Diels-Alder reactions of 1,1,3-triarylallyl cations: Determination of the free energy of concert

Article abstract of DOI:10.1039/b203555c

1,1,3-Triarylallyl cations react with 1,3-dienes to give Diels-Alder products in low to moderate yields. The observed second-order rate constants agree with those calculated for the stepwise processes by the correlation log k(20 °C) = s(N + E) [eqn. (1)]

Full text of DOI:10.1039/b203555c

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Diels–Alder reactions of 1,1,3-triarylallyl cations: determination
of the free energy of concert†‡
Claudia Fichtner§ and Herbert Mayr*
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
1,1,3-Triarylallyl cations react with 1,3-dienes to give Diels–Alder products in low to moderate yields. The observed
second-order rate constants agree with those calculated for the stepwise processes by the correlation log k(20 ЊC) =
s(N ϩ E) [eqn. (1)] indicating the absence of a significant free energy of concert. Eqn. (1) can, therefore, also be
employed for the prediction of absolute rate constants of Diels–Alder reactions of allyl cations.
with rate-determining formation of 3a/b, the observed reaction
rate should be predictable by eqn. (1), since in this case electro-
phile and nucleophile combine with the formation of one
Introduction
The acceleration of [4ϩ2]-cycloadditions by acceptor groups
in the dienophile (Scheme 1) has long been known and is
new σ-bond, like the reactions of carbocations with olefins
for which eqn. (1) was derived.4,5 The cycloaddition will only
proceed by a concerted mechanism if this reaction mode is
energetically more favourable than the stepwise mechanism.
The ratio between observed rate constant and rate constant
for the hypothetical stepwise reaction calculated by eqn. (1)
thus provides a measure for the “free energy of concert”10
Scheme 1 Diels–Alder reaction of an acceptor-substituted dienophile
with a diene.
(Scheme 2). For calculating the rate of formation of 3a/b, the
usually referred to as the “Alder rule”.1 PMO theory rational-
ises this phenomenon by the lowering of the LUMO of the
dienophile and the reduction of the energy difference between
HOMO (diene) and LUMO (dienophile).2 Relative reactivities
of dienophiles have been explained in this way.3
Recently we have demonstrated that the rates of the reactions
of carbocations with nucleophiles are given by the three-
parameter eqn. (1),4–6
log k(20 ЊC) = s(N ϩ E)
(1)
where s = nucleophile-specific slope parameter, N = nucleo-
philicity parameter and E = electrophilicity parameter.
Since the allyl cations 1 can be considered as acceptor substi-
tuted dienophiles,7–9 they represent a class of compounds where
kinetic aspects of Diels–Alder reactions can be investigated by
the use of the correlation eqn. (1).
Scheme 2 Stepwise and concerted pathways of the Diels–Alder
reactions of allyl cations 1 with dienes 2.
previously published N and s parameters of the 1,3-dienes 2
and the E parameters of the allyl cations 1a–d derived in the
preceding article11 can be used.
For our analysis, the following line of arguments is used: if
the cycloaddition of 1 with the 1,3-dienes 2 proceeds stepwise,
Results
† Dedicated to Professor W. Beck on the occasion of his 70th birthday.
‡ Electronic supplementary information (ESI) available: rate constants
and experimental conditions of the individual kinetic experiments. See
http://www.rsc.org/suppdata/p2/b2/b203555c/
§ Present address: CarboGen Laboratories, CH-5001 Aarau,
Switzerland.
Cycloaddition products
Combination of 1,3,3-triphenylprop-2-enyl acetate 5a with
ZnCl2ؒOEt2 at Ϫ70 ЊC in dichloromethane gave a dark-red
solution of 1a which reacted with isoprene (2a) and 2,3-
DOI: 10.1039/b203555c
J. Chem. Soc., Perkin Trans. 2, 2002, 1441–1444
This journal is © The Royal Society of Chemistry 2002
1441

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Supplementary Information. Because of the cyclisation12,13 of
1a above Ϫ30 ЊC, all kinetic experiments were performed
between Ϫ70 and Ϫ50 ЊC. The k(20 ЊC) values given in Table 1
were then extrapolated from the activation parameters deter-
mined in the low temperature range.
