Our model highlights the intricate interplay between electronic features of the primary amine and the steric hindrance of the aryl-bromide, both of which play pivotal roles in determining the success of Ullmann C-N couplings. Moreover, these features offer insights about the substrate chemical space of Ullmann coupling reactions. This is visually represented in the chemical space plot presented in Fig. . It is evident from the plot that our training set (depicted as circles) spans all four quadrants of the decision tree chemical space, with good representation in each. However, there were regions within the plot that remained unexplored, specifically those involving combinations of sterically hindered aryl-bromides and nucleophilic primary amines. The chemical space plot was thus used to guide the external validation of the classification model. For validation purposes, we employed four in-sample ligands L1, L21, L28, and L33 and out-of-sample substrates that were not encountered during the training phase. This involved introducing a set of 12 new aryl-bromides and six primary amines, resulting in a total of 20 new products (depicted as crosses in Fig. ). This comprehensive approach was designed to span a wide range of the defined chemical space by targeting underrepresented regions with examples of aryl bromides with significant steric hindrance and amines of varying nucleophilicity. The resulting accuracy of this validation phase was 87%, which is virtually identical to the accuracy achieved in the test set during the training phase (88%) and highlights the robustness and reliability of the classification model.