Trifluoromethylphenylcarbenes. Carbene-Carbene Interconversion on the Singlet Energy Surface and Rearrangement to Trifluorobenzocyclobutene, Trifluorostyrene, and Trifluoromethylfulvenallenes

Article abstract of DOI:10.1071/CH14097

The four isomeric α-, ortho-, meta-, and para-trifluoromethylphenylcarbenes were generated by photolysis of the corresponding 3-phenyl-3-trifluoromethyldiazirene 1 or the four isomeric trifluoromethylphenyldiazomethanes 2 and 4-6. The four corresponding triplet trifluoromethylphenylcarbenes 3 and 7-9 were observed by electron spin resonance (ESR) spectroscopy in Ar matrices at 14K. The α- and ortho-carbenes 3 and 7 and the ortho- and para-carbenes 7 and 9 interconvert partially when generated by short-wavelength photolysis (350nm) before intersystem crossing to the triplet states. The triplet states do not undergo further Carbene-Carbene interconversion. The interconversions are assumed to take place via the meta-trifluoromethylphenylcarbene 8. When the ortho- and para-carbenes are generated by long-wavelength photolysis (>450nm), the discrete, non-interconverting triplet carbenes are observed in the ESR spectra. Flash vacuum thermolysis of the diazirene 1 at 500°C afforded a mixture of bis(trifluoromethyl)heptafulvalenes 11, bis(trifluoromethyl)stilbenes 12, and bis(trifluoromethyl)anthracenes 13, and the presence of their likely precursor(s), trifluoromethylcycloheptatetraene(s), was confirmed by a peak at 1830cm^-1 in the Ar matrix IR spectrum. In addition, at 700°C, four monomeric carbene rearrangement products were isolated and characterised, viz. 1,1,2-trifluorobenzocyclobutene 14, 1′,2′,2′-trifluorostyrene 15, and 1- and 2-trifluoromethylfulvenallenes 16 and 17.

Full text of DOI:10.1071/CH14097

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2015, 68, 36–43
Full Paper
http://dx.doi.org/10.1071/CH14097
Trifluoromethylphenylcarbenes. Carbene–Carbene
Interconversion on the Singlet Energy Surface
and Rearrangement to Trifluorobenzocyclobutene,
Trifluorostyrene, and Trifluoromethylfulvenallenes
A
A B
,
Rodney J. Blanch and Curt Wentrup
A
School of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, Qld 4072, Australia.
B
Corresponding author. Email: wentrup@uq.edu.au
The four isomeric a-, ortho-, meta-, and para-trifluoromethylphenylcarbenes were generated by photolysis of the
corresponding 3-phenyl-3-trifluoromethyldiazirene 1 or the four isomeric trifluoromethylphenyldiazomethanes 2 and
4–6. The four corresponding triplet trifluoromethylphenylcarbenes 3 and 7–9 were observed by electron spin resonance
(ESR) spectroscopy in Ar matrices at 14 K. The a- and ortho-carbenes 3 and 7 and the ortho- and para-carbenes 7 and 9
interconvert partially when generated by short-wavelength photolysis (350 nm) before intersystem crossing to the triplet
states. The triplet states do not undergo further carbene–carbene interconversion. The interconversions are assumed to
take place via the meta-trifluoromethylphenylcarbene 8. When the ortho- and para-carbenes are generated by long-
wavelength photolysis (.450 nm), the discrete, non-interconverting triplet carbenes are observed in the ESR spectra.
Flash vacuum thermolysis of the diazirene 1 at 5008C afforded a mixture of bis(trifluoromethyl)heptafulvalenes 11,
bis(trifluoromethyl)stilbenes 12, and bis(trifluoromethyl)anthracenes 13, and the presence of their likely precursor(s),
trifluoromethylcycloheptatetraene(s), was confirmed by a peak at 1830 cmꢀ1 in the Ar matrix IR spectrum. In addition,
at 7008C, four monomeric carbene rearrangement products were isolated and characterised, viz. 1,1,2-trifluorobenzocy-
clobutene 14, 10,20,20-trifluorostyrene 15, and 1- and 2-trifluoromethylfulvenallenes 16 and 17.
Manuscript received: 25 February 2014.
Manuscript accepted: 9 April 2014.
Published online: 15 May 2014.
Introduction
The phenylcarbene rearrangement, which interconverts
phenylcarbene and cycloheptatetraene (Scheme 1) has been
described in several reviews.[10] The reaction usually takes
place in the gas-phase under the conditions of flash vacuum
thermolysis (FVT), and because it is reversible, it leads to
cycloperambulation of the carbene carbon with consequential
carbon scrambling in suitably labelled compounds. Thus, all
the tolylcarbenes afford benzocyclobutene and styrene as the
final products (Scheme 1).[11]
The recent resurgence of interest in organo-fluorine com-
pounds generally[1] and trifluoromethyl-substitution in parti-
cular,[2] prompts us to report our investigations of the isomeric
trifluoromethyl-phenylcarbenes.[3] The electronegativity of the
CF3 group and the consequential higher reactivity makes tri-
fluoromethylcarbenes interesting agents for photoactivity label-
ling.[4–6] p-Substituted 3-phenyl-3-(trifluoromethyl)diazirenes
were found to rearrange photochemically at 363–368nm to the
corresponding diazo compounds, which on further photolysis at
266 nm formed carbenes.[6] a-Trifluoromethylphenylcarbene
has been generated photolytically in a 4 K matrix, and its elec-
tron spin resonance (ESR) spectrum (D ¼ 0.5183 cmꢀ1, E ¼
Results
Matrix Photolysis
The IR spectrum of the Ar matrix following broad-band UV
photolysis of the 3-phenyl-3-(trifluoromethyl)diazirene
0.0313 cmꢀ1 [7] and IR spectrum[8] have been recorded. The
)
1
ESR spectrum suggests that the ground state is a triplet, and this
is supported by density functional theory (DFT) calculations,
which predict a singlet–triplet energy gap of 3–5 kcal molꢀ1
with the triplet as the ground state.[9] This carbene has also been
generated by supersonic jet flash pyrolysis of the diazirene, and
gas-phase UV absorption, time-of-flight (TOF)-mass spectra,
and time-resolved photoelectron spectra of it and/or its isom-
erized species were observed.[9] Nothing is known about the
possible interconversion of the a-, ortho-, meta-, and para-
trifluoromethylphenylcarbenes.
