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(Prn)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
1H, 13C{1H}, 15N, and 195Pt 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 trans-[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 15N,1H-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 31P{1H} 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 hydroxo-functionalized amidines 8−10 were converted into oxazolines (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.