The slightly higher reactivity of 2b over 2a shown in Table 1
is comparable to the relative reactivities of these two dienes
towards the 4-methoxy substituted benzhydryl cation (k2b/k2a
=
1.8, Ϫ70 ЊC)16 as well as towards the dienophiles maleic
anhydride (k2b/k2a = 2.2, 30 ЊC)17 and tetracyanoethylene (k2b/
k2a = 32, 20 ЊC). The activation entropies given in Table 1 closely
resemble those determined for the reactions of the dienes 2a,b
and other terminal olefins with benzhydryl cations16 as well as
those of concerted Diels–Alder reactions of non-charged
cycloaddends,3 indicating that ∆S‡ is not a reliable criterion to
differentiate between concerted and stepwise mechanisms.
Rate constants for the reactions of 1b with 2a,b could not be
determined. When 1b was treated with 100 equiv. of 2b (c = 35
mmol lϪ1) at Ϫ50 ЊC (CH2Cl2) no change of the intensity of the
absorbance of 1b could be detected within one day.
Because of the lower tendency of 1c and 1d to undergo cyclis-
ation, their reactions with Danishefsky’s diene 2c could be
investigated at 20 ЊC. Decolourisation of the green solution of
1c and the blue solution of 1d was followed photometrically
at 675 nm and 600 nm, respectively, under pseudo-first-order
conditions using the work-station described previously.15,18 The
second-order rate constants shown in Table 1 were obtained by
averaging the results of several of kinetic experiments, which
are documented in the Supplementary Information.
Scheme 3
dimethylbuta-1,3-diene (2b) at Ϫ30 ЊC to afford the cyclohexene
derivatives 6a and 6b, respectively (Scheme 3). Attempts to
trap the intermediate cations 4 by addition of 2 equiv. of
triethylamine–borane before aqueous work-up failed. It must,
therefore, be concluded that deprotonation of the carbocations
4 to give 6a and 6b occurs under the conditions of the cyclo-
additions and not during aqueous work-up.
The analogous reactions of 3,3-dianisyl-1-phenylprop-2-enyl
acetate 5b with 2a and 2b were very slow, and the cyclohexenes
6c,d were obtained in low yields after five days at Ϫ45 ЊC
(Scheme 3). Raising of the temperature was not attempted
because of the well-known tendency of the intermediate allyl
cations 1a,b to undergo electrocyclic ring-closure with form-
ation of indanyl cations.12,13 The 4-methylene-5-phenylcyclohex-
Discussion
1
1-ene structure of compounds 6a–d was recognised in the H
The eventual formation of the cations 3a,b from the dienophiles
1 and the dienes 2 closely resembles the electrophilic additions
of benzhydryl cations to these dienes. For that reason, the rate
constants kcalc(20 ЊC) for the one-bond attack of the allyl
cations 1a–d at the dienes 2a–c to give 3 can be calculated by
eqn. (1). One can recognise that the calculated rate constants
listed in the last column of Table 1 agree with the experi-
mentally obtained values within a factor of 3, i.e., the deviation
is comparable to the standard deviation for predictions by eqn.
(1) for reactions of these dienes with electrophiles other than
benzhydryl cations.4 It is, therefore, suggested that compounds
6 and 7, consecutive products of the cations 4, are produced
by stepwise processes via intermediates 3a,b or analogous allyl
cations. Possibly, these intermediates can also undergo altern-
ative consecutive reactions and thus account for the low yields
of products isolated in some cases. Competing stepwise and
concerted cycloadditions of the allyl cations 1a–d with the
dienes 2a–c are also in accord with the experimental findings.
The close similarity of kcalc and kexp rules out transition
states, however, that are considerably stabilised due to the sim-
ultaneous formation of two new σ-bonds.19 The reactions
definitely do not have a high degree of concertedness. Theor-
etical investigations of the Diels–Alder reaction of the parent
allyl cation with buta-1,3-diene came to the same conclusion.20
The failure to observe allyl cation diene combinations
with kcalc < 10Ϫ4 l molϪ1 sϪ1 has an interesting consequence:
obviously, a concerted mechanism cannot be turned on when
the stepwise process becomes energetically unfavourable. For
that reason, eqn. (1) can be used not only for the prediction
of absolute rate constants for reactions of the allyl cations 1
with one-bond nucleophiles but also for predicting rate
constants of their Diels–Alder reactions with 1,3-dienes.
