Cobalt-Mediated Synthesis of Angular [4]Phenylene: Structural Characterization of a Metallacyclopentadiene(Alkyne) Intermediate and Its Thermal and Photochemical Conversion

Article abstract of DOI:10.1021/ol025956o

(Matrix Presented) The first X-ray crystal structure of a mononuclear metallacyclopentadiene(alkyne) complex has been obtained. This type of metallacycle is believed to be the key intermediate in the cobalt-mediated [2 + 2 + 2]cycloaddition of alkynes. Th

Full text of DOI:10.1021/ol025956o

ORGANIC
LETTERS
2002
Vol. 4, No. 12
2075-2078
Cobalt-Mediated Synthesis of Angular
[4]Phenylene: Structural
Characterization of a
Metallacyclopentadiene(Alkyne)
Intermediate and Its Thermal and
Photochemical Conversion
Peter I. Dosa, Glenn D. Whitener, and K. Peter C. Vollhardt*
Center for New Directions in Organic Synthesis, Department of Chemistry,
UniVersity of California at Berkeley, and the Chemical Sciences DiVision,
Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460
Andrew D. Bond
UniVersity of Cambridge, Department of Chemistry, Lensfield Road,
Cambridge, CB2 1EW, UK
Simon J. Teat
CLRC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK
kpcV@uclink.berkeley.edu
Received April 1, 2002
ABSTRACT
The first X-ray crystal structure of a mononuclear metallacyclopentadiene(alkyne) complex has been obtained. This type of metallacycle is
believed to be the key intermediate in the cobalt-mediated [2 + 2 + 2]cycloaddition of alkynes. Thermal treatment leads to the generation of
angular [4]phenylene, the X-ray structural details of which are described. Under photochemical conditions, the cobaltacycle isomerizes to a
highly strained (cyclobutadieno)dibenzocyclooctatrienyne complex.
The phenylenes have been the object of intense study by
both theoretical and synthetic chemists due to their unique
combination of aromatic and antiaromatic properties.1 Al-
though a variety have been synthesized, few X-ray crystal
structures are available for phenylenes that are undistorted
by substituents.2 We report the crystal structure of angular
[4]phenylene (1) and that of its precursor metallacycle 2;
the latter is believed to be a prototype intermediate in the
cobalt-mediated [2 + 2 + 2]cycloaddition of alkynes.3 This
is the first X-ray crystal structure of a mononuclear metalla-
(1) Vollhardt, K. P. C.; Mohler, D. L. In AdVances in Strain in Organic
Chemistry; Halton, B., Ed.; JAI: London, 1996; pp 121-160.
10.1021/ol025956o CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/14/2002

