Synthesis and crystal structures of isolable terminal aryl hexatriyne and octatetrayne derivatives: Ar-(C≡C)nH (n = 3, 4)

Article abstract of DOI:10.1021/ol801121p

(Chemical Equation Presented) Unprecedented stability has been observed in terminal aryl hexatriyne and terminal aryl octatetrayne derivatives by judicious choice of a bulky, nonplanar headgroup [viz., 4-(3,6-di-tert-butyl-N- carbazolyl)phenyl] which hinders topochemical intermolecular interactions in the crystal lattice.

Full text of DOI:10.1021/ol801121p

ORGANIC
LETTERS
2008
Vol. 10, No. 14
3069-3072
Synthesis and Crystal Structures of
Isolable Terminal Aryl Hexatriyne and
Octatetrayne Derivatives:
Ar-(Ct C)nH (n ) 3, 4)
Changsheng Wang, Andrei S. Batsanov, Kara West, and Martin R. Bryce*
Department of Chemistry, Durham UniVersity, Science Site,
Durham, United Kingdom DH1 3LE
m.r.bryce@durham.ac.uk
Received May 15, 2008
ABSTRACT
Unprecedented stability has been observed in terminal aryl hexatriyne and terminal aryl octatetrayne derivatives by judicious choice of a
bulky, nonplanar headgroup [viz., 4-(3,6-di-tert-butyl-N-carbazolyl)phenyl] which hinders topochemical intermolecular interactions in the crystal
lattice.
It is generally accepted that alkyl and/or aryl terminal
oligoynes with more than two conjugated triple bonds are
unstable to routine isolation and purification, and these
compounds decompose quickly under ambient conditions.1
Phenylhexatriyne was reported to explode at 0 °C in the
absence of air during attempted isolation,2 while the unsub-
stituted hexatriyne exploded violently upon exposure to air.3
Kloster-Jensen et al. reported the isolation of the parent
butadiyne, hexatriyne, octatetrayne, and even decapentayne
[H-(Ct C)nH] (n ) 2-5). However, the only data to support
the structures were UV-vis spectra and 1H NMR spectra of
mixtures.4 Since the instability of oligoynes increases
dramatically as the sp-carbon chain elongates, aryl terminal
hexatriynes and higher homologues have normally been
obtained as solutions rather than isolated as their pure forms.5
Organometallic analogues are generally considered to be
significantly more stable than their organic counterparts,6 thus
terminal organometallo-hexatriynes have been isolated and
(4) (a) Kloster-Jensen, E. Angew. Chem., Int. Ed. 1972, 11, 438. (b)
Kloster-Jensen, E.; Haink, H.-J.; Christen, H. HelV. Chim. Acta 1974, 57,
1731.
(5) (a) Eastmond, R.; Walton, D. R. M. J. Chem. Soc., Chem. Commun.
1968, 204. (b) Eastmond, R.; Walton, D. R. M. Tetrahedron 1972, 28, 4591.
(c) Johnson, T. R.; Walton, D. R. M. Tetrahedron 1972, 28, 5221. (d) Luu,
T.; Morisaki, Y.; Cunningham, N.; Tykwinski, R. R. J. Org. Chem. 2007,
72, 9622.
(1) Morisaki, Y.; Luu, T.; Tykwinski, R. R. Org. Lett. 2006, 8, 689.
(2) Armitage, J. B.; Entwistle, N.; Jones, E. R. H.; Whiting, M. C.
J. Chem. Soc. 1954, 147.
(6) (a) Dembinski, R.; Lis, T.; Szafert, S.; Mayne, C. L.; Bartik, T.;
Gladysz, J. A. J. Organomet. Chem. 1999, 578, 229. (b) Dembinski, R.;
Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. J. Am. Chem. Soc. 2000,
122, 810. (c) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Chem.-Eur.
J. 2003, 9, 3324.
(3) Armitage, J. B.; Cook, C. L.; Jones, E. R. H.; Whiting, M. C.
J. Chem. Soc. 1952, 2010.
10.1021/ol801121p CCC: $40.75
Published on Web 06/13/2008
2008 American Chemical Society

well characterized.6,7 A Zn porphyrinylphenylhexatriyne has
also been isolated.8 Recently, we reported the synthesis and
X-ray crystal structures of a series of terminal aryl/heteroaryl
butadiynes which were stable to ambient isolation and
characterization.9 The crystal packing motifs of these mol-
ecules played important roles in their solid-state stability.