NMR spectra by the broad doublet absorption of the benzylic
proton 5-H (δ 4.05–4.12) and the multiplets at δ 2.42–2.80 due
to four allylic hydrogens (3-H2, 6-H2). Additional evidence
comes from the 13C NMR spectra of 6a–d with three sp3 and
three sp2 carbon resonances for the cyclohexene ring.
The low reactivity of the methyl substituted butadienes 2a,b
towards 1b suggested that there would not be any chance to
observe a reaction with the considerably less electrophilic11 allyl
cations 1c,d. Actually, we have not even been able to isolate
a product from 1c and the more nucleophilic 2-(trimethyl-
siloxy)buta-1,3-diene (N = 4.83, s = 0.90),14 but the reactions of
1c,d–BF4 with Danishefsky’s diene 2c (N = 8.57, s = 0.84)4 gave
low to moderate yields of the cyclohexenones 7a (51%) and 7b
(8%) (Scheme 4).
Scheme 4
Kinetics
When the purple solution of 1a, generated in dichloromethane
from 5a and ZnCl2ؒOEt2, was treated with 2a or 2b, the colour
disappeared, and the decay of the absorbance of 1a at 510 nm
was followed photometrically using the work-station described
previously.15 Since the dienes 2a,b were used in large excess over
1a, their concentrations remained almost constant during
the reactions, and the pseudo-first-order rate constants kobs
could be divided by the concentration of the dienes to yield
the second-order rate constants given in Tables S1–S3 of the
Experimental
General
NMR spectra were recorded with Bruker ARX 300, Varian
1
VRX 400S, and Varian INOVA-400 spectrometers. H NMR
1442
J. Chem. Soc., Perkin Trans. 2, 2002, 1441–1444

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Table 1 Observed and calculated rate constants for the reactions of the 1,1,3-triarylallyl cations 1a–d with 1,3-dienes 2a–c (in CH2Cl2)
kexp(20 ЊC)/
kcalc(20 ЊC)/
Diene
Na
sa
Dienophile
Eb
∆H‡/kJ molϪ1 g
∆S‡/J molϪ1 KϪ1 g
l molϪ1 sϪ1
l molϪ1 sϪ1 c
Isoprene (2a)
1.10
1.10
1.17e
1.17e
8.57
8.57
0.98
0.98
1.00e
1.00e
0.84
0.84
1a
1b
1a
1b
1c
1d
ϩ0.98
Ϫ2.67
ϩ0.98
Ϫ2.67
Ϫ8.97
Ϫ9.84
26.40 0.66
—
28.26 1.04
—
—
—
Ϫ124.7 3.0
—
Ϫ105.2 4.8
—
—
—
3.68 × 101 d
—
1.09 × 102
2.89 × 10Ϫ2
1.41 × 102
3.16 × 10Ϫ2
4.61 × 10Ϫ1
8.57 × 10Ϫ2
Isoprene (2a)
2,3-Dimethylbutadiene (2b)
2,3-Dimethylbutadiene (2b)
Danishefsky’s diene (2c)
Danishefsky’s diene (2c)
1.79 × 102 d
f
—
3.72 × 10Ϫ1
8.32 × 10Ϫ2
a Nucleophilicity parameters N and s from ref. 4. b E parameters from ref. 11. c Calculated from eqn. (1) for the formation of 3a/b. d Extrapolated
from experiments at Ϫ70 to Ϫ50 ЊC. e From rate constant in ref. 16 using the parametrisation described in ref. 4. f Using similar concentrations as for
the experiments with 1a, no change of absorbance was detectable within 22 h at Ϫ50 ЊC. g 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.
chemical shifts refer to tetramethylsilane (δH 0.00) and 13C
NMR chemical shifts refer to the solvent as internal standard
(CDCl3: δC 77.0). 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 photometer used for the kinetic studies of
reactions of 1a was a Schölly KGS III with band-pass filters by
Corion15 whereas reactions of cations 1c and 1d were followed
with a J&M TIDAS DAD connected to quartz-insertion probes
by Hellma.18 The kinetic measurements and the data-evaluation
were carried out as described previously.15
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
before use.
The compounds 1a–d and the precursors 5a,b were generated
as described before.11 Dichloromethane solutions of the Lewis
acid ZnCl2ؒOEt2 were prepared with a molar Et2O–ZnCl2 ratio
of 1.12 to achieve the highest catalytic activity according to the
literature.21 Dienes 2a–c are commercially available.
