The synthesis, spectroscopic observation and chemical reactivity of N-(2,2,6,6-tetramethylpiperidyl)nitrene
<p>1,1-Dialkyldiazenes (aminonitrenes, N-nitrenes) unlike their more stable 1,2-dialkyldiazetle isomers (azo compounds) have not yet been isolated or detected by spectroscopic methods, but rather are assumed intermediates based on a substantial body of chemical evidence. The first direct...
| Summary: | <p>1,1-Dialkyldiazenes (aminonitrenes, N-nitrenes)
unlike their more stable 1,2-dialkyldiazetle isomers
(azo compounds) have not yet been isolated or detected
by spectroscopic methods, but rather are assumed
intermediates based on a substantial body of chemical
evidence. The first direct observation of a 1,1-
dialkyldiazene is described here. The visible spectrum
at -78° of N-(2,2,6,6-tetramethylpiperidyl)nitrene (33)
(λ<sub>max</sub> = 541 nm) provides experimental evidence on (1)
the energy required for the n + π* electronic
transition, and (2) the vibrational spacing of the
first electronically excited state. The infrared spectrum
at -78° (<sup>14</sup>N=<sup>14</sup>N stretch at 1595 cm<sup>-1</sup>; <sup>14</sup>N=<sup>15</sup>N
stretch at 1569 cm<sup>-1</sup>) provides evidence that the 1,1-
diazene has considerable N=N double bond character
in the ground state.</p>
<p>The first kinetic study of the thermal decomposition
of a 1,1-dialkyldiazene is described. The
temperature dependence of the unimolecular rate (k<sub>1</sub>)
of fragmentation of 33 was examined in three different
solvents and kinetic evidence for a direct bimolecular
pathway for the formation of 1,1 "-azo-2,2,6,6-tetra-
methylpiperidine 41 from 33 is provided. The activation
parameters for the unimolecular fragmentations
are log A = 11.6 ± 0.5, Ea = 16.9 ± 0.7 in n-hexane,
log A = 13.7 ± 0.3, Ea = 20.0 ± 0.4 in Et<sub>2</sub>0, log A=
13.6 ± 0.3, Ea = 20.1 ± 0.4 kcal/mole in THF. Using
computer simulation it is found that the curved
portions of the tn A vs. time plots may be modelled as
competitive unimolecular and bimolecular reactions
(k<sub>obs</sub> = k<sub>1</sub> + k<sub>2</sub>[33]). In Et<sub>2</sub>0 at -16°, k<sub>1</sub> = 5.03 x
10<sup>-4</sup> sec<sup>-1</sup> and k<sub>2</sub> = 5.0 x 10<sup>-2</sup> liter/mole-sec.</p>
<p>Also reported are proton and carbon-13 nuclear
magnetic resonance data for 33, along with the results
of a preliminary study of its photoreactivity.</p> |
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