The cobalt-way to heterophenylenes: Syntheses of 2-thianorbiphenylenes, monoazabiphenylenes, and linear 1-aza[3]phenylene {biphenyleno[2,3-a] cyclobuta[1,2- b ]pyridine}

Article abstract of DOI:10.1055/s-0030-1259087

CpCo(CO)₂ catalyzes the cocyclization of ortho- diethynylthiophenes and -pyridines with alkynes to construct the corresponding thia- and azaphenylenes. This strategy is applied to the synthesis of linear 1-aza[3]phenylene, the first higher heterophenylene. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0030-1259087

280
LETTER
The Cobalt-Way to Heterophenylenes: Syntheses of 2-Thianorbiphenylenes,
Monoazabiphenylenes, and Linear 1-Aza[3]phenylene {Biphenyleno[2,3-a]-
cyclobuta[1,2-b]pyridine}
C
obalt-Catalyze
d
S
e
ynthesis of
r
H
eterop
e
henylenesna Engelhardt,a J. Gabriel Garcia,a Aude A. Hubaud,a Konstantin A. Lyssenko,b Spyros Spyroudis,a
Tatiana V. Timofeeva,b Paul Tongwa,b K. Peter C. Vollhardt*a
a
Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720-1460, USA
Fax +1(510)6435208; E-mail: kpcv@berkeley.edu
Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701, USA
b
Received 19 October 2010
70%) to give the known parent 3 (R = H), which had been
Abstract: CpCo(CO)2 catalyzes the cocyclization of ortho-diethy-
nylthiophenes and -pyridines with alkynes to construct the corre-
sponding thia- and azaphenylenes. This strategy is applied to the
synthesis of linear 1-aza[3]phenylene, the first higher heterophe-
nylene.
synthesized by the bis-Wittig reaction of 1,2-benzo-
cyclobutadienequinone with bis(triphenylphosphoranyl-
idenemethyl) sulfide.12 The success (if limited) of the
cocyclizations of 2b (6 equiv) and 2c (0.9 equiv), render-
ing 3b and 3c (Scheme 1),8 point to some generality of the
process. In an attempt to access the 2-thianor[3]phenylene
analogue of 3a, diyne 3c was protodesilylated and sub-
jected to cocyclotrimerization with 2a, resulting in
decomposition. The pyrrole relatives of 1, 3,4-
diethynylpyrrole12 and (hitherto unknown) 3,4-diethynyl-
1,2,5-trimethylpyrrole,8 failed to furnish 2-azanorbiphe-
nylenes.
Key words: alkynes, cobalt, cyclization, heterocycles, transition
metals
The [N]phenylenes are a new class of polycyclic cyclo-
hexatrienoid hydrocarbons, in which the alternating fu-
sion between benzene and cyclobutadiene rings causes
unusual activation of the p- and s-framework.1 As such,
they have garnered increasing recent attention as
electronically2 and otherwise functional materials.3 In a
similar vein, heteropolyarenes, in particular thiophene-
and pyridine-derived systems, are also the subject of in-
tense current scrutiny for such applications.4,5 Conse-
quently, heterophenylenes appear to be attractive targets
of investigation. However, few such systems are known
(or claimed), all restricted to the biphenylene nucleus.6
We report that cobalt-catalyzed cocyclization of ortho-
diethynylarenes with alkynes, a strategy employed for the
assembly of the all-carbon phenylenes,1 seems applicable
to the synthesis of heteroanalogues.
R
R (2),
R
R
CpCo(CO)2, Δ, hν
S
S
124°
2a, R = SiMe3, 10%
2b, R = COOMe, 18%
2c, R = C2TIPS, 5%
1
3
Scheme 1
The relatively poor outcome of the above experiments
notwithstanding, they suggested that employment of a less
sensitive heteroaromatic core might be more successful.
