Determination of the electrophilic reactivities of 1,1,3-triarylallyl cations

Article abstract of DOI:10.1039/b203554e

The 1,1,3-triphenylallyl cation and its p-methoxy and p-dimethylamino substituted derivatives have been generated in dichloromethane solution or as stable salts. Allylsilanes, allylstannanes, silylated enol ethers and ketene acetals, as well as enamines a

Full text of DOI:10.1039/b203554e

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Determination of the electrophilic reactivities of 1,1,3-triarylallyl
cations†‡
Herbert Mayr,* Claudia Fichtner§ and Armin R. Ofial
2
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5–13 (Haus F),
D-81377 München, Germany. E-mail: Herbert.Mayr@cup.uni-muenchen.de;
Fax: int. ϩ49 (0) 89-2180 7717
Received (in Cambridge, UK) 11th April 2002, Accepted 13th May 2002
First published as an Advance Article on the web 28th June 2002
The 1,1,3-triphenylallyl cation and its p-methoxy and p-dimethylamino substituted derivatives have been generated
in dichloromethane solution or as stable salts. Allylsilanes, allylstannanes, silylated enol ethers and ketene acetals,
as well as enamines attack the sterically less hindered 3-position of the allyl cation as derived from the structures of
the reaction products. Kinetic investigations of these reactions revealed that they follow the equation log k(20 ЊC) =
s(N ϩ E), which allows one to derive the electrophilicity parameters E of Ph C᎐CH–CHϩPh (E = ϩ0.98 0.20),
᎐
2
(p-MeOC H ) C᎐CH–CHϩPh (E = Ϫ2.67 0.30), (p-Me NC H ) C᎐CH–CHϩPh (E = Ϫ8.97 0.32) and
᎐
᎐
᎐
6
4
2
2
6
4 2
(p-Me NC H ) C᎐CH–CHϩ(p-Me NC H ) (E = Ϫ9.84 0.21).
2
6
4
2
2
6
4
the 3,3-diarylpropenals 2a,b11 were combined with phenyl-
lithium in THF to give the triarylallyl alcohols 3a,b. Acylation
with acetic anhydride in the presence of triethylamine and
4-(dimethylamino)pyridine (DMAP) converted the alcohols
3a,b into the acetates 4a,b10,12 (Scheme 1).
Introduction
1,1,3-Triaryl substituted allyl cations 1, diversely substituted at
the p-positions of the aromatic moieties have been known since
1941.1–4 Their use as sensitisers in electrophotographical
materials has been described.5
In a series of papers,6–9 we have recently demonstrated that
the rates of the reactions of carbocations with nucleophiles are
given by the three-parameter eqn. (1),
Scheme 1 Syntheses of the acetates 4
Because of their tendency to undergo cyclisation to indanyl
cations,13,14 the cations 1a and 1b were generated at low
temperature from the corresponding precursors 4a or 4b with
trimethylsilyl triflate (TMSOTf ) in dichloromethane and com-
bined with the nucleophilic reaction partners below Ϫ40 ЊC.
The 1,3-diarylpropenones 5a,b were obtained from
p-(dimethylamino)acetophenone15 and the corresponding
benzaldehydes by aldol condensation.16 Treatment with
p-(dimethylamino)phenyllithium,17–19 subsequent acidification
and addition of aqueous sodium tetrafluoroborate solution led
to the precipitation of 1c–BF4 and 1d–BF4,1 respectively, which
could be purified by recrystallisation from dichloromethane–
pentane mixtures (Scheme 2).
log k (20 ЊC) = s (N ϩ E)
(1)
where s = nucleophile-specific slope parameter, N = nucleo-
philicity parameter and E = electrophilicity parameter, and we
have defined reference nucleophiles,6 which are recommended
for the characterisation of further electrophiles.
We now report on the kinetics of the reactions of the triaryl-
allyl cations 1a–d with different types of nucleophiles and
employ these data for determining the E parameters of 1a–d.
Results and discussion
Synthesis of the triarylallyl cations 1a–d
Following a reaction sequence described by Williams et al.,10
† Dedicated to Professor W. Beck on the occasion of his 70th birthday.
‡ Electronic supplementary information (ESI) available: UV–VIS spec-
tra of cations 1a–d; synthesis of compounds 17, 18, 20–22, 24, 26, 28;
rate constants and experimental conditions of the individual kinetic
experiments. See http://www.rsc.org/suppdata/p2/b2/b203554e/
§ Present address: CarboGen Laboratories, CH-5001 Aarau,
Switzerland.
