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The Benzyne Story

Abstract and Figures

The history of o-benzyne from its early beginnings as an unobservable reactive intermediate until its present status as a very well characterized but still theoretically challenging molecule with important applications in synthesis is reviewed. The m- and p-benzynes, tridehydrobenzenes, and benzdiynes are also known, and p-benzyne is a key intermediate in the action of a potent class of ene-diyne anti-tumour compounds.
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RESEARCH FRONT
CSIRO PUBLISHING Essay
www.publish.csiro.au/journals/ajc Aust. J. Chem. 2010,63, 979–986
The Benzyne Story
Curt Wentrup
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane,
Qld 4072, Australia. Email: wentrup@uq.edu.au
The history of o-benzyne from its early beginnings as an unobservable reactive intermediate until its present status as a
very well characterized but still theoretically challenging molecule with important applications in synthesis is reviewed.
The m- and p-benzynes, tridehydrobenzenes, and benzdiynes are also known, and p-benzyne is a key intermediate in the
action of a potent class of ene-diyne anti-tumour compounds.
Manuscript received: 2 May 2010.
Manuscript accepted: 3 June 2010.
Benzyne (C6H4) was first proposed as a reactive intermedi-
ate by Bachmann and Clarke at the Eastman Kodak Co., who
investigated the reaction of sodium with boiling chlorobenzene
(a Wurtz–Fittig reaction), which produces benzene, biphenyl,
a trace of p-diphenylbenzene, and significant amounts of
o-diphenylbenzene, triphenylene, and o,o-diphenylbiphenyl.[1]
While the formation of the first three products could be explained
by means of ionic reactions of the types
C6H5Cl +2Na=C6H5Na +NaCl
C6H5Na +C6H5Cl =C6H5–C6H5+NaCl
C6H5Na +C6H5–C6H5=C6H6+C6H5–C6H4Na
C6H5–C6H4Na +C6H5Cl =C6H5–C6H4–C6H5+NaCl
the formation of the last three, in particular triphenylene, was
taken as ‘decisive in favour of the free radical explanation’:
2C
6H5=C6H4<+C6H6
3C
6H4<triphenylene
C6H4<+2C
6H5o-diphenylbenzene
+o,o-diphenylbiphenyl,
whereby C6H5– is the phenyl radical, and C6H4<was named
free phenylene. It was clear from the presentation that the free
phenylene was meant to be the biradical 1(Scheme 1).
Compelling experimental evidence was provided by Georg
Wittig, then at the University of Freiburg and soon afterwards
at Tübingen, in 1942.[2] Wittig and his students discovered
that the formation of biphenyl by reaction of the haloben-
zenes with phenyllithium was the fastest with fluorobenzene,
3
1 2
Scheme 1.
which was contrary to the expected difficulty of displacing
fluoride directly in a nucleophilic substitution reaction. A di-
polar form 2of benzyne was proposed as an intermediate in
the formation of biphenyl (Scheme 2a) because, in contrast
to the corresponding olefinic zwitterions, which would give
rise to open-chain acetylenes, ‘the zwitterion [2] would not
be able to form dehydrobenzene’, dehydrobenzene being the
triple-bonded molecule 3. The trimerization of this zwitterion to
triphenylene ‘under certain conditions’ was noted. In the same
year, Morton et al. at Massachusetts Institute of Technology
(MIT) had also proposed the ‘dipolar phenylene’ intermediate
[2], which ‘cannot be stabilized by double bond formation’ in
their investigation of the Wurtz reaction of chlorobenzene with
pentylsodium (Scheme 2b).[3] Schubert invoked o-phenylene to
explain the formation of triphenylene in the reactions of diphenyl
ethers with phenylpotassium in 1950 (Scheme 2c).[4]
In 1953, John D. Roberts and coworkers, then at MIT and soon
afterwards at CalTech, reported a classic 14C labelling exper-
iment, which necessitated the involvement of a symmetrical,
electrically neutral intermediate – formulated as benzyne 3–in
the reaction of chlorobenzene with potassium amide in liquid
ammonia (Scheme 3).[5] These authors also estimated that the
bent acetylene structure would lead to a strain energy comparable
to that of cyclopropene.[5b] In fact, the current values for these
X
H
X
M
2
R
M
R
RH
MX
RM
H2OMOH
RM
(a) X F, RM PhLi
(b) X Cl, RM C5H11Na
(c) X OPh, RM PhK
3
Scheme 2.
© CSIRO 2010 10.1071/CH10179 0004-9425/10/070979
RESEARCH FRONT
980 C. Wentrup
Cl
14C
3
KNH2/NH3NH3
NH2
NH2
1:1
Scheme 3.
FF
Li
LiF
PhLi
OMe OMe
OMe
Li
F
PhLi
OMe OMe
F
COOH
OMe
Ph
Ph
OMe
COOH
LiF
1. PhLi
2. CO2
3. H2O
1. PhLi
2. CO2
3. H2O4
5
Scheme 4.
