Cyclophyne

Article abstract of DOI:10.1039/a701712j

Use of a palladium/cobalt/copper alkyne reaction strategy provides the cobalt-stabilized complex of a highly strained cyclophane.

Full text of DOI:10.1039/a701712j

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Cyclophyne chemistry: synthesis and study of an octacobalt complex of
[8.8]paracyclophaneoctayne
Michael M. Haley* and Brandi L. Langsdorf
Department of Chemistry, University of Oregon, Eugene, Oregon 97403, USA
Use of a palladium/cobalt/copper alkyne reaction strategy
provides the cobalt-stabilized complex of a highly strained
cyclophane.
reagents and conditions explored, only oxidation by molecular
iodine was found to regenerate butadiyne 6 cleanly and in
excellent yield.
Application of the iodine-promoted decomplexation method
to 2 provided dppm as the sole organic-soluble component
along with a copious amount of insoluble material. Analysis of
an aliquot of the crude reaction mixture by 1H NMR showed a
broad signal at d 7.6 attributable to the phosphine ligand; no
other aromatic peaks were observed. Although disappointing,
this result is not wholly unexpected. A survey of the current
literature reveals that, despite significant efforts on the part of
several research groups, systems containing contiguous, bent
triynes and higher polyynes have proven elusive.5 Only strained
monoyne8,15 and diyne16 derivatives are known. The cyclo-
carbons represent the ultimate bent polyyne macrocycles; C18 is
predicted to exhibit C·C–C bond angles near 160°, leading to a
The recent discovery of the fullerenes1 and related carbon
nanotubes2 has prompted a flurry of activity directed toward the
synthesis and study of curved, fully conjugated p-systems.3 In
a similar regard, there have been numerous attempts to prepare
related belt-shaped molecules possessing conjugated p-electron
circuits; the various belt- and hoop-shaped compounds studied
include cyclacenes,4 [n]cyclocarbons,5 cyclic paraphenylenes6
and others.7 Recently, two groups reported the successful
preparation of fully conjugated, belt-shaped compounds. Oda
and co-workers produced cyclic paraphenylacetylenes contain-
ing six and eight subunits.8 Herges and co-workers isolated a
‘picotube’ based on four anthracene units.9 Our recent work on
polyacetylenic macrocycles10 spurred an interest in systems
possessing the cyclic p-orbital motif. We report herein our work
toward the synthesis of [8.8]paracyclophaneoctayne 1, a
cyclophane-cyclocarbon hybrid nicknamed a cyclophyne.11
strain energy of around 72 kcal mol21 5b
Cyclophyne 1 should
.
be similarly strained as the triple bond angles are calculated to
SiPri3
Co(CO)3
(CO)3Co
(CO)3Co
Co(CO)3
i, ii
SiPri3
SiPri3
3
4
iii–v
PPh2
Co(CO)2
Ph2P
(CO)2Co
Ph2P
PPh2
(CO)2Co
Co(CO)2
1
An octacobalt complex such as 2 was envisioned for the
immediate precursor to the target macrocycle as: (i) the carbon
skeleton could be assembled quickly and efficiently using
known methodology, and (ii) the alkyne C·C–C angles in this
type of (m-acetylene)dicobalt hexacarbonyl complex are re-
duced to ca. 140°, thus allowing the complex to participate in
reactions that might be difficult or otherwise impossible for the
rigid, unbound molecule to undergo. The synthesis of 2 is
depicted in Scheme 1. Palladium-catalysed alkynylation of
1,4-diiodobenzene with excess triisopropylsilylbutadiyne 312
furnished a pale yellow tetrayne, which in turn reacted with 2
equiv. of octacarbonyldicobalt to provide the sterically favoured
tetracobalt complex 4.† Attempted desilylation with tetra-
butylammonium fluoride led to complete decomposition of 4.
Ligand exchange5a with bis(diphenylphosphino)methane
(dppm) afforded a bridged complex (85% yield) stable to
fluoride ion; desilylation now proceeded in high yield ( > 95%).
Eglinton coupling13 of the terminal acetylenes provided fine,
deep maroon crystals of 2† in 47% yield. Spectroscopic data
supported formation of the strain-free dimeric structure.
Co(CO)2
PPh2
(CO)2Co
Ph2P
Co(CO)2 (CO)
2Co
Ph2P
PPh2
2
Scheme 1 Reagents and conditions: i, 1,4-diiodobenzene, Pd(PPh3)2Cl2,
CuI, Et3N, 73%; ii, Co2(CO)8, Et2O, 66%; iii, dppm, toluene, 85%; iv,
Bu4NF, THF, > 95%; v, Cu(OAc)2·H2O, py, 47%
PPh2
Co(CO)2
Ph
SiPri3
Ph2P
SiPri3
(CO)2Co
iii, iv
v
i, ii
Ph
SiPri3
In their work with the hexacobalt complex of [18]cyclocar-
bon, Diederich et al. were unable to free C18 of the protecting
groups by mild oxidation.5a,14 With this problem in mind, we
prepared model complex 5† (Scheme 2) in order to investigate
various methods to remove the cobalts. Of the numerous
6
5
Scheme 2 Reagents and conditions: i, CBr4, PPh3, CH2Cl2, 85%; ii,
phenylacetylene, CuCl, NH2OH·HCl, PrNH2, MeOH, H2O, 82%; iii,
Co2(CO)8, Et2O, 76%; iv, dppm, toluene, 87%; v, I2, THF, 85%
Chem. Commun., 1997
1121

