In this work, three speculative mechanisms of the reaction between triphenylphosphine and dimethyl acetylendicarboxylate in the presence of 3-chloropentane-2,4-dione were energetically and thermodynamically developed using quantum mechanical calculations and were profoundly compared with stopped-flow and UV spectrophotometry approaches. The third speculative mechanism that led to the five-membered ring structure was experimentally and theoretically favorable. The five-membered ring structure of product was characterized by X-ray crystallographic data. Also, steps 1 and 2 of the third mechanism were determined as fast and rate-determining steps, respectively. The experimental kinetic evidence of the formation and decay of intermediate in steps 1 and 2 (fast and rate-determining steps, respectively) was compatible with theoretical data. Experimental kinetic data were recognized for overall reaction along with activation parameters for fast and rate-determining steps of the reaction. Theoretical kinetic data (k and Ea) and activation parameters (ΔG≠, ΔS,≠ and ΔH≠) were calculated for each step and overall reactions.