Different behavior of nitrenes and carbenes on photolysis and thermolysis: Formation of azirine, ylidic cumulene, and cyclic ketenimine and the rearrangement of 6-phenanthridylcarbene to 9-phenanthrylnitrene

Article abstract of DOI:10.1021/jo050898g

Flash vacuum thermolysis (FVT) of 9-azidophenanthrene 8, 6-(5-tetrazolyl)phenanthridine 18, and [1,2,3]triazolo[l,5-f]phenanthridine 19 yields 9-cyanofluorene 12 as the principal product and 4-cyanofluorene as a minor product. In all cases, when the product is condensed at or below 77 K, the seven-membered ring ketenimine 24 is detectable by IR spectroscopy (1932 cm^-1) up to 200 K. Photolysis of Ar matrix isolated 8 at λ = 308 or 313 nm generates at first the azirine 26, rapidly followed by the ylidic cumulene 27. The latter reverts to azirine 26 at λ > 405 nm, and the azirine reverts to the ylidic cumulene at 313 nm. Nitrene 9 is observed by ESR spectroscopy following FVT of either azide 8, tetrazole 18, or triazole 19 with Ar matrix isolation of the products. Nitrene 9 and carbene 21 are observed by ESR spectroscopy in the Ar matrix photolyses of azide 8 and triazole 19, respectively.

Full text of DOI:10.1021/jo050898g

Different Behavior of Nitrenes and Carbenes on Photolysis and
Thermolysis: Formation of Azirine, Ylidic Cumulene, and Cyclic
Ketenimine and the Rearrangement of 6-Phenanthridylcarbene to
9-Phenanthrylnitrene
David Kvaskoff, Pawel Bednarek,† Lisa George,‡ Sreekumar Pankajakshan,§ and
Curt Wentrup*
Chemistry Building, School of Molecular and Microbial Sciences, The University of Queensland,
Brisbane, Queensland 4072, Australia
wentrup@uq.edu.au
Received May 5, 2005
Flash vacuum thermolysis (FVT) of 9-azidophenanthrene 8, 6-(5-tetrazolyl)phenanthridine 18, and
[1,2,3]triazolo[1,5-f]phenanthridine 19 yields 9-cyanofluorene 12 as the principal product and
4-cyanofluorene as a minor product. In all cases, when the product is condensed at or below 77 K,
the seven-membered ring ketenimine 24 is detectable by IR spectroscopy (1932 cm-1) up to 200 K.
Photolysis of Ar matrix isolated 8 at λ ) 308 or 313 nm generates at first the azirine 26, rapidly
followed by the ylidic cumulene 27. The latter reverts to azirine 26 at λ > 405 nm, and the azirine
reverts to the ylidic cumulene at 313 nm. Nitrene 9 is observed by ESR spectroscopy following
FVT of either azide 8, tetrazole 18, or triazole 19 with Ar matrix isolation of the products. Nitrene
9 and carbene 21 are observed by ESR spectroscopy in the Ar matrix photolyses of azide 8 and
triazole 19, respectively.
Introduction
The rearrangements and interconversions of aromatic
carbenes and nitrenes, e.g., phenylnitrene 1 and 2-py-
ridylcarbene (eq 1), have been the subject of intense
interest.1 Recently, DFT calculations2 revealed that an-
nelated derivatives of the cyclic ketenimine 2 may exist
in an ylidic (zwitterionic) form 6 rather than the classical
o-quinoid ketenimine form 7. The cost of charge separa-
tion in the ylide is compensated by the recovery of the
resonance energy of the benzene ring. 4-Quinolylnitrene
* Corresponding author.
† ARC postdoctoral research associate 2002-2003.
‡ ARC postdoctoral research associate 2004-2005.
§ Visiting scholar from Mahatma Gandhi University 2004-2005.
(1) (a) Wentrup, C. Top. Curr. Chem. 1976, 62, 173. (b) Wentrup,
C. Adv. Heterocycl. Chem. 1981, 28. 232. (c) Gritsan, N. P.; Platz, M.
S. Adv. Phys. Org. Chem. 2001, 36, 255. (d) Karney, W. L.; Borden, W.
T. Adv. Carbene Chem. 2001, 3, 205.
and 1-naphthylnitrene 3 can in principle undergo ring
expansion in two directions, either via the high energy
azirine 4a (which is a transition state) or the lower
(2) Kuhn, A.; Vosswinkel, M.; Wentrup, C. J. Org. Chem. 2002, 67,
9023.
10.1021/jo050898g CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/25/2005
J. Org. Chem. 2005, 70, 7947-7955
7947

Kvaskoff et al.
SCHEME 1
energy azirine 4b, to afford ketenimines 5 and/or 6/7 (eq
2). Calculations indicated that all three cumulenes (5, 6,
and 7) can exist as separate energy minima,2,3 and a
similar choice exists for 2-naphthylnitrene. Recent ex-
periments have demonstrated that the ylides such as 6
formed from 1- and 2-naphthylnitrenes are indeed spec-
troscopically observable.3 Here we report that different
cumulenes are obtained from 9-phenanthrylnitrene under
thermal and photochemical conditions. Furthermore, the
rearrangement of 6-phenanthridylcarbene to 9-phenan-
thrylnitrene is demonstrated.
