Efficient thia-bridged triarylamine heterohelicenes: synthesis, resolution, and absolute configuration determination

Article abstract of DOI:10.1002/chem.200800705

Researchers investigated the synthesis, resolution, and absolute configuration determination of efficient thia-bridged triarylamine heterohelicenes. It was found that the reaction of tris(4-methylphenyl) and tris(4-methyoxyphenyl)amine, with one equivalent of phthalimidesulfenyl chloride, occurred smoothly at room temperature, to produce mono-sulfenylation ortho to the amine nitrogen atom. The researchers also demonstrated that the electrophilic character of sulfenamide sulfur in N-thiopthalimides can be increased by using Lewis acids. The application of a straightforward procedure to tris(3,4,5-methyoxyphenyl)amine resulted in the formation of heterohelicenes, during the bis-sulfenylation process without the use any Lewis acid catalyst.

Full text of DOI:10.1002/chem.200800705

COMMUNICATION
DOI: 10.1002/chem.200800705
Efficient Thia-Bridged Triarylamine Heterohelicenes:
Synthesis, Resolution, and Absolute Configuration Determination
Giuseppe Lamanna,[a] Cristina Faggi,[a] FrancescoGasparrini, Alessia Ciogli,[b]
[b]
[b]
[a]
ClaudioVillani, Philip J. Stephens,[c] Frank J. Devlin,[c] and StefanoMenichetti*
Dedicated to the memory of Professor Giuseppe (Pino) Capozzi
For many years helicenes were regarded as an academic
curiosity and a nice example of chirality without stereogenic
centers. The situation changed dramatically during the last
decade when fascinating optical[1] and electronic[2] properties
as well as attractive applications in bioorganic chemistry[3]
and asymmetric synthesis[4] emerged for these helical-shaped
molecules. In parallel, new synthetic methods that circum-
vented the problems often related with the classical photo-
cyclization of stilbenes,[5] including Diels–Alder reactions,[6]
cyclotrimerization of alkynes,[7] carbenoid couplings,[8] radi-
cal cyclizations,[9] Pd-mediated methodologies,[10] and olefin
metathesis[11] were developed. Recently, bridged triaryl-
amine heterohelicenes of the type 1–3 have been pre-
pared[12,13] and studied[13,14] in the belief that the introduction
of molecular helicity, sterically driven by the increasing
overlap of the terminal a and b aryl rings, influences[2b] the
well known photochemical and physical properties of triaryl-
amines.[15]
pared by building the triarylamine skeleton by employing a
Buchwald–Hartwig type cross-coupling of an open iodo-ani-
line in the final step of a multistep reaction sequence.[13] We
envisaged that derivatives like 3 could be accessible by a
cascade of regioselective electrophilic aromatic sulfur inser-
tions,[17] by applying the chemistry of the phthalimidesulfen-
yl chloride (4) (PhtNSCl, Pht=phthaloyl) to properly substi-
tuted triarylamines.
The reaction of tris(4-methylphenyl)- (5a) and tris(4-me-
thoxyphenyl)amine (5b) with one equivalent of 4 occurred
smoothly at room temperature to give mono-sulfenylation
ortho to the amine nitrogen atom. The bis-sulfenylation can
however be achieved under more forcing reaction conditions
(Scheme 1), which allowed the isolation of derivatives 6a
and 6b in 83% yield. In line with our previous results,[18] the
introduction of a N-thiophthalimide group strongly deacti-
vates the aromatic system, preventing further substitutions.
Moreover, using triarylamines as substrates, protonation at
the amine nitrogen, due to the HCl formed during sulfenyla-
tion, represents a supplementary obstacle to the polysubsti-
tution. Indeed, we were unable to carry out an exhaustive
sulfenylation of all three aromatic rings of 5a or 5b.
