C15H10 and C15H12 Thermal Chemistry: Phenanthrylcarbene Isomers and Phenylindenes by Falling Solid Flash Vacuum Pyrolysis of Tetrazoles

Article abstract of DOI:10.1021/acs.joc.5b01007

2-Phenyl-5-(phenylethynyl)tetrazole 44 provides a new entry to the C₁₅H₁₀ energy surface. Flash vacuum pyrolysis of 44 using the falling solid flash vacuum pyrolysis (FS-FVP) method afforded cyclopenta[def]phenanthrene 31 and cyclope

Full text of DOI:10.1021/acs.joc.5b01007

Article
pubs.acs.org/joc
C15H10 and C15H12 Thermal Chemistry: Phenanthrylcarbene Isomers
and Phenylindenes by Falling Solid Flash Vacuum Pyrolysis of
Tetrazoles
Curt Wentrup,*,†,‡ Jurgen Becker,† and Manfred Diehl†
̈
†Fachbereich Chemie der Philipps-Universitat Marburg, D-35037 Marburg, Germany
̈
‡School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Qld 4072, Australia
S
* Supporting Information
ABSTRACT: 2-Phenyl-5-(phenylethynyl)tetrazole 44 provides a new entry to
the C15H10 energy surface. Flash vacuum pyrolysis of 44 using the falling solid
flash vacuum pyrolysis (FS-FVP) method afforded cyclopenta[def ]-
phenanthrene 31 and cyclopenta[jk]fluorene 52 as the principal products.
The products are explained in terms of the formation of N-phenyl-C-
phenylethynylnitrile imine/(phenylazo)(phenylethynyl)carbene 45 and its
cyclization to 3-(phenylethynyl)-3H-indazole 46b. Pyrolytic loss of N2 from
46b generates C15H10 intermediate 48. Cyclization of 48 to a dibenzocyclo-
heptatetraene derivative and further rearrangements with analogies in the
chemistry of phenylcarbene and the naphthylcarbenes leads to the final
products. Similar pyrolysis of 2-phenyl-5-styryltetrazole 43 afforded 3-styrylindazole 58, which on further pyrolysis eliminated N2
to generate 3- and 2-phenylindenes 61 and 62 via C15H12 intermediates.
been formed by intramolecular addition of the photochemically
generated arylcarbene 23 to the acetylene moiety.9 Compound
22 has also been generated in solution by photolysis of the
tosylhydrazone salt 26 and trapped by Diels−Alder addition of
cyclopentadiene, butadiene, and furan to the strained cyclo-
propene CC bond.10,11 Phenanthrylcarbene 20, formed by
rearrangement of the dibenzocycloheptatrienylidene 25 or
cycloheptatetraene 24 by thermolysis of 26 at 125 °C in
benzene solution, was trapped by addition to benzene, forming
9-(7-cycloheptatrienyl)phenanthrene.10 The rearrangement of
24 or 25 to 22 takes place at −60 °C upon photolysis of 26.
The top row in Scheme 3 connecting 20, 22, 24, and 25 is
analogous to the rearrangements of the naphthylcarbenes3−5
(Scheme 2). The reaction sequence 28 → 29 → 30 is a normal
phenylcarbene rearrangement, and the final cyclization to 31 is
expected to be rapid and highly exothermic.12 It is worth noting
that attempts to observe 4-phenanthrylcarbene (from 4-
diazomethylphenanthrene) by ESR spectroscopy failed, prob-
ably because of its cyclization to 31 under the photolysis
conditions.7
INTRODUCTION
■
The rich rearrangement chemistry of phenylcarbene1 (Scheme
1) and the naphthylcarbenes2 (Scheme 2) has been the subject
of detailed investigations under both thermal and photo-
chemical reaction conditions.
