Pentiptycene building blocks derived from nucleophilic aromatic substitution of pentiptycene triflates and halides

Article abstract of DOI:10.1021/jo100812z

(Figure presented) The nucleophilic aromatic substitution (S_NAr) reactions of the nitro-substituted pentiptycene triflate 15 with LiBr and LiI and the resulting halides 18 and 19 with N₃^-, CN ^-, and ArS^-

Full text of DOI:10.1021/jo100812z

pubs.acs.org/joc
molecular machines,3 molecular hosts,4 supramolecular cata-
Pentiptycene Building Blocks Derived from
Nucleophilic Aromatic Substitution of Pentiptycene
Triflates and Halides
lysts,5 crystal and thin-film engineering,6,7 and light-emitting
π-conjugated oligomers and polymers.8 The majority of these
applications are associated with derivatization of the central
phenylene ring. Thus, facile synthesis of the central-ring pre-
functionalized pentiptycene building blocks is crucial toward
the development of new pentiptycene-derived materials.
Sandip Kumar Kundu, Wei Shyang Tan, Jyu-Lun Yan,
and Jye-Shane Yang*
Department of Chemistry, National Taiwan University,
Taipei, Taiwan 10617
jsyang@ntu.edu.tw
Received April 26, 2010
The nucleophilic aromatic substitution (SNAr) reactions
of the nitro-substituted pentiptycene triflate 15 with LiBr
We have shown that the readily prepared pentiptycene
quinone 29,10 is an excellent precursor for a variety of central-
ring functionalizedpentiptycenes, including the symmetrically
disubstituted pentiptycene hydroquinone 3,7 diacetylene
4,9 dibromide 5, and diiodide 6,11 and a series of unsym-
metrically disubstituted pentiptycenes such as compounds
7-14.11,12 While most of the functional-group transforma-
tions gave an excellent yield (>85%) of the desired products,
the synthesis of pentiptycene dihalides 5, 6, and 14 from
pentiptycene triflate 15 has encountered some problems
(Scheme 1).11 For example, the triflate (15) to halide (12
and 13) transformation was not accomplished in one step but
through two individual ones: (i) reduction (detriflate reac-
tion, 15 f 16) and (ii) oxidation (halogenation, 17 f 12/13)
of the central phenylene ring. In addition, the nitro group in
15 must be reduced before carrying out the oxidation step.
More importantly, the reduction step has suffered from a
small reaction scale (e0.35 mmol) and low product yield
(e45%). We report herein an alternative route that circum-
vents these drawbacks and limitations through direct
nucleophilic aromatic substitution (SNAr) of 15 with bro-
mide and iodide ions. This reaction provides not only a
solution toward a large-scale synthesis of the pentiptycene
dihalides 5, 6, and 14 but also an easy access to new
pentiptycene building blocks with nitro, amino, and cyano
substituents.
-
and LiI and the resulting halides 18 and 19 with N3
,
CN-, and ArS- in DMF provide an efficient route
toward pentiptycene halides and dihalides and other
new pentiptycene building blocks. The reactivity of dia-
minopentiptycene in Pd-catalyzed C-N coupling reac-
tions is also demonstrated.
The rigid, aromatic, and H-shaped pentiptycene scaffold (1)1
has found many potential applications in molecular and su-
pramolecular chemistry, including fluorescent chemosensors,2
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The SNAr reaction of 15 with LiBr and LiI in DMF
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4640 J. Org. Chem. 2010, 75, 4640–4643
Published on Web 06/09/2010
DOI: 10.1021/jo100812z
r
2010 American Chemical Society

Kundu et al.
JOCNote
SCHEME 1
SCHEME 2
reflux temperatures. Evidently, the SNAr reactivity of 15 is
significantly lower than 21, attributable to the steric and electro-
nic perturbations of the iptycene substituents,19 and its SNAr
reaction in forming 18 and 19 is mainly driven by the novel SNAr
reagents LiBr/DMF and LiI/DMF. To the best of our knowl-
edge, the novel SNAr reactivity of LiBr or LiI in DMF has not
been recognized in the literature.20-22 We postulated that the
observed SNAr reactivity of 15 is associated with cation-π
interactions between Liþ and the pentiptycene phenylene rings.
More specifically, the SNAr reactions require a cooperated effect
between an effective cation for activating the phenylene carbon
through cation-π interactions and a soft anion for nucleophilic
attack on the C rather than the S center. Among the tested
halogen salts, Liþ possesses a much larger binding energy with a
benzene molecule than Naþ and Kþ.23 Among the tested
solvents, DMF is superior in dissolving lithium salts into free
cations and anions instead of solvated ion pairs or aggregates.24
A higher reaction temperature in DMF than in THF and aceto-
nitrile might also contribute to the observed SNAr reactions.
