Synthesis of cationic dibenzosemibullvalene-based phase-transfer catalysts by di-π-methane rearrangements of pyrrolinium-annelated dibenzobarrelene derivatives

Article abstract of DOI:10.3762/bjoc.7.17

Dibenzobarrelene derivatives, that are annelated with a pyrrolinium unit [N,N-dialkyl-3,4-(9′,10′-dihydro-9′,10′-anthraceno-3- pyrrolinium) derivatives], undergo a photo-induced di-π-methane rearrangement upon triplet sensitization to give the corresponding cationic dibenzosemibullvalene derivatives [N,N-dialkyl-3,4-{8c,8e-(4b,8b- dihydrodibenzo[a,f]cyclopropa[cd]pentaleno)}-pyrrolidinium derivatives]. Whereas the covalent attachment of a benzophenone functionality to the pyrrolinium nitrogen atom did not result in an internal triplet sensitization, the introduction of a benzophenone unit as part of the counter ion enables the di-π-methane rearrangement of the dibenzobarrelene derivative in the solid-state. Preliminary experiments indicate that a cationic pyrrolidinium-annelated dibenzosemibullvalene may act as phase-transfer catalyst in alkylation reactions.

Full text of DOI:10.3762/bjoc.7.17

Synthesis of cationic dibenzosemibullvalene-based
phase-transfer catalysts by di-π-methane
rearrangements of pyrrolinium-annelated
dibenzobarrelene derivatives
Heiko Ihmels* and Jia Luo
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 119–126.
Organic Chemistry II, University of Siegen, Adolf-Reichwein-Str. 2,
D-57068 Siegen, Germany
doi:10.3762/bjoc.7.17
Received: 29 October 2010
Accepted: 09 December 2010
Published: 26 January 2011
Email:
Heiko Ihmels* - ihmels@chemie.uni-siegen.de
* Corresponding author
This article is part of the Thematic Series "Photocycloadditions and
photorearrangements".
Keywords:
di-π-methane rearrangement; dibenzobarrelenes;
dibenzosemibullvalenes; phase-transfer catalysis; photochemistry;
polycylic compounds
Guest Editor: A. G. Griesbeck
© 2011 Ihmels and Luo; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Dibenzobarrelene derivatives, that are annelated with a pyrrolinium unit [N,N-dialkyl-3,4-(9',10'-dihydro-9',10'-anthraceno-3-
pyrrolinium) derivatives], undergo a photo-induced di-π-methane rearrangement upon triplet sensitization to give the corres-
ponding cationic dibenzosemibullvalene derivatives [N,N-dialkyl-3,4-{8c,8e-(4b,8b-dihydrodibenzo[a,f]cyclopropa[cd]pentaleno)}-
pyrrolidinium derivatives]. Whereas the covalent attachment of a benzophenone functionality to the pyrrolinium nitrogen atom did
not result in an internal triplet sensitization, the introduction of a benzophenone unit as part of the counter ion enables the
di-π-methane rearrangement of the dibenzobarrelene derivative in the solid-state. Preliminary experiments indicate that a cationic
pyrrolidinium-annelated dibenzosemibullvalene may act as phase-transfer catalyst in alkylation reactions.
Introduction
The di-π-methane (DPM) rearrangement is among the most rearrangement of dibenzobarrelene (DBB), also termed diben-
thoroughly investigated organic photoreactions [1,2]. Since the zobicyclo[2.2.2]octatriene, and its derivatives has been investi-
discovery of the DPM rearrangement of dibenzobarrelene [3], gated in detail and has provided significant insights into the
extensive studies have been carried out to assess the mecha- mechanism of the DPM rearrangement [6,7]. The DPM
nistic aspects of this reaction and to explore their application in rearrangement of DBB proceeds according to the mechanism
synthetic organic chemistry [1-5]. Along these lines, the DPM shown in Scheme 1 through biradical intermediates BR1 and
119

Beilstein J. Org. Chem. 2011, 7, 119–126.
