An unusual cobalt-mediated cleavage of a hindered alkyne

Article abstract of DOI:10.1016/S0040-4039(00)00667-5

When 1-(pentaphenylphenyl)-2-phenylacetylene (1) is heated with (η⁵- cyclopentadienyl)dicarbonylcobalt, 1,2,3,4-tetraphenylfluorene (3) and related minor products are formed rather than the expected tetraarylcyclobutadiene. The formation of 3 requires that a seven-carbon fragment (formally a phenylcarbyne) must be lost. Two minor products, however, result from intramolecular cyclization of the alkyne and these two retain all of the carbons from the starting material. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00667-5

Tetrahedron Letters 41 (2000) 4261±4264
An unusual cobalt-mediated cleavage of a hindered alkyne
Naoto Hayashi, Douglas M. Ho and Robert A. Pascal Jr.*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
Received 29 March 2000; accepted 20 April 2000
Abstract
When 1-(pentaphenylphenyl)-2-phenylacetylene (1) is heated with (Z5-cyclopentadienyl)dicarbonylcobalt,
1,2,3,4-tetraphenyl¯uorene (3) and related minor products are formed rather than the expected tetraaryl-
cyclobutadiene. The formation of 3 requires that a seven-carbon fragment (formally a phenylcarbyne) must
be lost. Two minor products, however, result from intramolecular cyclization of the alkyne and these two
retain all of the carbons from the starting material. # 2000 Elsevier Science Ltd. All rights reserved.
We recently described the synthesis 1,3,5-tris(pentaphenylphenyl)benzene, a crowded molecule
in which the central benzene ring is completely shielded from the surrounding solvent by the
pentaphenylphenyl groups.1 This tremendously bulky substituent is easy to prepare, and it should
admirably protect even extremely reactive functional groups. In order to test this hypothesis, we
attempted to prepare 1,3-bis(pentaphenylphenyl)-2,4-diphenylcyclobutadiene (2) by dimerization
of the readily available 1-(pentaphenylphenyl)-2-phenylacetylene2 (1). (Z5-Cyclopentadienyl)-
dicarbonylcobalt has been used for the conversion of other hindered, asymmetric alkynes,
including naphthylphenylacetylene and mesitylphenylacetylene, to the corresponding cyclo-
butadiene complexes;3,4 it is readily available, and so it seemed to be the reagent of choice for
such a synthesis.
For the dimerization of mesitylphenylacetylene, Rausch et al. heated 2 equivalents of the
acetylene with 1 equivalent of CpCo(CO)2 in re¯uxing xylenes for 72 h.4 When we attempted
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00667-5

4262
similar dimerizations of compound 1 under these conditions, the reaction was sluggish, even
when re¯uxing tetralin was substituted for xylenes. No organometallic products were isolated
from these mixtures, and the main organic product found in each case proved to be 1,2,3,4-
tetraphenyl¯uorene (3, C37H26), which necessitates the loss of a seven-carbon fragment from 1
(C44H30). This result was quite unexpected, and so we analyzed the products from one of the
large-scale reactions in detail. In this case 10.24 g (18.33 mmol) of 1 were heated with 1.65 g (9.16
mmol) of CpCo(CO)2 in 150 mL xylenes, and most of the starting material was consumed after
144 h at re¯ux. Three high molecular weight organic products were obtained after repeated silica
gel column chromatography: compounds 3 (2.05 g), 4 (80 mg), 5 (70 mg), and mixtures of the
three (0.85 g).
1
The H and 13C NMR spectra of compounds 3 and 4 exhibited resonances due to aliphatic
protons and carbons. For compound 3, a proton singlet at ꢀ 3.86 (2H), a carbon resonance at ꢀ
37.3, and a molecular ion at m/z 4705 led to its identi®cation as the known 1,2,3,4-tetraphenyl-
¯uorene.6,7 Precise mass data for 4 gave the formula C44H32,8 showing that it retains all the car-
1
bons from 1, but that two hydrogens have been added. The AMX system (3H) in its H NMR
spectrum and the presence of two aliphatic carbon resonances (ꢀ 38.4 and 49.2) strongly suggest
that this material is 9-benzyl-1,2,3,4-tetraphenyl¯uorene.8 The structure of 5 was not immediately
1
apparent from its spectral characteristics. Its H NMR spectrum contains only complex and
overlapping aromatic resonances, but a precise mass measurement gave the formula C44H28.9
Fortunately, a single crystal of 5 was obtained, and X-ray analysis10 showed it to be 1,2,3,8-
tetraphenylbenz[e]acephenanthrylene (Fig. 1), which results from a double cyclization and dehy-
drogenation of 1.
Figure 1. X-Ray structure of 1,2,3,8-tetraphenylbenz[e]acephenanthrylene (5). Thermal ellipsoids are drawn at the
50% probability level

