Herein, we report the Aza-Büchner–Curtius–Schlotterbeck reaction (ABCS)– a novel method for the uncatalyzed C─C insertion reaction of photogenerated donor-donor diazo compounds into aryl aldimines. The transformation proceeds under very mild conditions and high chemoselectivity, favoring C─C over C─H insertion pathways. In a single step, two C(sp2)─C(sp3) bonds are formed, granting access to homologated imines bearing a newly constructed quaternary carbon center in good yields. Subsequent hydrolysis gave homologated aldehydes, enabling versatile downstream transformations. The diazo intermediate is generated using a continuous-flow microreactor, enhancing both safety and scalability. Mechanistic insights, supported by density functional theory (DFT) calculations, reveal the origins of the observed selectivity and reactivity.