Diamides are ubiquitous in chemical structures of organic molecules with a variety of applications. Most of these diamides are accessed via acylation of diamines with corresponding acyl donors, using a protecting group, or with excess amounts of diamines to prevent unwanted diacylation reactions. Here we report a practical and atom-economical method to access monoamide and unsymmetric diamides with diamines and stoichiometric amounts of acyl chlorides - the most accessible and highly reactive acyl donor. The reactivity of diamines can be controlled by CO₂ as a green, temporary and traceless protecting group for monoacylation reactions, achieving high atom efficienty and low environmental factor. We demonstrated the utility of the method with a broad substrate scope and a large scale reaction, and performed control experiments to elucidate the origin of the high selectivity obtained with CO₂. We showed that ambient CO₂ from thin air affected the selectivity of monoacylation of diamines, implying the pervasive influence of atmospheric CO₂ in organic synthesis.