C8H6 thermal chemistry. 7-Methylenecyclohepta-1,3,5- dienyne (heptafulvyne) by flash vacuum thermolysis]matrix isolation. Chemical activation in the rearrangements of phenylenedicarbenes and of benzocyclobutadiene to phenylacetylene

Article abstract of DOI:10.1071/CH13670

Methylenecycloheptadienyne 11 (heptafulvyne) is obtained very cleanly by flash vacuum thermolysis (FVT) of the diazobenzocyclobutene precursor 8 at 400°C followed by isolation as a neat solid at 77K or in an Ar matrix at 7-10K. Compound 11 is a yellow solid, stable till ～-100°C in the neat state. The diazo compound itself (2) is observable by IR spectroscopy following mild decomposition of the tosylhydrazone salt 1 at 115°C. FVT of 8 at 200°C also generates diazo compound 2 as observed by IR spectroscopy and on-line mass spectrometry. FVT of 8 at 600-800°C causes rearrangement of 11 to phenylacetylene 12 and benzocyclobutadiene 13. Mechanisms for the rearrangements are proposed. Facile rearrangement of benzocyclobutadiene to phenylacetylene is ascribed to chemical activation, which is also seen to be involved in the rearrangement of p-, m-, and o-phenylenebiscarbenes 25-27 to phenylacetylene 12.

Full text of DOI:10.1071/CH13670

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2014, 67, 1174–1179
Full Paper
http://dx.doi.org/10.1071/CH13670
C8H6 Thermal Chemistry. 7-Methylenecyclohepta-
1,3,5-dienyne (Heptafulvyne) by Flash Vacuum
Thermolysis Matrix Isolation. Chemical Activation
]
in the Rearrangements of Phenylenedicarbenes
and of Benzocyclobutadiene to Phenylacetylene
A
A
B
A C
,
Arvid Kuhn, Daisuke Miura, Hideo Tomioka, and Curt Wentrup
A
School of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, Qld 4072, Australia.
B
Mie University, Tsu, Mie 514-8507, Japan.
C
Corresponding author. Email: wentrup@uq.edu.au
Methylenecycloheptadienyne 11 (heptafulvyne) is obtained very cleanly by flash vacuum thermolysis (FVT) of the
diazobenzocyclobutene precursor 8 at 4008C followed by isolation as a neat solid at 77 K or in an Ar matrix at 7–10 K.
Compound 11 is a yellow solid, stable till ,ꢀ1008C in the neat state. The diazo compound itself (2) is observable by IR
spectroscopy following mild decomposition of the tosylhydrazone salt 1 at 1158C. FVT of 8 at 2008C also generates diazo
compound 2 as observed by IR spectroscopy and on-line mass spectrometry. FVT of 8 at 600–8008C causes rearrangement
of 11 to phenylacetylene 12 and benzocyclobutadiene 13. Mechanisms for the rearrangements are proposed. Facile
rearrangement of benzocyclobutadiene to phenylacetylene is ascribed to chemical activation, which is also seen to be
involved in the rearrangement of p-, m-, and o-phenylenebiscarbenes 25–27 to phenylacetylene 12.
Manuscript received: 3 December 2013.
Manuscript accepted: 6 January 2014.
Published online: 28 January 2014.
Introduction
8, and 9 in Ar matrices led to the formation of the novel
methylenecycloheptadienyne 11 (heptafulvyne), possibly
formed in a vinylidene–acetylene rearrangement of the valence
isomer 10 (Scheme 2).[3] Neither 3 nor 10 were observable.
