Phosphinite Ligand Effects in Palladium( II )‐Catalysed Cycloisomerisation of 1,6‐Dienes: Bicyclo[3.2.0]heptanyl Diphosphinite (B[3.2.0]DPO) Ligands Exhibit Flexible Bite Angles, an Effect Derived from Conformational Changes (exo‐ or endo‐Envelope) in the Bicyclic Ligand Scaffold

Changes in bidentate ligand structure significantly affect catalytic activity in mono‐cationic Pd(II)‐catalysed 1,6‐diene cycloisomerisation processes to give cyclopentene products. A bicyclo[3.2.0]heptanyl diphosphinite ligand (B[3.2.0]DPO, 3) is the first phosphorus‐based bidentate ligand capable of promoting regioselective 1,6‐diene cycloisomerisation. Trace quantities of water are essential for catalytic activity, as is the precise order of mixing of 1,6‐diene, Pd(II) pro‐catalyst and additives. Conformational changes in the ligand backbone seem to be important in stabilising the active catalyst species, assumed to be a cationic Pd(II) hydride species. DFT calculations support a change in bite angle on the cationic Pd(II) hydride species from circa 90° (cis) to 170° (trans); in the latter geometry an agostic interaction of the C4 endo hydrogen of the bicyclic ring‐system with Pd(II) stabilises the cationic metal centre. This unique ligand property could be exploited in other transition metal catalysed processes.