Electrocyclization reactions of 1-Aza- and 1-oxapentadienyl and -heptatrienyl cations: Synthesis of pyrrole and furan derivatives

Article abstract of DOI:10.1002/1099-0690(200205)2002:9<1523::AID-EJOC1523>3.0.CO;2-S

Quantum chemical DFT calculations (B3LYP/6-31+G) have been used to gain insight into the conformational and energy properties of the 1-aza- and 1-oxapentadienyl and -heptatrienyl cations 1, 2, 3, and 4. The calculated thermodynamic and kinetic data of the ring-closure reactions giving the cyclic products 5-14 are reported and discussed with respect to the experimental results. Experimentally, synthetic routes to the α,β-unsaturated carbonyl compounds 24 and 27, each with a leaving group in the γ-position, have been developed. These compounds have been investigated with respect to their ability to undergo 1,5-electrocyclization reactions to yield 2,5-disubstituted furans 28 upon heating in the presence of acid, presumably through the intermediate formation of the 1-oxapentadienyl cations 2. From the corresponding imine 29a the pyrrole 30d was obtained after treatment with tetrakis (triphenylphosphane) palladium. In the presence of benzylamine and the Pd° catalyst, the corresponding pyrroles 30a-c were formed from 24 and 27. The homologous α,β,γ,δ-unsaturated carbonyl compounds 31 afforded 2-vinyl-substituted furans 32 upon heating with acid, and the 2-vinyl-substituted pyrroles 34 on treatment with benzylamine and the Pd catalyst. No seven-membered heterocyclic rings were formed. Surprisingly, the α,β-unsaturated carbonyl compounds with two phenyl substituents at the γ-position also provided pyrrole derivatives 40 through a formal dimerization. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0690(200205)2002:9<1523::AID-EJOC1523>3.0.CO;2-S

FULL PAPER
Electrocyclization Reactions of 1-Aza- and 1-Oxapentadienyl and -heptatrienyl
Cations: Synthesis of Pyrrole and Furan Derivatives
Dirk Alickmann,[a] Roland Fröhlich,[a] Andreas H. Maulitz,[a] and
Ernst-Ulrich Würthwein*[a]
Dedicated to Prof. Dr. H. J. Schäfer on the occasion of his 65th birthday
Keywords: Cations / Density functional calculations / Electrocyclic reactions / Nitrogen heterocycles / Oxygen heterocycles
Quantum chemical DFT calculations (B3LYP/6−31+G*) have
been used to gain insight into the conformational and energy
properties of the 1-aza- and 1-oxapentadienyl and -heptatri-
imine 29a the pyrrole 30d was obtained after treatment with
tetrakis(triphenylphosphane)palladium. In the presence of
benzylamine and the Pd0 catalyst, the corresponding pyr-
enyl cations 1, 2, 3, and 4. The calculated thermodynamic roles 30a−c were formed from 24 and 27. The homologous
and kinetic data of the ring-closure reactions giving the cyc-
lic products 5−14 are reported and discussed with respect to
α,β,γ,δ-unsaturated carbonyl compounds 31 afforded 2-vinyl-
substituted furans 32 upon heating with acid, and the 2-vi-
the experimental results. Experimentally, synthetic routes to nyl-substituted pyrroles 34 on treatment with benzylamine
the α,β-unsaturated carbonyl compounds 24 and 27, each and the Pd catalyst. No seven-membered heterocyclic rings
with a leaving group in the γ-position, have been developed.
These compounds have been investigated with respect to
were formed. Surprisingly, the α,β-unsaturated carbonyl
compounds with two phenyl substituents at the γ-position
their ability to undergo 1,5-electrocyclization reactions to also provided pyrrole derivatives 40 through a formal dimer-
yield 2,5-disubstituted furans 28 upon heating in the pres- ization.
ence of acid, presumably through the intermediate formation
(
Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
of the 1-oxapentadienyl cations 2. From the corresponding 2002)
Introduction
Like 1-aza- and 1-oxaallyl cations,[1,2] 1-azapentadienyl
and 1-oxapentadienyl cations (1 and 2) are destabilized con-
jugated cations.[3] Both systems are more electrophilic than
the pentadienyl cation itself, due to the electronegative het-
eroatom at the 1-position. The mesomeric structures 1Ј, 2Ј,
1ЈЈ, and 2ЈЈ indicate their reactivity either as nitrenium/ox-
enium species with six-electron configurations on the nitro-
gen/oxygen atom, or as allyl cations with electron-with-
drawing imino/carbonyl functions (Scheme 1). In contrast,
2-azapentadienyl cations are less reactive, adopting the val-
ence isomeric structure of 1-vinyl-2-azaallenium ions.[4] The
general stability of such isomeric cations is well explained
qualitatively by the ‘‘topology charge stabilization’’ prin-
ciple, developed by Gimarc for neutral heterocycles.[5]
Scheme 1
[a]
Institut für Organische Chemie, Westfälische Wilhelms-
Universität,
Corrensstraße 40, 48149 Münster, Germany
Fax: (internat.) ϩ 49-(0)251/83-39772
One predominant monomolecular way to overcome the
inherent electronic destabilization of 1-aza- and 1-oxapen-
tadienyl cations is a cyclization reaction. This results in five-
E-mail: wurthwe@uni-muenster.de
Supporting information for this article is available on the
WWW under http://www.eurjoc.com or from the author.
Eur. J. Org. Chem. 2002, 1523Ϫ1537
WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ193X/02/0509Ϫ1523 $ 20.00ϩ.50/0
1523

D. Alickmann, R. Fröhlich, A. H. Maulitz, E.-U. Würthwein
FULL PAPER
membered heterocyclic allyl cations with additional donor
stabilization, and these produce aromatic systems after pro-
ton elimination. The analogous reaction of the 3-hydroxy-
pentadienyl cation is known as the Nazarov cyclization.[6,7]
In this report we describe for the first time the structural
and electronic properties predicted for such reactive cations
by quantum chemical calculations, together with the syn-
thesis of their precursors, their in situ generation, and their
transformation into cyclic products. The study is then ex-
tended to the generation of 1-aza- and 1-oxaheptatrienyl
cations 3 and 4, and their corresponding cyclization reac-
tions.
Quantum Chemical Calculations
In comparison to the pentadienyl and heptatrienyl
cations,[8Ϫ10] the 1-aza- and 1-oxapentadienyl cations 1 and
2 and the 1-aza- and 1-oxaheptatrienyl cations 3 and 4 dis-
play strong polarization of their electronic structures, due to
the electronegative heteroatoms. As Figure 1 (AM1 results)
demonstrates, quite substantial deviations from the sym-
metrical electron density distribution of the parent systems
are indicated by the HOMO-1[11] (for 1a, 2b, 4b) or HOMO
(for 3a) coefficients obtained by quantum chemical calcula-
tions, resulting in enhanced electron density at the het-
eroatom and decreased electron density at C2. In the oxygen
Figure 1. Graphic presentation of the frontier π-orbitals of 1a, 2b,
3a, and 4b obtained from AM1 calculations (Program PERGRA:
[a]
R. Sustmann, W. Sicking, Univ. Essen);
For 3a the HOMO is
shown; the HOMOs of 1a, 2b, and 4b correspond to the lone pairs
centered at N or O, respectively
systems 2b and 4b, carbon atoms C3, which in the TS1a-1c) and 27.4 kcal/mol (through TS1d-5) are due to
pentadienyl system represents the position of the nodal the much less favorable 1-azaallyl-type moiety, which is
plane, bear quite significant HOMO-1 coefficients. In con- formed in the transition state during the vinyl group rota-
trast, the LUMOs of 1؊4 closely resemble those of the tion. Interestingly, the five-membered heterocycle is formed
pentadienyl and heptatrienyl cations, but have smaller coef- from both transition states (TS1c-5, TS1d-5), which would
ficients at the heteroatoms. According to the AM1 charge be expected to connect the sickle type structures 1c and 1d
distribution for planar,[12] all-trans cations (MOPAC 93[13]), through U-type structures, but without involvement of such
the picture is clearly that of an acceptor-substituted allyl or defined U-shaped minimum structures (1g, 1h). Structures
pentadienyl cation, with electrophilic centers in position C3, 1g and 1h do not correspond to minima on the energy
C5, and C7 (for heptatrienyl systems) (Table 1). Other hypersurface, and the electrocyclization proceeds without
quantum chemical methods and population analyses give any additional barrier. For the parent compound, this data
similar results.
indicates only slow ring-formation at room temperature.
The 1-azapentadienyl cation 1 may in principle adopt The derivatives studied experimentally contain additional
eight conformations (Scheme 2, Table 2). For the isolated substituents, which are well suited to lower the rotation
ion in the gas phase, B3LYP/6Ϫ31ϩG* calculations barriers.
(GAUSSIAN 98[14]) predict the W-shaped structure 1a with
As well as 1,5-electrocyclization reactions, 1,3-ring-clos-
the NH group in the exo position as the global minimum, ure reactions giving azirinium or cyclopropyl cation inter-
followed by sickle-type structures 1c and 1d, also with exo- mediates 6 or 7 also have to be taken into account
NH units. The endo-NH isomers 1b, 1e, and 1f are higher (Scheme 3).[15,16] Whereas the azirinium ion 6 is calculated
in energy (5Ϫ9 kcal/mol).
to be higher in energy than 1a by 13.8 kcal/mol, the bicyclic
The electrocyclization reaction of the W-shaped 1-azap- iminium system 7, which is predicted to be formed sub-
entadienyl cation 1a experiences a series of two subsequent sequently after the 1,3-ring closure, has a total energy sim-
bond rotations to reach a sufficiently bent conformation ilar to that of 1a (1.2 kcal/mol higher), despite the ring
capable of undergoing the ring-closure reaction (see Fig- strain in this system. However, the barrier to the formation
ure 2) to form the C-protonated pyrrole 5. These bond rota- of the intermediate cyclopropyl cation is calculated to be
tions have quite different activation barriers. According to high (35.8 kcal/mol).
DFT calculations, rotation about C2ϪC3 has barriers of 8.0
In contrast to the W-shaped structure of 1, the 1-oxapen-
and 9.9 kcal/mol (through TS1c-5,TS1a-1d); here, the fav- tadienyl cation 2 is predicted to have the sickle-shape struc-
orable allyl cation is preserved during the rotation of the ture 2a as the lowest-energy conformer (Scheme 2, Table 3).
CϭN group. Rotation around C3ϪC4 is much more diffi- The W conformation 2b is slightly higher in energy (0.4
cult to achieve. The calculated barriers of 25.5 (through kcal/mol). In analogy to the 1-azapentadienyl cation, 1a,
1524
Eur. J. Org. Chem. 2002, 1523Ϫ1537

1-Aza- and 1-Oxapentadienyl and -heptatrienyl Cations
FULL PAPER
Table 1. AM1 group charges, calculated for the cations 1a and 2b (XϪC2HϪC3HϪC4HϪC5H2ϩ) and for the cations 3a and 4b
(XϪC2HϪC3HϪC4HϪC5HϪC6HϪC7H2
)
ϩ
No.
X
C2H
C3H
C4H
C5H/C5H2
C6H
C7H2
1a (X ϭ NH)
3a (X ϭ NH)
2b (X ϭ O)
4b (X ϭ O)
0.1448
0.0860
Ϫ0.1206
Ϫ0.1618
0.0211
0.0282
0.3128
0.3096
0.4365
0.3268
0.3281
0.1958
Ϫ0.0887
Ϫ0.0854
Ϫ0.0718
Ϫ0.0531
0.4863
0.3582
0.5515
0.3789
Ϫ0.0820
Ϫ0.0822
0.3682
0.4128
Figure 2. Energy diagram for the conversion of all-trans-1a into 5
according to DFT calculations (B3LYP/6Ϫ31ϩG*//B3LYP/
6Ϫ31ϩG*)
has a very low barrier of only 0.9 kcal/mol, whereas rota-
tion about C3ϪC4 is more difficult, with a calculated bar-
rier of 21.0 to 21.8 kcal/mol (TS2b-2c, TS2a-2c; 2H-oxiren-
Scheme 2
2a is able to undergo a quite exothermic electrocyclization ium contribution in the transition state). The transition
reaction to give the 2H-furyl cation 8. However, the less state for the cyclization of 2c to 8 is calculated to be only
pronounced electron donor capacity of oxygen, relative to 3.3 kcal/mol higher than 2a.
nitrogen, is reflected in the smaller calculated reaction en-
Like the 1-azapentadienyl cation 1, the 1-azaheptatrienyl
thalpy for this process (Erel ϭ Ϫ40.0 kcal/mol). As with the cation 3 prefers the all-trans W-shaped structure 3a with an
1-azapentadienyl cation 1a, two barriers have to be over- exo-NH bond (Scheme 4, Table 4). Among the many (32 !)
come for cyclization of 2a. Again, rotation about the possible conformers, some sickle-type structures (3b,c,e,-
C2ϪC3 bond (TS2a-2b; allyl cation in the transition state) i,j,k) are also low in energy (0.6Ϫ5 kcal/mol), whereas U-
Table 2. Torsion angles [°], total energies Etot [a.u.] (with number of imaginary frequencies), energies at 0 K (E0 ϭ Etot ϩ ZPE) [a.u.],
and relative energies E0rel at 0 K [kcal/mol] for various conformations of 1, for 5؊7, and related transition states (B3LYP/6Ϫ31ϩG*//
B3LYP/6Ϫ31ϩG*)
No.
Torsion angles along
HϪNϪCHϪCHϪCHϪCH2
Etot
E0
E0rel
ϩ
1a
180/180/180
0/180/180
180/180/0
180/0/180
0/180/0
Ϫ1.04/8.45/Ϫ179.92
180/0/0
170/17/Ϫ102.06/Ϫ164.54
163.5/1.00/Ϫ113.1
179.34/175.58/93.06
180.24/Ϫ89.10/176.65
178.88/96.31/Ϫ1.47
181.00/Ϫ3.44/ 94.92
Ϫ210.43686 (0)
Ϫ210.42762 (0)
Ϫ210.43045 (0)
Ϫ210.43585 (0)
Ϫ210.42231 (0)
Ϫ210.42242 (0)
Ϫ210.52358 (0)
Ϫ210.41357 (0)
Ϫ210.43717 (0)
Ϫ210.39306 (1)
Ϫ210.41930 (1)
Ϫ210.41593 (1)
Ϫ210.38876 (1)
Ϫ210.34553
Ϫ210.33661
Ϫ210.33913
Ϫ210.34443
Ϫ210.33122
Ϫ210.33164
Ϫ210.42830
Ϫ210.32362
Ϫ210.34358
Ϫ210.30491
Ϫ210.32975
Ϫ210.32638
Ϫ210.30067
0.00
5.59
4.02
0.70
8.98
1b
1c
1d
1e
1f
8.72
5
Ϫ51.94
13.75
1.22
25.49
9.90
12.02
28.15
6
7
TS 1a-1c
TS 1a-1d
TS 1c-5
TS 1d-5
Eur. J. Org. Chem. 2002, 1523Ϫ1537
1525