Preparative flash thermolysis of 9 at 5008C with NMR spectro-
scopic analysis of the stable products afforded a low yield of a
mixture of phenylacetylene 12 and the dimer 14 of benzocyclo-
butadiene 13 in a ,7 : 1 ratio.[2]
Two dimers 4 and 5 and a trimer 6 of the unobserved benzo-
cyclobutenylidene 3 have been isolated in a combined yield of
65 % from photolysis of a suspension of the tosylhydrazone salt
1 in benzene (Scheme 1).[1]
Addition of benzocyclobutenylidene 3 to benzene takes
place in this reaction as well, and this has also been observed
in the thermal decomposition of 1 in benzene.[2]
Thermolysis of the tosylhydrazone salt 7 generated diazo
compound 2, which was observable by its IR absorption at
2042 cmꢀ1 in an Ar matrix. Photolyses of the three precursors 2,
In an endeavour to shed more light on this chemistry, we have
investigated the flash vacuum thermolysis (FVT) of 1 and 8 with
Ar matrix isolation of the products.
Naϩ
Results
N
N2
Gentle decomposition of the solid tosylhydrazone salt 1 at
1158C in high vacuum (5 ꢁ 10ꢀ4 hPa) with deposition of the
material at 77 K gave diazo compound 2 absorbing at 2041,
1589, 1584, 1430 cmꢀ1 (2047, 1599, 1595, 1443 cmꢀ1 by pho-
tolysis of 9[3]). The same diazo absorption was also obtained by
first depositing a sample of solid aziridine 8 at 77 K and then
photolyzing it with a high-pressure Hg/Xe lamp. Compounds 4,
5, and 6 were not obtained under these conditions, but FVT of 8
at 200–4008C with deposition of the neat product at 77 K
afforded a weak band at 2098 cmꢀ1 in the IR spectrum, assigned
to 11. This compound was stable to ,ꢀ1008C on subsequent
warm-up. This was the first observation of 11 in a purely thermal
reaction.
Ts
N
hν
PhH
1
2
3
4
5
6
Scheme 1. Dimers and trimer formed on photolysis of 1 as a suspension in
benzene.[1]
Journal compilation Ó CSIRO 2014
www.publish.csiro.au/journals/ajc

C8H6 Thermal Chemistry
1175
Further FVT reactions of 8 were carried out at 400, 600, and
8008C with isolation of the products in Ar matrices at ,7 K.
Apart from the necessary by-product styrene (15), the IR
spectrum of the products of the 4008C reaction showed clear
absorptions assigned to the cycloheptadienyne 11,[3] which is
formed very cleanly (Fig. 1; Scheme 3). The acetylenic CꢂC
stretch appears at 2102 cmꢀ1, and other absorptions at 1570,
848, 731, 618, and 609 cmꢀ1 are in good agreement with
previous observations. FVT at 600 and 8008C showed formation
of styrene 15, disappearance of 11, and formation of two new
products, which were identified by comparison with literature
data: phenylacetylene 12 and benzocyclobutadiene 13 (Fig. 2).
The intensities of bands attributable to 11 decrease as those of 12
and 13 increase. Compound 11 has largely disappeared at 600,
and completely at 8008C (Fig. 2).
The bands attributable to phenylacetylene 12 at 2122, 1490,
1446, 1218, 1028, 912, 758, and 648 cmꢀ1 were assigned by
comparison with data reported by Jørgensen et al.[4] and King
and So.[5] Benzocyclobutadiene 13 absorbed at 1430, 1400,
1280, 1240, 898, 735, and 690 cmꢀ1 in accord with a spectrum
reported by Chapman.[6]
A correlation between observed and calculated IR spectra of
11 (Fig. 3) is in excellent agreement with the previous observa-
tions of McMahon and co-workers.[3] Comparison with the
calculated IR spectra (B3LYP/6–31G*) of several other poten-
tial rearrangement products 16–20 is shown in Fig. S1 (Supple-
mentary Material). Their relative energies calculated at the
B3LYP/6–31G* level are shown in Chart 1. Energies and
structures of 17 and several other C8H6 isomers have been
calculated previously at the MNDO level.[7] The unstable Z,Z-
isomer of octadienediyne 19 has been observed to rearrange to
Naϩ
N
N
N
SO2
7
Δ
Ph
hν
N2
N
hν
8
9
2
3
10
11
hν
N
N
500ЊC
ϩ
13
14
12
Scheme 2. Formation of methylenecycloheptadienyne 11 by matrix
photolysis.[3]
S
(a)
S
A
S
A
S
S
S
A
S
A
A
A
A
S
A
S
A
S
S
A
A
S
S
A
A
(b)
Styrene
2200.0 2000
1800
1600
1400
1200
[cmϪ1
1000
800
600
400.0
]
Fig. 1. IR spectra (Ar matrices, ,7 K; Ar/substance ratio ,1000 : 1). (a) The product of FVT of 8 at 4008C. A ¼ methylenecycloheptadienyne 11.