As a continuation of our efforts related to sulfur heterocy-
cles chemistry,[16] we focused on compound 3, which was pre-
[a] Dr. G. Lamanna, Dr. C. Faggi, Prof. S. Menichetti
Dipartimento di Chimica Organica e Laboratorio di Progettazione
e Sintesi di Eterocicli Biologicamente Attivi (HeteroBioLab)
Università di Firenze
Via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze (Italy)
Fax : (+39)055-4573531
E-mail: stefano.menichetti@unifi.it
[b] Prof. F. Gasparrini, Dr. A. Ciogli, Prof. C. Villani
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze
Biologicamente Attive
Università di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma
(Italy)
The electrophilic character of sulfenamide sulfur in N-thi-
ophthalimides can be increased by using Lewis acids.[17] Sat-
isfyingly, reacting derivatives 6a and 6b with BF3·Et2O or
AlCl3 triggers an intramolecular attack of the aromatic ring
on the adjacent sulfur atom, leading to the formation of het-
ero[4]helicenes 7a and 7b in 85 and 87% yield, respectively
[c] Prof. P. J. Stephens, Dr. F. J. Devlin
Department of Chemistry, University of Southern California
Los Angeles, CA 90089 (USA)
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
Chem. Eur. J. 2008, 14, 5747 – 5750
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Scheme 1. Synthesis of thia-bridged hetero[4]helicenes from triarylamines
by four consecutive electrophilic aromatic sulfur insertions with phthal-
imidesulfenyl chloride.
Scheme 2. Access to hetero[6]helicene 12.
pared with the situation in derivatives 1 and 2[13] (Figure 1
and torsion angles in Table 1),[21] suggesting a peculiar stabil-
ity against a reversal of the helicity.
(Scheme 1). Applying this straightforward procedure to
tris(3,4,5-trimethoxyphenyl)amine 5c, we observed the for-
mation of heterohelicene 7c during the bis-sulfenylation
process, without the use of any Lewis acid as catalyst. We
reasoned that, after the initial introduction of the N-thio-
phthalimide groups, a possible protonation at the sulfena-
mide nitrogen atom[19] and the high nucleophilic character
of the trimethoxy-substituted aromatic ring can contribute
to activating the sulfur atom and promoting the intramolec-
ular SEAr processes.[20] In this way, the hetero[4]helicene 7c
was isolated in 63% yield as the result of four consecutive
one-pot regioselective electrophilic sulfur insertions
(Scheme 1).
A fairly simple modification of the reaction sequence al-
lowed access to hetero[6]helicenes (Scheme 2). Monosulfe-
nylation of amine 8 with 4 gave chemo-and regioselectively
sulfenamide 9 in 87% yield. Cyclization of 9 with an excess
of BF3·Et2O led to the two expected 1,4-thiazines 10 and 11
in a 10:1 ratio and 70% overall yield. Gratifyingly, the
major isomer 10 reacts with 4 under the above-described
conditions, and thereby undergoes sulfenylation followed by
a spontaneous intramolecular proton-mediated cyclization,
to afford hetero[6]helicene 12 in 75% yield (Scheme 2).
The assignments of the structures of the helicenes 7a–c
and 12 were supported by spectroscopic data and confirmed
for 7a, 7c, and 12 by single-crystal X-ray analysis (see the-
Table 1. X-ray torsion angles [8][21] between the planes of terminal a and
b phenyl rings in heterohelicenes 1,[12] 2,[13] 3,[13] 7a, 7c, and 12.
Helicene
1
2
3
7a
7c
12
angle [8]
41.9
43.0
62.3
61.4
65.3
74.5
The enantiomers of heterohelicenes 7a–c and 12 were ef-
fectively separated by HPLC using a column packed with an
amylose-based chiral stationary phase.[22] Positive identifica-
tion of the enantiomers was obtained by dual simultaneous
UV and CD detection that gave, at any wavelength between
254 and 400 nm, bisignate peaks of equal area, as expected
for a racemate (see the Supporting Information).
