Photosubstitution of Hexachlorobenzene with Cyanide Ion
J. Am. Chem. Soc., Vol. 120, No. 22, 1998 5467
Table 4. Sensitization of HCB Photocyanation by Acetone,
Acetophenone, and Oxygen Removal
accommodated by the SN2(Ar*) reaction, in which a Meisen-
heimer complex is formed between the excited substrate and
nucleophile, followed by departure of the substituent from the
complex. The SR+N1(Ar*) mechanism, with ionization of the
excited aromatic to a radical cation, is followed mainly for the
molecules activated by electron-donating groups. Examples
include photosubstitution of chlorobenzene14 and photocyanation
of chloro- and fluoroanisoles.6 In the electron transfer reaction,
which is promoted particularly in polar solvents, the formation
of radical cations proceeds via an electron transfer from the
excited aromatic to a ground state electron acceptor. It seems
unlikely that an electron-deficient compound such as HCB
would form a radical cation, and no obvious electron donors
are present to produce a radical anion.
ratio
Ci/Caerated
%
convn
sample
aerated
degassed
acetone
acetophenone
Creacted, M
6.47 × 10-5 ( 5.6 × 10-6
1.52 × 10-4 ( 5.6 × 10-6
2.42 × 10-4 ( 3.6 × 10-6
2.67 × 10-4 ( 6.63 × 10-6
1
25
54
87
99
2.35
3.74
4.13
with two having acetone (0.125 M, A300 ) 0.78) and the other two
having acetophenone (1.12 × 10-2 M, A300 ) 0.57) added. Six samples
and two more without sensitizers were degassed and sealed under
vacuum. The duplicate samples were irradiated at 300 nm for 7 min
and analyzed by HPLC (Table 4).
Quenching of HCB Photocyanation by Ferrocene. Photolyses
were done in acetonitrile, containing 5% water, with the initial
concentrations of HCB and sodium cyanide equal to 1.67 × 10-4 and
0.131 M, respectively. Degassed samples were irradiated for 20 min
and analyzed by HPLC. The Stern-Volmer plot was obtained by
plotting the ratio of quantum yields of reaction in the absence of the
quencher to that in the presence of the quencher (φo/φq) versus the
concentration of quencher.
Photolysis of HCB at Various Concentrations of Sodium Cya-
nide. Nondegassed solutions of HCB (2.56 × 10-4 M) in acetonitrile/
water (5%), having various concentrations of sodium cyanide, were
irradiated for 30, 50, and 80 min at 300 nm. Values for φdiss were
calculated for each run and averaged.
The novel aspects of this research are the discovery of the
nonexcimer mediated photocyanation of a polychlorinated
aromatic without the assistance of an added electron acceptor
and the observation of multiple photosubstitution steps, each
proceeding with high efficiency. The reaction reported here is
not unique to HCB but seems to be common to a wide variety
of polyhaloaromatic compounds. We are currently extending
this work to the study of successive photochemical cyanations
on lower chlorinated benzenes, highly chlorinated polycyclic
aromatics, such as biphenyls, naphthalenes, and dioxins, and
polyfluorinated aromatic compounds.
Synthesis and Ground-State Reactions of 1,4-Dichloro-2,3,5,6-
tetracyanobenzene and a Trichlorotricyanobenzene. To the mixture
of dichlorotricyanophenol 3 (3.1 mg) and phosphorus oxychloride (0.1
mL) at room temperature was added 14 µL of freshly distilled pyridine.
The reaction mixture was heated at 80 °C for 7 min, cooled to 25 °C,
and poured onto ice/water mixture (2.0 mL). Upon standing (45 min)
a precipitate was formed, filtered under vacuum, washed with a small
amount of cold water, and dried. The product was identified as a
trichlorotricyanobenzene by GC-MS: yield 1.8 mg. Chlorination of
1.8 mg of 2 by the same procedure yielded ca. 0.8 mg of 1,4-dichloro-
2,3,5,6-tetracyanobenzene 4.
The chlorination products were dissolved in acetonitrile (10.0 mL)
and 1.0 mL aliquots were mixed with 1.0 mL solutions of NaCN in
acetonitrile/water (0.262 M, 10% H2O), NaOH in water (2.5%), and
deionized water, respectively, at room temperature. The progress of
reactions was monitored by HPLC, and the mixtures were subjected
to ES-MS analysis after 1 h.