S ¼ styrene. (b) Pure styrene 15.

1176
A. Kuhn et al.
cycloocta-1,2,4,6,7-pentaene 18 and then to benzocyclobuta-
diene 13 at room temperature.[8] The calculated IR spectra of
16–20 (Fig. S1) do not match the experimental IR spectrum
ascribed to 11 (Figs. 1–3).
the collisional activation spectra of both 11 and 12 is m/z 76
(C6H4), which corresponds to the benzyne molecular ion or an ion
formed by ring opening of the benzyne ion.[11]
FVT of 8 at 5008C, isolation of the material at 77 K, warm-up
and extraction with hexane followed by gas chromatography–
mass spectrometry (GC-MS) analysis of the product afforded a
small amount of benzocyclobutadiene dimer 14. The two dimers
4 and 5 and the trimer 6 synthesized by Frimer et al.[1]
(Scheme 1) were resynthesized from 1, and their mass spectra
were shown to be different from that of 14 (Fig. S6 in the
Supplementary Material). Thus, it appears that benzocyclo-
butenylidene 3 survives in sufficient amount to dimerize and
trimerize when generated by photolysis in benzene at 0–108C;
but it does not survive the FVT conditions used by us or by
McMahon and co-workers.[3]
Interestingly, Du¨rr et al. reported the formation of low yields
of Frimer’s dimers 4 and 5 on ‘flash pyrolysis’ of the Li salt
corresponding to 1 at 3508C.[12] However, the reaction condi-
tions used by these workers involve dropping the solid salt into a
heated and evacuated pyrex flask at 0.05–0.1 hPa. This results in
Parallel to the FVT/matrix isolation–IR investigation, the FVT
reactions of 8 were also monitored by direct on-line mass
spectrometry. At a FVT temperature of 200–3008C, peaks at m/z
130 and 102 are observed, corresponding to 2þꢃ and [2þꢃ ꢀ N2]
(Fig. S2 in the Supplementary Material). At 4008C the spectrum
changes completely, the peak at m/z 130 disappears, and a peak at
m/z 28 increases (N2), and two major signals were recorded at m/z
104(styrene)andm/z102(Fig. S3intheSupplementaryMaterial).
The latter corresponds to 11, 12, and/or 13, and on the basis of the
IRinvestigationreportedabove,itisassumedthemassspectrumis
attributablelargely to11 at thistemperature. The mass spectrumis
very similar tothat of phenylacetylene 12, butminordifferencesin
relative abundances and in particular a strong peak at m/z 101
[Mþꢃ ꢀ H] in the collisional activation mass spectrum are com-
patible with 11þꢃ being the carrier (Figs S4 and S5 in the
Supplementary Material). The most abundant fragment peak in
Naϩ
N
N
TS
115ЊC
N2
1
FVT
FVT
ϩ
Ph
FVT
Ͼ500ЊC
400–600ЊC
N
N
11
13
12
2
8
15
Scheme 3. Thermal formation and rearrangement of 11 on flash vacuum thermolysis (FVT).
(a)
A
A
A
A
A
(b)
(c)
A
B
C
B
C
C
B
C
B
C
B
C
B
C
B
B
C
C C
1600.0
1500
1400
1300
1200
1100
1000
[cmϪ1
900
800
700
600
500
400.0
]
Fig. 2. Products of flash vacuum thermolysis (FVT) of 8 at (a) 400, (b) 600, and (c) 8008C. A ¼ methylenecycloheptadienyne 11.