For 7a the analytical separation was easily scaled up to
the milligram range, affording the individual enantiomers of
the heterohelicenes 7a with ee=99.9% and 97.6% for the
first and second eluted enantiomers, respectively. The first
and second eluted enantiomers of 7a exhibited [a]D values
of (+)-376 and (ꢀ)-376 (c=0.11, hexane), respectively.[23]
Racemization of the resolved (+)-7a and (ꢀ)-7a enantio-
mers was not achieved when they were heated for 8 h in
decalin at 958C. However, thermal racemization was ob-
served when (+)-7a was heated at 121, 135, and 1458C in
decalin. The decay of enantiomeric excess over time was
ꢀ
Supporting Information). The length of the Csp2 S bond
causes a greater overlay of the a and b aromatic rings com-
5748
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Chem. Eur. J. 2008, 14, 5747 – 5750

Thia-Bridged Triarylamine Heterohelicenes
COMMUNICATION
monitored by enantioselective HPLC. Good first-order ki-
netics were displayed by the experimental data points, from
which the rate constants of 2.2”10ꢀ5 (1218C), 1.2”10ꢀ4
(1358C), and 2.6”10ꢀ4 sꢀ1 (1458C) were calculated for the
racemization process.[24] The energy barriers of racemization
DG° =131.9–132.6ꢁ0.5 kJmolꢀ1 in the 121–1458C tempera-
ture range were obtained, with DH° =137ꢁ0.5 kJmolꢀ1 and
DS° =12ꢁ6 Jmolꢀ1 Kꢀ1. Thus, the energy barrier for the hel-
icity reversal in the thia-bridged hetero[4]helicene 7a is be-
tween that of the parent [5]helicene (DG° =101 kJmolꢀ1 at
208C)[5d] and [6]helicene (DG° =151 kJmolꢀ1 at 278C),[5d]
which allows an easy physical separation and chiroptical
characterization of the individual M/P enantiomers at room
temperature.
The absolute configuration of 7a was determined by com-
parison of the vibrational circular dichroism (VCD) spectra
of the two enantiomers, P-7a and M-7a, calculated by using
density functional theory (DFT), to the experimental VCD
spectrum of (+)-7a, a methodology used previously in deter-
mining the absolute configurations of chiral-chromatogra-
phy-resolved enantiopure molecules[25] (see the Supporting
Information). Conformational analysis of 7a, using the
MMFF94 force field, showed 7a to be a conformationally
rigid molecule. Reoptimization of the MMFF94 geometry,
using DFT at the B3PW91/TZ2P level, and calculation of
the B3PW91/TZ2P harmonic vibrational frequencies and ro-
tational strengths, led to the VCD spectrum of P-7a and to
the predicted structure of P-7a, which is shown in Figure 1
four consecutive (one-pot) electrophilic regioselective aro-
matic sulfur insertions. Owing to their remarkably high race-
mization barrier, these derivatives can be resolved by
HPLC, and the absolute configuration for 7a has been de-
termined as P-(+) by ab initio calculations and by experi-
mental measurement of VCD spectra. The synthesis of simi-
lar helicenes with potential application in asymmetric syn-
thesis or biorganic chemistry is ongoing.
Experimental Section
A detailed experimental procedure including synthesis, X-ray structure
determination, HPLC separation, and determination of absolute configu-
ration is available in the Supporting Information. The following proce-
dure for the synthesis of helicene 7a from amine 5a is reported as de-
monstrative.
Bis-N-thiophthalimide (6a): To
a
solution of tris(p-tolyl)amine (5a)
A
(1.0 mmol) in dry CHCl3 (15 mL) was added phthalimidesulfenyl chloride
(4) (2.3 mmol) under a nitrogen atmosphere. After stirring at 608C for
24 h, the reaction mixture was diluted with CH2Cl2 (5 mL), and washed
with a saturated NaHCO3 solution (2”30 mL) and water (2”30 mL).