for 30 min at 10 K is in good agreement with the previously
reported experimental IR spectrum[8a] and with the calculated
IR spectrum[9c] of the triplet phenyl(trifluoromethyl)carbene
(a-trifluorotolylcarbene) 3 (Scheme 2). A cycloheptatetraene,
a fulvenallene, trifluorostyrene, or trifluorobenzocyclobutene –
products described below – were not observable under these
conditions. A small amount of the a-diazo compound 2 was
produced (2092 cmꢀ1) (Fig. 1).
The UV absorption maximum of 3-phenyl-3-(trifluoro-
methyl)diazirene isolated in Ar at 14 K is at 350 nm. Photolysis
Journal compilation Ó CSIRO 2015
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Trifluoromethylphenylcarbenes
37
CH
CH
CH
CH
CH
CH
CH
CH3
CH3
CH3
CH3
Scheme 1. Phenylcarbene and tolylcarbene rearrangements.
CHN2
N2
N
N
CHN2
CHN2
CF3
CF3
CF3
hν
1
2
4
CF3
CH
6
CF3
5
hν
hν
hν
hν
hν
CH
CH
CF3
CF3
CF3
CF3
3
7
8
9
Scheme 2. Formation and interconversion of the trifluoromethylphenylcarbenes.
Z1
3
X2
2
g ϭ 2
Z2
1
Y2
2000
1600
1200
800
ν [cmϪ1
]
0
10000
Fig. 1. IR difference spectrum resulting from broadband UV photolysis of
3-phenyl-3-diazomethyldiazirene 1 in an Ar matrix at 10 K for 30 min.
Negative peaks are attributable to the starting material, 1. Positive peaks are
attributable to the photolysis product, phenyl(trifluoromethyl)carbene 3
together with a small amount of diazo compound 2 (2092 cmꢀ1).
Magnetic field [G]
Fig. 2. Electron spin resonance spectrum resulting from photolysis of
diazirene 1 at 350 nm (Ar, 14 K). Major triplet species assigned as 3:
Z1 ¼ 2104, Z2 ¼ 9067, X2 ¼ 4929, Y2 ¼ 6249 G; D ¼ 0.523; E ¼ 0.032 cmꢀ1
.
Minor species (assigned as 7): Z1 ¼ X2 ¼ 5064, Y2 ¼ 6092 G; D ¼ 0.515;
E ¼ 0.0240 cmꢀ1
.
of this matrix at 350 nm in a cryostat for ESR spectroscopy at
14 K gave rise to the ESR spectrum shown in Fig. 2. The same
spectrum was obtained by broadband UV photolysis of 1, and
also by photolysis of the diazo compound 2 at 350 nm (Fig. 3);
this diazo compound has an absorption minimum at 350 nm.[4]
The ESR spectrum obtained is a composite of the signals of
two triplet species. The zero-field splitting (ZFS) parameters
of the major species (D ¼ 0.523; E ¼ 0.032 cmꢀ1) are in good
agreement with Wasserman’s data[7] and are assigned to
a-(trifluoromethyl)phenylcarbene 3. The signals of the minor
species are assigned to o-(trifluoromethyl)phenylcarbene 7
because the same signals (Fig. 3) were obtained upon Ar matrix
photolysis of o-(trifluoromethyl)phenyldiazomethane 4, itself
generated by mild thermolysis of the tosylhydrazone salt. Thus,
these results indicate partial conversion of the a- into the ortho-
trifluoromethylphenylcarbene (3 - 7). The reaction is incom-
plete and does not progress further on continued irradiation at
the same wavelength. The same composite ESR spectrum is
formed rapidly on broadband UV photolysis, but very slowly
using l . 455 nm. As is commonly observed in matrix ESR
spectroscopy, varying amounts of free radical products and H
atoms are seen in the g ¼ 2 region in all the ESR spectra.

38
R. J. Blanch and C. Wentrup
g ϭ 2
g ϭ 2
Z1
Z1
X2
X2
X2
Y2
Z2
Z2
Y2
Y2
0
10000
Magnetic field [G]
0
10000
Magnetic field [G]
Fig. 3. ESR spectrum resulting from photolysis of diazo compound 2 at
l . 455 nm (Ar, 10K). Microwave frequency 9.7450 GHz. Major species
(assigned as 3): Z1 ¼ 2106, Z2 ¼ 9084, X2 ¼ 4942, Y2 ¼ 6266 G; D ¼ 0.523;
Fig. 5. Electron spin resonance spectrum resulting from broadband
UV photolysis of para-trifluoromethylphenyldiazomethane 6 (Ar, 14 K).
Microwave frequency 9.7454 GHz. Major species assigned as ortho-
carbene 7: Z1 ¼ 2018, Z2 ¼ 8993, X2 ¼ 5086, Y2 ¼ 6074 G; D ¼ 0.515;
E ¼ 0.0240 cmꢀ1; minor species para-carbene 9: X2 ¼ 5116, Y2 ¼ 6041 G;
D ¼ 0.512; E ¼ 0.0225 G (inset: X2 and Y2 signals).
E ¼ 0.032 cmꢀ1
.
Minor species (assigned as 7): D ¼ 0.515;
E ¼ 0.0240 cmꢀ1
.