Results and Discussion
FVT. 9-Azidophenanthrene 8 was subjected to flash
vacuum thermolysis (FVT) at temperatures between 450
and 600 °C (no significant reaction took place at lower
temperatures). The following products were isolated and
identified from the FVT reaction at 500 °C: 9-cyanofluo-
rene 12 (73%), phenanthrene (7%), 9,9′-azophenanthrene
11 (10%), and 9-aminophenanthrene 10 (10%). Com-
pounds 10 and 11 are the typical triplet arylnitrene
products, and their formation demonstrates that some
intermolecular reaction take place under FVT condi-
tions. Compound 12 is the expected product of ring
contraction.1a,4
temperatures, and such shifts can be accelerated strongly
by chemical activation,6 it was necessary to check the
FVT of 9-cyanofluorene 12. FVT of 12 at 600-800 °C did
not result in any isomerization. At 1000 °C a 5% rear-
rangement to 1-cyanofluorene 16 took place; 4-cyano-
fluorene was not detectable. A 1% yield of fluorene was
also obtained, which indicates the occurrence of a free
radical pathway. Therefore, in any FVT reaction where
9-cyanofluorene 12 is formed at elevated temperature,
and/or in strongly exothermic reactions, where chemical
activation is likely, minor amounts of 1-cyanofluorene 16
may be expected as a byproduct.
Formation of 9-cyanofluorene 12 could proceed via ring
opening to diradical 14 (Scheme 1), but calculations
reported below indicate that there is a lower energy
transition state for the concerted ring contraction, 9 f
12. CASPT2/CASSCF(8,8) calculations with the cc-PVDZ-
Dunning basis set for phenylnitrene 1 OSS (open-shell
singlet) give an activation energy of 31 kcal/mol for the
concerted ring contraction to 5-cyanocyclopentadiene.
This is further discussed in the Calculations section.
The mechanism of formation of 4-cyanofluorene 13 is
thought to involve ring opening to the carbene 15
(Scheme 1). Calculations reported below show that this
carbene can cyclize to 13 in a highly exothermic reaction
(∼92 kcal/mol). The cyclization of 2-biphenylylcarbene to
fluorene takes place via ”isofluorene” (9aH-fluorene) and
has a very small calculated barrier.7
When the FVT of 8 was carried out with isolation of
the products on a KBr target, either as a neat solid film
at 77 K or with Ar at 20 K for IR spectroscopic monitor-
ing, a new species absorbing strongly at 1932 cm-1 was
observed together with 9-cyanofluorene, which has char-
acteristic strong bands at 1451, 759, and 742 cm-1 and
CN stretching vibrations at 2262 and 2234 cm-1 (Figure
1).
Closer inspection of the Ar matrix IR spectra and GC/
MS analyses of the crude thermolysis products revealed
that minor amounts of 4-cyanofluorene 13 and 1-cyano-
fluorene 16 were also present (2.5% of each at 500 °C;
6.7% of 13 and 3.7% of 16 at 600 °C by GC).5 These
compounds were identified by direct comparison with
synthetic samples. Because it is known that cyano groups
can undergo migrations in aromatic systems at high
The 1932 cm-1 signal disappeared on warming between
150 and 200 K and is ascribed to the dibenzo[d,f]-1-
azacycloheptatraene 24 (Scheme 2). No discrete product
was identified as a result of the warming, but dimeriza-
tion analogous to that of the carbodiimide (dibenzo[d,f]-
1,3-diazacycloheptatetraene) analog5b is a possibility.
Compound 24 is the first seven-membered ring keten-
(3) Maltsev, A.; Bally, T.; Tsao, M.-L.; Platz, M. S.; Kuhn, A.;
Vosswinkel, M.; Wentrup, C. J. Am. Chem. Soc. 2004, 126, 237.
(4) (a) Crow, W. D.; Wentrup, C. Tetrahedron Lett. 1967, 4379. (b)
Wentrup, C. In Reactive Intermediates; Abramovitch, R. A., Ed.;
Plenum Press: New York, 1980; Vol 1, Chapter 4, p 263f. (c) Wentrup,
C. In Azides and Nitrenes; Scrivcn, E. F. V., Ed.; Academic Press: New
York, 1984; Chapter 8, p 395 f.
(5) The thermal ring contractions of 9-phenanthridylnitrene to a
mixture of 9- and 4-cyanocarbazoles, its ring expansion to dibenzo-
[d,f]-1,3-diazcycloheptatetraene, and the dimerization of the latter to
a stable 2,4-diimino-1,2-diazetidine derivative have been reported: (a)
Wentrup, C.; Thetaz, C.; Mayor, C. Helv. Chim. Acta 1972, 55, 2633.
(b) Wentrup, C.; Winter, H.-W. J. Am. Chem. Soc. 1980, 102, 6159.
