Jianke Liu

A Multilateral Mechanistic Study into Asymmetric Transfer Hydrogenation in Water

The mechanism of aqueous-phase asymmetric transfer hydrogenation (ATH) of acetophenone (acp) with HCOONa catalyzed by Ru-TsDPEN has been investigated by stoichiometric reactions, NMR probing, kinetic and isotope effect measurements, DFT modeling, and X-ray structure analysis. The chloride [RuCl(TsDPEN)(p-cymene)] (1), hydride [RuH(TsDPEN)(p-cymene)] (3), and the 16-electorn species [Ru(TsDPEN-H)(p-cymene)] (4) were shown to be involved in the aqueous ATH, with 1 being the precatalyst, and 3 as the active catalyst detectable by NMR in both stoichiometric and catalytic reactions. The formato complex [Ru(OCOH)(TsDPEN)(p-cymene)] (2) was not observed; its existence, however, was demonstrated by its reversible decarboxylation to form 3. Both 1 and 3 were protonated under acidic conditions, leading to ring opening of the TsDPEN ligand. 4 reacted with water, affording a hydroxyl species. In a homogeneous DMF/H(2)O solvent, the ATH was found to be first order in the concentration of catalyst and acp, and inhibited by CO(2). In conjunction with the NMR results, this suggests that hydrogen transfer to ketone is the rate-determining step. The addition of water stabilized the ruthenium catalyst and accelerated the ATH reaction; it does so by participating in the catalytic cycle. DFT calculations revealed that water hydrogen bonds to the ketone oxygen at the transition state of hydrogen transfer, lowering the energy barrier by about 4 kcal mol(-1). The calculations also suggested that the hydrogen transfer is more step-wise in nature rather than concerted. This is supported to some degree by the kinetic isotope effects, which were obscured by extensive H/D scrambling.

The Synthesis of, and Characterization of the Dynamic Processes Occurring in, Pd(II) Chelate Complexes of 2-pyridyldiphenylphosphine

Pd(II) complexes in which 2-pyridyldiphenylphosphine (Ph(2)Ppy) chelates the Pd(II) centre have been prepared and characterized by multinuclear NMR spectroscopy and by X-ray crystallographic analysis. trans-[Pd(kappa(1)-Ph(2)Ppy)(2)Cl(2)] is transformed into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl]Cl by the addition of a few drops of methanol to dichloromethane solutions, and into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl]X by addition of AgX or TlX, (X = BF(4)(-), CF(3)SO(3)(-) or MeSO(3)(-)). [Pd(kappa(1)-Ph(2)Ppy)(2)(p-benzoquinone)] can be transformed into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(MeSO(3))][MeSO(3)] by the addition of two equivalents of MeSO(3)H. Addition of further MeSO(3)H affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)PpyH)(MeSO(3))][MeSO(3)](2). Addition of two equivalents of CF(3)SO(3)H, MeSO(3)H or CF(3)CO(2)H and two equivalents of Ph(2)Ppy to [Pd(OAc)(2)] in CH(2)Cl(2) or CH(2)Cl(2)-MeOH affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)X]X, (X = CF(3)SO(3)(-), MeSO(3)(-) or CF(3)CO(2)(-)), however addition of two equivalents of HBF(4).Et(2)O affords a different complex, tentatively formulated as [Pd(kappa(2)-Ph(2)Ppy)(2)]X(2). Addition of excess acid results in the clean formation of [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)PpyH)(X)]X(2). In methanol, addition of MeSO(3)H and three equivalents of Ph(2)Ppy to [Pd(OAc)(2)] affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(2)][MeSO(3)](2) as the principal Pd-phosphine complex. The fluxional processes occuring in these complexes and in [Pd (kappa(1)-Ph(2)Ppy)(3)Cl]X, (X = Cl, OTf) and the potential for hemilability of the Ph(2)Ppy ligand has been investigated by variable-temperature NMR. The activation entropy and enthalpy for the regiospecific fluxional processes occuring in [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(2)][MeSO(3)](2) have been determined and are in the range -10 to -30 J mol(-1) K(-1) and ca. 30 kJ mol(-1) respectively, consistent with associative pathways being followed. The observed regioselectivities of the exchanges are attributed to the constraints imposed by microscopic reversibility and the small bite angle of the Ph(2)Ppy ligand. X-Ray crystal structure determinations of trans-[Pd(kappa(1)-Ph(2)Ppy)(2)Cl(2)], [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl][BF(4)], [Pd(kappa(1)-Ph(2)Ppy)(2)(p-benzoquinone)], trans-[Pd(kappa(1)-Ph(2)PpyH)(2)Cl(2)][MeSO(3)](2), and [Pd(kappa(1)-Ph(2)Ppy)(3)Cl](Cl) are reported. In [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl][BF(4)] a donor-acceptor interaction is seen between the pyridyl-N of the monodentate Ph(2)Ppy ligand and the phosphorus of the chelating Ph(2)Ppy resulting in a trigonal bipyramidal geometry at this phosphorus.

Methanolysis of acyl-Pd(II) complexes relevant to CO/ethene coupling reactions

Alcoholysis of acyl–Pd(II) complexes relevant to palladium catalysed CO/ethene coupling reactions such as polyketone synthesis/alkoxycarbonylation reactions is shown, in a highly active catalyst system, to proceed via coordination of methanol to the Pd centre prior to nucleophilic attack at the acyl carbon.

The Indirect Detection of Metal Nuclei by Correlation Spectroscopy (HSQC and HMQC)

Abstract This review discusses 2D HXQC methods for the indirect detection of the NMR spectrum of metal nuclei. The workings of several pulse sequences and of coherence selection are briefly outlined. The spin systems likely to be encountered in HXQC NMR studies of metal coordination and cluster compounds are analysed and appropriate modifications to the standard experiments discussed. In the second part, the literature describing the application of HXQC methods to the detection of the resonances of metal nuclei, since 1999 are reviewed and tables of data presented.