cyclopentadiene bearing a π-bound alkyne ligand.4 The
unusual photoconversion of 2 to the highly strained cyclo-
butadiene cobalt complex 3 is also described.
Scheme 2a
Phenylene 1 can be synthesized in nine steps from
1-bromo-2-iodobenzene (Scheme 1).5 In a modification of
Scheme 1a
a Reference 7.
analysis. This difficulty was overcome by using synchrotron
radiation (Station 9.8, SRS Daresbury, UK).
The crystal structure of 1 (Figure 1) reveals a significant
amount of bond alternation,8 especially in the internal
a Reaction Conditions: (a) (CH3)3SiCtCH, PdCl2(PPh3)2, CuI,
NEt3, 93%; (b) (CH3)2CHC(CH3)2Si(CH3)2CtCH, PdCl2(PPh3)2,
CuI, azacyclohexane, toluene, ∆, 79%; (c) K2CO3, MeOH, ether,
100%; (d) (CH3)3Si-CtCH; CpCo(CO)2, m-xylene, hν, ∆, 19%;
(e) ICl, CH2Cl2, 0 °C, 56%; (f) (CH3)3SiCtCH, PdCl2(PPh3)2, CuI,
NEt3, 64%; (g) 5, PdCl2(PPh3)2, CuI, NEt3, 76%; (h) Bu4N+F-,
THF, 96%; (i) CpCo(CO)2, m-xylene, hν, ∆, 30%.
this approach, the final step, cyclotrimerization of triyne 4
using CpCo(CO)2, was replaced by a two-step protocol
employing CpCo(C2H4)2, thus increasing the overall yield
of 1 from 30 to 51% (Scheme 2). In this sequence,
metallacycle 6 was generated first in the cold and subse-
quently decomposed at high temperatures in the presence of
1,3-cyclohexadiene to trap CpCo as it evolved from inter-
mediate A.4,6 Previous attempts to obtain an X-ray crystal
structure of 1 had been frustrated by its tendency to form
thin needlelike crystals too small for conventional X-ray
Figure 1. Crystal structure of 1: views from above (top) and the
side (bottom). Bond lengths (Å; (0.004).
(2) (a) For biphenylene, see: Fawcett, J. K.; Trotter, J. Acta Crystallogr.
1966, 20, 87. (b) For angular [3]phenylene, see: Diercks, R.; Vollhardt, K.
P. C. Angew. Chem., Int. Ed. Engl. 1986, 25, 266. (c) For linear
[3]phenylene, see: Schleifenbaum, A.; Feeder, N.; Vollhardt, K. P. C.
Tetrahedron Lett. 2001, 42, 7329. (d) For triangular [4]phenylene, see:
Holmes, D.; Kumaraswamy, S.; Matzger, A. J.; Vollhardt, K. P. C. Chem.
Eur. J. 1999, 5, 3399.
benzene rings of the molecule, which are 52% bond fixed.9
This phenomenon is less pronounced (25%) in the terminal
(3) For a recent theoretical study of this mechanism as it pertains to
CpCo, see: Hardesty, J. H.; Koerner, J. B.; Albright, T. A.; Lee, G.-Y. J.
Am. Chem. Soc. 1999, 121, 6055.
(4) See: Diercks, R.; Eaton, B. E.; Jalisatgi, S.; Matzger, A. J.; Radde,
R. H.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1998, 120, 8247 and references
therein.
(6) Dosa, P. I.; Schleifenbaum, A.; Vollhardt, K. P. C. Org. Lett. 2001,
3, 1017.
(7) NICS(1) values from: Schulman, J. M.; Disch, R. L.; Jiao, H.;
Schleyer, P. v. R. J. Phys. Chem. A 1998, 102, 8051. For additional
theoretical studies of the phenylenes, see: Schulman, J. M.; Disch, R. L. J.
Am. Chem. Soc. 1996, 118, 8470; J. Phys. Chem. A 1997, 101, 5596 and
references therein.
(5) Schmidt-Radde, R. H.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1992,
114, 9713.
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Org. Lett., Vol. 4, No. 12, 2002