The organic layer was dried over Na2SO4, filtered, concentrated under re-
duced pressure, and the crude material purified by flash chromatography
(CH2Cl2/petroleum ether 4:1) to provide the thiophthalimide 6a as a
yellow solid (83% yield). M.p. 259–2618C; 1H NMR (CDCl3, 400 MHz):
d=2.22 (s, 6H), 2.26 (s, 3H), 6.68–6.71 (m, 2H), 6.86 (d, J=1.2 Hz, 2H),
7.03–7.07 (m, 4H), 7.49 (d, J=8.0 Hz, 2H), 7.76–7.81 (m, 4H), 7.91–
7.96 ppm (m, 4H); 13C NMR (CDCl3, 50 MHz): d=20.92, 21.37, 118.42,
124.13, 127.10, 128.92, 129.83, 130.36, 130.56, 132.26, 133.75, 134.71,
136.35, 141.61, 146.31, 167.96 pm; IR (KBr): n˜ =1787 + 1742 + 1709
(C=O stretching PhtN), 1278 cmꢀ1; MS (70 eV): m/z (%): 641 (3) [M +],
C
147 (43), 76 (100), 50 (88); elemental analysis calcd (%) for
C37H27N3O4S2: C 69.25, H 4.24, N 6.55; found: C 69.38, H 4.10, N 6.12.
Hetero[4]helicene 7a: To
a solution of bis-N-thiophthalimide 6a
(1.0 mmol) in dry CH2Cl2 (50 mL) was added BF3·Et2O (40.0 mmol)
under a nitrogen atmosphere. After the mixture had been stirred for 3 h
at room temperature, the mixture was diluted with CH2Cl2 (15 mL) and
washed with a saturated Na2CO3 solution (2”60 mL) and a saturated
NaF solution (2”60 mL). The organic layer was dried over Na2SO4.
Evaporation of the solvent gave a crude product that was purified by
flash chromatography (petroleum ether/CH2Cl2 2:1) to afford the hetero-
helicene 7a as a white solid (85% yield) further purified by recrystalliza-
tion from CHCl3. M.p. 162–1648C; 1H NMR (C6D6, 400 MHz): d=1.80
(s, 3H), 1.96 (s, 6H), 6.55–6.59 (m, 4H), 6.87 (d, J=1.2 Hz, 2H),
6.95 ppm (d, J=8.4 Hz, 2H); 13C NMR (C6D6, 50 MHz): d=20.25, 20.56,
120.56, 125.98, 126.38, 127.31, 128.35, 134.07, 134.54, 137.78, 137.95,
Figure 1. B3PW91/TZ2Pstructure of P-7a (left), and X-ray structure of
rac-7a (right, P enantiomer chosen arbitrarily).
140.90 ppm; MS (70 eV): m/z (%): 347 (100) [M +], 315 (47), 158 (50); el-
C
emental analysis calcd (%) for C21H17NS2: C 72.58, H 4.93, N 4.03;
found: C 72.10, H, 4.98, N 3.99.
together with the X-ray structure of rac-7a (P enantiomer
was arbitrarily chosen among the two molecules included in
the asymmetric unit, see Supporting Information). The pre-
dicted VCD spectrum of P-7a is in excellent qualitative
agreement with the experimental VCD spectrum of (+)-7a,
leading to the conclusion that the absolute configuration of
7a is unambiguously P-(+). Further support for the reliabili-
ty of the DFT calculations for 7a is provided by the excel-
lent agreement of the predicted B3PW91/TZ2P equilibrium
geometry and the X-ray structure. The X-ray structure tor-
sion angle between the terminal a and b phenyl ring planes
of 7a is 61.48 (Table 1); the B3PW91/TZ2P torsion angle is
66.38 (Figure 1).
Acknowledgements
Financial support from MiUR (Research project “Stereoselezione in Sin-
tesi Organica. Metodologie ed Applicazioni” contract 2005035330), the
U.S. National Science Foundation, CHE-0209957 and CHE-0614577 are
acknowledged. We thank Brunella Innocenti and Maurizo Passaponti for
MS spectra and elemental analysis.
Keywords: aromatic substitution
·
chiral resolution
·
In summary we have shown a very practical access to
thia-bridged triarylamine hetero[4]- and -[6]helicenes by
heterohelicenes · sulfur heterocycles · vibrational circular
dichroism
Chem. Eur. J. 2008, 14, 5747 – 5750
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5749

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Received: April 14, 2008
Published online: May 13, 2008
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Chem. Eur. J. 2008, 14, 5747 – 5750