Switching regioselectivity in the sequential iodoplatination of methyl propiolate CC triple bond by PtIV iodo complexes

• Iodoplatination of HCC–C(O)OMe by PtMeI52− occurs against Markovnikov's rule.
• Chelation of Pt by COOMe group in the vinyl ligand explains unusual regioselectivity.
• Further iodoplatination by the metallocycle occurs according to Markovnikov's rule.
• Such chelation labilizes coordinated O–Me bond regarding its cleavage liberating MeI.

Stepwise iodoplatination of methyl propiolate CC triple bond in the system PtII – CH3I – NaI – acetone leads to the formation of methyl derivative of bis-σ-vinyl PtIV complex in which the platinum atom is attached simultaneously to the terminal and internal C(sp2)-atoms. The first iodoplatination step forms chelate metallocycle (1a) in which platinum atom is bound to the terminal C-atom of the vinyl ligand contrary to the expected for an alkyne with electron withdrawing substituent. The obvious reason for the unusual regioselectivity is more favorable thermodynamics of such mode of the reaction: chelation of the metal center by the oxygen atom of the carboxymethyl group in the σ-vinyl ligand forming metallocycle 1a provides an additional stabilization of the product. In the second iodoplatination step an opposite regioselectivity is observed: iodide ion addition occurs at the terminal carbon atom, and the metal center adds to the internal C-atom in formal agreement with Markovnikov's rule yielding product 2a with different regioselectivity in vinyl ligands. The formed organoplatinum(IV) derivatives contain auxiliary halogenide ligands similar to the key intermediates in the Shilov system and therefore can be considered as compounds modeling some peculiarities of Shilov chemistry.

Methyl propiolate CC bond iodoplatination by Pt(CH3)I52− formed in equilibrium via CH3I oxidative addition to PtI42− yields chelate metallocycle 1a with the opposite to Markovnikov's rule regioselectivity. Subsequent iodoplatination of the alkyne by the complex 1a occurs according to Markovnikov's rule yielding product 2a with different regioselectivity in vinyl ligands.Figure optionsDownload as PowerPoint slide