Intermolecular cross-linking of triple bonds is a major
pathway for the decomposition of oligoynes.10 Bulky aryl
head groups should stabilize longer sp homologues by
hindering the topochemical intermolecular interactions in the
crystal lattice, which are required for the head-tail π-π
cross-coupling.11
Scheme 1. Synthesis of the Terminal Aryl Oligoynes
With the aim of synthesizing longer all-organic terminal
oligoynes, we chose N-phenylcarbazole as a headgroup
because the N-phenyl substituent cannot be coplanar with
the carbazole moiety due to steric repulsion between peri-H
atoms, as proven by AM1 calculations (showing potential
minima at twist angles θ ) 45° and 135° with a torsion
barrier of 11 kcal/mol at θ ) 0°),12 NMR data, and crystal
structures of N-phenylcarbazole (θ ) 78° and 55°),12 N-(4-
cyanophenyl)carbazole (θ ) 48°),13 and 8 (θ ) 47°).14
We now report the successful synthesis, isolation, and
X-ray structural analyses of the terminal aryl hexatriyne 10
and the terminal aryl octatetrayne 7 both of which are
monocapped with an all-organic group, namely, 4-(3,6-di-
tert-butyl-N-carbazolyl)phenyl. To the best of our knowledge,
a pure terminal octatetrayne has not been reported previously.
The trimethylsilyl (TMS) protected decapentayne homologue
12 has also been isolated, although the corresponding
terminal pentayne 13 was unstable to isolation. Caution: The
stability of terminal oligoynes is dependent on the nature of
the end groups. Although no explosions were encountered
during this work, great care should be taken when carrying
out similar reactions.
of terminal 4-(3,6-di-tert-butyl-N-carbazolyl)phenyl alkynes
with 1-iodo-4-trimethylsilyl butadiyne (TMSBI)15 under
Sonogashira conditions, apart from the butadiyne derivative
4 which was prepared via precursor 3 using a 2-methyl-3,5-
hexadiyn-2-ol protocol.9,16 For the synthesis of the TMS-
protected hexatriyne 9, both methods, namely, the cross-
coupling of the monoalkyne 817 with TMSBI and the
cross-coupling of the iodo-butadiyne 5 with trimethylsily-
lacetylene (TMSA), gave 9 in comparable yields. However,
the former route was synthetically preferable as the cleaner
reaction mixture facilitated purification. Furthermore, the
monoalkyne 8 and its counterpart TMSBI are more readily
available than the diyne homologue 5. The terminal hexatriyne
10 was obtained by deprotecting 9 with potassium carbonate
in MeOH-CH2Cl2 at room temperature. Similarly, the tet-
rayne 7 was obtained by cross-coupling of diyne 4 with
TMSBI, followed by deprotection of 6. However, the
attempted cross-coupling of iodo-hexatriyne 11 with TMSA
under Sonogashira conditions yielded exclusively the oxi-
datively self-coupled diaryldodecahexayne derivative 14, as
dark yellow crystals which could be stored at room temper-
ature in the dark for 6 months without any change.18 The
same reaction under Hay conditions7 resulted in decomposi-
tion of 11 and self-coupling of TMSA. We, therefore,
abandoned this route.
Following the successful isolation of the terminal tetrayne
7, the synthesis of pentayne 13 was attempted. Terminal
hexatriyne 10 was cross-coupled with TMSBI under Sono-
gashira conditions to afford TMS-capped pentayne 12 in 10%
yield. The orange crystals of 12 were remarkably stable: a
sample was stored at rt in a flask wrapped with Al foil for
a week, then in a freezer (at ca. -20 °C) for several months
without any observable color change. It is likely that the low
yield of 12 was due to decomposition of 10 in the reaction
mixture. A large amount of insoluble dark-brown solid was
observed: this is characteristic of decomposed oligoynes. The
Our synthetic strategy toward the terminal oligoynes
(Scheme 1) was based on iterative cross-coupling reactions
(7) (a) Hartbaum, C.; Fischer, H. J. Organomet. Chem. 1999, 578, 186.
(b) Xu, G.-L.; Wang, C.-Y.; Ni, Y.-H.; Goodson, T. G., III; Ren, T.