Scheme 1. Rearrangement of Phenylcarbene 1 to
Cycloheptatetraene 2 and Fulvenallene 41
The thermal interconversion of 2- and 1-naphthylcarbenes 5
and 9, and their rearrangement to cyclobuta[de]naphthalene
10, is summarized in Scheme 2.3−5 DFT calculations indicated
that the tricyclic cyclopropenes 6TS and 8TS are transition
states.6
9-Phenanthrylcarbene 20 has been observed directly by ESR
spectroscopy following photolysis of diazo compound 18.7 This
carbene can be generated thermally by flash vacuum pyrolysis
(FVP) of either 18 or the tetrazole 19. Loss of N2 from
tetrazole 19 probably occurs via the 5H tautomer8 to generate
18. Thus, FVP of both 18 and 19 produced cyclobuta[de]-
phenanthrene 21, but in addition, a small amount of 4H-
cyclopenta[def ]phenanthrene 31 was also obtained (Scheme
3).4
The postulated carbene−carbene rearrangement 25 → 27 →
28 would pass through the strained intermediate or transition
state 27. We will refer to this type of reaction as a ring
interchange. In fact, there is excellent evidence for carbene−
carbene rearrangements via strained cyclopropenes of this
type.4,13 Two such examples are shown in Scheme 4. Carbene
33 has a triplet ground state,14 but the excited singlet15 and the
A likely mechanism for the formation of 31 is illustrated in
Scheme 3. Cyclopropene 22 is a known intermediate, which has
Received: May 6, 2015
© XXXX American Chemical Society
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Scheme 2. Rearrangements of the Naphthylcarbenes 5 and 9 to 10, 15, 16, and 17
Scheme 3. Phenanthrylcarbene Rearrangements
excited triplet states are also known.16 Generated by FVP of
diazo compound 32, it yields two products, 34 and 38.4 The
analogous rearrangement of 40 to 31 was observed upon FVP
of diazodibenzocycloheptene 39 (Scheme 4).13 Both reactions
require a ring interchange via 35 or 41. Early SCF calculations
indicated that the tetracyclic cyclopropene intermediate 35 lies
only ∼23 kcal/mol above 334 (i.e., these reactions are perfectly
feasible under FVP conditions).
The involvement of higher arylcarbenes and rearrangements of
arylcarbenes in PAH formation seem very likely. However,
there is no knowledge of this in the literature, and the
mechanisms of higher PAH formation are poorly understood.18
C15H10 isomer cyclopenta[def ]phenanthrene 31 was identified
recently as a product of pyrolysis of catechol, which was used as
a model for pyrolysis of solid fuels.19 Other methylene-bridged
PAHs were also characterized.19 Indene figures prominently in
models of combustion and pyrolysis reactions and the
formation of PAHs,20 and 2-phenylindene has been detected
in the pyrolysis oil from pine sawdust intended for use as
The importance of phenylcarbene 1 and fulvenallene 4 in the
formation of polycyclic aromatic hydrocarbons (PAHs) in
combustion processes has received much attention recently.17
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Scheme 4. Dibenzo[a,d]cyclohepten-5-ylidene
Scheme 5. Synthesis and Pyrolysis of 2-Phenyl-5-
(phenylethynyl)tetrazole 44
Rearrangements;4,31 Cyclic Carbenes Isomeric with the
Allenes 36 and 42 are Omitted for the Sake of Simplicity
52 in yields of 30 and 60%, respectively (Scheme 6). Carbene
48 may cyclize to the cycloheptatetraene intermediate 49. A
1,5-H shift in 49 can yield the more stable isomer 24, which can
now rearrange to 28 via strained tetracyclic cyclopropene 27,
biodiesel21 and in the pyrolysis of 1-phenyltetralin investigated
as a model for lignins responsible for coal formation.22 Here,
we report new results on the pyrolytic formation and
rearrangements of the C15H10 phenanthrylcarbene isomers
and C15H12 phenylindene isomers.
Scheme 6. New C15H10 Intermediates
RESULTS AND DISCUSSION
■
1. 2-Phenyl-5-(phenylethynyl)tetrazole 44. 2-Phenyl-5-
(phenylethynyl)tetrazole 44 provides a new entry to the C15H10
energy surface. This compound was prepared by the
straightforward addition of bromine to styryltetrazole 43
followed by elimination of HBr with sodium (Scheme 5).
