Cascade carbopalladation reaction between alkynes and gem -dibromoolefins: Facile access to monoannelated pentalenes

Article abstract of DOI:10.1021/ol301670d

A carbopalladation cascade reaction of easily accessible gem-dibromoolefins and alkynes furnishes monobenzo- and mononaphthopentalenes. The new chromophores accessed by this short route exhibit small HOMO-LUMO gaps and redox amphoteric behavior with tunable redox potentials.

Full text of DOI:10.1021/ol301670d

ORGANIC
LETTERS
2012
Vol. 14, No. 16
4066–4069
Cascade Carbopalladation Reaction
between Alkynes and gem-
Dibromoolefins: Facile Access
to Monoannelated Pentalenes
Pablo Rivera-Fuentes,†,§ Margarete von Wantoch Rekowski,†,§ W. Bernd Schweizer,†
Jean-Paul Gisselbrecht,‡ Corinne Boudon,‡ and Franc-ois Diederich*,†
€
€
Laboratorium fu€r Organische Chemie, ETH Zu€rich, Honggerberg, HCI, 8093 Zurich,
Switzerland, and Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
ꢀ
Institut de Chimie-UMR 7177, C.N.R.S., Universite de Strasbourg, 4 rue Blaise Pascal,
67081 Strasbourg Cedex, France
diederich@org.chem.ethz.ch
Received June 18, 2012
ABSTRACT
A carbopalladation cascade reaction of easily accessible gem-dibromoolefins and alkynes furnishes monobenzo- and mononaphthopentalenes. The
new chromophores accessed by this short route exhibit small HOMOꢀLUMO gaps and redox amphoteric behavior with tunable redox potentials.
Polycyclic aromatic hydrocarbons (PAHs) are ubiqui-
tous structures in molecular organic electronics.1 The vast
majority of these PAHs are based on fused six-membered
rings, while the opto-electronic properties of polycyclic
unsaturated hydrocarbons consisting of fused five-membered
rings (polyquinenes)2 have remained largely unexplored.3
Pentalene is the simplest polyquinene, and its derivatives
have recently attracted considerable attention due to their
interesting opto-electronic properties. Symmetrically substi-
tuted pentalenes fused to two other benzenoid aromatic
systems can be conveniently accessed by palladium-4 and
nickel-mediated5 homocoupling reactions. These materials
are both good electron donors and electron acceptors (redox
amphoteric behavior) with small HOMOꢀLUMO gaps,5,6
possessing interesting charge transport properties, and are
suitable for application in organic field-effect transistors
(OFETs).7
† ETH Z€urich.
‡
ꢀ
Universite de Strasbourg.
§ P.R.-F. and M.v.W.R. contributed equally.
€
(1) For recent reviews, see: (a) Pisula, W.; Feng, X.; Mullen, K. Chem.
Mater. 2011, 23, 554–567. (b) Hoheisel, T. N.; Schrettl, S.; Szilluweit, R.;
Frauenrath, H. Angew. Chem., Int. Ed. 2010, 49, 6496–6515.
(2) Moss, G. P.; Smith, P. A. S.; Tavernier, D. Pure Appl. Chem. 1995,
67, 1307–1375.
(3) For classic and recent papers on pentalene and polyquinene
synthesis, see: (a) Brand, K. Ber. Dtsch. Chem. Ges 1912, 45, 3071–
3077. (b) Bally, T.; Chai, S.; Neuenschwander, M.; Zhu, Z. J. Am. Chem.
Soc. 1997, 119, 1869–1875. (c) Saito, M.; Hashimoto, Y.; Tajima, T.;
Ishimura, K.; Nagase, S.; Minoura, M. Chem.;Asian. J. 2012, 7, 480–
We became interested in exploring the opto-electronic
properties of asymmetrically substituted pentalenes fused
to only one benzenoid aromatic system. However, only a
few approaches for the synthesis of such compounds have
€
483. (d) Hashmi, A. S. K.; Wieteck, M.; Braun, I.; Nosel, P.; Jongbloed,
L.; Rudolph, M.; Rominger, F. Adv. Synth. Catal. 2012, 354, 555–562.
