1
640
L. S. M. Miranda et al. / Tetrahedron Letters 52 (2011) 1639–1640
Table 1
Results on the MW assisted NBS or TBCA mediated Hofmann rearrangement with DBU
O
H
N
MeOH, halogenating agent,
O
NH2
DBU
O
MW, 5 Min
R
R
1-4
5-8
Entry
R
Halogenating reagent
Temperature (°C)
GC conversion (%)
1
2
3
4
5
6
7
8
H (1)
NBS
NBS
NBS
NBS
TBCA
TBCA
TBCA
TBCA
60
60
60
60
60
60
60
60
80 (5)
20 (6)
>99 (7)
90(8)
50 (5)
10 (6)
91 (7)
66 (8)
NO
2
(2)
OMe (3)
Cl (4)
H (1)
NO (2)
2
OMe (3)
Cl (4)
A hold time of 5 min was chosen to monitor the conversion to
the desired carbamates. Under these conditions, a complete con-
version of 4-methoxybenzamide to the corresponding methyl car-
bamate was observed with the use of NBS (Table 1), once the 4-
methoxybenzamide could not be detected in the crude reaction
mixture. In the case of benzamide and 4-chlorobenzamide, high
conversions were also observed with NBS (entries 1 and 4). In
the case of the 4-nitrobenzamide only 20% conversion was ob-
tained (entry 2), and no improvement was observed with increas-
ing reaction time.
Gratifyingly the reaction performed with TBCA afforded the de-
sired carbamates with the same conversion pattern as for NBS,
however with lower conversions (entries 5–8). This lower TBCA
performance can be explained by the concurrent reaction with
the DBU base. In the case with the electron withdrawing substi-
tuted benzamides where a slower rearrangement rate is expected
the decomposition of DBU is responsible for the low yields.
In order to circumvent this problem, the reaction with TBCA
was performed in the presence of KOH; results are shown in Table
ditions the reagent showed that high chemo-selectivity for no bro-
minated products could be detected.
Acknowledgements
We thank CAPES, CNPq and Faperj for financial support.
Supplementary data
References and notes
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3
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5
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Results on the MW assisted HR mediated by TBCA/KOH
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2
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N
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MeOH, TBCA, KOH
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o
O
MW, 5 Min, 60 C
R
R
8-11
4
-7
Entry
R
Product
GC conversion (%) (isolated yields)
1
2
3
4
H (1)
NO (2)
OMe (3)
Cl (4)
5
6
7
8
>99% (85%)
80% (75%)
>99% (95%)
97% (90%)
2
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