Chemistry Reference
In-Depth Information
Rhee and co-workers described the use of Au(I) catalysts for the synthesis of highly sub-
stituted 2-cyclopentenones from 5-siloxypent-3-en-1-ynes.
83
The mechanism of the reac-
tion could be easily rationalized by an initial siloxycyclization of
167
followed by ionization
of a C-O bond to generate a carbocation intermediate
169
. Intramolecular carbocyclization
and the subsequent elimination of cationic gold(I) via a cyclic intermediate
170
with the
assistance of iPrOH produces the highly substituted cyclopentenone (Scheme 10.49).
R
2
R
1
OSiR
3
O
SiR
3
R
2
R
1
R
2
Au
Au
R
1
R
3
R
4
Au
R
3
R
3
OSiR
3
R
4
R
4
169
167
168
172
O
SiOR
3
O
R
1
R
2
Au
i
PrOH
R
4
+
R
2
R
3
OSiR
3
R
1
Au
R
3
R
4
171
170
Scheme 10.49
Mechanism for the Au-catalyzed synthesis of cyclopentenones.
The reaction gives access to densely substituted cyclopentenones possessing a quaternary
carbon center, in good yields as shown in Scheme 10.50.
R
2
R
1
O
OSiR
3
AuCl[P(C
6
F
5
)
3
]AgSbF
5
R
4
R
2
R
3
CH
2
Cl
2
,
i
PrOH, r.t.
R
1
R
4
R
3
171a-c
167
171a
R
1
=Ph R
2
=Me R
3
=H R
4
=
n
C8H17 97% yield
171b
R
1
=Ph R
2
=Et R
3
=H R
4
=
n
C8H17 91%yield
171c
R
1
=allyl R
2
=Me R
3
=R
4
=-(CH2)3- 91%yield
Scheme 10.50
Cylopentanone synthesis reported by Rhee.
10.7 Conclusions
In this chapter, we presented a variety of transition-metal mediated alternatives to the
Pauson-Khand reaction for the synthesis of cyclopentenones. For example, the long-known
nickel-mediated carbonylation reaction represents a very attractive methodology and offers
a great degree of regioselectivity in the intramolecular version. Unfortunately, however,
nickel tetracarbonyl, which is required in stoichiometric amounts for the reaction to occur,
is very hazardous to handle and the scope of the reaction is very limited. In contrast, Pd
catalysis or alkenylchromium carbene compounds represent powerful alternatives to the
venerable Pauson-Khand reaction, complementing and improving it.