0.24 g, 2.9 mmol) were reacted as described above. Work-up
gave a yellow oil which contained polymer and the cyclic
product 6b (yield 33%, as calculated from the H NMR). A
1
small amount of purified 6b could be obtained by several
recrystallisations from acetonitrile in which the polymer is
poorly soluble, mp 128–130 ЊC (Found C, 92.30; H, 7.25. C27H26
requires C, 92.52; H, 7.48); δH (CDCl3, 300 MHz) 1.49, 1.76
(2 × 3 H, 2 s, 2 × CH3), 2.44–2.72 (4 H, m, 3-H2 and 6-H2), 4.05
(1 H, br d, J 5.4, 5-H), 7.10–7.36 (15 H, m, ArH); δC (CDCl3, 75
MHz) 18.82, 18.92 (2 q, 2 × CH3), 34.55, 36.94 (2 t, C-3 and
C-6), 41.56 (d, C-5), 123.85, 125.63 (2 s, C-1 and C-2), 125.72,
126.29, 126.45, 126.93, 127.85, 128.03, 128.27, 129.60, 129.82
(9 d, Ar), 135.43, 137.73, 142.42, 143.04, 144.21 (5 s).
4-[Bis(4-methoxyphenyl)methylene]-1-methyl-5-phenylcyclohex-
1-ene (6c)
A solution of 1b was produced at Ϫ70 ЊC from 5b (0.23 g,
21
0.59 mmol) and a dichloromethane solution of ZnCl2ؒOEt2
(0.40 ml, 0.77 mmol, c = 1.914 mol lϪ1) in dichloromethane
(10 ml) as described above. Then diene 2a (0.14 g, 2.1 mmol)
was added, and the reaction mixture was stirred for 5 d at Ϫ45
ЊC. Work-up as above gave the crude product which was further
purified by chromatography on neutral alumina with n-hexane–
diethyl ether (20 : 1) as eluent to yield 24.2 mg of a mixture of
The kinetic measurements and the data-evaluation were
carried out as described previously.11,15,16
4-(Diphenylmethylene)-1-methyl-5-phenylcyclohex-1-ene (6a)
1
polymer and 6c (yield 4%, as calculated from the H NMR)
A solution of 5a (0.22 g, 0.67 mmol) in dichloromethane (10
ml) was cooled to Ϫ70 ЊC. Then a solution of ZnCl2ؒOEt2 in
dichloromethane21 (1.0 ml, c = 1.20 mol lϪ1) was added and the
mixture was stirred for 10 min during which time its colour
turned purple. After the addition of diene 2a (0.22 ml, 0.15 g,
2.2 mmol) the reaction mixture was stirred for 21 h at Ϫ30 ЊC.
Then saturated aq. NaHCO3 (10 ml) was added with stirring
and the mixture was allowed to warm to ambient temperature.
The aqueous phase was separated and extracted with dichloro-
methane (10 ml). The combined organic phases were dried over
MgSO4 and filtered, and the solvent was evaporated in vacuo to
yield 268 mg of a colourless oil which consisted of a mixture of
polymer and cyclic product 6a (yield 32%, as calculated from
the 1H NMR). A small amount of purified 6a could be obtained
by several recrystallisations from acetonitrile in which the
polymer is poorly soluble, mp 121–123 ЊC (Found C, 92.50; H,
7.40. C26H24 requires C, 92.81; H, 7.19); δH (CDCl3, 300 MHz)22
1.80 (3 H, s, CH3), 2.45–2.78 (4 H, m, 3-H2 and 6-H2), 4.09 (1 H,
br d, J 5.5, 5-H), 5.25 (1 H, br s, 2-H), 7.09–7.37 (15 H, m,
ArH); δC (CDCl3, 75.5 MHz)22 23.34 (q, CH3), 28.59 (t, C-3),
35.48 (t, C-6), 41.22 (d, C-5), 121.09 (d, C-2), 125.84, 126.34,
126.50, 126.90, 127.88, 128.08, 128.30, 129.53, 129.68 (9 d, Ar),
132.26, 136.02, 136.87, 142.42, 142.96, 143.98 (6 s).
