Examination of the reaction scope reveals that structural features of the aniline component greatly impact the reaction yield. Thus, we wished to identify which of the reaction steps is influenced by the aniline substitution pattern. The role of aniline in the mechanism is unique, it first behaves as a nucleophile that adds to ketone to form imine; thus, anilines bearing electron-donating groups should increase reactivity in this step. However, electron-donating aniline-derived intermediates would not facilitate the following tautomerization and cycloaddition steps. Based on this dichotomy, aniline served as an ideal probe for exploring the proposed reaction mechanism and identifying which steps are associated with reactivity. We first attempted to correlate Hammett parameters to the yield of reaction using meta-and para-substituted anilines. In the absence of the m-anisidine, we identified a good correlation with σ+ values (see SI Figure for details). The slope of the plot was positive, which classically indicates the formation of a negative charge or the collapse of a positive charge at the rate-determining step. This analysis fits well with our observation that electron-withdrawing substituents increase reactivity given a collapse of positive charge at the postulated iminium transition state (Figure .d.). Thus, this analysis supports the formation of enamine as being rate-limiting. Moreover, the poor fit with σ values and the reduced correlation with m-anisidine included, both support the important contribution of resonance to the rate-determining transition state(s).