Synthesis, structure, and reactivity of naphthalyne-Co2(CO) 6 complexes

Article abstract of DOI:10.1021/ja801569q

Synthesis of naphthalyne-Co₂(CO)₆ complexes, the first example of the aryne-Co₂(CO)₆ complex, was achieved via cyclization of acyclic alkyne-Co₂(CO)₆ complex precursors containing an allyls

Full text of DOI:10.1021/ja801569q

Published on Web 04/24/2008
Synthesis, Structure, and Reactivity of Naphthalyne-Co2(CO)6 Complexes
Nobuharu Iwasawa,* Maiko Otsuka, Satomi Yamashita, Masao Aoki, and Jun Takaya
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
Received March 2, 2008; E-mail: niwasawa@chem.titech.ac.jp
Scheme 1. Preparation of Naphthalyne-Co2(CO)6 Complexes 5
Arynes are well-known, highly reactive transient species, which have
been employed for the preparation of various aromatic compounds.1
Interestingly, these highly reactive arynes can be stabilized and
sometimes become isolable by making a complex with transition
metals, and a variety of such complexes have been reported so far.2
However, it is
a little surprising that the corresponding
alkyne-Co2(CO)6 complexes, probably one of the most common and
stable alkyne complexes of transition metals,3 have not yet been
synthesized.4,5 In this paper, we would like to report the first example
of isolation and X-ray analysis of this type of complex, that is,
naphthalyne-Co2(CO)6 complex and its unique reactivity as an
alkyne-Co2(CO)6 complex.
After extensive fruitless trials for the preparation of aryne-Co2(CO)6
complexes from acyclic alkyne-Co2(CO)6 precursors utilizing cy-
clization reactions, such as coupling reactions, metathesis reactions,
and so on,6 we have succeeded in preparing the naphthalyne-Co2(CO)6
derivatives according to the following procedure utilizing cyclization
of benzene derivatives having an allylsilane and an alkynal-Co2(CO)6
moiety at the ortho-position,4b followed by dehydration of the produced
alcohols (Scheme 1).
The characteristic features of the complex are as follows. The
complex has a planar naphthalyne core, and the Co complex moiety
is mostly symmetrical concerning this plane. Furthermore, this
naphthalyne-Co2(CO)6 complex clearly showed no naphthalene
character but substituted benzene character, as obviously seen from
the bond length of the noncomplexed benzene part of the
naphthalyne-Co2(CO)6 complex. Usually, bond alternation is observed
for naphthalene derivatives, but in this complex, three consecutive
bonds had almost the same length around 1.38-1.39 Å and the other
three had longer length of 1.40-1.43 Å, probably due to the influence
of the strained alkyne-Co2(CO)6 moiety. Another feature is the bond
angle of 122-123° around the alkyne-Co2(CO)6 moiety, which is
highly deviated from the standard bond angle of around 140° for acyclic
alkyne-Co2(CO)6 complexes.
Thus, the cyclization precursor 3a (R ) H) was prepared in five
steps from o-iodobenzaldehyde 1a (R ) H, X ) I), and the cyclization
was carried out under strictly controlled conditions using 3.5 mol
amounts of TiCl4 at -40 °C followed by purification by silica gel
column chromatography under argon using precooled solvent. The
yield of the cyclized product 4a, a dark-red solid, is around 40% in a
small-scale experiment, but on a larger scale, the yield of the product
became lower probably due to decomposition of the product during
purification.7 This dihydronaphthalyne-Co2(CO)6 complex 4a de-
composed within 1 or 2 days in the air at room temperature, but the
decomposition could be mostly suppressed under Ar. Then dehydration
was examined under several conditions and was found to proceed
smoothly by carrying out the reaction using Tf2O and Hunig’s base in
CH2Cl2 at -40 °C. The dehydration reaction itself was clean by direct
measurement of the reaction mixture by low-temperature 1H NMR,8
but by increasing the temperature to 20 °C, decomposition of the
complex occurred in the reaction mixture. The most difficult point is
the instability of the final product, in particular, to oxygen during
isolation, and finally, we have succeeded in isolating the desired
naphthalyne-Co2(CO)6 complex 5a, a brownish yellow solid, by
rapidly carrying out a silica gel column chromatograph cooled to -10
°C using degassed, precooled solvent under Ar and removing the
solvent around 0 °C with sufficient care to the air. Roughly estimated
isolated yield is around 60%. We have also prepared MeO- and Br-
substituted naphthalyne-Co2(CO)6 complexes (5b: R ) OMe; 5c: R
) Br) in a similar manner,9 but the stability of the complexes did not
change to a great extent. Under strict exclusion of oxygen, the
complexes were stable even at room temperature in a solid state, but
mostly decomposed in the air at room temperature in 1 or 2 h, and
furthermore, when a hexane solution of naphthalyne-Co2(CO)6 was
left open in the air for 10 min at 0 °C, decomposition occurred rapidly
and insoluble red solids appeared. The structure of the products was
first determined by 1H, 13C, and HMBC spectra, and finally, we have
succeeded in obtaining a single crystal suitable for X-ray analysis for
the Br-substituted complex 5c by carefully recrystallizing from pentane
(Figure 1).10
We next examined the reactivity of the complex.11 First, the
reactivity to oxygen was scrutinized (Scheme 2). When the complex
5 was left in the air at room temperature, it changed its brownish yellow
color to red within 1 or 2 h and then to ocher after 1 day.12 Treatment
of the red solids A with H2O/acetone/AcOEt at room temperature in
the air gave naphthalene dicarboxylic acid anhydride 6, while treatment
with MeOH gave the corresponding dimethyl ester 7. On the other
hand, treatment of the ocher solids B under the same reaction conditions
Figure 1. X-ray structure of naphthalyne-Co2(CO)6 complex 5c.