D. Alickmann, R. Fröhlich, A. H. Maulitz, E.-U. Würthwein
FULL PAPER
Table 3. Torsion angles [°], total energies Etot [a.u.] (with number of imaginary frequencies), energies at 0 K (E0 ϭ Etot ϩ ZPE) [a.u.],
and relative energies E0rel at 0 K [kcal/mol] for various conformations of 2, for 8, and related transition states (B3LYP/6Ϫ31ϩG*//B3LYP/
6Ϫ31ϩG*)
No.
Torsion angles along
OϪCHϪCHϪCHϪCH2
Etot
E0
E0rel
ϩ
2a
0/180
180/180
Ϫ230.28750 (0)
Ϫ230.28680 (0)
Ϫ230.28186 (0)
Ϫ230.35550 (0)
Ϫ230.28563 (1)
Ϫ230.25166 (1)
Ϫ230.25291 (1)
Ϫ230.28166 (1)
Ϫ230.20957
Ϫ230.20893
Ϫ230.20415
Ϫ230.27326
Ϫ230.20812
Ϫ230.17476
Ϫ230.17618
Ϫ230.20426
0.00
0.40
3.40
2b
2c
8
101.20/9.55
0/0
70.65/Ϫ175.27
94.28/102.24
Ϫ96.61/87.42
Ϫ25.04/62.58
Ϫ39.97
0.91
21.84
20.95
3.33
TS 2a-2b
TS 2b-2c
TS 2a-2c
TS 2c-8
Scheme 3
shaped structures (3d,g,h,l,m,o) are mostly energy-rich
(8Ϫ15 kcal/mol, except 3n). The all-cis isomer collapses
during optimization, forming the seven-membered ring sys-
tem 9.
Three possible reaction channels have been investigated
computationally: the formation from their respective con-
formers of a seven-membered C-protonated azepine 9 and
of two five-membered systems, the cationic 2-vinyldihydro-
pyrrole derivative 10 and the cyclopentenyl cation 11
(Scheme 5). The most stable cyclization product is the
Scheme 4
seven-membered ring 9 (Ϫ32.9 kcal/mol relative to 3a), fol- trans structure 4b should be slightly higher in energy (0.3
lowed by the protonated pyrrole 10 (Ϫ25.0 kcal/mol; a cyc- kcal/mol, Scheme 4). The cyclic isomers of the 1-oxahepta-
lic amino-substituted allyl cation), whereas the cyclopen- trienyl cation 4a differ little in energy. All three are lower
tenyl system 11 is similar to 3a in energy (Ϫ3.9 kcal/mol, than 4a: by 15.8 kcal/mol [2H-oxepinium ion 12, due to
no stabilization by the heteroatom). The ring-opening of 11 the longer conjugated π-electron systems (thermodynamic
affords the highest transition state energy (22.7 kcal/mol, control)], 12.5 kcal/mol (2-vinyl-2H-furanyl cation 13), and
relative to 3a), that of 9 is 18.1 kcal/mol, whereas the ring- by 11.5 kcal/mol (4-formylcyclopentenyl cation 14)
opening of 10 represents the kinetically preferred reaction, (Scheme 6, Table 5). The transition state energies relative to
with a calculated transition state energy of only 4.3 kcal/ 4a for ring-opening reactions amount to 13.6 kcal/mol for
mol.
the seven-membered ring 12, to 15.1 kcal/mol for 4-formyl-
In analogy to 2a, the oxaheptatrienyl cation 4 also adopts cyclopentenyl cation 14, and to 1.9 kcal/mol for ring-open-
a minimum-energy structure (4a) with an s-cis arrangement ing of the 2-vinyl-2H-furanyl cation 13, kinetically the most
of the CϭO group. According to the calculations, the all- readily accessible isomer.
1526
Eur. J. Org. Chem. 2002, 1523Ϫ1537

1-Aza- and 1-Oxapentadienyl and -heptatrienyl Cations
FULL PAPER
Table 4. Torsion angles [°], total energies Etot [a.u.] (with number of imaginary frequencies), energies at 0 K (E0 ϭ Etot ϩ ZPE) [a.u.],
and relative energies E0rel at 0 K [kcal/mol] for various conformations of 3, for 9؊11, and related transition states (B3LYP/6Ϫ31ϩG*//
B3LYP/6Ϫ31ϩG*)
No.
Torsion angles along
HϪNϪCHϪCHϪCHϪCHϪCHϪCH2
Etot
E0
E0rel
ϩ
3a
180/180/180/180/180
180/180/180/180/0
180/180/180/0/180
Ϫ179.85/Ϫ179.18/Ϫ177.30/22.64/29.36
180/180/0/180/180
Ϫ287.87741(0)
Ϫ287.87111(0)
Ϫ287.87216(0)
Ϫ287.86398(0)
Ϫ287.87099(0)
Ϫ287.86497(0)
Ϫ287.86164(0)
Ϫ287.85345(0)
Ϫ287.87640(0)
Ϫ287.87106(0)
Ϫ287.87018(0)
Ϫ287.86224(0)
Ϫ287.87315(0)
Ϫ287.86661(0)
Ϫ287.86404(0)
Ϫ287.93538(0)
Ϫ287.91989(0)
Ϫ287.88466(0)
Ϫ287.84800(1)
Ϫ287.87030(1)
Ϫ287.84022(1)
Ϫ287.75185
Ϫ287.74559
Ϫ287.74652
Ϫ287.73846
Ϫ287.74538
Ϫ287.73941
Ϫ287.73589
Ϫ287.72807
Ϫ287.75077
Ϫ287.74536
Ϫ287.74453
Ϫ287.73670
Ϫ287.74756
Ϫ287.74108
Ϫ287.73790
Ϫ287.80431
Ϫ287.79161
Ϫ287.75798
Ϫ287.72309
Ϫ287.74507
Ϫ287.71562
0.00
3.93
3.34
8.40
4.06
3b
3c
3d
3e
3f
180/180/0/180/0
7.81
3g
Ϫ179.85/Ϫ177.18/9.42/14.72/Ϫ179.98
177.63/178.32/13.29/29.12/28.26
180/0/180/180/180
180/0/180/180/0
180/0/180/0/180
179.54/2/19/Ϫ179.38/24.68/29.66
179.86/17.97/3.58/Ϫ173.93/179.97
Ϫ179.60/22.82/5.76/Ϫ170.58/6.11
180/0/0/0/180
Ϫ171.20/32.00/Ϫ4.39/Ϫ28.27/Ϫ5.42
178.91/Ϫ0.63/0.07/124.98/Ϫ103.98
Ϫ179.63/0.02/126.56/Ϫ0.92/Ϫ1.17
Ϫ176.17/2.20/Ϫ11.33/Ϫ41.03/Ϫ19.75
173.17/Ϫ17.62/Ϫ8.13/145.21/172.42
0.31/163.16/154.38/Ϫ15.57/Ϫ17.89
10.01
14.92
0.68
4.07
4.60
9.51
2.69
6.76
8.75
Ϫ32.92
Ϫ24.95
Ϫ3.85
18.05
4.25
3h
3i
3j
3k
3l
3m
3n
3o
9
10
11
TS 9[a]
TS 10[b]
TS 11[c]
22.73
[a]
[b]
[c]
Transition state for ring-opening of 9. Transition state for ring-opening of 10. Transition state for ring-opening of 11.
Results and Discussion
For experimental investigation of intermediate 1-aza- and
1-oxapentadienyl cations 1 and 2, two classes of possible
precursors were envisaged: 1-aza- and 1-oxapentadienes
with leaving groups either in the 3- or in the 5-position
(Scheme 7). Because of their expected low stability, N-halo-
dienamines (leaving group in 1-position) were regarded as
less valuable starting compounds for 1-azapentadienyl cat-
ions.
For the synthesis of precursors with a leaving group in 3-
position, a three-step procedure starting from crotonalde-
hyde (15) was planned. Epoxidation and subsequent imin-
ation afforded the oxiranylimine 16, which was expected to
give the allyl alcohol 3-hydroxy-N-isopropyl-1-azapenta-
1,4-diene (17) upon base-induced oxirane ring-opening
(Scheme 8). However, treatment of 16 with LDA or LDA/
KOtBu gave the aziridine 18, which is interpreted as the
result of a base-induced heterochiral dimerization by an aza
Darzens mechanism.[17] This reaction seems to be induced
by a deprotonation at the oxirane ring at the position α to
the imino group.
Scheme 5
1-Oxa-1,4-pentadienes with acetoxy groups at their 3-po-
sitions are accessible from 1,2-diketones by treatment with
vinylmagnesium bromide, as described by Trost et al.[18] In
our experiments, however, benzil (19) gave the allyl acetate
20 only in a small yield of 11% (41Ϫ76% had been re-
ported). The main product of these experiments (56%) was
the stilbene derivative 21, which was not reported by Trost
et al. (Scheme 9). The structure of 21 was elucidated by X-
ray crystallography. The formation of 21 may be explained
Scheme 6
Eur. J. Org. Chem. 2002, 1523Ϫ1537
1527

D. Alickmann, R. Fröhlich, A. H. Maulitz, E.-U. Würthwein
FULL PAPER
Table 5. Torsion angles [°], total energies Etot [a.u.] (with number of imaginary frequencies), energies at 0 K (E0 ϭ Etot ϩ ZPE) [a.u.],
and relative energies E0rel at 0 K [kcal/mol] for various conformations of 4, for 12؊14, and related transition states (B3LYP/6Ϫ31ϩG*//
B3LYP/6Ϫ31ϩG*)
No.
Torsion angles along
OϪCHϪCHϪCHϪCHϪCHϪCH2
Etot
E0
E0rel
ϩ
4a
0/180/180/180
180/180/180/180
Ϫ19.00/Ϫ1.26/173.27/Ϫ179.66
Ϫ31.04/8.24/25.21/4.62
0.60/Ϫ0.71/Ϫ119.53/Ϫ130.10
77.62/Ϫ98.17/Ϫ6.27/Ϫ5.95
Ϫ88.75/174.18/177.81/179.80
47.43/90.00/155.56/Ϫ176.68
8.65/Ϫ7.32/Ϫ48.16/Ϫ15.45
Ϫ14.50/Ϫ4.70/142.11/176.96
Ϫ164.98/154.12/Ϫ16.13/Ϫ16.56
Ϫ307.73621(0)
Ϫ307.73561(0)
Ϫ307.73518(0)
Ϫ307.76613(0)
Ϫ307.75878(0)
Ϫ307.75636(0)
Ϫ307.72972(1)
Ϫ307.69176(1)
Ϫ307.71380(1)
Ϫ307.73319(1)
Ϫ307.71123(1)
Ϫ307.62373
Ϫ307.62321
Ϫ307.62265
Ϫ307.64863
Ϫ307.64368
Ϫ307.64208
Ϫ307.61810
Ϫ307.58067
Ϫ307.60210
Ϫ307.62077
Ϫ307.59971
0.00
0.33
0.68
4b
4c
12
Ϫ15.81
Ϫ12.52
Ϫ11.51
3.53
27.02
13.57
1.86
13
14
TS 4a-4b
TS 4a-4c
TS 12[a]
TS 13[b]
TS 14[c]
15.07
[a]
[b]
[c]
Transition state for ring-opening of 12. Transition state for ring-opening of 13. Transition state for ring-opening of 14.
In contrast, precursor α,β-unsaturated aldehydes with
leaving groups in their γ-positions were successfully synthe-
sized from crotonaldehyde (15) by a route summarized in
Scheme 10. The corresponding allyl alcohols 22 were ob-
tained by the addition of Grignard reagents. After
acetylation, the allyl acetates 23 were oxidized with selen-
ium dioxide to give the aldehydes 24.
Scheme 7
Scheme 10
Alternatively, for the corresponding α,β-unsaturated al-
dehydes and ketones, a straightforward reaction sequence
(Scheme 11) starting from allyl acetate 23a was used. This
compound was converted into α-acetoxy aldehyde 25 by
ozonolysis[19] and transformed into the α,β-unsaturated car-
bonyl compounds 24a and 27 by an arsa Wittig reaction as
introduced by Huang et al.[20Ϫ22] Here, for the olefination,
we took advantage of the high acidity of arsonium salts 26
in comparison to phosphonium salts, which allowed the use
of potassium carbonate in combination with traces of
water.[23] These conditions did not affect the rather sensitive
Scheme 8
Scheme 9
in terms of an electron-transfer reduction from vinylmag- α-acetoxy aldehydes, which otherwise easily rearranged to
nesium to benzil, giving the benzil radical anion (or mag- the corresponding α-acetoxy ketones. The ketones 27a and
nesium bromide compound) and subsequent radical com- 27b were obtained in yields of 30 and 62%. The aldehyde
bination either on the carbon or on the oxygen atom of 24a, however, was subject to further homologation, which
the intermediate benzil radical anion. We therefore did not diminished its yield and caused separation problems. Con-
continue with this route for the synthesis of 1-oxa- and 1- sequently, the first method (SeO2 oxidation) was better
azapentadienyl precursors.
suited (see above) for 24a.
1528
Eur. J. Org. Chem. 2002, 1523Ϫ1537

