Activation free energy irreversible process

The acetylene coordination to the active catalyst HCo(CO)3 forms the most stable 7i-complex (15a) with the C=C bond in the equatorial plane. The following step of acetylene insertion into Co-H bond leads to the stable vinyl complex (H2C=CH)Co(CO)3 (15b), which has a butterfly geometry with vinyl group in the axial site. In addition, acetylene insertion has an activation free energy of 28.5 kJ/mol, and is an exergonic (—84.9 kJ/mol) and irreversible process, in contrast to the thermal neutral process of alkenes. Therefore, it is expected that the regioselectivity of terminal alkynes RC=CH with HCo(CO)3 leading to linear or branched product should be controlled by this irreversible step. 

In general, the normal DA reaction mechanism is a domino process that is initialized by the polar reaction between the diene and the dienophile to give the primary cicloadduct. These DA reactions have a two-step non-intermediate mechanism characterized by the nucleophilic attack on the non-substituted methylene of the diene to the electrophilically activated position of the dienophile. The subsequent ring-closure affords the primary cicloadduct. This behavior makes the reaction to be regioselective. The latter concerted elimination of the nitrous acid from the primary cicloadduct yields the precursor of the final aromatic product. Spite of the large activation free energy associated with the DA reaction and the endergonic character of formation of the primary cicloadduct, the irreversible extrusion of the nitrous acid make feasible thermodynamically the domino reaction. 

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