, 2005, 15(5), 181–183
Chlorination of 2,2,2,5-tetrachloro-6-methylbenzo[d]-1,3,2-dioxaphosphole
Elena N. Varaksina,* Vladimir F. Mironov, Alfia A. Shtyrlina, Alexey B. Dobrynin, Igor A. Litvinov and
Alexander I. Konovalov
A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences,
420088 Kazan, Russian Federation. Fax: +7 8432 73 2253; e-mail: vlena@iopc.knc.ru
DOI: 10.1070/MC2005v015n05ABEH002170
An NMR investigation has shown that the reaction of 2,2,2,5-tetrachloro-6-methylbenzo[d]-1,3,2-dioxaphosphole 2 with an excess
of molecular chlorine occurs stepwise with high stereoselectivity to give 2,3,4,5,6,6-hexachloro-3-dichlorophosphoryl-5-methyl-
cyclohex-1-ene 4, which then undergoes stabilization via a 3,3-sigmatropic shift of the dichlorophosphonate fragment to give
1,2,4,4,5,6-hexachloro-3-dichlorophosphoryl-5-methylcyclohex-1-ene 5. The configuration of the hydrolysis product of the latter
(1,2,4,4,5,6-hexachloro-5-methyl-3-phosphorylcyclohex-1-ene 6) was established by single crystal X-ray diffraction.
Selective incorporation of halogen atoms into organic mole-
cules is a problem of current interest. In most cases, this is
achieved using mild halogenating agents in various solvents;1
the use of accessible halogens is restricted by the low selec-
tivity of halogenation.1 The reaction of chlorine with benzene
derivatives such as phenols, anilines and pyrocatechols usually
occurs as electrophilic substitution to give either mixtures of
mono-, di- and trihalogen derivatives or a single product
(exhaustive chlorination).2 The reactions of methyl-substituted
anilines and phenols can occur not only as ordinary substitu-
tion but also as a rare process of violation of benzene ring
aromaticity resulting in mixtures of possible diastereomers of
polychlorinated cyclohexen-3-ones.3 Aromaticity violation also
occurs in the case of benzene chlorination under UV irradiation
to give hexachlorocyclohexane.2 We found previously4 that
the reaction of chlorine with 2,2,2-trichloro-4-methyl- and
2,2,2-trichloro-5-methylbenzo[d]-1,3,2-dioxaphospholes in a 1:1
ratio results in 2,2,2,5-tetrachloro-4-methyl- and 2,2,2,5-tetra-
chloro-6-methylbenzo[d]-1,3,2-dioxaphospholes 1 and 2, respec-
tively, in yields higher than 90%.
In this study, we found that, in the presence of an excess of
chlorine, chlorination does not stop, and dioxaphosphole 2 can
be further chlorinated with molecular chlorine under mild
conditions; this involves aromaticity violation of the benzene
fragment, that is, an addition reaction occurs.† We traced the
reaction path by 31P NMR spectroscopy and showed that the
reaction steps can be distinguished based on the kinetics. The
structures of certain intermediates were found by 13C NMR
spectroscopy.
Judging from the chemical shift dP –31.5, the first step involves
the formation of compound 3 containing a pentacoordinated
phosphorus atom. The violation of the benzene ring aromaticity
and the formation of a diastereoisomer containing four chiral
centres [C(3a), C(4), C(5), C(7a)] was reliably established based
on the 13C-{1H} and 13C NMR spectra of this compound. In the
molecule of phosphorane 3, five of the seven carbon atoms
resonate in the high-field region typical of the sp3-hydridised
carbon atom. The presence of two signals at d 95–101 evidenced
chlorine addition to the OC(3a)=C(7a)O bond and formation of
a C–C(3a)(Cl)–C(7a)(Cl)–O fragment.5 The nuclei of the C(3a)
and C(7a) ipso-carbon atoms are distinguishable owing to a
difference in the electronic effects of the chloromethylene and
alkenyl substituents [stronger deshielding of the C(3a) carbon
atom]. The signal of the other sp3-hybridised nucleus [C(4)] is
observed in the 13C-{1H} NMR spectrum‡ as a doublet with a
characteristic trans-constant 3JPCCC (18.9 Hz). When the spectrum
is recorded without decoupling from protons, this doublet is
additionally split into a doublet of quartets, which is only pos-
sible if there is a methyl group at the C(5) atom. The multiplicity
of the remaining signals for the C(6), C(7) and C(5) nuclei
suggests that the location of the double bond is C(6)=C(7).
There is no subsequent addition of chlorine to the double
bond of compound 3. During the storage of phosphorane 3 for
3–4 days at 20 °C, it is gradually converted into phosphate 4
(dP –2.1) via a non-classical variant of the Arbuzov reaction.
Phosphate 4 was obtained as a thick colourless oil. Its structure
was established on the basis of the 13C and 13C-{1H} NMR
spectra.† Unlike compound 3, the spectrum of phosphate 4 con-
tains only one signal, which is characteristic of the O–C(Cl)–C
2
environment and belongs to the C(3) nucleus (doublet, JPOC
15.2 Hz). In this case, the two low-field signals corresponding
to the resonance of the carbon atoms at the double bond have
the form of doublets due to the spin–spin coupling with the
†
General Procedures. Solvents and commercial reagents were purified
by conventional methods. All experiments were performed under an
atmosphere of dry argon. Melting points are uncorrected. Measurements
involved a Boetius melting point apparatus. NMR spectra were recorded
on Bruker MSL-400 (1H, 400 MHz; 13C, 100.6 MHz), Bruker WM-250
(1H, 250 MHz) and Bruker CXP-100 (31P, 36.48 MHz) spectrometers.