F
Br or Mg
Li(Hg) O
O
H
OH
6
Scheme 5.
strain energies are 54 (cyclopropene) and 50kcalmol1(ben-
zyne) (1 cal =4.184 J).[6,7] The enthalpy of formation of benzyne
is 107 kcal mol1.[8]
Huisgen and Rist (University of Munich) provided further
evidence for the benzyne structure (3-methoxybenzyne 4)by
demonstrating that identical mixtures of carboxylic acids were
obtained on treatment of ortho- and meta-fluoroanisole with
phenyllithium followed by carboxylation with CO2(Scheme 4).
The existence of 1,2-naphthyne 5was demonstrated analogously
(Scheme 4).[9] Nevertheless, Huisgen expressed uncertainty as
to whether the symmetrical intermediates implied by the results
were really free arynes, and, at any rate, ‘an isolation is out of
the question’.[9a]
Further advances were made when it was discovered
that benzyne could be generated from o-dihalobenzenes (e.g.
o-bromofluorobenzene) with lithium amalgam or by forming the
Grignard reagent with magnesium.[10–12] The first Diels–Alder
reaction of benzyne, with furan, was performed in this manner,
giving a 76% yield of the adduct 6, which was converted to
1-naphthol with acid (Scheme 5).[10] In the absence of the furan
trapping agent, biphenylene and triphenylene were obtained in
yields of 24 and 3%, respectively.
The concept of mesomerism (resonance) had been developed
by Slater, Ingold, Hückel, Pauling, and Wheland in 1931–33[13]
and was slowly being adopted by organic chemists. Qualitative
valence bond theory was accepted fairly readily, but molecular
orbital (MO) theory only took hold in the 1950s. Huisgen and
Rist[9] formulated benzyne 3with a (weakened) in-plane πbond.
Lüttringhaus and Schubert were the first to explicitly formulate
mesomerism between the three limiting valence structures of
benzyne, i.e. the zwitterion 2, the biradical o-phenylene 1, and
the ‘acetylene’ 3(Eqn 1).[4b] Wittig used the two equivalent zwit-
terionic structures 2in mesomerism with the ‘cyclohexadienyne’
3(Eqn 2) at this time (1955)[10] and continued to do so for many
years (the term cyclohexadienyne is incorrect, as it implies that
benzyne lacks the resonance stabilization of benzene). Eugen
Müller[14] considered resonance between the singlet biradical
and the zwitterion, 12, and Heaney, Mann, and Millar[11]
formulated a resonance between the zwitterion and the aryne,
23.
3
12
(1)
3
22
(2)
In an early application of MO theory, Coulson[15] estimated
the structure of o-benzyne with a pronounced and localized but
rather too short C1C2triple bond (122 pm) and a too long C2C3
bond (144 pm) (the modern experimental values are 125.5 and
138.3 pm, respectively; see below). Clearly, in 1958 MO the-
ory was not yet capable of providing a satisfactory description
of benzyne. At the same time, Roberts admitted to not know-
ing the state of the two electrons formally forming the triple
bond.[16] The first detailed MO study of benzyne by Hoffmann,
Imamura, and Hehre was published in 1968.[17] An account by
Radom et al. in 1986 stressed that very little was still known about
the level of theory needed for a proper description of benzynes
but concluded from two-configurational calculations that while
the triple-bonded valence structure 3dominates, o-benzyne pos-
sesses non-negligible biradical character,i.e. 31; accordingly,
single-configuration descriptions are inadequate.[18] This theme
is developed thoroughly in this issue’s Research Front.[8,19,20]
An important experimental advance was made in 1960, when
Stiles and Miller discovered that o-benzenediazonium carboxy-
late 7is an excellent precursor of benzyne in organic solvents
at 40–60C. This precursor, easily obtained by diazotization of
anthranilic acid, allowed benzyne chemistry to move out of the
organometallic domain for the first time (Scheme 6).[21] A 30%
yield of biphenylene can be obtained by thermal decomposition
of 7in boiling 1,2-dichloroethane (84C).[22] The first UV
spectrum of gaseous benzyne (λmax =242 ±3 nm) was obtained
by photolysis of 7.[23] Fur ther details of the UV spectrum of ben-
zyne are reviewed herein.[24] A mass spectrum of benzyne (or
most likely a ring-opened[25] C6H+
4ion) was obtained in the
same manner.[26] Compound 7[22] continues to be an important
preparative source of benzyne.[27]
Diphenyliodonium carboxylate 8requires a higher tempera-
ture, 160C in diglyme, to form benzyne, iodobenzene, and CO2.
RESEARCH FRONT
The Benzyne Story 981
COO
N2
7
Δ
COO
I Ph
8
or hn
Δ
ΔAnthracene
Scheme 6.