rings, classifying them as more aromatic by this criterion.
This difference in aromatic character can also be observed
1
in the H NMR spectrum of 1: H5 and H6 (δ ) 6.31 ppm,
CD2Cl2) are shielded relative to H1-H4 (δavg ) 6.89 ppm),1,5
consistent with a more cyclohexatrienic environment for the
former.
It is instructive to compare 1 with its lower analogue,
angular [3]phenylene (7),2b in which the respective extent
of bond alternation is 62 and 22% and the central ring protons
are shielded (δ ) 6.18 ppm) relative to those in 1, in
agreement with nucleus-independent chemical shift (NICS)
calculations (Scheme 2).7 These trends can be understood if
one views 1 as the result of benzocyclobutadienofusion of
7. In 7, the terminal rings maximize their aromaticity by
acting in concert to induce bond alternation in their central
neighbor. Alteration of 7 to 1 induces increased bond
alternation in one of the formerly terminal rings, thus
relieving the “pressure” to bond fixation in the opposing
internal ring. Because of symmetry, the net result is a smaller
reduction in the aromatic character of the inside benzenes
in 1 relative to that in 7.
The geometry of 1 is unusually flat (Figure 1), with a
median angle of 0.7° between the least-squares planes of
each ring, ranging from 0.1 to 1.1°. In contrast, most
phenylenes show significantly larger deviations from planar-
ity, a testament to their ready deformability.2d For example,
in triangular [4]phenylene, an isomer of 1, the corresponding
median angle between planes is 2.0°, ranging from 0.7 to
3.5°.2d
Figure 2. Crystal structure of metallacycle 2. Selected bond lengths
(Å; (0.007): Co1-C11 1.920, Co1-C14 1.923, Co1-C21 2.039,
Co1-C22 2.042, C11-C12 1.358, C12-C13 1.455, C13-C14
1.353, C14-C15 1.483, C15-C20 1.417, C20-C21 1.469, C21-
C22 1.232, C22-C23 1.451, C23-C34 1.460, C11-C34 1.467,
Co1-Ct 1.721. Selected bond angles (deg): C20-C21-C22 164.7,
C21-C22-C23 165.7.
The geometry of the ligating triple bond in 2 closely
resembles that calculated for the analogous (CpCo) organo-
metallic precursor to angular [3]phenylene (CtC 1.242 Å,
Co-Ct 2.097 Å, CtCC 165.97°),4,10 as well as the parent
CpCo(C4H4)(C2H2).3 Significantly, all the Co-Calkyne bond
distances in 2 are large in comparison with other alkyne-
cobalt complexes,11 in agreement with the notion of an
intrinsically weak cobalt-alkyne bond.3
During attempts to improve yields of 1, complex 2 was
irradiated with a slide projector lamp. Surprisingly, the
product obtained from this reaction was not 1 but rather the
bright orange cyclobutadiene (Cb) complex 3 (Scheme 4).
The ready generation of metallacycle 6, formed in analogy
to its debenzocyclobutadieno relatives on route to 7,4 spurred
renewed efforts to obtain crystals suitable for an X-ray
analysis, a task that had proven impossible with several
previous derivatives. While 6 was again unsuitable, crystal-
Scheme 3
Scheme 4
lization of the Cp* analogue 2, prepared as in Scheme 3,
was successful (Figure 2).
(8) The structure is in excellent agreement with calculations at the
B3LYP/6-31G* level: Professor P. von Rague´ Schleyer and Dr. H. Jiao,
private communication. The maximum deviation of the calculated from the
experimental bond lengths is 0.025 Å, and the average deviation is 0.012
Å.
(9) The formula [Σ single-bond lengths - Σ double-bond lengths]/3
provides a quantitative measure of bond alternation in each ring. This
number was normalized using the exocyclic diene portion of 3,4-dimeth-
ylenecyclobutene (1.497, 1.338 Å) as a 100% standard. See: Beckhaus,
H.-D.; Faust, R.; Matzger, A. J.; Mohler, D. L.; Rogers, D. W.; Ru¨chardt,
C.; Sawhney, A. K.; Verevkin, S. P.; Vollhardt, K. P. C.; Wolff, S. J. Am.
Chem. Soc. 2000, 122, 7819 and references therein.
While CbCo complexes are common byproducts of cobalt-
mediated alkyne oligomerizations, no such species has ever
been observed during the synthesis of angular phenylenes.
(10) Alkyl substitution has a negligible effect on the Co-Calkyne bond
distances in a series of RxCpCo(CO)(alkyne) systems: Benisch, C.; Cha´vez,
J.; Gleiter, R.; Nuber, B.; Irngartinger, H.; Oeser, T.; Pritzkow, H.;
Rominger, F. Eur. J. Inorg. Chem. 1998, 629, 9.
Org. Lett., Vol. 4, No. 12, 2002
2077