4263
The cleavage of alkyne 1 to yield 3 is most surprising; we know of no comparable cobalt-
catalyzed reaction. It seems extremely unlikely that arylcarbynes could have been formed by the
direct insertion of cobalt into the alkyne, although such species might have led to the observed
products. Instead, we propose the following speculative mechanistic scheme (Fig. 2). The identity
of the active cobalt species is unknown, but it may be multinuclear, and so it is indicated merely
as `ConLm'. After an initial activation of 1 by cobalt complexation and ortho metallation, a
migratory insertion into one of the phenyl C±H bonds would yield the ¯uorene nucleus, perhaps
as part of a complex such as 6. A similar insertion into a second phenyl group would lead to the
minor product 5, but a hydride shift would give the carbene 7, and reduction or hydrolysis of 7
would yield the benzyl ¯uorene 4. More dicult to explain is the production of the major product
3, which must involve some type of ꢁ-bond activation. Perhaps it forms by cobalt insertion into
the ole®n of 8, followed by reduction of the metal ¯uorenylidene 9. Alternatively, under such
rigorous reaction conditions the weak, doubly-benzylic bond in 7 might fragment, and the
resulting radical 13 could scavenge hydrogen from the solvent to yield 3.
Figure 2. Hypothetical pathways for cobalt-mediated transformations of 1. Products isolated from the reaction mix-
ture are enclosed in boxes
The existence of metal carbenes in this reaction is supported by the ®nding of various alkene
metathesis products. A very nonpolar fraction (38 mg) was composed primarily of trans-stilbene
(11) and 1,2-diphenylethane (12) (cis-stilbene was not observed). These small organics were not