The benzyne was trapped with anthracene, giving triptycene
(Scheme 6). Flash vacuum pyrolysis (FVP) of 8at 325C yielded
biphenylene and iodobenzene.[28] Much later, the three-phase
test was used to demonstrate the occurrence of free o-benzyne
in the thermolysis of 8in solution.[29]
As more and more solid evidence for the existence of ben-
zyne accumulated, further efforts were made to demonstrate the
existence of the free molecule in the gas phase. Schüler and
coworkers generated benzyne from the monohalobenzenes and
even from benzene and phenylacetylene in an electrical glow
discharge, the evidence being the observation of biphenylene
by means of its gas-phase UV spectrum. They assumed that
benzyne existed in a biradical state (1) in the plasma.[30] This
method of benzyne generation languished for many years, but a
modern variation has risen to prominence recently: high quality
microwave spectra of benzyne have been produced by pulsed
electrical discharges (0.8–1.4 kV) in benzene vapour seeded in
neon, which had been cooled to a few degrees K in a free jet
expansion.[31]
Wittig and Ebel found that iodophenylmercuric iodide 9and
bis(2-iodophenyl)mercury 10 yielded biphenylene on gas phase
pyrolysis.[32] Ebel and Hoffmann[33] reported detailed experi-
ments on the gas-phase flow pyrolysis of 10 (Ar carrier gas,
60 hPa, 700C; residence time in the hot zone 8.5 ×103s),
where the benzyne so produced was trapped downstream with
furan vapour. Although the furan adduct 6was isolated in only
0.1% yield, these experiments demonstrated the existence of free
benzyne with a lifetime of at least 20 ms under the reaction condi-
tions. FVP of compound 10 was used later by Grützmacher[34] to
obtain the mass spectrum and an approximate ionization poten-
tial of benzyne (or most likely a ring-opened C6H+
4ion[25]),
whereas Fisher and Lossing[35] used FVP of o-diiodobenzene
for the same purpose. Günther demonstrated that FVP of
o-diiodobenzene over Zn at 500–550C(2hPaN
2carrier gas at a
rate of 120 cm s1) afforded a 21% yield of biphenylene and 10%
of triphenylene.[36] The relatively high yields of triphenylene in
some of these reactions indicate that it is not (only) formed by
free benzyne trimerizing in the gas phase. With H2as carrier gas
a 3–5% yield of benzene and 7% of biphenylene were obtained.
Co-pyrolysis with anthracene afforded the Diels–Alder trapping
product triptycene in 8% and biphenylene in 10% yields.[36]
Monitoring of the dimerization of gaseous benzyne produced by
flash photolysis of solid benzene diazonium carboxylate 5in an
evacuated tube by both UV and mass spectrometry permitted the
determination of a preliminary rate of dimerization to bipheny-
lene as equal to or greater than 8 ±108l mol1s1.[37] This value
was considered to be within a factor of 3 of the true bimolecular
rate constant, which is just a little short of a rate measured a few
years later by flash photolysis of phthalic anhydride vapour in
Ne and Ar, 4.6 ±1.2 ×109l mol1s1.[38]
The photolysis of either 9,10, or phthaloyl peroxide 11
in the presence of tetraphenylcyclopentadienone (tetracyclone)
Hg
I
I
10
I
HgI
9

Gas
phase O
Ph
PhPh
Ph
Ph
Ph
Ph
Ph
CO
or hnor hn
O
O
O
O
hn
11
Scheme 7.
S
O2
N
N
12
COOAg
ClR R
R
CO2
Cl
R
m 61%, p 39%
13
Δ
Δ
13
Scheme 8.
yielded 1,2,3,4-tetraphenylnaphthalene, formed via Diels–Alder
addition of benzyne followed by cheletropic extrusion of CO
(Scheme 7).[32] The fact that the reactions took place photo-
chemically caused the authors to propose a biradical state
(1; singlet or triplet) of dehydrobenzene.[32] Benzyne is now rec-
ognized to be a ground state singlet molecule, and the currently
accepted singlet-triplet splitting is 37.5 kcal mol1.[8]
Wittig and Hoffmann introduced yet another, convenient
albeit explosive, benzyne precursor, 1,2,3-benzothiadiazole S,S-
dioxide 12, which decomposes to benzyne at 10–20Cin
solution. Benzyne was trapped in Diels–Alder cycloaddition
reactions (Scheme 8).[39] When 12 was allowed to decompose
in substance in an evacuated flask at 60C, biphenylene was iso-
lated in 38–52% yield together with 2% triphenylene. Köbrich
provided evidence for a symmetrical benzyne intermediate in
the pyrolysis of the silver salts of o-chlorobenzoic acids 13 in a
sealed tube (Scheme 8).[40]
Huisgen and Knorr built a solid foundation for mechanis-
tic benzyne chemistry by using competition experiments to
demonstrate that the same reactive intermediate – interpreted as
benzyne – was formed from three different precursors, namely
thermolysis of 7and 10 and treatment of o-bromofluorobenzene
with Li(Hg). The competition constant for the Diels–Alder
cycloadditions with furan and 1,3-cyclohexadiene was 21.5
for all precursors under a variety of conditions.[41]
RESEARCH FRONT
982 C. Wentrup
N
N
N
NH2
N
N
N
N
LTA
or NiO22 N2
Scheme 9.
