Letho.10 To a solution of isopropyl benzene kept at 0 °C (35 mL,
0.25 mol in 75 mL of CHCl3) was slowly dropped the chlorosulfonic
acid (50 mL, 0.75 mol), keeping the temperature at 0 °C. The
mixture was stirred overnight at room temperature, then was poured
on crushed ice and extracted with CHCl3, and the collected organic
phase was dried on MgSO4 and concentrated at reduced pressure.
The crude product was distilled to yield 28 mL (62%) of pure
2,7-Diisopropylthianthrene-5,5,10,10-tetraoxide (3) was pre-
pared by oxidation of 2 with m-chloroperbenzoic acid.1c 1H NMR
(CDCl3, 600 MHz, 25 °C): δ 1.31 (d, 6H, J ) 6.9 Hz), 3.09 (septet,
1H, J ) 6.9 Hz), 7.64 (dd, 2H, J ) 1.7, 8.0 Hz), 8.09 (d, 2H, J )
1.7 Hz), 8.17 (d, 2H, J ) 8.0 Hz). 13C NMR (CDCl3, 150.8 MHz,
25 °C): δ 23.6 (2Me), 34.8 (CH), 124.2 (CH), 126.5 (CH), 131.7
(CH), 137.1 (quat), 140.0 (quat), 156.2 (quat). Anal. Calcd for
C18H20S2O4: C, 59.32; H, 5.53; S, 17.59. Found: C, 59.15; H, 5.50;
S, 17.45.
1
colorless liquid, bp5 131 °C. H NMR (CDCl3, 300 MHz, 25 °C):
δ 1.28 (d, 6H, J ) 7.0 Hz), 3.02 (septet, 1H, J ) 7.0 Hz), 7.42 (m,
2H), 7.94 (m, 2H). 13C NMR (CDCl3, 75.4 MHz, 25 °C): δ 23.8
(2 Me), 34.5 (CH), 127.4 (2CH), 127.9 (2CH), 142.1 (quat), 157.6
(quat).
NMR Measurements. NMR spectra were recorded at 600 MHz
for 1H and 150.8 MHz for 13C. The assignments were obtained by
DEPT and two-dimensional experiments (g-HSQC and g-HMBC).
The samples for the 13C NMR low-temperature measurements were
prepared by connecting to a vacuum line the NMR tubes containing
the compound and some C6D6 for locking purpose and condensing
therein the gaseous CHF2Cl and CHFCl2 under cooling with liquid
nitrogen. The tubes were subsequently sealed in vacuo and
introduced into the precooled probe of the spectrometer. The
temperatures were calibrated by substituting the sample with a
precision Cu/Ni thermocouple before the measurements. A complete
fitting of NMR line shapes was carried out using a PC version of
the DNMR-6 program.13
4-Isopropylbenzenethiol.9 To a suspension of LiAlH4 (12.0 g,
0.316 mol) in 50 mL of dry Et2O cooled to 0 °C was dropped, in
30 min, a solution of 4-isopropylbenzenesulfonyl chloride (20.0 g,
92 mmol, in 100 mL of Et2O). After refluxing for 3 h, the reaction
was complete. The mixture was again cooled to 5 °C, and the
exceeding hydride was destroyed by dropping carefully a saturated
aqueous solution of NH4Cl. The rough mass was filtered and the
organic phase separated, dried on MgSO4, and concentrated at
reduced pressure to yield 8.1 g (58%) of product that was directly
used in the next step. 1H NMR (CDCl3, 300 MHz, 25 °C): δ 1.11
(d, 6H, J ) 7.1 Hz), 2.82 (septet, 1H, J ) 7.1 Hz), 3.39 (s, 1H),
7.09 (m, 2H), 7.21 (m, 2H). 13C NMR (CDCl3, 75.4 MHz, 25 °C):
δ 24.0 (2Me), 33.7 (CH), 127.0 (quat), 127.3 (2CH), 129.9 (2CH),
146.5 (quat).
Computational Details. Ab initio computations of 1a, 2, and 3
were carried out at the B3LYP/6-31G(d) level by means of the
Gaussian 03 series of programs14 (the standard Berny algorithm in
redundant internal coordinates and default criteria of convergence
were employed). Harmonic vibrational frequencies were calculated
in order to ascertain the nature of the stationary points. For the
optimized ground states, the frequency analysis showed the absence
of imaginary frequencies, whereas for each transition state, the
frequency analysis showed a single imaginary frequency. The
corresponding optimized structures are reported in the Supporting
Information.