In addition to aryl triflates, activated aryl halides are good
candidates for SNAr reactions.16,25,26 Indeed, without the
good yields (the C-O bond cleavage of route a in Scheme 2).
The only observed side product is compound 8, which results
from the S-O bond cleavage (route b in Scheme 2). The
effect of nucleophiles on the C-O vs S-O bond cleavage
of activated aryl triflates (e.g., 20 and 21) has been well
documented.13-18 The C-O bond cleavage (the SNAr pro-
duct) is more favored with nucleophiles of higher polariz-
ability, and the reverse is true for the S-O bond cleavage.
This has been attributed to the lower positive charge density
on the C (soft) relative to the S (hard) center for nucleophilic
attack.13,14 Potential soft nucleophiles toward SNAr reactions
are amines (e.g., piperidine and morpholine),15,16 iodide,17
thiolate,13 and malonate18 ions. Although our observation of
a higher yield of 19 (Nu = I-) relative to 18 (Nu = Br-) is
consistent with this scenario, the SNAr efficiency of 15 is
highlydependent on the reactionconditions. A replacementof
the lithium salts LiBr and LiI with the other halogen salts,
including NaBr, NaI, KBr, KI, and MCl, and MF (M = Li,
-
Na, and K), or with other potential nucleophiles such as N3
,
CN-, PhO-, PhS-, piperidine, and morpholine all leadstothe
S-O bond cleaved product 8 in nearly quantitative yields
without any detectable SNAr products. In the cases of sec-
ondary amines and NaN3, the S-O bond cleavage reaction is
particularly efficient and proceeds to completion within 5 h at
room temperature. In addition, no reaction occurred bet-
ween 15 and LiBr or LiI when DMF was replaced by other
solvents such as THF, acetonitrile, or ethyl acetate under the
(20) The product and yield of a Heck-type reaction between 4-nitrophenyl
triflate (21) and (E)-1,2-bis(trimethylsilyl)ethylene (TMSE) in the presence of
LiI in DMF at 110 °C were found to be the same as those between 1-iodo-4-
nitrobenzene (INB) and TMSE under the same reaction condition. The same
reaction also worked for iodobenzene, but it did not work for phenyl triflate
in the presence of LiI. The authors have proposed a four-membered inter-
mediate for the reaction of 21. See: Karabelas, K.; Hallberg, A. J. Org. Chem.
1989, 54, 1773–1776. Alternatively, INB could be formed in situ through the
SNAr reaction in the case of the activated triflate 21 but not for the
nonactivated phenyl triflate. Our preliminary test on the relative reactivity
of LiI vs NaI on the SNAr reaction of 21 showed that a high yield of the
product INB was observed with LiI (95%) within 12 h but it requires 24 h to
reach a yield of 79% with NaI.
(13) Bunnett, J. F.; Bassett, J. Y., Jr. J. Am. Chem. Soc. 1959, 81, 2104–
2109.
(14) (a) Zhu, J.; Bigot, A.; Tran Huu Dau, M. E. Tetrahedron Lett. 1997,
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facilitate the oxidative addition of vinyl and aryl triflates to the palladium(0)
catalyst in Stille coupling ractions. See: Scott, W. J.; Stille, J. K. J. Am. Chem.
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A. Angew. Chem., Int. Ed. Engl. 1993, 32, 404–406. (c) Schio, L.; Lemoine, G.;
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2004, 6, 985–987.