BR2 and leads to the formation of the corresponding diben-
zosemibullvalene (DBS) [6,7]. Notably, the photoreactivity of
the dibenzobarrelene system is multiplicity-dependent: Upon
photoinduced triplet sensitization in the presence of an appro-
priate sensitizer, such as acetone or benzophenone, dibenzo-
barrelene (DBB) rearranges to dibenzosemibullvalene (DBS),
whereas the direct excitation leads to dibenzocyclooctene
(DBC) through the singlet excited-state (Scheme 1) [4,5,7]. The
DPM photorearrangement of dibenzobarrelene and its deriva-
tives is a synthetically useful reaction, because it allows the
preparation of polycyclic structures which are relatively diffi-
cult to obtain by ground-state transformations [8].
Figure 1: General structure of pyrrolidinium-annelated diben-
zosemibullvalenes (pyDBS).
Note that according to the IUPAC rules the compounds
During our attempts to develop novel ammonium-based phase- presented in the following should be classified as pyrrolidinium
transfer catalysts [9-11] that are embedded within a rigid struc- or pyrrolinium derivatives because of the higher priority of the
ture, we noticed that the complex dibenzosemibullvalene struc- cationic heterocycles; however, for clarity and to keep the focus
ture may constitute a reasonable starting point. We proposed on the photochemical reaction we chose to name these com-
that the general structure pyDBS (Figure 1), as established by pounds dibenzobarrelene and dibenzosemibullvalene deriva-
the annelation of a pyrrolidinium unit to the dibenzosemibullva- tives throughout the text.
lene structure [12], represents an amphiphilic tetraalkyl-
ammonium derivative that exhibits three different benzene- Results and Discussion
containing concave sites to which an organic substrate may as- The cationic dibenzobarrelene derivatives 2a–g were synthe-
sociate by attractive van der Waals interactions or π-stacking. sized by the base-catalyzed reaction between the dibro-
Moreover, additional functionalities, e.g., hydroxy or amino momethyl-substituted dibenzobarrelene derivative 1 [14] with
groups, may be attached to the benzene rings of the semibullva- selected secondary amines (Scheme 2). The reaction was
lene structure to enable additional hydrogen bonding with the initially performed with a slight excess of the amine and DBU
substrate. Also, the benzene rings may be further annelated with as a base in dichloromethane at room temperature, but under
additional aromatic units to increase the potential π-stacking these conditions the products could not be completely sep-
area [13]. Most notably, dibenzosemibullvalenes are chiral arated from the remaining amine or the DBU catalyst, even by
compounds, so that the propensity of an enantiopure pyDBS column chromatography. Nevertheless, the products were avail-
derivative to act as phase-transfer catalyst in stereoselective able in reasonable yields (47–85%) when the reaction was
reactions may be considered in long-term studies. Herein we performed with polymer-bound DBU as catalyst (2a–f: 0.1
demonstrate that the pyrrolidinium-annelated dibenzosemibull- equiv; 2g: 0.5 equiv) or when the quaternary ammonium deriva-
valene structure is indeed available via the DPM rearrangement tives were precipitated from aqueous solutions by the addition
of appropriately substituted dibenzobarrelene derivatives and of perchloric acid to the reaction mixture and, if necessary,
that such a compound may act as phase-transfer catalyst in subsequent column chromatography. All products were identi-
alkylation reactions.
fied and fully characterized by NMR spectroscopy, mass spec-
Scheme 1: Photorearrangements of dibenzobarrelene (DBB).
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Beilstein J. Org. Chem. 2011, 7, 119–126.
Scheme 2: Synthesis of dibenzobarrelene derivatives 2a–g.
trometry and elemental analyses. In general, the perchlorate zylic position at C4b (ca. 4.9 ppm) and of the cyclopropyl
salts of 2a–g have good solubility in aprotic polar solvents, protons at C8b (ca. 4.0 ppm) (Scheme 3). Direct irradiation
especially in acetone and acetonitrile. To confer water-solu- (quartz filter, λ > 254 nm) of the dibenzobarrelene derivatives
bility on the cationic dibenzobarrelene derivatives 2b and 2d, 2a–d resulted in complex reaction mixtures in which the char-
the perchlorate salts were converted quantitatively to chlorides acteristic 1H NMR signals of dibenzosemibullvalene or diben-
by ion exchange with chloride ions on an ion exchange resin.
