Chemistry Reference
In-Depth Information
R
L
R
S
OMe
OMe
OMe
± CO
coordination
(OC)
4
Cr
(OC)
5
Cr
R
L
R
S
8.43
8.42
8.44
MeO
OMe
OMe
alkyne
insertion
CO
insertion
R
S
CO
(OC)
5
Cr
(OC)
4
Cr
(OC)
4
Cr
R
L
R
L
R
S
R
L
R
S
O
8.46
8.45
8.47
MeO
OMe
OMe
electrocyclic
ring closure
R
S
R
S
R
S
tautomerism
R
L
•
R
L
R
L
O
Cr(CO)
4
Cr(CO)
4
O
HO
Cr(CO)
4
8.48
8
.
4
8
5
0
Scheme 8.16
formed. This is useful in the case of acid-sensitive substrates and products. Silylation can be done similarly.
The donor substituent of the carbene (OMe below) will be
p
- to the phenol, the large substituent of the alkyne
will be
o
- to the phenol.
The mechanism of the Dotz reaction involves substitution of a CO ligand by the alkyne to give a
2
-
intermediate
8.44
(Scheme 8.16). The alkyne can then insert into a Cr-C bond. This leaves the other Cr-C
bond intact in a process that is a formal [2
2] cycloaddition giving a metallacyclobutene
8.46
after re-
coordination of CO. This step determines the regioselectivity of the reaction: the smaller of the alkyne
substituents is favoured to be next to the quaternary centre. The chromium is very electron poor at this
stage as it is no longer benefiting from donation of the oxygen lone pair. CO insertion results, giving a
metallacyclopentenone
8.47
that opens to a coordinated vinyl ketene
8.48
. Electrocyclic ring closure of
this ketone gives a dienone complex
8.49
that gives the phenol product as its Cr(CO)
3
complex
8.50
after
tautomerism.
In some cases, the
+
6
-complex
8.50
is labile and decomplexes under the reaction conditions, in other cases
the free arene can only be obtained by oxidative decomplexation. As the free arene is highly electron rich,
however, this is usually accompanied by oxidation to the quinone
8.51
(Scheme 8.17). As there is considerable
interest in quinone antibiotics, this can be an advantage! On the other hand, decomplexation may sometimes
be achieved by heating with CO. This procedure generates Cr(CO)
6
as a by-product, which may be recovered.
O
OMe
R
S
R
S
Ce(NH
4
)
2
(NO
3
)
6
R
L
R
L
O
HO
Cr(CO)
4
8.50
8.51
Scheme 8.17