1-Aza- and 1-Oxapentadienyl and -heptatrienyl Cations
FULL PAPER
Scheme 12
ponding pyrroles 30aϪc were obtained directly from the
carbonyl compounds when a mixture of these with benzyl-
amine was treated with the palladium catalyst as suggested
by Trost[18,24] for an isomeric α-acetoxy-β,γ-unsaturated
carbonyl compound. The mechanism was probably similar
to that mentioned above, involving an initial Pd-catalyzed
nucleophilic substitution and a subsequent ring-closure re-
action. Removal of acetic acid and of Pd affords the ob-
served pyrroles. As in Trost’s work, we observed much
better yields for ketones than for aldehydes.
Scheme 11
In a further experiment we also tried to extend the cat-
ionic electrocyclization method to 2,4-hexadien-1-ones with
As predicted by the quantum chemical calculations, the leaving groups at their 6-positions (Scheme 13). When start-
enones 24 and 27 proved to be valuable precursors for elec- ing from the corresponding anions (or lithium compounds),
trocyclization reactions. Treatment of these with aqueous we had previously observed the formation of seven-mem-
sulfuric acid yielded the furans 28 in 32Ϫ77% yield after bered rings (dihydroazepines).[25,26] The arsa Wittig reac-
workup. Obviously, under the reaction conditions, the ester tion served as an excellent homologization tool for the syn-
was subject to solvolysis, producing the 1-oxapentadienyl- thesis of the 6-acetoxy-2,4-hexadien-1-ones 31aϪc from the
ium intermediate, and isomerism to the sickle- or U-shaped aldehyde 24a. On treatment with dilute sulfuric acid, the 2-
cation then took place. The electrocyclization reaction and vinyl-substituted furans 32aϪc were obtained in moderate
a final proton elimination were the subsequent steps in yield. As predicted by the quantum chemical results, no
this sequence.
formation of the seven-membered ring systems (oxepines)
For the synthesis of pyrroles by electrocyclization, the al- was observed. Similarly, the 1-azahexadiene 33, obtained
dehydes 24 were first converted into the imines 29 by mo- from the aldehyde 31a by amine condensation, afforded the
lecular sieve assisted condensation with primary amines. 2-vinylpyrrole 34a in low yield (5%) upon treatment with
Treatment of aldimine 29a with catalytic amounts of tetra- tetrakis(triphenylphosphane)palladium. Polymeric residues
kis(triphenylphosphane)palladium in refluxing toluene pro- were the main products of this reaction. Much better yields
vided the corresponding pyrrole 30d in low yield (ca. 10%, of pyrroles 34b and 34c were again obtained from the ke-
Scheme 12). We assume the intermediate formation of a cat- tones 31b and 31c upon treatment with benzylamine and
ionic (1-azapentadienyl)palladium complex, which un- tetrakis(triphenylphosphane)palladium in refluxing toluene
dergoes the cyclization reaction. Polymeric products were (45 and 58%).
also formed.
All the electrocyclization reactions so far reported here
We were not able to isolate satisfactorily pure ketimines were accompanied by final proton elimination from the 2-
from ketones 27 and primary amines. However, the corres- position to produce the aromatic furan and pyrrole electron
Eur. J. Org. Chem. 2002, 1523Ϫ1537
1529

D. Alickmann, R. Fröhlich, A. H. Maulitz, E.-U. Würthwein
FULL PAPER
Figure 3. Molecular structure of 40a obtained by X-ray crystallo-
˚
graphy; selected bond lengths [A], bond angles [°], and torsion
Scheme 13
angles [°]: N1ϪC2 1.369(3), N1ϪCiPr 1.522(5), C2ϪC3 1.381(3),
C3ϪC4 1.437(4), C3ϪCα 1.455(3), CαϪCβ 1.346(3), CβϪCPh
1.490(3); N1ϪC2ϪC3 108.49(23), C2ϪN1ϪCiPr 111.72(20),
C2ϪN1ϪCiPr
A
138.64(23) (mean 125.18), C2ϪC3ϪCα
128.93(21), CβϪCαϪC3 131.68(21), CαϪCβϪCPh 119.83(19);
N1ϪC2ϪC3ϪCα Ϫ175.59(0.21), C2ϪC3ϪCαϪCβ Ϫ1.23(0.44),
C3ϪCαϪCβϪCPh 174.39(0.23), CiPrϪN1ϪC2ϪC3 175.11(0.29);
large thermal displacement parameters indicate splitting of the
central carbon atom of the isopropyl group (iPr, iPr A)
and Rao[28] (oxidative decarboxylation with lead tetraacet-
ate) for the preparation of 37. The diphenyl-substituted α,β-
unsaturated carbonyl compounds 38 were again synthesized
by an arsa Wittig route. Condensation with isopropylamine
produced the 1-azabutadiene 39. The conformation and
configuration of 39 in the solid state were determined by
X-ray crystallography.[29]
Treatment of 39 with the tetrakis(triphenylphosphane)-
palladium catalyst in toluene surprisingly once again pro-
duced a pyrrole derivative 40a, in 28% yield. Its structure
in the crystalline state (X-ray diffraction) is shown in Fig-
ure 3. The formation of 40a includes a dimerization of 39
and loss of an amine unit, probably by a redox process.
However, the mechanism of this unexpected reaction re-
mains to be clarified. Pyrrole 40b was obtained in low yield
(12%) directly from ketone 38b by treatment with benzyl-
amine in the presence of the Pd catalyst.
Conclusion
Scheme 14
Through the use of quantum chemical DFT calculations
we have investigated the structural, thermodynamic, and
systems. We were interested in investigating which reaction kinetic aspects of electrocyclization reactions of 1-aza- and
products might be observed if no proton was available for 1-oxapentadienyl and -hepatrienyl cations 1, 2 and 3, 4. Ex-
elimination. To answer this question, the aldehyde 38a and perimentally, suitable precursors for such cations were
the ketone 38b were prepared as depicted in Scheme 14, ac- found to be α,β-unsaturated carbonyl compounds and im-
cording to procedures reported by Johnson[27] (Darzens re- ines with leaving groups at the γ-position for 1 and 2 and
action and saponification) for 35 and 36 and by Kulkarni α,β,γ,δ-unsaturated carbonyl compounds and imines with
1530
Eur. J. Org. Chem. 2002, 1523Ϫ1537

1-Aza- and 1-Oxapentadienyl and -heptatrienyl Cations
FULL PAPER
δ ϭ 20.8 (CH3), 93.4 (CH2), 126.9 (CHarom., 2 C), 128.1 (CHarom.
,
leaving groups at the ε-position for 3 and 4. Cyclization
reactions were induced by protonation and solvolysis or by
Pd0 catalysis. 1,5-Cyclization reactions yielding furans 28
and 32 and pyrroles 30 and 34 were favored over 1,7-cyc-
lization to give seven-membered heterocycles, which was
not observed. 5,5-Diphenyl-substituted α,β-unsaturated
carbonyl compounds and imines with leaving groups at
their γ-positions surprisingly underwent dimerization to
give 3,4-bis(diphenylvinyl)-substituted pyrroles 40.
2 C), 128.2 (CHarom., 2 C), 128.25 (CHarom., 1 C), 128.3 (CHarom.
,
2 C), 128.8 (CHarom., 1 C), 133.2 (Cq), 133.3 (Cq), 137.3 (Cq), 144.0
(Cq), 149.1 (CHolef.), 169.3 (CϭO). GC-MS (70 eV): m/z (%) ϭ 280
(8) [Mϩ], 238 (84), 223 (14), 178 (4), 165 (25), 105 (100), 77 (43),
51 (8), 43 (12).C18H16O3 (280.32): calcd. C 77.12, H 5.75; found C
77.09, H 5.82. This compound has also been characterized in the
solid state by X-ray crystallography.[29]
(2E)-4-Acetoxy-4-phenylbut-2-enal (24a): 1-Acetoxy-1-phenylbut-2-
ene [23a, 15.2 g, 79.9 mmol, produced from crotonaldehyde (15)
and phenylmagnesium bromide and subsequent acetylation] was
dissolved in dry toluene (60 mL) and treated with selenium dioxide
(13.3 g, 120 mmol). The reaction mixture was heated for 2 h at re-
flux under argon. The selenium was filtered off and the solvent
was removed under reduced pressure. The product was purified by
column chromatography (petroleum ether/ethyl acetate, 5:1). Light
yellow oil, 2.25 g (11.0 mmol, 14%), b.p. 140 °C/0.2 mbar, Rf
(TLC) ϭ 0.35 (silica gel, petroleum ether/ethyl acetate, 5:1), Rt
Experimental Section
Materials and Methods: IR: PerkinϪElmer PE 298. 1H NMR:
Bruker WM 300 (300.13 MHz) and Varian Unity plus
(599.86 MHz), internal reference tetramethylsilane. 13C NMR:
Bruker WM 300 (75.47 MHz), Bruker AM 360 (360.13 MHz) and
Varian Unity plus (150.85 MHz), internal reference tetramethylsil-
ane or solvent. GC: HewlettϪPackard 6890 with HP5 quartz capil-
lary (30 m). GC/MS: Varian MAT CH7A with GC Varian 1400
plus data system SS 200; Varian MAT 8230 with GC Varian 3400
plus data system SS 300. CHN: PerkinϪElmer Dia CHN 240. Col-
umn chromatography: Kieselgel 60 (Merck), 0.063Ϫ0.200 mm.
Melting points are uncorrected. All solvents were rigorously dried
by standard methods. When necessary, the experiments were car-
ried out with complete exclusion of moisture (argon, septum-syr-
inge technique) in glassware that had been thoroughly dried by
repeated heating under argon and subsequent evacuation.
˜
(GC) ϭ 13.72 min [40Ϫ10/min-280(5)]. IR (neat): ν ϭ 3020 (w)
cmϪ1, 2920 (w), 2810 (w), 2730 (w), 1730 (vs, OϭCϪO), 1685 (vs,
CHO), 1630 (m), 1600 (sh), 1490 (w), 1440 (w), 1370 (m), 1300
1
(sh), 1220 (s), 1190 (sh), 1120 (m), 1020 (m). H NMR (600 MHz,
CDCl3): δ ϭ 2.14 (s, 3 H, CH3CϭO), 6.28 (dd, 3J ϭ 7.6, 3J ϭ
15.8 Hz, 1 H, (CHϭCHCHO), 6.49 (d, 3J ϭ 4.6 Hz, 1 H, CHϪPh),
3
3
6.86 (dd, J ϭ 4.6, J ϭ 15.8 Hz, 1 H, CHϭCHCHO), 7.33Ϫ7.45
(m, 5 H, CHarom.), 9.57 (d, 3J ϭ 7.6 Hz, 1 H, CHO). 13C NMR
(150 MHz, CDCl3): δ ϭ 20.9 (CH3), 74.0 (CHϪPh), 127.4 (o/m-
CHarom.), 128.9 (o/m-CHarom.), 129.0 (p-CHarom.), 131.2 (CHCHO),
136.5 (Cipso), 152.7 (CHϭCHCHO), 169.5 (OϭCϪO), 192.7
(CHO). GC-MS (70 eV): m/z (%) ϭ 204 (1) [Mϩ], 175 (11), 162
(72), 144 (45), 133 (86), 116 (32), 115 (100), 105 (38), 91 (14), 77
(23), 55 (14), 43 (81). C12H12O3 (204.22): calcd. C 70.57, H 5.92;
found C 70.34, H 6.12.
(E)-1-Acetoxy-2-vinyloxystilbene (21): Under argon, a solution of
vinylmagnesium bromide (1
 solution in THF, 45.7 mL,
45.7 mmol) was added dropwise to a solution of benzil (19, 9.60 g,
45.7 mmol) in THF (50 mL). The reaction mixture was then heated
to reflux for 2 h. After the mixture had cooled to room temper-
ature, a saturated aqueous solution of ammonium chloride (50 mL)
was slowly added. Hydrochloric acid (2 ) was added dropwise
until the precipitate had dissolved completely, and the layers were
separated. The aqueous layer was extracted with three portions (30
mL) of diethyl ether. The combined organic layers were dried with
magnesium sulfate. After removal of the solvent under reduced
pressure, the crude product was dissolved in dichloromethane (100
mL). This solution was cooled to 0 °C and treated with triethylam-
ine (5.50 g, 54.5 mmol), acetic anhydride (5.55 g, 54.5 mmol), and
4-(dimethylamino)pyridine (0.500 g, 4.1 mmol), and stirred for
10 h. After addition of water (20 mL) and diethyl ether (50 mL)
and stirring for 10 min, the layers were separated. The organic layer
(2E)-4-Acetoxy-hex-2-enal (24b): This compound was prepared in
a similar manner by literature procedures.[37,38]
General Procedure for the Synthesis of Unsaturated Carbonyl Com-
pounds 27 and 31: The aldehyde (1.4Ϫ20.0 mmol) was dissolved
in diethyl ether (10Ϫ50 mL). Triphenylarsonium bromide (26, 1.5
equiv.), potassium carbonate (1.5 equiv.), and water (2 equiv.) were
added. The reaction mixture was stirred under argon at room tem-
perature for 14 h. The inorganic salts were removed by filtration
through silica gel, with diethyl ether as eluent. The crude product
was purified by column chromatography.
(2E)-4-Acetoxy-1,4-diphenylbut-2-en-1-one (27a): Treatment of al-
was washed with hydrochloric acid (2 , 20 mL), sodium hydrogen dehyde 25[19] (1.78 g, 10.0 mmol) with (2-oxo-2-phenylethyl)tri-
carbonate solution (10%), and water. After drying of the organic
layer with magnesium sulfate, the solvent was removed under re-
duced pressure. The crude product was purified by column chroma-
tography (silica gel, petroleum ether/ethyl acetate, 5:1). Colorless
crystals, 5.79 g (20.7 mmol, 45%). M.p. 98 °C, Rf (TLC) ϭ 0.37
phenylarsonium bromide (26b, from triphenylarsane and α-bro-
moacetophenone in refluxing dioxane, compare ref.[32]) (7.58 g,
15.0 mmol) afforded 27a as colorless crystals (1.74 g, 6.2 mmol,
62%) after column chromatography (petroleum ether/ethyl acetate,
5:1). M.p. 76 °C, Rf (TLC) ϭ 0.35 (silica gel, petroleum ether/ethyl
(silica gel, petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ 17.66 min acetate, 5:1), Rt (GC) ϭ 20.34 min [40Ϫ10/min-280(5)]. IR (neat):
[40Ϫ10/min-280(5)]. As a minor product, 20[18] was isolated in a ν ϭ 3064 (w) cmϪ1, 2938 (w), 1739 (vs), 1653 (s), 1624 (s), 1594
˜
yield of 11%. IR (KBr): ν ϭ 3067 (w) cmϪ1, 1962 (w), 1896 (w),
(m), 1578 (m), 1494 (m), 1448 (w), 1373 (m), 1335 (m), 1287 (s),
˜
1758 (vs), 1626 (s), 1598 (m) 1575 (m), 1494 (s), 1448 (vs), 1427 1231 (vs), 1211 (s), 1089 (m), 1070 (m), 1010 (w). 1H NMR
(m), 1371 (s), 1323 (m), 1313 (m), 1261 (m), 1224 (vs), 1178 (s), (300 MHz, CDCl3): δ ϭ 2.16 (CH3), 6.50 (m, 1 H, PhCH), 7.06
1095 (s), 1072 (s), 1043 (m), 1028 (m), 1007 (m), 1000 (m). 1H (m, 2 H, CHolef.), 7.29Ϫ7.59 (m, 8 H, CHarom.), 7.90 (m, 2 H, o-
3
NMR (400 MHz, CDCl3): δ ϭ 2.08 (s, 3 H, CH3), 4.22 (dd, J ϭ CHarom.). 13C NMR (75 MHz, CDCl3): δ ϭ 21.0 (CH3), 74.7
2
3
2
16.4, J ϭ 2.0 Hz, 1 H, HCHtransϭCH), 4.60 (dd, J ϭ 14.0, J ϭ (PhCH), 125.7 [CHϪC(O)ϪPh], 127.4 (o/m-CHarom.), 128.6 (o/m-
2.0 Hz, 1 H, HCHcisϭCH), 6.31 (dd, 3J ϭ 14.0, 3J ϭ 16.4 Hz, 1 CHarom., 4 C), 128.7 (p-CHarom.), 128.8 (o/m-CHarom.), 133.0 (p-
H, HCHϭCH), 7.28Ϫ7.42 (m, 6 H, CHarom.), 7.51 (m, 2 H, CHarom.), 137.4 (Cipso), 137.5 (Cipso), 144.3 (PhCHCH), 169.6
CHarom.), 7.61 (m, 2 H, CHarom.). 13C NMR (100 MHz, CDCl3): (CH3ϪCO2), 190.1 [CHϪC(O)ϪPh]. GC-MS (70 eV): m/z (%) ϭ
Eur. J. Org. Chem. 2002, 1523Ϫ1537
1531

