Selectivity in nickel-catalyzed rearrangements of cyclopropylen-ynes

The Ni/NHC-catalyzed rearrangement of cyclopropylen-ynes is described. Two different heterocycles, cyclopentane- or cycloheptene-based, were obtained. However, by employing ItBu as the NHC ligand, cyclopentane products were obtained selectively under mild conditions.     

A nickel-catalyzed route to pyridines

A mild and general route for preparing pyridines from nitriles and diynes is described. Ni/imidazolyidene complexes were used to mediate cyclization alkynes and both aryl and alkyl nitriles at ambient temperature. In addition, the efficacy of this protocol allows for the preparation of a fused seven-membered pyridone and for intermolecular cyclizations. When an asymmetrical diyne was employed, cyclization afforded a single pyridine regioisomer.     

Nickel-catalyzed cycloaddition of unsaturated hydrocarbons and carbonyl compounds

A mild and general route for preparing pyrans and dienones from carbonyls and diynes is described. Ni imidazolylidene complexes were used to mediate cyclizations between diynes and aldehydes. The reaction of an enyne and an aldehyde afforded a mixture of cyclized products. In addition, a spiropyran was prepared from the cycloaddition of a diyne and cyclohexanone. [reaction: see text]     

Synthesis and characterization of a redox- and light-sensitive MRI contrast agent

A redox- and light-sensitive, T₁-weighted magnetic resonance imaging (MRI) contrast agent, which tethers a spiropyran (SP)/merocyanine (MC) motif to a Gd-DO3A moiety was synthesized and characterized. When in the dark, the probe is in its MC form, which has an r₁ relaxivity of 2.51 m M⁻¹ s⁻¹ (60 MHz, 37 °C). After irradiation with visible light or mixing with NADH, the probe experiences an isomerization and the r₁ relaxivity decreased 18% and 26%, respectively. Additionally, the signal intensity in MRI showed an observable decrease after the compound was mixed with NADH.     

The Discovery of [Ni(NHC)RCN](2) Species and Their Role as Cycloaddition Catalysts for the Formation of Pyridines

The reaction of Ni(COD)(2), IPr, and nitrile affords dimeric [Ni(IPr)RCN](2) in high yields. X-ray analysis revealed these species display simultaneous η(1)- and η(2)-nitrile binding modes. These dimers are catalytically competent in the formation of pyridines from the cycloaddition of diynes and nitriles. Kinetic analysis showed the reaction to be first order in [Ni(IPr)RCN](2), zeroth order in added IPr, zeroth order in nitrile, and zeroth order in diyne. Extensive stoichiometric competition studies were performed, and selective incorporation of the exogenous, not dimer bound, nitrile was observed. Post cycloaddition, the dimeric state was found to be largely preserved. Nitrile and ligand exchange experiments were performed and found to be inoperative in the catalytic cycle. These observations suggest a mechanism whereby the catalyst is activated by partial dimer-opening followed by binding of exogenous nitrile and subsequent oxidative heterocoupling.