(6) (a) Lan, N. M.; Burgard, R.; Wentrup, C. J. Org. Chem. 2004,
69, 3022. (b) Wentrup, C.; Crow, W. D. Tetrahedron 1970, 26, 3965.
(7) Monguchi, K.; Itoh, T.; Hirai, K.; Tomioka, H. J. Am. Chem. Soc.
2004, 126, 11900.
7948 J. Org. Chem., Vol. 70, No. 20, 2005

Photolysis and Thermolysis of Nitrenes and Carbenes
that arylnitrenes actually undergo ring expansion to aza-
cycloheptatetraenes in the gas phase under thermal con-
ditions. The two benzo rings confer considerable stability
to the molecule, just as is the case for the cyclic carbo-
diimide analogue.5 Nevertheless, 24 is a delicate com-
pound. It was observable only when an “internal” oven
was used, consisting of a 10 cm long, 0.7 cm i.d., electri-
cally heated quartz tube suspended in a vacuum chamber
directly flanged onto the cryostat cold head so that the
FVT products can travel to the cold KBr target without
any wall collisions occurring. If an “external” oven was
used, consisting of a similar quartz tube but with a 5 cm
unheated length connecting it to the cold head, no trace
of the 1932 cm-1 absorption was detectable.
The wavenumber of the ketenimine function in 24 is
in line with expectations; 1895 cm-1 for 2; 1902-1909
for 5 (X ) N)2 and 1913-1926 for 5 (X ) CH).3,6,8 The
calculated IR spectrum of 24 (Figure 1a) predicts the
cumulenic stretching vibration at 1947 cm-1 at the
B3LYP/6-31G* level. At the B3LYP/6-31+G* level it is
at 1934 cm-1. The o-quinoid ketenimine ”bond-shift”
isomer 28 (Scheme 2) is not an energy minimum, and
hence its IR spectrum was not calculated. However,
judging from our computational experience with the ring
expansion products of the quinolyl- and naphthylni-
trenes,2,3 o-quinoid ketenimines of this type, e.g., 7 (eq
2), should absorb strongly around 1800 cm-1, i.e., in
between the aromatic ketenimines such as 5 and 24 and
ylidic cumulenes such as 6 and 27 (see below).
We have reported the aromatic carbene-nitrene rear-
rangement (eq 1)9 and demonstrated that nitrenes are
inherently more stable than the isomeric carbenes.1a
Thus, 2-pyridylcarbene rearranges thermally to phe-
nylnitrene 1. Modern calculations have confirmed this.10
Arylcarbenes can be generated by FVT of 5-aryltetrazoles
by consecutive loss of two nitrogen molecules, a reaction
leading to aryldiazomethanes initially.6a,11 The same
aryldiazomethanes may be formed by reversible ring
opening of triazoloazines.6a,11 Therefore, to generate
carbene 21, tetrazole 18 was prepared by addition of HN3
to phenanthridine-6-carbonitrile 17, and triazole 19 by
oxidation of the hydrazone of phenanthridine-6-carbox-
aldehyde with MnO2.
FIGURE 1. (a) Calculated IR spectrum of dibenzoazacyclo-
heptatraene 24 (B3LYP/6-31G* level with a scaling factor of
0.9613 for wavenumbers). (b) IR spectrum after FVT of
9-azidophenanthrene 8. A strong ketenimine band at 1932
cm-1 is assigned to species 24; bands belonging to 9-cyano-
fluorene 12 are at 2262, 2234, 1451, 759, and 742 cm-1; the
increased intensity at 2234 cm-1 is due to 4-cyanofluorene 13;
the asterisk depicts a trace of the precursor, 8. (c) IR spectrum
of matrix-isolated 9-cyanofluorene 12 with characteristic bands
at 2262, 2234, 1455, 1451, 759, 742 cm-1. The abscissa is in
wavenumbers.
SCHEME 2
Preparative FVT of tetrazole 18 at 600 °C resulted in
partial cycloreversion to nitrile 17 (3-10%), but the main
product was 9-cyanofluorene 12 (isolated in 68% yield),
which suggests that a very efficient carbene-nitrene
rearrangement (21 f 9) is taking place. 6-Methylphenan-
thridine 22 and small amounts of 4-cyanofluorene 13,
1-cyanofluorene 16, and 9-aminophenanthrene 10 were
also formed (eq 3). The former product (22) suggests
H-abstraction by 6-phenanthridylcarbene 21, again re-
vealing the occurrence of intermolecular reactions under
(8) Dunkin, I.; Thomson, P. C. P. Chem. Commun. 1980, 499.
(9) (a) Wentrup, C. Chem. Commun. 1969, 1386. (b) Crow, W. D.;
Wentrup, C. Tetrahedron Lett. 1968, 6149.
imine to be stable enough to be detected in an FVT
experiment without recourse to matrix isolation photo-
chemistry. In other words, this is the first direct proof
(10) Kemnitz, C. R.; Karney, W. L.; Borden, W. T. J. Am. Chem.
Soc. 1998, 120, 3499.
(11) Wentrup, C. Helv. Chim. Acta 1978, 61, 1755.