Because of the low volatility of 44, we applied the falling solid
flash vacuum pyrolysis (FS-FVP) method.23 Using this
technique, the solid tetrazole was pyrolyzed at 400−500 °C
at a dynamic pressure between 10−3 and 10−1 hPa.
As is common for 2,5-disubstituted tetrazoles,24 the pyrolysis
of 44 should result in the elimination of N2 with the formation
of nitrile imine 45a. Several mesomeric structures and
conformations of this compound can be formulated,24 including
the Z-conformer of the carbene canonical structure 45b, which
can now cyclize to indazole 46a in a 6-electron electro-
cyclization analogous to previously reported cyclizations of
nitrile imines.24,25 A 1,5-H shift affords 3H-indazole 46b
(Scheme 5). Further isomerization leads to indazole 47, but
1
this reaction may not be unimolecular. In this event, the H
NMR and mass spectra clearly indicated that 47 was present in
the products of pyrolysis at 400 °C, but at 500 °C, it had
disappeared. This is ascribed to tautomer 46b eliminating a
second molecule of N2, thereby forming carbene 48 and
entering the C15H10 energy surface.
The main products of the FS-FVP reaction at 500 °C were
cyclopenta[def ]phenanthrene 31 and cyclopenta[jk]fluorene
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which was discussed above in connection with Schemes 3 and
4. Further rearrangement of 28 to 31 is a straightforward
phenylcarbene-type1 rearrangement (Scheme 6).
Scheme 8. Unobserved 1,3-H Shift in 49
Cyclic allene 24 may also undergo a transannular cyclization
similar to that of cycloheptatetraene (2 → 3, Scheme 1)1 and
the naphthylcarbenes3−6 (Scheme 2); this will lead to
compound 50 (Scheme 6). Further ring opening to 51 and
cyclization of 51 to 52 are completely analogous to the
reactions taking place on the naphthylcarbene energy surface27
(compare Schemes 2 and 6). Thus, the products of the FS-FVP
of 44 are readily explained in terms of known types of
processes.1,3−6,27
It is instructive to consider some known, calculated activation
barriers for the types of rearrangements put forward here. The
ring expansion of singlet phenylcarbene 1 to cycloheptatetraene
2 (Scheme 1) via bicyclo[4.1.0]hepta-2,4,7-triene has a barrier
of 15−20 kcal/mol.1,26 The transannular cyclization to 3 is the
highest barrier for the ring contraction to fulvenallene 4, ∼46
kcal/mol at the B3LYP/6-31G* level.1 Both reactions take
place on FVP in our apparatus at 600 °C and above.1
The barrier for interconversion of the singlet naphthylcar-
benes 9 and 5 depicted in Scheme 2 is ∼24 kcal/mol at the
B3LYP/6-31G** level.6,27 For the cyclization to cyclobuta[de]-
naphthalene 10 it is almost the same, 25 kcal/mol,6,27 and both
processes take place very easily under FVP conditions.3,4
Nonaromatic cycloheptatetraene 12 (Scheme 2) lies only ∼15
kcal/mol higher than aromatic 7.6,27 The barriers for the
formation of 13−17 were evaluated recently in the context of
the azulenylcarbene rearrangements, and the highest barrier is
∼44 kcal/mol relative to the singlet state of the E-isomer of 1-
naphthylcarbene 9.27 The formation of 15−17 is known to take
place on FVP at 600 °C and above.4,5
NMR spectroscopy (the NMR spectra of 21 obtained by FVP
of 19 are shown in the Supporting Information for
comparison). Thus, the 1,5-H shift (Scheme 6) is energetically
preferred over the 1,3-H shift (Scheme 8) in agreement with
the Woodward−Hoffmann rules, even though the 1,3-shift in
49 should be possible as a pseudopericyclic reaction because of
the presence of orthogonal orbitals in the 1,2,3-triene moiety.