as a pale yellow oil. δH (400 MHz, CDCl3)22 1.80 (3 H, s, CH3),
2.46–2.80 (4 H, m, 3-H2 and 6-H2), 3.77, 3.80 (2 × 3 H, 2 s,
2 × OCH3), 4.12 (1 H, br d, J 5.5, 5-H), 5.24 (1 H, br s, 2-H),
6.76–6.80, 6.85–6.89, 6.98–7.02 (3 × 2 H, 3 m, ArH),
7.12–7.26 (m, 7 H, ArH); δC (100 MHz, CDCl3)22 23.34 (q,
CH3), 28.76 (t, C-3), 35.45 (t, C-6), 41.25 (d, C-5), 55.16, 55.20
(2 q, 2 × OCH3), 113.20, 113.67 (2 d, Ar), 121.24 (d, C-2),
125.76, 126.88, 128.03, 130.59, 130.85 (5 d, Ar), 132.20, 135.06,
135.24, 135.74, 135.96, 144.21, 158.01, 158.18 (8 s).
4-[Bis(4-methoxyphenyl)methylene]-1,2-dimethyl-5-phenyl-
cyclohex-1-ene (6d)
Compound 5b (0.22 g, 0.57 mmol), a dichloromethane solution
21
of ZnCl2ؒOEt2 (0.40 ml, 0.77 mmol, c = 1.914 mol lϪ1), and
diene 2b (0.15 g, 1.8 mmol) were allowed to react for 5 d at Ϫ45
ЊC as described above for the formation of 6c. Work-up as
above yielded a crude product which was purified by chrom-
atography on neutral alumina with n-hexane–diethyl ether
(20 : 1) as eluent to give 39.5 mg of a mixture of polymer and 6d
(yield 9%, as calculated from the 1H NMR) as a pale yellow oil.
δH (400 MHz, CDCl3)22 1.49, 1.75 (2 × 3 H, 2 s, 2 × CH3),
2.42–2.66 (4 H, m, 3-H2 and 6-H2), 3.77, 3.79 (2 × 3 H, 2 s,
2 × OCH3), 4.08 (1 H, br d, J 5.5, 5-H), 6.77–6.81, 6.84–6.89,
6.99–7.03 (3 × 2 H, 3 m, ArH), 7.12–7.23 (7 H, m, ArH); δC (100
MHz, CDCl3)22 18.82, 18.93 (2 q, 2 × CH3), 34.73, 36.91 (2 t,
C-3 and C-6), 41.57 (d, C-5), 55.13, 55.18 (2 q, 2 × OCH3),
113.16, 113.63 (2 d, Ar), 123.78 (s, C-1 or C-2), 125.63 (d, Ar),
4-(Diphenylmethylene)-1,2-dimethyl-5-phenylcyclohex-1-ene
(6b)
Compound 5a (0.28 g, 0.85 mmol), a dichloromethane solution
21
of ZnCl2ؒOEt2 (1.00 ml, c = 1.20 mol lϪ1), and 2b (0.33 ml,
J. Chem. Soc., Perkin Trans. 2, 2002, 1441–1444
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125.74 (s, C-1 or C-2), 126.90, 127.98, 130.64, 130.96 (4 d, Ar),
134.47, 135.24, 135.79, 136.81, 144.43, 157.94, 158.13 (7 s).
References
1 K. Alder, Experientia Supplementum II, 1955, 86, as quoted in
J. Sauer, Angew. Chem., 1967, 79, 76; J. Sauer, Angew. Chem., Int. Ed.
Engl., 1967, 6, 16.
4-[Bis(4-dimethylaminophenyl)methylene]-5-phenylcyclohex-2-
2 I. Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley,
Chichester, 1976.
3 J. Sauer and R. Sustmann, Angew. Chem., 1980, 92, 773; J. Sauer and
R. Sustmann, Angew. Chem., Int. Ed. Engl., 1980, 19, 779.
4 H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker,
B. Kempf, R. Loos, A. R. Ofial, G. Remennikov and H. Schimmel,
J. Am. Chem. Soc., 2001, 123, 9500.
5 (a) H. Mayr and M. Patz, Angew. Chem., 1994, 106, 990; H. Mayr
and M. Patz, Angew. Chem., Int. Ed. Engl., 1994, 33, 938; (b)
H. Mayr, O. Kuhn, M. F. Gotta and M. Patz, J. Phys. Org. Chem.,
1998, 11, 642; (c) H. Mayr, M. Patz, M. F. Gotta and A. R. Ofial,
Pure Appl. Chem., 1998, 70, 1993.
6 R. Lucius, R. Loos and H. Mayr, Angew. Chem., 2002, 114, 97;
R. Lucius, R. Loos and H. Mayr, Angew. Chem., Int. Ed., 2002, 41,
91.
