suggest either that the electron is localized in the π system
of one carbonyl or that a product less symmetrical than 3•
-
(
e.g., the third nitrogen is missing) has formed. When the
reduction of C
2
H
5
NCO is carried out in a THF-d
8
solution,
-
followed by reoxidation with iodine (3C
2
H
5
NCO + e f
•
-
-
3
+ 1/2 I
2
f 3 + I ), NMR analysis reveals that 3 is the
dominant product, with an 86% yield (Figure 3). The same
.
Geometries predicted by B3LYP/6-31G* for 3 and 3•
-
Figure 4
carbon ) black, nitrogen ) blue, and oxygen ) red; the hydrogens
(
on the ethyl groups have been removed for clarity). In the
isocyanurate ring of 3, all six bond lengths are the same, all three
NCN bond angles are 116°, and all three CNC bond angles are
•
-
1
24°. In 3 the predicted spin densities on all ring carbon and
(Upper) 400 MHz 1H NMR spectrum of a THF-d
.
8
nitrogen atoms are in parentheses. The carbonyl with high spin
density is now pyramidalized, whereas the other two COs are nearly
coplanar with the nitrogens.
Figure 3
solution containing ethyl isocyanate with an excess of 18-crown-6
(
1
∼3.6 ppm). (Lower) 400 MHz H NMR spectrum of the same
THF-d solution after reduction of the ethyl isocyanate with Na
metal followed by reoxidation with I . The 18-crown-6 is needed
to generate solvated electrons in THF.
8
2
indicate that the D3h isocyanurate ring in 3 loses planarity
•
-
upon electron attachment; this would imply that 3 under-
1
3
goes a first order Jahn-Teller (J-T) distortion. This
distortion reduces the symmetry of the isocyanurate ring
resulting in two or more conformers that differ in geometry,
6
procedure was carried out with the C H11NCO system, and
1
0
4
was generated with a 81% yield.
As further confirmation that the anion radicals of 3 and 4
13
symmetry and spin distribution. Our calculations have
are generated by the electron-initiated reduction of their
found one of these conformers for which the isocyanurate
respective isocyanates, we synthesized 3 using known
•-
ring in 3 has a boatlike geometry (C
s
symmetry) with two
1
1
synthetic procedures. The reduction of authentic 3 and 4
in HMPA gave the identical EPR spectrum as that generated
by the reduction of ethyl isocyanate and cyclohexyl isocy-
anate, respectively. These results leave little doubt that the
anion radicals of 3 and 4 form upon reduction of their
isocyanate precursors.
short and four long N-C bonds, and the carbonyl with high
spin density is now pyramidalized while the other two COs
14
remain nearly coplanar (Figure 4). However, the line-width
effect observed in the EPR spectrum (Figure 2) suggests that
•-
a second similar, but not identical, J-T conformer of 3 is
also present.
•-
B3LYP/6-31G* geometry optimizations for 3 and 3 show
that the gas phase anion radical is unstable relative to the
neutral molecule by 40.8 kJ/mol, indicating that solvation
The line-width alternation effect detected in the EPR
spectrum recorded at 295 K (Figure 2) becomes more
pronounced upon cooling the HMPA solution, confirming
•-
effects are important to the stability of 3 . Despite this, we
find that the predicted spin densities for the ring carbons
and nitrogens correlate remarkably well with the measured
•-
that there are indeed two J-T conformers of 3 in equilib-
rium, undergoing fast exchange on the EPR time scale (see
1
2
coupling constants (Figure 4). These calculations also
(13) For a discussion and examples of systems that undergo J-T distortion
upon electron addtion, see: (a) Jahn, H. A.; Teller, E. Proc. R. Soc. London
Ser. A 1937, 161, 220. (b) Horvat, D. A.; Hammons, J. H.; Stevenson, C. D.;
Borden, W. T. J. Am. Chem. Soc. 1997, 119, 9523–9526. (c) Kurth, T. L.;
Brown, E. C.; Hattan, C. M.; Reiter, R. C.; Stevenson, C. D. J. Phys. Chem.
A 2002, 106, 478–481. (d) Gerson, F.; Huber, W. Electron Spin Resonance
Spectroscopy of Organic Radicals; Wiley-VCH: Weinheim, 2003; p 161.
(
10) (a) 1H chemical shift assignments are known. See: Tang, J.;
Verkade, J. G. J. Org. Chem, 1994, 59, 4931–4938. (b) Duong, H. A.; Cross,
M. J.; Louie, J. Org. Lett. 2004, 6, 4679–4681.
(
11) Ovchinnikova, N. A.; Sinyakov, A. E.; Reznik, A. M.; Sakharov,
S. G.; Gorbunova, Yu. E.; Mikhailov, Yu. N.; Kanishcheva, A. S.; Butskii,
•-
V. D. Koord. Khim. 2002, 28, 673–677.
(14) The symmetry assignments assigned to 3 and 3 are of the
•
-
•-
(
12) Calculations on 4 and 4 give similar results, with 4 being more
isocyanurate ring only. DFT calculations on 4 and 4 also show that the
•
-
stable by only 33 kJ/mol; 4 has calculated spin densities similar to those
neutral isocyanurate ring undergoes a boatlike molecular distortion upon
electron reduction.
of 3•
-
.
Org. Lett., Vol. 10, No. 20, 2008
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