2. 2-Phenyl-5-styryltetrazole 43. 5-Styryltetrazole 43 was
also subjected to FS-FVP under the same conditions used for
44. In this case, indazole 58 (Scheme 9) was isolated in 69%
Scheme 9. 3-Styrylindazole 58 and 3- and 2-Phenylindenes
61 and 62 from FVP of 2-Phenyl-5-styryltetrazole 43
The reactions become a little more complicated when we
consider ring interchange reactions, such as 24 → 27 → 28
(Schemes 3 and 6). Early force field − SCF calculations
indicated that tetracyclic cyclopropene 35, formed from 5-
diazo-9,10-dihydrodibenzo[a,d]cycloheptene 32, lies ∼23 kcal/
mol above carbene 33 (Scheme 4).4 The tricyclic cyclopropene
intermediate 53 in the ring-interchange in the C11H8 system
(12, Scheme 7) was calculated to lie 36 kcal/mol above 12.6
Scheme 7. Ring Interchange in C11H8
Accordingly, the interconversions of 24 and 28 via 27 in
Scheme 3 and Scheme 6 appear to be perfectly reasonable
under the FVP conditions, where experience shows that
activation energies on the order of 50 kcal/mol are readily
achievable.
The ratios of products 31 and 52 in Scheme 6 (1:6 at 400 °C
and 1:2 at 500 °C) indicate that the transannular cyclization 24
→ 50 is faster than the ring interchange reaction 24 → 27 →
28.
It should be noted that one could also have expected a 1,3-H
shift (which can also be formulated as a 1,7-H shift) in
cycloheptatetraene 49, yielding 54 and then, in principle, 9-
phenanthrylcarbene 20 (Scheme 8). However, this process can
be excluded because the known product of 9-phenanthrylcar-
bene, cyclobuta[de]phenanthrene 21 (Scheme 3), could not be
detected as a product of the FS-FVP of 44 by either GC or
yield and fully characterized when using a pyrolysis temperature
of 360 °C. Further FS-FVP of 43 at 400−800 °C afforded
increasing amounts of 3- and 2-phenylindenes 61 and 62 and
decreasing amounts of 58. At 800 °C, a nearly 1:1 mixture of 3-
and 2-phenylindenes 61 and 62 was obtained in 89% yield, and
only ∼7% of 58 remained (Scheme 9). Unlike ethynyl analogue
48 in Scheme 6, carbene intermediate 59 can cyclize to 1-
phenylindene 60, which then isomerizes to the more stable,
conjugated 3- and 2-phenylindenes 61 and 62 by means of
sequential 1,5-H and 1,5-Ph shifts (Scheme 9).
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filtered and dried in vacuo at 10−2 hPa to yield 1.0 g (87%) of 2-
It is known that 3- and 2-phenylindenes interconvert
thermally; von Braun and Manz observed the isomerization
of 3-phenylindene to 2-phenylindene (but not the reverse)
upon passing the vapors in a stream of CO2 over pumice in a
dark-red glowing tube.28 Koelsch and Johnson confirmed the
phenyl migration from the 1- to the 2-position in the pyrolysis
of some polysubstituted phenylindenes in a steam of N2 at
450−490 °C.29 Miller and Boyer determined the activation
parameters for the hydrogen shifts in 1-phenylindene through
two sequential 1,5-H shifts to yield 3-phenylindene in a
diphenyl ether solution at 140−160 °C as ΔH‡ = 33 kcal mol−1
and ΔS‡ = −2.3 cal K−1 mol−1.30 In substituted 1-phenyl-
indenes (e.g., 1,1-diphenylindene and 1-methyl-1-phenyl-
indene), they concurred with the previous authors28,29 that
phenyl migration takes place specifically to the 2-position and
reported an activation enthalpy of ∼28 kcal mol−1 and a
strongly negative activation entropy ΔS‡ = −25 to −27 cal K−1
mol−1 for the phenyl migration in solution at an average
temperature of 265 °C.30 This translates to a free energy of
activation for phenyl migration of ∼42 kcal mol−1. These values
are very similar to those calculated for the corresponding
interconversion of 3- and 2-cyanoindenes (33 kcal mol−1 for
1,5-H migration; 46 kcal mol−1 for 1,5-CN migration starting
from 1-cyanoindene)31 and are in line with the observation that
high temperatures are required for equilibration of the 2- and 3-
isomers. Brown, Eastwood, and Jackman obtained a mixture of
2- and 3-phenylindenes in a ratio of 12:7 upon FVP of
phenyl(o-tolyl)acetylene at 790 °C. Similar FVP of 3-phenyl-
indene at 710 °C also afforded a mixture of 2- and 3-
phenylindenes in a ratio of 12:7, and 2-phenylindene at 700 °C
afforded a mixture of 2- and 3-phenylindenes in a ratio of
17:8.32 As mentioned above, we obtained a nearly 1:1 ratio at
800 °C.