D. Alickmann, R. Fröhlich, A. H. Maulitz, E.-U. Würthwein
FULL PAPER
280 (0) [Mϩ], 262 (13), 238 (6) [Mϩ Ϫ Ac ϩ Hϩ], 221 (19) [Mϩ
Ϫ
138.2 (Cipso), 140.4 (PhCHCH), 143.0 (PhCHCHCHCH), 169.7
OAc], 209 (4), 191 (4), 133 (19), 115 (24), 105 (100), 91 (3), 77 (38), (OϭCϪOCH3), 190.3 (PhϪCϭO). GC-MS (70 eV): m/z (%) ϭ 306
51 (10), 43 (22). C16H18O3 (280.32): calcd. C 77.12, H 5.75; found (1) [Mϩ], 264 (26), 159 (63), 105 (100), 77 (56), 43 (44). C20H18O3
C 76.87, H 5.87.
(306.36): calcd. C 78.41, H 5.92; found C 78.30, H 5.59.
(3E)-5-Acetoxy-5-phenylpent-3-en-2-one (27b): Treatment of alde-
hyde 25[19] (3.56 g, 20.0 mmol) with (2-oxopropyl)triphenylarson-
ium bromide (26c,[22,30] 13.3 g, 30.0 mmol) afforded 27b (1.31 g,
6.00 mmol, 30%) as a yellow oil[31] after column chromatography
(petroleum ether/ethyl acetate, 5:1). Rf (TLC) ϭ 0.33 (silica gel,
petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ 14.60 min [40Ϫ10/
(3E,5E)-7-Acetoxy-7-phenylhepta-3,5-dien-2-one (31c): Treatment
of aldehyde 24a (1.03 g, 5.05 mmol) with (2-oxopropyl)triphenylar-
sonium bromide (26c, 3.32 g, 7.50 mmol) afforded 31c (692 mg,
2.83 mmol, 56%) as a yellow oil after column chromatography (pet-
roleum ether/ethyl acetate, 5:1). Rf (TLC) ϭ 0.24 (silica gel, petro-
leum ether/ethyl acetate, 5:1), Rt (GC) ϭ 17.52 min [40Ϫ10/min-
min-280(5)]. IR (neat): ν ϭ 3010 (w) cmϪ1, 2980 (w), 2900 (w),
280(5)]. IR (neat): ν ϭ 3010 (w) cmϪ1, 2980 (w), 2900 (w), 1720
˜
˜
1720 (s), 1660 (s), 1620 (s), 1480 (w), 1415 (w), 1350 (m), 1220 (s),
1160 (sh), 1060 (m), 1010 (m). H NMR (300 MHz, CDCl3): δ ϭ
2.12 (s, 3 H, CH3ϪCO2), 2.25 [s, 3 H, CHϪC(O)ϪCH3], 6.25 (dd,
(s), 1660 (s), 1620 (s), 1480 (w), 1415 (w), 1350 (m), 1220 (s), 1160
(sh), 1060 (m), 1010 (m). 1H NMR (300 MHz, CDCl3): δ ϭ 2.12 (s,
CH3ϪCO2), 2.25 [s, 3 H, CHϪC(O)ϪCH3], 6.17 [d, 3J ϭ 15.7 Hz, 1
H, CHϪC(O)ϪCH3], 6.25 (dd, 3J ϭ 15.5, 3J ϭ 5.5 Hz, 1 H,
PhCHCH), 6.36 (d, 3J ϭ 5.5 Hz, 1 H, PhCH), 6.37 (dd, 3J ϭ 15.5,
3J ϭ 10.0 Hz, 1 H, PhCHCHCH), 7.08 [dd, 3J ϭ 15.7, 3J ϭ
10.0 Hz, 1 H, CHCHC(O)ϪCH3], 7.28Ϫ7.42 (m, 5 H, CHarom.).
1
4
3
4
3J ϭ 16.2, J ϭ 1.6 Hz, 1 H, CHϪCϭO), 6.40 (dd, J ϭ 5.2, J ϭ
1.6 Hz, 1 H, PhCH), 6.81 (dd, 3J ϭ 16.2, 3J ϭ 5.2 Hz, 1 H,
PhCHCH), 7.28Ϫ7.42 (m, 5 H, CHarom.). 13C NMR (75 MHz,
CDCl3): δ ϭ 20.9 (CH3ϪCO2), 27.2 [CHϪC(O)ϪCH3], 72.2
(PhCH), 127.3 (o/m-CHarom.), 128.7 (p-CHarom.), 128.8 (o/m-
CHarom.), 130.1 [CHϪC(O)ϪCH3], 137.2 (Cipso), 143.2 (PhCHCH),
169.5 (CH3ϪCO2), 197.7 [CHϪC(O)ϪCH3]. GC-MS (70 eV): m/z
(%) ϭ 218 (1) [Mϩ], 200 (6), 176 (93), 158 (100), 147 (100), 133
(100), 117 (48), 115 (100), 105 (100), 91 (28), 77 (54), 43 (100).
13C NMR (75 MHz, CDCl3):
δ ϭ 20.9 (CH3ϪCO2), 27.2
[CHϪC(O)ϪCH3], 72.2 (PhCH), 127.1 (o/m-CHarom.), 128.5 (p-
CHarom.), 128.7 (o/m-CHarom.), 129.7 (PhCHCHCH), 131.6
[CHϪC(O)ϪCH3], 138.1 (Cipso), 140.0 (PhCHCH), 141.6
[CHCHϪC(O)ϪCH3],
169.7
(CH3ϪCO2),
198.0
[CHϪC(O)ϪCH3]. GC-MS (70 eV): m/z (%) ϭ 218 (1) [Mϩ], 200
(6), 176 (93), 158 (100), 147 (100), 133 (100), 117 (48), 115 (100),
105 (100), 91 (28), 77 (54), 43 (100).
(2E,4E)-6-Acetoxy-6-phenylhexa-2,4-dienal (31a): Treatment of al-
dehyde 24a (290 mg, 1.42 mmol) with (2-oxoethyl)triphenylarson-
ium bromide (26a, 914 mg, 2.13 mmol) afforded 31a (110 mg,
0.48 mmol, 34%) as an oil after column chromatography (petro-
leum ether/ethyl acetate, 10:1). Rf (TLC) ϭ 0.29 (silica gel, petro-
leum ether/ethyl acetate, 5:1), Rt (GC) ϭ 16.65 min [40Ϫ10/min-
General Procedure for the Synthesis of Furans 28 and 32. 2-
Phenylfuran (28a): Compound 24a (328 mg, 1.61 mmol) was dis-
solved in dioxane (10 mL) and water (10 mL) and added to 2 
sulfuric acid (10 mL). The reaction mixture was heated to reflux
for 30 min. After this had cooled to room temperature, diethyl
ether (30 mL) was added. The layers were separated and the or-
ganic layer was washed with water (10 mL) and saturated sodium
hydrogen carbonate solution (10 mL) and dried with magnesium
sulfate. Column chromatography (petroleum ether/ethyl acetate,
10:1) yielded the furan 28a (74 mg, 0.51 mmol, 32%) as a yellow
oil.[33] Rf (TLC) ϭ 0.74 (silica gel, petroleum ether/ethyl acetate,
5:1), Rt (GC) ϭ 9.54 min [40Ϫ10/min-280(5)]. IR (neat): ν˜ ϭ 3100
(sh) cmϪ1, 3040 (w), 2940 (m), 2850 (sh), 1600 (m), 1500 (w), 1465
(m), 1435 (w), 1270 (w), 1250 (s), 1210 (w), 1150 (m), 1060 (m),
1000 (vs), 950 (sh). 1H NMR: (300 MHz, CDCl3): δ ϭ 6.46 (dd,
280(5)]. IR (neat): ν ϭ 3020 (m, Ph) cmϪ1, 2980 (w, CH3), 2800 (w,
˜
CHO), 2720 (w, CHO), 1725 (s, OϭCϪO), 1670 (s, CHO), 1630 (s,
CϭC), 1595 (m, Ph), 1485 (m, Ph), 1440 (m), 1360 (s, CH3), 1220
(s, OϪCϭO), 1150 (s), 1110 (m), 1010 (s). 1H NMR (600 MHz,
3
3
CDCl3): δ ϭ 2.13 (s, 3 H, CH3), 6.17 (dd, J ϭ 7.8, J ϭ 15.6 Hz,
1
H, CHCHO), 6.34 [dd, 3J ϭ 15.2, 3J ϭ 6.0 Hz,
1 H,
PhC(OAc)HCH], 6.38 [d, 3J ϭ 6.0 Hz, 1 H, PhC(OAc)H], 6.50 (dd,
3J ϭ 10.7, 3J ϭ 15.2 Hz, 1 H, CHCHCHCHO), 7.08 (dd, 3J ϭ
10.7, 3J ϭ 15.6 Hz, 1 H, CHCHCHO), 7.30Ϫ7.42 (m, 5 H,
CHarom.), 9.54 (d, J ϭ 7.8 Hz, 1 H, CHO). 13C NMR (150 MHz,
3
CDCl3): δ ϭ 21.1 (CH3), 74.9 (PhCOAc), 127.2 (o/m-CHarom.),
128.7 (p-CHarom.), 128.8 (o/m-CHarom.), 129.1 (CHCHCHCHO),
132.8 (CHCHO), 137.8 (Cipso), 141.3 (CHCHCHCHCHO), 150.2
(CHCHCHO), 169.7 (COOCH3), 193.3 (CHO). GC-MS (70 eV):
3
3
4
3J ϭ 1.7, J ϭ 3.3 Hz, 1 H, 4-H), 6.64 (dd, J ϭ 3.3, J ϭ 0.7 Hz,
1 H, 3-H), 7.25 (m, 1 H, Carom.), 7.37 (m, 2 H, Carom.), 7.46 (dd,
3J ϭ 1.7, 4J ϭ 0.7 Hz, 1 H, 5-H), 7.67 (m, 2 H, Carom.). 13C NMR:
(75 MHz, CDCl3): δ ϭ 104.9 (C3), 111.6 (C4), 123.8 (o/m-Carom.),
127.3 (p-Carom.), 128.7 (o/m-Carom.), 131.0 (Cipso), 142.0 (C5), 154.0
(C2). GC-MS (70 eV): m/z (%) ϭ 144 (94) [Mϩ], 116 (38), 115 (100),
89 (23), 77 (5), 63 (22).
m/z (%) ϭ 230 (2) [Mϩ], 201 (19) [Mϩ Ϫ CHO], 188 (20) [Mϩ
Ϫ
OAc ϩ Hϩ], 170 (12) [Mϩ Ϫ CHO Ϫ OAc], 159 (100), 142 (27),
141 (42), 115 (38), 105 (50), 91 (30), 81 (92), 77 (25), 43 (91).
C14H14O3 (230.26): calcd. C 73.03, H 6.13; found C 72.44, H 6.10.
(2E,4E)-6-Acetoxy-1,6-diphenylhexa-2,4-dien-1-one (31b): Treat-
ment of aldehyde 24a (1.02 g, 5.0 mmol) with (2-oxo-2-phenyl-
ethyl)triphenylarsonium bromide (26b, 3.79 g, 7.50 mmol) afforded
31b (579 mg, 1.89 mmol, 38%) as a yellow oil after column chroma-
tography (petroleum ether/ethyl acetate, 5:1). Rf (TLC) ϭ 0.30 (sil-
ica gel, petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ 22.55 min
2,5-Diphenylfuran (28b): This compound was synthesized from 27a
(200 mg, 0.71 mmol). Colorless crystals, 121 mg (0.55 mmol, 77%),
m.p. 87 °C, Rf (TLC) ϭ 0.77 (silica gel, petroleum ether/ethyl acet-
1
ate, 5:1). H NMR (300 MHz, CDCl3): δ ϭ 6.67 (s, 2 H, CHfuran),
7.24 (m, 2 H, p-CHarom.), 7.36 (m, 4 H, o/m-CHarom.), 7.71 (m, 4
[40Ϫ10/min-280(5)]. IR (neat): ν ϭ 3038 (m) cmϪ1, 1700 (s), 1684
˜
H, o/m-CHarom.). 13C NMR (75 MHz, CDCl3):
δ ϭ 107.2
(s), 1600 (m), 1577 (m), 1494 (m). 1H NMR (600 MHz, CDCl3):
δ ϭ 2.13 (s, 3 H, CH3), 6.31 (dd, 3J ϭ 6.0, 3J ϭ 15.0 Hz, 1 H,
PhCHCH), 6.38 (d, 3J ϭ 6.0 Hz, 1 H, PhCH), 6.50 (dd, 3J ϭ 10.8,
3J ϭ 15.0 Hz, 1 H, PhCHCHCH), 7.00 [d, 3J ϭ 15.0 Hz, 1 H,
(CHfuran), 126.7 (o-CHarom.), 127.3 (p-CHarom.), 128.6 (m-CHarom.),
130.8 (Cipso,arom.), 153.4 (Cipso,furan).
5-Methyl-2-phenylfuran (28c): This compound was synthesized
CHC(O)Ph], 7.29Ϫ7.58 (m, 9 H, PhCHCHCHCH, CHarom.), 7.92 from 27b (200 mg, 0.92 mmol). Light yellow crystals, 72 mg
(m, 2 H, CHarom.). 13C NMR (150 MHz, CDCl3): δ ϭ 21.1 (CH3), (0.46 mmol, 50%), m.p. 38 °C (ref.:[34] 38Ϫ39 °C), Rf (TLC) ϭ 0.77
75.2 [PhCH(OAc)], 126.9 [CHC(O)Ph], 127.2, 128.3, 128.4, 128.5,
(silica gel, petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ 10.79 min
128.6, 128.7, (CHarom.), 130.1 (PhCHCHCH), 132.7, 137.9 (Cipso), [40Ϫ10/min-280(5)]. IR (neat): ν ϭ 3059 (w) cmϪ1, 2919 (w), 1596
˜
1532
Eur. J. Org. Chem. 2002, 1523Ϫ1537