Large-Scale Photolysis of HCB. In a typical experiment 100 mg
(0.35 mmol) of HCB and 450 mg (9.2 mmol) of NaCN were dissolved
in 250 mL of acetonitrile, containing 5% water using a model FS-3
sonicator (Fisher). The solution was placed into four 25 mm o.d. Pyrex
test tubes and irradiated for 8 h at 300 nm in a merry-go-round. The
combined solutions were extracted with hexane (3 × 70 mL) and stirred
in a well ventilated fume cabinet at room temperature in the presence
of 5.6 g of Dowex 50W (50X4-100, Sigma). The solution was stirred
for additional 30 min at 50 °C to remove dissolved hydrogen cyanide.
Upon cooling to room temperature, the resin was removed by a vacuum
filtration. The solution was dried overnight over activated 4 Å
molecular sieves. After drying and filtering, acetonitrile was removed
on a rotary evaporator; the residue was taken up on 5 g of silica gel
(60-200 mesh), which was then placed onto a bed of preequilibrated
silica gel and eluted with ethyl acetate-acetonitrile mixture (4:1).
Collected fractions (typically 15 fractions of 10-12 mL) were subjected
to analysis by TLC and combined, if similar. The purity of isolated
products was checked by ES-MS. The hexane extracts were shown to
contain only unreacted HCB with a typical recovery of ca. 40 mg of
the starting material. The products 2 and 3 were isolated with 60%
raw yield based on reacted HCB.
Experimental Section
General Methods. HPLC grade acetonitrile, methanol, and ethyl
acetate were obtained from Fisher. All solvents were used as received.
Potassium cyanide 15N (99%+ isotopic purity) was purchased from
Isotech Inc. Hexachlorobenzene (Aldrich) was twice recrystallized from
chloroform/methanol before use.
Irradiations. Solutions of HCB and NaCN were photolyzed in 8
mm o.d. Pyrex test tubes in a Rayonet RPR photochemical reactor
equipped with 16 RPR-3000 low-pressure mercury lamps which emit
a band at 300 nm with a bandwidth of 25 nm at a half-intensity. When
necessary, samples were degassed and sealed on a vacuum manifold
using four freeze-pump-thaw cycles (last two cycles using an oil-
diffusion pump, P < 0.01 Torr).
Measurement of Quantum Yield of Disappearance of HCB.
Quantum yields of substrate disappearance were determined by
potassium ferrioxalate actinometry16 using solutions of potassium
chromate and nickel sulfate17 as a filter. The actinometer in water and
the test solution in acetonitrile, containing 5% water, were placed in 8
mm o.d. Pyrex test tubes which were positioned coaxially in 25 mm
o.d. Pyrex test tubes filled with optical filter. The filter transmitted in
the 285-335 nm region, with maximum transmittance of approximately
20% at 313 nm. The actinometer and test solutions were irradiated
simultaneously in the photoreactor using a merry-go-round to ensure
equal light intensity incident on all solutions. All the light was absorbed
by the actinometer, and the photon intensity absorbed by the actinometer
(Io ) 7.09 × 10-7 ( 7.55 × 10-8 mol min-1) was calculated based on
φ313 ) 1.24 for potassium ferrioxalate.12
The intensity absorbed by the test solution was determined in a
separate experiment. Solutions of HCB (3.88 × 10-4 mol L-1, A313
)
0.01) acetonitrile/water (5% H2O) were degassed, sealed under vacuum,
and irradiated for various time intervals, ranging from 10 min to 2 h.
After irradiation, the ampules were opened, and disappearance of HCB
was determined by HPLC analysis. The apparent quantum yields were
calculated and plotted against Iabs
.
Sensitization of HCB Photocyanation by Acetone, Acetophenone,
and Oxygen Removal. Concentrations of acetone and acetophenone
were chosen such that most of the incident light was absorbed by the
sensitizers. Eight solutions of HCB (2.73 × 10-4 M) in acetonitrile/
water (5% H2O), containing NaCN (8.06 × 10-2 M) were prepared,
(16) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. A. 1956, 236, 518.
(17) Gould, I. R. In Handbook of Organic Photochemistry; Scaiano, J.
C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 1, p 47.
2,3,4,5,6-Pentacyanophenol (not isolated). Mass spectrum, m/z (%)
218 (M-H) (100), 219 (13.9), 220 (1.9); MS/MS, Daughters of m/z
218: m/z (%) 190 (46), 164 (5.1), 138 (2.6), 114 (46.2).
4-Chloro-2,3,5,6-tetracyanophenol (2) (sodium salt). Approxi-
mately 20 mg (38% yield, based on reacted HCB) was obtained with