9
6328 J. AM. CHEM. SOC. 2008, 130, 6328–6329
10.1021/ja801569q CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Reaction of Naphthalyne-Co2(CO)6 Complex 5a under
Air
Supporting Information Available: Preparative methods and spectral
and analytical data of compounds 1-9, X-ray data and a CIF file for 5c,
and IR data for oxidation intermediates A and B. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) For recent reviews, see: (a) Pellissier, H.; Santelli, M. Tetrahedron 2003, 59,
701. (b) Dyke, A. M.; Hester, A. J.; Lloyd-Jones, G. C. Synthesis 2006, 4093.
(2) For examples of transition metal complexes of arynes, see: (a) Jones, W. M.;
Klosin, J. AdV. Organomet. Chem. 1998, 42, 147. (b) Hughes, R. P.;
Laritchev, R. B.; Williamson, A.; Incarvito, C. D.; Zakharov, L. N.;
Rheingold, A. L. Organometallics 2002, 21, 4873. (c) Hughes, R. P.;
Laritchev, R. B.; Williamson, A. L.; Incarvito, C. D.; Zakharov, L. N.;
Rheingold, A. L. Organometallics 2003, 22, 2134. (d) Wada, K.; Pamplin,
C. B.; Legzdins, P.; Patrick, B. O.; Tsyba, I.; Bau, R. J. Am. Chem. Soc.
2003, 125, 7035. (e) Kabir, S. E.; Begum, N.; Manjur, H. M.; Iqbal, H.;
Nur, H.; Bennett, D. W.; Siddiquee, T. A.; Haworth, D. T.; Rosenberg, E.
J. Organomet. Chem. 2004, 689, 1569. (f) Keen, A. L.; Johnson, S. A
J. Am. Chem. Soc. 2006, 128, 1806. and references cited therein.
(3) Kimmitt, R. D. W.; Russell, D. R. In ComprehenisVe Organometallic
Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, 1982; Vol. 5, pp 195-
209.
(4) For preparation of six-membered alkyne-Co2(CO)6 complexes, see: (a)
Hunt, R. L.; Wilkinson, G. Inorg. Chem. 1965, 4, 1270. (b) Schreiber, S. L.;
Sammakia, T.; Crowe, W. E. J. Am. Chem. Soc. 1986, 108, 3128. (c)
Magnus, P.; Carter, R.; Davies, M.; Elliott, J.; Pitterna, T. Tetrahedron
1996, 52, 6283.
(5) It was reported that decomposition of C6F5MgBr in dioxane in the presence
of Co2(CO)8 gave a tetranuclear C6F4-Co4(CO)10 complex, although no
X-ray analysis was carried out. See: Roe, D. M.; Massey, A. G. J.
Organomet. Chem. 1970, 23, 517.
(6) For a previous trial, see: Iwasawa, N.; Satoh, H. J. Am. Chem. Soc. 1999,
121, 7951.
(7) In general, 50 mg (0.095 mmol) of 3 was cyclized to give about 15-20 mg
of 4 (around 40% yield) after chromatography. Complex 4 was stable for a
short period of time even in the air at room temperature in a solid state, but a
solution of 4 was more sensitive to the air, in particular, on silica gel.
(8) The yield of the complex 5 in the NMR tube was around 90% based on
the internal standard method.
(9) The overall yield of 5 from 1: 13% for 5a; 19% for 5b; 13% for 5c.
(10) Crystal data and structure refinement for 5a. T ) 173(2) K, space group )
P21/n, unit cell dimensions ) a ) 15.931(5) Å, R ) 90°, b ) 7.498(3) Å,
ꢀ ) 110.193(13)°, c ) 16.677(6) Å, γ ) 90°, Z ) 4, reflections collected
) 17 486, Data/restraints/parameters ) 4241/0/272, final R indices [I >
2σ(I)], R1 ) 0.0307, R2 ) 0.0700, R indices (all data), R1 ) 0.0366, R2
) 0.0733, Goodness-of-fit on F2 ) 1.046.
gave none of the above products, but on treatment with 1 N HCl,
dicarboxylic acid 8 was obtained in good yield. Although the exact
mechanism of the reaction is not clear, we currently suppose that
carbonyl insertion occurred by oxidation of the complex 5 with
molecular oxygen to give acylcobalt intermediate,12–14 which gave
acid anhydride or dimethyl ester as described above.15 Further exposure
to the air induced the reaction of acylcobalt intermediate with oxygen
to give ocher dicarboxylic acid cobalt salt B,12 which gave dicarboxylic
acid on treatment with 1 N HCl.