J. Org. Chem, Vol. 70, No. 20, 2005 7949

Kvaskoff et al.
FVT conditions. The presence of a small amount of
9-aminophenanthrene is important evidence for the
formation of nitrene 9. When this reaction was carried
out at 600 °C using the internal oven with product
isolation at 20 K for IR spectroscopy, 9-cyanofluorene 12
(2262, 742 cm-1), 4-cyanofluorene (2235 cm-1), and the
cyclic ketenimine 24 (1932 cm-1; observable until 200 K
on warm up without Ar) were recorded (the IR spectrum
is shown as Figure S1 in the Supporting Information).
GC/MS of the thermolysate also revealed the formation
of substantial amounts of triazolophenanthridine 19,
formed by loss of N2 from the tetrazole 18 and recycliza-
tion of the diazo compound 20 so produced.
Similarly, FVT of triazole 19 at 600-750 °C followed
by GC/MS analysis afforded 9-cyanofluorene 12, 6-me-
thylphenanthridine, and small amounts of 1- and 4-cy-
anofluorenes and 9-aminophenanthrene. FVT at 550 °C
using the internal oven and Ar matrix isolation of the
products at 12 K allowed the detection of the cyclic
ketenimine 24 (1932 cm-1) and 9-cyanofluorene 12 (2262,
742 cm-1) by IR spectroscopy.
Photolysis. The matrix photolysis of azide 8 was quite
different. The ketenimine 24 was not observed at all.
Instead, brief photolysis of 8 (Ar, 7 K, 313 nm, 30 s)
resulted in a new species absorbing at 1742 cm-1. In the
course of further photolysis another new species rapidly
grew in at 1702 cm-1, and this was the main species after
14 h, i.e., the time required to fully destroy the azide.
Photolysis of the 1702 cm-1 species at λ > 405 nm
regenerated the 1742 cm-1 species, which could again be
converted to the 1702 cm-1 species at 313 nm. It was
possible to cycle between these two species several times.
Suitable spectral subtraction generated the spectra of the
individual compounds, and the good agreement with the
DFT-calculated spectra identify them as azirine 26 (νCd
FIGURE 2. (a) Calculated IR spectrum of ylidic cumulene
27 (B3LYP/6-31G* with a scaling factor of 0.9613 for wave-
numbers). (b) Difference IR spectrum after photolysis at λ )
313 nm (monochromator) showing the formation of cumulene
27 and disappearance of azirine 26. (c) Difference IR spectrum
after photolysis at λ > 405 nm showing the formation of azirine
26 and vanishing of ylidic cumulene 27. Spectra b and c are
mirror images and show reversible interconversion of 26 and
27. (d) Calculated IR spectrum of azirine 26 (B3LYP/6-31G*
level with a scaling factor of 0.9613 for wavenumbers). The
abscissa is in wavenumbers.
) 1742 cm-1) and ylidic cumulene 27 (1702 cm-1),
N
respectively (Scheme 2 and Figure 2). The corresponding
azirine 4b derived from 1-naphthylnitrene 3 (eq 2; X )
CH) has νCdN ∼ 1730 cm-1, and the ylidic cumulene 6
absorbs at ca. 1700 cm-1.3 The analogous azirine and
ylidic cumulene from 2-naphthylnitrene absorb at ca.
1725-1730 and 1680 cm-1, respectively.3 The azirine 29
The tetrazole 18 was photochemically inert, and the
triazole 19 photolyzed extremely sluggishly. Such behav-
ior has been observed for other triazoloazines.6a A diazo
compound (20) was observed at 2080-2097 cm-1 on
photolysis at 222 nm, but the intensities of subsequent
photoproducts in the matrix IR spectrum were too weak
for a clear-cut demonstration of the rearrangement of
carbene 21 to nitrene 9 (via 25, 24, and 23, Scheme 2).
However, this rearrangement was revealed by ESR
spectroscopy, which is more sensitive than IR.
ESR Spectroscopy. Photolysis of triazole 19 in an Ar
matrix at 15 K using a 308 nm lamp afforded the carbene
21 (Figure 4a; D ) 0.5161 cm-1; E ) 0.0257 cm-1). In
contrast, FVT of either tetrazole 18 or triazole 19 at 550
°C with Ar matrix isolation at 15 K afforded the ESR
spectrum of the nitrene 9 (Figure 4b and Figures S2 and
S4 in the Supporting Information; D ) 0.8110 cm-1; E e
0.0023 cm-1; averages of several measurements) (the
carbene was not detectable under FVT conditions). The
thermolysate from the tetrazole was recovered from the
cryostat, and examination by GC/MS revealed triazol-
ophenanthridine 19, 9-cyanofluorene 12, 4-cyanofluorene
formed by cyclization of 3-isoquinolylnitrene has νCdN
)
1725 cm-1 in an Ar matrix.4c
As explained above, the o-quinoid ketenimine ”bond-
shift” isomer 28 (Scheme 2) is not a calculated energy
minimum, but it would be expected to absorb in the 1800
cm-1 range in the IR spectrum.2,3
Similar matrix photolysis with UV-vis detection re-
vealed a yellow intermediate with an absorption maxi-
mum at 450 nm, which is assigned to the ylidic cumulene
27 (Figure 3). This absorption disappeared again on
bleaching at λ > 405 nm. The initial UV spectrum shows
fine structure in the 325-360 nm region due to azide 8.