1-Aza- and 1-Oxapentadienyl and -heptatrienyl Cations
(m), 1549 (s), 1488 (m), 1446 (w), 1204 (w), 1068 (w), 1022 (s). H
FULL PAPER
3020 (w) cmϪ1, 2950 (s), 2840 (m), 1730 (vs, OϭCϪO), 1650 (m),
1
NMR (300 MHz, CDCl3): δ ϭ 2.05 (s, 3 H, CH3), 6.03 (d, 3J ϭ 1620 (m), 1490 (w), 1450 (m), 1360 (m), 1220 (vs), 1150 (w), 1060
3.3 Hz, 1 H, CHfuran), 6.51 (d, 3J ϭ 3.3 Hz, 1 H, CHfuran), 7.19 (m, (w), 1015 (m). 1H NMR (300 MHz, CDCl3): δ ϭ 1.17 [d, 3J ϭ
1 H, CHarom.), 7.33 (m, 2 H, CHarom.), 7.62 (m, 2 H, CHarom.). 13C 6.2 Hz, 6 H, (CH3)2ϪCH], 2.11 (s, 3 H, CH3CϭO), 3.34 [sept, 3J ϭ
NMR (75 MHz, CDCl3): δ ϭ 13.6 (CH3), 105.8 (CHfuran), 107.7 6.2 Hz, 1 H, (CH3)2ϪCH], 6.25 (m, 1 H, CHNϪCHϭCH),
(CHfuran), 123.3 (o/m-CHarom.), 126.7 (p-CHarom.), 128.6 (o/m- 6.34Ϫ6.44 (m, 2 H, CHNϪCHϭCHϪCH), 7.28Ϫ7.38 (m, 5 H,
3
CHarom.), 131.2 (Cipso,arom.), 151.9 (Cipso, furan), 152.3 (Cipso,furan).
CHarom.), 7.89 (d, J ϭ 8.4 Hz, 1 H, CHN). 13C NMR (150 MHz,
GC-MS (70 eV): m/z (%) ϭ 158 (100) [Mϩ], 129 (16), 115 (51), 105 CDCl3):
δ
ϭ 21.0 (CH3ϪCϭO), 23.9 [(CH3)2ϪCH], 61.2
(19), 89 (8), 79 (17), 77 (21), 64 (12), 51 (26), 43 (70).
[(CH3)2ϪCH], 74.7 (CH-Ph), 127.2 (o/m-Carom.), 128.4 (p-Carom.),
128.7 (o/m-Carom.), 131.0 (CHϪCHN), 137.9 (Cipso), 140.5
(CHNϪCHϭCH), 158.8 (CHN), 169.7 (OϭCϪO). GC-MS (70
eV): m/z (%) ϭ 245 (6) [Mϩ], 230 (4) [Mϩ Ϫ CH3], 203 (31) [Mϩ
ϩ H ϪC(O)ϪCH3], 186 (100) [Mϩ Ϫ OϪCOϪCH3], 174 (21), 170
(23) [186 Ϫ CH4], 160 (25), 144 (44), 143 (30), 115 (38), 105 (7), 98
(20), 91 (9), 82 (9), 77 (8), 43 (35).
(E)-2-(2-Phenylvinyl)furan (32a):[35] This compound was synthe-
sized from 31a (230 mg, 1.00 mmol). Light yellow oil, 14 mg
(0.082 mmol, 8.2%), Rf (TLC) ϭ 0.64 (silica gel, petroleum ether/
ethyl acetate, 5:1), Rt (GC) ϭ 13.02 min [40Ϫ10/min-280(5)]. 1H
NMR (300 MHz, CDCl3): δ ϭ 6.35 (m, 1 H, 3-Hfuran), 6.42 (dd,
4
3
3J ϭ 3.3, J ϭ 1.7 Hz, 1 H, 4-Hfuran), 6.89 (d, J ϭ 16.2 Hz, 1 H,
3
(1E,3E)-5-Acetoxy-N-isopropyl-1-azahepta-1,3-diene (29b): This
compound was synthesized from 24b[37,38] (293 mg, 1.88 mmol) and
isopropylamine (133 mg, 2.25 mmol), in almost quantitative yield
and good purity (1H NMR). Colorless crystals were obtained after
recrystallization from chloroform/petroleum ether. 351 mg
(1.78 mmol, 95%), m.p. 91 °C, Rt (GC) ϭ 10.57 min [40Ϫ10/min-
280(5)]. IR (neat): ν˜ ϭ 2950 (s) cmϪ1, 2920 (m), 2850 (m), 1730
(vs), 1680 (sh), 1650 (m), 1610 (m), 1450 (m), 1430 (sh), 1360 (s),
1305 (w), 1230 (vs), 1120 (m), 1100 (w), 1070 (w), 1040 (sh), 1010
CHolef.), 7.04 (d, J ϭ 16.2 Hz, 1 H, CHolef.), 7.20Ϫ7.49 (m, 6 H,
CHarom., 5-Hfuran). GC-MS (70 eV): m/z (%) ϭ 170 (100) [Mϩ], 141
(81), 46 (115), 105 (8), 57 (36).
(E)-5-Phenyl-2-(2-phenylvinyl)furan (32b):[36] This compound was
synthesized from 31b (360 mg, 1.18 mmol). White crystals, 51 mg
(0.207 mmol, 18%), m.p. 86 °C (ref.[36] 87Ϫ88 °C), Rf (TLC) ϭ 0.79
(silica gel, petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ 20.64 min
[40Ϫ10/min-280(5)]. IR (neat): ν˜ ϭ 3060 (w) cmϪ1, 3024 (w), 2918
(w), 2860 (w), 1675 (w), 1600 (m), 1482 (s), 1450 (s), 1260 (w), 1072
(m), 1023 (s). 1H NMR (360 MHz, CDCl3): δ ϭ 6.46 (d, 3J ϭ
1
3
(m). H NMR (300 MHz, CDCl3): δ ϭ 0.93 (t, J ϭ 7.4 Hz, 3 H,
CH3CH2), 1.18 [d, 3J ϭ 6.2 Hz, 6 H, CH(CH3)2], 1.70 (m, 2 H,
CH2), 2.08 (m, 3 H, CH3CϭO), 3.35 [sept, 3J ϭ 6.2 Hz, 6 H,
CH(CH3)2], 5.33 (m, 1 H, HCOAc), 6.08 (dd, 3J ϭ 15.7, 3J ϭ
3
3.2 Hz, 1 H, CHfuran), 6.71 (d, J ϭ 3.2 Hz, 1 H, CHfuran), 6.93 (d,
3J ϭ 16.2 Hz, 1 H, CHCHPh), 7.16 (d, 3J ϭ 16.2 Hz, 1 H,
CHCHPh), 7.22Ϫ7.58 (m, 8 H, CHarom.), 7.76 (m, 2 H, CHarom.).
13C NMR (90 MHz, CDCl3): δ ϭ 107.3 (CHfuran), 111.1 (CHfuran),
116.4 (CHCHPh), 123.8 (o/m-CHarom.), 126.3 (o/m-CHarom.), 127.0,
127.4, 127.5 (2 p-CHarom., CHCHPh), 128.7 (4 o/m-CHarom.), 130.6
(Cipso,arom.), 137.1 (Cipso,arom.), 152.8 (Cipso,furan), 153.5 (Cipso,furan).
GC-MS (70 eV): m/z (%) ϭ 246 (100) [Mϩ], 217 (21), 202 (15), 168
(13), 141 (32), 123 (20), 115 (26), 105 (50), 77 (46), 51 (14).
3
3
5.5 Hz, 1 H, CHCHCHN), 6.34 (dd, J ϭ 15.7, J ϭ 8.8 Hz, 1 H,
CHCHN), 7.88 (d, J ϭ 8.8 Hz, 1 H, CHN). 13C NMR (75 MHz,
3
CDCl3): δ ϭ 9.3 (CH3CH2), 21.0 (CH3CϭO), 24.0 [CH(CH3)2],
27.1 (CH2), 61.1 [CH(CH3)2], 74.1 (CHϪOAc), 131.1 (CHCHN),
140.7 (CHCHCHN), 158.9 (CHN), 170.1 (OϪCϭO). GC-MS (70
eV): m/z (%) ϭ 197 (1) [Mϩ], 182 (4), 155 (100), 154 (62), 140 (42),
138 (77), 126 (33), 122 (28), 112 (84), 98 (65), 96 (50), 82 (33), 56
(16), 43 (69).
(E)-5-Methyl-2-(2-phenylvinyl)furan (32c):[34] This compound was
synthesized from 31c (140 mg, 0.57 mmol). Light yellow oil, 30 mg
(0.16 mmol, 28%), Rf (TLC) ϭ 0.68 (silica gel, petroleum ether/
ethyl acetate, 5:1), Rt (GC) ϭ 14.32 min [40Ϫ10/min-280(5)]. IR
(1E,3E,5E)-7-Acetoxy-N-isopropyl-7-phenyl-1-azahepta-1,3,5-triene
(33): This compound was synthesized from 31a (110 mg,
0.48 mmol), dissolved in dichloromethane (10 mL), molecular
(neat): ν ϭ 3024 (w) cmϪ1, 2962 (m), 2930 (m), 2856 (w), 1645 (w),
˚
˜
sieves (4 A, 260 mg) and isopropylamine (32 mg, 0.54 mmol) at Ϫ5
1597 (m), 1576 (w), 1532 (w), 1494 (m), 1447 (s), 1364 (w), 1260
(s), 1184 (m), 1028 (s). H NMR (600 MHz, CDCl3): δ ϭ 2.36 (s,
°C. Light brown oil, 128 mg (0.47 mmol, 98%), Rt (GC) ϭ
1
Ϫ1
˜
18.43 min [40Ϫ10/min-280(5)]. IR (neat): ν ϭ 3020 (w, Ph) cm
,
3 H, CH3), 6.01 (d, 3J ϭ 3.2 Hz, 1 H, CHfuran), 6.24 (d, 3J ϭ 3.2 Hz,
1 H, CHfuran), 6.83 (d, 3J ϭ 16.2 Hz, 1 H, CHCHPh), 6.95 (d, 3J ϭ
16.2 Hz, 1 H, CHCHPh), 7.22 (m, 1 H, CHarom.), 7.33 (m, 2 H,
CHarom.), 7.45 (m, 2 H, CHarom.). 13C NMR (150 MHz, CDCl3):
δ ϭ 13.8 (CH3), 107.8 (CHfuran), 109.9 (CHfuran), 116.7 (CHCHPh),
125.4 (CHCHPh), 126.1 (o/m-CHarom.), 127.2 (p-CHarom.), 128.6 (o/
m-CHarom.), 137.3 (Cipso,arom.), 151.7 (Cipso,furan), 152.3 (Cipso,furan).
GC-MS (70 eV): m/z (%) ϭ 184 (76) [Mϩ], 169 (29), 165 (12), 155
(16), 141 (100), 129 (13), 115 (45), 103 (6), 92 (7), 77 (19), 63 (11),
51 (13), 43 (36).
2966 (m), 2927 (m), 2861 (m), 1739 (vs), 1630 (s), 1520 (w), 1466
(m), 1370 (m), 1232 (vs). 1H NMR (400 MHz, CDCl3): δ ϭ 1.18
[d, 3J ϭ 6.4 Hz, 6 H, CH(CH3)2], 2.10 (s, 3 H, CH3CϭO), 3.35
[sept, 3J ϭ 6.4 Hz, 1 H, CH(CH3)2], 6.02 (dd, 3J ϭ 15.0, 3J ϭ
6.6 Hz, 1 H, CHCHCHCHCHN), 6.35 (d, 1 H, PhCH), 6.35 (dd,
1 H, CHCHN), 6.40 (dd, 3J ϭ 10.7 Hz, 1 H, CHCHCHCHN),
3
3
6.55 (dd, J ϭ 15.3, J ϭ 10.7 Hz, 1 H, CHCHCHN), 7.27Ϫ7.40
3
(CHarom.), 7.88 (d, J ϭ 8.9 Hz, 1 H, CHN). 13C NMR (100 MHz,
CDCl3): δ ϭ 21.0 (CH3), 24.0 [(CH3)2CH], 61.2 [(CH3)2CH], 75.3
(PhCH), 126.9 (o/m-CHarom.), 128.2 (p-CHarom.), 128.5 (o/m-
CHarom.), 130.9 (CHCHCHCHN), 133.1 (CHCHN), 135.0
(CHCHCHCHCHN), 138.6 (Cipso), 139.4 (CHCHCHN), 159.3
(CϭN), 169.6 (COOCH3). GC-MS (70 eV): m/z (%) ϭ 271 (0.3)
[Mϩ], 211 (17), 196 (4), 168 (22), 141 (10), 122 (100), 105 (5), 80
(92).
General Procedure for the Synthesis of Imines 29 and 33. (1E,3E)-
5-Acetoxy-N-isopropyl-5-phenyl-1-azapenta-1,3-diene (29a): Com-
pound 24a (0.500 g, 2.45 mmol) was dissolved in dichloromethane
˚
(20 mL). Molecular sieves (4A, 1 g) were added. At Ϫ10 °C the
reaction mixture was treated with isopropylamine (0.159 g,
2.70 mmol). After 10 h, the molecular sieves were removed by fil-
1-Isopropyl-2-phenylpyrrole (30d): Compound 29a (245 mg,
tration and washed with dichloromethane. The solvent was re- 1.00 mmol) was dissolved in dry toluene (10 mL) and treated under
moved under reduced pressure. The crude product was of good
purity; it was used for the pyrrole synthesis without further puri-
argon with tetrakis(triphenylphosphane)palladium(0) (0.116 g,
0.10 mmol). The reaction mixture was stirred at room temperature
for 20 min and then heated to reflux for 1 h. The solvent was re-
˜
fication. Light brown oil, 0.593 g (2.42 mmol, 99%). IR (neat): ν ϭ
Eur. J. Org. Chem. 2002, 1523Ϫ1537
1533

