Pop-the-Cork Strategy in Synthetic Utilization of Imines:  Stabilization by Complexation and Activation via Liberation of the Ligated Species

• Published in: Inorganic chemistry 2003-06, Vol.42 (11), p.3602-3608
• Main Authors: Bokach, Nadezhda A, Kukushkin, Vadim Yu, Haukka, Matti, Fraústo da Silva, João J. R, Pombeiro, Armando J. L
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic034086j

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]Cl2 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.