Chemistry Reference
In-Depth Information
1.4.2 Ultrasound Techniques
Metal-carbonyl bond cleavage, which is believed to be the requisite first step in the P-K
annulation, is promoted by ultrasonication.
25
In relation to this, the use of low powered
sonication (e.g. an ultrasound cleaning bath) has been applied to both inter-
7d
and in-
tramolecular
26
PK processes. Other than notably decreasing the required reaction times,
these methods have had, at best, little general effect on the product yields. In contrast,
studies from our own laboratory have shown that the use of high intensity ultrasound, in
combination with other reaction promoters, can appreciably improve cyclisation efficien-
cies.
27
As shown in Scheme 1.13, the use of a titanium ultrasonic horn (Vibracell VC 50)
alone reduces the reaction time for the reaction of complex
1
and norbornene from 4 h to
10 min, along with an uplift in reaction yield. When this sonication method is combined with
an amine
N
-oxide promoter (
vide infra
) the intermolecular cyclisation is further enhanced
in terms of both reaction time and cyclopentenone yield.
Ph
+
Ph
H
Co
2
(CO)
6
O
1
H
H
Conditions
Thermal: toluene, 60-70 °C, 4 h: 75%
Ultrasonication: high intensity ))), 10 min: 84%
Ultrasonication and
N
-oxide promoter:
high intensity ))), Me
3
NO.2H
2
O, 6 min: 95%
Scheme 1.13
1.4.3 Microwave Promotion
Over recent years and as an alternative to thermal heating, organic transformations have
been subjected to irradiation within tuned microwave reactors, which has delivered gener-
ally elevated reaction efficiencies, particularly in relation to appreciably reduced reaction
times.
28
In 2002, Evans reported the application of microwave (MW) techniques to a series
of PK reactions, with the cyclopentenone products being delivered in good to excellent
yields, as illustrated in Scheme 1.14.
29
In the same year, Groth
et al
. reported on a MW-
promoted catalytic protocol, with cyclohexylamine as a reaction additive;
30
in general and
despite the introduction of no additional CO gas being required, when compared to other
methods this technique provided relatively moderate efficiencies with a series of normally
reactive substrates.
Ph
MW, 1,2-DCE,
90 °C, 20 min
+
Ph
H
Co
2
(CO)
6
O
1
H
90%
H
Scheme 1.14
