Pre-catalyst resting states: a kinetic, thermodynamic and quantum mechanical analyses of [PdCl2(2-oxazoline)2] complexes.

摘要: The treatment of cold (∼3 °C) methanolic solutions Li2PdCl4 with two equivalents 2-phenyl-2-oxazoline (Phox) results in the isolation [PdCl2(Phox)2] (3). This complex undergoes remarkably slow isomerisation (CHCl3-d) at room temperature to a corresponding thermodynamic form. In addition theoretical (DFT), behaviour has been analysed both kinetically and thermodynamically. These investigations lead conclusion that initially formed (i.e. kinetic) isomer 3 is cis-form which conversion trans-form via dissociative (D) mechanism involving loss Phox ligand. activation parameters ΔS‡ ΔH‡ are found be +304 (±3) J K−1 mol−1 +176 (±1) kJ mol−1, respectively indicate high barrier Pd–N bond cleavage under these conditions. show expected endothermic nature this process (+140 ± 17 mol−1) slight positive overall entropy (ΔS° = +17 2 mol−1); latter parameter presumably due formation lower dipole moment trans-product when compared cis-isomer. Calculated (DFT) values ΔG‡ excellent agreement those experimentally. Further investigation suggests 14-electron three-coordinate T-shaped transition states (i.e., [PdCl2(Phox)]‡) involved; form pre-disposed yield following re-attachment released oxazoline energetically favoured. analogous alkyloxazoline system [PdCl2(Meox)2] (4: Meox 2-methyl-2-oxazoline) likewise investigated. material gives no indication cis–transisomerisation solution (NMR) shown exist (X-ray) solid-state (as do previously reported crystalline samples 3). A DFT study 4 reveals similar if D type were operating rapidly convert cis- trans-4. However, significantly higher stability trans-isomer relative revealed versus calculations derivative 3. possibility (i) reactions may directly or alternatively (ii) complexes such as have different, much more rapid, for isomerisation. placed into context behaviour, lack thereof, could play key preliminary role later substrate modification. fact [PdX2(oxazoline)2] compounds well-known (pre-)catalysts C–C forming chemistry.