Chemistry Reference
In-Depth Information
CO
CO
CO
CO
OC
OC
Co
Co
NMO, CH
2
Cl
2
, rt
CO
S
Co
Co
CO
CO
OC
CO
S
S
S
Me
Me
Me
Me
NMO, CH
2
Cl
2
, rt
S
Co(CO)
3
Co(CO)
3
S
SMe
Co
Co(CO)
3
Me
OC
CO
Scheme 2.8
Chelation-aided decarbonylation of PK substrates.
has an activation energy lower than that of olefin decomplexation. For this reason, olefin
complexation is a kinetically controlled process and the resulting Co
2
(CO)
5
(alkyne)(alkene)
complexes cannot, in general, be detected as intermediates.
X
X
CO
CO
CO
CO
R
R
Co
Co
Co
CO
Co
CO
H
H
CO
CO
CO
CO
Scheme 2.9
Olefin complexation.
There are, however, some known exceptions to this behavior. Evans and McGlinchey
isolated and characterized by X-ray diffraction Co
2
(CO)
5
(enyne) complexes (Figure 2.2,
left), where an olefin is intramolecularly coordinated to a cobalt atom.
17
It is noteworthy
that these complexes do not evolve to the corresponding PKR products, and this behavior
is attributed to two factors: the severe strain in the possible reaction product, and the
pseudoequatorial coordination of the alkene in the isolated complexes. More recently, an
alkyne tetracarbonyl dicobalt complex with a chelated phosphine-alkene ligand, in which
the phosphorus atom and the alkene moieties of the ligand are attached to the same cobalt
atom, has been characterized by X-ray diffraction by Riera, Verdaguer, and co-workers
18
(Figure 2.2, center). Very interestingly, although the alkene fragment is located in this
complex in an equatorial coordination site with an appropriate orientation and, therefore,
should undergo insertion into a C(sp)-Co bond leading to cobaltacycle formation, it fails to
provide the PKR product upon either thermal or
N
-methylmorpholine
N
-oxide activation.
On the other hand, a similar Co
2
(CO)
5
(phosphine) complex containing a terminal alkene as
one of the phosphine substituents, failed to afford any isolatable chelated phosphine-alkene
complex (Figure 2.2, right), and readily provided the corresponding PKR product. This
change in reactivity was attributed by the authors to the different ability of the olefins
involved to undergo 1,2-insertion.
