Platinum(IV)‐Mediated Nitrile–Amidoxime Coupling Reactions: Insights into the Mechanism for the Generation of 1,2,4‐Oxadiazoles

Abstract

AbstractThe nucleophilic addition of amidoximes R′C(NH2)NOH (4: R′=Me, 5: CH2Ph, 6: Ph) to coordinated nitriles in the platinum(IV) complexes trans‐[PtCl4(RCN)2] (1: R=Et, 2: Ph, 3: NMe2) proceeds in a 1:1 molar ratio and leads to the monoaddition products [PtCl4(RCN){HNC(R)ONC(R′)NH2}] (7: R/R′: Et/CH2Ph, 8: Et/Ph, 9: NMe2/CH2Ph, 10: NMe2/Ph). Meanwhile, if the nucleophilic addition proceeds in a 2:1 molar ratio the reaction gives the bisaddition species [PtCl4{HNC(R)ONC(R′)NH2}2] (11: R/R′=Et/Me, 12: Et/CH2Ph, 13: Et/Ph, 14: Ph/Ph, 15: NMe2/Me, 16: NMe2/CH2Ph, 17: NMe2/Ph). All complexes 7–17 bear nitrogen‐bound O‐iminoacylated amidoxime groups. The addition of one equivalent of the corresponding amidoxime to each of 7–10 leads to 12, 13, 16, and 17, respectively. Complex [PtCl4(NCNMe2){HNC(NMe2)ONC(Ph)NH2}] (10), when dissolved in MeNO2, gave mer‐[PtCl3{HNC(NMe2)ONC(Ph)NHC(NMe2)NH}] (18) with the newly formed tridentate ligand derived from an unexpected coupling between two Me2NCN ligands and the N and the O centers of the amidoxime. The O‐imidoylamidoxime compounds R′C(NH2)NOCR(NH) (19–25) were liberated from the corresponding complexes 11–17 by treatment with excess NaCN and these metal‐free species were characterized by 1H and 13C{1H} NMR spectroscopy. The conversion of 19–25 into the 3,5‐substituted 1,2,4‐oxadiazole compounds OaNC(R′)NCb(R)(a–b) (26: R/R′=Me/Et, 27: PhCH2/Et, 28: Ph/Et, 29: Ph/Ph, 30: Me/NMe2, 31: PhCH2/NMe2, 32: Ph/NMe2) occurs at room temperature and the cyclization is promoted by strong acceptor substituents R′.