(e) LeGoff, E. J. Am. Chem. Soc. 1962, 84, 3975–3976. (f) Hafner, K.;
Bangert, K. F.; Orfanos, V. Angew. Chem., Int. Ed. 1967, 6, 451–452. (g)
(4) Levi, Z. U.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131, 2796–2797.
(5) Kawase, T.; Konishi, A.; Hirao, Y.; Matsumoto, K.; Kurata, H.;
Kubo, T. Chem.;Eur. J. 2009, 15, 2653–2661.
(6) Levi, Z. U.; Tilley, T. D. J. Am. Chem. Soc. 2010, 132, 11012–
11014.
(7) Kawase, T.; Fujiwara, T.; Kitamura, C.; Konishi, A.; Hirao, Y.;
Matsumoto, K.; Kurata, H.; Kubo, T.; Shinamura, S.; Mori, H.;
Miyazaki, E.; Takimiya, K. Angew. Chem., Int. Ed. 2010, 49, 7728–7732.
€
Hafner, K.; Suss, H. U. Angew. Chem., Int. Ed. 1973, 12, 575–577. (h)
Hafner, K.; Suda, M. Angew. Chem., Int. Ed. 1976, 15, 314–315. (i)
Hafner, K.; Goltz, M. Angew. Chem., Int. Ed. 1982, 21, 695–696. (j) Fu,
X.; Cook, J. M. Aldrichimica Acta 1992, 25, 43–54. (k) Cao, H.; Flippen-
Anderson, J.; Cook, J. M. J. Am. Chem. Soc. 2003, 125, 3230–3231. (l)
Cao, H.; Van Ornum, S. G.; Deschamps, J.; Flippen-Anderson, J.; Laib,
F.; Cook, J. M. J. Am. Chem. Soc. 2005, 127, 933–943.
r
10.1021/ol301670d
Published on Web 07/31/2012
2012 American Chemical Society

been reported using pyrolysis8aꢀe or multistep syntheses.8fꢀj
Hence, we decided to develop a simple and rapid method
that allows preparation of this kind of molecules starting
from readily available reactants. Carbopalladation cascade
reactions are powerful methods for the construction of
carbocycles,9 usually consisting of an initial oxidative addi-
tion, one or more carbopalladation steps, and a reductive
elimination. In this sense, carbopalladation cascades often
rely on the presence of carbonꢀhalide bonds for the
oxidative addition and multiple carbonꢀcarbon bonds
for the carbopalladation steps. Among the many orga-
nohalide derivatives that can be used to start the cascade,
gem-dibromoolefins10 are especially interesting and have
been used in a variety of palladium-catalyzed cascades to
afford N-fused heterocyclic systems,11 methyleneindenes,12
isocoumarins,13 benzofurans,14 oxazolones,15 methylenei-
soindolin-1-ones,16 indoles,17 and thiophenes,18 among
many other heterocycles. To the best of our knowledge,
gem-dibromoolefins have never been used for the con-
struction of pentalene derivatives.
We began our investigations on the reaction of gem-
dibromoolefin 1a and diphenylacetylene 2a (Table 1).
Initial experiments using catalytic or stochiometric amounts
of palladium did not deliver any reaction product (Table 1,
entries 1 and 2). We noticed that the reaction requires
addition of a reducing agent (1,4-hydroquinone, isopro-
panol, or zinc dust) to proceed; however, only zinc dust
gave reasonable yields (Table 1, entries 3ꢀ8). On the
other hand, a control experiment using only zinc dust
proved the necessity of the palladium catalyst for product
formation (Table 1, entry 9). With these results in hand, we
started to explore the optimal conditions. Different reac-
tion parameters such as solvent, temperature, and source
of palladium (Table 1, entries 10ꢀ13) were examined;
however, no improvement was observed.