D. Alickmann, R. Fröhlich, A. H. Maulitz, E.-U. Würthwein
(w), 1602 (m), 1513 (m), 1495 (m), 1476 (m), 1452 (s), 1444 (s),
FULL PAPER
moved under reduced pressure. The crude product was purified by
column chromatography. Yellow oil,[39] 18.7 mg (0.10 mmol, 10%), 1438 (m), 1402 (s), 1355 (m), 1313 (m), 1197 (w), 1184 (w), 1119
Rf (TLC) ϭ 0.74 (silica gel, petroleum ether/ethyl acetate, 5:1), Rt
(w), 1074 (w), 1028 (m). 1H NMR (300 MHz, CDCl3): δ ϭ 2.14 (s,
(GC) ϭ 12.16 min [40Ϫ10/min-280(5)]. IR (neat): ν˜ ϭ 3058 (w) 3 H, CH3), 5.13 (s, 2 H, CH2), 6.05 (d, 3J ϭ 3.3 Hz, 1 H, CHpyrrole),
cmϪ1, 3029 (w), 2965 (m), 2924 (m), 2862 (w), 1596 (m), 1579 (m), 6.23 (d, 3J ϭ 3.3 Hz, 1 H, CHpyrrole), 6.93 (m, 2 H, o-CHbenzyl),
1509 (s), 1491 (s), 1465 (s), 1383 (w), 1365 (w), 1316 (m), 1287 (m), 7.17Ϫ7.34 (m, 8 H, CHarom.). 13C NMR (75 MHz, CDCl3): δ ϭ
1
1176 (m), 1084 (w), 1008 (w). H NMR (300 MHz, CDCl3): δ ϭ 12.6 (CH3), 47.7 (CH2), 107.2 (CHpyrrole), 108.0 (CHpyrrole), 125.6
1.40 [d, J ϭ 6.9 Hz, 6 H, (CH3)2ϪCH], 4.49 [sept, J ϭ 6.9 Hz, 1 (CHarom., 2 C), 126.6 (CHarom., 2 C), 127.0 (CHarom., 1 C), 128.3
3
3
H, (CH3)2ϪCH], 6.14 (dd, J ϭ 3.3, J ϭ 1.7 Hz, 3-Hpyrrole), 6.26 (CHarom., 2 C), 128.7 (CHarom., 3 C), 130.4 (C5pyrrole), 133.9 (Cq),
(dd, 3J ϭ 3.3, 3J ϭ 2.9 Hz, 4-Hpyrrole), 6.89 (dd, 3J ϭ 2.9, 4J ϭ 134.7 (Cq), 139.0 (Cq). GC-MS (70 eV): m/z (%) ϭ 247 (100) [Mϩ],
1.7 Hz, 5-Hpyrrole), 7.29Ϫ7.44 (m, 5 H, CHarom.). 13C NMR 156 (71), 128 (8), 91 (58), 65 (5). C18H17N (247.33): calcd. C 87.41,
(75 MHz, CDCl3): δ ϭ 24.1 [(CH3)2ϪCH], 47.1 [(CH3)2ϪCH], H 6.93, N 5.66; found C 87.55, H 7.39, N 5.82.
108.1 (CHpyrrole), 108.3 (CHpyrrole), 117.1 (CHpyrrole), 126.9 (p-Ca-
3
4
(E)-1-Isopropyl-2-(2-phenylvinyl)pyrrole (34a): This compound was
rom.), 128.3 (o/m-Carom.), 129.3 (o/m-Carom.), 134.0 (Cq), 134.0 (Cq).
synthesized from 33 (480 mg, 1.77 mmol) and tetrakis(triphenyl-
GC-MS (70 eV): m/z (%) ϭ 185 (76) [Mϩ], 170 (12) [Mϩ Ϫ CH3],
phosphane)palladium(0) (204 mg, 0.177 mmol), dissolved in dry
143 (100) [Mϩ Ϫ C3H6], 115 (45) 89 (8), 77 (6).
toluene (10 mL). Reflux under argon for 2 h. Purification by col-
General Procedure for the Synthesis of Pyrroles 30 and 34. 1-Benzyl-
umn chromatography (petroleum ether/ethyl acetate, 10:1). Yellow
2-phenylpyrrole (30a): Compound 24a (300 mg, 1.47 mmol) was oil, 20 mg (0.094 mmol, 5.3%), Rf (TLC) ϭ 0.75 (silica gel, petro-
dissolved in dry toluene (10 mL) and treated with benzylamine leum ether/ethyl acetate, 5:1), Rt (GC) ϭ 16.67 min [40Ϫ10/min-
280(5)]. IR (neat): ν ϭ 3024 (w) cmϪ1, 2973 (m), 2928 (m), 2870
˜
(270 mg, 2.52 mmol) and tetrakis(triphenylphosphane)pallad-
ium(0) (174 mg, 0.15 mmol). The reaction mixture was stirred at (sh), 1625 (w), 1596 (m), 1576 (m), 1493 (m), 1464 (s), 1367 (w),
room temperature for 20 min and then heated to reflux for 1 h. The 1288 (s), 1236 (w), 1176 (w), 1069 (w). 1H NMR (300 MHz,
solvent was removed under reduced pressure. The crude product is
purified by column chromatography (petroleum ether/ethyl acetate,
50:1). Yellow oil,[40] 41 mg (0.18 mmol, 12%), Rf (TLC) ϭ 0.78 (sil-
CDCl3): δ ϭ 1.48 [d, 3J ϭ 6.7 Hz, 6 H, CH(CH3)2], 4.53 [sept, 3J ϭ
6.4 Hz, 1 H, CH(CH3)2], 6.19 (m, 1 H, CHpyrrole), 6.48 (m, 1 H,
CHpyrrole), 6.80 (m, 1 H, CHpyrrole), 6.89 (d, 3J ϭ 16.0 Hz, 1 H,
3
ica gel, petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ 17.58 min CHCH), 7.02 (d, J ϭ 16.0 Hz, 1 H, CHCH), 7.16Ϫ7.48 (m, 5 H,
[40Ϫ10/min-280(5)]. IR (neat): ν ϭ 3061 (w, Ph) cmϪ1, 3028 (w), CHarom.). 13C NMR (150 MHz, CDCl3): δ ϭ 23.7 [(CH3)2ϪCH],
˜
2924 (w), 1602 (m), 1587 (m), 1495 (s), 1471 (m), 1451 (s), 1416 47.0 [(CH3)2ϪCH], 106.3 (CHpyrrole), 108.4 (CHpyrrole), 117.1, 117.7
1
(w), 1356 (w), 1310 (w), 1074 (w), 1028 (w). H NMR (300 MHz,
(CHpyrrol, CHCHPh), 126.0 (o/m-CHarom.), 126.3, 126.9 (p-CHarom.,
CDCl3): δ ϭ 5.06 (s, 2 H, CH2), 6.19 (m, 1 H, CHpyrrole), 6.20 (m, CHCHPh), 128.6 (o/m-CHarom.), 131.1 (Cipso,pyrrole), 138.0
1 H, CHpyrrole), 6.66 (t, 3J ϭ 2.2 Hz, 1 H, CHpyrrole), 6.90Ϫ6.95 (m, (Cipso,arom.). GC-MS (70 eV): m/z (%) ϭ 211 (99) [Mϩ], 196 (10),
2 H, o-CHbenzyl), 7.14Ϫ7.26 (m, 8 H, CHarom.). 13C NMR [Mϩ Ϫ CH3], 169 (40) [Mϩ Ϫ C3H6], 168 (100), 167 (39), 154 (5),
(150 MHz, CDCl3): δ ϭ 50.7 (CH2), 108.5 (CHpyrrole), 108.9 152 (4), 141 (14), 139 (8), 120 (21), 115 (13), 80 (7), 77 (5).
(CHpyrrole), 122.9 (C5Hpyrrole), 126.5 (CHarom.), 126.9 (CHarom.),
(E)-1-Benzyl-5-phenyl-2-(2-phenylvinyl)pyrrole (34b): This com-
pound was synthesized from 31b (300 mg, 0.98 mmol), benzylam-
127.3 (CHarom.), 128.4 (CHarom.), 128.7 (CHarom.), 128.9 (CHarom.),
133.3 (Cq), 135.0 (Cq), 138.8 (Cq). GC-MS (70 eV): m/z (%) ϭ 233
ine (170 mg, 1.59 mmol), and tetrakis(triphenylphosphane)pallad-
(55) [Mϩ], 155 (5), 142 (9), 115 (15), 99 (17), 91 (100), 85 (11), 83
ium(0) (105 mg, 0.09 mmol). Yellow solid after column chromato-
(15), 81 (10), 77 (6), 71 (20), 69 (24), 57 (36), 55 (37).
graphy, 190 mg (0.57 mmol, 58%), m.p. 105 °C, Rf (TLC) ϭ 0.62
˜
(silica gel, petroleum ether/ethyl acetate, 5:1). IR (neat): ν ϭ 3020
1-Benzyl-2,5-diphenylpyrrole (30b): This compound was synthesized
from 27b (224 mg, 0.80 mmol), benzylamine (145 mg, 1.36 mmol), (m) cmϪ1, 2922 (s), 1627 (m), 1598 (m), 1575 (m), 1494 (m), 1456
and tetrakis(triphenylphosphane)palladium(0) (92 mg, 0.08 mmol).
Colorless solid after column chromatography (petroleum ether/
ethyl acetate, 10:1).[41,42] 0.142 g (0.46 mmol, 57%), m.p. 140 °C
(s), 1416 (w), 1360 (w), 1224 (w). 1H NMR (300 MHz, CDCl3):
δ ϭ 5.23 (CH2), 6.35 (d, 3J ϭ 3.8 Hz, 1 H, CHpyrrole), 6.66 (d, 3J ϭ
3.8 Hz, 1 H, CHpyrrole), 6.80 (d, 3J ϭ 16.2 Hz, 1 H, CHCHPh),
(ref.[42] 144 °C), Rf (TLC) ϭ 0.58 (silica gel, petroleum ether/ethyl 6.90 (d, 3J ϭ 16.2 Hz, 1 H, CHCHPh), 7.02 (m, 2 H, CHarom.),
acetate, 5:1), Rt (GC) ϭ 22.48 min [40Ϫ10/min-280(5)]. IR (neat):
7.25 (m, 13 H, CHarom.). 13C NMR (75 MHz, CDCl3): δ ϭ 47.9
ν˜ ϭ 3065 (w) cmϪ1, 3026 (w), 1602 (m), 1495 (m), 1482 (m), 1463 (CH2), 107.2 (CHpyrrole), 109.9 (CHpyrrole), 117.6 (PhCHCH), 125.7,
(w), 1450 (s), 1433 (m), 1394 (w), 1361 (m), 1322 (m), 1218 (w), 125.9, 126.5, 126.9, 127.1, 127.2, 128.4, 128.5, 128.7, 128.8
1185 (w), 1120 (w), 1074 (m), 1055 (w), 1030 (m). 1H NMR (CHarom., PhCHCH), 133.1 (Cq), 133.4 (Cq), 136.6 (Cq), 137.8 (Cq),
(300 MHz, CDCl3): δ ϭ 5.22 (s, 2 H, CH2), 6.35 (s, 2 H, CHpyrrole), 138.8 (Cq). C25H21N (335.44): calcd. C 89.51, H 6.31, N 4.18; found
6.64 (m, 2 H, o-CHbenzyl), 7.05Ϫ7.40 (m, 13 H, CHarom.). 13C NMR
(75 MHz, CDCl3): δ ϭ 48.7 (CH2), 109.7 (CHpyrrole), 126.0 (o/m-
C 89.08, H 6.81, N 3.98.
(E)-1-Benzyl-5-methyl-2-(2-phenylvinyl)pyrrole (34c): This com-
pound was synthesized from 31c (202 mg, 0.827 mmol) in dry tolu-
ene (5 mL), tetrakis(triphenylphosphane)palladium(0) (113 mg,
0.098 mmol), and benzylamine (150 mg, 1.40 mmol). Light yellow
needles after column chromatography (petroleum ether/ethyl acet-
ate, 10:1). 102 mg (0.373 mmol, 45%), m.p. 104 °C, Rf (TLC) ϭ
0.73 (silica gel, petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ
CHbenzyl), 126.7 (p-CHbenzyl), 127.0 (o/m-CHbenzyl), 128.3 (CHarom.
,
6 C), 129.1 (CHarom., 4 C), 133.8 (Cq), 136.8 (Cq), 139.3 (Cq). GC-
MS (70 eV): m/z (%) ϭ 309 (71) [Mϩ], 277 (18), 218 (100), 165 (5),
91 (38), 77 (12), 65 (20), 57 (17).
1-Benzyl-5-methyl-2-phenylpyrrole (30c): This compound was syn-
thesized from 24c (180 mg, 0.825 mmol), dissolved in dry toluene
(5 mL), tetrakis(triphenylphosphane)palladium(0) (113 mg, 21.85 min [40Ϫ10/min-280(5)]. IR (KBr): ν˜ ϭ 3078 (w) cmϪ1, 3062
0.098 mmol), and benzylamine (150 mg, 1.40 mmol). Purification
by kugelrohr distillation. Light yellow oil, 133 mg (0.538 mmol,
(w), 3027 (m), 2931 (m), 2909 (w), 1628 (m), 1596 (m), 1494 (m),
1478 (s), 1452 (s), 1442 (m), 1413 (s), 1356 (m), 1308 (m), 1025 (w).
65%), b.p. 150 °C/0.1 mbar, Rt (GC) ϭ 18.38 min [40Ϫ10/min- 1H NMR (600 MHz, CDCl3): δ ϭ 2.11 (s, 3 H, CH3), 5.13 (s, 2 H,
280(5)]. IR (neat): ν ϭ 3061 (m) cmϪ1, 3029 (m), 2931 (w), 2914 CH2), 6.00 (d, J ϭ 3.3 Hz, 1 H, 4-Hpyrrole), 6.51 (d, J ϭ 3.3 Hz,
3
3
˜
1534 Eur. J. Org. Chem. 2002, 1523Ϫ1537

