Non-redox oxy-insertion via organometallic Baeyer-Villiger transformations: A computational Hammett study of platinum(II) complexes

摘要

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 = p-X-C_6H_4; Y = 4-X-pyridine; ~Xbpy = 4,4′-X-bpy; X = NO_2, H, NMe_2) to the corresponding [(~Xbpy)Pt(OR)]~+ complexes via an organometallic Baeyer-Villiger (BV) pathway. Computational modeling predicts that incorporation of an electron-deficient NO_2 group at the 4-position of pyridine-N-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 NMe_2 at the para 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 p-NO_2/p-NMe_2-substituted aryl migrating groups (R), the δδG? for X = NMe_2 versus X = NO_2 is 12 kcal/mol, which is three times larger than that calculated for the changes that occur upon substitution of NO_2 and NMe_2 groups (δδG? ≈ 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 δδG? ≈ 0.4 kcal/mol for the oxy-insertion of bpy ligands substituted at the 4/4′ positions with NMe_2 and NO_2 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 δG?'s are as low as 32 kcal/mol.