Secondary Interactions or Ligand Scrambling? Subtle Steric Effects Govern the Iridium(I) Coordination Chemistry of Phosphoramidite Ligands

Abstract

The like and unlike isomers of phosphoramidite (P*) ligands are found to react differently with iridium(I), which is a key to explaining the apparently inconsistent results obtained by us and other research groups in a variety of catalytic reactions. Thus, the unlike diastereoisomer (aR,S,S)-[IrCl(cod)(1 a)] (2 a; cod=1,5-cyclooctadiene, 1 a=(aR,S,S)-(1,1'-binaphthalene)-2,2'-diyl bis(1-phenylethyl)phosphoramidite) forms, upon chloride abstraction, the monosubstituted complex (aR,S,S)-[Ir(cod)(1,2-eta-1 a,kappaP)](+) (3 a), which contains a chelating P* ligand that features an eta(2) interaction between a dangling phenyl group and iridium. Under analogous conditions, the like analogue (aR,R,R)-1 a' gives the disubstituted species (aR,R,R)-[Ir(cod)(1 a',kappaP)(2)](+) (4 a') with monodentate P* ligands. The structure of 3 a was assessed by a combination of X-ray and NMR spectroscopic studies, which indicate that it is the configuration of the binaphthol moiety (and not that of the dangling benzyl N groups) that determines the configuration of the complex. The effect of the relative configuration of the P* ligand on its iridium(I) coordination chemistry is discussed in the context of our preliminary catalytic results and of apparently random results obtained by other groups in the iridium(I)-catalyzed asymmetric allylic alkylation of allylic acetates and in rhodium(I)-catalyzed asymmetric cycloaddition reactions. Further studies with the unlike ligand (aS,R,R)-(1,1'-binaphthalene)-2,2'-diyl bis{[1-(1-naphthalene-1-yl)ethyl]phosphoramidite} (1 b) showed a yet different coordination mode, that is, the eta(4)-arene-metal interaction in (aS,R,R)-[Ir(cod)(1,2,3,4-eta-1 b,kappaP)](+) (3 b).

Unravelling the Reaction Path of Rhodium–MonoPhos‐Catalysed Olefin Hydrogenation

“…The chemical shift, δ =−620 ppm (compare with the chemical shift of δ =−268 ppm for 8 b ),24 is in the range of known values for arene diphosphine Rh complexes 25. The observation of cross‐peaks in the 1 H, 103 Rh HMQC spectrum for the aromatic protons confirmed the coordination of the “outer” ring of the naphthyl group (see the Supporting Information) 26…”

Iridium-Catalyzed Allylic Alkylation Reaction with N-Aryl Phosphoramidite Ligands: Scope and Mechanistic Studies

“…The distance between Ir and the nearest proton on the methyl group of the ligand is 2.522 Å. Different from the case for ligand 1e′ , the Ir center in this system is favored to coordinate with the phenyl ring of the chiral amine part rather than either of the C–H bonds to form the stabilized complex III-1a′- s p 2 . This complex is set as the energy reference, and the relative energies of C-2 and the agostic complex ( III-1a′- s p 3 ) in which the Ir center interacts with the methyl C(sp 3 )–H bond of the ligand are 1.0 and 7.4 kcal/mol, respectively.…”

Ir‐Catalysed Asymmetric Allylic Substitutions with Cyclometalated (Phosphoramidite)Ir Complexes—Resting States, Catalytically Active (π‐Allyl)Ir Complexes and Computational Exploration

“…Two likely reaction pathways for their formation are described in Scheme 3, which cannot be distinguished presently. [14] The solution B (Figure 3) was treated with cinnamyl methyl carbonate (1 a), whereupon the two 31 P NMR singlets vanished over a period of 14 h and a singlet at 120.0 ppm appeared (D in Figure 3). The corresponding compound was identified by NMR experiments as well as ESI-MS, and a considerable time later also by X-ray crystallography, as the (p-allyl)Ir complex K4 a (Scheme 4).…”