80. Scheetz, P. M.; Chachula, S.
T.; Hughes, R.
P.; Glueck, D.
S.; Moore, C.
E.; Gembicky,
M.; Rheingold,
A. L.
Synthesis,
Structure,
Dynamics, and
Enantioface-Selective
η3-Benzyl
Coordination
in the Chiral
Rhodium
Complexes
Rh(diphos*)(η3-CH2Ph).
Organometallics
2020, 39,
3802–3816.
Abstract
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 C2-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 C1-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.