The oxidation of 1-aminobenzotriazole with lead tetraacetate
or nickel peroxide constitutes an extremely mild method for gen-
erating benzyne by loss of two molecules of N2. In this way
benzyne chemistry can be carried out well below room temper-
ature (Scheme 9).[42] This continues to be an important method
in preparative benzyne chemistry.[27] It is interesting to note
that benzyne undergoes click chemistry; thus, the cycloaddition
of azides to benzynes is a convenient method of synthesis of
benzotriazoles.[27]
Three groups reported the formation of benzyne by pyrolysis
of carbonyl compounds in 1965–66: the gas-phase pyrolysis
of indanetrione (FVP over quartz tubings at 600C/0.2 hPa)
yielded up to 40% of biphenylene and 4% of triphenylene,[43]
the solution-phase pyrolysis of phthalic anhydride in benzene
afforded 1% biphenylene and 1% triphenylene,[44] and flow
pyrolysis of phthalic anhydride (N2carrier gas, 50 hPa, 800C)
yielded 10–15% biphenylene.[45] In the latter work, a 30% yield
of octachlorobiphenylene was isolated from a similar pyrolysis
of tetrachlorophthalic anhydride.[45] The formation of benzyne
by microwave flash pyrolysis of phthalic anhydride is reviewed
herein.[7] The mechanism of the deceptively simple forma-
tion of benzyne in the thermal reactions (Eqn 3) is still under
debate.[19,43c,46]
O
O
O
Δ or hn
CO2, CO (3)
The photolysis of phthalic anhydride in the gas phase and in
matrices also affords benzyne. The measurement of the rate of
dimerization of gaseous benzyne formed by microsecond flash
photolysis was mentioned above.[38] The reaction is reported to
take place via a hot, highly vibrationally excited, ground state of
phthalic anhydride when using ArF laser photolysis at 193 nm
on the nanosecond timescale.[47]
The first matrix-isolation experiments with IR-spectroscopic
observation of benzyne were reported in 1973 using phthaloyl
proxide and benzocyclobutenedione as precursors.[48]
o-Benzyne has now been characterized very thoroughly by
IR spectroscopy[49] in noble gas matrices, which is reviewed
herein,[19,25] as are the gas-phase microwave,[31,50] photoelec-
tron and UV spectroscopic,[24] and mass spectrometric[25] inves-
tigations. Also the 1,2-naphthyne 5and 2,3-naphthyne have been
characterized by IR and UV spectroscopy.[51] Moreover, the 13C
dipolar NMR spectrum of benzyne-1,2-13C2was obtained in
an Ar matrix at 20K and yielded the important acetylenic
C1C2distance as 124 ±2 pm,[52] which is in agreement with
the more accurate value obtained by microwave spectroscopy,
125.5 pm.[31,50] This bond distance lies between the values for
ethyne (120.3pm) and ethene (133.2pm), being closer to the
ethyne value, and the C2C3bonds (138.3pm) are a little shorter
than the CC bonds in benzene (139.1 pm).[31] The 1H and 13C
NMR spectra of benzyne in solution were obtained after gen-
erating the molecule inside a hemi-carcerand, a large molecular
container.[53] In this environment benzyne is stable at 100C
for several hours, sufficiently for the recording of its NMR spec-
tra. At higher temperatures it reacts irreversibly with the hemi-
carcerand host via Diels–Alder addition to benzene moieties.[53]
The combined evidence from microwave,[31,50] NMR,[53a] and
IR spectroscopy[48,49] as well as theoretical calculations[17,53b]
indicates that o-benzyne can be described as a resonance hybrid
of the acetylene 3and the cumulene 14, being closer to the
acetylenic structure (Eqn 4).
314
(4)
Stable transition metal complexes of benzyne have been pre-
pared and investigated by X-ray crystallography.They have much
more normal bond lengths than benzyne itself, e.g. C1C2is
ethene-like (133–136 pm), all other CC bonds are very much
benzene-like (138–140 pm) in the Zr and Ni complexes, and
they are best described as metallacyclopropenes (e.g. the Ni(0)
complex shown in Eqn 5).[54]
Ni
P
P
Cy2
Cy2
Ni
P
P
Cy2
Cy2
Cy cycloalkyl
(5)
The m- and p-benzynes 15 and 16,[8,19,55] tridehydrobenzenes
1719,[19] and tetradehydrobenzenes (benzdiynes), e.g. 20[20]
(Scheme 10) are described herein. Several resonance struc-
tures can be formulated for these species as illustrated for 20
as an example (Scheme 10). Therefore, for a proper quantum-
mechanical description, it is preferable to use multireference
methods.[8,19,20,56] Sato and Niino present experimental evi-
dence for the multireference character of benzdiynes.[20] Also
the highly reactive 1,4-benzdiynes can be stabilized by com-
plexation with transition metals, thus turning them into isolable
compounds.[54,57] X-ray crystallography of such complexes
indicate that the C1C2and C4C5bonds are ethene-like (131–
135 pm), and the C2C3bonds are benzene-like (139–140 pm).