Z1
X2
g ϭ 2
X2
g ϭ 2
Z1
Y2
X2
Y2
X2
Z2
Z2
Y2
Y2
0
10000
Magnetic field [G]
0
10000
Magnetic field [G]
Fig. 4. Electron spin resonance spectrum resulting from broadband UV
photolysis of ortho-trifluoromethylphenyldiazomethane 4 (Ar, 14 K). Micro-
wave frequency 9.7455 GHz. Major species ortho-carbene 7: Z1 ¼ 2018,
Z2 ¼ 8986, X2 ¼ 5087, Y2 ¼ 6076 G; D ¼ 0.515; E ¼ 0.0240 cmꢀ1; minor
species assigned as para-carbene 9: X2 ¼ 5113, Y2 ¼ 6042 G; D ¼ 0.514;
E ¼ 0.0240 G (inset: X2 and Y2 signals).
Fig. 6. Electron spin resonance spectrum resulting from photolysis of
ortho-trifluoromethylphenyldiazomethane 4 at l . 455 nm (Ar, 14 K).
Microwave frequency 9.7440 GHz. Spectrum ascribed to ortho-carbene 7,
Z1 ¼ 2019,
Z2 ¼ 8989,
X2 ¼ 5087,
Y2 ¼ 6078
G;
D ¼ 0.515;
E ¼ 0.0240 cmꢀ1
.
Similar photolysis of Ar matrices of o-(trifluoromethyl)
phenyldiazomethane 4 and p-(trifluoromethyl)phenyldiazo-
methane 6 gave rise to virtually identical spectra, again com-
posed of two species (Fig. 4 and Fig. 5). The major signals are
For the para-isomer, the previously minor signals ascribed to the
p-carbene now became the major signals, but there was still a
splitting of the Y2 signal, indicating that a minor amount of the
ortho-carbene was still formed (Fig. 7). Importantly, additional
irradiation of these matrices with broad-band UV light did not
cause any further interconversion of the carbenes.
Matrix photolysis of the meta-isomer 5 afforded discrete
signals assigned to meta-trifluoromethylphenylcarbene 8 (D ¼
0.5235; E ¼ 0.0243 cmꢀ1, Fig. 8). In this case, the presence of
geometrical isomers (s-E and s-Z conformers) is possible. When
a very dilute Ar matrix of 5 was photolyzed, a splitting of the
Z1 line was observed (see inset in Fig. 8). Because the field
positions of the meta- and ortho-carbenes 8 and 7 are very
assigned to the ortho-carbene 7 (D ¼ 0.515; E ¼ 0.0240 cmꢀ1
)
in both cases, and the minor signals to the para-carbene 9
(D ¼ 0.513; E ¼ 0.0225 cmꢀ1). It could be thought that the
splitting of signals observed for 7 could be attributable to the
presence of geometrical isomers, but the following experiments
indicate that this is not the case. The a- and para-isomers cannot,
of course, form different geometrical isomers. When these
photolyses were carried out at lower energy, l . 455 nm, the
ortho-diazo compound afforded only one set of signals, the ones
ascribed to the o-carbene (probably the s-E-conformer) (Fig. 6).

Trifluoromethylphenylcarbenes
39
g ϭ 2
para-isomers 7 and 9 interconvert partially with each other
when generated at short wavelength, but not or only to a small
extent when generated at long wavelength. The triplet meta-
carbene 8 is probably also involved, but the field positions of the
X2 and Y2 transitions are too close to those of 7 to make a clear
distinction.
X2
Z1
X2
Y2
In order to shed more light on the isomerizations, FVT
experiments were performed and substantiated the rearrange-
ment of the a-trifluoromethylphenylcarbene 3 to the ortho-
isomer 7 with formation of trifluorobenzocyclobutene and
trifluorostyrene as described below.
Z2
FVT–Matrix Isolation
Y2
FVT of the diazirene 1 at 5008C with isolation of the products in
an Ar matrix at ,10 K resulted in the appearance of a new,
weak, and somewhat broad peak at 1830 cmꢀ1, as expected for
a cycloheptatetraene (CHT) or mixture of isomeric CHTs 10
(Fig. 9b and Scheme 3). At 700 and 8008C this peak was no
longer present, and new peaks in the allene region, at 1960
(major) and 1915 (minor) cmꢀ1, appeared instead (Fig. 9c).
After warm-up to room temperature and dissolution in CHCl3,
0
10000
Magnetic field [G]
Fig. 7. Electron spin resonance spectrum resulting from photolysis
of para-trifluoromethylphenyldiazomethane 6 at l . 455 nm (Ar, 14 K).
Microwave frequency 9.7453 GHz. Major signals ascribed to para-
carbene 9, Z1 ¼ 1989, Z2 ¼ 8963, X2 ¼ 5096, Y2 ¼ 6046 G; D ¼ 0.513; E ¼
0.0225 cmꢀ1. The minor signals are ascribed to the ortho-carbene 7 (Y2 6090
G, inset).
these peaks appeared at 1952 (major) and 1906 (minor) cmꢀ1
,
and they are ascribed to the trifluoromethylfulvenallenes 16
and 17, which are further characterised by NMR spectroscopy
below. While the weakness of the 1830 cmꢀ1 peak and the
presence of other compounds prevent a proper characterization
of the CHT(s), the isolation of heptafulvalenes 11 described in
the next section provides strong evidence.
g ϭ 2
X2
Z1
X2
Carbenes were not observed by ESR spectroscopy following
FVT of the phenyl(trifluoromethyl)diazo compounds and isola-
tion of the products in Ar matrices. This is in line with our
previous observations[12] that nitrenes survive, but carbenes do
not survive FVT experiments, except when pulsed pyrolysis
with rapid cooling of the products is used.