Table 1. Investigated Reaction Conditions for the Carbopalla-
dation Cascade Reaction
temp
reducing
agentb
yieldc
[%]
entry
catalysta
[°C]
solvent
1
2
3
4
5
[Pd(PPh3)2Cl2]
100 toluene
none
0
[Pd(PPh3)2Cl2]d 100 toluene
none
0
[Pd(PPh3)2Cl2]
25
toluene
zinc dust
tracese
[Pd(PPh3)2Cl2] 100 toluene
zinc dust 42
1,4-hydro- 14
quinone
[Pd(PPh3)2Cl2]
[Pd(dba)2]
100 toluene
100 toluene
6
1,4-hydro- 10
quinone
7
8
9
[Pd(PPh3)2Cl2]
[Pd(dba)2]
none
100 toluene
100 toluene
100 toluene
isopropanol
8
isopropanol tracese
zinc dust
0
10 [Pd(PPh3)2Cl2]
11 [Pd(PPh3)2Cl2]
12 [Pd(MeCN)2Cl2] 100 toluene
13 [Pd(dba)2] 100 toluene
100 1,4-dioxane zinc dust
tracese
tracese
25
130 DMF
zinc dust
zinc dust
zinc dust
(8) For papers on the synthesis of monoannelated pentalenes: (a)
Brown, R. F. C.; Eastwood, F. W. Pure Appl. Chem. 1996, 68, 261–266.
(b) Bapat, J. B.; Brown, R. F. C.; Bulmer, G. H.; Childs, T.; Coulston,
K. J.; Eastwood, F. W.; Taylar, D. K. Aust. J. Chem. 1997, 50, 1159–
1182. (c) Blake, M. E.; Bartlett, K. L.; Jones, M., Jr. J. Am. Chem. Soc.
2003, 125, 6485–6490. (d) Shukla, B.; Tsuchiya, K.; Koshi, M. J. Phys.
Chem. A 2011, 115, 5284–5293. (e) Comandini, A.; Malewicki, T.;
Brezinsky, K. J. Phys. Chem. A 2012, 116, 2409–2434. (f) LeGoff, E.
J. Am. Chem. Soc. 1962, 84, 1505–1506. (g) Ried, W.; Freitag, D.
Tetrahedron Lett. 1967, 8, 3135–3136. (h) Ried, W.; Freitag, D. Chem.
Ber. 1968, 101, 756–762. (i) Ried, W.; Schaefer, H.-J. Synthesis 1970, 3,
142. (j) Wu, T.-C.; Tai, C.-C.; Tiao, H.-C.; Chang, Y.-T.; Liu, C.-C.; Li,
C.-H.; Huang, C.-H.; Kuo, M.-Y.; Wu, Y.-T. Chem.;Eur. J. 2011, 17,
7220–7227.
22
a 0.05 equiv (unless otherwise stated). b 1 equiv. c Isolatedyields(unless
otherwise stated). d 1 equiv. e Traces were detected by TLC and 1H NMR
but no product was isolated. dba = dibenzylideneacetone. DMF = N,N-
dimethylformamide.
We next investigated the generality of this carbopallada-
tion cascade using various gem-dibromoolefins 1 and
internal alkynes 2 (Table 2). The transformation indeed
provides a variety of pentalene derivatives that are other-
wise inaccessible or tedious to prepare. Another advantage
of this method is the availability of the starting materials,
which are either commercially available or can be prepared
in a few steps following well-established protocols.
€
(9) For a review on carbopalladation cascade reactions, see: Brase, S.;
de Meijere, A. In Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; John Wiley & Sons, Inc.: New York, 2002; Vol. 1,
pp 1369ꢀ1403.
(10) The chemistry of gem-dihaloolefins has been recently reviewed:
Chelucci, G. Chem. Rev. 2012, 112, 1344–1462.
(11) Chai, D. I.; Lautens, M. J. Org. Chem. 2009, 74, 3054–3061.