zocyclooctene products were not detected. Presumably, the
photo-induced cleavage of the C–N bonds, i.e., a well-known
The photolysis of the cationic dibenzobarrelene derivatives photo-fragmentation of quaternary ammonium salts to give
2a–d, which bear two identical N-alkyl substituents, was carried highly reactive radical intermediates [15,16], is a significant
out in acetone solutions (λ > 310 nm), and the conversion of the side-reaction during the direct photolysis of compounds 2a–d,
dibenzobarrelene was monitored by 1H NMR spectroscopy. The leading to the formation of complex reaction mixtures. The
dibenzosemibullvalene derivatives 3a–d were isolated by crys- triplet-sensitized photoreaction of the dibenzobarrelene deriva-
tallization directly from the photolysate in 67–85% yield. The tives 2e,f, which carry two different alkyl substituents at the
photoproducts were identified and characterized by 1H NMR quaternary pyrrolidinium, gave two isomeric dibenzosemibull-
and 13C NMR spectroscopy, mass spectrometry, and elemental valene products 3eI,3fI and 3eII,3fII , in a ratio of approxi-
analyses. The dibenzosemibullvalene structure of the photo- mately 60:40 as determined by 1H NMR spectroscopic analysis
products 3a–d was confirmed by 1H NMR spectroscopy, specif- of the reaction mixture (Scheme 3). The comparison was based
ically by the characteristic singlets of the protons at the diben- on the two characteristic singlets of the protons at C4b and C8b
Scheme 3: Di-π-methane rearrangements of dibenzobarrelene derivatives 2a–f (counter ions omitted for clarity).
121

Beilstein J. Org. Chem. 2011, 7, 119–126.
of the dibenzosemibullvalene ring (3eI: 4.30, 4.75; 3eII: 4.17, The analysis of the photolysate by 1H NMR spectroscopy
4.91; 3fI: 4.31, 5.05 ppm; 3fII: 4.20, 5.02, in CDCl3). After revealed that independent of the solvent, the two semibull-
column chromatography and subsequent recrystallization, the valenes 3gI and 3gII were formed in a 45:55 ratio; however,
major photoproducts 3eI and 3fI were isolated in low yields attempts to separate the two regioisomers by chromatography
(15% and 18%). The relative configuration of the ammonium were unsuccessful (Scheme 4). Derivative 2g was also
functionality was deduced from NOE experiments. Specifically, photoinert in the solid-state.
the close proximity between the methyl group and the methine
proton (8b’-H) of the cyclopropane ring gave rise to an NOE in The lack of internal photosensitization of derivative 2g resem-
3eI and 3fI (Scheme 3). The structure of the minor products 3eII bles the behavior of the hydrochloride salt of a cationic ammo-
and 3fII, although not isolated, were determined by 1H NMR niummethyl-substituted dibenzobarrelene derivative [18] which
spectroscopic analyses of the photolysates. In these cases, the is also photoinert in the presence of benzophenone and even
dibenzosemibullvalene structure was indicated by the character- acetone. It may be assumed that attachment of the benzophe-
istic 1H NMR spectroscopic shifts of the 4b-H and 8b-H none unit to the cationic dibenzobarrelene structure has an
protons.
influence on the intersystem-crossing (ISC) rate or the triplet
energy of the carbonyl functionality leading to insufficient
It has been demonstrated with several examples that solid-state sensitization. In addition, competing deactivation pathways of
photoreactions are an excellent tool to induce highly selective the excited ketone may be considered, such as rotation about the
di-π-methane rearrangements of dibenzobarrelene derivatives Car–CH2 group or reaction with the chloride counter ion.
[17], because the constrained medium within the crystal lattice However, further attempts to clarify this aspect or to optimize
allows only limited molecular movement, so that the reaction the conditions for sensitization were not made, because the
pathway with the least required molecular motion is preferred. photoreaction of 2g upon external sensitization by acetone
Accordingly, the solid-state photoreactivity of the dibenzo- showed that the products of the DPM rearrangement of 2g are
barrelene derivatives 2a–f was investigated; but unfortunately, formed only slowly in relatively low yield and cannot be sep-
all tested derivatives, either as chloride or as perchlorate salts, arated.