Organometallics 2005, 24, 3247
.
(8) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen,
L.; Tom, J. P. C.; Fazio, M. A.; Schuster, D. I. Chem.-Eur. J. 2005, 11,
3375.
(9) (a) Wang, C.; Pa˚lsson, L.-O.; Batsanov, A. S.; Bryce, M. R. J. Am.
Chem. Soc. 2006, 128, 3789. (b) West, K.; Wang, C.; Batsanov, A. S.;
Bryce, M. R. J. Org. Chem. 2006, 71, 8541.
(10) Klinger, C.; Vostrowsky, O.; Hirsch, A. Eur. J. Org. Chem. 2006,
1508.
(11) (a) Baughman, R. H. J. Appl. Phys. 1972, 43, 4362. (b) Enkelmann,
V. Chem. Mater. 1994, 6, 1337. (c) Coates, G. W.; Dunn, A. R.; Henling,
L. M.; Dougherty, D. A.; Grubbs, R. H. Angew. Chem., Int. Ed. 1997, 36,
248. (d) Xiao, J.; Yang, M.; Lauher, J. W.; Fowler, F. W. Angew. Chem.,
Int. Ed. 2000, 39, 2132.
(12) Avendano, C.; Espada, M.; Ocana, B.; Garcia-Granda, S.; del
Rosario Diaz, M.; Tejirina, B.; Gomez-Beltran, F.; Martinez, A.; Elguero,
J. J. Chem. Soc., Perkin Trans. 2 1993, 1547.
(13) Saha, S.; Samanta, A. Acta Crystallogr. 1999, C55, 1299.
(14) Lin, H.-Y.; Zhang, X.-B.; Li, P. Acta Crystallogr. 2005, E61, o3597.
(15) (a) Graaf, W. D.; Smits, A.; Bowsma, J.; Koten, G. v. Tetrahedron
1988, 44, 6699. (b) Holmes, A. B.; Jones, G. E. Tetrahedron Lett. 1980,
21, 3111.
(16) Dabdoub, M. J.; Dabdoub, V. B.; Lenarda˜o, E. J. Tetrahedron Lett.
2001, 42, 1807.
(17) Liu, Y.; Nishiura, M.; Hou, Z. J. Am. Chem. Soc. 2006, 128, 5592.
(18) Alternative syntheses of diarylpolyynes. (a) Luu, T.; Elliott, E.;
Slepkov, A. D.; Eisler, S.; McDonald, R.; Hegmann, F. A.; Tykwinski,
R. R. Org. Lett. 2005, 7, 51. (b) Simpkins, S. M. E.; Weller, M. D.; Cox,
L. R. Chem. Commun. 2007, 4035.
3070
Org. Lett., Vol. 10, No. 14, 2008

attempted cross-coupling of tetrayne 7 with iodo-TMSA
yielded a dark brown solid from which no product could be
isolated. The attempted isolation of the terminal pentayne
13 from the deprotection of 12 was unsuccessful, although
the UV-vis absorption spectrum (Figure 1b) obtained from
When the TMS protecting groups are removed, the 13C
chemical shifts of the terminal acetylenic carbons show the
same trend, i.e., 77.9 (8, n ) 1), 71.7 (4, n ) 2), 69.2 (10,
n ) 3), and 68.2 (7, n ) 4) ppm. This shielding effect is in
agreement with the report of Ziessel et al. for pyridyl
oligoynes21 but is opposite to that reported for [H-(Ct C)n-
H] (n ) 1-4) where the acetylenic protons were deshielded
by additional triple bonds.4
The solution UV-vis absorption spectra of the terminal
oligoynes are shown in Figure 1. Unlike their mesityl
analogues,4 the vibronic features of the low energy oligoynic
bands for 4 and 10 were not clearly visible. In particular,
butadiyne 4 exhibited a very intense single peak at λmax
)
349 nm with a small shoulder on the high energy side. The
loss of the fine structure could be due to overlap of the n-π*
transition of the N-phenylcarbazole unit (λmax ) 343 nm for
1, Figure 1a; and λmax ) 343 nm for N-phenylcarbazole in
cyclohexane22) with the π-π* transitions of the oligoynic
chromophores. For the tetrayne 7 and pentayne 13, the
vibronic fine structures with λmax ) 348, 371, and 395 nm
(for 7) and 362, 386, and 416 nm (for 13), respectively, were
well-defined. In addition, the carbazole-derived high intensity
π-π* transitions below 300 nm for these two compounds
also became vibronic (Figure 1b).
