Inner‐Sphere Oxygen Activation Promoting Outer‐Sphere Nucleophilic Attack on Olefins

Abstract

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 ORC8H11 (R=Me, Et) resulted, and the reaction is enantioselective if a chiral amino acid, such as l‐proline, is used instead of Hpic.

What Makes a Good (Computed) Energy Profile?

“…The importance of the proper description of the methanol solvent can be appreciated comparing the relative Gibbs energies in methanol solvent of the ion pair ([Ir(pic)2(cod)] + ,[OCH3] -) with respect to the neutral complex [Ir(pic)2(MeOC8H12)] (2a) (Figure 16). With a pure continuum model the ion pair lies 28.4 kcal mol .1 above 2a, but when a cluster of five molecules of methanol was incorporated to stabilize methoxide anion it was found to be only 11.0 kcal mol -1 above 2a, in much better agreement with the experimental evidences [106].…”

“…A proper understanding of reactions in hydrogen-bonded solvents requires taking into account the nearest solvating molecules on an atomistic level. ), in methanol solvent, of the ion pair ([Ir(pic)2(cod)] + ,[OCH3] -) with respect the neutral complex [Ir(pic)2(MeOC8H12)] (2a), with a continuum solvent model and with a cluster-continuum solvent model which incorporates five solvent molecules [106]. Hydrogen atoms attached to carbons have been omitted for clarity.…”

Beyond Continuum Solvent Models in Computational Homogeneous Catalysis

“…For instance, complex [Ir(pic) 2 (MeOC 8 H 12 )] (pic = picolinate ligand) undergoes a fast isomerization between 2a and 2b structures (Fig. 8) [50]. A plausible pathway for this isomerization would entail the cleavage of the C-OCH 3 bond to form the ionic pair [Ir(pic) 2 (cod)][OCH 3 ].…”

Iridium Complex of N‐Fused Bilatrienone: Oxidative Cleavage of N‐Fused Porphyrin Induced by Iridium‐Cyclooctadiene Complexation

“…At present, the mechanistic insight into the formation of the bilatrienone complex Ir ‐ 3 remains unclear. The in situ formation of the (cod)iridium‐peroxide species was assumed to involve an attack at a specific alkene moiety in the electron‐rich NFP skeleton, which resulted in the oxidative degradation of the macrocycle (Scheme S3) [39,40] …”