Scheme 2 Conditions: i, 4-bromo-N,N-dimethylaniline, lithium, Et2O–
THF; ii, AcOH, NaBF4, H2O.
DOI: 10.1039/b203554e
J. Chem. Soc., Perkin Trans. 2, 2002, 1435–1440
This journal is © The Royal Society of Chemistry 2002
1435

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Table 1 Products and kinetics of the reactions of the triarylallyl cations 1a–d with various nucleophiles
Electrophile
Nucleophile
Product (yield)
∆H‡/kJ molϪ1 a
∆S‡/J molϪ1 KϪ1 a
k(20 ЊC)/l molϪ1 sϪ1
1a
6
7
16 (75%)
24.17 0.87
27.05 1.22
20.40 1.24
32.66 1.72
27.57 1.12
31.12 1.75
20.87 0.94
—
Ϫ116.4 4.1
Ϫ106.3 5.9
Ϫ106.4 5.8
Ϫ77.3 7.9
Ϫ90.0 5.3
Ϫ106.7 8.1
Ϫ95.6 4.5
—
2.51 × 102
2.58 × 102
3.93 × 103
8.48 × 102
1.50 × 103
4.67 × 101
1.18 × 104
6.58 × 10Ϫ2
1.46 × 102
1.46 × 102
5.77 × 10Ϫ1
1.16 × 10Ϫ2
2.05 × 101
4.92 × 101
1.69 × 10Ϫ1
17 (64%)
8
17 (66%)
1b
1c
1d
9
18 (40%)
10
11
12
12
13
14
15
12
13
14
15
19 (21%)
20 (42%)
20 (40%)
21 (52%)
23 (24%, 65 : 35)
25 (67%, 53 : 47)
27 (24%)
28.79 2.19
30.19 0.60
—
Ϫ105.2 8.8
Ϫ100.4 2.5
—
22 (37%)
—
—
24 (5%, 71 : 29)
26 (37%, 58 : 42)
28 (30 %)
18.03 2.03
29.37 0.48
—
Ϫ158.2 8.0
Ϫ112.2 2.0
—
a Since the standard deviations of ∆H‡ and ∆S‡ do not include systematic errors, the actual uncertainties are greater than indicated in this column.
For that reason the listed decimals are meaningless by themselves but are needed to reproduce the rate constants given.
Reactions with nucleophiles
at 20 ЊC. Low diastereoselectivities were observed in the
reactions of the prochiral electrophiles 1c,d with the prochiral
π-systems 13 and 14. While the five-membered ring compound
14 gave rise to a ratio of diastereoisomers of ca. 1 : 1,
morpholinocyclohexene 13 gave 2 : 1 mixtures of diastereomers
which were not separated. Since only low yields of 23–26 were
isolated, an interpretation of these diastereomeric ratios is not
possible.
Treatment of the triphenylallyl acetate 4a with trimethylsilyl
triflate and the nucleophiles 6–8 at Ϫ70 ЊC gave rise to the
formation of compounds 16 and 17 indicating the exclusive
nucleophilic attack at the monosubstituted allylic terminus of
1a20 (Table 1). An analogous reaction pathway was derived
from the formation of compounds 18–20 from the trimethylsilyl
triflate induced reactions of 4b with 9–12 at Ϫ45 ЊC. Though
only moderate yields of the pure products were isolated, we
did not find evidence for nucleophilic attack at the diaryl
substituted allylic terminus.
Kinetics
The rate constants of the reactions of the triarylallyl cations
1a–d with the nucleophiles 6–15 were determined by monitor-
ing the decay of the electrophile absorbances, using the
equipment and the data evaluation procedures described
previously.21
Because of the previously mentioned tendency of the allyl
cations 1a and 1b to undergo cyclisation, solutions of 1a,b-OTf
were generated from the allyl acetates 4a,b and trimethylsilyl
triflate in dichloromethane at T < Ϫ40 ЊC. Kinetic investiga-
tions of the reactions of 1a,b-OTf with the nucleophiles 6–12
were performed at Ϫ70 ЊC to Ϫ50 ЊC and monitored at λ = 510
5 nm (1a) or 470 5 nm (1b).22 The rate constants referring to
20 ЊC (Table 1) were then calculated from the Eyring activation
parameters determined in the low-temperature range. Since the
allyl cations 1c and 1d do not undergo intramolecular cyclis-
ation reactions,23 the kinetics of their reactions with the
π-nucleophiles 12–14 as well as with tributylstannane (15) could
be studied in a wider temperature range or even at 20 ЊC at
λ = 675 nm for 1c and 600 nm for 1d.22 In analogy to the
reactions of benzhydrylium triflates and tetrafluoroborates
with the nucleophiles 6–15,6 all reactions listed in Table 1 follow
second-order kinetics, first order with respect to carbenium
ion concentration and first order with respect to nucleophile
concentration. Since kinetic investigations of the reactions
of benzhydrylium triflates and tetrafluoroborates with 6–15
showed independence of the rate constants of the nature of the
counterions,21,24,25 rate-determining electrophile–nucleophile
combination can also be assumed for the reactions of 1a,b-OTf
and 1c,d–BF4 with 6–15.