Attention was therefore directed at the synthesis of aza-
phenylenes from appropriate ethynylpyridines. Thus, pro-
todesilylation of 3,4-bis[(trimethylsilyl)ethynyl]pyridine13
[K2(CO)3, MeOH–Et2O (1:3)] generated (unisolated) 4,
which was immediately cocyclized with 2a to assemble
6,7-bis(trimethylsilyl)-2-azabiphenylene (5) in 61% yield
(Scheme 2).8 Protodesilylation (TfOH–cyclohexane,
69%) gave the known parent system.6,14
An initial foray exploring this notion started with 3,4-di-
ethynylthiophene (1)7 and its reaction with irradiated (hn),
boiling (D) bis(trimethylsilyl)acetylene (BTMSA, 2a) as
the solvent in the presence of catalytic CpCo(CO)2 (ca. 15
mol%). Gratifyingly (Scheme 1), the desired 2-thianorbi-
phenylene 3a formed, but only in 10% yield, in addition
to an array of CpCo(cyclobutadiene) and diyne autocy-
clization products.8 The corresponding reaction of 2 with
o-diethynylbenzene is nearly quantitative,9 and the poor
outcome in the case of 1 may be the result of the cumula-
tively deleterious effect of the widened enediyne bond an-
gles,10 the potential interference of the metal with the
sulfur frame,11 and the activated nature of the benzocy-
clobutadieno-fused thiophene ring. Chemical structure
proof was attained by protodesilylation (TFA–CH2Cl2,
Similarly, 2,3-bis[(trimethylsilyl)ethynyl]pyridine, ac-
cessed more directly than reported,15 namely by Sono-
gashira trimethylsilylethynylation of commercial 2,3-
2a (solvent),
CpCo(CO)2
(15 mol%),
SiMe3
SiMe3
Δ, hν
61%
N
N
SYNLETT 2011, No. 2, pp 0280–0284
Advanced online publication: 07.12.2010
x
x.
x
x.
2
0
1
1
4
5
DOI: 10.1055/s-0030-1259087; Art ID: S07710ST
© Georg Thieme Verlag Stuttgart · New York
Scheme 2

LETTER
Cobalt-Catalyzed Synthesis of Heterophenylenes
281
dibromopyridine,8 was converted into the 1-aza derivative
7 via (unisolated) diyne 6 (Scheme 3).8 Again, protodesi-
lylation (as above for 5; 81%) led to the known 1-azabi-
phenylene.6,14 Iododesilylation (ICl, CCl4, 58%) furnished
6,7-diiodo-1-azabiphenylene.8 These results, while pre-
liminary, make the underlying strategy clearly the ap-
proach of choice for the synthesis of substituted
derivatives of both azabiphenylenes.
1. KF⋅2H2O (43 equiv),
Me3Si
SiMe3
18-crown-6, DME, r.t.
2. 2a (solvent),
CpCo(CO)2 (0.9 equiv), Δ, hν
11%
N
Me3Si
SiMe3
8
Me3Si
SiMe3
SiMe3
2a (solvent),
Me3Si
N
CpCo(CO)2
4
5
8
(15 mol%),
SiMe3
SiMe3
4a 4b
6
7
3
2
Δ, hν
9 (not detected)
64%
SiMe3
8b 8a
N
N
1
Hb
Ha
SiMe3
6
7
Me3Si
Me3Si
Scheme 3
N
SiMe3
SiMe3
Encouraged by these findings, we embarked on the more
ambitious goal of extending the series to the first higher
heterophenylenes, specifically substituted linear 5-aza-
and 1-aza[3]phenylenes 9 and 13, respectively. Approach-
ing the first entailed quadruple alkynylation of 2,3,5,6-
tetrabromopyridine16 to produce 8,8 which was deprotect-
ed (KF·2H2O, 18-crown-6, DME) in situ and reacted with
2a in the presence of CpCo(CO)2. Surprisingly, the only
product that could be detected was 10, evidently derived
from 9 by insertion of 2a into the four-membered-ring
bond proximal to nitrogen (Scheme 4). Its structural, in
particular regiochemical, assignment rests on the spectral
data.8 Most clearly diagnostic is the NOE between Ha and
Hb. The formation of 10 can be viewed in the context of
the well-precedented activation of the strained ring in bi-
phenylene by transition metals toward insertion of unsat-
urated moieties17 and the relatively high reactivity of the
a-position in pyridine in metal-mediated transforma-
tions.18 Considering the relative stability of 5 and 7 to the
conditions of their generation, it appears that the second
strained ring in 9 labilizes the s frame of the first. Such an
effect has also been observed for the all-carbon analogue,
linear [3]phenylene.9,19
10
Scheme 4
TIPS
TIPS
TIPS
CpCo(CO)2
(0.9 equiv),
toluene, Δ, hν
6
+
27%
N
12
TIPS
11 (0.6 equiv)
1. TBAF (2.8 equiv),
THF, r.t.