(22) For examples of NaI in SNAr reactions of aryl triflates, see: (a)
Prugh, J. D.; Alberts, A. W.; Deana, A. A.; Gilfillian, J. L.; Huff, J. W.;
Smith, R. L.; Wiggins, J. M. J. Med. Chem. 1990, 33, 758–765. (b) Wang, Z.;
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ꢀ
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SCHEME 3
SCHEME 4
TABLE 1. Optimum Yields of the Pentiptycene Dihalides 5, 6, and 14
from Deaminative Halogenation of Precursors 12, 13, and 26
entry reactant halide salta tempb (°C) product yieldc (%)
1
2d
3d
4
5
6
12
12
13
13
26
26
CuBr2
KI
KI
CuBr2
CuBr2
KI
65
65
65
65
25
25
5
14
6
14
5
6
91
62
50
60
83
55
aSolvent is MeCN, another reagent is tert-butyl nitrite. bReaction
complication of the S-O bond cleavage as seen in pentiptycene
triflate 15, the pentiptycene halides 18 and 19 reacted with
NaN3, CuCN, and 4-methoxyphenylthiol to form pentipty-
cenes 22, 23, and 24, respectively (Scheme 3). The mechanistic
aspects of these reactions deserve further comments. In the case
of NaN3, the initially formed azide-substituted product could
further react with DMF to form the imine intermediate 25 and
then hydrolyzed to form 22 during the aqueous workup
according to the literature.25 For the cyanation of aryl halides
(the Rosenmund-von Braun reaction), several different me-
chanisms, including an SNAr-like one, have been proposed.27,28
The crucial role of Cu(I) in the cyanation of 19 is borne out by a
failure of the reaction with NaCN but a success with NaCN in
the presence of a catalytic amount of CuI.29 The reaction
mechanism between aryl halides and alkyl- or arylthiolate ions
could be versatile as well.25 In addition to the SNAr mechanism,
the radical-chain mechanism SRN1 could take place. Never-
theless, there was no evidence of an SRN1 mechanism in the
reactions of 18 or 19 f 24, as neither 8 nor 16 was detected11,12
in the products.
time is 16 h. cIsolated yield. dFrom ref 11.
SCHEME 5
To further convert 18 and 19 to the desired pentiptycene
dihalides 5, 6, and 14, the nitro group in 18 and 19 needed to be
selectively reduced to form 12 and 13, respectively, so that the
subsequent deaminative halogenation can be conducted. In-
deed, this can be achieved with indium metal in the presence of
ammonium chloride (Scheme 4).30 Other reducing agents such
as SnCl2/THF, Sn/NH4Cl, Zn/HCl, and H2/Pd/C gave a
mixture of both the desired products 12 and 13 and the
unwanted dehalogenated product 17, which is hardly separated.
Reduction of the nitro group in 22 and 23 leads to new
aminopentiptycenes 26 and 27, respectively (Scheme 4). This
can be conducted simply with SnCl2 to reach an excellent
yield. The new compound diaminopentiptycene 26 is parti-
cularly interesting, because it was the first proposed
precursor31 for the syntheis of pentiptycene dihalides. The
deaminative bromination and iodination of 26 indeed pro-
ceed smoothly with tert-butyl nitrite and halide salts CuBr2
and KI. The optimum reaction yields toward pentiptycene
dihalides from different starting materials 12, 13, and 26 are
reported in Table 1.
Scheme 5 summarizes the possible routes toward pentip-
tyence dibromide 5 and diiodide 6 from 15. The SNAr
reaction routes (routes b and c) provide higher yields
(46-56% vs 37% for 5 and 30-35% vs 12% for 6) than
the previous method (route a).11 More importantly, the
reaction scale in routes b and c are larger by more than 10-
fold than that in route a.
(26) (a) Montanari, S.; Paradisi, C.; Scorrano, G. J. Org. Chem. 1991, 56,
4274–4279. (b) Montanari, S.; Paradisi, C.; Scorrano, G. J. Org. Chem. 1993,
58, 5628–5631. and references cited therein.
(27) Ellis, G. P.; Romney-Alexander, T. M. Chem. Rev. 1987, 87,
779–794.
(28) (a) Zanon, J.; Klapars, A.; Buchwald, S. L. J. Am. Chem. Soc. 2003,
125, 2890–2891. (b) Cristau, H.-J.; Ouali, A.; Spindler, J.-F.; Taillefer, M.
Chem.;Eur. J. 2005, 11, 2483–2492.
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389. (b) Arvela, R. K.; Leadbeater, N. E.; Torenius, H. M.; Tye, H. Org.
Biomol. Chem. 2003, 1, 1119–1121.
The reactivity of 26 in Pd-catalyzed Buchwald-Hartwig
C-N coupling reactions32 has also been investigated. As shown
(30) (a) Moody, C. J.; Pitts, M. R. Synlett 1998, 1028. (b) Moody, C. J.;
Pitts, M. R. Synlett 1998, 1029–1030.
(31) Williams, V. E.; Swager, T. M. Macromolecules 2000, 33, 4069–4073.
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Kundu et al.