The cyclic C6(hexadehydrobenzene), which according to cal-
culations has D3h symmetry (structure similar to that shown for
m-benzyne in Fig. 1), is also known,[20,58] as are linear C6and
other carbon chain clusters.[59]
The structure of m-benzyne has been the subject of much
debate, partly because there is experimental evidence for the
20
17 18 19
15 16
Where
Scheme 10.
RESEARCH FRONT
The Benzyne Story 983
Br
Br
LiNMe2
TMEDA
78°C
HNMe2
NMe2
Cl
LiNMe2
10°C
22
23
HNMe2
NMe2
NMe2
O
Ph
Ph
NMe2
O
Ph
Ph
21
Scheme 11.
140.3
140.2
137.8
137.7
137.2
137.5
205.8
210.4
Fig. 1. m-Benzyne. Calculated bond distances at the CCSD(T)/cc-pVDZ
(upright numbers) and CAS-RS2/cc-pVDZ (italics).[19]
existence of an isomer, bicyclo[3.1.0]hexatriene 22 as a reac-
tive intermediate formed by base treatment of the bicyclohexene
21 (Scheme 11).[60] Similarly, there is good experimental evi-
dence for the existence of butalene 23, an isomer of p-benzyne
(Scheme 11).[61] However, both these reactions are complex and
not fully understood. Similar experiments on a benzologue of 21
indicated that a m-naphthyne was formed rather than a benzo-
logue of 22.[62] Early Hückel MO calculations indicated, not
surprisingly, that 22, being a non-alternant ‘azulenoid’ 6πelec-
tron system, retains a major portion of the benzenoid resonance
energy.[63] In spite of its high strain energy, it is, therefore, a rea-
sonable intermediate. In contrast, butalene 23 is a cyclobutadiene
derivative with antiaromatic character and consequential high
energy. MINDO/3 calculations indicated the separate existence
of m- and p-benzynes 15 and 16 as well as the bicyclic structures
22 and 23.[64] General valence bond calculations predicted that
the lowest energy forms of the 1,3- and 1,4-dehydrobenzenes
are not bicyclic but monocyclic structures with considerable
biradical character. However, the m-isomer 15 collapses to the
bicyclic structure 22 at the single-determinant level.[65] Thus,
again, multireference theoretical methods are required in order
to handle these systems correctly. Recent calculations reveal a
1,3-bonding interaction resulting in an interradical distance of
200–210 pm for the singlet state of m-benzyne, depending on
the computational method used (Fig. 1).[19,66] Due to this bond-
ing interaction, m-benzyne has much less biradical character
and a larger singlet-triplet splitting than p-benzyne. The calcu-
lated IR spectra of m-benzyne and tetrafluoro-p-benzyne at the
CCSD(T) level are in very good agreement with the matrix-
isolation IR spectra.[19] To summarize, the species observed in
matrix isolation[19] as well as gas phase[8] experiments definitely
possess the monocyclic aryne structures 15 and 16, although
they are not regular hexagons.The bicyclic structures 22 and 23
do not correspond to the observed species. The natures of the
16
24
HN OCOOH
COOH
OOH
OH O OH 25 Dynemicin A
C
26 27
Scheme 12.
intermediates generated in the reactions shown in Scheme 11
remain to be clarified.