Except for the hydride donor 15, all nucleophiles 6–14 which
were used for kinetic investigations in this work (Table 1) belong
to a list of reference nucleophiles which have recently been
recommended as reaction partners for the determination of
further E parameters.6 Therefore, the rate constants of the
reactions of the π-nucleophiles 6–14 with the allyl cations 1a–d
given in Table 1 have been selected for the calculation of E
parameters for 1a–d. Table 2 demonstrates that for each allyl
cation the E parameters derived from reactions with different
nucleophiles are in close agreement, corroborating that the
Products 21–28 were obtained by combining the stable
tetrafluoroborate salts of 1c and 1d with the nucleophiles 12–15
1436
J. Chem. Soc., Perkin Trans. 2, 2002, 1435–1440

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Table 2 Determination of the electrophilicity parameters E of 1a–d from reactions with different reference nucleophiles
Electrophile
Nucleophile
Na
sa
log (k/l molϪ1 sϪ1
)
E
1a
6
7
8
1.26
1.79
3.09
0.96
0.94
0.90
2.40
2.41
3.59
ϩ1.25
ϩ0.77
ϩ0.89
E(1a) = ϩ0.98 0.20b
1b
9
10
11
12
5.46
6.22
4.41
7.48
0.89
0.96
0.96
0.89
2.93
3.18
1.67
4.07
Ϫ2.16
Ϫ2.90
Ϫ2.68
Ϫ2.90
E(1b) = Ϫ2.67 0.30b
1c
1d
12
13
14
7.48
11.40
12.56
0.89
0.83
0.70
Ϫ1.18
2.16
2.16
Ϫ8.81
Ϫ8.79
Ϫ9.47
E(1c) = Ϫ8.97 0.32b
12
13
14
7.48
11.40
12.56
0.89
0.83
0.70
Ϫ1.94
1.31
1.69
Ϫ9.65
Ϫ9.82
Ϫ10.14
E(1d) = Ϫ9.84 0.21b
a From ref. 6. b The E parameter is obtained by minimizing ∆2 = Σ[log ki Ϫ si(E ϩ Ni)]2 and deviates slightly from the arithmetic mean of E derived
from the individual reactions. The calculations were actually performed with more decimals of log k, N, and s than indicated in the table. The use of
log k, N, and s given in the table leads to slightly deviating results.
nucleophilicity parameters derived from reactions with benz-
hydrylium ions6 also hold for the reactions with this class of
electrophiles.
shrinks to ∆E = Ϫ2.67 and Ϫ1.95 for the methoxy and dimeth-
ylamino substituted systems because of the reduced electron
demand in the latter carbenium ions.
The wide applicability of these reactivity parameters is
further demonstrated by the fact that the rate constants for the
hydride transfer from tributylstannane (15) to 1c (kcalc = 3.50 l
Remarkably, replacement of phenyl by p-(dimethylamino)-
phenyl (1c 1d) reduces the electrophilicity by less than one
order of magnitude. This difference is comparable to that
between malachite green [(p-Me2NC6H4)2PhCϩ] and crystal
violet [(p-Me2NC6H4)3Cϩ],27 though in the case of the tritylium
ions, the introduction of a third strongly donating arene ring
is accompanied by a reduction of the π-overlap with the other
two rings.
molϪ1 Ϫ1) and 1d (kcalc = 1.16 l molϪ1 Ϫ1), calculated from
s s
eqn. (1) using the E values of these electrophiles (Table 2) and
the reactivity parameters of hydride donor 15 (N = 9.96, s =
0.55)6 deviate by less than a factor of 7 from the experimental
numbers in Table 1.
The triarylallyl cations 1a–d can be considered as vinylogous
tritylium ions. Because steric effects have to be considered,
reactions of tritylium ions with π-nucleophiles cannot be
treated by eqn. (1), but for reactions with hydride donors an
electrophilicity parameter of E(Ph3Cϩ) = Ϫ0.69 has been
derived.26 Comparison with the E value of the triphenylallyl
cation 1a indicates that the electrophilicity of Ph3Cϩ is
enhanced by 1.7 logarithmic units upon insertion of an
additional double bond.
