Chemistry Reference
In-Depth Information
NHR'
OMe
OMe
NHR'
R'NH
2
(OC)
5
Cr
(OC)
5
Cr
(OC)
5
Cr
R
R
R
NHR'
OMe
OMe
NHR'
R'NH
2
O
O
O
R
R
R
Scheme 8.5
OMe
OMe
O
O
7 days
25 °C
70:30
OMe
O
OMe
OMe
3 hours
25 °C
92:8
(OC)
5
Cr
(OC)
5
Cr
OMe
(OC)
5
Cr
8.20
"
para
"
"
ortho
"
Scheme 8.6
OEt
(OC)
5
Cr
Cr(CO)
5
N
(OC)
5
Cr
N
+
N
N
N
EtO
N
N
OEt
Ph
N
Ph
Ph
8.21
8.22
Scheme 8.7
The mechanisms are also parallel. The reaction between an alkoxy carbene and an amine to give an amino
carbene is equivalent to the exchange reaction between an ester and an amine to give an amide (Scheme 8.5).
The parallel can be extended. Vinyl carbenes are powerful dienophiles in Diels-Alder reactions, better than
the corresponding esters or amides (Scheme 8.6).
6
For the reaction between methyl acrylate and isoprene,
compared to the reaction between vinyl carbene
8.20
and isoprene, the rate enhancement is about 2
×
10
4
; in comparison, a rate enhancement of 7
10
5
is obtained by adding AlCl
3
to the methyl acrylate
reaction. They are also excellent Michael acceptors (Scheme 8.7). The alkynyl carbenes are also very good in
both regards.
7
The
×
- to an ester carbonyl group are
acidic. Measurement of pK
a
values for carbene complexes is not straightforward as such numbers are often
determined in an aqueous environment in which the conjugate base of the carbene is unstable. A value of 12.5
for the methoxy substituted methyl chromium carbene in 1:1 H
2
O/ CH
3
CN appears reasonable. Values for
-protons of the alkyl substituent are acidic, just as the protons
