Structural Studies on Nitrosobenzene Cycloadducts
1697
Conclusion
NMR(CDCl3):δ = 7.23–7.19(2H, m), 7.02–7.00(2H, m), 6.94–
6.91 (1H, m), 6.60–6.57 (1H, m), 6.16–6.13 (1H, m), 4.72–4.70
(1H, m), 4.44–4.42 (1H, m), 2.30–2.24 (2H, m), 1.43–1.37 (2H,
m). 13C NMR (CDCl3): δ = 152.3, 131.6, 129.9, 128.3, 122.0,
117.4, 69.1, 56.5, 24.0, 21.3.
Crystal structure analysis of nitrosobenzene cycloadducts 5–7
show structural effects consistent with the early stages of frag-
mentation. In the case of compound 5, which is predicted to
undergo a thermally allowed concerted retro Diels–Alder reac-
tion, both the C-O and C-N bonds were significantly lengthened
compared with standard values. In contrast only the C-O bond
was lengthened in the structures of compounds 6 and 7. We
therefore predict that these molecules fragment by a two-step
process involving C-O bond cleavage, followed by C-N bond
breaking.
(2Z,4Z)-7-Phenyl-8-oxa-7-azabicyclo[4.2.1]
nona-2,4-diene (6)
Cycloheptatriene (0.24 mL, 2.33 mmol) was added to a solu-
tion of nitrosobenzene (0.25 g, 2.33 mmol) in chloroform (7 mL)
at 0◦C and stirred for 3 h, while monitored by TLC. The solu-
tion was stirred for an additional 16 h at room temperature. The
solvent was removed under reduced pressure giving the crude
product, which was separated by a dry flash column (petroleum
spirit/diethyl ether) and recrystallized from pentane giving 6 as
a white solid (0.16 g, 35%) mp 47–48◦C (lit.[13] 41–42◦C). 1H
NMR(CDCl3):δ = 7.28–7.24(2H, m), 7.06–7.03(2H, m), 6.99–
6.96 (1H, m), 6.21–6.12 (2H, m), 6.07–5.99 (2H, m), 4.89–4.86
(1H, m), 4.28–4.25 (1H, m), 2.39–2.34 (1H, m), 2.30–2.28 (1H,
m). 13C NMR (CDCl3): δ = 151.9, 133.8, 132.1, 128.9, 128.7,
127.7, 122.6, 115.4, 72.5, 64.0, 31.1.
Experimental
(1) Crystallography
Intensity data for 6–8 were collected with an Oxford Diffraction
Sapphire CCD diffractometer using Cu-Kα radiation (graphite
crystal monochromator λ = 1.54184 Å). Intensity data for 5
were collected with a Bruker SMART Apex CCD detector
using Mo-Kα radiation (graphite crystal monochromator
λ =0.71073 Å). The temperature during data collection was
maintained at 130.0(1) K.
Crystal data for 5. C12H13NO, M = 187.23, T = 130.0(2) K,
λ = 0.71073 Å, monoclinic, space group P21/c, a = 9.515(1),
b = 12.206(2), c = 8.616(1) Å, β = 104.546(2)◦, V 968.6(2) Å3,
Z = 4, Dc = 1.284 mg M−3, µ(Mo-Kα) 0.082 mm−1, F(000) =
400, crystal size 0.5 × 0.35 × 0.30 mm. 5982 reflections
measured, 2189 independent reflections (Rint = 0.019) the final
R was 0.0428, [I > 2σ(I)] and wR(F2) was 0.1125 (all data).
Crystal data for 6. C13H13NO, M = 199.24, T = 130.0(2) K,
λ=1.54184Å, orthorhombic, space group Pbca, a=11.5795(3),
b = 9.1698(2), c = 19.4705(4) Å, V 2067.41(8) Å3, Z = 8,
Dc = 1.280 mg M−3, µ(Cu-Kα) 0.640 mm−1, F(000) = 848,
crystal size 0.57 × 0.35 × 0.04 mm. 5503 reflections measured,
1850 independent reflections (Rint = 0.034) the final R was
0.0420, [I > 2σ(I)] and wR(F2) was 0.1190 (all data).
1-Methoxy-3-phenyl-2-oxa-3-azabicyclo[2.2.2]
oct-5-ene (7)
A solution of nitrosobenzene (0.25 g, 2.33 mmol) in chloro-
form (5 mL) was cooled to 0◦C and to this 65% 1-methoxy-1,
2-cyclohexadiene (0.639 mL, 3.50 mmol) was added. The
solution was stirred below 0◦C for 2 h under nitrogen and then
warmed to 25◦C and stirred for an additional 5 h. The solvent
was removed at a low temperature under reduced pressure giv-
ing a yellow solid, which was recrystallized from toluene at low
temperature (−18◦C) (0.5 g, 99%) mp 67–68◦C. HRMS (ESI)
(m/z) calc for [C13H15NO2 + H]+ 218.1176, found 218.1176.