The dP values were determined relative to an external standard (H3PO4).
The dC and dH values were determined relative to an internal standard
(HMDS).
Chlorination of 2,2,2,5-tetrachloro-6-methylbenzo[d]-1,3,2-dioxa-
phosphole. A chlorine solution (1.9 g in 20 ml of dichloromethane,
–5 °C) was added to a stirred solution of phosphole 2 (6.5 g in 20 ml
of dichloromethane) in an argon atmosphere at –50 °C. A mixture of
compounds 2 (d –23.2) and 3 in a ratio of 3:2 was obtained in three days.
After six days, the ratio of compounds 2 and 3 was 1:6. Compound 3
was completely converted into phosphate 4 in two weeks (25 °C).
2,2,2,3a,4,5,6,7a-Octachloro-5-methyl-3a,7a,4,5-tetrahydrobenzo[d]-
1,3,2-dioxaphosphole 3. 13C NMR (hereinafter, the description of a
signal in 13C-{1H} NMR spectrum is given in parentheses) (CDCl3) d:
3
2
2
95.35 [ddd (d), C(7a), JHC(4)CC(7a) 2.3 Hz, JPOC(7a) 1.2 Hz, JHC(7)C(7a)
1.0–1.1 Hz], 122.94 [dd (s), C(7), 1JHC(7) 177.5 Hz, 4JHC(4)CCC(7) 1.4 Hz],
139.22 [dq (s), C(6), 2JHC(7)C(6) 5.4 Hz, 3JHC(8)CC(6) 4.3 Hz], 69.95 [m (s),
C(5)], 69.12 [ddq (d), C(4), 1JHC(4) 156.1 Hz, 3JPOCC(4) 18.9 Hz, 3JHC(8)CC(4)
2
3
3.5 Hz], 100.77 [ddd (d), C(3a), JPOC(3a) 5.9 Hz, JHC(7)CC(3a) 5.9 Hz,
2JHC(4)C(3a) 2.5 Hz], 25.38 [qd (s), C(8)H3, JHC(8) 132.7 Hz, JHC(4)CC(8)
1
3
3.1 Hz]. 31P-{1H} NMR (CDCl3) dP: –31.5.
2,3,4,5,6,6-Hexachloro-3-dichlorophosphoryl-5-methylcyclohex-1-ene
4. 13C NMR (CDCl3) d: 132.30 [dd (d), C(1), 1JHC(1) 180.4 Hz, 4JPOCCC(1)
1.4 Hz], 130.13 [dd (d), C(2), 2JHC(4)C(2) 5.1 Hz, 3JPOCC(2) 3.0 Hz], 96.91
2
3
2
[ddd (d), C(3), JPOC(3) 15.2 Hz, JHC(4)CC(3) 11.3 Hz, JHCC(3) 1.2 Hz],
71.16 [ddq (d), C(4), 1JHC(4) 158.5 Hz, 3JPOCC(4) 3.5 Hz, 2JHCC(4) 3.3 Hz],
75.37 [ddq (s), C(5), 3JHC(4)CC(5) 5.3 Hz, 2JHCC(5) 5.3 Hz, 2JHCC(5) 4.2 Hz],
87.97 [m (s), C(6), 3JHC(8)CC(6) 4.2 Hz, 3JHCCC(6) 2.7 Hz, 2JHCC(6) 1.5 Hz],
23.00 [qd (s), C(7), 1JHC(7) 132.7 Hz, 3JHC(4)CC(7) 4.4 Hz]. 31P-{1H} NMR
(CDCl3) dP: –1.3.
1,2,4,4,5,6-Hexachloro-3-dichlorophosphoryl-5-methylcyclohex-1-ene
5 was obtained by heating (180 °C, 5 min) of phosphate 4, as the yellow
1
viscous oil with 90% content (3.5 g), bp 145–148 °C (1 Torr). H NMR
(400 MHz, CDCl3) d: 5.94 [br. d, 1H, H(3), 3JPOCH 12.0 Hz], 5.03 [br. s,
1H, H(6)], 2.13 [br. s, 3H, Me]. 13C NMR (CDCl3) d: 126.62 [br. s (br. s),
1
C(1)], 132.00 [br. s (br. s), C(2)], 84.26 [dd (d), C(3), JHC(3) 162.3 Hz,
2JPOC(3) 7.8 Hz], 89.83 [br. m (d), C(4), 3JPOCC(4) 2.5 Hz], 73.95 [br. s (s),
1
C(5)], 65.39 [br. d (br. s), C(6), JHC(6) 163.4 Hz], 25.63 [br. q (br. s),
C(7)H3, 1JHC(7) 132.8 Hz]. 31P NMR (CDCl3) dP: 13.2 (d, 3JPOCH 12.0 Hz).
1,2,4,4,5,6-Hexachloro-5-methyl-3-phosphorylcyclohex-1-ene
6.
Colourless crystals of compound 6 (mp 211 °C) were obtained by
hydrolysis of phosphate 5 in aqueous acetone. 1H NMR (400 MHz,
[2H6]DMSO) d: 2.08 (br. s, 3H, Me), 5.60 [d, 1H, H(6), 3JPOCH 11.7 Hz],
6
5.69 [d, 1H, H(3), JPOCCCCH 1.1 Hz]. 31P NMR ([2H6]DMSO) dP: –2.7
(d, 3JPOCH 11.6 Hz).
Mendeleev Commun. 2005 181