Pop-the-cork strategy in synthetic utilization of imines: Stabilization by complexation and activation via liberation of the ligated species

Treatment of trans-[PtCl4(RCN)(2)] (R = Me, Et) with ethanol allowed the isolation of trans-[PtCl4{E-NH=C(R)OEt}(2)]. The latter were reduced selectively, by the ylide Ph3P=CHCO2Me, to trans-[PtCl2{E-NH=C(R)OEt}(2)]. The complexed imino esters NH=C(R)OEt were liberated from the platinum(II) complexes by reaction with 2 equiv of 1,2-bis(diphenylphosphino)ethane (dppe) in chloroform; the cationic complex [Pt(dppe)(2)]Cl-2 precipitates almost quantitatively from the reaction mixture and can be easily separated by filtration to give a solution of NH=C(R)OEt with a known concentration of the imino ester. The imino esters efficiently couple with the coordinated nitriles in trans-[PtCl4(EtCN)(2)] to give, as the dominant product, [PtCl4{NH=C(Et)N=C(R)OEt}(2)] containing a previously unknown linkage, i.e., ligated N-(1-imino-propyl)-alkylimidic acid ethyl esters. In addition to [PtCl4{NH=C(Et)N=C(Et)OEt}(2)], another compound was generated as the minor product, i.e., [PtCl4(EtCN){NH=C(Et)N=C(Et)OEt}], which was reduced to [PtCl2(EtCN){NH=C(Et)N=C(Et)OEt}], and this complex was characterized by X-ray single-crystal diffraction. The platinum(IV) complexes [PtCl4{NH=C(Et)N=C(R)OEt}(2)] are unstable toward hydrolysis and give EtOH and the acylamidine complexes trans-[PtCl4{Z-NH=C(Et)NHC(R)=O}(2)], where the coordination to the Pt center results in the predominant stabilization of the imino tautomer NH=C(Et)NHC(R)=O over the other form, i.e., NH2C(Et)=NC(R)=O, which is the major one for free acylamidines. The structures of trans-[PtCl4{Z-NH=C(Et)NHC(R)=O}(2)] (R = Me, Et) were determined by X-ray studies. The complexes [PtCl4{NH=C(Et)N=C(R)OEt}(2)] were reduced to the appropriate platinum(II) compounds [PtCl2{NH=C(Et)N=C(R)OEt}(2)], which, similarly to the appropriate Pt(IV) compounds, rapidly hydrolyze to yield the acylamidine complexes [PtCl2{NH=C(Et)NHC(R)=O}(2)] and EtOH. The latter acylamidine compounds were also prepared by an alternative route upon reduction of the corresponding platinum(IV) complexes. Besides the first observation of the platinum(IV)-mediated nitrile-imine ester integration, this work demonstrates that the application of metal complexes gives new opportunities for the generation of a great variety of imines (sometimes unreachable in pure organic chemistry) in metal-mediated conversions of organonitriles, the "storage" of imino species in the complexed form, and their synthetic utilization after liberation.