Bromination of Cyclohexene (5) Using Ethyl 3-Bromo-2,4-dioxo-1,2,3,4-tetrahydroquinoline-
3-carboxylate (4). A mixture of the 3-bromo-substituted ester 4 (3.12 g, 0.01 mol), cyclohexene freshly distilled
over P2O5 (1.1 ml, 0.01 mol), and several crystals of diperoxyazelaic acid in anhydrous CCl4 (15 ml) was
refluxed for 20 h. The product was cooled to room temperature and held at -25ºC for 1 day. The precipitated
solid was filtered off and washed with a small amount of cold CCl4. Crystallization from alcohol gave ethyl
4-hydroxy-2-oxo-1,2-dihydroquinoline-3-carboxylate (8) 1.03 g identified by the absence of a depression of
melting point when mixed with a known sample [10]. Solvent and unreacted cyclohexene were distilled off from
the combined filtrates at about 100ºC. The residue (0.76 g) underwent chromato-mass spectrometric
investigation. The content of the 3-bromocyclohexene (6) was 11%. Mass spectrum, m/z (Irel, %): 160 [M]+ (2.6),
81 [M-Br]+ (100), 65 (5), 53 (12). The content of trans-1,2-dibromocyclohexane (7) was 78%. Mass spectrum:
240 [M]+ (4), 161 [M-Br]+ (37), 81 (M-Br-Br)+ (100). In both case the m/z values are given only for the 79Br
isotope. The experimental mass spectra of the 3-bromocyclohexene (6) and trans-1,2-dibromo-cyclohexane (7)
are identical to the spectra of the corresponding standard samples taken from a mass spectrometry library. For
comparison it should be noted that in the mass spectrum of cis-1,2-dibromo-cyclohexane the intensities of the
molecular ion [M]+ and [M-Br]+ fragment ions are identical at 16%.
X-Ray Structural Investigation. Crystals of the mixed ester 3 are monoclinic, at 20ºC: a = 8.937(3),
b = 16.615(5), c = 11.025(3) Å, β = 111.10(2)º, V = 1527.4(8) Å3, Mr = 365.84, Z = 4, space group P21/n,
d
calc = 1.591 g/cm3, µ(MoKα) = 3.092 mm-1, F(000) = 740. Unit cell parameters and intensities of 4514
reflections (2660 independent with Rint = 0.097) were measured on a Siemens P3/PC, four circle, automatic
diffractometer (MoKα radiation, graphite monochromator, θ/2θ scanning to 2θmax = 50º).
The structure was solved by a direct method using the SHELXTL program package [11]. Absorption
was included by a semi-empirical method via Ψ-scanning (Tmin = 0.661, Tmax = 0.987). For refinement of the
disordered fragment and the second bromine atom limits were placed on the bond lengths of Csp3–Csp3 1.54(0.01)
and Car–Br 1.90(0.005) Å. The positions of the hydrogen atoms were revealed from electron density difference
synthesis and refined using the "riding" model with Uiso = nUeq for a non-hydrogen atom bonded to the given
hydrogen (n = 1.5 for methyl and hydroxyl groups and 1.2 for the remaining hydrogen atoms). The structure was
refined in F2 full matrix least squares analysis in the anisotropic approximation for non-hydrogen atoms with
wR2 = 0.145 for 2489 reflections (R1 = 0.063 for 1765 reflections with F>4σ(F), S = 1.026). The complete
crystallographic information has been placed in the Cambridge structural data bank, reference CCDC 608697.
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