The addition products 2 of a secondary amine to a coordinated olefin, in the cationic complexes [PtCl(η 2 -CH 2 =CHR)-(tmeda)] + (tmeda = N,N,NЈ,NЈ-tetramethylethylenediamine; R = Me, 1a; Ph, 1b, H, 1c), undergo in basic medium an intramolecular nucleophilic substitution with elimination of the chlorido ligand and formation of an azaplatinacyclobutane ring 3. The ring-closing process occurs notwithstanding the absence of a labilizing ligand trans to the leaving chlorido ligand and of bulky substituents on the amino-ethanide chain. If the addition product 2 is a mixture of Markovnikov and anti-Markovnikov isomers, the ring-closing reaction is faster for the anti-Markovnikov form, and this leads to an

Reaction of [PtCl(η2‐CH2=CHAr)(tmeda)](ClO4) (1) [Ar = C6H5 (1a), 4‐CH3OC6H4 (1b), 3‐NO2C6H4 (1c); tmeda = N,N,N′,N′‐tetramethylethylenediamine) with triethylamine, or even with inorganic carbonate, leads to the abstraction of a vinylic proton and formation of the (alkenyl)platinum species [PtCl{(E)‐CH=CHAr}(tmeda)] (2) in high yield. The initially formed (E) isomer, in solution of chlorinated solvents, partly isomerizes into the (Z) form [equilibrium ratio (E)/(Z) ≈︁ 4:1]; the isomerization reaction is rather slow and is catalyzed by visible light. The nature of the phenyl substituent (electron donor OMe or electron‐withdrawing NO2 group) does not appear to affect the rate of deprotonation. The nature and stereochemistry of the formed (alkenyl)platinum species has been elucidated through 1D and 2D 1H and 13C NMR spectroscopy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Rate constants for guanine rotation about the Pt-N7 bond in (R,S,S,R)-Me(2)DABPtG(2) complexes (Me(2)DAB = N,N'-dimethyl-2,3-diaminobutane; G = 9-EtG, 3'-GMP, and 5'-GMP) were evaluated from line-shape analysis of H8 resonances. Three diastereomers, two in head-to-tail (DeltaHT and LambdaHT) and one in head-to-head (HH) conformations, exist in equilibrium in solution. The two guanines are equivalent in DeltaHT and LambdaHT conformers and nonequivalent in the HH form; therefore, four rate constants (k(Delta)(HT), k(Lambda)(HT), k(HH)()s, and k(HH)()d; sub-subscripts s and d stand for H8-shielded and -deshielded guanine, respectively) were evaluated. Activation parameters (DeltaH and DeltaS) were evaluated from the rate constant dependence on temperature. High values of DeltaH (78-93 kJ mol(-)(1)) and DeltaS (51-71 J K(-)(1) mol(-)(1)) were found for G rotation in the preferred DeltaHT rotamer having the six-membered ring of each guanine more canted toward the cis-G and a favorable dipole-dipole internucleotide interaction. Lower values of DeltaH() (64-76 kJ mol(-)(1)) and very small values of DeltaS() (-7-11 J K(-)(1) mol(-)(1)) were found for G rotation in the less favorable LambdaHT rotamer, indicating that the ground-state entropy of this rotamer is close to that of the activated complex and the ground-state enthalpy closer to that of the activated complex than for the DeltaHT rotamer. For the two guanines in the HH rotamer there is no large difference in activation parameters. In general DeltaH falls in the range 66-84 kJ mol(-)(1) (rather close to the values for the LambdaHT rotamer) and DeltaS in the range 14-41 J K(-)(1) mol(-)(1). The equilibrium constant between HT and HH rotamers was also evaluated together with the corresponding thermodynamic parameters (DeltaH and DeltaS). It is found that the low enthalpy is the major stabilizing factor for DeltaHT as compared to HH, while the entropy factor would favor the latter rotamer. In contrast the greater entropy is the stabilizing factor for the LambdaHT rotamer (the second most abundant conformer for 9-EtG and 3'-GMP) over the HH rotamer. In the latter case the enthalpy would favor the HH rotamer. In the case of the 5'-GMP derivative the greater entropy of the LambdaHT rotamer is not such to compensate for the lower enthalpy of the HH rotamer, and the latter remains the second most abundant rotamer. This investigation has allowed for the first time the enthalpic and entropic contributions favoring different rotamers to be distinguished.

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