New, weak, fine structure grows in on photolysis, and
by analogy with other arylnitrenes,3 this may be due to
the presence of nitrene 9T. More substantial evidence
for the nitrene is given by the ESR spectrum below.
7950 J. Org. Chem., Vol. 70, No. 20, 2005

Photolysis and Thermolysis of Nitrenes and Carbenes
FIGURE 3. UV-vis spectra resulting from the photolysis of 9-azidophenanthrene 8 at λ ) 313 nm showing formation of ylide
27 with an absorption maximum at 450 nm. The heavy bars represent the calculated optical spectrum of ylide 27. The open bar
is for azirine 26 (time-dependent B3LYP/6-31G*). The abscissa is in nanometers; the ordinate is in arbitrary intensity units.
Photolysis of azide 8 in an Ar matrix at 15 K at 308
nm also afforded the ESR spectrum of nitrene 9, and
thermolysis of this azide in a stream of Ar at 550 °C using
the internal oven and condensation of the product to form
an Ar matrix at 15 K afforded the ESR spectrum of
nitrene 9 (Figure S3 in the Supporting Information).
The zero-field splitting parameters D for both the
nitrene 9T and the carbene 21T are in very good
agreement with expectations based on correlations with
natural (or Mulliken) spin densities calculated with the
UB3LYP/EPR-II program within Gaussian 03. Smolin-
sky et al. were the first to report a correlation between
D values and HMO spin densities in arylnitrenes.12 We
have found excellent linear correlations between the
measured values of D (cm-1) and the calculated natural
and Mulliken spin densities on the nitrene nitrogen (F)
in a wide range of aryl- and heteroarylnitrenes, and
similarly between D and the spin density on the carbene
carbon in aryl- and heteroarylcarbenes.13
Calculations. It is now well established that for
arylnitrenes (unlike the corresponding carbenes) the
open-shell singlet (OSS) states have lower energies than
the closed-shell singlets (CSS).14 It is not possible to
calculate the open-shell singlet nitrene 9 OSS using DFT
(12) Smolinsky, G.; Snyder. L. C.; Wasserman, E. Rev. Mod. Phys.
1963, 35, 576.
(13) (a) Bednarek, P.; Kvaskoff, D.; Wentrup, C. The University of
Queensland, Brisbane, Australia. Unpublished results, 2004-2005. (b)
Kvaskoff, D. Ph.D. Thesis, The University of Queensland, Brisbane,
Australia, 2005. (c) Selected examples of (D,F) values for several
nitrenes: 2-pyrimidyl (1.217, 1.703), 4-pyridyl (1.107, 1.6312), 3-py-
ridazinyl (1.066, 1.6374), 2-pyridyl (1.051, 1.6393), phenyl (0.998,
1.5993), 2-naphthyl (0.925, 1.5605), 9-phenanthryl 9T (0.811, 1.4567),
1-naphthyl (0.793, 1.4891), 1-anthryl (0.65, 1.3919), 9-anthryl (0.470,
1.2837). Selected examples of (D,F) values for several carbenes:
2-pyrimidyl (0.565, 1.7373), 4-pyrimidyl (0.555, 1.7312 (Z)), 3-pyridazi-
nyl (0.544, 1.7066 (E), 1.7180 (Z)), 2-pyridyl (0.537, 1.7000 (E), 1.7230
(Z)), 4-quinazolyl (0.5253, 1.6737 (Z)), phenyl (0.517, 1.6841), 6-phenan-
thridyl 21T (0.5161, 1.6575 (E), 1.6581 (Z)), (E)-2-naphthyl (0.4926,
1.6538), (E)-1-naphthyl (0.4347, 1.5973), 9-anthryl (0.3008, 1.4592).
(14) Karney. W. L.; Borden, W. T. J. Am. Chem. Soc. 1997, 119,
1378.
FIGURE 4. (a) ESR spectrum of carbene 21 obtained after a
45 min photolysis of 19 at 308 nm. (b) ESR spectrum of nitrene
9 obtained by FVT of 19 at 550 °C.
13, 1-cyanofluorene 16, 6-methylphenanthridine 22, and
9-aminophenanthrene 10.
J. Org. Chem, Vol. 70, No. 20, 2005 7951

Kvaskoff et al.
