Non-redox Oxy-Insertion via Organometallic Baeyer–Villiger Transformations: A Computational Hammett Study of Platinum(II) Complexes

A Hammett analysis of platinum-mediated oxy-insertion into Pt–aryl bonds is performed using DFT calculations. Modeled transformations involve the conversion of cationic Pt^II-aryl complexes [(^Xbpy)Pt(R)(OY)]⁺ (R = <i>p</i>-X-C₆H₄; Y = 4-X-pyridine; ^Xbpy = 4,4′-X-bpy; X = NO₂, H, NMe₂) to the corresponding [(^Xbpy)Pt(OR)]⁺ complexes via an organometallic Baeyer–Villiger (BV) pathway. Computational modeling predicts that incorporation of an electron-deficient NO₂ group at the 4-position of pyridine-<i>N</i>-oxide lowers the activation barrier to the organometallic BV transformation. In contrast, computational studies reveal that increasing the donor ability of the migrating aryl group, by placement of NMe₂ at the <i>para</i> position, lowers the activation barrier to the oxy-insertion step. The impact on the calculated activation barrier is greater for variation of the R group than for modification of Y of the oxygen delivery reagent. For the <i>p</i>-NO₂/<i>p</i>-NMe₂-substituted aryl migrating groups (R), the ΔΔ<i>G</i>^‡ for X = NMe₂ versus X = NO₂ is 12 kcal/mol, which is three times larger than that calculated for the changes that occur upon substitution of NO₂ and NMe₂ groups (ΔΔ<i>G</i>^‡ ≈ 4 kcal/mol) at the 4-position of the pyridine group. For these Pt^II complexes with bipyridine (bpy) supporting ligands, the influence of modification of the bpy ligand is calculated to be minimal with ΔΔ<i>G</i>^‡ ≈ 0.4 kcal/mol for the oxy-insertion of bpy ligands substituted at the 4/4′ positions with NMe₂ and NO₂ groups. Overall, the predicted activation barriers for oxy-insertion (from the YO adducts [(^Xbpy)Pt(R)(OY)]⁺) are large and in most cases are >40 kcal/mol, although some calculated Δ<i>G</i>^‡'s are as low as 32 kcal/mol.