4264
present in sucient quantity to account for all of the `lost' seven-carbon fragments; however, it is
clear that some hydrogenation is occurring in this reaction mixture, and so it is likely that small
carbenes such as 10 would simply be reduced to toluene, just as the carbenes 7 and 9 would be
reduced to the observed 3 and 4. Whatever the mechanism of formation of 3, it is remarkable that
such a C±C cleavage route is more than competitive with the simpler cyclization reactions
occurring in this reaction.
Acknowledgements
This work was supported by NSF Grant CHE-9707958 which is gratefully acknowledged. We
thank S. C. Lee for helpful discussions.
References
1. Tong, L.; Ho, D. M.; Vogelaar, N. J.; Schutt, C. E.; Pascal Jr., R. A. J. Am. Chem. Soc. 1997, 119, 7291±7302.
2. Dilthey, W.; Schommer, W.; Hoschen, W.; Dierichs, H. Ber. 1935, 68, 1159±1162.
3. Rausch, M. D.; Tokas, E. F.; Mintz, E. A.; Clear®eld, A.; Mangion, M.; Bernal, I. J. Organomet. Chem. 1979, 172,
109±131.
4. Rausch, M. D.; Westover, G. F.; Mintz, E.; Reisner, G. M.; Bernal, I.; Clear®eld, A.; Troup, J. M. Inorg. Chem.
1979, 18, 2605±2615.
5. For 3: mp 237±240ꢀC [lit.6 246ꢀC, lit.7 225±226ꢀC]; H NMR (CDCl3, 500 MHz) ꢀ 3.86 (s, 2H), 6.43 (d, J=8 Hz,
1
1H), 6.89 (m, 10H), 7.02 (t, J=8 Hz, 1H), 7.20 (t, J=8 Hz, 1H), 7.27 (m, 10H), 7.45 (d, J=8 Hz, 1H); 13C NMR
(CDCl3) ꢀ 37.3, 123.1, 124.5, 125.28, 125.31, 126.26, 126.28, 126.35, 126.5, 126.6, 126.7, 127.8, 128.0, 129.9, 130.2,
131.40, 131.42, 135.9, 137.8, 138.4, 139.1, 139.8, 140.0, 140.1, 140.2, 140.6, 141.70, 141.73, 144.1 (29 of 29 expected
peaks observed); FAB MS, m/z 470 (M+, 100).
6. Ghosh, K.; Bhattacharya, A. J. Indian J. Chem. 1977, 15B, 32±35.
7. Sengupta, S. C.; Mitra, A. J. Indian Chem. Soc. 1959, 36, 825±829.
8. For 4: mp 273±274ꢀC; 1H NMR (CDCl3, 500 MHz) ꢀ 2.03 (dd, J=13, 10 Hz, 1H), 3.00 (dd, J=13, 3 Hz, 1H), 4.38
(dd, J=10, 3 Hz, 1H), 6.19 (d, J=8 Hz, 1H), 6.55 (d, J=8 Hz, 1H), 6.64 (m, 3H), 6.83 (m, 12H), 6.99 (t, J=8 Hz,
1H), 7.11 (m, 7H), 7.27 (t, J=8 Hz, 1H), 7.32 (d, J=8 Hz, 1H), 7.44 (t, J=8 Hz, 1H), 7.64 (d, J=8 Hz, 1H); 13C
NMR (CDCl3) ꢀ 38.4, 49.2, 123.2, 125.0, 125.5, 126.1, 126.2, 126.4, 126.6, 126.8, 126.9, 127.7, 128.1, 128.4, 129.7,
129.9, 130.0, 130.9, 131.1, 131.4, 131.5, 131.7, 132.1, 136.2, 137.8, 138.1, 139.7, 140.1, 140.2, 140.4, 141.1, 145.2,
147.9 (33 of 34 expected peaks observed); MS, m/z 600 (M+, 100); HRMS: 560.2499; calcd for C44H32: 560.2504.
9. For 5: mp >400ꢀC; H NMR (CDCl3, 500 MHz) ꢀ 6.4±7.7 (m); MS, m/z 556 (M+, 100); HRMS: 556.2196; calcd
1
for C44H28: 556.2191.
10. A crystal of 5 measuring 0.43Â0.30Â0.04 mm was used for X-ray measurements. Crystal data: C44H28; triclinic,
ꢀ
ꢀ
ꢀ
space group P1 (No. 2); a=9.8213 (4) A, b=12.1285 (7) A, c=13.6158 (7) A, ꢂ=64.967 (2) , ꢃ=86.852 (3) ,
ꢄ=83.841 (3)ꢀ, V=1461.0 (1) A3, Z=2, Dcalcd=1.265 g/cm3. MoKa radiation (l=0.71073 A) was employed for
data collection (ꢅmax=22.44ꢀ) at 200 K on a Nonius KappaCCD di€ractometer. A total of 9046 re¯ections were
indexed, integrated, and corrected for Lorentz and polarization e€ects by using the program DENZO,11 and then
were merged to 3783 unique re¯ections (Rint=0.036) by using SCALEPACK.11 The structure was solved by direct
methods (SHELXTL12) and re®ned by full-matrix least-squares on F2. The molecule is disordered over two
positions in an 83:17 ratio; this disorder is evident only for the benz[e]acephenanthrylene core and the C(8)-phenyl
group. In the ®nal crystallographic model, all carbons of the major component were re®ned anisotropically, with
hydrogens riding [C±H=0.95 A, U(H)=1.2U(C)]; the minor component was re®ned isotropically as two rigid
bodies with light restraints on the thermal parameters. The re®nement converged to R(F)=0.050, wR(F2)=0.117,
and S=1.10 for 2842 re¯ections with F>4ꢁ(F), and R(F)=0.075, wR(F2)=0.130, and S=1.04 for 3783 unique
re¯ections, 437 variables, and 30 restraints.
11. Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307±326.
12. Sheldrick, G. M. SHELXTL Version 5; Siemens Analytical X-ray Instruments: Madison, Wisconsin, 1996.