Replacement of the benzhydryl hydrogen in benzhydrylium
ions by a styryl group reduces the electrophilicity by 2 to 5
orders of magnitude (Table 3). As expected, the effect is largest
for the unsubstituted benzhydrylium ion (∆E = Ϫ4.92) and
As the reduction of electrophilicity due to replacement of
p-methoxy by p-dimethylamino turned out to be comparable in
the benzhydryl and the 1,1-diaryl-3-phenylallyl series (Table 3),
one can expect that the exchange of the p-dimethylaminophenyl
groups in 1c by stronger electron donors such as the julolidin-9-
yl or the lilolidin-8-yl group would have similar effects as in
the benzhydryl series.6 Sterically non-shielded carbocations
with electrophilicity parameters as low as E = Ϫ12 would
thus become available for kinetic investigations of stronger
nucleophiles.
Experimental
General
Table 3 Comparison of the electrophilicities of the triarylallyl cations
with those of structurally analogous benzhydrylium ions
NMR spectra were recorded with Varian Mercury 200, Bruker
ARX 300, Varian VRX 400S, and Varian INOVA-400 spectro-
meters. 1H NMR chemical shifts refer to tetramethylsilane
(δH 0.00) and 13C NMR chemical shifts refer to the solvent as
internal standard (CDCl3: δ 77.0; CD3CN: δ 1.3). Coupling
constants J are given in Hz. Mass spectra (EI, 70 eV) were
obtained with a Varian MAT 90 or MAT 95 instrument.
Melting points (uncorrected) were determined with a Reichert
Thermovar apparatus.
The UV–VIS photometers used for the kinetic studies were
Schölly KGS III with band-pass filters by Corion21 and J&M
TIDAS DAD with insertion probes by Hellma.28 The kinetic
measurements and the data-evaluation were carried out as
described previously.21
All reactions were performed with exclusion of moisture
in an atmosphere of dry nitrogen in carefully dried Schlenk
glassware. Dichloromethane was freshly distilled from CaH2
X
Ea
Eb
∆E
H
OMe
NMe2
ϩ5.90
0.00
Ϫ7.02
ϩ0.98
Ϫ2.67
Ϫ8.97
Ϫ4.92
Ϫ2.67
Ϫ1.95
a From ref. 6. b This work.
J. Chem. Soc., Perkin Trans. 2, 2002, 1435–1440
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before use. THF and diethyl ether were dried over KOH and
Na–benzophenone. Pentane was dried over KOH and Na.
Allyl acetate 4a,10 allylsilane 11,25 allylstannane 12,25 and silyl
ketene acetal 1429 were synthesised according to literature
methods. All other nucleophiles are commercially available.
2-Methyl-5-(1,3,3-triphenylprop-2-enyl)thiophene (16)
Compound 4a (0.65 g, 2.0 mmol) was dissolved in dichloro-
methane(10ml)undernitrogenandcooledtoϪ70ЊC. Trimethyl-
silyl triflate (0.39 ml, 2.2 mmol) was added to give a red solution
of 1a-OTf. After the addition of arene 6 (0.25 ml, 2.6 mmol)
the solution was stirred at Ϫ70 ЊC until the red colour of
1a disappeared (3 d). The mixture was allowed to warm to
ambient temperature and hydrolysed over 15 min by adding aq.
