Chemistry Reference
In-Depth Information
+
+
PPh
2
PPh
2
CO
Ph
P
Pt
Ph
P
Pt
OAr
C(O)OAr
(6.66)
PPh
2
PPh
2
where
H, Me, OMe
.
OAr
=
O
R,
R
=
CO
! Ir
CO
PPh
3
2
fC
O
OPhg
Ir
CO
PPh
3
2
OPh
6
.
67
CO
!
dppe
PtfC
O
OPhg
2
dppe
Pt
OPh
2
6
.
68
Mechanistic studies show that the first reaction proceeds
via
a migratory insertion
reaction. In contrast the second was shown to proceed
via
initial displacement of
aryloxide by CO followed by nucleophilic attack at the coordinated carbonyl ligand.
In the case of the bis(aryloxycarbonyl), subsequent thermolysis under CO leads to
formation of a metal dicarbonyl, CO
2
,ArCO
2
Ar and deoxygenation of one of the
initial aryloxide ligands. A competing reaction is elimination of phenol and formation
of a metallolactone.
The addition of CO to the compound
cis
-[Pt(OC
6
H
5
)Me(PMe
3
2
] was found to
lead to the isomerized product
trans
-[Pt(OC
6
H
5
)Me(PMe
3
2
] which did not undergo
insertion. Interestingly the compound
trans
-[Pt(OC
6
H
5
)Me(PEt
3
2
] was found to insert
CO into the metal- methyl bond.
240
5.2.2
Insertion of Carbon Dioxide and Heterocumulenes
This is a very general reaction for metal aryloxides, and has been observed for both
early and late d-block metal centres as well as for the p-block metals.
The reactivity of the carbonyl anions [M(CO)
5
(OAr)]
(M D Cr, Mo, W) towards
CO
2
and thio analogues has been extensively studied by Darensbourg
et al
.The
phenoxides [M(CO)
5
(OPh)]
(M D Cr, W) readily insert CO
2
(reversibly), SCO, and
CS
2
to produce the corresponding phenyl carbonate, thiocarbonate, and dithiocarbonate
respectively (Eq. 6.69).
241
[
OC
5
WOPh]
C CO
2
D [
OC
5
WfOC
O
OPhg]
[
OC
5
WOPh]
C SCO D [
OC
5
WfSC
O
OPhg]
[
OC
5
WOPh]
C CS
2
D [
OC
5
WfSC
S
OPhg]
6
.
69
The relative rates of the insertion reaction were found to be CS
2
>
SCO
>
CO
2
with Cr
>
W. The substitution compounds [M(CO)
4
(L)(OAr)]
were found to undergo
insertion of CO
2
much more slowly than the parent carbonyl with L D P(OMe)
3
>
PMe
3
>
PPh
3
andinthiscaseW
>
Cr. It was argued there was a steric inhibition
of the insertion reaction. It was also found that the more bulky aryloxide compound
[W(CO)
5
(OC
6
H
3
Ph
2
-2,6)]
did not react with CO
2
but would cleanly form the thio-
carbonate (S-bound) with SCO.
242
The mechanism of these reactions is believed to