Y2
Z1
Z2
FVT and Product Studies
FVT at 5008C with isolation of the products at 77 K gave rise to a
yellow pyrolyzate, which quickly darkened on warming and was
essentially black at room temperature. Gas chromatography–
mass spectrometry (GC-MS) analysis revealed 21 isomers with
m/z 316 (heptafulvalenes 11 and styrenes 12, Scheme 3) corre-
sponding to 90 % of the product based on the uncorrected GC
responses. An additional four compounds had m/z 314 (anthra-
cenes 13) and account for 10 % of the total product. Formation of
styrenes and anthracenes from the heptafulvalenes and/or phe-
nylcarbenes is known.[13] In conformity with this, the 13C NMR
spectrum of the total pyrolyzate showed signals at 142–105 and
95–93 ppm.
Y2
0
10000
Magnetic field [G]
Fig. 8. Electron spin resonance spectrum resulting from broadband UV
photolysis of meta-trifluoromethylphenyldiazomethane 5 (Ar, 14K). Micro-
wave frequency 9.7452GHz. The major signals are ascribed to the meta-
carbene 8, Z1 ¼ 2083, Z2 ¼ 9037, X2 ¼ 5098, Y2 ¼ 6110 G; D ¼ 0.5235; E ¼
0.0243 cmꢀ1. Upper inset: X2 and Y2 signals. Lower inset: Z1 signal (see text).
After FVT at 7008C 70 % of the pyrolyzate consisted of
the dimers (19 with m/z 316 and five with m/z 314) together with
two m/z 246 isomers (formally formed by loss of CF3 and
addition of H). Importantly, an additional four monomeric
products with m/z 158, accounting for 28 % of the pyrolyzate,
were obtained.
similar, it is not possible to differentiate the X2 and Y2 signals of
these species. The biggest difference is in the Z1 signals.
Therefore, it is possible that the splitting of the Z1 line for 8 is
due to its isomerization to 7. Geometrical isomerization or
rearrangement to 7, or both, may be responsible.
It is noted that the interconversions are expected to occur
on the singlet energy surface (see Discussion), whereas the
species observed by ESR spectroscopy are necessarily the triplet
ground state carbenes. Therefore, depending on the individual
rates of reaction and intersystem crossing, ESR spectroscopy
may not be able to reveal all carbenes that are involved on the
singlet energy surface.
The mixture of four monomeric products was isolated by
bulb-to-bulb distillation, and two of these components
were separated by preparative gas chromatography and identi-
fied as trifluorobenzocyclobutene 14 and trifluorostyrene 15[14]
by 1H, 13C, and 19F NMR spectroscopy (see Experimental and
Figs S1–S5, Supplementary Material, for details). The other two
components, which did not survive the GC conditions, were
identified as the 1- and 2-trifluoromethylfulvenallenes 16 and 17
What is certain is that a-trifluoromethylphenylcarbene 3
isomerizes in part to the ortho-isomer 7; the ortho- and
1
on the basis of their H and 13C NMR spectra (Figs S6–S9,

40
R. J. Blanch and C. Wentrup
Supplementary Material, and Scheme 3). These two com-
pounds were relatively stable at ꢀ208C, but one of them
disappeared in the course of 3 days in solution at room
temperature, thereby facilitating the assignment of the NMR
signals (see Fig. S6, Supplementary Material). The IR spectra
(see Fig. 9c) indicate the presence of a major (2-CF3) and a
minor (1-CF3) trifluoromethylfulvenallene (1960 and
1915 cmꢀ1), and the 1H NMR spectra indicate a relatively
modest preponderance of one isomer, which is the more stable
one (2-CF3) (Fig. S7, Supplementary Material). Based on the
GC-MS and NMR data, the overall yields of trifluorobenzo-
cyclobutene 14, trifluorostyrene 15, and 1-and 2-trifluoromethyl-
fulvenallenes 16 and 17 can be given as ,13, 4, 11, and 15 %,
respectively. Ethynyl(trifluoromethyl)cyclopentadienes were
not detected, nor was 7-trifluoromethylfulvenallene.
(a)
Abs
Discussion
2300
800
The photochemical carbene–carbene rearrangement intercon-
verting 3 and 7–9 is wavelength dependent. It does not take
place, or does so only to a smaller extent, when the carbenes are
generated by long-wavelength photolysis. Rearrangement must
take place in an initially formed singlet excited state (S1,
S2ꢁꢁꢁSn). Once this has been used up and/or when all the
singlets have undergone intersystem crossing (ISC) to the T0
ground state, no further rearrangement takes place, even on
prolonged irradiation, at either long or short wavelength, i.e. the
triplets do not readily return to the singlet state(s). A mixture of
Sn and T0 may also be formed initially, and the same argument
would apply. If rearrangement in the Sn state(s) is competitive
with ISC, then partial interconversion will be seen. Because of
the possibility of reaction in excited states (RIES),[15] there may
be more than one carbene-forming process, and their weights
may be wavelength-dependent.
