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H
2
N
O
C
C
O
O
O
O
Ph
C
C
C
N
N
N
N
O
N
N
N
O
O
O
Figure4.10
TheknownE-diazeniumdiolates.
result from the addition of nucleophiles (Equation 4.23) adopt the Z geometry.
170
Recently, a series of
tetradentate amine derivatives with a terminal diazeniumdiolate were proposed to have a coordinated
E-diazeniumdiolate (Figure 4.10) but this was not structurally or spectroscopically confirmed.
181
Ab initio
calculations indicate that the Z isomers are more stable than the E isomers by between 4 - 10 kJ mol
−
1
, but
the barriers for interconversion
179
by nitroso group rotation are often high, 120 - 160 kJ mol
−
1
. The origin
of the stereoselectivity in these reactions remains an important question. A priori there is little reason for
the apparent thermodynamic control in these reactions unless it corresponds to the same product from the
kinetic control. As with nitric oxide oxidation, two mechanisms have been proposed involving (i) rate
limiting nucleophile addition to the dimer (Equations 4.24 - 4.26) or (ii) reaction of the second NO to the
pre-equilibrated HR
2
NNO
•
radical (Equations 4.27 - 4.29). Initial mechanistic studies
175
were confused by
oxygen contamination
145
and subsequent studies have demonstrated that both mechanisms operate, with a
strong solvent dependence.
116
The current interpretation is that at higher pressures and with methanol as
a solvent, a bimolecular mechanism (Equations 4.24 - 4.26) predominates.
O
2NO
N
[R
2
NH
2
]
+
2R
2
NH
N
ether, R.T., 24 hr.
R
2
NO
(4.23)
H
2
O, pH ~ 7
Dimer
(4.24)
N
2
O
2
2 NO
k
2
r.d.s.
(4.25)
N
2
O
2
+
R
2
NH
HR
2
NN
2
O
2
[R
2
NN
2
O
2
]
−
[R
2
NH]
+
(4.26)
HR
2
NN
2
O
2
+
R
2
NH
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