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

Abstract

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-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 908 (cis) to 1708 (trans); in the latter geometry an agostic interaction of the C4 endo hydrogen of the bicyclic ringsystem with Pd(II) stabilises the cationic metal centre. This unique ligand property could be exploited in other transition metal catalysed processes.

Transition-Metal-Catalyzed Cycloisomerizations of α,ω-Dienes

“…Interestingly, the bicyclo[4.3.0]nonene derivative 100 was quantitatively obtained from 99 using this method. Fairlamb and co-workers also obtained cyclopentenes 60-Me and 96 with high yields and selectivity using the bis(phosphinite) ligand L8 with a bicyclo[3.2.0]heptane backbone (Scheme ) . In contrast, cyclohexane-1,3-dione derivative 101 underwent less selective cyclization to afford 102 in a lower yield of 45%.…”

A DFT Study on the Mechanism of Palladium-Catalyzed Alkyne Hydrogenation: Neutral versus Cationic Pathways

“…Results map appropriate transition states and associated energy barriers both in the gas phase and in methanol (via a polarizable continuum model (PCM)). Mechanisms involving neutral palladium intermediates are explicitly compared with those predicted for a reaction proceeding via a palladium monohydride cation. – These two schemes are indicated in Figure with the associated NMR trace illustrating the PHIP effect seen in the 1 H resonances of the cis -alkene product. , …”

Conformational Flexibility in a Carbobicyclic Diphosphinite Ligand

“…2 – 4 , where the ligand backbone possesses an endo ‐envelope conformation ( I ). Crucially, theoretical calculations support a conformational change from I to II , the latter an exo ‐envelope, for the stabilisation of cationic [Pd II (H)L 2 ] species, which are proposed key intermediates in 1,6‐diene cycloisomerization reactions 15. Direct experimental evidence for the existence of conformation II is presented in this communication.…”