Chemistry Reference
In-Depth Information
O
5 mol% [RhCl(CO)
2
]
2
10 mol% (
S
)-DifluorPhos
12 mol% AgOTf
F
F
Ph
Ph
O
PPh
2
PPh
2
O
O
O
O
THF, 18-20
°
C
F
F
H
Ar:CO = 10:1.1 atm
O
91% yield
95%
de
(
S
)-DifluorPhos
Scheme 7.7
Asymmetric Rh-catalyzed desymmetrization of dienynes.
In 2010, Ratovelomanan-Vidal and Jeong
et al.
showed an alternative asymmetric Rh-
catalyzed PKR of 1,6-enynes using sterically hindered (
S
)-dtbm-MeO-Biphep as the chiral
ligand (Scheme 7.8).
11
This catalytic system was an improvement on the previously reported
system (c.f. Scheme 7.6). The product yield and/or enantioselectivity were higher, especially
for the substrates containing an aryl substituent on an alkyne terminus, though a higher
reaction temperature was required for this system.
5 mol% [RhCl(CO)
2
]
2
10 mol% (
S
)-dtbm-MeO-Biphep
15 mol% AgOTf
R
Ar =
R
MeO
PAr
2
PAr
2
Z
O
Z
MeO
OMe
°
C
THF, 40 -80
H
Ar:CO = 10:1, 1 atm
R = H, alkyl, allyl, aryl
Z = O, NTs, C(CO
2
Et)
2
41-99%
ee
34-99% yield
(
S
)-dtbm-MeO-Biphep
Scheme 7.8
Asymmetric Rh-catalyzed PKR of enynes.
To illustrate the applicability of the Pauson-Khand reaction in the synthesis of natural
compounds, the same research group reported an efficient asymmetric desymmetrization
of the prochiral acetal.
12
The desymmetrization of acetal was sensitive to the reaction
conditions and decomposition of the acetal occurred under thermal reaction conditions.
Interestingly, cinnamaldehyde can serve as the CO surrogate
18
for the carbonylative cy-
cloaddition to give the target products with satisfactory yields and moderate
ee
value
(Table 7.1).
12
Apart from the chiral bidentate phosphine ligands, Zhou and co-workers developed a
chiral monodentate phosphoramidite ligand (
SIPHOS
) which was found to be effective
in the Rh-catalyzed asymmetric carbonylative cyclization (Scheme 7.9).
13
The finding
represented the first successful example of chiral monodentate ligand in asymmetric Rh-
catalyzed intramolecular PKR. Thus the chiral monodentate ligands have a high potential
for the application in related enanatioselective reactions.
Transition metal-catalyzed carbonylation has proved successful in the area of organic
synthesis, and has been recognized as a powerful protocol for the direct synthesis of a wide
variety of carbonyl-containing compounds.
14
However, the use of highly poisonous carbon
monoxide constitutes a drawback to these methodologies. In 2002, two independent groups,
Morimoto/Kakiuchi
15
and Shibata,
16
respectively, illustrated a conceptual evolution in the
