The rhodium benzyl complexes Rh(diphos*)(η3-CH2Ph) (1–14,
diphos* = chiral
bis(phosphine)), potential precursors for asymmetric catalysis, were
prepared either by treatment of Rh(COD)(η3-CH2Ph) (15, COD = 1,5-cyclooctadiene) with diphos*
or from the reaction of [Rh(diphos*)(Cl)]2 (16–20) with PhCH2MgCl, and their structures
and dynamics were investigated. For C
2-symmetric diphos* (BPE and DuPhos derivatives, Me-FerroLANE, Et-FerroTANE,
DIOP, BINAP), observation of one set of NMR signals for complexes 1–12 suggested that the two diastereomers
in which different η3-benzyl enantiofaces were coordinated
to rhodium interconverted rapidly on the NMR time scale via suprafacial
shifts; observation of five inequivalent aryl 1H NMR signals
showed that antarafacial shifts were slow on the NMR time scale. With
the C
1-symmetric ligands (R,S)-CyPF-t-Bu and (S,R)-Me-BoPhoz, complexes 13 and 14 gave rise to two sets of NMR signals, consistent with fast
suprafacial shifts but slow antarafacial shifts on the NMR time scale.
Density functional theory studies of the Me-DuPhos, Me-BPE, Ph-BPE,
Me-FerroLANE, and CyPF-t-Bu benzyl complexes 1, 4, 7, 11, and 13 showed that enantioface-selective benzyl coordination involved
small energy differences (0.4–2.7 kcal/mol). The barrier to
interconversion between these isomers by suprafacial shifts was also
low (2.2–7.1 kcal/mol), and the computed barrier for antarafacial
shifts in Me-DuPhos complex 1 was significantly higher.
Treatment of [Rh(COD)(Cl)]2 with PhCH2MgCl gave 15; excess Grignard reagent yielded the ate complex [Mg2Cl3(THF)6][Rh(COD)(η1-CH2Ph)2] (21). Benzyl complexes 11 and 13, 21, and dimers 17–19 (diphos* = (R,R)-i-Pr-DuPhos, (R,R)-Me-FerroLANE, (R,R)-Ph-BPE)
were structurally characterized by X-ray crystallography.