FIGURE 5. Potential energy diagram for phenanthrylnitrene 9, its rearrangement products, and the corresponding transition
states. The energies are in kcal/mol relative to 9 OSS. The numbers in italics are CASPT2/CASSCF(8,8) calculations. All other
energies are normalized to triplet 3A′′ phenanthrylnitrene 9T + S-T gap from the CASPT2 calculation (15.8 kcal/mol); the roman
numbers are B3LYP/6-31G* except TS5, which is from a UB3LYP/6-31G* calculation; numbers in brackets are estimates, see
text. OSS ) open-shell (1A′′) singlet; CSS ) closed-shell singlet.
methods. An approximation described by Cramer15 was
used to estimate the energy of 9 OSS at the B3LYP/6-
31G* level in Figure 5. This places 9 OSS 19.2 kcal/mol
below 9 CSS and 12.5 kcal/mol above 9T. A CASPT2/
CASSCF(8,8) calculation with the ANO basis set places
9 OSS 20.1 kcal/mol below 9 CSS and 15.8 kcal/mol
above 9T (Figure 5). We have used the latter value for
the singlet-triplet (S-T) gap in 9. To calculate the
transition states connecting 9 OSS with the interesting
products 12, 24, and 26, CASSCF calculations would be
required, but this would be beyond our current computing
resources. DFT calculations of the transition states TS1
(27 kcal/mol) and TS4 (32 kcal/mol on the energy scale
of Figure 5) connecting 12 and 24 with the closed-shell
singlet, 9 CSS, have been performed, but these energy
values can be expected to be too high. In the case of the
open-shell naphthylnitrenes, it was found that UB3LYP
calculations gave transition state energies for the cy-
clization to azirines which were in good agreement with
CASPT2 calculations.3 The CASPT2//CASSCF(12,12)/6-
31G* calculations on the open-shell 1- and 2-naphthylni-
trenes gave barriers of 3-7 kcal/mol for cyclization to
azirines analogous to 26, e.g., 4b (eq 2).3 A UB3LYP/6-
31G* calculation of TS5 connecting 9 OSS and 26 gives
a value of 2.5 kcal/mol (Figure 5) with a spin contamina-
tion factor <S2> of 0.26. Thus, estimates based on the
naphthylnitrenes seem justified. For comparison, activa-
tion barriers of 6-9 kcal/mol were found for cyclizations
of 2-biphenylnitrene to azirines at CASPT2/CASSCF and
CASPT2/DFT levels of theory16 and 4-8 kcal/mol for
cyclization of 2,6-dialkylphenylnitrenes17 and the parent
phenylnitrene at the DFT/CASPT2 level. Cyclization to
nonaromatic azirines such as 4a (eq 2) and 23 (Scheme
2) have much higher activation energies due to the loss
of the resonance energy of a second benzene ring. On the
basis of the comparison with the naphthylnitrenes,2,3 TS1
connecting 9 OSS and ketenimine 24 is estimated as ca.
20 kcal/mol (Figure 5). This nonaromatic transition state
TS1 has the structure of azirine 23 in Scheme 2, and
this azirine is not found to be a minimum at the DFT
level. Since CASPT2 calculations may overestimate the
stability of open-shell singlet nitrenes relative to azir-
ines,3,14 the true energies of TS1 and TS5 may be a few
kcal/mol lower. This would make the cyclization of 9 OSS
to azirine 26 virtually barrierless.
A proper description of the ring contraction of nitrene
9 to 9-cyanofluorene 12 via TS4 again requires CASSCF
calculations. We have investigated the analogous ring
contraction in phenylnitrene 1 OSS using CASPT2//
CASSCF(8,8) calculations and found a transition state
for the concerted ring contraction to 5-cyanocyclopenta-
diene 30 with a barrier of 30.9 kcal/mol (eq 4).18 Assuming
a similar barrier for phenanthrylnitrene 9 OSS, this
places TS4 at ca. 31 kcal/mol (Figure 5). Ring contraction
to 5-cyanocyclopentadiene can also take place from the
seven-membered ring ketenimine 2, albeit with a drasti-
cally higher calculated activation energy (68 kcal/mol; eq
(15) Johnson, W. T. G.; Sullivan, M. B.; Cramer, C. J. Int. J.
Quantum Chem. 2001, 85, 492.
(16) Tsao, M.-L.; Gritsan, N.; James, T. R.; Platz, M. S.; Hrovat, D.
A.; Borden, W. T. J. Am. Chem. Soc. 2003, 125, 9343.
(17) Tsao, M.-L.; Platz, M. S. J. Am. Chem. Soc. 2003, 125, 12014.
7952 J. Org. Chem., Vol. 70, No. 20, 2005

Photolysis and Thermolysis of Nitrenes and Carbenes
4). Thus, starting from the carbene side, 21, it is possible
that ketenimine 24 may undergo some direct ring con-
traction to 9-cyanofluorene 12. The diradical 14 could
form by ring opening of either 9 or 24, and it would also
be able to ring close to 12. The triplet state 14T is
calculated to be 44.2 kcal/mol above 9T (28.4 kcal/mol
above 9 OSS), and 14 CSS is 55.9 kcal/mol above 9 OSS
(Figure 5). The energy of 14 OSS cannot be calculated
directly using DFT methods, but its energy is estimated
as ca. 45 kcal/mol using Cramer’s method,15 whereby it
is necessary to constrain the molecule to planarity.