The stability of the crystalline butadiyne 4 is remarkable.
A gram-scale sample was stored for many months at -20
°C without any noticeable color change. Triyne 10 decom-
posed faster in chloroform solution than 4. Nonetheless, an
NMR sample of 10 [ca. 5 mg in CDCl3 (0.5 mL)] survived
>6 h inside the spectrometer at 20 °C. A single crystal of
10 was obtained by an accelerated evaporation process. As
expected, the tetrayne 7 was less stable than the triyne 10.
Crystals of 7 were stable in a freezer at -20 °C in the dark,
although storage at 20 °C under standard room light resulted
in darkening within ca. 10 min. Predictably, the pentayne
homologue 13 was the least stable in the series.
Figure 1. UV-vis absorption spectra: (a) 1, terminal mono- (8),
di- (4), tri- (10), tetra- (7), and penta- (13) ynes (1 × 10-5 M in
dichloromethane); (b) comparison of the pentaynes 12 and 13. The
concentration of pentayne 13 is arbitrary.
a dilute solution of the reaction mixture was consistent with
the formation of 13. The deprotection resulted in an 18 nm
blue shift in the oligoynic λmax which is larger than that
reported for a mesityl analogue (∆λmax 8 nm in MeOH).5c
However, Maldi-Tof mass spectrometry could not detect any
expected signal for 13 from the solution of deprotected 12.
Presumably, 13 decomposed too rapidly as the solvents were
evaporated during the matrix preparation. Therefore, the
synthesis of higher homologues was not attempted.
The November 2007 release of the CSD23 contains no
structure with a (Ct C)4H moiety and only two with a
(Ct C)3H, viz. (tBuNO)ClC6H3(Ct C)3H (which polymerized
under X-rays even at 220 K, hence only the lattice parameters
were obtained)24 and (PhN-C5H4N)4Ru2(Ct C)3H, where the
oligoyne chain may be stabilized by pπ-dπ interaction with
The successful isolation of the series of oligoynes 4, 10,
and 7 has enabled for the first time an analysis of how the
1
acetylenic H NMR chemical shift changes with extending
the sp-carbon chain length up to a C8H system. In marked
contrast to the red shifts of the UV-vis absorption spectra,
which follow the Lewis-Calvin rule,19 the additional shield-
ing effects caused by extending a triple bond decreased
dramatically as the chain length increased. The elongation
from monoyne 8 (δ ) 3.20 ppm) to diyne 4 (δ ) 2.54 ppm)
to triyne 10 (δ ) 2.31 ppm) to tetrayne (δ ) 2.23 ppm)
resulted in 0.66, 0.23, and 0.08 ppm upfield shifts, respec-
tively. This progressively reduced shielding effect of the
additional triple bonds can also be seen from the 13C chemical
shifts of the TMS-protected oligoynes [di(t-butyl)carba-
zolylphenyl-(Ct C)n-TMS], where the δ values of the
methyl-C of the TMS group are 0.02 (n ) 1),20 -0.39 (n )
2, Figure S35), -0.51 (n ) 3, Figure S20), -0.59 (n ) 4,
Figure S11), and -0.64 (n ) 5, Figure S29), and thus the
upfield shifts are 0.41, 0.12, 0.08, and 0.05, respectively.
(21) Ziessel, R.; Suffert, J.; Youinou, M.-T. J. Org. Chem. 1996, 61,
6535.
(22) Piet, J. J.; Biemans, H. A. M.; Warman, J. M.; Meijer, E. W. Chem.
Phys. Lett. 1998, 289, 13.
(23) Allen, F. H.; Taylor, R. Chem. Soc. ReV. 2004, 33, 463.
(24) Inoue, K.; Iwamura, H. AdV. Mater. 1992, 4, 801.
(25) X-ray experiments: SMART 6K CCD area detector diffractometer,
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å), SHELXTL
6.14 software (Bruker AXS, Madison WI, USA, 2003). At room temper-
ature, single crystals deteriorated rapidly, both in and out of mother liquor.