(12) (a) Bryan, C. S.; Lautens, M. Org. Lett. 2010, 12, 2754–2757. (b)
Ye, S.; Ren, H.; Wu, J. J. Comb. Chem. 2010, 12, 670–675. (c) Ye, S.;
Yang, X.; Wu, J. Chem. Commun. 2010, 46, 2950–2952.
(13) Wang, L.; Shen, W. Tetrahedron Lett. 1998, 39, 7625–7628.
(14) Thielges, S.; Meddah, E.; Bisseret, P.; Eustache, J. Tetrahedron
Lett. 2004, 45, 907–910.
The reaction possibly follows a mechanism similar to
that proposed for the homocoupling of haloenynes re-
ported by Levi and Tilley,4 although the requirement of
zinc dust suggests that zinc might trigger an alternative
mechanistic pathway. We propose that the first step in the
catalytic cycle (Scheme 1) involves coordination of palla-
dium to the alkyne moiety of the gem-dibromoolefin (A),
directing the oxidative addition to the desired CBr bond
(B).19 The next step is the intramolecular carbopalladation
(15) Chai, D. I.; Hoffmeister, L.; Lautens, M. Org. Lett. 2011, 13,
106–109.
(16) Sun, C.; Xu, B. J. Org. Chem. 2008, 73, 7361–7364.
(17) (a) Fang, Y.-Q.; Lautens, M. J. Org. Chem. 2008, 73, 538–549.
(b) Fang, Y.-Q.; Lautens, M. Org. Lett. 2005, 7, 3549–3552. (c) Fayol,
A.; Fang, Y.-Q.; Lautens, M. Org. Lett. 2006, 8, 4203–4206. (d) Bryan,
C. S.; Lautens, M. Org. Lett. 2008, 10, 4633–4636. (e) Arthuis, M.;
Pontikis, R.; Florent, J.-C. Org. Lett. 2009, 11, 4608–4611.
(18) (a) Bryan, C. S.; Braunger, J. A.; Lautens, M. Angew. Chem., Int.
Ed. 2009, 48, 7064–7068. (b) Zeng, F.; Alper, H. Org. Lett. 2011, 13,
2868–2871.
(19) A similar directing effect has been observed in related substrates:
(a) Nuss, J. M.; Rennels, R. A.; Levine, B. H. J. Am. Chem. Soc. 1993,
115, 6991–6992. (b) Torii, S.; Okumoto, H.; Tadokoro, T.; Nishimura,
A.; Rashid, M. A. Tetrahedron Lett. 1993, 34, 2139–2142.
Org. Lett., Vol. 14, No. 16, 2012
4067

Table 2. Carbopalladation Cascade Reaction To Access
Monoannelated Pentalenes 3
Scheme 1. Proposed Catalytic Cycle with Three Plausible
Termination Steps
yielda
[%]
entry gem-dibromoolefin
alkyne
product
1
2
3
4
5
6
7
8
9
1a: phenyl, R1 = H
1a: phenyl, R1 = H
1a: phenyl, R1 = H
1b: phenyl, R1 = F
1b: phenyl, R1 = F
1b: phenyl, R1 = F
2a: R2 = H 3a: R1 = H, R2 = H
2b: R2 = Me 3b: R1 = H, R2 = Me
2c: R2 = F 3c: R1 = H, R2 = F
2a: R2 = H 3d: R1 = F, R2 = H
2b: R2 = Me 3e: R1 = F, R2 = Me
2c: R2 = F 3f: R1 = F, R2 = F
42
32
21
40
27
17
35
1c: phenyl, R1 = OMe 2a: R2 = H 3g: R1 = OMe, R2 = H
1c: phenyl, R1 = OMe 2b: R2 = Me 3h: R1 = OMe, R2 = Me 20
1c: phenyl, R1 = OMe 2c: R2 = F 3i: R1 = OMe, R2 = F
20
23
20
10 1d: naphthyl, R1 = H 2a: R2 = H 3j: R1 = H, R2 = H
11 1d: naphthyl, R1 = H 2b: R2 = Me 3k: R1 = H, R2 = Me
a Yields correspond to isolated, pure products. Reagents and condi-
tions: [Pd(PPh3)2Cl2] (0.05 equiv), zinc dust (1 equiv), alkyne (20 equiv),
toluene, 100 °C, 24 h.