NaHCO3 solution (30 ml).32 The aqueous layer was separated
and extracted with dichloromethane (10 ml). The combined
organic layers were dried over MgSO4 and filtered. The solvent
was removed in vacuo to yield the crude product which was
recrystallised to give 16 (0.55 g, 75%) as light brown crystals,
mp 127–129 ЊC (from diethyl ether–n-pentane) (Found: C,
85.31; H, 6.08; S, 8.64. C26H22S requires C, 85.20; H, 6.05; S,
8.75 %); δH (CDCl3, 400 MHz) 2.40 (3 H, s, 2-Me), 4.89 (1 H, d,
Synthesis of 3,3-bis(4-methoxyphenyl)-1-phenylallyl acetate (4b)
In analogy to the report by Williams et al.,10 reaction of alde-
hyde 2b11,30 with phenyllithium yielded alcohol 3b which was
converted into the acetate 4b. Alcohol 3b (0.54 g, 1.6 mmol) was
dissolved in a mixture of acetic anhydride (0.22 ml, 2.3 mmol),
triethylamine (0.35 ml, 2.5 mmol), 4-(dimethylamino)pyridine
(40 mg, 0.33 mmol), and dichloromethane (30 ml). The solution
was stirred at ambient temperature for 22 h and then worked up
as described10 to yield 4b (0.51 g, 82%) as an orange oil; δH (300
MHz; CDCl3)31 2.06 (3 H, s, CH3), 3.77, 3.84 (2 × 3 H, 2 s,
2 × OMe), 6.17 (1 H, d, J 9.4, 2-H), 6.31 (1 H, d, J 9.4, 1-H),
6.76–6.83, 6.89–6.94, 7.10–7.14, 7.15–7.19 (4 × 2 H, 4 m, ArH),
7.26–7.34 (5 H, m, ArH); δC (75.5 MHz, CDCl3)31 21.4 (q,
CH3), 55.2, 55.3 (2 q, OMe), 74.4 (d, C-1), 113.4, 113.5 (2 d,
Ar), 124.2 (d, C-2), 126.9, 127.9, 128.6, 128.8, 130.9 (5 d, Ar),
131.3, 134.3, 140.5, 143.9, 158.6, 158.9 (6 s, C-3 and Ar), 169.8
(COO); m/z 388 (Mϩ, 24%), 345 (16), 329 (100), 221 (14).
J 10.4, 5-CHCH᎐C), 6.49–6.56 (3 H, m, 3-H, 4-H, and
᎐
5-CHCH᎐C), 7.19–7.39 (15 H, m, ArH); δ (CDCl3, 100.6
᎐
C
MHz) 15.31 (q, 2-Me), 46.51 (d, 5-CHCH᎐C), 124.27, 124.66
᎐
(2 d, C-3, C-4), 126.64, 127.31, 127.36, 127.52, 127.92, 128.11,
128.30, 128.56, 129.76 (9 d, Ar), 130.45 (d, 5-CHCH᎐C),
᎐
138.59 (s, C-2), 139.40, 141.54, 142.03, 143.91 (4 s, Ar and
5-CHCH᎐C), 146.22 (s, C-5); m/z 366 (Mϩ, 100%), 351 (10),
᎐
289 (12), 275 (16), 268 (19), 187 (19).
Synthesis of 3,3-bis(4-dimethylaminophenyl)-1-phenylallyl tetra-
fluoroborate (1c–BF4)
1,1,3-Triphenylhexa-1,5-diene (17)
As described above for the formation of 16, acetate 4a (0.35 g,
1.1 mmol), trimethylsilyl triflate (0.06 ml, 0.3 mmol) and
allylsilane 7 (0.22 ml, 0.16 g, 1.4 mmol) reacted for 21 h. Work-
up gave a crude product which was purified by chromatography
on neutral alumina with n-hexane–diethyl ether (5 : 2) as eluent
to yield 17 (0.22 g, 64%) as a yellow oil which crystallised on
standing, mp 56–57 ЊC (lit.,33 53–55 ЊC); δH (CDCl3, 400
MHz)31 2.48–2.52 (2 H, m, 4-H2), 3.48–3.55 (1 H, m, 3-H),
4.93–5.02 (2 H, m, 6-H2), 5.61–5.71 (1 H, m, 5-H), 6.24 (1 H, d,
J 10.5, 2-H), 7.12–7.37 (15 H, m, ArH); δC (CDCl3, 100.6
MHz)31 41.70 (t, C-4), 45.12 (d, C-3), 116.22 (t, C-6), 126.11,
127.05, 127.29, 127.38, 128.06, 128.15, 128.50, 129.87 (8 d, Ar),
132.26 (d, C-2), 136.32 (d, C-5), 140.04, 141.46, 142.46, 144.70
(4 s, C-1 and Ar).
A solution of 4-(dimethylamino)phenyllithium in diethyl ether
(45 ml) was prepared from 4-bromo-N,N-dimethylaniline
(9.91 g, 49.5 mmol) and lithium (0.64 g, 92 mmol) as described
in the literature.17,18 Then a solution of ketone 5a16 (5.00 g,
19.9 mmol) in dry THF (20 ml) was added at ambient temper-
ature over 1.5 h. The mixture was stirred for another 30 min,
then it was acidified with glacial acetic acid (30 ml) and poured
into a 2 M aqueous solution of NaBF4 (200 ml). Small portions
of the mixture were diluted with water to precipitate a blue
solid which was filtered off, washed with water and dried
in vacuo. Recrystallisation from CH2Cl2–n-pentane yielded
1c–BF4 (6.83 g, 78%) as blue metallic needles, mp 146–147 ЊC
(decomp.); δH (CD2Cl2, 300 MHz)31 3.30 (12 H, s, 2 ×
NMe2), 6.91–6.96 (4 H, m, ArH), 7.20 (1 H, d, J 15.5, 1-H),
7.48–7.50 (3 H, m, ArH), 7.58–7.62 (4 H, m, ArH), 7.69–7.73 (2
H, m, ArH), 7.79 (1 H, d, J 15.5 Hz, 2-H); δC (CD2Cl2, 75.5
MHz)31 41.0 (q, NMe2), 113.8 (d, Ar), 126.9 (s, Ar), 128.1 (d,
C-2), 129.6, 129.7, 132.3 (3 d, Ar), 135.9 (s, Ar), 139.3 (d, Ar),
153.8 (d, C-1), 157.1 (s, Ar), 173.8 (s, C-3).