were photoinert in the crystalline state. Since one of the
possible reasons for this photoinertness may be the lack of Since the covalent attachment of a sensitizer unit to the
proper sensitization, the dibenzobarrelene derivative 2g was dibenzobarrelene chromophore did not induce the desired triplet
prepared with a triplet-sensitizing functionality attached, sensitization, the ionic auxiliary strategy [19] was applied to
namely the benzophenone unit. Although in acetone the achieve triplet-sensitization in the solid-state, i.e., the sensitizer
dibenzobarrelene 2g underwent a DPM rearrangement with full was introduced as counter anion. For this purpose, an anionic
conversion (Scheme 4), the irradiation of 2g in water or acetoni- derivative of benzophenone was prepared and associated with
trile solution in the absence of an external sensitizer only the ammonium functionality of the dibenzobarrelene 2b by
induced a relatively low conversion of 2g. Thus, irradiation of anion metathesis (Scheme 5). The known sulfonic acid 4 [20]
2g in acetonitrile for 12 h led to ca. 10% conversion with ca. was transformed to the corresponding silver salt Ag-4 by reac-
60% dibenzosemibullvalenes formed. At the same time, several tion with Ag2O, and subsequent ion metathesis upon treatment
unidentified byproducts were formed in significant amounts. with 2b-Cl in acetonitrile gave the salt 2b-4 in 73% yield.
Scheme 4: Di-π-methane rearrangement of dibenzobarrelene derivative 2g.
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Beilstein J. Org. Chem. 2011, 7, 119–126.
Scheme 5: Synthesis and solid-state photoreactivity of the sulfonate salt 2b-4.
Irradiation of ground crystals of the salt 2b-4 for 2 h induced a barrelene derivatives may be achieved in a solid-state reaction
DPM rearrangement to the dibenzosemibullvalene derivative 3b with a sensitizing counter ion; however, the sensitization by
with full conversion and without any detectable byproducts as acetone solvent appears to be more practical as it allows the
determined by 1H NMR spectroscopy. A solid-state photoreac- handling of larger scales.
tion was also performed by the irradiation of a suspension of
crystalline 2b-4 in diethyl ether. The solid products were The general catalytic ability of the dibenzosemibullvalene salt
dissolved and precipitated as perchlorate salts. Although the 3d in phase-transfer catalyzed nucleophilic substitution reac-
dibenzosemibullvalene 3b was formed (68%), 1H NMR spec- tions was investigated. Compounds 5 [21], 7, 9, and 11 were
troscopic analysis of the photolysate indicated the additional chosen as substrates in alkylation reactions under phase-transfer
formation of significant amounts of byproducts (ca. 15%). conditions, and the catalytic activity of 3d was compared with a
These results indicate that, in principle, the internal sensitiza- tetrabutylammonium salt (Scheme 6, Table 1; TBAB = tetra-
tion of the DPM rearrangement of the cationic dibenzo- butylammonium bromide; TBAC = tetrabutylammonium chlo-
ride.). All four reactions under investigation were significantly
accelerated in the presence of substoichiometric amounts of the
quaternary ammonium catalysts. Notably, under identical condi-
tions the dibenzosemibullvalene salt 3d induced higher conver-
sions in the phase-transfer reactions than the tetrabutylammo-
nium salts, except for the alkylation of compound 5, in which
Table 1: Phase-transfer catalyzed alkylation reactions according to
Scheme 6.
Substrate
Catalysta
Conv.b / %
5
5
–
TBAB
3d
5c
92c
32c
2
5
7
–
7
TBAC
3d
36
82
5
7
9
–
9
TBAC
3d
81
95
4
9
11
11
11
–
TBAC
3d
49
>97
aTBAB = tetrabutylammonium bromide; TBAC = tetrabutylammonium
chloride. bConversion determined by 1H NMR spectroscopic analysis
of the reaction mixture, relative to 2,7-dimethyl naphthalene as internal
standard; estimated error: ±3% of the given value. cDetermined by GC
analysis, relative to ethyl acetate as internal standard.
Scheme 6: Phase-transfer catalyzed alkylation reactions (see Table 1
for details).
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Beilstein J. Org. Chem. 2011, 7, 119–126.
