Nucleophilic Addition to Alkynes Hg, Pd

The addition of H2O to alkynes is catalyzed by Hg(II) under acidic conditions. (The uncatalyzed reaction is extremely difficult to carry out due to the instability of alkenyl cations.) 

The mechanism begins the same way as the Hg-mediated nucleophilic addition to alkenes. In the first step, an electrophilic tt complex forms between the alkyne and Hg(II). Water attacks one of the C s of the rr complex in Markovnikov fashion to give a 2-hydroxy-1-alkenylmercury(II) compound, an enol, which is protonated to give a carbocation. Fragmentative loss of Hg(II) then occurs to give a metal-free enol, which tautomerizes to give the ketone product. 

Pd(Il) salts catalyze the addition of nucleophiles to alkynes by exactly the same mechanism. The reaction is useful for intramolecular cyclization, such as the isomerization of 2-alkynylphenols to benzofurans. 

Transition-metal porphyrin complexes are found in such important proteins as hemoglobin, chlorophyll, and the cytochromes. Because of their biological significance, porphyrins and their simpler analogs have been extensively studied in the laboratory. Porphines and their close cousins the salens (salicylaldehyde eth-ylenediamine) are tetradentate, six-electron ligands, with two one-electron donors and two two-electron donors. Porphines are macrocycles made up of four pyrroles 

There is some controversy about the mechanisms of the Fe-catalyzed reactions and especially about the oxidation state of the metal in the catalytic cycle. The Fe complexes may be seen to proceed through an Fe(in)/(V) cycle or an Fe(II)/(IV) cycle. The situation is complicated by the fact that the tt system of the porphine or salen ligand can act as a one-electron reducing agent. 

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