Until recently, ammonia had rarely succumbed to catalytic transformations with homogeneous catalysts, and the development of such reactions that are selective for the formation of single products under mild conditions has encountered numerous challenges. However, recently developed catalysts have allowed several classes of reactions to create products with nitrogen-containing functional groups from ammonia. These reactions include hydroaminomethylation, reductive amination, alkylation, allylic substitution, hydroamination, and cross-coupling. This Minireview describes examples of these processes and the factors that control catalyst activity and selectivity.

We report reductive eliminations of primary arylamines from a series of bisphosphine-ligated arylpalladium(II) parent amido complexes that counter several established trends. In contrast to arylamido and alkylamido complexes of the aromatic bisphosphines DPPF and BINAP, parent amido complexes, do not form or undergo reductive elimination of monoarylamines. However, arylpalladium parent amido complexes ligated by the alkylbisphosphine CyPF-t-Bu form in good yield and undergo reductive elimination. Despite the basicity of parent amido ligand and the typically faster reductive elimination from complexes containing more basic amido ligands, the CyPF-t-Bu-ligated arylpalladium parent amido complexes undergo reductive elimination much more slowly than the analogous complexes containing arylamido or alkylamido ligands. Moreover, the parent amido complexes form more rapidly and are more stable thermodynamically in a series of exchange processes than the arylamido complexes. Computational studies support the overriding influence of steric effects on the stability and reactivity of the parent amido complex. The slow rate of reductive elimination causes the arylpalladium amido complex to be the resting state of the coupling of aryl halides with ammonia catalyzed by CyPF-t-Bu-ligated palladium, and this resting state contrasts the Pd(0) or arylpalladium(II) resting states of reactions of aryl halides with amines catalyzed by most palladium complexes.

We report the isolation and characterization of arylpalladium cyanide complexes that undergo reductive elimination to form arylnitriles. The rates of reductive elimination from a series of arylpalladium cyanide complexes reveal that the electronic effects on the reductive elimination from arylpalladium cyanide complexes are distinct from those on reductive reductive eliminations from arylpalladium alkoxo, amido, thiolate, and enolate complexes. Arylpalladium cyanide complexes containing aryl ligands with electron-donating substituents undergo faster reductive elimination of aromatic nitriles than complexes containing aryl ligands with electron-withdrawing substituents. In addition, the transition state for the reductive elimination of the aromatic nitrile is much different from that for reductive eliminations that occur from most other arylpalladium complexes. Computational studies indicate that the reductive elimination of an arylnitrile from Pd(II) occurs through a transition state more closely related in structure and electronic distribution to that for the insertion of CO into a palladiumaryl bond.

Block copolymers incorporating biocompatible poly(ethylene glycol) (PEG) and poly(lactide) (PLA) polymers and labile metal complexes can serve as stimuli responsive biomaterials. Bipyridine-centered PEG-PLA materials were prepared via ring opening polymerization of ethylene oxide and lactide (D,L and L). Macroligands and their iron complexes [Fe{bpy(PEG-PLA) 2 } 3 ]Cl 2 were characterized by gel permeation chromatography, 1 H NMR, and UV-vis spectroscopy. Solvent and counterion effects were also explored by UV-vis spectroscopy. Extinction coefficients drop 3-fold in CH 2 Cl 2 ; values typical for iron(II) tris(bipyridine) complexes are restored by addition of very small quantities of MeOH. Thermal properties of bpy(PEG-PLA) 2 and [Fe{bpy(PEG-PLA) 2 } 3 ] 2+ were investigated by thermogravimetric analysis and differential scanning calorimetry, showing values typical for the polymer blocks and little effect of incorporated ligand or metal sites. Micelle-like nanoparticles of block copolymer macroligands and iron star polymer complexes were prepared via nanoprecipitation. Dynamic light scattering and transmission electron microscopy analyses confirm the presence of spheroid particles (macroligands, 111-253 nm in diameter; iron complexes, 37-75 nm in diameter).