92% conversion was achieved using TBAB, whereas under column chromatography was performed on MN Silica Gel 60 M
identical conditions a conversion of only 32% was obtained (particle size 0.04–0.063 mm, 230–440 mesh).
with dibenzosemibullvalene 3d as the catalyst. In contrast, the
alkylation of the β-oxoester 7 in the presence of 3d gave the Synthesis of dibenzobarrelene derivatives
product in 82% conversion after 2 h at room temperature, General procedure for the preparation of N,N-dialkyl-3,4-
whereas with TBAC as catalyst the conversion was only 36% (9',10'-dihydro-9',10'-anthraceno-3-pyrrolinium derivatives
under identical conditions. The dibenzosemibullvalene salt 3d (GP-1): A mixture of 11,12-bis(bromomethyl)-9,10-dihydro-
was also found to be more efficient in the alkylation of cyclic 9,10-ethenoanthracene (1, 1.0–5.0 mmol), the corresponding
β-oxoesters 9 and 11, as compared with the TBAC catalyst secondary amine (1.2 equiv) and catalytic amount of DBU (0.1
(Table 1). Interestingly, the alkylation of the indanone deriva- equiv or otherwise explicitly specified) in dichloromethane (10
tive 11 is less efficient in the presence of TBAC as compared ml/mmol of 1) was stirred at room temperature for 3–6 days
with the non-aromatic cyclopentanone derivative 9, whereas in until TLC analysis indicated the full conversion of 1. The
the presence of dibenzosemibullvalene 3d both substrates are solvent was removed in vacuo and the residue re-dissolved in
alkylated with comparable efficiency.
methanol (10–25 ml depending on the scale and solubility); if
necessary, gentle heating was applied to dissolve the residue.
Aqueous perchloric acid (60%, 1–3 ml) was added to the mix-
Conclusion
In summary, it was shown that cationic pyrrolinium-annelated ture. The perchlorate salt of the ammonium-dibenzobarrelene
dibenzosemibullvalene derivatives are accessable by the derivative precipitated spontaneously, or upon subsequent add-
photoinduced di-π-methane rearrangement of appropriately ition of a few drops of water. The solid was collected by filtra-
substituted dibenzobarrelene substrates. With one represen- tion, washed with cold methanol and recrystallized from
tative example, it was demonstrated that these compounds may acetone. In those cases when no solid product precipitated from
act as phase-transfer catalysts in alkylation reactions with the methanol solution, water was added and the mixture
comparable or even better performance than the commonly extracted with dichloromethane. The organic layer was concen-
employed tetrabutylammonium salts. It is therefore concluded trated and the product purified by column chromatography
that this class of chiral compounds may be used as a promising (SiO2; 3–5% MeOH in dichloromethane).
platform for the development of versatile phase-transfer cata-
lysts.
The corresponding chloride salts were prepared by dissolving
the perchlorate salt in MeCN and passing the solution through
an ion-exchange column (DOWEX 1 x 8, Cl− form). After
Experimental
General remarks: NMR spectra were recorded on a Bruker removal of the solvent in vacuo, the residue was crystallized
Avance 400 (1H NMR: 400 MHz; 13C NMR: 100 MHz) and a from Et2O/MeOH.