phenyl-5-(2-phenyl-1,2-dibromoethyl)tetrazole as light rosa colored
1
crystals; mp 173−175 °C. H NMR (CDCl3): δ 8.19−8.16 (m, 2H),
7.60−7.38 (m, 8H), 5.95 (d, J = 11.8 Hz, 1H), 5.87 (d, J = 11.8 Hz,
1H).
13C NMR (DMSO-d6): 165.9, 138.8, 135.7, 130.6, 130.2, 129.1,
128.6, 128.3, 119.9, 53.3, 41.3. IR (KBr/cm−1): 3070 w, 2995 m, 1600
m, 1500 s, 1485 s, 1470 s, 1455 s, 1210 s, 1180 s, 1140 s, 1000 s, 790 s,
680 s. MS (FD) m/z: 301 ([M − 79Br − H]+, 25%), 299 ([M − 81Br −
H]+, 28), 221 (10), 220 (70), 219 (15), 115 (5), 91 (100). Anal. Calcd
for C15H12N4Br2: C, 44.11; H, 2.96; N, 13.76%. Found: C, 44.04; H,
2.78; N, 13.48%.
2-Phenyl-5-(phenylethynyl)tetrazole 44. A solution of 2-phenyl-5-
(2-phenyl-1,2-dibromoethyl)tetrazole (800 mg; 2 mmol) in 10 mL of
dry tert-butanol was added slowly to a stirred, boiling solution of 200
mg (8.6 mmol) of Na in 30 mL of dry tert-butanol. The resulting
mixture was refluxed for another 30 min, cooled to RT, and 10 mL of
water was added slowly with stirring. This mixture was evaporated to
dryness in vacuo; the residue was taken up in diethyl ether, and the
solution was dried over MgSO4. Filtering and removal of the ether in
vacuo afforded 350 mg (73%) of white crystals; mp 131−133 °C after
1
recrystallization from petroleum ether. H NMR (CDCl3): δ 8.15−
8.12 (m, 2H), 7.66−7.36 (m, 8H). 13C NMR (CDCl3): δ 151.1 (C),
136.4 (C), 132.1 (CH), 12.9 (CH), 129.8 (CH), 129.6 (CH), 128.4
(CH), 120.7 (C), 119.7 (CH), 94.5 (C), 75.9 (C). IR (KBr/cm−1):
3050 w, 2240 s, 1600 s, 1515 s, 1495 m, 1210 s, 1100 s, 1000 s, 760 s,
705 s, 690 s, 680 s. MS (FD) m/z: 246 (M+.). Anal. Calcd for
C15H10N4: C, 73.16; H, 4.10; N, 22.75%. Found: C, 73.16; H, 3.91; N,
22.70%.
