Chemistry Reference
In-Depth Information
Microwave heating has been applied to many different areas of chemical research in the
last decade.
33
Since the pioneering reports on microwave-accelerated organic transforma-
tion by the groups of Gedye and Giguere/Majetich in 1986,
34
the application of microwave
dielectric heating to microwave-assisted organic synthesis (MAOS) has attracted consid-
erable attention in recent years.
35
In 2008, Kwong and co-workers reported a microwave-
assisted Rh-diphosphine complex-catalyzed PKR.
36, 37
The cooperative process provided
[2
1] cycloadducts by sequential decarbonylation of aldehyde or formate and carbony-
lation of enynes within a short period of time. Various
O
-,
N
- and
C
-tethered enynes were
transformed to corresponding products in good yields. This catalysis represents the first
enantioselective version of a microwave-accelerated cascade cyclization. In the presence
of the chiral complex of Rh-L
∗
+
2
+
[L
∗
=
(
S
)-BisbenzodioxanPhos], up to 90% ee of bicyclic
cyclopentenones were achieved (Scheme 7.16).
37
Rh-dppp
R
R
O
Microwave irradiation
R'
t-amyl
alcohol, 100-120
°
C
*
Cinnamylaldehyde
X
R'
X
R = aryl, alkyl
X = O, NTs, C(COOEt)
2
Up to 90%
ee
87% yield
Rh-L*
Scheme 7.16
The microwave-assisted asymmetric PKR.
In 2008, Evans and Baik reported a mechanistic study on the diastereoselective Rh-
catalyzed PKR with the role of a coordination number for governing the stereo-outcomes.
38
They figured out a theoretical analysis of Rh-catalyzed PKR that could provide two mech-
anistic scenarios for the origin of diastereoselectivity (Table 7.2). The optimum selectivity
could be attributed to a five- rather than a four-coordinating organorhodium complex.
These findings offer a foundation to determine relevant controlling factors for improving
the stereochemical outcome of future Pauson-Khand reactions.
Tab l e 7 . 2
The diastereoselectivity of PKR adducts governed by CO/Ar ratio.
Me
Me
Me
3 mol% [RhCl(CO)(dppp)]
2
O
O
O
O
O
+
CO/Ar, xylene, 110
°
C
H
H
Me
Me
Me
A
B
CO pressure
Ar pressure
%yield
dr (A/B)
1.00
0.00
88
99.1
0.10
0.90
51
58.1
0.05
0.95
44
57.1
