Chemistry Reference
In-Depth Information
H
H
O
N
O
N
O
N
O
N
R
R
R
R
R
R
R
R
Scheme 6.4
6.4 Other reactions and non-reactions of di-
tert
-butyliminoxyl
The reaction of this iminoxyl with nitrogen monoxide,
•
NO, goes through the colorful sequence: blue
→
colorless, and the final product is the nitrimine, (Me
3
C)
2
C
=
NNO
2
.
19
green
→
yellowish brown
→
There
=
is, presumably, an initial coupling to form (Me
3
C)
2
C
NONO, and this is followed by decomposition to
•
NO
2
and the iminyl, (Me
3
C)
2
C
=
N
•
, which then couple to the nitrimine (Equation 6.9). In contrast to this
NO
•
does not react with the DPPH
•
radical, nor with stable
nitroxides.
19
This iminoxyl is also unreactive towards styrene, vinyl acetate, methyl iodide and triphenyl
phosphine, but it does react with triethyl phosphate, potassium iodide-starch paper, bromine vapor, and
ceric ammonium nitrate.
19
facile radical
+
radical reaction, (Me
3
C)
2
C
=
None of these last reactions has been examined in any detail.
6.5 Di-
tert
-alkyliminoxyls more sterically crowded than di-
tert
-butyliminoxyl
Since (Me
3
C)
2
C
=
NO
•
decays via an irreversible head-to-tail dimerization, it was hypothesized that the
replacement of one or both
tert
-butyl groups by sterically more demanding substituents would hinder
dimer formation and, hence, yield much more persistent, and possibly stable, iminoxyls.
20
Unfortunately,
this was not the case.
20
The total steric effect associated with a substituent, relative to the steric effect of a methyl group,
is best quantified by Taft's (logarithmic) steric substituents constants,
E
s
.
36
Va l u e s o f
E
s
for some
of the alkyl substituents of the iminoxyls listed in Table 6.1 (which gives the rate constants for
their second order decay) are: Me, 0.00; Me
2
CH,
1.95
37
;PhCMe
2
,
−
0.47; Me
3
C,
−
1.54; EtCMe
2
,
−
2.06
37
;Et
3
C,
−
−
3.8. The following sterically-protected iminoxyls have been isolated as blue liquids:
=
NO
•
,(EtCMe
2
)
2
C
=
NO
•
,andMe
3
C(Et
3
C)C
=
NO
•
.
20
Me
3
C(EtCMe
2
)
C
Although
they
are
more
sterically congested than (Me
3
C)
2
C
=
NO
•
, the second order rate constants (2
k
13
)
for the decay of two
C
=
NO
•
and (EtCMe
2
)
2
C
=
NO
•
, are about three times as great as 2
k
13
for
di-
tert
-butyliminoxyl
23
(Table 6.1). Only Me
3
C(Et
3
C)C
=
NO
•
decayed at a slightly lower rate than
(Me
3
C)
2
C
=
NO
•
(Table 6.1). However, the kinetics of decay of this iminoxyl did not correspond to a
simple second order process and gave between 9 and 11 products (HPLC) with traces of pivalonitrile
and triethylacetonitrile, but no detectable (
of them, Me
3
C(EtCMe
2
)
1 %) 4,4-diethyl-2,2-dimethylhexan-3-one.
23
In contrast,
the decay of (Me
3
C)
2
C
=
NO
•
gave 4 % pivalonitrile and 40 % di-
tert
-butyl ketone (
vide supra
).
19
At high concentrations of Me
3
C(Et
3
C)C
<
=
NO
•
(0.2 M) decay followed reasonable first order kinetics
(
k
obs
10
−
5
s
−
1
in acetonitrile at 23
◦
C), probably via an intramolecular hydrogen atom abstraction
(Equations 6.27 and 6.28):
=
2
.
3
×
C
=
NO
•
C
•
HCH
3
)
(
=
NOH
Me
3
C
(
Et
3
C
)
→
Me
3
CC
)
CEt
2
(
(6.27)
C
•
HCH
3
)
+
C
=
NO
•
(
=
NOH
Me
3
CC
)
CEt
2
(
Me
3
C
(
Et
3
C
)
→
(
=
NOH
)
CEt
2
(
CHCH
3
ON
=
C
(
)
CEt
3
)
→→→
Me
3
CC
Me
3
C
9 to 11 products
(6.28)
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