Pyrolysis of 2-Phenyl-5-(phenylethynyl)tetrazole 44. (a) A
sample of 250 mg of the solid, powdered tetrazole was subjected to
FS-FVP at 500 °C at a pressure varying between 10−3 and 10−1 hPa in
the course of 60 min. The resulting products were examined by GC
(SE52, 200 °C isothermally) and 1H NMR spectroscopy and identified
by comparison with the compounds isolated previously.4,34 The
following products were obtained: 9H-cyclopenta[def ]phenanthrene
31, 30%; 2H-cyclopenta[jk]fluorene 52, 60%. Data for 9H-cyclopenta-
[def ]phenanthrene 31 as follows. 1H NMR (CDCl3): δ 7.88−7.66 (m,
8H), 4.36 (s, 2H). 13C NMR (CDCl3): δ 141.8, 138.4, 127.9, 127.2,
125.3, 122.5, 121.2, 37.4 (t, JH = 131 Hz).
CONCLUSION
■
Falling solid flash vacuum pyrolysis (FS-FVP) of 44 proceeds
via N-phenyl-C-(phenylethynyl)nitrile imine/(phenylazo)-
(phenylethynyl)carbene 45 and 3-(phenylethynyl)-3H-indazole
46b to generate carbene 48 as the first C14H10 intermediate.
Two series of pericyclic reactions yields cyclopenta[def ]-
phenanthrene 31 and cyclopenta[jk]fluorene 52 as final
products. An analogous but much simpler reaction is the
formation of 3-styrylindazole 58, 3-phenylindene 61, and 2-
phenylindene 62 from 2-phenyl-5-styryltetrazole 43.
The data are in agreement with the literature,4,5 and the identity of
the compound was confirmed by coinjection of an authentic sample on
SE30 and SE52 GC columns.
Data for 2H-cyclopenta[jk]fluorene 52 as follows. 1H NMR
(CDCl3): δ 7.72−7.34 (m, 7H), 6.77 (t, J = 1.4 Hz, 1H), 4.07 (d, J
= 1.4 Hz, 2H). 13C NMR (CDCl3): δ 147.4, 142.8, 135.3, 134.9, 130.3,
127.45, 127.4, 126.5, 126.3, 125.8, 125.4, 124.8, 122.5, 118.3, 46.0 (dt,
JH = 9 and 130 Hz). MS (EI) m/z: 190 (M+., 100%), 189 (70), 188
(11), 187 (17), 163 (10), 95 (10), 94 (10). HRMS (EI) m/z:
190.0767; calcd for C15H10: 190.0782. The data are in agreement with
the literature.34
All of the rearrangement mechanisms depicted in Schemes 3,
4, 6, 7, and 9 are estimated to have activation barriers <50 kcal/
mol and therefore to be perfectly accessible under flash vacuum
pyrolysis conditions in the 500−800 °C temperature range.
(b) A sample of 100 mg of the tetrazole was pyrolyzed at 400 °C.
1H and mass spectra of the product indicated the presence of a mixture
of 3-(phenylethynyl)indazole 47, cyclopenta[def ]phenanthrene 31,
and cyclopenta[jk]fluorene 52 at a ratio of 1:1:6, which was separated
by flash chromatography on silica gel.
EXPERIMENTAL SECTION
■
Data for 3-(phenylethynyl)indazole 47 as follows. 1H NMR
(CDCl3): δ 10.5 (broad s, NH), 7.1−7.6 (m). MS (EI) m/z: 218
(M+., 100%), 190 (11), 189 (35), 109 (6), 89 (6), 69 (5). HRMS (EI)
m/z: 218.0840; calcd for C15H10N2: 218.08439. Anal. Calcd for
C15H10N2: C, 82.55; H, 4.62; N, 12.84%. Found: C, 82.61; H, 4.59; N,
12.76%.
The apparatus and procedure for falling solid flash vacuum pyrolysis
(FS-FVP) have been described.23 All HRMS measurements were
carried out using a conventional double-focusing sector mass
spectrometer of Mattauch−Herzog geometry. Electron ionization at
70 eV (EI) and field desorption (FD) is indicated where appropriate.
Synthesis of 2-Phenyl-5-styryltetrazole 43. This compound
was prepared according to the procedure of Ito et al.;33 mp 88−90 °C
[lit.33 90 °C].