The reactivity of carbene 3 in liquid solution was examined
by Brunner et al.[4] who found that insertion into the O–H bond
of MeOH was much faster than insertion into a C–H bond in
cyclohexane and concluded that the O–H insertion was probably
a singlet reaction, but no conclusion was made regarding the
slower C–H insertion. Insertion of trifluoromethylphenylcar-
benes into O–H bonds in cyclodextrin was also proposed to be a
singlet reaction.[16] In an investigation of the addition of
substituted arylcarbenes to olefins it was concluded that lone
pair-donating substituents (OMe) stabilize the singlet state, and
electron withdrawing substituents (NO2) promote ISC to the
triplet state. The trifluoromethylphenylcarbenes were thought to
react largely from the triplet state, but some addition of singlet
carbenes might also occur.[17]
[cmϪ1
[cmϪ1
[cmϪ1
]
]
]
(b)
Abs
A
2300
800
(c)
Geise and Hadad calculated the activation energy (Ea) for ring
expansion in singlet p-CF3-phenylcarbene as 12.8kcalmolꢀ1
,
B
Abs
and the exothermicity of the reaction as 15.2 kcal molꢀ1 at the
DFT B3LYP level of theory.[18] Similar values were derived for
p-CN and p-NO2, and there was no dramatic effect of substi-
tuents. For the unsubstituted phenylcarbene: Ea ¼ 14.8, exo-
thermicity 13.6 kcal molꢀ1; for p-F-phenylcarbene: 16.1 and
12.5 kcal molꢀ1; for syn-meta-CF3-phenylcarbene: 13.8 and
C
ꢀ15.6 kcal molꢀ1
; for anti-meta-CF3-phenylcarbene: 13.7
and 14.6 kcal molꢀ1; for syn-ortho-CF3-phenylcarbene: 10.6
and 19.3 kcal molꢀ1; and for anti-ortho-CF3-phenylcarbene,
12.8 and 19.9 kcal molꢀ1. The calculated singlet triplet splittings
in phenylcarbene and o-, m-, and p-CF3-phenylcarbenes are ,6,
9, 7, and 10 kcal molꢀ1, respectively,[19] and for 3 a value of
,4 kcal molꢀ1 was calculated, the triplet being the ground state
in each case.[9b] In summary, all the carbenes considered here
2300
800
Fig. 9. (a) IR spectrum of 1 before flash vacuum thermolysis (FVT). (b)
After FVT at 5008C (peak labelled A at 1830 cmꢀ1 ascribed to 10). (c) After
FVT at 8008C (peaks labelled B and C at 1960 and 1915 cmꢀ1 ascribed to 16
and 17). All spectra are in an Ar matrix at 10 K.

Trifluoromethylphenylcarbenes
41
F
F
F
ϩ
CH2
C
CH2
C
F
F
CH3
F
CF3
14
15
16
17
700ЊC
FVT
700ЊC
700ЊC
FVT
FVT
N
N
CF3
FVT
CH
CH
1
CH
CF3 FVT
FVT
FVT
N2
CF3
CF3
3
7
CF3
CF3
8
9
2
FVT 500ЊC
ϫ 2
CF3
CF3
F3C
CF3
10
11
ϩ
F3C
12
13
ϩ
F3C
CF3
Scheme 3. Thermal rearrangement products derived from the trifluoromethylphenylcarbenes (FVT: flash vacuum
thermolysis).
have triplet ground states, and all the singlet carbene ring
expansions are predicted to be exothermic and to have very
modest activation energies.
Since trifluorobenzocyclobutene is a major product in our
thermolyses of 3-phenyl-3-trifluoromethyldiazirene, it is
likely that it is also formed in the jet pyrolysis reaction,[9]
although it was not reported. The thermal formation of the
trifluoromethylfulvenallenes 16 and 17 corresponds to ring
contraction of 7, 8, and probably also 9 in analogy with the
high-temperature FVT reaction of phenylcarbene.[10,13,22] The
absence of 7-trifluoromethylfulvenallene indicates that the
a-carbene 3 does not undergo ring contraction, probably
because the activation energy for the F-shift producing 15
is lower.
Fischer et al. described initial deactivation within 60 fs of an
excited state produced by photolysis of a-trifluoromethyltolyl-
carbene 3 at 254 nm (the carbene having been prepared by jet
flash pyrolysis). This initial decay was followed by relaxation
occurring with a time constant of ,500 fs. Further deactivation
to the hot ground state took place on a timescale of ,3 ps.[9b]
The authors did not consider carbene–carbene rearrangements,
but our results presented here make it likely that such rearrange-
ments occur under both thermal and photochemical conditions.
Elsewhere we have described the diphenylcarbene–
biphenylylcarbene rearrangement plus formation of the fluorene
product taking place within the ,15 ns duration of one laser
pulse.[20] The results reported here provide further evidence
for rapid carbene–carbene rearrangement on the singlet
energy surface prior to and competitive with deactivation to
the triplet state.
The thermal formation of trifluorobenzocyclobutene 14 and
trifluorostyrene 15 is readily understood in terms of rearrange-
ments of o- and a-tolylcarbenes, respectively (Scheme 3).
Although the strong C–F bond (110–130 kcal molꢀ1) makes
migrations of fluorine less common than those of other ele-
ments, many C–F migrations are known.[21] Interestingly, no
evidence for rearrangement to trifluorostyrene was found in the
photochemical reaction in solution at room temperature.[4]
Conclusion and Outlook
The ESR investigations lead to the conclusion that the
carbene–carbene rearrangements (via trifluoromethylcyclo-
heptatetraenes) take place on a singlet energy surface, and the
singlet–triplet intersystem crossing has competitive rates. It is
likely that carbene–carbene rearrangements in other systems
also take place on excited singlet state surfaces, although it was
not possible to determine this in the parent phenylcarbene sys-
tem (Scheme 1) or the 4-, 3-, 2-pyridylcarbene–phenylnitrene
system, where intersystem crossing between triplet and singlet
carbenes and nitrenes appears to be rapid under the matrix
photolysis conditions.[23] It would be of interest to perform
further photochemical studies of the rearrangements of the
trifluoromethylphenylcarbenes, for example in xenon and
mixed argon–xenon matrices in an attempt to modulate the

42
R. J. Blanch and C. Wentrup
singlet–triplet intersystem crossing rates by the external heavy-
atom effect[24] and thereby influence product compositions.
1,1,2-Trifluorobenzocyclobutene 14[29,30]
dH (400 MHz)7.65–7.45(m, 4H), 6.06(ddd, JHF 58,1.9,0.5, 1H).
dC (100 MHz) 140.9 (ddd, JCF 22.85, 20.84, 10.42), 140.8 (td, JCF
25.88, 10.42), 133.6 (dt, JCF 2.35, 1.09), 132.4 (dt, JCF 2.91, 1),
124.3 (dd, JCF 4.87, 2.53), 122.3 (s), 118.1 (ddd, JCF 286.35,
283.83, 20.59), 93.7 (ddd, JCF 227.5, 223.2, 30.6). dF (189 MHz)
ꢀ96.18 (ddd, JFF 211.7, 10.5, 1.5), ꢀ108 (dd, JFF 212.5, 13.7),
ꢀ176.11 (dddd, JHF 58.0, 1.2; JFF 13.4, 10.4) (see Fig. S9 for the
2D COSY spectrum and Figs S3 and S4 for the assignments).
