Kinetic and Thermodynamic Aspects of the Regioselective Addition of Bifunctional Hydroxylaminooxime-type HO-Nucleophiles to Pt-Complexed Nitriles

The coupling between coordinated propiononitriles in trans-[PtCl_n(EtCN)₂] (n = 2, 4) and the 1,2-hydroxylaminooximes HON(H)CMe₂C(R)NOH (R = Ph 1, Me 2) proceeds smoothly in CHCl₃ at ca. 40−45 °C and gives trans-[PtCl_n{NHC(Et)ON(H)CMe₂C(R)NOH}₂] (n = 2, R = Ph 5, Me 6; n = 4, R = Ph 7, Me 8) in 80−85% isolated yields. The reaction is highly regioselective, and both spectroscopic (IR; FAB⁺-MS; 1D ¹H, ¹³C{¹H}, and ¹⁹⁵Pt NMR; and 2D ¹H,¹³C HMQC, ¹H,¹³C HMBC, and ¹H,¹⁵N HMQC NMR) and X-ray data for 6−8 suggest that the addition proceeds exclusively via the hydroxylamine moiety of the 1,2-hydroxylaminooxime species; the existence of an oxime group remote from the nucleophile was also confirmed. Heating of 6 in air leads to its conversion to the unusual nitrosoalkane complex [PtCl₂{HONC(Me)C(Me)₂NO}] (9), whereas in the case of 5, only the metal-free salt [H₃NC(Me)₂C(Ph)NOH]₂(NO₃)Cl·H₂O (10) was isolated. To compare the kinetic aspects and trends in the addition of both types of nucleophiles (oximes and hydroxylamines; for the latter, see our recent work:  Inorg. Chem. 2005, 44, 2944) to coordinated nitriles, a kinetic study of the addition of HONC(CH₂Ph)₂ to [Ph₃PCH₂Ph][PtCl₅(EtCN)] (11) to give [Ph₃PCH₂Ph][PtCl₅{NHC(Et)ONC(CH₂Ph)₂}] (12) was performed. The calculated rate constant k₂ of 3.9 × 10^-6 M^-1 s^-1 at −20 °C for the addition of the oxime indicates that the hydroxylamine is, by a factor 1.7 × 10⁴, more reactive toward the addition to nitriles than the oxime. Results of the synthetic, kinetic, and theoretical (at the B3LYP level of theory) studies have demonstrated that the high regioselectivity of the reactions of the 1,2-hydroxylaminooximes with ligated nitriles is both kinetically and thermodynamically controlled.