J = 8 Hz), 5.82 (d, 2 H, J = 8 Hz), 7.2–7.4 (mult., 3 H) and 7.45–
7.55 (mult., 2 H); HRMS (El, 70 eV): m/z, 200.1200. C14H16O
requires 200.1201.
sary with NaCl. The measured pH was corrected by the subtrac-
tion of 0.08. This factor was based on the measured pH of 1.92
for a solution of 0.01 mol dm3 HCl–0.09 mol dm3 NaCl in 20%
acetonitrile. The pKa was then calculated using the Henderson–
Hasselbach equation; 5.20 for CH3COOH and 4.46 for HN3.
6,6-Dimethyl-3-phenylcyclohexa-2,4-dien-1-ol (8) and 3,4-
dimethylbiphenyl (10). When 4,4-Dimethyl-1-phenylcyclo-
hexa-2,5-dien-1-ol (7) contains trace amounts of solvents (hex-
ane and/or ethyl acetate) it slowly isomerizes to 8 and loses
water to form 10. Thus, after standing for several days a mixture
of the three compounds was obtained. These could be separ-
ated by chromatography on silica gel with 15% ethyl acetate–
85% hexanes; 10 elutes first as a colourless liquid, then 7 and
then 8 elutes as a sticky oil.
8: δH (CDCl3, 200 MHz) 1.11 (s, 3 H), 1.19 (s, 3 H), 1.7 (d, 1 H,
J = 7 Hz, OH), 4.08 (dd, 1 H, J = 7 Hz and JЈ = 3 Hz), 5.82 (d, 1
H, JЉ = 8 Hz), 6.24 (dd, 1 H, JЈ = 3 Hz and Jٞ = 1 Hz), 6.52
(JЉ = 8 Hz and Jٞ = 1 Hz) and 7.3–7.5 (mult., 5 H); HRMS
(El, 70 eV): m/z, 200.1205. C14H16O requires 200.1201.
10: δH (CDCl3, 200 MHz) 2.13 (s, 3 H), 2.15 (s, 3 H) and 7.2–
7.6 (mult., 8 H); HRMS (El, 70 eV): m/z, 182.1902. C14H14
requires 182.1096.
Computations
Semi-empirical calculations were carried out using the MOPAC
93 suite of programs27 on a Hewlett-Packard work station run-
ning the Unix operating system. Geometry optimizations, force
calculations and thermodynamic calculations were accom-
plished using the AM1 semi-empirical Hamiltonian.28 The
geometry was initially optimized using high levels of precision
(precise, GNORM = 0.0 and SCFCRT = 1D-15) to obtain
enthalpies of formation. The optimized structure was then
submitted to a force calculation and thermodynamical calcu-
lation to obtain thermodynamic parameters necessary for the
change in Gibbs’ energy.
Acknowledgements
6,6-Dimethyl-3-phenylcyclohexa-2,4-dien-1-yl acetate (9).
Sodium hydride (0.4 g of 60% dispersion in oil, 10.5 mmol) was
washed three times with dry hexanes, and after evaporation of
the hexanes, 2 cm3 of dry THF was added. 4,4-Dimethyl-1-
phenylcyclohexa-2,5-dien-1-ol (7 (10.0 mmol) in 10 cm3 of dry
THF was then slowly added. After stirring at 40 ЊC for 2 h, the
mixture was cooled to Ϫ78 ЊC and acetic anhydride (10.6
mmol) in 5 cm3 of THF was slowly added. After warming to
room temperature, aqueous K2CO3 was added, and the mixture
extracted with diethyl ether. After drying over CaCl2 and
removal of the solvents, chromatography on silica gel with 20%
ethyl acetate–80% hexanes afforded 1.24 g of 9 as a colourless
oil.
The continued financial support of the Natural Sciences and
Engineering Research Council of Canada is gratefully
acknowledged.
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HPLC
These experiments were performed with a Waters 600E system
with UV detector. Conditions:1 × 10 cm C18 column, flow
rate = 2 cm3 minϪ1, isocratic 30:70 methanol:water for 6 min
followed by a linear gradient from 6–9 min to 15:85 meth-
anol:water followed by isocratic 15:85 methanol:water from
9–16 min, detector wavelength at 220 nm for 0–6 min followed
by 235 nm from 6–16 min. Retention times: 4.8 min (7), 5.2 min
(8), 10.5 (9), 13.8 min (12) and 14.5 min (10). Peak areas were
converted into absolute quantities by use of response factors
determined by injecting known amounts of the authentic
samples of 7–10. Solutions of substrates of concentration 1–
2 × 10Ϫ4 mol dm3 were placed in a constant temperature bath at
25 ЊC. At appropriate times 0.080 cm3 were removed and
injected directly to the HPLC.
28 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart,
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Acid dissociation constants in 20% acetonitrile
These were determined by measuring the pH with a combin-
ation electrode for solutions containing known amounts of
CH3COOH :CH3COONa or HN3 :NaN3 in 20% acetonitrile,
with the ionic strength at 0.1 mol dm3 maintained where neces-
Paper 6/06786G
Received 3rd October 1996
Accepted 5th November 1996
456
J. Chem. Soc., Perkin Trans. 2, 1997