1-Aza- and 1-Oxapentadienyl and -heptatrienyl Cations
1 H, 3-Hpyrrole), 6.81Ϫ6.85 (m, 2 H, CHCHPh), 6.95Ϫ7.02 (m, 2 CϪO), 192.4 (CHO). GC-MS (70 eV): m/z (%) ϭ 225 (18) [Mϩ
FULL PAPER
Ϫ
H, o-CHbenzyl), 7.10Ϫ7.17 (m, 1 H, CHarom.), 7.20Ϫ7.34 (m, 7 H, CHO], 183 (100) [Mϩ Ϫ CHO Ϫ C(O)CH3], 166 (36) [Mϩ Ϫ CHO
CHarom.). 13C NMR (150 MHz, CDCl3): δ ϭ 12.5 (CH3), 46.7
(CH2), 105.9 (CHpyrrole), 107.9 (CHpyrrole), 117.5 (CHCHPh), 125.1
(CHCHPh), 125.69 (p-CHarom.), 125.74 (o/m-CHarom.), 126.6 (p-
CHarom.), 127.2 (o/m-CHarom.), 128.5 (o/m-CHarom.), 128.8 (o/m-
CHarom.), 130.7 (C5Hpyrrole), 131.4 (C2Hpyrrole), 137.9 (Cipso,arom.),
138.1 (Cipso,arom.). GC-MS (70 eV): m/z (%) ϭ 273 (88) [Mϩ], 182
(79), 167 (100), 152 (5), 139 (6), 115 (8), 91 (33), 77 (2), 65 (6).
Ϫ OϭCϪOCH3], 105 (71) [Ph2COϩ], 77 (35) [Phϩ], 51 (7) [C4H4؉].
(2E)-4-Acetoxy-4,4-diphenylbut-2-enal (38a): Compound 26a
(2.57 g, 6.00 mmol) and potassium carbonate (840 mg, 6.00 mmol)
were added to a solution of 37 (1.016 g, 4.00 mmol) in diethyl ether
(20 mL) and water (0.2 mL). After the mixture was stirred over-
night (argon) the triphenylarsinic oxide was removed by column
filtration. The crude product was purified by column chromato-
graphy (silica gel, petroleum ether/ethyl acetate, 5:1). Colorless oil,
0.274 g (0.98 mmol, 24%). Rf (TLC) ϭ 0.45 (silica gel, petroleum
ether/ethyl acetate, 5:1), Rt (GC) ϭ 19.04 min [40Ϫ10/min-280(5)].
Ethyl 2,3-Epoxy-3,3-diphenylpropionate (35):[27] A solution of pot-
assium tert-butoxide (9.25 g, 83 mmol) in tert-butyl alcohol (65
mL) was added dropwise at 10 °C to a solution of benzophenone
(13.6 g, 75 mmol) in ethyl chloroacetate (10.1 g, 82.4 mmol). After IR (neat): ν˜ ϭ 3040 (w) cmϪ1, 3010 (w), 2950 (w), 2910 (w), 2800
this was stirred for 10 h, the solvent was removed under reduced
pressure, and the residue was dissolved in diethyl ether and washed
with water. The organic layer was dried with magnesium sulfate. NMR: (300 MHz, CDCl3): δ ϭ 2.16 (s, 3 H, CH3), 5.93 (dd, J ϭ
(w), 2700 (w), 1730 (vs), 1680 (vs), 1480 (m), 1440 (m), 1360 (s),
1230 (vs), 1170 (sh), 1130 (m), 1110 (m), 1060 (m), 1010 (s). 1H
3
3
After removal of the solvent under reduced pressure, the crude 7.6, J ϭ 16.0 Hz, 1 H, CHCHO), 7.20Ϫ7.45 (m, 10 H, CHarom.),
product was purified by distillation. Colorless oil, 13.82 g
7.77 (d, 3J ϭ 16.0 Hz, 1 H, CHCq), 9.67 (d, 3J ϭ 7.6 Hz, 1 H,
(51.5 mmol, 69%), b.p. 110 °C/0.02 mbar (ref.[43] 152Ϫ153 °C/ CHO). 13C NMR (75 MHz, CDCl3): δ ϭ 21.8 (CH3), 85.7 (Cq),
˜
1 Torr), Rt (GC) ϭ 17.87 min [40Ϫ10/min-280(5)]. IR (neat): ν ϭ
126.9 (o/m-CHarom.), 128.0 (p-CHarom.), 128.3 (o/m-CHarom.), 133.7
3061 (m) cmϪ1, 3030 (w), 2982 (m), 2936 (w), 1756 (s), 1728 (s), (CHϪCHO), 141.2 (Cipso), 155.7 (Cq-CH), 168.6 (OϭCϪO), 192.9
1665 (w), 1600 (w), 1497 (m), 1449 (s), 1402 (m), 1387 (m), 1369 (CHO). GC-MS (70 eV): m/z (%) ϭ 280 (12) [Mϩ], 251 (8) [Mϩ
Ϫ
(m), 1276 (s), 1200 (s), 1156 (w), 1031 (s). 1H NMR (300 MHz, CHO], 238 (10), 220 (23), 209 (60), 207 (36), 192 (156), 191 (59),
3
CDCl3): δ ϭ 0.95 (t, J ϭ 7.2 Hz, 3 H, CH3), 3.96 (s, 1 H, CH), 178 (39), 165 (39), 133 (38), 115 (60), 105 (100), 91 (19), 77 (39), 43
3.97 (q, 3J ϭ 7.2 Hz, 2 H, CH2), 7.25Ϫ7.48 (m, 10 H, CHarom.). (59). C18H16NO2 (280.32): calcd. C 77.12, H 5.75; found C 76.89, H
13C NMR (75 MHz, CDCl3): δ ϭ 13.7 (CH3), 61.1 (CH2), 61.9 5.85.
(CH), 66.3 (Cq), 126.8 (CHarom., 2 C), 127.9 (CHarom., 2 C), 128.0
(CHarom., 2 C), 128.2 (CHarom., 1 C), 128.4 (CHarom., 3 C), 135.4
(3E)-5-Acetoxy-5,5-diphenylpent-3-en-2-one (38b): Compound 37
(2.54 g, 10.0 mmol), dissolved in diethyl ether (30 mL), was treated
(Cipso), 138.8 (Cipso), 166.7 (CϭO). GC-MS (70 eV): m/z (%) ϭ 267
with 26c, (6.65 g, 15.0 mmol), potassium carbonate (2.07 g,
(5) [Mϩ Ϫ 1], 252 (7), 240 (14), 207 (13), 194 (75), 165 (100), 152
15.0 mmol), and water (540 mg, 30.0 mmol). The reaction mixture
(14), 105 (24), 77 (14), 51 (5).
was stirred under argon for 14 h. Inorganic salts were removed by
Sodium 2,3-Epoxy-3,3-diphenylpropionate (36):[27] Compound 35 column filtration (diethyl ether). The crude product was purified
(6.30 g, 23.5 mmol) and water (0.45 g, 25 mmol) were added to a by column chromatography (silica gel, petroleum ether/ethyl acet-
solution of sodium ethoxide [from sodium (0.58 g, 25 mmol) and
anhydrous ethanol (12 mL)]. After the mixture was stirred for
ate, 5:1). Light brown solid, 1.90 g (6.45 mmol, 65%), m.p. 101 °C,
Rf (TLC) ϭ 0.40 (silica gel, petroleum ether/ethyl acetate, 5:1), Rt
˜
5 min, a colorless precipitate had formed, and this was filtered off, (GC) ϭ 19.46 min [40Ϫ10/min-280(5)]. IR (KBr): ν ϭ 3055 (w)
washed with diethyl ether, and dried at 60 °C. Colorless solid, cmϪ1, 1748 (vs), 1678 (vs), 1593 (w), 1497 (w), 1488 (w), 1368 (m),
5.68 g (21.6 mmol, 92%). M.p. 271 °C (decomp.). 1H NMR 1448 (m), 1256 (s), 1228 (vs), 1204 (m), 1175 (m), 1131 (m), 1068
(300 MHz, D2O/[D6]acetone): δ ϭ 3.83 (s, 1 H, CH), 7.15Ϫ7.45
(w), 1000 (s). 1H NMR (300 MHz, CDCl3): δ ϭ 2.15 [s, 3 H,
3
(m, 10 H, CHarom.). 13C NMR (75 MHz, D2O/[D6]acetone): δ ϭ CHϪC(O)ϪCH3], 2.30 (s, 3 H, CH3ϪCO2), 5.87 (d, J ϭ 16.2 Hz,
65.1 (CH), 67.23 (Cq), 127.9 (CHarom., 2 C), 128.6 (CHarom., 2 C), 1 H, Ph2CCHCH), 7.22Ϫ7.38 (m, 10 H, CHarom.), 7.73 (d, 3J ϭ
129.1 (CHarom., 1 C), 129.3 (CHarom., 3 C), 129.6 (CHarom., 2 C), 16.2 Hz, 1 H, Ph2CCH). 13C NMR (75 MHz, CDCl3): δ ϭ 22.0
138.0 (Cipso), 141.4 (Cipso), 173.9 (COOϪ).
(CH3ϪCO2), 27.3 [CHϪC(O)ϪCH3], 86.0 (Ph2Cq), 127.0 (o/m-
CHarom.), 127.9 (p-CHarom.), 128.3 (o/m-CHarom.), 132.2
[CHϪC(O)ϪCH3], 141.8 (Cipso), 146.5 (Ph2CqCH), 168.7
(CH3ϪCO2), 198.2 [CHϪC(O)ϪCH3]. GC-MS (70 eV): m/z (%) ϭ
294 (5) [Mϩ], 252 (6), 251 (11), 234 (15), 209 (100), 193 (80), 191
(67), 178 (28), 147 (44), 115 (51), 105 (34), 91 (19), 77 (15), 43 (61).
C19H18O3 (294.35): calcd. C 77.53, H 6.16; found C 77.40, H 6.12.
This compound has also been characterized in the solid state by
X-ray crystallography.[29]
2-Acetoxy-2,2-diphenylethanal (37): See also ref.[28] A mixture of
36 (5.24 g, 20 mmol), pyridine (1.58 g, 20 mmol), lead tetraacetate
(8.86 g, 20 mmol), and dry toluene (80 mL) was stirred under argon
for 30 min and then heated to reflux for 70 min. The reaction mix-
ture was treated with ethylene glycol in order to decompose surplus
lead tetraacetate. The precipitate was filtered off and the filtrate
was washed with water, 2  hydrochloric acid solution, and again
with water. The organic layer was dried with magnesium sulfate.
The solvent was removed under reduced pressure. The crude prod-
uct was purified by column chromatography (silica gel, petroleum
ether/ethyl acetate, 5:1). Colorless crystals at 0° C, oil at room tem-
(1E,3E)-5-Acetoxy-N-isopropyl-5,5-diphenyl-1-azapenta-1,3-diene
(39): A mixture of 38a (0.231 g, 0.824 mmol), dissolved in dry
˚
dichloromethane (10 mL), and molecular sieves (4 A, 1 g) was
perature, 3.21 g (12.6 mmol, 63%) (ref.:[28] 55%), Rf (TLC) ϭ 0.38 treated at Ϫ10 °C, with slow stirring, with isopropylamine (53 mg,
(silica gel, petroleum ether/ethyl acetate, 5:1), Rt (GC) ϭ 17.08 min 0.90 mmol). After the mixture was stirred overnight, the molecular
[40Ϫ10/min-280(5)]. IR (neat): ν ϭ 3040 (w) cmϪ1, 2800 (w), 2700 sieves were filtered off and washed with dichloromethane. The solv-
˜
(w), 1720 (vs), 1480 (m), 1440 (m), 1360 (s), 1230 (vs), 1170 (s), ent was removed under reduced pressure. The crude product was of
1010 (s). 1H NMR (300 MHz, CDCl3): δ ϭ 2.20 (s, 3 H, CH3), good purity (1H NMR) and was used without further purification.
7.28Ϫ7.48 (m, 10 H, CHarom.), 9.70 (s, 1 H, CHO). 13C NMR Recrystallisation from dichloromethane/petroleum ether yielded
(75 MHz, CDCl3): δ ϭ 21.0 (CH3), 87.7 (Cq), 127.9 (o/m-CHarom.),
128.4 (o/m-CHarom.), 128.5 (p-CHarom.), 137.3 (Cipso), 169.6 (Oϭ
colorless rectangular crystals, 0.252 mg (0.784 mmol, 95%). m.p.
115Ϫ118 °C. IR (neat): ν ϭ 3040 (w) cmϪ1, 3010 (w), 2940 (s),
˜
Eur. J. Org. Chem. 2002, 1523Ϫ1537
1535

