The diastereomerically and enantiomerically pure amido complex
(SR)-(η5-C5H5)Re(NO)(PPh3)(N̈HCH(CH3)Ph)
((SR)-5) converts to (RR)-5
(inversion at rhenium, retention at carbon)
in THF-d
8 at 49.4 °C with
k
1 = 2.34 × 10-4
s-1 and
k
-
1 = 0.90 ×
10-4 s-1.
Similarly, (SS)-5
converts to (RS)-5 with k
1
= 0.90 × 10-4
s-1 and
k
-
1 = 2.30 ×
10-4 s-1. Both
epimerizations
give equilibrium ratios (RR/SR or
SS/RS) of 70:30. Reactions with HOTf yield
[(η5-C5H5)Re(NO)(PPh3)(NH2CH(CH3)Ph)]+TfO-,
and subsequent additions of
Et4N+CN-
afford
(η5-C5H5)Re(NO)(PPh3)(CN)
and NH2CH(CH3)Ph (all steps with
retention at rhenium and
carbon). Enantiomeric purities and absolute configurations are
assayed by chiral NMR shift
reagents and (−)-menthyl chloroformate derivatives, respectively,
establishing configurations
of epimerized 5. Reaction of
(SR)-5 and P(p-tol)3 in
THF-d
8 at 49.4 °C gives
(η5-C5H5)Re(NO)(P(p-tol)3)(N̈HCH(CH3)Ph)
(50:50 (t
0) → 66:34
(t
∞) RR/SR) with k =
4.6 × 10-4 s-1,
twice
that for the conversion of (SR)-5 to
(RR)-5. Rate data for the latter at
32.3−59.1 °C give
ΔH
⧧ = 26 kcal/mol and
ΔS
⧧ = 6 eu. These results are best
modeled by mechanisms involving
initial and rate determining PPh3 dissociation, with
anchimeric assistance by the amido
lone pair, to give an intermediate that is trigonal planar at rhenium
and combines with
PAr3 without significant diastereoselectivity.
Reactions of n-BuLi with 5 and
related
complexes give N̈LiR species from which PPh3 is lost
at lower temperatures, and are
presumed to be much less configurationally stable.