1H NMR (CDCl3): δ = 7.24–7.20 (2H, m), 7.04–7.02 (2H, m),
6.95–6.91 (1H, m), 6.52 (1H, d, J = 8.99 Hz), 6.22–6.20 (1H,
dd, J = 8.7, 5.0 Hz), 4.47–4.44 (1H, m), 3.66 (3H, s), 2.43–2.38
(1H, m), 2.22–2.17 (1H, m), 1.70–1.63 (1H, m), 1.58–1.53 (1H,
m). 13C NMR (CDCl3): δ = 152.2, 131.4, 131.3, 128.4, 121.9,
116.7, 101.8, 56.5, 52.1, 28.7, 22.7.
Crystal data for 7. C13H15NO2, M = 217.26, T = 130.0(2) K,
λ = 1.54184 Å, monoclinic, space group P21/c, a = 18.5752(2),
b = 17.4530(1),
c = 15.0817(1) Å,
β = 113.937(1)◦,
V
4468.85(6) Å3, Z = 16, Dc = 1.292 mg M−3
, µ(Cu-Kα)
3-Phenyl-2-oxa-3-azabicyclo[2.2.2]octane (8)
0.702 mm−1, F(000) = 1856, crystal size 0.4 × 0.4 × 0.20 mm.
20653 reflections measured, 8053 independent reflections
(Rint = 0.034) the final R was 0.052, [I > 2σ(I)] and wR(F2)
was 0.1507 (all data).
A solution of adduct 5 (0.39 g, 2.098 mmol) in ethanol (8 mL)
was chilled to 0◦C. Hydrazine hydrate (0.75 mL) was added
alongwith1%cupricsulfatepentahydratewithvigorousstirring.
30% hydrogen peroxide (0.38 mL) was slowly added and the
resultingsolutionstirredfor3 hat0◦C. NMRshowed∼50%con-
version to 8, so the reaction was continued with twice the amount
of each reactant, however further conversion was not achieved.
The reduction product 8 was separated from the starting mate-
rial 5 via a dry flash column impregnated with silver nitrate
(0.02 g, 5%) mp 48–49◦C (from pentane). HRMS (ESI) (m/z)
calc for [C12H15NO + H]+ 190.1226, found 190.1227. 1H NMR
(CDCl3): δ = 7.28–7.25 (2H, m), 7.11–7.10 (2H, dd, J = 7.99,
1.0 Hz), 6.92–6.89 (1H, dd, J = 7.5, 1.0 Hz), 4.18–4.17 (1H, m),
3.78–3.76 (1H, m), 2.19–2.11 (4H, m), 1.76–1.62 (4H, m).
13C NMR (CDCl3): δ = 152.1, 128.7, 120.8, 115.7, 69.8, 52.6,
25.2, 22.7.
Crystal data for 8. C12H15NO, M = 189.25, T =
130.0(2) K, λ = 1.54184 Å, monoclinic, space group P21/c, a =
9.5075(2), b = 11.4893(2), c = 9.2650(2) Å, β = 104.842(3)◦,
V
978.29(3) Å3, Z = 4, Dc = 1.285 mg M−3
,
µ(Cu-Kα)
0.640 mm−1, F(000) = 408, crystal size 0.28 × 0.16 × 0.11 mm.
4525 reflections measured, 1902 independent reflections
(Rint = 0.037) the final R was 0.0476, [I > 2σ(I)] and wR(F2)
was 0.1262 (all data).
(2) Synthesis. General Synthetic Details and Procedures
are Reported Elsewhere[11]
3-Phenyl-2-oxa-3-azabicyclo[2.2.2]oct-5-ene (5)
1,3-cyclohexadiene (0.45 mL, 4.67 mL) was added to a solu-
tion of nitrosobenzene (0.5 g, 4.67 mmol) in chloroform (10 mL)
and stirred at 25◦C for 2 h. The solvent was removed under
reduced pressure producing a white solid, which was recrys-
tallized from dichloromethane to give 5 clear block-shaped
crystals (0.87 g, >99%) mp 65–66◦C (lit.[12] 65–66◦C). 1H
Supporting Information
Crystallographic information files have been deposited with
the Cambridge Crystallographic Data Centre and assigned
the deposit codes 715311–715314 respectively. These can