The Bergman-cyclization[67] of 3-hexene-1,5-diynes(e.g. 24)
to p-benzynes (e.g. 16) (Scheme 12) is biologically important
because several natural products with powerful anti-tumour,
and/or antibiotic properties such as the calicheamicins, esperam-
icins, lidamycin, and dynemicins (e.g. 25, Scheme 12) con-
tain such enediyne moieties, and their biological activity can
be ascribed to formation of transient p-benzynes. Hydrogen
abstraction by the p-benzyne generates a more reactive phenyl
radical, which abstract a second H atom from DNA, resulting
in DNA cleavage.[68,69] There is a related cyclization of enyne-
allenes 26 to α,3-didehydrotoluenes 27 or 1,4-didehydroindenes,
known as the Myers–Saito cyclization, which is involved in
the mechanism of action of the neocarcinostatin anti-cancer
agents. Examples of photochemically triggered Bergman and
Myers–Saito rearrangements are reviewed herein.[68d]
The possible interconversion of the o-,m-, and p-benzynes
has been evaluated theoretically.[56] Experiments using femto-
second gas-phase laser irradiation to generate the o-,m-, and
p-benzynes from the corresponding dibromobenzenes, together
with femtosecond mass spectrometric product observation, indi-
cate lifetimes of 400 ps for each benzyne isomer under the
reaction conditions used. The experiments also suggested that
the o- and m-isomers may rearrange to the p-isomer by H-atom
tunnelling; the p-isomer then undergoes the Bergman ring
opening.[56a]
Several reviews of benzyne chemistry have been published
previously.[6a,16,55,67b,70]
Outlook
In spite of extensive experimentation, many challenges remain
in benzyne-related chemistry.A better experimental and theoret-
ical understanding of m- and p-benzynes and their relationships
with the bicyclic isomers 22 and 23 is needed. It is an exper-
imental challenge but highly desirable to obtain microwave
spectra and hence exact geometrical parameters of m- and p-
benzynes. The benzynes have always been a testing ground
for computational methods (as have carbenes and nitrenes,
for example) and continue to be so;[66] their multireference
character puts serious demands on the theoretical methods.
The interplay between experiment and theory will undoubt-
edly continue, and systems such as α,n-didehydrotoluenes
27 and 1,n-didehydroindenes involved in Myers–Saito and
related rearrangements,[68] didehydrophenyl cations,[67b] and
RESEARCH FRONT
984 C. Wentrup
hetarynes[71] need further characterization by matrix-isolation
spectroscopy and computational methods. A deeper insight
into the mechanisms of rearrangement and interconversion
of arynes[25,43c,46,56a] will require exacting experimental and
computational investigations. Benzynes continue to be applied
widely in synthesis by using classical procedures as well as mod-
ern methods, such as the CsF-induced reactions of o-(trimethyl-
silyl)aryl triflates and o-(trimethylsilyl)aryliodonium triflates,
described in this issue.[72] New chemotherapeutic drugs based
on enediyne/p-benzyne chemistry are being developed, and
their modes of action are being investigated experimentally and
computationally.[73] There is fascinating potential for synthetic
applications of aryne and cycloalkyne transition metal com-
plexes either as isolable compounds or reactive intermediates
under stoichiometric,[54d,74] or catalytic conditions.[75]
References
[1] (a) W. E. Bachmann, H. T. Clarke, J. Am. Chem. Soc. 1927,49, 2089.
doi:10.1021/JA01407A038
(b) See also A.A. Morton, J. T. Massengale, G. M. Richardson, J.Am.
Chem. Soc. 1940,62, 126. doi:10.1021/JA01858A033
[2] G. Wittig, Naturwissenschaften 1942,30, 696. doi:10.1007/
BF01489519
[3] A. A. Morton, J. B. Davidson, B. L. Hakan, J. Am. Chem. Soc. 1942,
64, 2242. doi:10.1021/JA01262A003
[4] (a) K. Schubert, Dissertation: Über die Einwirkung von Kalium und
von Phenylkalium auf aromatische Äther 1950 (Universität Halle-
Wittenberg: Germany).
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... Die elektronische Struktur sowie die Eigenschaften dieses Moleküls sind Bestandteil der schon seitüber 100 Jahren anhaltenden Forschung, [142] was sich in der hohen Anzahl an Publikationen in den verschiedensten chemischen Disziplinen widerspiegelt. [143] Für Wissenschaftler aus der organischen Synthese [144]ü ber die Katalyse [145] bis hin zur Pharmazie [146] und Astrochemie [147] ist dieses Molekül von großem Interesse. In der Verbrennungsforschung als Teilbereich der physikalischen Chemie wird seine Rolle als reaktives Intermediat bei der Bildung von polyzyklischen aromatischen Kohlenwasserstoffen diskutiert und gibt zahlreiche Anstöße für Untersuchungen bei hohen Temperaturen. ...
... Es besitzt aufgrund seiner D 3h Symmetrie nur eine geringe Anzahl an Symmetrie erlaubten Banden, die ein charakteristisches IR-Spektrum zeigen. Auch die Bildung aus drei ortho-Benz-in XIV Einheiten erscheint logisch und konnte bereits sowohl in der flüssigen Phase bei ortho-Benz-in XIV Intermediaten [143] als auch bei der Selbstreaktion von Phenyl-Radikalen nach dem PAC-Mechanismus beobachtet werden. [ Reagiert Benzol mit XIV, wird Naphthalin (IR) und Ethin in einer exothermen Reaktion mit einer berechneten Barriere von lediglich +28 kJ / mol gebildet. ...