The formation of 4-cyanofluorene 13 can take place via
ring opening to the singlet 2-(2-cyanophenyl)phenylcar-
bene 15S, which in its s-trans conformation (43 degrees
twist between phenyl rings) is a minimum at ca. 37.3
kcal/mol above 9 OSS (the s-cis conformation is not a
minimum; it ring closes to 13). TS4 lies significantly
below 15S in agreement with the observation that 12 is
the major thermal product even though 13 has a lower
absolute energy.
TS2 (37.3 kcal/mol) connects the closed-shell singlet
phenanthridylcarbene 21S with cyclic ketenimine 24 and
has a structure corresponding to the nonaromatic and
nonobserved cyclopropene 25 (Scheme 2). The ketenimine
24 is calculated to be the global minimum among the
isomeric singlet species in Scheme 2 prior to ring
contraction to cyanofluorenes. This molecule (24) pre-
serves two aromatic rings and has no charge separation.
TS3 (26.9 kcal/mol) connects azirine 26 and the ylidic
cumulene 27. The azirine 26 is calculated to be 11.4 kcal/
mol lower in energy than the ylide 27 (Figure 5). The
o-quinoid ketenimine 28 (Scheme 2) was not observed in
any of the experiments (it would be expected to absorb
in the 1800 cm-1 range in the IR spectrum; see above2).
It is not an energy minimum at the DFT computational
level, but at the Hartree-Fock level it is ca. 10 kcal/mol
less stable than 27 and thus not energetically competitive
with the observed cumulenes 24 and 27.
FVT conditions it is likely that all the intermediates, 24,
26, and 27 can form, but thermochemistry will favor the
former. Although the barrier for ring contraction of
9-phenanthrylnitrene 9 to 9-cyanofluorene 12 is higher
than that for ring expansion to ketenimine 24, 12 is the
thermodynamic sink, and it cannot be avoided that it is
formed under FVT conditions.
6-Phenanthridylcarbene 21 is observed by ESR spec-
troscopy when photochemically generated (D ) 0.5161
cm-1; E ) 0.0257 cm-1), and it rearranges thermally to
9-phenanthrylnitene 9, which is also observed by ESR
spectroscopy (D ) 0.8110 cm-1; E e 0.0023 cm-1); thermal
rearrangement to ketenimine 24 is observed by IR
spectroscopy, and 9-cyanofluorene 12 is a major product
of preparative FVT.
Computational Methods
The energies of the relevant species isomeric with phenan-
thrylnitrene were calculated at the B3LYP/6-31G* level of
theory using the Gaussian 98 suite of programs.19 The ener-
gies, together with those of the triplet nitrene (9T), the closed-
shell singlet nitrene (9 CSS), and the open-shell singlet nitrene
(9 OSS) are depicted in Figure 5. The energy of the open-shell
singlet nitrene was estimated using the method of Cramer.15
The energies of the open-shell singlet and triplet nitrenes were
also calculated at the CASPT2 level using the MOLCAS20 suite
of programs. According to CASPT2/CASSCF(8,8) with the ANO
basis set, the triplet is 15.8 kcal/mol more stable than the open-
shell singlet. The active space for CASSCF and CASPT2
calculations was constructed from 8 orbitals, 3 π(A′′), 1 p(σ)-
(A′) on N, and 4 π*(A′′), with 8 electrons. All reference weights
were above 0.6, which is assumed as enough reference weight
for the CASSCF wave function in CASPT2 for this size of
molecule.
The geometry was optimized with CASSCF(8,8)/ANO, and
the single-point CASPT2 energy was obtained for this geom-
etry (the description of states is included in the Supporting
Information).
The UV-vis spectra were predicted using density functional
theory based on time-dependent (TD) response theory.21 We
used the TD-DFT implementation in Gaussian 98 as described
recently by Stratmann et al.22
Under FVT conditions, all the (singlet) species can be
expected to be in thermal equilibrium, including 24, 26,
and 27 (and perhaps even 28), but the thermodynami-
cally preferred and observable intermediate 24 will
dominate prior to ring contraction to the cyanofluorenes
12 and 13. The latter are thermodynamic sinks, and
under preparative FVT conditions they become the
exclusive rearrangement products.
Mulliken and natural spin densities in nitrenes and car-
benes were calculated using UB3LYP/EPR-II within Gaussian
03, revision C.01. Zero-field splitting parameters of nitrenes
and carbenes were calculated using Wasserman’s equations.23
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.:
Pittsburgh, PA, 1998.
Conclusion
Both 9-phenanthrylnitrene 9 and 6-phenanthridylcar-
bene 21 undergo ring expansion in the gas phase under
thermal conditions to give dibenzoazacycloheptatetraene
24 (νmax 1932 cm-1). In sharp contrast, under matrix
photochemical conditions, the nitrene cyclizes to the
azirine 26 (νCdN 1742 cm-1), which can be photochemi-
cally interconverted with the novel ylidic cumulene 27
(νmax 1702 cm-1, λmax 450 nm). DFT calculations indicate
that 24 is the product of thermodynamic control. Under
(20) Andersson, K.; Blomberg, M. R. A.; Fulscher, M. P.; Kell, V.;
Lindh, R.; Malmqvist, P.-A¨ .; Noga, J.; Olson, J.; Roos, B. O.; Sadlej,
A,; Siegbahn, P. E. M.; Urban, M.; Widmark, P.-O. MOLCAS, version
4; University of Lund: Sweden, 1998.
