[2+3] cycloaddition of nitrones to platinum-bound organonitriles: Effect of metal oxidation state and of nitrile substituent

The ligated benzonitriles in the platinum(II) complex [PtCl2(PhCN)(2)] undergo metal-mediated [2+3] cycloaddition with nitrones -ON+(R-3)=C(R-1)(R-2) [R-1/R-2/R-3 = H/Ph/Me, H/p-MeC6H4/Me, H/Ph/CH2Ph] to give Delta (4)-1,2,4-oxadiazoline complexes, [PtCl2{N=C(Ph)O-N(R-3)-C(R-1)(R-2)}(2)] (2a, 4a, 6a), as a 1:1 mixture of two diastereoisomers, in 60-75%, while [PtCl2(MeCN)(2)] is inactive toward the addition. However, a strong activation of acetonitrile was reached by application of the platinum(IV) complex [PtCl4(MeCN)(2)] and both [PtCl4(RCN)(2)] (R = Me, Ph) react smoothly with various nitrones to give [PtCl4{N=C(R)O-N(R-3)-C(R-1)(R-2)}(2)] (1b-6b). The latter were reduced to the corresponding platinum(II) complexes [PtCl2{N=C(R)O-N(R-3)-C(R-1)(R-2)}(2)] (1a-6a) exhibit different kinetic labilities, and liberation of the Delta (4)-1,2,4-oxadiazolines by substitution with 1,2-bis(diphenylphosphino)ethane (dppe) in CDCl3 proceeds at different rates to give free N=C(R)O-N(R-3)-C(R-1)(R-2) and [PtCl2(dppe)] in almost quantitative NMR yield. All prepared compounds were characterized by elemental analyses, FAB mass spectrometry, and IR and H-1, C-13{H-1}, and 195 Pt (metal complexes) NMR spectroscopies; X-ray structure determination of the first (Delta (4)-1,2,4-oxadiazoline)Pt(II) complexes was performed for (S,S)/(R,R)-rac-[PtCl2{N=C(Me)O-C(H)Ph-2}(2)] (1a) (a = 9.3562(4), b = 9.8046(3), c = 13.1146(5) Angstrom; alpha = 76.155(2), beta = 83.421(2), gamma = 73.285(2)degrees; V = 1117.39(7) Angstrom (3); triclinic P (1) over bar, Z = 2), (R,S)-meso-[PtCl2-(N=C(Ph)O-N(Me)-C(H)Ph)(2)] (2a) (a = 8.9689(9), b = 9.1365(5), c = 10.1846(10) Angstrom; alpha = 64.328(6), beta = 72.532(4), gamma = 67.744(6)degrees; V = 686.82(11) Angstrom (3); triclinic, P (1) over bar, Z = 1), (S,S)/(R,R)-rac-[PtCl2(N=C(Me)O-N(Me)-C(H)(p-C6H4Me))(2)] (3a) (a = 11.6378(2), b = 19.0767(7), c = 11.5782(40 Angstrom; beta = 111.062(2)degrees; V = 2398(13) Angstrom (3); monoclinic, P2(1)/c, Z = 4), and (S,S)/(R,R)-rac-[PtCl2(N=C(Me)O-N(CH2Ph)-C(H)Ph)-C(H)Ph-2] (5a) (a = 10.664(2), b = 10.879(2), c = 14.388(3) Angstrom; alpha = 73.11(3), beta = 78.30(3), gamma = 88.88(30 degrees; V = 1562.6(6) Angstrom (3); triclinic, P (1) over bar, Z = 2).