We next examined the reactivity of the complex 5 with several
kinds of alkenes and alkynes. Although the complexes did not react
cleanly with alkenes, such as norbornene, styrene, maleic anhydride,
etc., they reacted with terminal alkynes in a [2 + 2 + 1] manner
to give 1H-cyclopenta[a]naphthalen-1-one derivatives 9 in good
yield by carrying out the reaction using an excess of alkynes in
CH2Cl2 at -40 °C to room temperature (eq 1).11,16 The same type
of reaction with alkenes is the Pauson-Khand reaction, a well-known
transformation of the alkyne-Co2(CO)6 complex, but the reaction
with alkynes to give cyclopentadienone derivatives has rarely been
achieved.17 Furthermore, this reaction is specific to the
naphthalyne-Co2(CO)6 complexes, and the corresponding
dihydronaphthalyne-Co2(CO)6 complex did not give the same kind
(11) In a solid state, the naphthalyne-Co2(CO)6 complex 5 could be handled
in the air at room temperature for a few minutes; however, to avoid the
decomposition of the complex due to its inherent instability to oxygen,
examination of the reactions of the naphthalyne-Co2(CO)6 complex 5 was
carried out without the exact weight of 5 directly using the chromatographed
and concentrated complex obtained from about 15-20 mg of 4. Thus the
yields were based on the dihydronaphthalyne-Co2(CO)6 complex 4.
(12) We have not yet succeeded in characterizing these complexes, partly due
to their insolubility in most solvents. The red solid A has IR absorptions
around 2000 cm-1 for carbonyl ligands but does not seem to have an
absorption around 1600 cm-1 corresponding to acyl carbonyl. Thus, at
present, we suppose the red solid A is the precursor of acylcobalt
intermediate and treatment of A with H2O or MeOH in the air induced
acylcobalt formation followed by nucleophilic addition. In fact, no reaction
occurred on treatment of A with H2O or MeOH under argon. The ocher
solids B have an IR absorption at 1560 cm-1 which could be that of
carboxylate salt and no absorption for carbonyl ligands.
of
product
under
similar
conditions.
Thus,
the
naphthalyne-Co2(CO)6 complexes showed unique reactivities,
which have not been achieved by the standard alkyne-Co2(CO)6
complexes.
(13) For the reaction of alkyne-Co2(CO)6 complex with O2, see: Hamajima,
A.; Nakata, H.; Goto, M.; Isobe, M. Chem. Lett. 2006, 35, 464.
(14) For acyl cobalt intermediates, see: (a) Liebeskind, L. S.; Jewell, C. F., Jr
J. Organomet. Chem. 1985, 285, 305. (b) Kovacs, I.; Ungvary, F. Coord.
Chem. ReV. 1997, 161, 1.
(15) It was reported that acid anhydrides were obtained on treatment of cyclic
alkyne-Co2(CO)6 complexes with CAN, although its mechanism has not
been made clear. See: Tanino, K.; Shimizu, T.; Miyama, M.; Kuwajima, I.
J. Am. Chem. Soc. 2000, 122, 6116.
In conclusion, we have succeeded in preparing and isolating
naphthalyne-Co2(CO)6 complexes for the first time. Their unique
reactivities are also disclosed. We are currently trying to prepare other
aryne-Co2(CO)6 complexes.
(16) A small amount of regioisomer was also produced in most cases. The regio-
chemistry of the major products was determined by NOE. As the yield of the
first dehydration step was about 60%, the [2 + 2 + 1] cycloaddition proceeded
in about 40-60% yield. The remainder was mainly the oxidized product.
(17) Only a few examples are known for this type of reaction utilizing alkyne-
Co2(CO)6 complexes. See: (a) Rajesh, T.; Periasamy, M. Organometallics
1999, 18, 5709. (b) Hong, S. H.; Kim, J. W.; Choi, D. S.; Chung, Y. K.;
Lee, S.-G. Chem. Commun. 1999, 2099. (c) Shibata, T.; Yamashita, K.;
Takagi, K.; Ohta, T.; Soai, K. Tetrahedron 2000, 56, 9259–9267. (d)
Sugihara, T.; Wakabayashi, A.; Takao, H.; Imagawa, H.; Nishizawa, M.
Chem. Commun. 2001, 2456.
Acknowledgment. This research was partly supported by a Grant-
in-Aid for Scientific Research from Ministry of Education, Culture,
Sports, Science and Technology of Japan. We thank Central Glass
Co., Ltd. for the generous gift of trifluoromethanesulfonic acid
anhydride. We thank Mr. Takeshi Katayama for his contribution to
the initial phase of this research. We also thank Professor Hidehiro
Uekusa and Mr. Kotaro Fujii for performing X-ray analysis.
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