B ¼ benzocyclobutadiene 13. C ¼ phenylacetylene 12. The acetylenic band at 2102 cmꢀ1 (Fig. 1) also disappears at 600–8008C. Unmarked
bands are attributable to styrene 15 (compare Fig. 1).

C8H6 Thermal Chemistry
1177
(a)
A
(b)
A
W
A
A
A
A
A
A
A
A
A
A
A
A
A
4000.0
3000
2000
1500
1000
500 400.0
[cmϪ1
]
Fig. 3. (a) Calculated IR spectrum of 11 (B3LYP/6–31G*; wavenumbers scaled by 0.95). (b) Experimental spectrum from the flash vacuum
thermolysis (FVT) of 8 at 4008C. A ¼ 11, W ¼ water.
11 (0)
12 (Ϫ51.2)
13 (Ϫ29.3)
97Ϯ4
73.3
Scheme 4. Roger Brown rearrangements.
C
C
CH2
Discussion
The rearrangement 10 - 11 is formally a vinylidene–acetylene
rearrangement, i.e. a Roger Brown rearrangement.[13] The
corresponding phenyl group shift from phenylvinylidene to
phenylacetylene (Scheme 4) has a calculated barrier of
,10 kcal molꢀ1; i.e. it is very rapid under FVT conditions, and it
is exothermic by ,47 kcal molꢀ1 (both H and phenyl can
undergo the 1,2-migration).[14] The rearrangement of fulven-6-
ylidene to o-benzyne (Scheme 4) is even faster, with a calculated
16 (Ϫ18.7)
17 (ϩ24.8)
18 (ϩ9.4)
20 (ϩ8.4)
19 (ϩ13.4)
barrier of only ,2 kcal molꢀ1 [15]
.
Chart 1. Energies of C8H6 isomers 11, 12, and 13 and potential structures
16–20. Values in parentheses are relative energies in kcal molꢀ1 at the
B3LYP/6–31G* level. Other values in plain font for 12 and 13 are
Our matrix–IR investigation shows that the cyclohepta-
dienyne 11 is formed in high yield on FVT at 4008C, but it
rearranges to phenylacetylene 12 and benzocyclobutadiene 13 at
higher temperatures.
experimental enthalpies of formation in kcal molꢀ1 [9,10]
.
The calculated activation barrier of ,31 kcal molꢀ1 [3] sug-
gests the possibility of a thermal reversion of 11 to carbene 10,
which is endothermic by ,24 kcal molꢀ1. A negligible barrier
then permits the cyclization of 10 to 3 (exothermic by
15 kcal molꢀ1). A 1,2-H shift in 3 leading to benzocyclo-
butadiene 13 has a calculated barrier[3] of ,28 kcal molꢀ1
(Scheme 5). Therefore, the highest barrier for the formation of
13 lies ,37 kcal molꢀ1 above 11. The low pressure FVT con-
ditions do not favour dimerization of the elusive carbene 3;
hence Frimer’s products 4, 5, and 6 are not obtained. But the
an immediate pressure rise due to pyrolysis in the solid state.
These conditions cannot be compared with conventional low-
pressure FVT. It thus appears that concentrations of benzo-
cyclobutenylidene (3) sufficiently high for its dimerization are
produced from the solid salt, either by photolysis or thermolysis
of a suspension, and by pyrolysis of the bulk material. It is worth
noting that reaction between the tosylhydrazone salt and benzo-
cyclobutenylidene as well as benzocyclobutenone azine was
recorded by Frimer et al.[1]

1178
A. Kuhn et al.
CH
CH
31
37
E
CH
CH
11 (0)
~60
10 (24)
Ͼ65
3 (9.2)
13 (Ϫ29.3)
CH
CH
~79
~30
50
30
27
24
26
25
C
C
~H
10
10
0
Ϫ10
3
22
23
12 (Ϫ51.2)
24
21
11
28
Ϫ30
Ϫ50
Scheme 5. Potential mechanisms for the thermal rearrangements of 11 to
benzocyclobutadiene 13 and phenylacetylene 12. Ground state energies in
kcal molꢀ1 relative to 11 calculated at the B3LYP/6–31G* level. Transition
state energies estimated (see text).