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be between 161.7 and 163.6°.17 Whether the failure to prepare
these two classes of macrocycles is due to the extreme reactivity
of the distorted polyyne moiety or to the lack of a viable
synthetic route is not certain. Thus isolation and character-
ization of smaller bent hexatriyne- and octatetrayne-containing
systems should help answer these questions. To this end, we are
currently studying dehydrobenzoannulenes possessing higher
polyynes distorted on one or more edge.18 These results will be
reported in due course.
1992, 31, 1101 and references cited therein; (c) Y. Tobe, T. Fulii,
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Chem. Soc., 1996, 118, 2758; (d) Y. Tobe, H. Matsumoto, K. Naemura,
Y. Achiba and T. Wakabayashi, Angew. Chem., Int., Ed. Engl., 1996,
35, 1800.
6 R. Fiederich, M. Nieger and F. Vo¨gtle, Chem. Ber., 1993, 126, 1723.
7 F. Vo¨gtle, A. Schro¨der and D. Karbach, Angew. Chem., Int. Ed. Engl.,
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M. Przybylski, F. Beer and K. Mu¨llen, Chem. Ber., 1994, 127, 2089;
Y. Rubin, T. C. Parker, S. I. Khan, C. L. Holliman and S. W. McElvany,
J. Am. Chem. Soc., 1996, 118, 5308.
We acknowledge the donors of The Petroleum Research
Fund, administered by the American Chemical Society, and the
University of Oregon for generous support of this research.
8 T. Kawase, H. R. Darabi and M. Oda, Angew. Chem., Int. Ed. Engl.,
1996, 35, 2664.
Footnotes
9 S. Kammermeier, P. G. Jones and R. Herges, Angew. Chem., Int. Ed.
Engl., 1996, 35, 2669.
10 M. M. Haley, M. L. Bell, J. J. English, C. A. Johnson and
T. J. R. Weakley, J. Am. Chem. Soc., 1997, 119, 2956; M. M. Haley,
S. C. Brand and J. J. Pak, Angew. Chem., Int. Ed. Engl., 1997, 36,
836.
11 This work was presented in part at the 8th International Symposium on
Novel Aromatics in Braunschweig, Germany on 30.07.95, poster 35.
12 L. Blanco, H. E. Helson, M. Hirthammer, H. Mestdagh, S. Spyroudis
and K. P. C. Vollhardt, Angew. Chem., Int. Ed. Engl., 1987, 26, 1246.
13 G. Eglinton and W. McRae, Adv. Org. Chem., 1963, 4, 225.
14 Y. Rubin, Ph.D. Thesis, University of California, Los Angeles, 1991.
The thesis states that only three different methods were tried in the
attempts to free C18 from the cobalt atoms; apparently, no model
systems were investigated.
* E-mail: haley@oregon.voregon.edu
† Selected spectral data (1H NMR: 300.13 MHz, 13C NMR: 75.5 MHz). For
2 dH (CD2Cl2) 7.93 (8 H, s), 7.53 (16 H, m), 7.45–7.12 (64 H, m), 3.72 (4
H, dt), 3.39 (4 H, dt); dC (CD2Cl2) 206.61, 201.59, 142.67, 137.32, 135.03,
132.85, 131.78, 130.45, 130.03, 129.56, 128.89, 128.72, 100.11, 87.19,
83.50, 65.60, 37.73; nmax (KBr)/cm21 3033, 2154, 2089, 2055, 2023 For 4
d
H (CDCl3) 7.65 (4 H, s), 1.14 (42 H, s); dC (CDCl3) 138.36, 129.77, 105.30,
104.03, 88.93, 69.59, 18.65, 11.37; nmax (KBr)/cm21 3069, 3056, 2130,
2092, 2038, 2006, 1976. For 5 dH (CD2Cl2) 7.87 (2 H, d), 7.51 (4 H, m),
7.42–7.16 (19 H, m), 3.66 (1 H, dt), 3.36 (1 H, dt), 1.12 (21 H, s); dC
(CD2Cl2) 207.25, 201.38, 143.91, 139.00, 133.44, 131.37, 130.64, 129.87,
129.39, 128.97 (2), 128.94, 128.64, 127.11, 110.33, 99.04, 96.89, 65.36,
34.80, 19.21, 12.16; nmax (KBr)/cm21 3056, 2102, 2062, 2027, 2004,
1975.
15 Recent examples of strained monoynes: X. M. Wang, R. J. Wang,
T. C. W. Mak and H. N. C. Wong, J. Am. Chem. Soc., 1990, 112, 7790;
T. Kawase, N. Ueda, H. R. Darabi and M. Oda, Angew. Chem., Int. Ed.
Engl., 1996, 35, 1556.
16 Recent examples of strained diynes: Q. Zhou, P. J. Carroll and
T. M. Swager, J. Org. Chem., 1994, 59, 1294; K. P. Baldwin,
A. J. Matzger, D. A. Scheiman, C. A. Tessier, K. P. C. Vollhardt and
W. J. Youngs, Synlett, 1995, 1215; Y. Tobe, N. Utsumi, K. Kawabata
and K. Naemura, Tetrahedron Lett., 1996, 52, 9325.
17 Calculations performed at the ab initio HF/6-31G(d) level using
GAUSSIAN 92. We thank Dr Andreas Maulitz, University of Essen, for
obtaining these data.
18 We have recently prepared the first examples of triyne- and tetrayne-
linked dehydrobenzoannulenes. These systems, however, are relatively
strain-free: M. M. Haley, M. L. Bell and D. B. Kimball, unpublished
work.
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Received in Corvallis, OR, USA, 10th March 1997; Com.
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