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Inner‐Sphere Oxygen Activation Promoting Outer‐Sphere Nucleophilic Attack on Olefins
Author(s) -
Abril Paula,
Río M. Pilar,
López José A.,
Lledós Agustí,
Ciriano Miguel A.,
Tejel Cristina
Publication year - 2019
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201903068
Subject(s) - chemistry , iridium , nucleophile , medicinal chemistry , olefin fiber , picolinic acid , cationic polymerization , hydroxylation , alkoxy group , rhodium , peroxide , photochemistry , stereochemistry , organic chemistry , catalysis , alkyl , enzyme
Alkoxylation and hydroxylation reactions of 1,5‐cyclooctadiene (cod) in an iridium complex with alcohols and water promoted by the reduction of oxygen to hydrogen peroxide are described. The exo configuration of the OH/OR groups in the products agrees with nucleophilic attack at the external face of the olefin as the key step. The reactions also require the presence of a coordinating protic acid (such as picolinic acid (Hpic)) and involve the participation of a cationic diolefin iridium(III) complex, [Ir(cod)(pic) 2 ] + , which has been isolated. Independently, this cation is also involved in easy alkoxy group exchange reactions, which are very unusual for organic ethers. DFT studies on the mechanism of olefin alkoxylation mediated by oxygen show a low‐energy proton‐coupled electron‐transfer step connecting a superoxide–iridium(II) complex with hydroperoxide–iridium(III) intermediates, rather than peroxide complexes. Accordingly, a more complex reaction, with up to four different products, occurred upon reacting the diolefin–peroxide iridium(III) complex with Hpic. Moreover, such hydroperoxide intermediates are the origin of the regio‐ and stereoselectivity of the hydroxylation/alkoxylation reactions. If this protocol is applied to the diolefin–rhodium(I) complex [Rh(pic)(cod)], free alkyl ethers ORC 8 H 11 (R=Me, Et) resulted, and the reaction is enantioselective if a chiral amino acid, such as l ‐proline, is used instead of Hpic.