13
12
benzocyclobutadiene 13 formed in the gas-phase will survive to
the cold trap and dimerize to 14 on subsequent warm-up.
Compounds 11, 10, 3, and 13 can all in principle rearrange to
phenylacetylene 12 via ring opening (Scheme 5). Breaking of
the C1–C7 bond in 11 would lead to the diradical 21. Cyclization
to the benzenoid diradical 23 followed by a single H-shift leads
to phenylacetylene 12. Ring opening of 10 to the allenylidene 22
may be able to provide another route to 23. Benzocyclobuteny-
lidene 3 can ring open directly to 23. Normal vinylic C–C bond
dissociation energies are ,90 kcal molꢀ1. The strain energy of
Fig. 4. Schematic energy profiles for the reactions in Schemes 5 and 6.
Energies (E) in kcal molꢀ1 relative to heptafulvyne 11. The energies of 11,
10, and 3 are at the G2(MP2,SVP) level from Ref. [3]. The energies of 12 and
13 are based on the experimental enthalpies of formation.[9,10] The energies
of 24–27 are thermochemical estimates.[19] The barriers to ring expansion–
ring contraction in the phenylenebiscarbenes are assumed to be similar to
those in phenylcarbenes (13–15 kcal molꢀ1).[23] The barrier 12 - 28 is
based on B3LYP/6–31G* calculations on related molecules by Mackie
and Johnson.[14]
cycloheptyne is calculated as 31 kcal mol .
ꢀ1 [16] This would still
CH
suggest a high energy requirement of the order of 60 kcal molꢀ1
for ring opening 11 - 21, thereby making this path unlikely.
The ring opening of cyclobutane to 1,4-butanediyl has a barrier
CH
CH
CH
of 57 kcal molꢀ1 [16]
and a similar or higher barrier can be
,
CH
CH
27
13
expected for a direct ring opening of 3 to 23. This ring
opening can also be estimated as an aryl–C bond breakage
(,100 kcal molꢀ1) minus the release of ,30 kcal molꢀ1 of
25
26
cyclobutene strain energy) ¼ ,70 kcal molꢀ1 [16]
.
Adding the
places 23
calculated energy of
3
(9.2 kcal molꢀ1
)
,79 kcal molꢀ1 above 11. Such a reaction is not likely under
the reaction conditions. The ring opening 10 - 22 is expected
12
24
to be endothermic by at least 50 kcal mol .
ꢀ1 [17] For comparison,
the parent 6p electrocyclic, endothermic ring opening of
cyclohexadiene to Z-hexatriene has an activation energy of
,45 kcal molꢀ1 [18]
(24 kcal molꢀ1 [3]
.
Adding the calculated energy of 10
places 22 at least 65 kcal molꢀ1 above 11.
28
)
Thus, at any rate, reaction 10 - 22 is not likely to be competi-
tive. Finally, a C–C bond cleavage 13 - 24 is expected to have a
barrier of ,60 kcal molꢀ1 (the C–C bond dissociation energy,
,100 kcal molꢀ1, minus the strain energy of benzocyclobuta-
diene (13),[8] 40 kcal molꢀ1).[19] The low enthalpy of formation
of benzocyclobutadiene (13) then places this transition state at
,30 kcal molꢀ1 relative to 11, and this therefore becomes the
most favourable path to 12. In summary, thermochemical
arguments indicate that the rearrangement 11 - 13 will have
an overall activation barrier of ,37 kcal molꢀ1 relative to 11.
