Chemistry Reference
In-Depth Information
methods (
vide supra
) and, in most instances, significant enhancement in the yield of
cyclopentenone can be realised. It should also be noted that similar reductive outcomes
have been observed in PK reactions with vinyl halide substrates.
9, 39
O
Ph
NMO.H
2
O,
DCM, r.t., 16 h
+
Ph
H
Co
2
(CO)
6
BzO
80%
34d,e
1
16
O
Ph
C
2
H
4
(25-30 atm), Me
3
NO.2H
2
O,
Tol./MeOH, 40 °C, 7 h
Ph
H
Co
2
(CO)
6
71%
34b,c
1
Scheme 1.18
1.4.5
Sulfide Promoters
In 1997, Sugihara and Yamaguchi introduced the concept of employing Lewis basic ad-
ditives to promote the PK reaction, and demonstrated this with the use of, in particular,
cyclohexylamine to enhance a series of both intra- and intermolecular examples,
40
albeit un-
der considerably greater forcing temperatures than those used with amine
N
-oxides. Despite
the apparent success of the amine additive method, less reactive alkenes did not perform
well under the developed conditions and cleavage of carbon-heteroatom bonds was also
observed. Accordingly, further advances by the same researchers using sulfide-based pro-
moters have become significantly more widely employed by the preparative community.
41
An example using the recommended
n
-butyl methyl sulfide promoter compared with the cy-
clohexylamine additive in reaction with the allyl propargyl amine is shown in Scheme 1.19.
CyNH
2
, 1,2-DCE,
83 °C, 30 min
p-TsN
15%
Co
2
(CO)
6
N
Ts-
p
n-BuSMe, 1,2-DCE,
83 °C, 30 min
p-TsN
O
79%
Scheme 1.19
Despite the more elevated temperatures that are required, this sulfide-based method
appears to be complementary with the amine
N
-oxide techniques. Having stated this,
some appreciable practical drawbacks are encountered when employing this low molecular
weight sulfide, which possesses a rather unpleasant smell and is lachrymatory in nature.
Accordingly and in seeking to establish a more user-friendly protocol, work from our own
laboratory has shown that the inexpensive and stench-free alternative,
n
-dodecyl methyl