... Letzteres wird durch die Ringöffnung des Biphenylens zum Biphenyl-Biradikal und Addition mit XIV generiert. Beide Moleküle konnten zwar bereits sowohl in der Gas-als auch in der Flüssigphase nachgewiesen werden,[143,179,180] wurden hier aber erstmals in einem Pyrolysemikroreaktor bei sehr hohen Temperaturen und kurzen Verweilzeiten im Reaktor detektiert. ...
Thesis
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Die vorliegenden Arbeit behandelt VUV Valenz-Photoionisations-Experimente in der Gasphase. Zunächst wird die Photoionisation von stickstoffhaltigen Radikalen und deren Pyrolyseprodukten untersucht. Im Anschluss werden molekulare Biradikale betrachtet. Da in der Literatur bislang nur wenige solcher Biradikale als Intermediate experimentell zugänglich waren, war es das Ziel dieser Arbeit, neue reaktive Spezies dieser Substanzklassen in der Gasphase zu isolieren und deren Struktur, Eigenschaften und Reaktivität besser zu verstehen. Im Mittelpunkt stehen dabei Intermediate, die als echte Biradikale, Biradikaloide oder Triplett Carbene auftreten. Zu letzteren zählen das Methylbismut sowie die Pentadiinylidene. Biradikale bilden in Verbrennungsprozessen sehr effizient Ruß(vorläufer), was anhand des ortho-Benz-ins dargelegt wurde, indem dessen Pyrolyseprodukte charakterisiert und mögliche PAH-Bildungswege aufgezeigt wurden. Vakuum Flash Pyrolyse wurde verwendet, um in situ aus den geeigneten Vorläufermolekülen die radikalischen und biradikalischen Intermediate zu erzeugen. Während für biradikalische Zwischenstufen meist spezielle Verbindungen als Vorläufer synthetisiert werden müssen, waren die verwendeten Vorläufer für die stickstoffhaltigen Radikale kommerziell erhältlich. Die reaktiven Spezies wurden alle mittels monochromatischer VUV Synchrotronstrahlung an der Swiss Light Source in Villigen/ Schweiz ionisiert. Die Ionisationsereignisse wurden mit der Schwellenphotoelektronen-Photoionen-Koinzidenz (TPEPICO) Technik detektiert und ausgewertet. Anhand der resultierenden massenselektiven Schwellenphotoelektronenspektren wurden die Ionisierungsenergien der (Bi)radikale bestimmt und die Schwingungsstruktur der jeweiligen Kationen analysiert. Die erhaltenen Spektren und Daten wurden in Zusammenarbeit mit der theoretischen Chemie interpretiert. Wichtige Erkenntnisse • Es wurde die Ionisierungsenergie der 2-, 3- und 4-Picolylradikale auf 7.70\pm 0.02 eV, 7.59\pm 0.01 eV und 8.01\pm 0.01 eV bestimmt. Diese wurden in der Pyrolyse selektiv aus ihren zugehörigen Picolylaminen erzeugt. Zudem wurde analog zum Benzyl-Radikal für alle drei Radikale eine ausgeprägte Schwingungsprogression ermittelt, die der totalsymmetrischen Deformationsmode des aromatischen Rings entspricht. • Die Picolyl-Radikale dissoziieren in der Pyrolyse thermisch zu weiteren Produkten. Die Fragmentierung verläuft dabei isomerenunabhängig über ein stickstoffhaltiges Siebenringintermediat, dem Azepinyl-Radikal. Der Fragmentierungsmechanismus wurde mit dem von Benzyl verglichen. Die gewonnenen Erkenntnisse haben Relevanz für Verbrennungsprozesse, beispielsweise von Biokraftstoffen.Im ersten Schritt entstehen vier Isomere, das Cyclopenta-1,4-dien-1-carbonitril, das Cyclopenta-1,3-dien-1-carbonitril, das 2-Ethynyl-1H-pyrrol und das3-Ethynyl-1H-pyrrol mit den zugehörigen Ionisierungsenergien von 9.25\pm 0.02 eV, 9.14\pm 0.02 eV, 7.99\pm 0.02 eV und 8.12\pm 0.02 eV. Durch einen zweiten H-Verlust konnte das Cyanocyclopentadienyl-Radikal mit einer Ionisierungsenergie für die zwei niedrigsten Zustände im Kation mit 9.07\pm 0.02 eV (T0) und 9.21\pm 0.02 eV (S1) untersucht werden. Weitere Pyrolyseprodukte, deren Ionisierungsenergien bereits literaturbekannt sind und die bestätigt wurden, sind das Cyclopentadienyl-Radikal, das Cyclopenta-1,3-dien, das Propargyl-Radikal, das Penta-1,3-diin und das Cyanopropenyl. • Das ortho-Benz-in wurde pyrolytisch aus dem selbst synthetisierten Benzocyclobutendion erzeugt und ein Schwellenphotoelektronenspektrum frei von Störsignalen konnte aufgenommen werden. Mit Hilfe von Rechnungen aufCASPT2(11,14) Niveau, die neben dem elektronischen Übergang in den kationischen Grundzustand noch die Übergänge in zwei weitere angeregte kationische Zustände beinhalten, wurde die Ionisierungsenergie im Vergleich zu früheren Experimenten auf 9.51 eV revidiert. Eine verdrillte Geometrie für den kationischen Grundzustand