at the alkyne, forming a transient fulvene (C), followed by
intermolecular carbopalladation with an alkyne (D). For
termination, we suggest three plausible routes: (a) Oxida-
tive addition of zinc to the CBr bond (E), followed by an
intramolecular Negishi-type sequence; (b) Intramolecular
carbopalladation at the fulvene (F), followed by zinc-
assisted reductive elimination; (c) Two-electron reduction
of the Pd(II) intermediate by zinc, a second oxidative
addition, formation of a palladacycle (G), and reductive
elimination. Further mechanistic investigations are cur-
rently underway.
Monobenzopentalenes20 3aꢀi were isolated as dark
green solids that are stable both in solution and in the
solid state under ambient conditions. Mononaphthopen-
talenes 3j,k are greenish-brown solids that are stable in the
solid state but decompose slowly when kept in solution at
room temperature. Crystals suitable for X-ray diffraction
were grown of 3a and 3j (Figure 1),21 confirming the planar
structure of the tri- and tetracyclic scaffolds, respectively.
Pentalenes and some of their annelated derivatives are
planar, cyclic hydrocarbons with 4n π-electrons and can be
considered antiaromatic. The X-ray data of 3a and 3j show
bond length alternation in the pentalene cores (see the
Supporting Information (SI)), which is consistent with
antiaromaticity (for NICS values, see the SI).
Figure 1. X-ray crystal structures of 3a (a) and 3j (b). Arbitrary
numbering, atomic displacement parameters obtained at 100 K
are drawn at 30% probability level (for details, including bond
lengths, see the SI).
UV/vis spectra (see the SI) of the synthesized mono-
annelated pentalene derivatives display broad low-energy
absorptions with λmax values around 600 nm for 3aꢀi and
660 nm for 3j,k, with end absorptions reaching beyond
1000 nm. Time-dependent DFT (TD-DFT) calculations22
at the CAM-B3LYP/6-31G* level of theory revealed that
this absorption is associated with the HOMOꢀLUMO
transition (see the SI), which is symmetry forbidden (as is
usually the case for 4n π-chromophores) and therefore has
a low extinction coefficient. These results account for
optical HOMOꢀLUMO gaps of ca. 2.1 and 1.8 eV for
compounds 3aꢀi and 3j,k, respectively. These values are
smaller than those of dibenzo- and dinaphthopentalenes5
and comparable to those of the recently reported
(20) Monobenzo- and mononaphthopentalene are trivial names. For
IUPAC names of these compounds, see the Supporting Information.
(21) CCDC 871620 (3a) and 871621 (3j) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ or via www.ccdc.cam.ac.uk/data_request/
cif.
(22) All calculations were carried out using Gaussian 09, Revision
A.02, Frisch, M. J. et al. Gaussian, Inc., Wallington, CT, 2009. For the
full reference, see the Supporting Information.
4068
Org. Lett., Vol. 14, No. 16, 2012

6,12-diethynylindeno[1,2-b]fluorenes and indeno[2,1-
a]fluorenes (∼2.1 eV in both cases).23
substituents in compound 3f gives a reduction potential
of ꢀ1.45 V (compared to ꢀ1.51 V in 3a). Interestingly,
substituents increase or decrease the reduction and oxida-
tion potentials in a concerted manner, allowing the
HOMOꢀLUMO gap to remain essentially unchanged.
The electrochemical energy gaps around 2.1 eV for deriv-
atives 3aꢀi are consistent with the optical values (Table 3).
Mononaphthopentalene 3j undergoes oxidation at þ0.44 V
and reduction at ꢀ1.44 V, giving a smaller HOMOꢀ
LUMO gap of 1.88 eV, also in agreement with the
optical value (Table 3).
