Importance of Platinum(II)-Assisted Platinum(IV) Substitution for the Oxidation of Guanosine Derivatives by Platinum(IV) Complexes

Guanosine derivatives with a nucleophilic group at the 5′ position (G-5′) are oxidized by the Pt^IV complex Pt(d,l)(1,2-(NH₂)₂C₆H₁₀)Cl₄ ([Pt^IV(dach)Cl₄]). The overall redox reaction is autocatalytic, consisting of the Pt^II-catalyzed Pt^IV substitution and two-electron transfer between Pt^IV and the bound G-5′. In this paper, we extend the study to improve understanding of the redox reaction, particularly the substitution step. The [Pt^II(NH₃)₂(CBDCA-O,O′)] (CBDCA = cyclobutane-1,1-dicarboxylate) complex effectively accelerates the reactions of [Pt^IV(dach)Cl₄] with 5′-dGMP and with cGMP, indicating that the Pt^II complex does not need to be a Pt^IV analogue to accelerate the substitution. Liquid chromatography/mass spectroscopy (LC/MS) analysis showed that the [Pt^IV(dach)Cl₄]/[Pt^II(NH₃)₂(CBDCA-O,O′)]/cGMP reaction mixture contained two Pt^IVcGMP adducts, [Pt^IV(NH₃)₂(cGMP)(Cl)(CBDCA-O,O′)] and [Pt^IV(dach)(cGMP)Cl₃]. The LC/MS studies also indicated that the trans,cis-[Pt^IV(dach)(³⁷Cl)₂(³⁵Cl)₂]/[Pt^II(en)(³⁵Cl)₂]/9-EtG mixture contained two Pt^IV-9-EtG adducts, [Pt^IV(en)(9-EtG)(³⁷Cl)(³⁵Cl)₂] and [Pt^IV(dach)(9-EtG)(³⁷Cl)(³⁵Cl)₂]. These Pt^IVG products are predicted by the Basolo−Pearson (BP) Pt^II-catalyzed Pt^IV-substitution scheme. The substitution can be envisioned as an oxidative addition reaction of the planar Pt^II complex where the entering ligand G and the chloro ligand from the axial position of the Pt^IV complex are added to Pt^II in the axial positions. From the point of view of reactant Pt^IV, an axial chloro ligand is thought to be substituted by the entering ligand G. The Pt^IV complexes without halo axial ligands such as trans,cis-[Pt(en)(OH)₂Cl₂], trans,cis-[Pt(en)(OCOCF₃)₂Cl₂], and cis,trans,cis-[Pt(NH₃)(C₆H₁₁NH₂)(OCOCH₃)₂Cl₂] ([Pt^IV(a,cha)(OCOCH₃)₂Cl₂], satraplatin) did not react with 5′-dGMP. The bromo complex, [Pt^IV(en)Br₄], showed a significantly faster substitution rate than the chloro complexes, [Pt^IV(en)Cl₄] and [Pt^IV(dach)Cl₄]. The results indicate that the axial halo ligands are essential for substitution and the Pt^IV complexes with larger axial halo ligands have faster rates. When the Pt^IV complexes with different carrier ligands were compared, the substitution rates increased in the order [Pt^IV(dach)Cl₄] < [Pt^IV(en)Cl₄] < [Pt^IV(NH₃)₂Cl₄], which is in reverse order to the carrier ligand size. These axial and carrier ligand effects on the substitution rates are consistent with the BP mechanism. Larger axial halo ligands can form a better bridging ligand, which facilitates the electron-transfer process from the Pt^II to Pt^IV center. Smaller carrier ligands exert less steric hindrance for the bridge formation.