Yao and Pollack
635
Scheme 1.
(1a), 6-nitro-2-tetralone(1b), 6-methoxy-2-tetralone (1d),
7
-nitro-2-tetralone (1e), 7-chloro-2-tetralone (1f), and 5,7-
dinitro-2-tetralone (1g) were available from previous work
9). 6-Chloro-2-tetralone (1c) was synthesized as before, mp
7–68°C (lit. (9a) 67.0–67.5°C). Recrystallizations from
(
6
mixed solvents were performed by adding cosolvent until
the solution became cloudy, followed by heating until clari-
fication.
7
-Methyl-2-tetralone (1h)
A mixture of m-methylphenylacetic acid (3.40 g, 22.6 mmol)
and 100 mL 2 M thionyl chloride in dichloromethane solu-
tion (200 mmol) was stirred overnight. The solvent and ex-
cess thionyl chloride were distilled off. The resulting
m-methylphenylacetic chloride was vacuum distilled and
dissolved in ca. 20 mL anhydrous dichloromethane. This so-
lution was added dropwise under argon to a stirring suspen-
sion of anhydrous aluminum chloride (4 g, 30 mmol) in
1
20 mL anhydrous dichloromethane cooled at –15°C, while
minimal (1), whereas enolization results in stabilization due
to conjugation between the carbon–carbon double bond of
the enol and the phenyl group. On the basis of the dependence
of the enol content of ring-substituted 2-phenylpropionalde-
ethylene was bubbled through the reaction mixture. The re-
action was stopped after 1.5 h. After work-up as for 1f (9a),
column chromatography (silica, Merck 60, 1:4 ethyl ace-
tate:hexane) gave a 1:2 mixture of 5-methyl-2-tetralone and
–
hydes in DMSO on σ (ρ = 0.72) (5), Toullec (1) suggested
7
-methyl-2-tetralone (estimated from integration of protons
that the conjugation of the aryl ring with the double bond in-
volves little π electron transfer (6). This conclusion is some-
what surprising considering the possibility of direct
resonance interaction between the aryl ring and the hydroxyl
group through the double bond. Rappoport, on the other
hand, has interpreted the same data to suggest that there is
1
on C-1 carbon from H NMR), 1.76 g (combined yield 52.1%).
-Methyl-2-tetralone was obtained from two recrystalliza-
tions of this mixture from methanol–water (1.03 g, 28%),
7
1
mp 57.5–59.5°C (lit. (10) 57–59°C). H NMR (CDCl ) δ:
3
2
6
.33 (s, 3H, CH ), 2.57 (t, J = 6.0 Hz, 2H, H4), 3.03 (t, J =
3
.0 Hz, 2H, H3), 3.55 (s, 2H, H1), 6.95 (s, 1H, H8), 7.03 (d,
“
an important dipolar contribution to the enol ground state
J = 7.5, 1H, H5), 7.13 (d, J = 7.5 Hz, 1H, H6). Anal. calcd.
having a negative charge on the β-aryl group” (4d, 7). This
system, however, is complicated by the potentially less than
optimal coplanarity of the p orbitals of the phenyl ring with
those of the double bond, which might cause a diminished
interaction between the ring substituents and the OH group
of the enol.
To isolate electronic effects on keto–enol equilibria and
ionization of enols, we have examined the dependence of the
equilibrium constants on substituent for substituted 2-tetra-
lones (1a–1j) (Scheme 1). As a fused α -aryl ketone, 2-
tetralone has no direct resonance between the phenyl ring
and the carbonyl group, and therefore the electronic effect
on the ketone stability is minimal. In addition, the carbon–
carbon double bond and the phenyl ring of the enol should
be almost coplanar (8), which allows maximal resonance
for C H O: C 82.46, H 7.55%; found: C 82.28, H 7.61%.
11 12
6-Iodo-2-tetralone (1i)
p-Iodophenylacetic chloride was obtained using a similar
procedure as above, from p-iodophenylacetic acid (150 mg,
0.57 mmol) and 1.2 mL 2 M thionyl chloride in dichloro-
methane solution (2.3 mmol). With excess anhydrous alumi-
num chloride (3.0 g, 20 mmol), the acid chloride was
reacted with ethylene to give 6-iodo-2-tetralone. Chromatog-
raphy (hexane:ethyl acetate, 10:1.5) and recrystallization from
hexane and ethyl acetate gave pure 1i (58 mg, yield 37%),
1
mp 60.5–62.0°C. UV (1 N NaOH): 312 nm. H NMR
(CDCl3) δ: 2.53 (t, J = 6.6 Hz, 2H, H4), 3.02 (t, J = 6.6 Hz,
2H, H3), 3.53 (s, 2H, H1), 6.88 (d, J = 7.8, 1H, H8), 7.55 (d,
J = 7.8 Hz, 1H, H7), 7.51 (s, 1H, H5). Anal. calcd. for
C10H9IO: C 44.14, H 3.33%; found: C 44.22, H 3.38%.
and charge transfer. We report here the pK ’s of both the
a
K
E
ketones (pK ) and the enols (pK ) of a variety of substi-
a
a
tuted 2-tetralones, from which the substituent effect on the
keto–enol tautomerization can be derived. This study repre-
sents the first systematic study of electronic effects on equi-
libria among ketone, enol, and enolate in aqueous solution.
6-Chloro-7-nitro-2-tetralone (1j)
6-Chloro-2-tetralone (1.1 g, 6.1 mmol) was slowly added
to 8 mL of 90% nitric acid at –25°C, with nitrogen bubbling
through the solution. After addition was completed, the reac-
tion mixture was stirred for 8 min, poured onto a mixture of
sodium hydroxide (6.5 g) in water and ice (50 mL) and
200 mL ethyl acetate, and then shaken vigorously. Work-up
followed the procedure for the preparation of 1e (9a). The
major product from chromatography was 6-chloro-7-nitro-2-
tetralone, which was recrystallized from hexane – ethyl ace-
Experimental
Materials
Unless otherwise mentioned, all chemicals were reagent
grade or better, purchased commercially and used without
1
further purification. H NMR spectra were recorded in
CDCl with TMS as an internal standard on a General Elec-
tric QE-300 spectrometer. Melting points were determined
on a Mel-Temp apparatus and are uncorrected. 2-Tetralone
tate (0.19 g, yield 14%), mp 130–132°C. UV (1 N NaOH):
3
1
317 nm. H NMR (CDCl ) δ: 2.60 (t, J = 6.6 Hz, 2H, H4),
3
3.14 (t, J = 6.6 Hz, 2H, H3), 3.63 (s, 2H, H1), 7.46 (s, 1H,
©
1999 NRC Canada