Pyrolysis of 2-Phenyl-5-styryltetrazole 43. Samples of 100−
409 mg (0.27−1.65 mmol) of 43 were pyrolyzed in the range of 360−
800 °C/10−3 hPa.
(a) Pyrolysis of 409 mg (1.65 mmol) of tetrazole 43 at 360 °C
afforded almost pure 3-styrylindazole 58. Recrystallization from CHCl3
yielded 249 mg (69%) of 3-styrylindazole 58 as white needles; mp
174−175 °C. 1H NMR (CDCl3): δ 10.8 (broad s, NH), 8.02 (d, J = 17
Hz, 1H), 7.65 (d, J = 17 Hz, 1H), 7.1−7.6 (m, 9 H). 13C NMR
Synthesis of 2-Phenyl-5-(phenylethynyl)tetrazole 44. 2-
Phenyl-5-(2-phenyl)-1,2-dibromoethyl)tetrazole. Bromine (4.5 g;
28 mmol) in 10 mL of glacial acetic acid was added slowly to a
stirred solution of 43 (700 mg; 2.82 mmol) in 50 mL of glacial acetic
acid containing a trace catalytic amount of LiBr. The resulting mixture
was allowed to stand for 6 h at RT, and the precipitated product was
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(CDCl3; coupling pattern determined by off-resonance decoupling): δ
141.9 (C), 141.0 (C), 136.9 (C), 129.0 (CH), 128.4 (CH), 127.3
(CH), 126.1 (CH), 126.0 (CH), 120.55 (CH), 120.5 (CH), 120.45
(C), 120.4 (CH), 110.2 (CH). MS m/z: 220 (M+., 48%), 219 (100),
109 (8), 108 (9), 77 (6). IR (KBr): 3260 (broad), 1620 s, 1600 m,
1500 s, 1480 m, 1460 m, 1350 s, 1280 s, 1240 s, 1060 s, 960 s, 770 s,
740 s, 690 s cm−1. Anal. Calcd for C15H12N2: C, 81.79; H, 5.49; N,
12.72%. Found: C, 81.86; H, 5.39; N, 12.66%.
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(21) Huang, Y.; Wei, L.; Julson, J.; Gao, Y.; Zhao, Z. J. Anal. Appl.
Pyrolysis 2015, 111, 148.
NMR spectra with those of authentic materials.
ASSOCIATED CONTENT
* Supporting Information
■
S
NMR spectra of pyrolysis products 21, 31, 52, 58, 61, and 62.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b01007.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wentrup@uq.edu.au.
■
(22) Wilshire, J. F. K. Aust. J. Chem. 1962, 15, 538.
(23) (a) Wentrup, C.; Becker, J.; Winter, H.-W. Angew. Chem., Int.
Ed. 2015, 54, 5702. (b) Wentrup, C. Aust. J. Chem. 2014, 67, 1150.
Notes
The authors declare no competing financial interest.
(24) (a) Beg
134, 5339. (b) Beg
(25) Wentrup, C.; Damerius, A.; Reichen, W. J. Org. Chem. 1978, 43,
2037.
(26) (a) 16.5 kcal/mol at the CASPT2(8,8)/6-31G*//
CASSCF(8,8)/6−32G* level: Karney, W. L.; Borden, W. T. Adv.
Carbene Chem. 2001, 3, 205−250. (b) 14.8 kcal/mol at the B3LYP/6-
311+G** level: Geise, C. M.; Hadad, C. M. J. Org. Chem. 2002, 67,
2532. (c) 20.7 kcal/mol at the B3LYP/6-31G* + ZPVE level, see ref
1.
́
ue,
́
D.; Qiao, G. G.; Wentrup, C. J. Am. Chem. Soc. 2012,
ACKNOWLEDGMENTS
■
́
ue, D.; Wentrup, C. J. Org. Chem. 2014, 79, 1418.
́
This work was supported by the Deutsche Forschungsgemein-
schaft, the Fonds der Chemischen Industrie, and The
University of Queensland.
REFERENCES
■
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DOI: 10.1021/acs.joc.5b01007
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