Experimental
Procedures for matrix isolation, flash vacuum thermolysis,
photolysis, and ESR and IR spectroscopy have been pub-
lished.[25] Photolyses were carried out using a 1000 W high
pressure Hg/Xe lamp equipped with appropriate filters. A 10 cm
water filter was inserted in the optical path to absorb heat
radiation. Preparative GC separations were carried out on an
OV-101 column, 1 cm internal diameter, 2 m length, at 608C.
The NMR solvent was CDCl3. The reference was TMS for 1H
and 13C NMR spectra, and CFCl3 for 19F NMR spectroscopy.
3-Phenyl-3-trifluoromethyldiazirene[4] 1 and phenyltrifluoro-
diazoethane[26] 2 were prepared according to the literature.
(Trifluoromethylphenyl)diazomethanes were prepared from
the tosylhydrazone soldium salts as described by Creary.[17,27]
Deposition of the diazo compounds at 77 K and recording the IR
spectra confirmed the presence of strong diazo group absorp-
10,20,2-Trifluorostyrene 15[31]
dH (400 MHz) 7.5–7.3 (m). dF (189 MHz) ꢀ100.55 (dd, JFF 71.5,
31.7), ꢀ115.27 (dd, JFF 109.5, 71.5), ꢀ177.35 (dd, JFF 109.5,
31.7).
1-Trifluoromethylfulvenallene 16
dH (400MHz) 6.80 (sextet, JHF 1.9, ring proton), 6.51 (m, 1H),
6.45 (m, 1H), 5.45 (quintet, JHF 1.5, 2 ꢂ allenic H); the ass-
ignments are based on selective decoupling experiments.
dC (100 MHz, see COSY Fig. S9 and Fig. S10 for assignments)
126.65 (s, attached to proton at dH 6.51, C3), 126.07 (q, JCF 5.7,
attached to proton at dH 6.80, C2), 125.74 (q, JCF 2.0, attached to
protonatdH 6.54, C4), 122.41 (q, JCF 26.8, CF3), 76.95(s,attached
to proton at dH 5.45, terminal allenic C); central allenic C not
clearly observed. dF (189 MHz, 1H decoupled, CFCl3 reference)
ꢀ63.41 (s). dF (1H coupled) ꢀ63.41 (apparent quintet JHF ,1.5).
tions: 2, 2088 cmꢀ1; ortho 4, 2068 cmꢀ1; meta 5, 2070 cmꢀ1
and para 6, 2072 cmꢀ1
;
.
The diazo compounds were deposited with Ar on a Cu rod
at 14 K for ESR spectroscopy[25] and then photolyzed at the
wavelengths indicated. |D/hc| and |E/hc| values (abbreviated
D and E in this paper) for the resulting triplet carbenes were
calculated using Wasserman’s equations and method of
defining field positions.[28]
2-Trifluoromethylfulvenallene 17
dH (400 MHz) 6.83 (m, JHF 1.8, 1H), 6.59 (dd, JHH 5.1, 1.8, 1H),
6.32 (m, JHF 0.7, 1H), 5.51 (octet, J 0.7, 2 ꢂ allenic H); the
assignments are based on selective decoupling experiments.
dC NMR (100 MHz, see COSY Fig. S9 and Fig. S10 for
assignments) 212.31 (s, no attached proton, central allenic C),
133.01 (q, JCF 4.8 Hz, attached to proton at dH 6.83, C1), 129.43
(q, JCF 1.7 Hz, attached to proton at dH 6.59, C3), 127.30 (q, JCF
0.7, attached to proton at dH 6.32, C4), 126.33 (q, JHF 10.4, C2),
122.52 (q, JCF 267.9, CF3), 107.60 (q, JCF 1.4, C5), 78.40 (s,
attached to proton at dH 5.51, terminal allenic C). dF (189 MHz,
1H decoupled, CFCl3 reference) ꢀ58.79 (s). dF (1H coupled)
ꢀ58.79 (d of q JHF 1.58 and 0.7).
Preparative FVT of Diazirene 1
Method A
Compound 1 (100 mg) was introduced into the FVT tube
from a sample reservoir held at ꢀ208C. FVT was carried out at
5008C at 5 ꢂ 10ꢀ3 hPa. The products were collected in a liq. N2
cold trap. The dark brown colour of the product suggested the
presence of heptafulvalenes. The total crude product was exam-
ined by GC-MS, which indicated that at 5008C 90 % of the
product had a molecular mass of 316 in agreement with a
mixture of bis-trifluoromethyl-heptafulvalenes and -stilbenes.
The remaining 10 % of the product had a molecular mass of 314
corresponding to a mixture of bistrifluoromethylanthracenes
(see Table S1, Supplementary Material, for details).
Detailed NMR spectra and assignments are shown in
the Supplementary Material (Figs S2–S10). From the com-
bined GC and NMR data, the following ratio of monomeric
products was derived for the FVT reaction at 7008C:
14 : 15 : 16 : 17 ¼ 13 : 4 : 11 : 15 %.
Method B
In an analogous FVT reaction at 7008C, the yellowish
product darkened only modestly on warming of the cold trap,
thus suggesting that products other than heptafulvalenes were
present. GC-MS indicated that 70 % of the product was still
attributable to bis-trifluoromethyl-heptafulvalenes, -stilbenes,
and -anthracenes, together with minor amounts of two com-
pounds as a result of loss of a CF3 group (m/z 246) (Table S2,
Supplementary Material). The remaining 30 % of the product
observable by GC-MS was attributable to two monomeric
compounds with molecular masses of 158. These two com-
pounds were separated by distillation of the product, purified by
preparative GC and identified as 1,1,2-trifluorobenzocyclobu-
tene 14 and 10,20,20-trifluorostyrene 15. In addition, the distillate
contained two further compounds, which did not survive GC,
which were identified as the 1- and 2-trifluoromethylfulvenal-
lenes 16 and 17. The analysis of the NMR spectra of the
fulvenallenes was aided by the observation that the 1-isomer
was less stable thermally and disappeared from the NMR
solution in the course of 3 days at 208C.
