Modeling Complex Ligands for High Oxidation State Catalysis: Titanium Hydroamination with Unsymmetrical Ligands.

A method for modeling high oxidation state catalysts is used on precatalysts with unsymmetrical and symmetrical bidentate ligands to get a more detailed understanding of how changes to ancillary ligands affect the hydroamination of alkynes catalyzed by titanium. To model the electronic donor ability, the ligand donor parameter (LDP) was used, and to model the steric effects, percent buried volume (% Vbur) was employed. For the modeling study, 7 previously unpublished unsymmetrical Ti(XX')(NMe2)2 precatalysts were prepared, where XX' is a chelating ligand with pyrrolyl/indolyl linkages. The rates of these unsymmetrical and 10 previously reported symmetrical precatalysts were used with the model kobs = a + b(LDP)1 + c(LDP)2 + d(% Vbur)1 + e(% Vbur)2, where a-e were found through least-squares refinement. The model suggests that (1) the two attachment points of the bidentate ligand XX' are in different environments on the metal (e.g., axial and equatorial in a trigonal bipyramidal or square pyramidal structure), (2) the position of the unsymmetrical ligand on the metal is determined by the electronics of the ligand rather than the sterics, and (3) that one side of the chelating ligand's electronics strongly influences the rate, while the other side's sterics more strongly influences the rate. From these studies, we were able to generate catalysts fitting to this model with rate constants larger than the fastest symmetrical catalyst tested.

A rare isocyanide derived from an unprecedented neutral yttrium(ii) bis(amide) complex.

A room temperature stable complex formulated as Y(NHAr*)2 has been prepared, where Ar* = 2,6-(2,4,6-(iPr)3C6H2)C6H3, by KC8 reduction of ClY(NHAr*)2. Based on EPR evidence, Y(NHAr*)2 is an example of a d1 Y(ii) complex with significant delocalization of the unpaired electron density from the metal to the ligand. The isolation of molecular divalent metal complexes is challenging for rare earth elements such as yttrium. In fact, stabilization of the divalent state requires judicious ligand design that allows the metal center to be coordinatively saturated. Divalent rare earth elements tend to be reactive towards various substrates. Interestingly, Y(NHAr*)2 reacts as a radical donor towards t BuNC to generate an unusual yttrium isocyanide complex, CNY(NHAr*)2, based on spectroscopic evidence and single-crystal X-ray diffraction data.

Substituted pyridines from isoxazoles: scope and mechanism.

Treatment of isoxazoles with enamines leads to an inverse electron-demand hetero-Diels-Alder reaction that produces substituted pyridines in the presence of TiCl4(THF)2 and titanium powder. The reaction is highly regioselective with only a single isomer of the product observed by GC/MS and tolerant of many common functional groups. The transformation was examined computationally, and it was found that TiCl4 (or a similar Lewis acid) likely acts to catalyze the reaction. After the initial [4 + 2]-cycloaddition, the oxaza-[2.2.1]-bicycle produced likely ring opens before amine loss to give an N-oxide. The pyridine is then obtained after reduction with TiCl4 and titanium powder.