2. 2a (solvent),
CpCo(CO)2
(0.25 equiv), Δ, hν
SiMe3
SiMe3
65%
N
13
t-BuOK (10 equiv),
t-BuOH, DMSO,
THF, r.t.
67%
N
In light of the detrimental positioning of the nitrogen in 5-
azaphenylene 9, the core of the target was changed to that
of its 1-azaisomer (Scheme 5). For this purpose, 6 was co-
cyclized with triyne 1120 to provide 12.8 In situ protodesi-
lylation, followed by another cyclization, now with 2a,
gratifyingly engendered the novel 1-aza[3]phenylene 13.8
Attaining the ultimate goal, parent 14, was approached
with trepidation, since desilylation of 13 with acid was ex-
pected to lead to addition reactions to the central activated
ring,1,9 and treatment with base, successful in the synthe-
sis of the carbon analogue linear [3]phenylene, was con-
sidered risky in view of the hydrolytic instability of
azabiphenylene derivatives.21 In the event, the latter con-
cerns proved unfounded and 14 evolved from 13 through
the action of potassium tert-butoxide.8
14
Scheme 5
duced by the presence of the nitrogen atom. Specifically,
the NMR signals of the nuclei adjacent to the cyclobuta-
dienoid rings are unusually shielded, as exemplified by
d
H-5 = 6.40 ppm, dH-10 = 6.48 ppm (CDCl3), dC-5 = 112.2
ppm, and dC-10 = 111.4 ppm (CD2Cl2) for 14 (compare the
corresponding values for linear [3]phenylene: d = 6.24
and 111.2 ppm). The electronically activated nature of
these molecules is also confirmed by their deep red color,
quantified by their longest wavelength UV/vis absorp-
tions at 437 (13) and 433 nm (14), respectively. The ba-
thochromic shift experienced by the 1-azabiphenylene
frame on benzocyclobutannelation to 14 is 75 nm, identi-
cal to that observed when going from biphenylene to lin-
ear [3]phenylene. It thus seems that the azaderivatives
The spectral properties of 13 and 14 parallel those of their
carbon counterparts,1a,9 except for the perturbation in-
Synlett 2011, No. 2, 280–284 © Thieme Stuttgart · New York

282
V. Engelhardt et al.
LETTER
have potential as building blocks for interesting materials, Turning to Figure 2, analysis of the experimental structure
similar to that of their carbocyclic analogues, with the of 14 (see a) is obfuscated by the disorder of the nitrogen
added advantage of bearing the tunable heteroatom.
over all four topological-symmetry-equivalent positions.
The unit cell features two independent flat molecules, one
containing an inversion center. The packing shows a sand-
wich-herringbone motif,27 depicted in Figure 3, with near-
est neighbor planes at an angle of 94°.
Because of the novelty of these systems,22 X-ray crystal-
lographic analyses of 12 and 14 were performed, in addi-
tion to DFT calculations (B3LYP/6-311G**) of the parent
1-azabiphenylene and 14 (Figures 1 and 2).
Figure 1 Structural details (bond lengths and angles) of a: pyridine
in the gas phase (‘best fit’ data;23 top left), b: 1-azabiphenylene by
computation (B3LYP/6-311G**), c: biphenylene in the crystal [stan-
dard deviations 0.002–0.004 (bond lengths) and 0.2 (angles)],24 and d:
12 in the crystal [data are the mean values from two independent mo-
lecules in the unit cell; standard deviations 0.006–0.007 (bond
lengths) and 0.4–0.5 (angles)]. All four-membered-ring angles devia-
te <1° from 90°.