2,7-Diisopropylthianthrene (2). A mixture of 4-isopropylben-
zenethiol (2.0 g, 13.1 mmol) and 8 mL of 98% H2SO4 was stirred
overnight at room temperature, then cooled with ice, carefully
neutralized with a solution of KOH, and extracted with Et2O. The
organic layers were collected and dried on MgSO4. The purification
of the crude product by chromatography, on a silica gel column
eluted with hexane/ethyl acetate 95/5, yielded 500 mg (25%) of 2
as a white solid that was further purified by semipreparative HPLC
(Waters NovaPack, 5 µm silica, 300 × 19 mm, 24 mL/min,
1
n-hexane/2-propanol 99/1). H NMR (CDCl3, 600 MHz, 25 °C):
δ 1.21 (d, 6H, J ) 7.0 Hz), 2.85 (septet, 1H, J ) 7.0 Hz), 7.10
(dd, 2H, J ) 2.1, 8.2 Hz), 7.36 (d, 2H, J ) 2.1 Hz), 7.41(d, 2H, J
) 8.0 Hz). 13C NMR (CDCl3, 150.8 MHz, 25 °C): δ 23.9 (2Me),
33.8 (CH), 126.0 (CH), 126.8 (CH), 128.6 (CH), 132.6 (quat), 135.9
(quat), 148.9 (quat). Anal. Calcd for C18H20S2: C, 71.95; H, 6.71;
S, 21.34. Found: C, 71.84; H, 6.67; S, 21.29.
Acknowledgment. L.L. and A.M. received financial support
from the University of Bologna (Funds For Selected Research
Topics and RFO) and from MIUR-COFIN 2003, Rome (Na-
tional Project “Stereoselection in Organic Synthesis”).
2,7-Dimethylthianthrene11 was prepared from p-thiocresol with
1
the same procedure described for 2. H NMR (CDCl3, 600 MHz,
25 °C): δ 2.28 (s, 3H), 6.99 (dd, 1H, J ) 1.6, 7.9 Hz), 7.26 (d,
1H, J ) 1.6 Hz), 7.31 (d, 1H, J ) 7.9 Hz). 13C NMR (CDCl3,
150.8 MHz, 25 °C): δ 20.9 (2Me), 128.4 (CH), 128.5 (CH), 129.3
(CH), 132.2 (quat), 135.9 (quat), 137.7 (quat).
Supporting Information Available: X-ray structure of com-
pound 2 and ab initio computational data for compounds 1a, 2,
and 3. This material is available free of charge via the Internet at
2,7-Dimethylthianthrene-5,10-dioxide-trans (1a) and -cis (1b)12
were prepared by oxidation of the 2,7-dimethylthianthrene with
1
JO0606610
hydrogen peroxide in acidic media.1c 1a: H NMR (CDCl3, 600
MHz, 25 °C) δ 2.47 (s, 3H), 7.46 (dd, 1H, dd, 1H, J ) 1.7, 7.8
Hz), 7.89 (d, 1H, J ) 1.7 Hz), 7.95 (d, 1H, J ) 7.8 Hz); 13C NMR
(CDCl3, 150.8 MHz, 25 °C) δ 21.75 (2Me), 127.71 (CH), 128.25
(CH), 131.72 CH), 139.46 (quat), 142.52 (quat), 142.95 (quat).
1b: 1H NMR (CDCl3, 600 MHz, 25 °C) δ 2.50 (s, 3H), 7.52 (d,
1H, J ) 7.8 Hz), 7.82 (d, 1H, J ) 1.5 Hz), 7.87 (dd, 1H, J ) 7.8,
1.5 Hz); 13C NMR (CDCl3, 150.8 MHz, 25 °C) δ 21.7 (2Me), 123.5
(CH), 123.9 (CH), 131.4 (CH), 134.4 (quat), 137.7 (quat), 141.8
(quat).
(13) QCPE program No. 633, Indiana University, Bloomington, IN.
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6250 J. Org. Chem., Vol. 71, No. 16, 2006