Supplementary Material
GC-MS of 14, 1H, 13C, and 19F NMR spectra of 14, 15, 16, and
17, and GC-MS data for the heptafulvalenes, stilbenes, and
anthracenes are available from the Journal’s website.
Acknowledgement
This work was supported by the Australian Research Council, The Uni-
versity of Queensland, and the National Computing Infrastructure facility
supported by the Australian Government (MAS grant g 01).
References
[1] Fluorinated Heterocyclic Compounds (Ed. V. A. Petrov) 2009 (Wiley:
Hoboken, NJ).
[2] (a) S. Ritter, Chem. Eng. News 2012, 90, 7. doi:10.1021/CEN-09038-
NOTW4
(b) Z. Jin, G. B. Hammond, B. Xu, Aldrichim Acta 2012, 45, 67.
(c) Y. Li, L. Wu, H. Neumann, M. Beller, Chem. Commun. 2013, 49,
2628. doi:10.1039/C2CC36554E

Trifluoromethylphenylcarbenes
43
(d) M. Chen, S. L. Buchwald, Angew. Chem. Int. Ed. 2013, 52, 11628.
doi:10.1002/ANIE.201306094
(e) A. Lishchynskyi, M. A. Novikov, E. Martin, E. C. Escudero-Ada´n,
P. Nova´k, V. V. Grushin, J. Org. Chem. 2013, 78, 11126. doi:10.1021/
JO401423H
(f) N. O. Ilchenko, P. G. Janson, K. J. Szabo´, J. Org. Chem. 2013, 78,
11087. doi:10.1021/JO401831T
(g) Q. Wang, X. Dong, T. Xiao, L. Zhou, Org. Lett. 2013, 15, 4846.
doi:10.1021/OL4022589
doi:10.1021/JA00337A056
(h) W. S. Trahanovsky, M. E. Scribner, J. Am. Chem. Soc. 1984, 106,
7976. doi:10.1021/JA00337A057
(i) O. L. Chapman, C. J. Abelt, J. W. Johnson, C. L. Kreil,
J.-P. LeRoux, R. J. McMahon, A. M. Mooring, P. R. West, J. Am.
Chem. Soc. 1988, 110, 501. doi:10.1021/JA00210A032
[12] (a) M. Kuzaj, H. Lu¨erssen, C. Wentrup, Angew. Chem. Int. Ed. 1986,
25, 480. doi:10.1002/ANIE.198604801
(b) P. Kvaskoff, L. G. Bednarek, S. Pankarakshan, C. Wentrup, J. Org.
Chem. 2005, 70, 7947. doi:10.1021/JO050898G
[13] C. Wentrup, K. Wilczek, Helv. Chim. Acta 1970, 53, 1459.
doi:10.1002/HLCA.19700530628
[3] R. J. Blanch, The Generation and Direct Observation of Selected
Reactive Intermediates 1991, Ph.D. thesis, The University of
Queensland.
[4] J. Brunner, H. Senn, F. M. Richards, J. Biol. Chem. 1980, 255, 3313.
[5] (a) S. A. Fleming, Tetrahedron 1995, 51, 12479. doi:10.1016/0040-
4020(95)00598-3
[14] P. L. Heinze, D. J. Burton, J. Org. Chem. 1988, 53, 2714. doi:10.1021/
JO00247A010
[15] Y. Zhang, G. Burdzinski, J. Kubicki, M. S. Platz, J. Am. Chem. Soc.
2009, 131, 9646. doi:10.1021/JA904009X
[16] S. H. Smith, S. M. Forrest, D. C. Williams, Jr, M. F. Cabell,
M. F. Acquavella, C. J. Abelt, Carbohydr. Res. 1992, 230, 289.
doi:10.1016/0008-6215(92)84039-U
(b) A. Blencowe, W. Hayes, Soft Matter 2005, 1, 178. doi:10.1039/
B501989C
(c) E. L. Vodovozova, Biochemistry (Moscow) 2007, 72, 1.
doi:10.1134/S0006297907010014
(d) K. Nakamoto, Y. Ueno, J. Org. Chem. 2014, 79, doi:10.1021/
JO402738T
[17] X. Creary, J. Am. Chem. Soc. 1980, 102, 1611. doi:10.1021/
JA00525A025
[6] A. Ru¨hmann, C. Wentrup, Tetrahedron 1994, 50, 3785. doi:10.1016/
S0040-4020(01)90398-9
[18] C. M. Geise, C. M. Hadad, J. Org. Chem. 2002, 67, 2532. doi:10.1021/
JO0162181
[7] E. Wasserman, L. Barash, W. A. Yager, J. Am. Chem. Soc. 1965, 87,
4774.
[19] C. M. Geise, C. M. Hadad, J. Org. Chem. 2000, 65, 8348. doi:10.1021/
JO005655X
[8] (a) W. W. Sander, J. Org. Chem. 1988, 53, 121. doi:10.1021/
JO00236A024
[20] M. J. Regimbald-Krnel, C. Wentrup, J. Org. Chem. 2013, 78, 8789.
doi:10.1021/JO401607M
(b) P. S. Zuev, O. M. Nefedov, Russ. Chem. Rev. 1989, 58, 636.
doi:10.1070/RC1989V058N07ABEH003466
[21] (a) L. S. Kobrina, V. N. Kovtonyuk, Russ. Chem. Rev. 1988, 57, 62.
doi:10.1070/RC1988V057N01ABEH003335
(c) A. K. Mal’tsev, V. A. Korolov, O. M. Nefedov, Bull. Acad. Sci.