Figure 2 Structural details (bond lengths and angles) of a: linear 1-
aza[3]phenylene (14) in the crystal [the nitrogen atom is disordered
over four positions, modeled as the superposition of 0.25 N and 0.75
C; data are the mean values from two independent molecules in the
unit cell; standard deviations 0.002 (bond lengths) and 0.1–0.2 (an-
gles)], b: computed data for 14 (B3LYP/6-311G**), and c: X-ray data
for linear [3]phenylene (center of inversion).28 All four-membered-
ring angles deviate <1° from 90°.
Inspection of the structure of 12 in Figure 1 (d) reveals
that the presence of the nitrogen accentuates the distor-
tions induced by the cyclobutadienoid ring in biphenylene
(see c).1a,6,25 Thus, there is pronounced bond alternation of
the pyridine nucleus toward an azacyclohexatriene con-
figuration with a bond localization index Li = 0.22.26 For
comparison, the corresponding values for pyridine (see a)
and biphenylene are 0.15 and 0.14, respectively. This fea-
ture is also apparent in the calculated structure of the par-
ent 1-azabiphenylene (see b), which also provided the
opportunity to compare the geometric details of the un-
substituted cyclohexatrienoid ring with that in biphe-
nylene. The two are remarkably similar (Li = 0.11 and
0.14, respectively), indicating that the nitrogen atom ex-
erts its effect primarily on its immediate surroundings.
Because of the disorder of the N atom in a, recourse was
taken to a computed structure of 14 (see b) for a compar-
ison of bond lengths and angles with those in linear
[3]phenylene (see c).28 Averaging the structural details of
b to reflect the disorder in a reproduced the data for the
latter fairly well (e.g., averaged bonds to N in b: 1.348 and
1.405 Å; versus those in a: 1.346 and 1.415 Å, respective-
ly; averaged angle at N: 114.4° versus 114.0°). Analogous
to the findings pertaining to Figure 1, introducing the het-
eroatom into the linear [3]phenylene frame affects the het-
erocycle the most (Li = 0.19). The center ring retains its
characteristic bis-p-allyl frame and is unaltered (Li = 0.07
in both b and c), similar to the terminal carbocycle
(Li = 0.14 vs. 0.15).
Synlett 2011, No. 2, 280–284 © Thieme Stuttgart · New York

LETTER
Cobalt-Catalyzed Synthesis of Heterophenylenes
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Figure 3 Crystal packing diagram of 14
In summary, we have shown that CpCo(CO)2 educes the
potential of ortho-diethynylthiophenes and -pyridines to
function as cooligomerization partners with alkynes on
route to the corresponding heterophenylenes, a transfor-
mation that should be general for a variety of derivatives.
An iterative sequence gives rise to linear 1-aza[3]phe-
nylene, the first higher heterophenylene. Its properties re-
semble those of the carbon analogue, with the exception
of the local perturbations induced by the heteroatom. One
can anticipate that the chemistry disclosed herein is appli-
cable to the assembly of other members of the family of
heterophenylenes.
(6) Shepherd, M. K. Cyclobutarenes. The Chemistry of
Benzocyclobutene, Biphenylene, and Related Compounds;
Elsevier: Amsterdam, 1991.
(7) Sugihara, Y.; Yagi, T.; Murata, I.; Imamura, A. J. Am. Chem.
Soc. 1992, 114, 1479.
(8) The structures of all new compounds (Scifinder) were in
accord with their analytical and/or spectroscopic properties;
see Supporting Information. CCDC 797197 and CCDC
797198 contain the supplementary crystallographic data for
12 and 14, respectively. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This study benefited from financial aid of the NSF (CHE-0907800;
KPCV; STC MDITR, DMR-0120967, PREM DMR-0934212;
TVT).
Selected Data
Compound 3a: colorless crystals; mp 80–81 °C (EtOH–
H2O). IR (hexanes): 3010, 1364, 1250, 1064, 1031, 854 cm–
1. 1H NMR (300 MHz, CDCl3): d = 7.18 (s, 2 H), 6.53 (s, 2
H), 0.32 (s, 18 H). 13C NMR (75 MHz, CDCl3): d = 148.2,
147.7, 144.8, 124.1, 113.2, 2.63. MS (EI, 70 eV): m/z
(%) = 302 (100) [M]+, 273 (12), 272 (22), 271 (85), 215 (13),
73 (58). UV/Vis (EtOH): lmax (log e) = 257 sh (5.04), 262
(5.16), 337 (4.37), 353 (4.43) nm. Anal. Calcd for
C16H22SSi2: C, 63.51; H, 7.33. Found: C, 63.84; H, 7.58.
