| 20013762 |
Osswald T, Ruegger H, Mezzetti A: Secondary interactions or ligand scrambling? Subtle steric effects govern the iridium (I) coordination chemistry of phosphoramidite ligands. Chemistry. 2010 Jan 25;16(4):1388-97.
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). |
82(1,1,1,2) |