USSR Div. Chem. Sci. (Engl. Transl.) 1984, 33, 510. doi:10.1007/
BF00995688
(b) K. Uneyama, Organofluorine Chemistry 2006 (Blackwell:
Oxford).
[22] C. Wentrup, E. Wentrup-Byrne, P. Mu¨ller, J. Chem. Soc., Chem.
Commun. 1977, 210. doi:10.1039/C39770000210
[23] O. L. Chapman, R. S. Sheridan, J.-P. LeRoux, J. Am. Chem. Soc. 1978,
100, 6245. doi:10.1021/JA00487A054
[24] J. Lundell, M. Ra¨sa¨nen, J. Mol. Struct. 1997, 436–437, 349.
doi:10.1016/S0022-2860(97)00115-4
[9] (a) B. Noller, L. Poisson, R. Maksimenka, I. Fischer, J.-M. Mestdagh,
J. Am. Chem. Soc. 2008, 130, 14908. doi:10.1021/JA804133C
(b) B. Noller, L. Poisson, R. Maksimenka, O. Gobert, I. Fischer,
J. M. Mestdagh, J. Phys. Chem. A 2009, 113, 3041. doi:10.1021/
JP810974M
(c) K. H. Fischer, P. Hemberger, I. Fischer, A. M. Rijs, Chem-
PhysChem 2010, 11, 3228. doi:10.1002/CPHC.201000415
[10] (a) C. Wentrup, Top. Curr. Chem. 1976, 62, 173.
(b) W. M. Jones, Acc. Chem. Res. 1977, 10, 353. doi:10.1021/
AR50118A001
[25] (a) C. Wentrup, D. Kvaskoff, Aust. J. Chem. 2013, 66, 286.
doi:10.1071/CH12502
(b) S. Torker, D. Kvaskoff, C. Wentrup, J. Org. Chem. 2014, 79, 1758.
doi:10.1021/JO4027897
(c) C. Wentrup, Aust. J. Chem. 2014, 67, 1150. doi:10.1071/CH14096
[26] (a) G. Shi, Y. Xu, J. Fluor. Chem. 1990, 46, 173. doi:10.1016/S0022-
1139(00)81567-5
(c) W. M. Jones, U. H. Brinker, in Pericyclic Reactions
(Eds A. P. Marchand, A. E. Lehr) 1977, Vol. 1, Ch. 3, pp. 109–198
(Academic Press: New York, NY).
(b) G. Diderich, Helv. Chim. Acta 1972, 55, 2103. doi:10.1002/HLCA.
19720550629
(c) R. A. Shepard, S. E. Wentworth, J. Org. Chem. 1967, 32, 3197.
doi:10.1021/JO01285A059
(d) C. Wentrup, in Reactive Intermediates (Ed. R. A. Abramovich)
1980, Vol. 1, Ch. 4, pp. 263–319 (Plenum: New York, NY).
(e) C. Wentrup, Reactive Molecules 1985 (Wiley-Interscience:
New York, NY).
[27] X. Creary, Org. Synth. 1986, 64, 207. doi:10.15227/ORGSYN.
064.0207
[28] E. Wasserman, L. C. Snyder, W. A. Yager, J. Chem. Phys. 1964, 41,
(f) R. A. Moss, M. Jones, Jr, Reactive Intermediates 1985, Vol. 3
(Wiley: New York, NY).
(g) C. Wentrup, in Methoden der Organischen Chemie (Houben-Weyl)
(Ed. M. Regitz) 1989, Vol. E19b, pp. 824–976 (Thieme: Stuttgart).
[11] (a) W. J. Baron, M. Jones, Jr, P. P. Gaspar, J. Am. Chem. Soc. 1970, 92,
4739. doi:10.1021/JA00718A600
1763. doi:10.1063/1.1726156
[29] R. J. Shozda, R. E. Putnam, J. Org. Chem. 1962, 27, 1557. doi:10.1021/
JO01052A016
[30] For analogous 19F NMR data on 3,3,4-trifluorocyclobutenes, see:
M. F. Kuehnel, P. Holstein, M. Kliche, J. Kru¨ger, S. Matthies,
D. Nitsch, J. Schutt, M. Sparenberg, D. Lentz, Chem. – Eur. J. 2012,
18, 10701. doi:10.1002/CHEM.201201125
(b) E. Hedaya, M. E. Kent, J. Am. Chem. Soc. 1971, 93, 3283.
doi:10.1021/JA00742A032
(c) R. Gleiter, W. Rettig, C. Wentrup, Helv. Chim. Acta 1974, 57, 2111.
doi:10.1002/HLCA.19740570726
(d) C. Mayor, C. Wentrup, J. Am. Chem. Soc. 1975, 97, 7467.
doi:10.1021/JA00859A014
(e) P. P. Gaspar, J.-P. Hsu, S. Chari, M. Jones, Jr, Tetrahedron 1985,
41, 1479. doi:10.1016/S0040-4020(01)96389-6
[31] (a) P. L. Heinze, D. J. Burton, J. Fluor. Chem. 1986, 31, 115.
doi:10.1016/S0022-1139(00)85092-7
(b) A. Raghavanpillai, D. J. Burton, J. Org. Chem. 2004, 69, 7083.
doi:10.1021/JO049179C
(c) M. Ohashi, H. Saijo, M. Shibata, S. Ogoshi, Eur. J. Org. Chem.
2013, 443. doi:10.1002/EJOC.201201405
(f) O. L. Chapman, R. J. McMahon, P. R. West, J. Am. Chem. Soc.
1984, 106, 7973. doi:10.1021/JA00337A055
(g) O. L. Chapman, U. P. E. Tsou, J. Am. Chem. Soc. 1984, 106, 7974.