A new route for the thermal isomerization of a highly substituted hexadienyne derivative

Article abstract of DOI:10.1002/ejoc.200400288

Whereas the thermal isomerization of perphenylbutenyne (11) at 290 °C in toluene leads to the benzannelated semibullvalene derivative 13, pyrolysis at 360 °C furnishes the triphenylnaphthalene 12. The mechanisms of both cycloisomerizations, which presumably proceed via the isobenzene intermediate 14, are discussed. The structure of the rearrangement product 13 was proven by single-crystal X-ray analysis. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejoc.200400288

SHORT COMMUNICATION
A New Route for the Thermal Isomerization of a Highly Substituted
Hexadienyne Derivative[‡]
Harald Berger,[a] Henning Hopf,*[a] Ina Dix,[a] and Peter G. Jones[b]
Keywords: Thermal isomerization
/ Isobenzene / Cycloaromatization / Condensed aromatic hydrocarbons /
Semibullvalene / Cyclization
Whereas the thermal isomerization of perphenylbutenyne
(11) at 290 °C in toluene leads to the benzannelated semi-
bullvalene derivative 13, pyrolysis at 360 °C furnishes the
triphenylnaphthalene 12. The mechanisms of both cycloiso-
intermediate 14, are discussed. The structure of the re-
arrangement product 13 was proven by single-crystal X-ray
analysis.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
merizations, which presumably proceed via the isobenzene Germany, 2004)
Introduction
On heating to about 200 °C, hexadienynes such as 1 (with
R ϭ H, D, CH3, Br etc.), in the gas phase, cyclize in excel-
lent yield to the corresponding benzene derivatives 3.[2,3] As
shown by trapping reactions for the parent system (R ϭ
H), the isomerization takes place in two steps proceeding
via the intermediate isobenzene[4] derivative 2.[5] In experi-
ments using fluorene derivatives such as 4, in which one of
the double bonds of the substrate is part of a benzene ring,
it was shown that at higher temperatures (350 °C, in tolu-
ene, sealed ampoule) the aromatic isomerization product,
here 6, is formed in a good yield, but that at lower tempera-
tures (250 °C, toluene) a surprising new thermal process
took place to yield the bis(methylene)cyclobutene derivative
7. Raising the temperature to the initial value of 350 °C
resulted in the cleavage of this dimer of 4 and provided
the condensed aromatic hydrocarbon
6 once more
(Scheme 1);[5] it is very likely that the isobenzene intermedi-
ate 5, which in this case is also an o-isotoluene derivative,[6]
is involved in both thermal processes. In the present com-
munication we show that a slight modification of the sub-
strate structure, namely the removal of the single bond con-
necting the two benzene rings of 4, leads to another drastic
change in mechanism, and demonstrates that the 1,3-hexa-
[‡]
Thermal Isomerizations, XXXIII. Part XXXII: Ref.[1]
[a]
Institut für Organische Chemie, Technische Universität
Scheme 1. Thermal isomerization of hexadienynes: known cycliza-
tion routes
Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: (internat.) ϩ 49-531-391-5388
E-mail: h.hopf@tu-bs.de
[b]
Institut für Anorganische und Analytische Chemie, Technische
Universität Braunschweig,
dien-5-yne unit can be induced to isomerize along totally
different pathways, as is the case for the even more highly
unsaturated analog 3-hexen-1,5-diyne.[7,8]
Postfach 3329, 38023 Braunschweig, Germany
Fax: (internat.) ϩ 49-531-391-5387
E-mail: p.jones@tu-bs.de
Eur. J. Org. Chem. 2004, 3401Ϫ3403
DOI: 10.1002/ejoc.200400288
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3401

