Qualitative and quantitative discrimination of fake and true alkene rotation processes in pd(η2-olefin) complexes. A new bimolecular mechanism

Abstract

H NMR spectroscopy and quantitatively determined by line-shape analyses (for 2) and selective inversion recovery experiments (for 3). The coalescence of the AB multiplet of fn hydrogens of 2 is concentration dependent and presents a strongly negative DS -, suggesting the intermediacy of a dimeric complex and ruling out the hypothesis of olefin rotation. The accurate evaluation of all spectral features also allows determination of the approaching mode of the monomeric units. The inversion transfer between the tmetc methyls of 3 reveals a true propeller-like olefin rotation. The presence of a nucleophilic electron pair at sulfur in 2 triggers the formation of the dimeric intermediate.

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…noticed that the apparent olefin rotation can also originate from the collapse of a dimeric species produced by action of the lone pair on sulfur which attacks another palladium centre via a bimolecular mechanism [51,52]. In these cases it has been demonstrated that the authentic olefin propeller-like rotation occurs at a negligible rate with respect to that observed when the associative mechanism is operative.…”

Quantum mechanical study on the different fluxional mechanisms in a [Pd(η2-olefin) complex

“…Using experimental evidence and reasons of symmetry, Canovese et al showed that a mechanism including intermediate S9 could not be correct. Only the collapse of the intermediate S6 allows the exchange between the ligand coordinating atoms and the consequent olefin pseudo rotation independently of the olefin enantioface (Scheme 5) [10,11]. Our calculations showed as well that S7 ( Figure 5) is more stable than S9 ( Figure 6) Figure 6 shows the optimised structure of TSS6S7.…”

“…The fluxional processes of palladium(0) complexes of type [Pd(η 2 -olefin)(L-L´)] and their mechanisms have been studied in detail experimentally [10,11]. The mechanisms put forward for the fluxional movements in olefin complexes of Pd are listed below [3,10,11].…”

“…The mechanisms put forward for the fluxional movements in olefin complexes of Pd are listed below [3,10,11].…”

“…(7) Intermolecular associative process with free alkene. www.prkm.co.uk Luchini et al showed that the concentration dependence could not be attributed to the action of an adventitious free olefin or free ancillary ligand and consequently the associative mechanisms 6 and 7 were rejected [11]. Although these mechanisms are not equivalent, a suitable binary combination of them could explain the fluxional behaviour of each species [3].…”