Ian J. Casely

Intramolecular hydroamination of aminoalkenes by calcium and magnesium complexes: a synthetic and mechanistic study.

The beta-diketiminate-stabilized calcium amide complex [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe(3))(2)}(THF)] (Ar = 2,6-diisopropylphenyl) and magnesium methyl complex [{ArNC(Me)CHC(Me)NAr}Mg(Me)(THF)] are reported as efficient precatalysts for hydroamination/cyclization of aminoalkenes. The reactions proceeded under mild conditions, allowing the synthesis of five-, six-, and seven-membered heterocyclic compounds. Qualitative assessment of these reactions revealed that the ease of catalytic turnover increases (i) for smaller ring sizes (5 > 6 > 7), (ii) substrates that benefit from favorable Thorpe-Ingold effects, and (iii) substrates that do not possess additional substitution on the alkene entity. Prochiral substrates may undergo diastereoselective hydroamination/cyclization depending upon the position of the existing stereocenter. Furthermore, a number of minor byproducts of these reactions, arising from competitive alkene isomerization reactions, were identified. A series of stoichiometric reactions between the precatalysts and primary amines provided an important model for catalyst initiation and suggested that these reactions are facile at room temperature, with the reaction of the calcium precatalyst with benzylamine proceeding with DeltaG(o)(298 K) = -2.7 kcal mol(-1). Both external amine/amide exchange and coordinated amine/amide exchange were observed in model complexes, and the data suggest that these processes occur via low-activation-energy pathways. As a result of the formation of potentially reactive byproducts such as hexamethyldisilazane, calcium-catalyst initiation is reversible, whereas for the magnesium precatalyst, this process is nonreversible. Further stoichiometric reactions of the two precatalysts with 1-amino-2,2-diphenyl-4-pentene demonstrated that the alkene insertion step proceeds via a highly reactive transient alkylmetal intermediate that readily reacts with N-H sigma bonds under catalytically relevant conditions. The results of deuterium-labeling studies are consistent with the formation of a single transient alkyl complex for both the magnesium and calcium precatalysts. Kinetic analysis of the nonreversible magnesium system revealed that the reaction rate depends directly upon catalyst concentration and inversely upon substrate concentration, suggesting that substrate-inhibited alkene insertion is rate-determining.

Mild sp2Carbon–Oxygen Bond Activation by an Isolable Ruthenium(II) Bis(dinitrogen) Complex: Experiment and Theory

The isolable ruthenium(II) bis(dinitrogen) complex [Ru(H)2(N2)2(PCy3)2] (1) reacts with aryl ethers (Ar–OR, R = Me and Ar) containing a ketone directing group to effect sp2C–O bond activation at temperatures below 40 °C. DFT studies support a low-energy Ru(II)/Ru(IV) pathway for C–O bond activation: oxidative addition of the C–O bond to Ru(II) occurs in an asynchronous manner with Ru–C bond formation preceding C–O bond breaking. Alternative pathways based on a Ru(0)/Ru(II) couple are competitive but less accessible due to the high energy of the Ru(0) precursors. Both experimentally and by DFT calculations, sp2C–H bond activation is shown to be more facile than sp2C–O bond activation. The kinetic preference for C–H bond activation over C–O activation is attributed to unfavorable approach of the C–O bond toward the metal in the selectivity determining step of the reaction pathway.

Porous zinc and cobalt 2-nitroimidazolate frameworks with six-membered ring windows and a layered cobalt 2-nitroimidazolate polymorph

Polymorphs of Zn(2-nIm)2 (compound 1) and Co(2-nIm)2 (compounds 2 and 3) (2-nIm = 2-nitroimidazole) have been prepared by two routes: solvothermal synthesis and recrystallisation of ZIF-65(Zn/Co). Compounds 1 and 2 are isostructural, with a tetrahedrally-connected framework topology related to, but different from, that of tridymite (lonsdaleite). Single crystal X-ray diffraction analysis showed that in compound 1 (Pccn, Z = 8; a = 8.462(8) A, b = 14.549(15) A, c = 18.799(18) A, V = 2314(4) A3) there is rotational disorder for two of the three crystallographically-distinct linker types, which has been investigated computationally and by solid-state NMR spectroscopy. Detailed adsorption studies on a sample of 1 prepared by recrystallisation show 1.1 mmol g−1 uptake of CO2 at 0.1 bar (25 °C) with high affinity for CO2 over CH4 and N2 (adsorption enthalpies of 39.5, 26.0 and 18.5 kJ mol−1, respectively). A cobalt analogue (compound 2) with improved water stability (but lower porosity) has also been prepared. Changing the conditions of synthesis and recrystallisation gives rise to a cobalt 2-nitroimidazolate (Co(2-nIm)2, compound 3), which has a layered structure (I41/amd, a = 6.025(18) A, c = 26.95(8) A, V = 978.3(5) A3) containing sheets of tetrahedrally-connected Co2+ cations composed of four membered rings, without porosity.