D. Alickmann, R. Fröhlich, A. H. Maulitz, E.-U. Würthwein
FULL PAPER
2910 (w), 2840 (w), 1730 (vs), 1635 (w), 1600 (m), 1480 (m), 1460
10.9 (CH3), 46.7 (CH2), 118.1 (C3pyrrole), 123.5 (CHolef.), 124.9
(w), 1440 (m), 1360 (m), 1230 (vs), 1200 (m), 1140 (w), 1080 (w), (C2pyrroleϪCH3), 125.5 (o-CHbenzyl), 126.2 (2 C), 126.8 (2 C), 126.9
1060 (w), 1010 (m). 1H NMR: (300 MHz, CDCl3): δ ϭ 1.15 [d, (p-CHbenzyl), 127.79 (o/m-CHphenyl), 127.80 (o/m-CHphenyl), 128.1
3J ϭ 6.4 Hz, 6 H, CHϪ(CH3)2], 2.12 (s, 3 H, OϭCϪCH3), 3.34 (o/m-CHphenyl), 128.5 (2 C), 130.8 (o/m-CHphenyl), 138.0 (CH2Cipso),
[sept, 3J ϭ 6.4 Hz, 1 H, CHϪ(CH3)2], 5.99 (dd, 3J ϭ 9.0, 3J ϭ
141.2 (Cipso,phenyl), 141.9 (Cipso,phenyl), 144.2 (Cq,olef.). MS (70 eV):
16.0 Hz, 1 H, CHCHN), 7.20Ϫ7.38 (m, 11 H, CHarom., CqϪCH), m/z (%) ϭ 541 (100) [Mϩ], 450 (5), 435 (7), 167 (31), 105 (21), 91
8.00 (d, 3J ϭ 9.0 Hz, 1 H, CHN). 13C NMR: (75 MHz, CDCl3): (50), 77 (13), 57 (19), 55 (10). MS (high resolution, 70 eV): calcd.
δ ϭ 22.0 (CH3CϭO), 23.9 [CHϪ(CH3)2], 61.1 [CHϪ(CH3)2], 86.6 541.27698; found 541.27571. As a by-product, 5,5-diphenylpent-4-
(Cq), 126.9 (o/m-CHarom.), 128.0 (p-CHarom.), 128.3 (o/m-CHarom.), en-2-one[44] was separated by column chromatography. White solid,
133.7 (Cipso), 141.2, 155.7 (CHϭCHϪCHO), 159.2 (CHN), 168.8 65 mg (0.275 mmol, 11%), Rf (TLC) ϭ 0.45 (silica gel, petroleum
(OϭCϪO). MS (70 eV): m/z (%) ϭ 321 (46) [Mϩ], 279 (40), 278 ether/ethyl acetate, 5:1). 1H NMR (300 MHz, CDCl3): δ ϭ 2.09 (s,
3
3
(56), 263 (20), 262 (26), 250 (31), 236 (65), 220 (24), 209 (18), 203 3 H, CH3), 3.25 (d, J ϭ 7.3 Hz, 2 H, CH2), 6.28 (t, J ϭ 7.3 Hz,
(19), 192 (49), 191 (100), 178 (14), 174 (31), 167 (33), 165 (25), 139 2 H, CHCH2), 7.10Ϫ7.39 (m, 10 H, CHarom.). 13C NMR (75 MHz,
(16), 115 (46), 105 (43), 91 (14), 77 (30), 70 (33). C21H23NO2 CDCl3): δ ϭ 29.8 (CH3), 44.7 (CH2), 120.5 (CHolef.), 127.3
(321.41): calcd. C 78.47, H 7.21, N 4.36; found C 77.61, H 7.31, N
(CHarom., 4 C), 128.1 (CHarom., 2 C), 128.4 (CHarom., 2 C), 129.6
3.99. This compound has also been characterized in the solid state (CHarom., 2 C), 139.5 (Cipso), 141.9 (Cipso), 144.9 (CqCHCH2), 206.4
by X-ray crystallography.[29]
(CϭO).
3,4-Bis(2,2-diphenylvinyl)-1-isopropylpyrrole (40a): Under argon,
compound 39 (180 mg, 0.56 mmol), dissolved in dry toluene (10
mL), was treated with tetrakis(triphenylphosphane)palladium(0)
(65 mg, 0.056 mmol). The reaction mixture was stirred at room
temperature for 20 min. The mixture was then heated to reflux for
2 h. The solvent was removed under reduced pressure. The crude
product was purified by column chromatography (silica gel, petro-
leum ether/ethyl acetate, 50:1). Colorless solid, 37 mg (0.079 mmol,
Acknowledgments
Financial support by the Deutsche Forschungsgemeinschaft (Grad-
uiertenkolleg ‘‘Hochreaktive Mehrfachbindungssysteme’’ and
Sonderforschungsbereich 424) and by the Fonds der Chemischen
Industrie (Frankfurt) is gratefully acknowledged.
28%), m.p. 170 °C. IR (neat): ν ϭ 3075 (w) cmϪ1, 3025 (w), 2969
˜
[1]
(m), 2926 (w), 1661 (w), 1617 (w), 1593 (m), 1487 (s), 1445 (s), 1418
(s), 1368 (m), 1277 (w), 1188 (m), 1135 (m), 1071 (m), 1031 (w).
1H NMR (300 MHz, CDCl3): δ ϭ 0.97 [d, 3J ϭ 6.7 Hz, 6 H,
´
N. De Kimpe, R. Verhe, L. De Buyck, N. Schamp, Tetrahedron
Lett. 1982, 23, 2853Ϫ2856.
[2]
R. Flammang, S. Lacombe, A. Laurent, A. Maquestiau, B.
Marquet, S. Novkova, Tetrahedron 1986, 42, 315Ϫ328.
X. Creary, Chem. Rev. 1991, 91, 1625Ϫ1678.
3
CH(CH3)2], 3.60 [sept, J ϭ 6.7 Hz, 1 H, CH(CH3)2], 5.46 (s, 2 H,
[3]
[4]
CHpyrrole), 6.69 (s, 1 H,), 7.10Ϫ7.80 (m, 22 H, CHarom., CHolef.).
13C NMR (75 MHz, CDCl3): δ ϭ 23.1 (CH3), 46.7 [CH(CH3)2],
117.5, 119.6, 120.3 (Cq, pyrrol), 126.5, 126.8 (o/m-CHarom.), 126.9,
128.1 (o/m-CHarom.), 128.8 (o/m-CHarom.), 130.2 (o/m-CHarom.),
137.7 (Cq), 142.1 (Cq), 143.3 (Cq). MS (70 eV): m/z (%) ϭ 465 (100)
[Mϩ], 422 (3), 344 (3), 167 (5), 57 (9). C35H31N (465.63): calcd. C
90.28, H 6.71, N 3.01; found C 89.63, H 6.67, N 2.65.
J.-P. Moldenhauer, M. H. Möller, U. Rodewald, E.-U.
Würthwein, Liebigs Ann. 1995, 1015Ϫ1025.
B. Gimarc, J. Am. Chem. Soc. 1983, 105, 1979Ϫ1984.
K. L. Habermas, S. E. Denmark, T. K. Jones, Org. React. 1994,
54, 1Ϫ158.
[5]
[6]
[7]
[8]
D. A. Smith, C. W. Ulmer, II, J. Org. Chem. 1997, 62,
5110Ϫ5115.
T. S. Sorensen in: G. A. Olah Carbonium Ions (Ed.: P. v. R.
Schleyer), 1970, vol. 2, §19, p. 807Ϫ835.
E. A. Kallel, K. N. Houk, J. Org. Chem. 1989, 54, 6006Ϫ6008.
L. M. Pratt, A. Streitwieser, J. Org. Chem. 2000, 65, 290Ϫ294.
The AM1-HOMOs of 1a, 2b, and 4b correspond to lone pairs
at the heteroatoms.
Full AM1 geometry optimizations of 2a and 4a produce non-
planar structures with perpendicular conformations of the Cϭ
O groups, which is in contrast to the DFT results (see Table 2).
MOPAC93: J. J. P. Stewart, Fujitsu Limited, Tokyo, Japan,
1993.
X-ray Crystal Structure Analysis of 40a:[29] Empirical formula
C35H31N, M ϭ 465.61, colorless crystal 0.35 ϫ 0.20 ϫ 0.05 mm,
[9]
3
˚
˚
[10]
[11]
a ϭ 6.022(2), b ϭ 18.801(5), c ϭ 23.227(4) A, V ϭ 2629.8(12) A ,
ρcalcd. ϭ 1.176 g cmϪ3, µ ϭ 5.08 cmϪ1, no absorption correction
(0.842 Յ T Յ 0.975), Z ϭ 4, orthorhombic, space group Pbcn (No.
[12]
˚
60), λ ϭ 1.54178 A, T ϭ 223 K, ω/2θ scans, 2691 reflections col-
lected (ϩh, Ϫk, Ϫl), [(sinθ)/λ] ϭ 0.62 AϪ1, 2691 independent and
˚
1800 observed reflections [I Ն 2 σ(I)], 169 refined parameters, R ϭ
[13]
[14]
0.074, wR2 ϭ 0.182, max. residual electron density 0.43 (Ϫ0.33)
e AϪ3, due to symmetry disorder in the isopropyl group, hydrogen
˚
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.
A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgom-
ery, Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Mil-
lam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J.
Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Po-
melli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P.
Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B.
B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.
Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres,
C. Gonzalez, M. Head-Gordon, E. S. Replogle, and J. A. Pople,
Gaussian98, Revision A.6, Gaussian, Inc., Pittsburgh, PA, 1998,
and earlier versions of this program.
atoms calculated and refined as riding atoms.
1-Benzyl-3,4-bis(2,2-diphenylvinyl)-2,5-dimethylpyrrole (40b): Com-
pound 38b (738 mg, 2.51 mmol), dissolved in dry toluene (10 mL),
was treated with tetrakis(triphenylphosphane)palladium(0)
(400 mg, 0.35 mmol) and benzylamine (455 mg, 4.25 mmol). The
reaction mixture was heated to reflux for 1 h. After removal of the
solvent under reduced pressure, the crude product was purified by
column chromatography (silica gel, petroleum ether/ethyl acetate,
10:1). Yellow oil, 84 mg (0.155 mmol, 12%), Rf (TLC) ϭ 0.58 (silica
˜
gel, petroleum ether/ethyl acetate, 5:1). IR (neat): ν ϭ 3055 (w)
cmϪ1, 3025 (w), 2973 (m), 2926 (m), 2854 (m), 1596 (m), 1494 (s),
1453 (s), 1408 (m), 1382 (m), 1373 (m), 1351 (m), 1118 (m), 1078
(w), 1029 (w). 1H NMR (600 MHz, CDCl3): δ ϭ 1.57 (s, 6 H, CH3),
4.76 (s, 2 H, CH2), 6.66 (m, 2 H, o-CHbenzyl), 6.69 (s, 1 H, CHolef.),
7.10Ϫ7.30 (m, 23 H, CHarom.). 13C NMR (150 MHz, CDCl3): δ ϭ
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Received October 8, 2001
[O01483]
Eur. J. Org. Chem. 2002, 1523Ϫ1537
1537