Such activation energies are readily accessed under modest FVT
conditions (above 4008C). It must be kept in mind that benzo-
cyclobutadiene 13 formed from 11 in a low pressure FVT
reaction will be chemically activated[20] by ,66 kcal molꢀ1
(37 þ 29 kcal molꢀ1; Scheme 5), which will facilitate the pas-
sage over the 60 kcal molꢀ1 barrier to 12 (Fig. 4).[21]
Scheme 6. p-Phenylenebiscarbene -m-phenylenebiscarbene -o-phenyl-
enebiscarbene-benzocyclobutadiene-phenylacetylene-phenylvinylidene
rearrangement.
distribution or the 13C-labels in phenylacetylene 12 was in accord
with the carbene–carbene rarrangements to 26 and 27 via ring
expansion–ring contraction, cyclization to benzocyclobutadiene
13, isomerization to phenylacetylene 12, and finally the Roger
Brown rearrangement interconverting the two acetylenic carbons
in12 via phenylvinylidene 28(both Hand phenylcan undergo the
1,2-migration). Here too, the rearrangement 27 - 13 will be
massively exothermic – by ,80kcal molꢀ1 [19] thereby allowing
a chemically activated isomerization 13 - 12.
Conclusion
The rearrangement of benzocyclobutadiene 13 to phenyl-
acetylene 12 under FVT conditions has been proposed
previouslybasedonadouble13C-labellingexperimentonanother
C8H6 isomer, benzene-1,4-biscarbene 25 (Scheme 6).[22] The
Mild thermolysis of the tosylhydrazone salt 1 (1508C) yields
diazobenzocyclobutene 2, which is also obtained by elimination
of styrene from 8 under mild FVT conditions. FVT of 8 at 4008C

C8H6 Thermal Chemistry
1179
results in clean formation of the methylenecycloheptadienyne
11 (heptafulvyne). FVT at higher temperatures (600–8008C)
causes rearrangement of 11 to benzocyclobutadiene 13 and
phenylacetylene 12. The facile rearrangement of 13 to 12 in the
rearrangement of 11, and the occurrence of a phenylacetylene–
phenylvinylidene interconversion (12 - 28) in the rearrange-
ment of p-, m-, and o-phenylenebiscarbenes 25–27 are ascribed
to chemically activated benzocyclobutadiene 13.
[6] O. L. Chapman, C. C. Chang, N. R. Rosenquist, J. Am. Chem. Soc.
1976, 98, 261. doi:10.1021/JA00417A056
[7] M. J. S. Dewar, K. M. Merz, Jr, J. Am. Chem. Soc. 1985, 107, 6175.
doi:10.1021/JA00308A005
[8] G. H. Mitchell, F. Sondheimer, J. Am. Chem. Soc. 1969, 91, 7520.
doi:10.1021/JA01054A055
[9] Enthalpy of formation of benzocyclobutadiene 97 ꢄ 4 kcal molꢀ1
:
K. M. Broadus, S. R. Kass, J. Am. Chem. Soc. 2000, 122, 10697.
doi:10.1021/JA002352B
[10] Enthalpy of formation of phenylacetylene 73.3 ꢄ 0.4 kcal molꢀ1
:
Experimental
NIST Database, Available at www.webbook.nist.gov (accessed 8
August 2013).
General procedures for FVT and matrix isolation[24] and
descriptions, drawings, and images of the FVT apparatus[24c]
have been published. The FVT apparatus consisted of a 10 cm
long, 0.7 cm internal diameter, electrically heated quartz tube
suspended in a vacuum chamber directly flanged to the cryostat
cold head, with a wall-free flight path of ,3 cm between the exit
of the quartz tube and the cold KBr target at ,10ꢀ5 hPa. Ar
matrices were deposited onto KBr disks at ,22 K, and IR
spectra were recorded at 7–10 K with a resolution of 1 cmꢀ1. Ar
was ultra high purity. For isolation of FVT products at 77 K the
same pyrolysis apparatus was flanged to a liquid N2-cooled
cryostat. GC-MS data were obtained using a Zebron capillary
GC column ZB-5 (30 m length). Mass spectra were recorded on
Kratos MS25RFA or Waters AutoSpec 6F mass spectrometers
in electron ionization mode at 70 eV. Both instruments had an
FVT oven directly attached to the ion source.[11,25,26] For the
AutoSpec 6F of EBEEBE geometry typical ion source condi-
tions were 8 kV accelerating voltage and 200 mA trap current.
