Reactions of Carbocations with Hydrocarbons
A R T I C L E S
Table 8. Relative Reactivities of 9,10-Dihydroanthracene (5) and
9,9,10,10-Tetradeuterio-9,10-dihydroanthracene (5-D4) toward the
Benzhydrylium Ions 1a+ and 1e+ (CH2Cl2, 20 °C)
2-4 equiv of Lewis acid in dry CH2Cl2. Next 2-3 equiv of a solution
of the nucleophile in a small amount of CH2Cl2 was added. The reaction
mixture was stirred until the color of the reaction mixture had almost
faded. For hydrolysis, aqueous NH4OH solution (30 mL) was added.
The resulting mixture was filtered through a layer of Celite. The organic
phase was separated, and the aqueous phase was extracted with CH2-
Cl2 (2 × 20 mL). The combined organic fractions were dried (MgSO4),
filtered, and concentrated in vacuo. The crude product mixture was
then analyzed by 1H NMR spectroscopy and in part by HPLC (column,
Nucleosil RP-C18 100/7 µm; eluent, acetonitrile:water ) 95:5; flow
rate, 1.0 mL/min) or GC (SE 30 column). Diarylmethanes 1-H were
separated from the crude product by Kugelrohr distillation or column
chromatography (silica gel 60, n-hexane:ethyl acetate ) 10:1).
Details and results of the preparative studies are displayed in Table
7.
Bis(4-methoxyphenyl)methane (1a-H).73 1H NMR (CDCl3, 300
MHz): δ 3.75 (s, 6 H), 3.85 (s, 2 H), 6.78-6.83, 7.05-7.10 (2 m, 2
× 4 H). 13C NMR (CDCl3, 75.5 MHz): δ 40.1 (t), 55.2 (q), 113.8,
129.7 (2 d), 133.7, 157.9 (2 s).
(4-Methoxyphenyl)(4-methylphenyl)methane (1b-H). 1H NMR
(CDCl3, 300 MHz): δ 2.19 (s, 3 H), 3.62 (s, 3 H), 3.76 (s, 2 H), 6.67-
6.72 (m, 2 H), 6.92-7.03 (m, 6 H). 13C NMR (CDCl3, 75.5 MHz): δ
20.9 (q), 40.5 (t), 55.1 (q), 113.7, 128.6, 129.0, 129.7 (4 d), 133.4,
135.3, 138.4, 157.8 (4 s).
1-Cl,
g (mmol)
Lewis acid,
mL (mmol)
5,
5-D ,
g (mmol)
[1-H]/
[1-D] kH/kD
4
g (mmol)
1a-Cl, 0.127 (0.483) BCl3, 25 (1.12) 0.270 (1.50) 1.57 (8.50) 0.66a 3.9
1e-Cl, 0.115 (0.567) TiCl4, 0.20 (1.82) 0.227 (1.26) 0.314 (1.70) 1.95b 3.0
1e-Cl, 0.130 (0.641) TiCl4, 0.20 (1.82) 0.150 (0.832) 0.268 (1.45) 1.41b 3.0
1e-Cl, 0.132 (0.651) TiCl4, 0.20 (1.82) 0.215 (1.19) 0.195 (1.06) 2.79b 2.9
a Determined from the ratio of the NMR integrals at δ 3.81 (t, J ) 2.1
Hz, CHD) and 3.83 (s, CH2). b Determined from the ratio of the NMR
integrals at δ 3.93 (t, J ) 2.1 Hz, CHD) and 3.95 (s, CH2).
Reaction of 1d-BCl4 with 4-Allyltoluene. A solution of 1d-Cl
(598 mg, 2.59 mmol) and benzyltriethylammonium chloride
(590 mg, 2.59 mmol) in dichloromethane (30 mL) was cooled to -78
°C. Boron trichloride gas (200 mL, 8.90 mmol) and 4-allyltoluene (397
mg, 3.00 mmol) were then added, and the reaction mixture was stirred
for 18 h at -78 °C. Workup as described above for the analogous
reaction of 13 yielded a 84:16 mixture of 3-chloro-1,1,4-tris(p-tolyl)-
butane and 1d-H (determined by 1H NMR spectroscopy) as a yellowish
oil (838 mg) which was analyzed by HPLC (column, Nucleosil RP-
C18 100/7 µm; eluent, acetonitrile:water ) 95:5; flow rate, 1.0 mL/
min; tR, 3.6 min, 1d-H; 4.9 min, 3-chloro-1,1,4-tris(p-tolyl)butane).
From the NMR spectra of the crude product the following signals were
assigned to 3-chloro-1,1,4-tris(p-tolyl)butane. 1H NMR (300 MHz,
CDCl3): δ 2.15, 2.17, 2.20 (3 s, 3 × 3 H, 3 × Me), 2.33-2.42 (m, 2
H, 2-H2), 2.82-2.96 (m, 2 H, 4-H2), 3.72-3.81 (m, 1 H, 3-H), 4.18
(dd, J ) 11.1 Hz, 4.5 Hz, 1 H, Ar2CH), 6.86-7.21 (m, 12 H, ArH).
13C NMR (75.5 MHz, CDCl3): δ 20.9, 21.0, 21.1 (3 q, 3 × Me), 43.4,
44.7 (2 t, C-2 and C-4), 47.1 (d, Ar2CH), 61.7 (d, C-3), 127.4, 127.9,
129.1, 129.2 (4 d, Ar), 134.5, 135.4, 135.6, 135.9, 141.8, 144.5 (6 s,
Ar).
