Chemistry Reference
In-Depth Information
diazotization methods. Nevertheless, once formed the salts show enhanced
thermal stability because a strong donor effect of the hydroxyl group in-
creases the strength of the C-N bond, so that the respective phenolates
behave more like a quinoid diazo derivative. Such diazonium salts were
shown by Schmidt et al. to give dramatically better yields of Heck arylation
products in a simple catalytic system - both base-assisted and base-free
conditions work comparably well, as compared with simpler alkoxy deriva-
tives.
261
With standard olefins and acrylates, the yields are close to quanti-
tative, with only 2.5 mol% Pd being used either as Pd(OAc)
2
or Pd
2
(dba)
3
precatalysts. Arylation of more challenging olefins such as acet-
amidoacrylates also gives good yields (Scheme 9.72). However, with o-or
m-aminophenols this trend failed.
262
Successful arylation of amidoacrylates opens up a new route to amino
acids and an effective protocol not limited to some specific diazonium salt
was developed by de Azambuja and Correia.
263
The reaction required the
non-nucleophilic sterically hindered base di-tert-butylmethylpyridine
(DTBMP) and methanol as solvent. Interestingly, in this case the products
were not the expected amidocinnamates, but their tautomers instead, as if
the elimination of PdH takes place not in a regular way to give a C=C bond,
but rather to give a C=N bond, and the intermediate imine binds methanol
(Scheme 9.73). The formation of these and similar adducts often takes place
in base-free reactions of arenediazonium salts and in this case the addition
is likely to be catalysed by the conjugate acid of weak base DTBMP.
N
2
BF
4
COOMe
Pd(OA c)
2
(2.5 mol%)
NaOAc, EtOH
µ
W 80°C, 15 min
+
NHAc
RO
COOMe
R=Me-0%
R=H-51%
NHAc
RO
Scheme 9.72 Example of arylation by phenoldiazonium salts.
MeO
N
2
BF
4
CO
2
Me
CO
2
Me
Pd (OAc)
2
DT BM P
MeOH, 65°C
+
NHAc
NHAc
R
R
MeO
C
O
2
Me
CO
2
Me
CO
2
Me
-PdH
+
MeOH
Pd
+
Ar
NHAc
Ar
NAc
Ar
NHAc
CO
2
Me
Ar
NHAc
Scheme 9.73 Arylation of amidoacrylates.
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