7 (a) H. Vathke-Ernst and H. M. R. Hoffmann, Chem. Ber., 1981, 114,
1464; (b) H. M. R. Hoffmann and H. Vathke-Ernst, Chem. Ber.,
1981, 114, 2208; (c) review: H. M. R. Hoffmann, Angew. Chem.,
1984, 96, 29; H. M. R. Hoffmann, Angew. Chem., Int. Ed. Engl.,
1984, 23, 1.
8 For intramolecular Diels–Alder reactions of allyl cations, see (a)
P. G. Gassman and D. A. Singleton, J. Am. Chem. Soc., 1984, 106,
6085; (b) P. G. Gassman and D. A. Singleton, J. Org. Chem., 1986,
51, 3075; (c) P. G. Gassman and D. G. Gorman, J. Am. Chem. Soc.,
1990, 112, 8623; (d ) P. G. Gassman and D. G. Gorman, J. Am.
Chem. Soc., 1990, 112, 8624; (e) D. B. Gorman and P. G. Gassman,
J. Org. Chem., 1995, 60, 977.
9 For Diels–Alder reactions of heteroatom-stabilized allyl cations,
see (a) P. G. Gassman, D. A. Singleton, J. J. Wilwerding and
S. P. Chavan, J. Am. Chem. Soc., 1987, 109, 2182; (b) P. A. Grieco,
M. D. Kaufman, J. F. Daeuble and N. Saito, J. Am. Chem. Soc.,
1996, 118, 2095; (c) P. A. Grieco and Y. Dai, J. Am. Chem. Soc.,
1998, 120, 5128; (d ) P. A. Grieco and M. D. Kaufman, Tetrahedron
Lett., 1999, 40, 1265.
enone (7a)
A mixture of 1c–BF4 (0.43 g, 0.97 mmol) and diene 2c (0.40 ml,
0.36 g, 2.1 mmol) in dichloromethane (10 ml) was stirred for 1 h
at ambient temperature. After addition of 2 M NaOH (10 ml)
the aqueous phase was separated and extracted with dichloro-
methane (3 × 10 ml). The combined organic layers were dried
(Na2SO4), filtered, and the solvent was evaporated in vacuo. The
crude product was purified by column chromatography on
neutral alumina with n-hexane–ethyl acetate (3 : 2) as eluent to
yield 7a as a brown solid (0.21 g, 51%), mp >90 ЊC (decomp.);
δH (400 MHz, CDCl3)22 2.79 (1 H, d with fine-coupling, 2J 16.0,
6-H), 2.90, 2.99 (2 × 6 H, 2 s, 2 × NMe2), 3.07 (1 H, dd, 2J 16.0,
3J 5.7, 6-H), 4.38 (1 H, br d, J 5.7, 5-H), 5.83 (1 H, dd, J 10.0,
J 0.9, 2-H), 6.47 (2 H, AAЈBBЈ system with JAB 9.0, ArH), 6.67
(2 H, AAЈBBЈ system with JAB 8.8, ArH), 6.87 (2 H, AAЈBBЈ
system with JAB 9.0, ArH), 7.07 (2 H, AAЈBBЈ system with
JAB 8.8, ArH), 7.16–7.21 (1 H, m, ArH), 7.22–7.29 (4 H, m,
ArH), 7.47 (1 H, dd, J 10.0, J 1.4, 3-H); δC (100 MHz, CDCl3)22
40.08, 40.19 (2 q, 2 × NMe2), 45.53 (d, C-5), 45.65 (t, C-6),
110.91, 111.02 (2 d, Ar), 124.85 (d, C-2), 126.14, 127.78
(2 d, Ar), 128.38 (br s), 128.42 (d, Ar), 129.23, 128.88 (2 s),
132.66, 130.93 (2 d, Ar), 143.66 (s), 148.14 (d, C-3), 149.90,
150.27 (2 s), 198.06 (s, C-1); m/z (EI): 422 (Mϩ, 100%), 345 (6),
253 (11).