Compound 5: opaque oil. 1H NMR (400 MHz, CDCl3):
d = 8.12 (d, J = 5 Hz, 1 H), 7.60 (br s, 1 H), 7.32 (s, 1 H),
7.27 (s, 1 H), 6.38 (d, J = 5 Hz, 1 H), 0.30 (s, 18 H). MS (EI,
70 eV): m/z (%) = 297 (100) [M]+, 282 (70), 266 (20), 239
(12), 73 (53). HRMS (EI): m/z [M]+ calcd for C17H23NSi2:
297.1369; found: 297.1361. UV/Vis (hexane): lmax (log
References and Notes
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Synlett 2011, No. 2, 280–284 © Thieme Stuttgart · New York

284
V. Engelhardt et al.
LETTER
(15) Kim, C.-S.; Russell, K. C. J. Org. Chem. 1998, 63, 8229.
e) = 231 (3.56), 301 (3.02), 325 (3.25), 340 (3.31) nm.
Compound 7: opaque oil. 1H NMR (500 MHz, CDCl3;
assignments by 2D NMR): d = 7.64 (d, J = 5.5 Hz, 1 H, H2),
7.26 (s, 1 H, H8), 7.16 (s, 1 H, H5), 6.74 (d, J = 6.5 Hz, 1 H,
H4), 6.51 (dd, J = 6.25, 6.25 Hz, 1 H, H3), 0.34 (s, 9 H,
Me3Si), 0.33 (s, 9 H, Me3Si). 13C NMR (100 MHz, CDCl3):
d = 173.1 (C8b), 152.7 (C8a), 150.9 (C6/7), 150.5 (C4b), 150.2
(C4a), 149.3 (C6/7), 145.7 (C2), 124.6 (C5), 124.3 (C8), 122.4
(C3), 122.0 (C4), 2.00 (SiCH3), 1.99 (SiCH3). MS (EI, 70
eV): m/z (%) = 297 (100) [M]+, 282 (72), 266 (32), 239 (17),
73 (55). HRMS (EI): m/z [M]+ calcd for C17H23NSi2:
297.1369; found: 297.1361. UV/Vis (hexane): lmax (log
e) = 241 (4.01), 270 (3.76), 347 (3.50), 362 nm (3.40).
Compound 14: deep red crystals; mp 225–226 °C (dec.;
hexane). IR (neat): 3069, 2929, 2855, 1643, 1585, 1415,
1349, 1246, 1186, 1153, 865, 733 cm–1. 1H NMR (400 MHz,
CDCl3; assignments by 2D NMR): d = 7.48 (dd, J = 6.0, 1.2
Hz, 1 H, H2), 6.72 (m, 2 H, H7,8), 6.53 (m, 2 H, H6,9), 6.48 (d,
J = 1.2 Hz, 1 H, H10), 6.45 (dd, J = 6.8, 1.2 Hz, 1 H, H4), 6.40
(s, 1 H, H5), 6.39 (dd, J = 6.4, 6.4 Hz, 1 H, H3). 13C NMR
(150 MHz, CD2Cl2): d = 172.2 (C10b), 155.4 (C4b/10a), 155.0
(C4b/10a), 153.9 (C5a/9b), 153.6 (C5a/9b), 150.4 (C5b/9a), 150.1
(C5b/9a), 148.9 (C4a), 145.2 (C2), 129.2 (C7/8), 129.0 (C7/8),
122.3 (C3), 119.2 (C4), 117.1 (C6/9), 117.0 (C6/9), 112.2 (C5),
111.4 (C10). MS (EI, 70 eV): m/z (%) = 227 (100) [M]+, 201
(30), 174 (7), 150 (4), 123 (3), 113 (14), 100 (22). HRMS
(EI): m/z [M]+ calcd for C17H9N: 227.0735; found:
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