The solid samples were introduced with a direct insertion probe.
[11] P. Gerbaux, C. Wentrup, Aust. J. Chem. 2012, 65, 1655. doi:10.1071/
CH12327
[12] H. Du¨rr, H. Nickels, L. A. Pacala, M. Jones, Jr, J. Org. Chem. 1980, 45,
973. doi:10.1021/JO01294A012
[13] (a) R. F. C. Brown, K. J. Harrington, G. L. McMullen, J. Chem. Soc.,
Chem. Commun. 1974, 123. doi:10.1039/C39740000123
(b) R. F. C. Brown, F. W. Eastwood, G. P. Jackman, Aust. J. Chem.
1977, 30, 1757. doi:10.1071/CH9771757
(c) R. F. C. Brown, F. W. Eastwood, G. P. Jackman, Aust. J. Chem.
1978, 31, 579. doi:10.1071/CH9780579
(d) R. F. C. Brown, Aust. J. Chem. 2010, 63, 1002. doi:10.1071/
CH10086
[14] I. D. Mackie, R. P. Johnson, J. Org. Chem. 2009, 74, 499. doi:10.1021/
JO802259H
[15] M. Winkler, W. Sander, Aust. J. Chem. 2010, 63, 1013. doi:10.1071/
CH10113
[16] See C. Wentrup, Reactive Molecules 1984 (Wiley: New York, NY),
and references therein.
[17] (a) J. A. Miller, S. J. Klippenstein, J. Phys. Chem. A 2001, 105, 7254
and references therein. doi:10.1021/JP0102973
(b) H. Hopf, G. Markopoulos, Beilstein J. Org. Chem. 2012, 8, 1936
and references therein. doi:10.3762/BJOC.8.225
Materials
[18] J. J. Gajewski, Hydrocarbon Thermal Isomerizations 2004, 2nd edn
(Elsevier: San Diego, CA), and references therein.
[19] Enthalpies of formation are estimated using the method of C. Wentrup,
Tetrahedron, 1974, 30, 1301. Thus DH0f of 24 ¼ 153 and 26
,175 kcal molꢀ1 are derived.
Compounds 1 and 8 and benzocyclobutadiene dimers and trimer
4, 5, and 6 were prepared according to literature methods.[1,3]
Supplementary Material
[20] C. Wentrup, Aust. J. Chem. 2013, 66, 852. doi:10.1071/CH13283
[21] More precise activation energies for the reactions depicted in
Scheme 5 may be obtained through quantum-chemical calculations.
However, multiconfigurational methods will be required in order to
treat open-shell diradical states correctly.
Figs S1 (calculated IR spectra of 11 and 16–20), S2–S3 (mass
spectra from the FVT of 8 at 200 and 4008C, S4-S5 (collisional
activation mass spectra of fragment ion m/z 102 from phenyl-
acetylene and from the FVT of 8), and S6 (mass spectra of 4, 5,
and 14) and computed vibrational data are available on the
Journal’s website.
[22] M. W. Baum, J. L. Font, M. E. Meislich, C. Wentrup, M. Jones, Jr,
J. Am. Chem. Soc. 1987, 109, 2534. doi:10.1021/JA00242A060
[23] (a) S. Matzinger, T. Bally, E. V. Patterson, R. J. McMahon, J. Am.
Chem. Soc. 1996, 118, 1535. doi:10.1021/JA953579N
(b) M. W. Wong, C. Wentrup, J. Org. Chem. 1996, 61, 7022.
doi:10.1021/JO960806A
Acknowledgements
Daisuke Miura acknowledges the award of a travelling stipend, which
enabled him to participate in this project in Brisbane. The authors are
indebted to the Australian Research Council for a Research Associateship
for Dr Arvid Kuhn, to the National Computing Infrastructure Merit Allo-
cation Scheme (project g01) supported by the Australian Government for
financial assistance, and to the late Professor Robert Flammang, Universite´
de Mons-Hainaut, Belgium, for the recording of mass spectra on the Auto-
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