the generation of radical carbenoids species – a radical adjacent to an excellent leaving group- by photoredox catalysis. These species enabled a stereoconvegent cyclopropanation of styrenes using commercially available or easy-to-synthesize gem-diiodoalkanes and E/Z-mixtures of alkenes. Our new cyclopropanation reaction overcomes the limitations of current methods regarding functional group tolerance, safety, and ability to functionalize biomolecule derivatives. Secondly, we pioneered the generation of neutral carbyne equivalents by means of photoredox catalysis. This study revealed the underexploited dual carbene/radical behavior and ability of neutral carbynes to form three new bonds; and provided the fundaments to an assembly-point functionalization for chiral center sequencing through diazo compounds in simple arenes and medically relevant agents.