konnte erstmals nachgewiesen werden. Zusätzlich wurden die offenkettigen Isomere cis- und trans-Hexa-1,5-diin-3-en im Spektrum detektiert und zugeordnet. • Die Auftrittsenergien aus der DPI des Vorläufermoleküls Benzocyclobutendion betragen für den ersten CO-Verlust 9.62\pm 0.05 eV und für den zweiten CO-Verlust 12.14\pm 0.10 eV. Damit konnte über einen thermochemischen Kreisprozess eine Bindungsdissoziationsenergie für die Ph-CO Bindung im Benzoylkation von 2.52 eV berechnet werden. • Verschiedenen Pyrolyseprodukte des ortho-Benz-ins, wie Ethin, Buta-1,3-diin, Benzol, Biphenylen und 2-Ethinylnaphthalin, werden entweder in bimolekularen Reaktionen gebildet oder ortho-Benz-in fragmentiert unimolekular zu diesen. Die beiden kompetitiven Reaktionspfade tragen zur PAH-Bildung des ortho-Benz-ins bei. • Die Triplett-Carbene Pentadiinyliden, Methylpentadiinyliden und Dimethylpentadiinyliden wurden als Pyrolyseprodukt aus ihren zugehörigen Diazovorläufern identifiziert und die Ionisierungsenergien mit 8.36\pm 0.03 eV, 7.77\pm 0.04 eV und 7.27\pm 0.06 eV bestimmt. Jede Methylierung stabilisiert folglich das Carben. Zusätzlich konnte ein weiteres C5H2 Isomer, das 3-(Didehydrovinyliden)cyclopropen, mit einer Ionisierungsenergie von 8.60\pm 0.03 eV charakterisiert werden. • Zwei bismuthaltige, reaktive Spezies, das Dimethylbismut-Radikal\cdot BiMe2 (IE = 7.27\pm 0.04 eV) und das Methylbismut-Carben :BiMe(IE = 7.88\pm 0.02 eV) wurden als Pyrolyseprodukte aus dem BiMe3 identifiziert. Beide Verbindungen zeigen eine ausgeprägte Schwingungsstruktur, die der Bi-C Streckschwingung zugeordnet wurde. Weiterhin wurden elementares Bismut Bi und das Bismut-Dimer Bi2 nachgewiesen. • Die homolytische Dissoziation der ersten Me2Bi-CH3 Bindung im BiMe3 wurde untersucht und eine BDE von 210\pm 7 kJ/ mol bestimmt. Sie liegt um +15 % bzw. +28 kJ/ mol über dem aus der Literatur abgeschätzten Wert.
... Diradicals Finally, we apply the CVS-MR-ADC methods to investigate the carbon K-edge XPS spectra of three benzyne diradicals (ortho-, meta-, and para-isomers) shown in Figure 9. Benzynes are highly reactive intermediates that are commonly formed in organic and combustion reactions [143][144][145][146][147] and can act as precursors in the formation of polycyclic aromatic hydrocarbons. [148][149][150][151][152] Due to their open-shell singlet character, the electronic structure and properties of benzynes have been studied using a variety of quantum chemical methods. ...
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... Diradicals Encouraged by the performance of CVS-MR-ADC for ozone, in this section we apply these methods to investigate the carbon K-edge XPS spectra of three benzyne diradicals (ortho-, meta-, and para-isomers) shown in Figure 6. Benzynes are highly reactive intermediates that are commonly formed in organic and combustion reactions [140][141][142][143][144] and can act as precursors in the for- mation of polycyclic aromatic hydrocarbons. [145][146][147][148][149] Due to their open-shell singlet character, the electronic structure and properties of benzynes have been studied using a variety of quantum chemical methods. ...
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We present a new theoretical approach for the simulations of X-ray photoelectron spectra of strongly correlated molecular systems that combines multireference algebraic diagrammatic construction theory (MR-ADC) [J. Chem. Phys., 2018, 149, 204113] with core-valence separation (CVS) technique. The resulting CVS-MR-ADC approach has a low computational cost while overcoming many challenges of the conventional multireference theories associated with the calculations of excitations from inner-shell and core molecular orbitals. Our results demonstrate that the CVS-MR-ADC methods are as accurate as single-reference ADC approximations for predicting core ionization energies of weakly-correlated molecules, but are more accurate and reliable for systems with multireference character, such as ozone and isomers of benzyne diradical. We also highlight the importance of multireference effects for the description of core-hole screening that determines the relative spacing and order of peaks in the XPS spectra of strongly correlated systems.
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Chapter
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