H. Berger, H. Hopf, I. Dix, P. G. Jones
SHORT COMMUNICATION
Results and Discussion
mal isomerization product surprisingly is a derivative of
semibullvalene![11]
In the three-membered ring of the dimer 13, the bond
lengths are C4ϪC5 1.533(2), C4ϪC6 1.571(2), C5ϪC6
Thermal Isomerization of Perphenylbutenyne (11)
Hydrocarbon 11 was prepared according to a published
procedure[9] by adding phenylethynyllithium (9) in a diethyl
ether solution to benzhydryl phenyl ketone (8), and by de-
hydration of the thus formed propargyl alcohol 10 in tolu-
ene in the presence of potassium bisulfate at 120 °C
(Scheme 2).
˚
˚
1.503(2) A; the bond C1ϪC5 [1.586(2) A] is somewhat
longer than a standard single bond. A search of the Cam-
bridge Database[12] revealed 22 ordered structures including
a carbon skeleton as defined by C1ϪC8; the bond corre-
sponding to C4ϪC6 was the longest, but showed a large
˚
˚
scatter of 1.51Ϫ1.85 A (mean 1.62 A). This should be com-
pared with the ‘‘standard’’ bond length of 1.509 A, which
˚
was suggested by Allen[13] based on X-ray studies of small
cyclopropane derivatives. Substituents about the double
bond C7ϪC8 are coplanar [cf. torsion angle
C6ϪC7ϪC8ϪC1 Ϫ1.2(2)°], but there is slight rotation
about C13ϪC14 [C39ϪC13ϪC14ϪC45 9.4(3)°]. The mo-
lecular packing involves one short CϪH···π contact from
H47 to the centroid (Cg) of C21Ϫ26, with H···Cg 2.79,
CϪH···Cg 158°, operator 1 Ϫ x, 1 Ϫ y, 1 Ϫ z.
Mechanism of Formation of Semibullvalene 13
A mechanism to rationalize the formation of 13 is pre-
sented in Scheme 3. The isomerization/dimerization begins
with the electrocyclic ring closure of 11 to generate the iso-
benzene 14. This can either undergo the usual hydrogen shift
to form the naphthalene 12,[14] or collapse to the carbene
16, formally a reverse of the classical DoeringϪMooreϪ
Skattebøl allene synthesis. To generate a precursor suitable
Scheme 2. A novel route for thermal cyclization of hexadienyne de-
rivatives
Heating of 11 in toluene in a sealed ampoule at 360 °C
for 7 h gave 1,3,4-triphenylnaphthalene (12) in 32% yield, a
known compound[10] that was characterized by its analyti-
cal and spectroscopic data, and is the obvious result of a
cycloaromatization process as discussed in Scheme 1. How-
ever, when the reaction time and temperature were lowered
(290 °C, 5 h) another hydrocarbon product was isolated
(31% yield), which was shown by mass spectrometry to be
a dimer of the substrate 11, but the exact structure of which
could only be elucidated after single crystals suitable for an
X-ray structural investigation were obtained by recrystalli-
zation from pentane. As shown in Figure 1, this novel ther-
Figure 1. The molecule of compound 13 in the crystal; thermal
ellipsoids are drawn at the 30% probability level
Scheme 3. Mechanism of semibullvalene formation from a hexadi-
enyne
3402
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 3401Ϫ3403