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SCHEME 6
yield 86%, white solid, mp >300 °C; 1H NMR (400 MHz,
CDCl3) δ 5.74 (s, 2H), 5.96 (s, 2H), 7.03-7.07 (m, 8H), 7.41-
7.46 (m, 8H); 13C NMR (100 MHz, CDCl3) δ 50.1, 59.0, 98.5,
123.6, 123.9, 125.5, 125.6, 135.9, 142.8, 143.4, 147.8; IR (KBr)
1524, 1357 cm-1; HRMS (FAB) m/z calcd for C34H20INO2
601.0539, found 601.0541.
Procedure for the SNAr Reaction of 18 and 19. This is
illustrated with the reaction between 19 and azide. To a stirred
solution of 19 (1.02 g, 1.7 mmol) was added sodium azide
(0.32 g, 4.99 mmol) in dry DMF (5.0 mL). It was heated for
12 h at 140 °C, and then cooled to room temperature. Water was
added to the solution and the resulting precipitate was collected
by filtration, washed with water, and dried under vacuum to
afford crude product. Purification was carried out with silica gel
column chromatography, using CH2Cl2/hexane as an eluent to
afford compound 22: yield 67%, yellow solid, mp >300 o C; 1H
NMR (400 MHz, CDCl3) δ 4.49 (br, 2H), 5.42 (s, 2H), 6.00 (s,
2H), 6.99-7.04 (m, 8H), 7.34-7.44 (m, 8H);13CNMR (100MHz,
CDCl3) δ 47.9, 49.7, 123.2, 124.3, 125.5, 125.6, 129.8, 136.9, 138.3,
138.8, 144.0, 144.3; IR (KBr) 3487, 3402, 1526, 1350 cm-1; HRMS
(FAB) m/z calcd for C34H22N2O2 490.1681, found 490.1683.
General Procedure for Reduction of the Nitro Group in 18
and 19. This is illustrated with the case of compound 19. To a
stirred solution of 19 (0.51 g, 0.85 mmol) was added indium
powder (0.28 g, 2.5 mmol) and saturated ammonium chloride
(2.0 mL) solution in THF (10.0 mL). It was then refluxed for 72 h
at 100 °C, cooled to room temperature, and concentrated under
reduce pressure. The reaction mixture was diluted with water,
extracted with CH2Cl2, and dried under vacuum to afford crude
product. Purification was carried out with silica gel column
chromatography, using CH2Cl2/hexane as an eluent. Charac-
terization data for the resulting products 12 and 13 have been
reported.11
in Scheme 6, the reaction with bromobenzene afforded the
expected product 28 in a good yield (87%).
In conclusion, the unique SNAr reactivity of LiBr/DMF and
LiI/DMF uncovered in this work expedites the synthesis of the
central-ring dihalogenated pentiptycenes as well as other new
pentiptycene building blocks. These results should facilitate
the progress of pentiptycene-based functional materials.
Experimental Section
The key reactions of this work, which are the SNAr reaction of
15 to form 18 and 19, the SNAr reaction of 18 and 19 to form 22,
and the nitro group reduction of 18 and 19 to form 12 and 13, are
described in the following text. The procedures and character-
ization data for the other reactions are supplied as Supporting
Information.
The SNAr Reaction of 15. To a stirred solution of 15 (2.01 g,
3.2 mmol) was added LiBr (0.83 g, 9.6 mmol) or LiI (1.29 g,
9.6 mmol) in anhydrous DMF (5.0 mL). It was heated for 24 h at
150 °C and then cooled to room temperature. Water was added
to the solution and the resulting precipitate was collected by
filtration, washed with water, and dried under vacuum to afford
crude product. Further purification was carried out with silica
gel column chromatography, using CH2Cl2/hexane as an eluent.
Compound 18: yield 74%, white solid, mp >300 °C; 1H NMR
(400 MHz, CDCl3) δ 5.81 (s, 2H), 6.03 (s, 2H), 7.05-7.07 (m,
8H), 7.43-7.45 (m, 8H); 13C NMR (100 MHz, CDCl3) δ 50.0,
53.7, 119.6, 123.8, 123.9, 125.5, 125.6, 137.0, 142.8, 143.2,
144.2; IR (KBr): 1525, 1351 cm-1; HRMS (FAB) m/z calcd
for C34H20BrNO2 553.0677, found 553.0679. Compound 19:
Acknowledgment. We thank the National Science Council
of Taiwan (NSC-98-2119-M-002-002-MY3) and Academia
Sinica for financial support.
Supporting Information Available: Experimental proce-
dures and characterization data for 23, 24, and 26-28 and 1H
and 13C NMR spectra of 18, 19, 22-24, and 26-28. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(32) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805–818. (b) Hartwig, J. F. Angew. Chem., Int. Ed. 1998,
37, 2046–2067.
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