New Chiral Palladium(0) and -(II) Complexes of (Aminoferrocenyl)phosphine Ligands PPFA and PTFA. X-ray Crystal Structure Analysis and Fluxional Behavior Involving Alkene Rotation, Pd−N Bond Rupture, and Selective η³−η¹−η³ Allyl Isomerization

Abstract

(Aminoferrocenyl)phosphine ligands 2-(1-(dimethylamino)ethyl)-1-(diphenylphosphino)ferrocene (PPFA) and [η5-cyclopentadienyl][η5-4-(endo-dimethylamino)-3-(diphenylphosphino)-4,5,6,7-tetrahydro-1H-indenyl]iron(II) (PTFA), were used as ligands in palladium(0) and -(II) complexes. The reaction of Pd2(dba)3·CHCl3 with PPFA or PTFA in the presence of the electron-withdrawing olefins maleic anhydride (MA) and dimethyl fumarate (DMFU) gave the complexes Pd(PTFA)(DMFU) (2), Pd(PPFA)(MA) (3), and Pd(PPFA)(DMFU) (4). Allylic complexes [Pd(η3-2-Me-C3H4)(PTFA)]Tf (5) and [Pd(η3-2-Me-C3H4)(PPFA)]Tf (6) (Tf = triflate) were obtained by reaction of PTFA or PPFA with [Pd(η3-2-Me-C3H4)Cl]2 in the presence of AgTf. In solution all these compounds exist as mixtures of two diastereomers, with either the alkene or the allyl group differently oriented with respect to the aminophosphine ligand. 1H NMR variable-temperature studies have been carried out for 2−6 and for Pd(PTFA)(MA) (1). Rotation of the alkene was observed for complexes 1−4 on the NMR time scale. ΔG ⧧ c has been calculated and values between 57.6 kJ mol -1 (298 K) and 76.6 kJ mol -1 (373 K) have been obtained. A Pd−N bond rupture which interchanges the two amino methyl groups is observed (ΔG ⧧ 328 = 63.9 kJ mol-1 to ΔG ⧧ 368 = 74.9 kJ mol-1) for derivatives of PPFA, but not for complexes containing PTFA. An EXSY experiment carried out on complex 5 has evidenced a selective η3−η1−η3 (carbon cis to phosphorus) allyl isomerization. Molecular structures of 4 and 6 were determined by X-ray structure analysis.

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

“…Again, irrespectively of the stabilizing olefin the bond lengths are very similar among the fourteen complexes examined with the Pd-N bond length usually spanning within 2.11 and 2.25 Å. In two cases, however, when the ancillary ligands are ferrocenyl derivatives the Pd-N bond length reaches $2.5 Å [42,92]. The large majority of the Pd-P lengths lies in the proximity of 2.28 Å (2.27-2.29 Å).…”

“…In contrast with the compounds just described, hemi-labile auxiliary ligands containing phosphorus and nitrogen were more extensively studied [26,42,43,[90][91][92][93][94]. Again, irrespectively of the stabilizing olefin the bond lengths are very similar among the fourteen complexes examined with the Pd-N bond length usually spanning within 2.11 and 2.25 Å.…”

“…On the contrary, when N-N, N-S and N-P molecules are employed as ancillary ligands their 1 H NMR signals are all shifted downfield, indicating the predominant r-nature of the ensuing bonds. The formation of different isomers as a function of the olefin structure and of the symmetry of the ancillary ligand can be observed (Scheme 3) [4,30,32,34,[40][41][42][43]. Usually these species undergo extensive fluxionality and the RT 1 H NMR spectra of the complexes are characterized by rapid interconversion among isomers and only averaged spectra are detectable.…”

“…This is the earliest and most frequently proposed mechanism [30,42,43,[46][47][48]; (2) olefin-metal dissociation followed by recombination [30]; (3) intermolecular associative process with free alkene, via (4) metal-L or metal-L 0 bond dissociation followed by rotation of the ancillary ligand and recombination (olefin pseudorotation) [42,43]; (5) intermolecular exchange with the free ancillary ligand L-L 0 . In this case, a mechanism has been proposed which involves an olefin-metal dissociation step [30].…”

Using a Tripod as a Chiral Chelating Ligand: Chemical Exchange Between Equivalent Molecular Structures in Palladium Catalysis with 1,1,1‐Tris(oxazolinyl)ethane (“Trisox”)

“…The isomer interconversion may, in principle, be caused by either an olefin rotation or a decoordination-coordination process that involves the alkene and/or the ligand. The DG°values reported in the literature for alkene rotation are usually around 60-70 kJ mol À1 ; [28] however, energy barriers of 50 kJ mol À1 have also been noted. [29] In the case of complex 3 b, it is therefore difficult to conclude on the exact mechanism, since the ligand may be involved in the process.…”