Thermal Isomerization of a Highly Substituted Hexadienyne Derivative
SHORT COMMUNICATION
X-ray Structure Determination of 13: Crystal data: C56H40, triclinic,
for the formation of the ultimately isolated product 13, a
hydrogen shift is required (16 Ǟ 15), which can lead to
the resonance-stabilized diradical 15 ↔ 17. With this, the
aromaticity of the benzene ring is restored, thus providing
the driving force for this bond reorganization. In a final
trapping step with the triple bond of the starting material
11, the semibullvalene 13 is then produced.
˚
¯
space group P1, a ϭ 9.712(3), b ϭ 12.520(2), c ϭ 17.608(2) A, α ϭ
3
˚
75.852(8), β ϭ 74.494(12), γ ϭ 73.336(14)°, V ϭ 1943.5 A , Z ϭ
2, µ(Mo-Kα) ϭ 0.07 mmϪ1, T ϭ Ϫ100 °C. Data collection: A color-
less prism ca. 0.65 ϫ 0.4 ϫ 0.25 mm was used to record 7259 inten-
sities with a Siemens P4 diffractometer (Mo-Kα radiation, 2θmax. ϭ
50°). Structure refinement: The structure was refined aniso-
tropically on F2 (program SHELXL-97),[15] to wR2 ϭ 0.096, R1 ϭ
0.041 for 505 parameters and all 6818 unique reflections; S ϭ 0.90,
In summary, depending on the type and degree of substi-
tution, a remarkable molecular complexity can be generated
max. ∆ρ ϭ 0.18 e·AϪ3. Hydrogen atoms were included using a
˚
under simple laboratory conditions from easily available riding model. CCDC-236684 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk].
precursors that contain a 1,3-hexadien-5-yne subsystem.
Experimental Section
General Remarks: 1,1,2,4-Tetraphenyl-1-buten-3-yne (11) was pre-
pared as described in the literature[9] and characterized by the usual
spectroscopic data. Pyrolyses were carried out in anhydrous toluene
in heavy-walled glass ampoules that were sealed after several freeze-
pump-thaw cycles and placed in a steel bomb tube for protection.
[1]
I. Dix, C. Doll, H. Hopf, P. G. Jones, Eur. J. Org. Chem.
2002, 2547Ϫ2556.
[2]
H. Hopf, H. Musso, Angew. Chem. 1969, 81, 704; Angew.
Chem. Int. Ed. Engl. 1969, 8, 680; and H. Hopf, unpublished
results.
[3]
U. Nüchter, H. Hopf, G. Zimmermann, V. Francke, Liebigs
Pyrolysis of 11 at 360 °C: The starting hydrocarbon 11 (360 mg,
1.01 mmol) was dissolved in toluene (4 mL), and the solution was
heated for 6 h. After cooling and opening of the ampoule, the sol-
vent was removed in vacuo, and the residue was purified by chro-
matography (silica gel, pentane) and recrystallization (ethanol).
Yield: 115 mg (32%) of 1,2,4-triphenylnaphthalene (12). M.p. 159
°C, ref.[10] m.p. 159Ϫ161 °C. The spectroscopic data are identical
with those reported in ref.[10]
Ann./Recueil 1997, 1505Ϫ1515. For a summary on the scope
and mechanism of this cycloaromatization process, see: G.
Zimmermann, Eur. J. Org. Chem. 2001, 457Ϫ471.
[4]
M. Christl, M. Braun, G. Müller, Angew. Chem. 1992, 104,
471Ϫ473; Angew. Chem. Int. Ed. Engl. 1992, 31, 473Ϫ475.
[5]
H. Hopf, H. Berger, G. Zimermann, U. Nüchter, P. G. Jones,
I. Dix, Angew. Chem. 1997, 109, 1236Ϫ1238; Angew. Chem.
Int. Ed. Engl. 1997, 36, 1187Ϫ1190.
[6]
W. J. Bailey, R. A. Baylouney, J. Org. Chem. 1962, 27,
Pyrolysis of 11 at 290 °C: As described above, 11 (384 mg,
1.08 mmol) was pyrolyzed for 4 h. After workup (see above),
120 mg (31%) of the semibullvalene dimer 13 were obtained; recrys-
tallization from pentane provided crystals suitable for X-ray struc-
ture determination (see below). M.p. 289 °C. 1H NMR (CDCl3,
400.1 MHz, int. TMS): δ ϭ 7.24Ϫ7.28 (m, 1 H), 7.02Ϫ7.16 (m, 10
H), 6.93Ϫ7.01 (m, 12 H), 6.80Ϫ6.87 (m, 10 H), 6.60Ϫ6.65 (m, 4
H), 6.31Ϫ6.33 (d, J ϭ 7.4 Hz, 2 H), 3.67 (s, 1 H) ppm. 13C NMR
(CDCl3, 100.6 MHz, int. TMS): δ ϭ 142.79, 138.40, 137.96, 135.73,
130.92, 130.64, 130.51, 129.72, 127.44, 127.32, 127.22, 127.17,
126.77, 126.73, 126.53, 126.32, 126.04, 125.96, 125.92, 125.78,
125.14, 77.29 ppm. IR (diamond ATR): ν˜ ϭ 3082 (w), 3054 (w),
3021 (w), 2951 (w), 2926 (w), 2855 (w), 1573 (w), 1490 (m), 1467
(w), 1441 (m), 1155 (w), 1074 (m), 1027 (m), 913 (w), 841 (w), 785
(w), 756 (m), 696 (s) cmϪ1. UV (CDCl3): λmax (log ε) ϭ 329 nm
(4.01), 240 (4.53). MS (EI, 70 eV): m/z (%) ϭ 712 (100) [Mϩ], 621
(9), 545 (10), 467 (9), 367 (25), 356 (88), 339 (10), 279 (51), 265
(12), 239 (16), 178 (16), 167 (11), 138 (8). C56H40 (712.88): calcd.
C 94.34, H 5.66; found C 93.71, H 5.72.
3476Ϫ3478.
[7]
R. R. Jones, R. G. Bergman, J. Am. Chem. Soc. 1972, 94,
660Ϫ661; cf.: R. G. Bergman, Acc. Chem. Res. 1973, 6, 25Ϫ31.
[8]
For a recent review on the Bergman cyclization and related
processes (MyersϪSaito and Schmittel rearrangements, respec-
tively) see: M. Winkler, H. H. Wenk, W. Sander in Reactive
Intermediate Chemistry (Eds.: R. A. Moss, M. S. Platz, M.
Jones, Jr.), John Wiley & Sons, New York, 2004, chapter 16,
pp. 741Ϫ794.
[9]
F. Marcuzzi, U. Azzena, G. Melloni, J. Chem. Soc., Perkin
Trans. 1 1993, 2957Ϫ2960.
[10]
R. Larock, Q. Tian, J. Org. Chem. 1998, 63, 2002Ϫ2009.
[11]
For other benzannelated semibullvalenes see: E. Ciganek, J.
Am. Chem. Soc. 1966, 88, 2882Ϫ2883 and refs. cited therein.
[12]
F. H. Allen, Acta Crystallogr., Sect. B 2002, 58, 380Ϫ388.
[13]
F. H. Allen, Acta Crystallogr., Sect. B 1980, 36, 81Ϫ96.
[14]
M. Prall, A. Krüger, P. R. Schreiner, H. Hopf, Chem. Eur. J.
2001, 7, 4386Ϫ4394.
G. M. Sheldrick, SHELXL-97, University of Göttingen, Ger-
[15]
many, 1997.
Received April 28, 2004
Eur. J. Org. Chem. 2004, 3401Ϫ3403
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3403