4-[Bis(4-dimethylaminophenyl)methylene]-5-(4-dimethylamino-
phenyl)cyclohex-2-enone (7b)
A mixture of 1d–BF4 (0.50 g, 1.03 mmol) and diene 2c (0.40 ml,
0.36 g, 2.1 mmol) in dichloromethane (10 ml) was stirred for
1 h at ambient temperature. Work-up and chromatographic
purification as described for the isolation of 7a gave an orange
oil which could be crystallised from a dichloromethane–
pentane mixture to give yellow needles of 7b (35.8 mg, 8%); mp
>92 ЊC (decomp.); δH (400 MHz, CDCl3)22 2.74–3.10 (2 H, m,
superimposed by signals of NMe2, 6-H2), 2.918, 2.920, 3.004
(3 × 6 H, 3 s, 3 × NMe2), 4.31 (1 H, br d, J 4.7, 5-H), 5.81 (1 H,
dd, J 10.0, J 0.9, 2-H), 6.50 (2 H, AAЈBBЈ system with JAB 9.0,
ArH), 6.66–6.69 (4 H, m, ArH), 6.94 (2 H, AAЈBBЈ system with
JAB 9.0, ArH), 7.07 (2 H, AAЈBBЈ system with JAB 9.0, ArH),
7.12 (2 H, AAЈBBЈ system with JAB 8.8, ArH), 7.42 (1 H, dd,
J 10.0, J 1.4, 3-H); δC (100 MHz, CDCl3)22 40.20, 40.31, 40.69
(3 q, 3 × NMe2), 44.61 (d, C-5), 45.81 (t, C-6), 111.03, 111.12,
112.89 (3 d, Ar), 124.94 (d, C-2), 128.49 (d, Ar), 129.22, 129.43,
129.59 (3 s), 131.12 (d, Ar), 131.54 (s), 132.73 (d, Ar), 148.15 (d,
C-3), 149.02, 149.16, 149.90, 150.25 (4 s), 198.76 (s, C-1); m/z
(EI) 465 (Mϩ, 100%), 253 (15) (HRMS: Found m/z 465.2747.
C31H35N3O requires 465.2780).
10 W. v. E. Doering, W. R. Roth, R. Breuckmann, L. Figge,
H.-W. Lennartz, W.-D. Fessner and H. Prinzbach, Chem. Ber., 1988,
121, 1.
11 H. Mayr, C. Fichtner and A. R. Ofial, J. Chem. Soc., Perkin Trans. 2,
2002, preceding paper (DOI: 10.1039/b203554e).
12 C. U. Pittman and W. G. Miller, J. Am. Chem. Soc., 1973, 95, 2947.
13 A. C. Hopkinson, E. Lee-Ruff, T. W. Toone, P. G. Khazanie and
L. H. Dao, J. Chem. Soc., Perkin Trans. 2, 1979, 1395.
14 J. Burfeindt, M. Patz, M. Müller and H. Mayr, J. Am. Chem. Soc.,
1998, 120, 3629 with N and s adjusted to the revised parameter set in
ref. 4.
15 H. Mayr, R. Schneider, C. Schade, J. Bartl and R. Bederke, J. Am.
Chem. Soc., 1990, 112, 4446.
16 H. Mayr, R. Schneider, B. Irrgang and C. Schade, J. Am. Chem. Soc.,
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17 C. Rücker, D. Lang, J. Sauer, H. Friege and R. Sustmann, Chem.
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18 For further details, see A. D. Dilman, S. L. Ioffe and H. Mayr,
J. Org. Chem., 2001, 66, 3196.
19 For related mechanistic analyses of other Diels–Alder reactions, see
(a) H. Mayr, A. R. Ofial, J. Sauer and B. Schmied, Eur. J. Org.
Chem., 2000, 2013; (b) H. Mayr and J. Henninger, Eur. J. Org.
Chem., 1998, 1919; (c) M. Hartnagel, K. Grimm and H. Mayr,
Liebigs Ann. Chem., 1997, 71.
20 B. de Pascual-Teresa and K. N. Houk, Tetrahedron Lett., 1996, 37,
Acknowledgements
1759.
21 (a) H. Mayr and G. Hagen-Bartl, in Encyclopedia of Reagents for
Organic Reactions, ed. L. A. Paquette, Wiley, New York, 1995, vol.
8, pp. 5552–5554; (b) H. Mayr and W. Striepe, J. Org. Chem., 1985,
50, 2995.
Financial support from the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully
acknowledged. We thank Clemens Schlierf (München) for
experimental assistance.
22 Signal assignments are based on 1H,13C-HETCOR experiments.
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J. Chem. Soc., Perkin Trans. 2, 2002, 1441–1444