The diffraction of 4 and 7 was weak, mean I/σ(I) ) 5.0 and 4.3, respectively.
Crystal data for C30H29N (4), C34H29N (7), and C32H29N (10): M ) 403.54/
451.58/427.56, T ) 120 K, monoclinic, space group P21/c (no. 14), a )
19.295(2)/21.065(3)/5.824(1), b ) 5.7668(9)/5.6665(8)/19.988(2), c )
22.291(3)/22.090(4)/21.420(2) Å, ꢀ ) 107.45(1)/98.48(5)/96.61(1)°, V )
2366.2(5)/2607.9(7)/2476.9(5) Å3, Z ) 4, Dc ) 1.133/1.150/1.147 g/cm,
13441/13239/24480 reflections measured, 5416/4588/6544 unique, of which
2698/2068/4950 with I > 2σ(I), Rint ) 10.0/12.7/4.2%, R1[I > 2σ(I)] )
5.3/6.0/4.7%, wR(F2) ) 11.8/10.5/12.8% (all data). CCDC 679671 (4),
679672 (7), 679673 (10) contain the supplementary crystallographic data
for this paper, which can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(19) Lewis, G. N.; Calvin, M. Chem. ReV. 1939, 25, 273.
(20) Zhu, Z.; Moore, J. S. J. Org. Chem. 2000, 65, 116.
Org. Lett., Vol. 10, No. 14, 2008
3071

the σ-bonded Ru atom.7b Indeed, two-thirds (28 out of 41)
of the R(Ct C)2H derivatives also have the R unit linked to
the butadiyne unit through a (transition) metal atom.
X-ray structural studies of 4, 7, and 10 (Figure 2) showed
the expected twisted conformation (θ ) 58, 50, and 51°,
of 4 Å required for solid-state polymerization.26 The shortest
distances in 7, viz., C1···C4, C2···C5, C3···C6, C4···C7, and
C5···C8, of 3.73-3.88 Å, are within this limit but not
topologically suitable for a regular polymerization without
large displacements of entire molecules.10 Such displace-
ments, or even smaller (but still substantial) atomic displace-
ment occurring during a “good” topotactic polymerization
of butadiynes, would be seriously hindered by the bulky and
nonplanar di(t-butyl)carbazolylphenyl substituent.11a Struc-
ture 10 has both problems simultaneously, the shortest
contacts C1···C4, C2···C5, and C3···C6 being 4.06-4.17 Å.
In summary, unprecedented stability has been observed
in all-organic terminal aryl hexatriyne and octatetrayne
derivatives by judicious choice of a bulky, nonplanar
headgroup [viz., 4-(3,6-di-tert-butyl-N-carbazolyl)phenyl]
which serves to hinder the topochemical intermolecular
interactions in the crystal lattice which are required for the
head-tail cross-linking of the oligoyne units. New acetylenic
building blocks which are suitable for the assembly of
polyyne chains and other carbon-rich scaffolds27 have been
synthesized.
Acknowledgment. We thank EPSRC for funding this
work.
Figure 2. X-ray molecular structures of 4, 7, and 10.
Supporting Information Available: Supporting Informa-
tion for this article including synthetic details, NMR, and
MS spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
respectively).25 The oligoyne rods are slightly bent, the
terminal bonds forming an angle of 172, 174, and 164°,
respectively. In each structure, it is possible to discern arrays
of parallel oligoyne rods, related by the b translations in 4
and 7 and by the a translation in 10. The translation
(“stacking”) vectors of 5.77, 5.67, and 5.82 Å are inclined
to the rod directions by 48, 42, and 45°, respectively. The
packing does not favor polymerization. In 4 the, shortest
inter-rod distances (C1···C4) of 4.30 Å exceed the threshold
OL801121P
(26) Enkelmann, V. AdV. Polym. Sci. 1984, 63, 91.
(27) (a) Carbon-Rich Compounds: From Molecules to Materials; Haley,
M. M., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, Germany, 2006.
(b) Acetylene Chemistry: Chemistry, Biology and Material Science; Dieder-
ich, F., Stang, P. J., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim,
Germany, 2005. (c) Nielsen, M. B.; Diederich, F Chem. ReV. 2005, 105,
1837. (d) Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 28,
107.
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Org. Lett., Vol. 10, No. 14, 2008