Varian NMR system 600 (1H NMR: 600 MHz; 13C NMR: 150
MHz). 1H NMR chemical shifts are given relative to δTMS = N,N-Di-n-butyl-3,4-(9',10'-dihydro-9',10'-anthraceno)-3-
0.00 ppm, and 13C NMR chemical shifts refer to either the TMS pyrrolinium perchlorate (2d): Prepared from dibenzo-
signal (δTMS = 0.00 ppm) or the solvent signals [CDCl3: 77.0 barrelene 1 (1.60 g, 4.08 mmol) according to GP-1, yield 1.46 g
ppm; CD3OD: 49.0 ppm; (CD3)2CO: 29.8 ppm, CD3CN: 118.2 (3.18 mmol, 78%), colorless prisms, mp 173–174 °C. 1H NMR
ppm, (CD3)2SO: 39.5 ppm]. Melting points were determined [400 MHz, (CD3)2CO]: δ = 0.75 (t, J = 7 Hz, 6H, CH3),
with a Büchi 510K and are uncorrected. Mass spectra were 1.20–1.26 (m, 4H, CH2CH3), 1.42–1.46 (m, 4H, CH2CH2CH3),
recorded on a Hewlett-Packard HP 5968 (EI) and a Finnigan 3.52–3.56 (m, 4H, NCH2CH2), 4.72 (s, 4H, C=CCH2N), 5.39
LCQ Deca instrument (ESI). Elemental analyses were (s, 2H, CH), 6.99, 7.39 (AA’BB’-system, 8H, CHar). 13C NMR
performed on a KEKA-tech EuroEA combustion analyzer by [100 MHz, (CD3)2CO]: δ = 14.1 (CH3), 20.7 (CH2), 26.5
Mr. H. Bodenstedt, Organic Chemistry I, University of Siegen. (CH2), 49.8 (CH2), 65.6 (CH2), 69.0 (CH), 124.8 (CHar), 126.1
TLC analyses were performed on silica-gel sheets (Macherey- (CHar), 144.6 (Cq), 146.9 (Cq). UV (CH2Cl2): λmax (log ε) =
Nagel Polygram Sil G/UV254). Unless otherwise noted, 273 (4.00), 280 (4.17). MS (ESI+): m/z (%) = 358 (100). Anal.
commercially available chemicals were reagent grade and used Calcd for C26H32ClNO4 (458.0): C, 68.18; H, 7.04; N, 3.06.
without further purification. Polymer-bound DBU (polystyrene Found: C, 68.22; H, 7.12; N, 3.05.
cross-linked with 1% divinylbenzene; loading: 1.15 mmol/g)
was obtained from Aldrich. Anhydrous THF and diethyl ether N,N-Di-n-butyl-3,4-(9',10'-dihydro-9',10'-anthraceno-3-
were obtained by distillation from sodium wire, and anhydrous pyrrolinium chloride (2d-Cl): Obtained quantitatively (1.37 g,
CH2Cl2 was distilled from calcium hydride. Distilled water was 3.47 mmol) as a white powder by ion exchange of the perchlo-
used for all the synthetic reactions performed. Preparative rate 2d (1.59 g, 3.47 mmol; eluent: acetonitrile; DOWEX resin).
124

Beilstein J. Org. Chem. 2011, 7, 119–126.
mp 194–195 °C. 1H NMR (400 MHz, CD3CN): δ = 0.78 (t, J = solution (4 ml, 60%) was stirred for 2 h at 20 °C. Water (10 ml)
7 Hz, 6H, CH3), 1.16–1.34 (m, 8H, CH2CH2CH3), 3.28–3.30 and toluene (10 ml) were added and the mixture was thor-
(m, 4H, NCH2CH2, overlapped with the solvent signal), 4.48 (s, oughly shaken. After phase separation, the organic layer was
4H, C=CCH2N), 5.12 (s, 2H, CH), 6.99, 7.35 (AA’BB’-system, analyzed by gas chromatography.
8H, CHar). 13C NMR (100 MHz, CD3CN): δ = 13.9 (CH3), 20.8
(CH2), 26.5 (CH2), 50.0 (CH2), 65.7 (CH2), 68.7 (CH), 126.2 Stationary Phase: HP-5 PhMe-Silica, crosslinked 5%, 25 m;
(CHar), 126.7 (CHar), 144.6 (Cq), 147.0 (Cq).
Mobile Phase: Argon gas; Flow Pressure: 30 kPa; Tempera-
ture: Oven 220 °C, Injector 250 °C, Detector 250 °C; Retention
time: PhCH2CN 9.6 min; PhCH(CH2CH3)CN 10.6 min.
