Chemistry Reference
In-Depth Information
OMOM
OMOM
OMOM
Bu
3
SnH (5.3 eq.)
TEMPO (6 eq.)
60
O
N
I
Zn /
AcOH
OH
°
C
Boc
Boc
Boc
Scheme 5.38
Aryl radical cyclization and subsequent trapping by TEMPO
R
R
R
O
O
O
TEMPO
O
B
R
B
H
B
B
O
N
+
O
N
O
O
O
O
R
R
+
N
O
N
O
Scheme 5.39
derivative with tributyltin hydride in the presence of TEMPO leading, after fast 5-exo-cyclization, to an
alkoxyamine that can be cleaved to afford an alcohol in a high overall yield.
213
Addition of an olefin to catecholborane followed by the reaction of the resulting B-alkylcatecholborane
with TEMPO is a very interesting method to generate selectively alkyl radicals. B-alkylboranes (and
not trialkylboranes) react with two equivalents of TEMPO to afford in good yield the corresponding
alkoxyamine (Scheme 5.39).
214
Radical hydroxylation reactions of alkenes via the corresponding
B
-alkylcatecholboranes in the presence
of TEMPO are an interesting alternative to the classical hydroboration followed by oxidative work-up pro-
cedure (Scheme 5.40).
215
-pinene affords with high diastereoselectivity
the alkoxyamine, which can then be cleaved to afford the alcohol.
With specific substrates, the radical-mediated oxidation of organoboranes leads to products differing
from those obtained using the classical oxidation reaction due to rearrangement of the intermediate radical.
This strategy was also used for tandem process; however, this reaction is limited to radical addition to
highly reactive olefins to avoid the TEMPO trapping of the initial radical.
The reaction of TEMPO with ketenes to give the 1,2-bis adducts (Scheme 5.41) has been extensively
studied on a variety of ketenes and by theoretical calculations.
216 - 218
It was found that the first TEMPO
addition occurs at the carbon - carbon double bond, leading to the most stabilized radical.
For instance, the hydroboration of
α
O
1)
BH
O
O
MeCONMe
2
2) TEMPO (2. equiv.)
N
82 % (ds > 98 %)
Scheme 5.40
TEMPO-mediated oxidation of
α
-pinene
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