5,5-Bis(4-methoxyphenyl)-1,3-diphenylpent-4-en-1-one (19)
Compound 4b (0.12 g, 0.31 mmol) was dissolved in dichloro-
methane (10 ml) under nitrogen and cooled to Ϫ70 ЊC.
Trimethylsilyl triflate (80 µl, 0.44 mmol) was added to give a red
solution of 1b-OTf. After the addition of silyl enol ether 10 (80
µl, 0.39 mmol) the stirred solution was allowed to warm to Ϫ45
ЊC. After 3 d the reaction mixture was worked up as above, and
the crude product was purified by chromatography on neutral
alumina with n-hexane–diethyl ether (10 : 1) as eluent to yield
19 (29.2 mg, 21%) as an orange oil; δH (300 MHz, CDCl3)31 3.31
(1 H, dd, 2J 15.3, J 7.9, 2-H), 3.42 (1 H, dd, 2J 15.3, J 6.7, 2-H),
3.76, 3.82 (2 × 3 H, 2 s, 2 × OMe), 4.12–4.20 (1 H, m, 3-H),
6.14 (1 H, d, J 10.4, 4-H), 6.73–6.84 (4 H, m, ArH), 6.89–6.98,
7.05–7.11 (2 × 2 H, 2 m, ArH), 7.15–7.31 (5 H, m, ArH), 7.33–
7.40 (2 H, m, ArH), 7.45–7.53 (1 H, m, ArH), 7.80–7.84 (2 H,
m, ArH); δC (75.5 MHz, CDCl3)31 41.88 (d, C-3), 46.41 (t, C-2),
55.18, 55.24 (2 q, 2 × OMe), 113.38, 113.53, 126.33, 127.29,
128.17, 128.45, 128.49, 128.65 (8 d, Ar), 128.98 (d, C-4), 130.76
(d, Ar), 132.10 (s), 132.82 (d, Ar), 135.19, 137.01 (2 s), 141.18,
144.32, 158.61, 158.92 (4 s), 198.33 (s, C-1); m/z (EI) 448 (Mϩ,
8%), 330 (24), 329 (100), 328 (72), 221 (17), 121 (14) (HRMS:
Found m/z 448.2031. C31H28O3 requires 448.2038).
Synthesis of 1,3,3-tris(4-dimethylaminophenyl)allyl tetrafluoro-
borate (1d–BF4)
A solution of 4-(dimethylamino)phenyllithium in diethyl ether
(20 ml) was prepared from 4-bromo-N,N-dimethylaniline
(4.81 g, 24.0 mmol) and lithium (0.42 g, 61 mmol) as described
in the literature.17,18 Excess lithium was removed after the
reaction had been stirred for 21 h at ambient temperature. Then
a suspension of ketone 5b16 (5.89 g, 20.0 mmol) in dry THF (80
ml) was added. The mixture was heated to reflux for 3 h, then it
was cooled to ambient temperature, acidified with glacial acetic
acid (30 ml) and poured into a 5 M aqueous solution of NaBF4
(200 ml). Small portions of the mixture were diluted with water
and neutralised with aq. NaHCO3 to precipitate a blue solid
which was filtered off and dried in vacuo. Recrystallisation from
CH2Cl2–n-hexane yielded 1d–BF4 (3.69 g, 38%) as golden
needles, mp 134–138 ЊC; δH (400 MHz, CD2Cl2)31 3.20 (18 H, br
s, 3 × NMe2), 6.79–6.84 (6 H, m, ArH), 7.37 (1 H, d, J 14.4,
1-H), 7.47–7.50 (4 H, m, ArH), 7.63 (1 H, d, J 14.4, 2-H),
7.67–7.69 (2 H, m, ArH); δC (100.6 MHz, CD2Cl2)31 40.6,
40.7 (2 q, NMe2), 112.6, 113.3 (2 d, Ar), 121.7 (d, C-2), 125.2,
126.6 (2 s, Ar), 134.9, 137.1 (2 d, Ar), 155.5 (s, Ar), 158.7 (d, C-
1), 174.2 (s, C-3).