Synthesis of dibenzosemibullvalene deriva-
tives
General procedure for the photolysis in solution (GP-2): Solu- Three PTC reactions were performed in parallel with a) no cata-
tions of the substrates (10−2 to 10−3 mol/l) were placed in a 200 lyst, b) TBAB as the catalyst, or c) 3d as the catalyst.
ml Duran flask (acetone) or a quartz test tube (other solvents),
and argon gas was bubbled through the solution for at least 20 NMR-spectroscopically monitored PTC reactions: A
min. The solution was stirred and irradiated for 4–15 h until the biphasic mixture of the quaternary ammonium catalyst (0.03
starting material was fully converted, as determined by TLC or mmol), the substrate (0.10 mmol), benzylchloride (1.25–1.50
1H NMR spectroscopic analysis. After evaporation of the equiv) and 2,7-dimethylnaphthalene (as internal standard, 0.025
solvent in vacuo, the photolysate was analyzed by 1H NMR mmol) in toluene (5 ml) and aqueous KOH solution (5 ml, 10%)
spectroscopy, or, in preparative experiments, isolated by recrys- was stirred for 2 h at 20 °C. At the end of reaction, water (10
tallization or column chromatography to obtain the photo- ml) and toluene (10 ml) were added and the mixture was thor-
product.
oughly shaken. The separated organic phase was analyzed by
1H NMR spectroscopy, and the conversion calculated by com-
rac-N,N-Di-n-butyl-4b’,8b’,8c’,8e’-dibenzo[a,f]cyclo- parison with the signal of the methyl protons of 2,7-dimethyl-
propa[cd]pentaleno-pyrrolidinium perchlorate (3d): naphthalene (δ = 2.49 ppm, in CDCl3).
Prepared from 2d (50.0 mg, 0.11 mmol) according to GP-2 in
acetone solution as colorless prisms (42.0 mg, 0.09 mmol, Three reactions were run in parallel with a) no catalyst, b)
85%), mp 176–178 °C. 1H NMR (400 MHz, CD3CN): δ 0.61 (t, TBAC as the catalyst or c) 3d as the catalyst.
J = 7 Hz, 3H, CH3), 0.74 (t, J = 7 Hz, 3H, CH3), 0.85–0.94 (m,
1H, CH2), 0.99–1.07 (m, 1H, CH2), 1.16–1.33 (m, 4H, CH2),
Supporting Information
1.33–1.39 (m, 2H, CH2), 2.74–2.82 (m, 1H, CH2), 2.88–2.95
(m, 2H, CH2), 3.48 (d, J = 13 Hz, 1H, NCHHC), 3.73 (d, J = 13
Supporting Information File 1
Hz, 1H, NCHHC), 4.04 (s, 1 H, CH), 4.24 (d, J = 13 Hz, 1H,
NCHHC), 4.57 (d, J = 13 Hz, 1H, NCHHC), 4.91 (s, 1H, CH),
6.97–7.01 (m, 2H, CHar), 7.09–7.17 (m, 4H, CHar), 7.34–7.37
(m, 1H, CHar), 7.39–7.41 (m, 1 H, CHar), 7.38–7.40 (m, 1H,
CHar). 13C NMR (100 MHz, CD3CN): δ 13.0 (CH3), 13.6
(CH3), 20.1 (CH2), 20.3 (CH2), 24.8 (CH2), 25.8 (CH2), 55.5
(CH2), 55.7 (CH2), 59.8 (CH2), 60.0 (CH2), 61.9 (Cq), 67.3
(CH), 69.4 (Cq), 70.1 (CH), 122.5 (CHar), 123.3 (CHar), 125.8
(CHar), 125.9 (CHar), 127.7 (CHar), 128.3 (CHar), 128.4 (CHar),
128.9 (CHar), 137.0 (Cq), 137.1 (Cq), 152.0 (Cq), 154.5 (Cq).
MS (ESI+): m/z (%) = 358 (100). Anal. Calcd for C26H32ClNO4
(458.0): C, 68.18; H, 7.04; N, 3.06. Found: C, 68.22; H, 7.14;
N, 3.06.
Experimental procedures, characterization data and copies
of 1H NMR and 13C NMR spectra of compounds 2a–g and
3a–f.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-17-S1.pdf]
Acknowledgements
We thank the Deutsche Akademische Austauschdienst for a
fellowship to J. L. (DAAD-Abschluss-Stipendium) and Mr.
Maoqun Tian, University of Siegen, for assistance during the
preparation of this manuscript.
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a) Analysis with gas chromatography: A biphasic mixture of
the quaternary ammonium catalyst 3d or TBAC (0.05 mmol),
phenylacetonitrile (5, 506 mg, 4.32 mmol) and bromoethane
(808 mg, 4.75 mmol) in toluene (0.85 ml) and aqueous KOH
125

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