Chemistry Reference
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cinnamaldehyde was the best CO donor (Scheme 7.11). Moreover, Shibata
et al.
later
developed a protocol using (
S
)-tol-BINAP as chiral ligand for asymmetric Pauson-Khand-
type reaction to give the desired product in up to 90% ee.
19
Shibata protocol
Morimoto /Kakiuchi protocol
Ph
Ph
5 mol% [RhCl(cod)]
2
11 mol% dppp
Ph
5 mol% Rh(dppp)
2
Cl
O
O
O
O
O
Xylene, 130
C, 2 h
120
°
C, 2 h
solvent-free
°
98% yield
91% yield
2 eq. C
6
F
5
-CHO
20 eq.
CHO
Ph
Scheme 7.11
Sequential decarbonylation of aldehyde and carbonylation of enyne.
Apart from aromatic aldehyde, the Morimoto/Kakiuchi group further reported that
formaldehyde (in water) was an effective CO reservoir for Rh-catalyzed PKR.
20
They
used dppp/TPPTS (an organic-soluble and a water-soluble ligand, respectively) as a mixed-
phosphine catalyst system with the aid of SDS to obtain the desired cycloadduct in up to
97% chemical yield. They proposed a micelle formation
21
during the course of the reaction,
and the tandem decarbonylation and the carbonylation processes took place independently
in the aqueous and micellar phase, respectively. In 2004, the same research group further
disclosed the enantioselective version of this reaction by employing (
S
)-tol-BINAP as a
catalyst to give the desired product good to excellent enantioselectivity (up to 95% ee).
22
Recently, considerable attention has been given to aqueous transition metal-catalyzed re-
actions.
23
However, no catalytic asymmetric systems that enable the use of water as the only
solvent (without a surfactant) in the PKR have been developed since 2005. Kwong/Chan
et al.
did indeed present a Rh-catalyzed asymmetric PKR in water using chiral dipyridyl
diphosphine ligand, P-Phos (Scheme 7.12).
24
Varieties of
O
-,
N
-,and
C
-tethered 1,6-enynes
were converted to the corresponding cyclopentenones with up to 95% ee.
OMe
N
3 mol% [Rh(cod)Cl]
2
6 mol% (
S
)-P-Phos
Cinnamylaldehyde
Water
, 100
X
R
X
MeO
PPh
2
PPh
2
R'
R
MeO
*
O
R'
°
C, 36 h
N
R = aryl, alkyl
R' = H, Me
X = O, NTs, C(COOEt)
2
Up to 95%
ee
OMe
(
S
)-P-Phos
Scheme 7.12
Asymmetric Rh-catalyzed PKR of 1,6-enynes in water.
In addition to the aqueous system, Kwong/Chan also demonstrated a homogeneous
asymmetric Rh-catalyzed PKR using an environmentally benign alcoholic solvent. The
combination of (
S
)-BisbenzodioxanPhos (also named (
S
)-SYNPHOS by Genet
et al
.) with
