Here we carried out quantum chemistry calculations to study the proposed mechanism of the [4+2+1] reaction of CP-capped diene-ynes shown in Scheme 1c and Figure . A very important question to answer here is why [2+2+1] reaction did not happen for the CP capped dienes. We proposed that the catalytic species of this reaction is a monomer of the catalyst, [Rh(CO2)Cl], similar to that in the [4+2+1] and [5+2+1] reactions. The reaction starts from generating complex IN1 from the dimeric [Rh(CO)2Cl]2 and model substrate with an oxygen tether. Then IN1 which can dissociate a CO to generate IN2. This step has an estimated free energy of 17.0 kcal/mol (via TSCO-diss) if we assume its reverse reaction of CO coordination is diffusioncontrolled (the CO concentration is estimated to be 5.5 mM). 17 . Therefore, the most difficult step in the catalytic cycle is reductive elimination with an activation free energy of 20.0 kcal/mol (From IN8 to TS4). The resting state of this reaction is not known and we expected that the real activation free energy of the present [4+2+1] could be higher than 20.0 kcal/mol.