Amidines derived from Pt(IV)-mediated nitrile-amino alcohol coupling and their Zn(II)-catalyzed conversion into oxazolines

The reaction between the platinum(IV) complex trans-[PtCl4(EtCN)(2)] and the amino alcohols NH2CH2CH2OH, NH2CH2CH(Me)OH-(R)-(-), NH2CH(Ph)CH2OH-(R)-(-), NH2CH(Et)CH2OH-(R)-(-), NH2CH(Et)CH2OH-(S)-(+), and NH2CH(Pr-n)CH2OH proceeds rapidly at room temperature in CH2Cl2 to furnish the amidine complexes [PtCl4-{HN=C(Et)NH OH}(2)] (1-6) in good yield (70-80%). The related reaction between the platinum(II) complex trans[PtCl2(EtCN)(2)] and monoethanolamine in a molar ratio of 1:2 in CH2Cl2 results in the addition of 4 equiv of NH2CH2CH2OH per mole of complex to give [Pt{HN=C(Et)NHCH2CH2OH}(2)(NH2CH2CH2OH)(2)](2+) (7), Formulation of 1-6 is based upon satisfactory C, H, N elemental analyses, electrospray mass spectrometry, IR spectroscopy, and H-1, C-13{H-1}, N-15, and Pt-195 NMR spectroscopies, while the structures of trans-[PtCl4{(Z)-NH=C(Et)NHCH2CH2OH}(2)] (1), trans-[PtCl4{(Z)-NH = C(Et)NHCH2CH(Me)OH-(R)-(-)}(2)] (2), and 'rans-[PtCl4{(Z)-NH=C(Et)NHCH(Et)CH2OH-(R)-(-)}(2)] (4) were determined by X-ray single-crystal diffraction. The Z-amidine configuration of the ligands is preserved in CDCl3 solutions as confirmed by gradient-enhanced N-15,H-1-HMQC spectroscopy and NOE experiments. The amidines, formed upon Pt(IV)-mediated nitrile-amino alcohol coupling, were liberated from their platinum(IV) complexes 1, 3, and 4 by reaction with Ph2PCH2CH2PPh2 (dppe) giving free NH=C(Et)NHCHRCH2OH (R = H 8, Et 9, Ph 10), with the substituents R of different types, and dppe oxides; the P-containing species were identified by P-31{H-1} NMR spectroscopy. NOESY spectroscopy indicates that the liberated amidines retained the same configuration relative to the C=N double bond, i.e., syn-(H,Et)-NH=C(Et)NHCHRCH2OH. The liberated hydroxofunctionalized amidines 8-10 were converted into oxazolines N=C(D)OCKCH(R) (11-13) in the presence of a catalytic amount of ZnCl2. A similar catalytic effect has also been reached using anhydrous MSO4 (M = Cu, CO, Cd), CdCl2, and AlCl3.