(4-Methoxyphenyl)phenylmethane (1c-H).71,74,75 1H NMR (CDCl3,
300 MHz): δ 3.74 (s, 3 H), 3.90 (s, 2 H), 6.79-6.83, 7.06-7.11 (2 m,
2 × 2 H), 7.14-7.29 (m, 5 H). 13C NMR (CDCl3, 75.5 MHz): δ 41.0
(t), 55.2 (q), 113.8, 125.9, 128.4, 128.8, 129.8 (5 d), 133.2, 141.5, 157.9
(3 s).
Bis(4-methylphenyl)methane (1d-H).73 1H NMR (CDCl3, 300
MHz): δ 2.30 (s, 3 H), 3.89 (s, 2 H), 7.03-7.12 (m, 8 H). 13C NMR
(CDCl3, 75.5 MHz): δ 21.0 (q), 41.1 (t), 128.7, 129.1 (2 d), 135.4,
138.3 (2 s).
Determination of Kinetic Isotope Effects by Competition Experi-
ments. The benzhydryl chloride 1-Cl and a mixture of the 9,10-
dihydroanthracenes 5 and 5-D4 were dissolved in dry dichloromethane
(20 mL) at 20 °C. After the addition of the Lewis acid, the reaction
mixture was stirred for 1 h. Aqueous NH4OH solution was then added,
and the mixture was filtered through Celite. The layers were separated,
and the aqueous layer was extracted with dichloromethane. The organic
fractions were combined and dried (MgSO4). After the evaporation of
Reaction of 1d-BCl4 with Allylbenzene (13). A solution of bis(4-
methylphenyl)methyl chloride 1d-Cl (671 mg, 2.91 mmol) and ben-
zyltriethylammonium chloride (663 mg, 2.91 mmol) in dichloromethane
(30 mL) was cooled to -78 °C. Boron trichloride gas (150 mL, 6.70
mmol) and allylbenzene 13 (400 µL, 356 mg, 3.01 mmol) were then
added, and the reaction mixture was stirred for 18 h at -78 °C. For
hydrolysis, aqueous NH4OH solution (8 M, 30 mL) was added. The
organic phase was separated, and the aqueous phase was extracted with
CH2Cl2 (2 × 20 mL). The combined organic fractions were dried
(MgSO4), filtered, and concentrated in vacuo. The residue was filtered
through a short column of silica gel to remove ammonium salts before
it was analyzed by NMR spectroscopy and HPLC (column, Nucleosil
RP-C18 100/7 µm; eluent, acetonitrile:water ) 95:5; flow rate, 1.0 mL/
min; tR, 3.6 min, 1d-H; 4.4 min, 14). In the crude product a 10:90
1
the solvent in vacuo, the residue was analyzed by H NMR spectro-
scopy.
In a first competition experiment, a mixture of 5 and 5-D4 was reacted
with 1a+ (generated from 1a-Cl and BCl3) to give kH/kD ) 3.9 (Table
8),76 in good agreement with the reactivity ratio determined from direct
kinetic measurements (Table 3). With the method thus confirmed, the
competition experiment 5/5-D4 + 1e+ (generated from 1e-Cl and TiCl4)
was performed. An averaged reactivity ratio of kH/kD ) 3.0 was
calculated76 from three independent experiments (Table 8).
1
ratio of 1d-H and 14 was determined by H NMR spectroscopy.
The hydride transfer product 1d-H (58 mg, 10%) was separated by
Kugelrohr distillation (130-140 °C/1 × 10-3 mbar). The distillation
residue was recrystallized from pentane to yield 1,1-bis(4-methylphen-
yl)-3-chloro-4-phenylbutane 14 (802 mg, 79%) as a colorless solid;
mp 74-75 °C. 1H NMR (300 MHz, CDCl3): δ 2.19, 2.21 (2 s, 2 × 3
H, Me), 2.12-2.22, 2.31-2.45 (2 m, 2 H, 2-H2), 2.90-3.04 (m, 2 H,
4-H2), 3.76-3.85 (m, 1 H, 3-H), 4.20 (dd, J ) 11.1 Hz, 4.5 Hz, 1 H,
Ar2CH), 6.90-7.23 (m, 13 H, ArH). 13C NMR (75.5 MHz, CDCl3): δ
20.9, 21.0 (2 q, 2 × Me), 43.4, 45.2 (2 t, C-2 and C-4), 47.1 (d, C-1),
61.5 (d, C-3), 126.7, 127.4, 127.9, 128.3, 129.1, 129.3, 129.4 (7 d,
Ar), 135.7, 135.9, 137.6, 139.7, 141.8 (5 s, Ar). Anal. Calcd for C24H25-
Cl (348.92): C, 82.62; H, 7.22. Found: C, 82.87; H, 7.30.
Acknowledgment. This and the succeeding paper are dedi-
cated to Professor George A. Olah on the occasion of his 75th
birthday. Financial support by the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie is gratefully
acknowledged. We thank Dr. Michael Roth for performing the
experiments with 3-propylcyclopentene.
Supporting Information Available: Details of the kinetic
experiments (concentrations, rate constants at variable temper-
atures, and activation parameters) (PDF). This material is
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