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R
(Me
3
C)
2
C
(Me
3
C)
2
C
NO
(Me
3
C)
2
C
NO
O
O
Me
NO
R
A
(
R
= H, Me
3
C)
B
Figure6.1
4-methyl-4-iminoxycyclohexadiones,
A
,and4,4-bis-oximes,
B
.
4-methyl-4-iminoxycyclohexadiones,
A
(Figure 6.1), were formed with
para
-cresol and 2,6-di-
tert
-butyl-
4-methylphenol (BHT) in yields of 43 and 78 %, respectively; phenol and 1-naphthol gave the 4,4-bis-
oximes, for example,
B
(Figure 6.1), in 58 and 87 % yields, respectively; whereas 2-naphthol gave 1,2-
naphthoquinone in 82 % yield.
6.3.4 Oxidation of amines with di-
tert
-butyliminoxyl
Primary and secondary amines were converted into imines in good yields in a few hours at room temperature
in pentane or hexane.
31
Due to the high inherent reactivity of most imines, the majority of the imine
products were transformed
in situ
into the 2.4-dinitrophenylhydrazine derivatives of the corresponding
carbonyl compounds. For example,
31
the isolated yields of 2,4-dinitrophenylhydrazones from Ph
2
CHNH
2
,
PhCH
2
NH
2
, and (PhCH
2
)
2
NH were 79, 68, and 78 %, respectively. However,
N
-benzylidinemethylamine,
PhCH
=
NMe, was actually isolated from the reaction of PhCH
2
NHMe with (Me
3
C)
2
C
=
NO
•
in yields
(reaction time) of 49 % (30 min), 64 % (4 hours), and 76 % (24 hours) from reactions carried out at 69, 25,
and
5
◦
C, respectively.
31
In retrospect, it seems likely that other imines could probably also have been
isolated by a careful choice of reaction conditions and work-up procedures.
−
6.3.5 Oxidation of di-
tert
-butylketoxime with di-
tert
-butyliminoxyl
This hydrogen atom abstraction reaction is a
self-exchange
,or
identity
, reaction. Self-exchange reactions
are precisely thermoneutral and their study has provided a wealth of understanding about reaction mecha-
nisms, “intrinsic” activation energies, and so on. One remarkable series of observations has been that the
self-exchange reactions of oxygen-centered
radicals (Equation 6.21), and other thermoneutral (or near
thermoneutral) reactions involving the destruction and formation of oxygen-centered
π
π
radicals (Equation
1 kcal/mol)
15
∼
±
6.22), have remarkably low activation energies (typically
2
in comparison with the activa-
π
tion energies for self-exchange reactions involving carbon-centered
radicals, for example, 14.6 kcal/mol
for the CH
3
•
CH
3
•
reaction.
32
+
→
+
CH
4
CH
4
-RO
•
-RO
•
π
+
ROH
→
ROH
+
π
(6.21)
R
OH
-R
O
•
-RO
•
π
+
→
ROH
+
π
(6.22)
phenol self-exchange by Mayer
et al
.
33
revealed an
exciting fact. This self-exchange is
not
a'simple'
hydrogen atom transfer
(HAT) with the proton being
transferred together with one of its bonding electrons. Instead, the PhO
•
A detailed theoretical study of the
π
-phenoxyl
+
PhOH self-exchange is a
proton-coupled electron transfer
(PCET), with the proton and electron being transferred between different
sets of orbitals. There is an initial formation of a PhO
•
---
HOPh complex in which the phenol forms a
+
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