1,1-Bis(4-methoxyphenyl)-5-methyl-3-phenylhexa-1,5-diene (20)
As described for the formation of 19, compound 4b (0.12 g,
0.31 mmol), trimethylsilyl triflate (70 µl, 0.39 mmol), and
allylstannane 12 (0.19 g, 0.55 mmol) reacted for 4 h while the
1438
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1
temperature was raised from Ϫ70 ЊC to ambient. The crude
product was purified by chromatography on neutral alumina
with n-hexane–diethyl ether (15 : 1) as eluent to yield 20 (47.6
mg, 40%) as a colourless oil (Found C, 84.20; H, 7.64. C27H28O2
requires C, 84.34; H, 7.34%); δH (300 MHz, CDCl3)31 1.52 (3 H,
br s, 5-Me), 2.43 (2 H, d, J 7.5, 4-H2), 3.61 (1 H, dt, J 10.3, J 7.5,
3-H), 3.76, 3.83 (2 × 3 H, 2 s, 2 × OMe), 4.65–4.71 (2 H, m,
6-H2), 6.08 (2 H, d, J 10.3, 2-H), 6.76–6.80, 6.86–6.89, 6.99–7.04
(3 × 2 H, 3 m, ArH), 7.11–7.31 (7 H, m, Ar); δC (75.5 MHz,
CDCl3)31 22.22 (q, 5-Me), 43.56 (d, C-3), 46.59 (t, C-4), 55.18,
55.24 (2 q, 2 × OMe), 112.27 (t, C-6), 113.38, 125.94, 127.36,
128.46, 128.48 (5 d, Ar), 130.71 (d, C-2), 131.01 (d, Ar), 132.54,
135.73, 140.32, 143.61, 145.63, 158.57, 158.79 (7 s, C-1, C-5 and
Ar); m/z (EI) 384 (Mϩ, 2), 330 (27), 329 (100), 221 (13).
