In the context of [3+6+3+6] macrocyclization reactions, precise kinetics of α-diazo-β-ketoester decomposition were measured by in situ infrared (IR) monitoring. Dirhodium complexes of Ikegami–Hashimoto type—and perchlorinated phthalimido derivatives in particular—performed better than classical achiral complexes. Clear correlations were found between speciation among dirhodium species and catalytic activity. With these results, novel cyclohexyl-derived catalysts were developed, affording good yields of macrocycles (up to 78%), at low catalyst loadings (from 0.01 mol % to 0.001 mol %) and on a large scale (from 1 g to 20 g).