Chemistry Reference
In-Depth Information
In the same year, the Chung group reported a relatively more environmentally benign
Rh-catalyzed PKR under a water/dioxane reaction medium.
7
Surfactant (e.g. SDS) was
necessary for this successful transformation.
Consiglio
et al.
disclosed an interesting system for the Pauson-Khand-type chemistry.
8
They employed CO-saturated molecular sieves as a CO reservoir for the carbonylative
cyclization of 1,6-enynes. Particularly noteworthy is that this protocol represented the
lowest temperature of Pauson-Khand-type reaction achieved so far (Scheme 7.5). At 0
◦
C,
the desired product was obtained in 97% ee with 99% substrate conversion.
Me
PPh
2
PPh
2
(
R
)-Biphemp
Me
H
EtO
2
C
EtO
2
C
EtO
2
C
EtO
2
C
[Rh(COD)(
R
)-Biphemp]OTf
2 mol%
O
CO-Saturated MS
H
Toluene, 0
°
C, 4 h
99% Conv., 97%
ee
(33 yield)
Scheme 7.5
Rh-catalyzed PKR with a molecular sieve as the CO reservoir.
In 2008, Ratovelomanan-Vidal, Genet, and Jeong reported an efficient asymmet-
ric Pauson-Khand-type reaction mediated by a Rh(I) catalyst at ambient temperatures
(18-20
◦
C) under a reduced partial pressure of CO.
9
They employed (
R
)-xylyl-BINAP
and [RhCl(CO)
2
]
2
as the catalyst system with 0.1 atm CO partial pressure to give bicyclic
cyclopentenones in up to 99% yield and enantioselectivity (Scheme 7.6). In addition to 1,6-
enynes substrates, the asymmetric desymmetrization of dienynes was also accomplished
by a Rh-catalyzed enantioselective Pauson-Khand-type reaction at ambient temperatures,
in which (
S
)-DifluorPhos was found to be the best ligand to obtain the target adducts
in up to 95%
ds
(Scheme 7.7).
9
In 2009, Jeong
et al.
further demonstrated that solvents
could significantly influence this asymmetric reaction.
10
The coordinating solvents such as
THF provided a faster rate of reaction and better enantioselectivity than non-coordinating
solvents.
5 mol% [RhCl(CO)
2
]
2
10 mol% (
R
)-xylyl-BINAP
12 mol% AgOTf
R
PA r
2
PA r
2
R
Z
O
Z
THF, 18-20
°
C
H
Ar:CO = 10:1, 1 atm
60-99%
ee
18-99% yield
Ar = 3,5-diMe-C
6
H
3
(
R
)-xylyl-BINAP
R = alkyl, aryl
Z = C(CO
2
Et)
2
, CMe
2
, O, NTs
Scheme 7.6
Rh-catalyzed PKR under a reduced partial CO pressure.