Cyclic voltammetry (CV) and rotatingdiskvoltammetry
(RDV) were measured for derivatives 3aꢀj. CV results for
some selected compounds are summarized in Table 3 (for
the full data set and RDV results, see the SI).
Table 3. Overview of the First Reduction and Oxidation
Potentials from Cyclic Voltammetry (CV) and Summary of
Electrochemical and Optical Energy Gaps
In summary, we developed a simple and rapid synthesis
of monobenzo- and mononaphthopentalenes based on a
carbopalladation cascade reaction between gem-dibromo-
olefins and alkynes. This methodology allows for the
preparation of asymmetrically substituted pentalene deriv-
atives with only one fused aromatic system, starting from
readily available reactants. The pentalenes prepared by
this method display small HOMOꢀLUMO gaps and
redox amphoteric behavior. Further investigations in our
laboratories include the incorporation of pentalene units in
pushꢀpull chromophores and extension of this methodol-
ogy to the preparation of polyquinenes containing two or
more fused pentalene units.
entry
3
Eox [V]a
ERed [V]a
ΔEredox [V]d
ΔEopt [V]e
1
2
3
4
3a
3f
þ0.61b
þ0.67b
þ0.46b
þ0.44b
ꢀ1.51c
ꢀ1.45c
ꢀ1.57c
ꢀ1.44b
2.12
2.12
2.03
1.88
2.22
2.07
2.04
1.84
3g
3j
a Electrochemical data obtained at a scan rate of 0.1 V sꢀ1 in CH2Cl2
þ 0.1 M Bu4NPF6 on a glassy carbon working electrode. All potentials
are given versus the Fcþ/Fc couple used as internal standard. Complete
set of electrochemical data, including second oxidation and reduction
potentials (when detected), and RDV data can be found in the SI.
b Redox potential for reversible electron transfer. c Irreversible peak
potential. d The electrochemical gap, ΔEredox, is defined as the potential
difference between the first oxidative and first reductive redox poten-
tials. e The optical gap, ΔEopt, is defined as the energy corresponding to
the lowest-energy absorption (λmax).
Acknowledgment. This work was supported by a grant
from the Swiss National Science Foundation. P.R.-F.
thanks the Stipendienfonds der Schweizerischen Chemischen
Industrie (SSCI) for a doctoral fellowship. Dedicated to
Prof. Dr. Dieter Seebach on the occasion of his 75th
birthday.
The values shown in Table 3 reveal that derivatives 3a
and 3j display a strong redox amphoteric behavior similar
to dibenzo[a,e]pentalenes.5 Derivative 3a shows an oxida-
tionpotential of þ0.61V that can be lowered toþ0.46V by
incorporation of a single methoxy group in compound 3g.
Analogously, the presence of fluorine and p-fluorophenyl
Supporting Information Available. Synthesis, NMR
spectra, X-ray crystallographic details, CIF file, DFT
and TD-DFT calculations, UV/vis spectra, and full elec-
trochemical data. This material is available free of charge
via the Internet at http://pubs.acs.org
(23) (a) Chase, D. T.; Fix, A. G.; Rose, B. D.; Weber, C. D.; Nobusue,
S.; Stockwell, C. E.; Zakharov, L. N.; Lonergan, M. C.; Haley, M. M.
Angew. Chem., Int. Ed. 2011, 50, 11103–11106. (b) Shimizu, A.; Tobe, Y.
Angew. Chem., Int. Ed. 2011, 50, 6906–6910. (c) Chase, D. T.; Fix, A. G.;
Kang, S. J.; Rose, B. D.; Weber, C. D.; Zhong, Y.; Zakharov, L. N.;
Lonergan, M. C.; Nuckolls, C.; Haley, M. M. J. Am. Chem. Soc. 2012,
134, 10349–10352. (d) Nishida, J.-i.; Tsukaguchi, S.; Yamashita, Y.
Chem.;Eur. J. 2012, 18, 8964–8970.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 16, 2012
4069