mers 53 : 47 (determined by H NMR spectroscopy); δH (400
MHz, CDCl3)31 1.90–2.31 (2 H, m, 4-H2), 2.91/2.92 (6 H, s,
NMe2), 2.93–2.95 (1 H, m, 3-H), 2.96/2.97 (6 H, s, NMe2),
3.92–4.14 (3 H, m, 5-H2 and 1Ј-H), 6.20 (0.53 H, d, J 10.6, 2Ј-H,
major diastereomer) and 6.30 (0.47 H, d, J 10.7, 2Ј-H, minor
diastereomer), 6.60–6.72 (4 H, m, ArH), 6.92–6.99, 7.12–7.19
(2 × 2 H, 2 m, ArH), 7.20–7.33 (5 H, m, ArЈH); δC (100.6 MHz,
CDCl3)31 26.21, 26.29 (2 t, C-4), 40.48*, 40.51, 40.54 (3 q,
NMe2), 45.04, 45.25 (2 d, C-1Ј), 45.19, 45.47 (2 d, C-3), 66.33
(2 t, C-5), 111.93*, 111.97, 112.00 (3 d, Ar), 122.90, 123.57 (2 d,
C-2Ј), 126.54, 126.56 (2 d, ArЈ), 127.92 (s, Ar), 128.00, 128.27,
128.36, 128.46, 128.50, 128.58, 130.56* (7 d, Ar and ArЈ),
131.28, 131.39, 142.33, 142.47, 143.31, 143.62, 149.53, 149.91
(8 s, C-3Ј, Ar and ArЈ), 177.27, 177.29 (2 s, C-2), signals with
double intensity are marked with an asterisk; m/z (EI): 440 (Mϩ,
27%), 355 (100), 234 (11), 210 (11) (HRMS: Found m/z
440.2451. C29H32N2O2 requires 440.2464).
2-[3,3-Bis(4-dimethylaminophenyl)-1-phenylprop-2-enyl]cyclo-
hexanone (23)
1,1-Bis(4-dimethylaminophenyl)-3-phenylprop-1-ene (27)
A solution of 1c–BF4 (0.27 g, 0.61 mmol) and stannane 15
(0.18 ml, 0.20 g, 0.68 mmol) in dichloromethane (10 ml) was
stirred for 4 d at ambient temperature. Then the solvent was
removed in vacuo and the residue was purified by chrom-
atography on neutral alumina with n-hexane–diethyl ether
(1 : 1) as eluent to yield 27 (52.9 mg, 24%) as colourless crystals,
mp 98–99 ЊC (Found: C, 83.92; H, 7.89; N, 7.69. C25H28N2
requires C, 84.23; H, 7.92; N, 7.86%); δH (300 MHz, CDCl3)
2.91, 2.96 (2 × 6 H, 2 s, 2 × NMe2), 3.51 (2 H, d, J 7.5, 3-H2),
6.04 (1 H, t, J 7.5, 2-H), 6.60–6.64, 6.65–6.75 (2 × 2 H, 2 m,
ArH), 7.10–7.30 (9 H, m, ArH); δC (75.5 MHz, CDCl3) 35.98
(t, C-3), 40.54, 40.58 (2 q, 2 × NMe2), 112.01, 112.08 (2 d, Ar),
123.55 (d, C-2), 125.65, 128.27, 128.28, 128.42, 130.78 (5 d,
ArH), 131.82, 141.97, 142.24, 149.41, 149.66 (5 s); m/z (EI): 356
(Mϩ, 100%), 355 (53), 312 (14), 235 (10), 234 (11), 149 (10).
A solution of 1c–BF4 (0.27 g, 0.61 mmol) and enamine 13 (0.27
g, 1.6 mmol) in dichloromethane (10 ml) was stirred for 7 d at
ambient temperature. Then 2 M HCl (10 ml) and water (30 ml)
were added. The phases were separated, and the aqueous layer
was extracted with dichloromethane (10 ml). The combined
organic layers were washed with saturated aqueous NaHCO3
(2 × 10 ml) and water (2 × 10 ml), dried (MgSO4) and filtered,
and the solvent was evaporated in vacuo. The residue was puri-
fied by chromatography34 on neutral alumina (activity III) with
n-hexane–diethyl ether (1 : 1) as eluent to yield 23 (67 mg, 24%)
as a colourless oil, ratio of diastereomers 65 : 35 (determined by
1H NMR spectroscopy) (Found C, 82.16; H, 8.24; N, 5.82.
C31H36N2O requires C, 82.26; H, 8.02; N, 6.19%); δH (300 MHz,
CDCl3)31 1.42–1.80 (6 H, m, 3-H2, 4-H2, 5-H2), 2.10–2.20 (2 H,
m, 6-H2), 2.66–2.90 (1 H, m, 2-H), 2.89 (6 H, s, NMe2), 2.99
(0.65 × 6 H, s, NMe2, major diastereomer), 3.00 (0.35 × 6 H, s,
NMe2, minor diastereomer), 3.86–3.94 (1 H, m, 1Ј-H), 5.94
(0.35 H, d, J 10.7, 2Ј-H, minor diastereomer), 6.07 (0.65 H, d,
J 10.5, 2Ј-H, major diastereomer), 6.57–6.62, 6.68–6.77 (2 × 2
H, 2 m, ArH), 6.94–7.32 (9 H, m, ArH and ArЈH); δC (75.5
MHz, CDCl3)31 major diastereomer: 21.68 (t, C-4), 27.91 (t,
C-5), 30.17 (t, C-3), 40.41 (t, C-6), 40.55 (br q, NMe2), 44.97
(d, C-1Ј), 57.89 (d, C-2), 111.80, 112.02, 112.08 (3 d, Ar), 126.01
(d, ArЈ), 126.11 (d, C-2Ј), 128.19 (d), 128.32 (s), 128.47, 128.50,
130.92 (3 d), 131.83, 141.73, 143.50, 149.45, 149.79 (5 s), 213.31
(C-1), additional signals of the minor diastereomer: 23.61 (t,
C-4), 28.50 (t, C-5), 32.01 (t, C-3), 41.94 (t, C-6), 44.94 (d, C-1Ј),
57.05 (d, C-2), 125.86 (d), 126.50 (d, C-2Ј), 127.83, 127.97,
128.31 (3 d), 128.40 (s), 130.68 (d), 131.47, 142.10, 144.02,
149.43, 149.73 (5 s), 212.19 (s, C-1); m/z (EI) 452 (Mϩ, 8%), 356
(26), 355 (100).
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully
acknowledged. We thank Mrs T. Hübscher (München) for
experimental contributions.
References
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J. Chem. Soc., Perkin Trans. 2, 2002, 1435–1440
1439

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34 During chromatography, the column turns slightly blue probably
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for the low yield.
1440
J. Chem. Soc., Perkin Trans. 2, 2002, 1435–1440