Reductive elimination may occur from a coordinatively
unsaturated cobaltacyclopentadiene to furnish the four-
membered ring.3 In the case of 2, light may cause dissociation
of the alkyne ligand as a first step toward the formation of
3, although a direct photoisomerization is entirely possible.
ylenic bond angles of 154°.12b The strain inherent in the
molecule also causes the cyclobutadiene ring to distort into
a trapezoidal configuration: the bond of fusion (C11-C14)
is significantly longer and C12-C13 shorter than normal
(∼1.46 Å).13 This distortion is not present in 8, a closely
related complex lacking a triple bond.14 In this molecule,
the carbon-carbon bond distances in the four-membered
rings all fall in a narrow normal range (1.460-1.477 Å).
The structure of 3 was confirmed by X-ray crystallography
(Figure 3). Constraining the triple bond into an eight-
In conclusion, the crystal structures of angular [4]phen-
ylene and two mechanistically relevant organometallic
complexes have been obtained. These structures further
understanding of the phenylenes and the cobalt-mediated [2
+ 2 + 2]cycloaddition that produces them.
Acknowledgment. The NSF (CHE-0071887) funded this
work. The Center for New Directions in Organic Synthesis
is supported by Bristol-Myers Squibb as a Sponsoring
Member and Novartis as a Supporting Member. G.D.W.
thanks the NSF and Bayer-Miles for predoctoral fellowships.
A.D.B. thanks the Engineering and Physical Sciences
Research Council (EPSRC, U.K.) (Grant GR/R08919) and
The Biotechnology and Biological Sciences Research Coun-
cil (BBSRC, U.K.) (Grant 8/B11586) for funding access to
the synchrotron. We are indebted to Professor P. von R.
Schleyer for the communication of unpublished data and to
Personal Chemistry, Inc., for access to their Smith Synthe-
sizerTM.
Figure 3. Crystal structure of 3. Selected bond lengths (Å;
(0.005): C11-C12 1.455, C12-C13 1.421, C13-C14 1.461,
C11-C14 1.500, C21-C22 1.196. Selected bond angles (deg):
C20-C21-C22 157.6, C21-C22-C23 156.3, C12-C11-C14
88.9, C11-C14-C13 88.0, C11-C12-C13 91.3, C12-C13-C14
91.8.
membered ring leads to bond angles (C21-C22-C23 156.3°,
C20-C21-C22 157.6°) that deviate significantly from
linearity. Similar distortions are found in related cyclynes12a
such as 1,2,5,6-dibenzocyclooct-3-en-7-yne, which has acet-
Supporting Information Available: Detailed experi-
mental procedures, full characterization of all new com-
pounds, and crystallographic data for 1, 2‚0.5(toluene), and
3‚0.5(THF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(11) A search of the Cambridge Structural Database failed to uncover a
structurally characterized Co(III)-alkyne complex. For examples of Co(I)
complexes with shorter Co-alkyne bonds, see: (a) ref 10. (b) Foerstner,
J.; Kettenbach, R.; Goddard, R.; Butenscho¨n, H. Chem. Ber. 1996, 129,
319. (c) Okuda, J.; Zimmermann, K. H.; Herdtweck, E. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 430. For a discussion of how bond distances may
vary with an oxidation state, see: Frenking, G.; Fro¨hlich, N. Chem. ReV.
2000, 100, 717.
(12) (a) Gleiter, R.; Merger, R. In Modern Acetylene Chemistry; Stang,
P. J., Diederich, F., Eds.; VCH: Weinheim, Germany, 1995; pp 285-319.
(b) De Graaff, R. A. G.; Gorter, S.; Romers, C.; Wong, H. N. C.;
Sondheimer, F. J. Chem. Soc., Perkin Trans. 1 1981, 478.
OL025956O
(13) For a tabulation, see: Benisch, C.; Gleiter, R.; Staeb, T. H.; Nuber,
B.; Oeser, T.; Pritzkow, H.; Rominger, F. J. Organomet. Chem. 2002, 641,
102.
(14) Rausch, M. D.; Tokas, E. F.; Gardner, S. A.; Clearfield, A.; Chinn,
J. W., Jr.; Bernal, I. J. Organomet. Chem. 1981, 212, 247.
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