Chemistry Reference
In-Depth Information
O
N
MeO
H
H
N
H
MeO
O
H
O
Figure 2.1
Structure of brucine
N
-oxide (BNO).
Other approaches to the promotion of the decarbonylation of dicobalt hexacarbonyl
complexes have been explored. Photochemical promotion,
11
ultrasound promotion,
12
and
the use of TEMPO to promote single-electron activation of cobalt-carbonyl bonds
13
have
been studied, and even mechanistic proposals have been formulated for some of these
approaches.
11b,13
However, synthetic use of these activation methods has been up to now
very limited and will not be discussed in detail here.
The pentacarbonyl complex resulting from CO loss of the initial Co
2
(CO)
6
(alkyne)
complex is a high energy species that can only be observed under special conditions. Thus,
photolysis
14
of Co
2
(CO)
6
(alkyne) complexes, involving alkynes such as acetylene, simple
symmetrically disubstituted alkynes or phenylacetylene, in frozen Nujol at ca. 90 K results
in loss of CO to give the coordinatively unsaturated Co
2
(CO)
5
(alkyne) complexes, where
CO loss appears to have occurred from the axial position. However, complete conversion
to a second isomer, presumably having the coordination vacancy in an equatorial position,
is observed for all compounds upon annealing the matrices to ca. 140 K (Scheme 2.7).
R
R
R
eq
OC
CO
eq
eq
OC
CO
eq
eq
OC
CO
eq
140 K
h
Nujol, 90 K
ax
OC
Co
Co
CO
ax
ax
OC
Co
Co
CO
ax
ax
OC
Co
Co
eq
OC
eq
OC
CO
eq
eq
OC
CO
eq
R
R
R
Scheme 2.7
Detection of the coordinatively unsaturated dicobalt pentacarbonyl species.
In addition, Co
2
(CO)
6
(alkyne) complexes where the alkyne bears properly placed
thioether
15b
or thioacetal
15a
substituents afford, upon decarbonylation, thermally stable,
yet reactive in PKR, pentacarbonyl complexes involving the thioether function as a hemil-
abile ligand
15
(Scheme 2.8).
Coordination of the reactant alkene to the coordinatively unsaturated Co
2
(CO)
5
(alkyne)
intermediates is an energetically favorable process taking place with very low (if any)
activation energy. Product stereochemistry in PKR involving alkenes with diastereotopic
faces (for instance, norbornene and norbornadiene derivatives) indicates that coordination
takes place involving the less hindered face of the olefin (Scheme 2.9). According to
theoretical studies
16
(see below), even with poorly reacting olefins the energy barrier
associated to the next step in the PKR mechanistic pathway (cobaltacycle formation)