(18) Bednarek, P.; George, L.; Wentrup, C. To be published. The
calculated activation barriers for the 1,5-H shift converting 5-cyano-
cyclpentadiene to 1-cyanocyclopentadiene is 22.6 kcal/mol and that for
conversion of 1-cyanocyclopentadiene to 2-cyanocyclopentadiene is 17.8
kcal/mol. The calculated energies of 5-, 1-, and 2-cyanocyclopentadienes
are -40.3, -49.6, and -47.7 kcal/mol, respectively, relative to 1 OSS
(B3LYP/6-31G*).
(21) Casida, M. E. In Recent Advances in Density Functional
Methods, Part I; Chong, D. P., Ed; World Scientific: Singapore, 1995;
p. 155.
(22) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218.
(23) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys.
1964, 41, 1763-1772.
J. Org. Chem, Vol. 70, No. 20, 2005 7953

Kvaskoff et al.
Fluorene-1-carbonitrile 16. This was prepared analo-
gously to 13 from 210 mg (1 mmol) of fluorene-1-carboxylic
acid with chlorosulfonyl isocyanate to yield 121 mg (63%); mp
92 °C (literature value,31 94-94.5 °C); GC/MS Rt 10.97 min;
m/z 191.
9-Azidophenanthrene 8. This was initially prepared by
lithiation of 9-bromophenanthrene in ether with butyllithium
in hexane at reflux followed by addition to tosyl azide in ether
at 0 °C by adaptation of literature procedures.32 However, this
procedure affords as a byproduct a butylated azidophenan-
threne which, if not removed completely, gives rise to some
butylated cyanofluorene on FVT (detected by GC/MS and IR
but not examined further).
Experimental Section
The apparatus and procedures for preparative FVT,24 for
isolation at 77 K,24 and for Ar matrix isolation25 were as
previously described. The internal oven employed a 10 cm long,
0.7 cm i.d., electrically heated quartz tube suspended in a
vacuum chamber directly flanged to the cryostat cold head,
with a wall-free flight path of ca. 3 cm between the exit of the
quartz tube and the cold target (KBr or CsI for IR spectroscopy,
quartz for UV spectroscopy, and Cu for ESR spectroscopy). The
external oven consisted of a 20 cm (0.7 cm i.d.) quartz tube
ending in a quartz flange directly flanged to the cryostat cold
head; this tube was heated on a 10 cm length and had a ca. 5
cm unheated length connecting it to the cold head. For matrix
isolation and FVT/matrix isolation, ca. 10 mg of the compounds
were sublimed in a stream or Ar at 60-100 °C (azide 8), 70-
250 °C (tetrazole 18), 90-110 °C (triazole 19), and 120 °C (9-
cyanofluorene 12). FVT products were isolated in liquid
nitrogen (77 K) in the preparative thermolyses, at 12-20 K
with Ar for matrix isolation IR and UV experiments, and at
15 K with Ar for ESR experiments. Photolysis experiments
were carried out in Ar matrixes at 7 K for 8 and at 25 K for
19. A 1000 W high-pressure Xe/Hg lamp equipped with a
monochromator and appropriate filters, a 75 W low-pressure
Hg lamp (254 nm), and excimer lamps operating at 222 nm
(25 mW/cm2) and 308 nm (50 mW/cm2) were used for the
photolyses. IR spectra were recorded with a resolution of 1
cm-1. GC for GC/MS analysis was performed on a 30 m
capillary column, with an injector port temperature of 200 °C
and a temperature program of 100 °C for 2 min, then 16 °C/
min to 270 °C. 9-Aminophenanthrene 10 and 6-methylphenan-
thridine 22 were commercial samples. 9,9′-Azophenenthrene
11 was prepared according to Schmidt and Strobel26 and had
a mp of 270-275 °C (literature value,26 270 °C (dec without
melting completely)), and 6-phenanthridinecarbonitrile 17 was
prepared in 85% yield from the N-oxide according to the
method of Fife27a and had a mp of 136 °C (literature value,27b
135 °C). Melting points are uncorrected.
Pure 9-azidophenanthrene was prepared as follows: 9-ami-
nophenanthrene (0.525 g, 2.7 mmol) was dissolved in a warm
(60 °C) solution of concentrated sulfuric acid (3 mL) in water
(18 mL) and diazotized with sodium nitrite (0.225 g, 3.3 mmol)
in water (12 mL) at 0 °C. After stirring for 30 min, active
charcoal (0.3 g) was added, and the mixture was stirred for
another 10 min at 0 °C. The cold yellow solution of diazonium
salt was filtered on Celite by suction. Sodium azide (0.300 g,
4.6 mmol) in water (10 mL) was added dropwise to the filtrate
at 0 °C. After 3 h, the precipitated azide was collected and
recrystallized from methanol/water (20:1) to yield 9-azi-
dophenanthrene as white shiny needles (0.356 g, 60%); mp
112-113°C (literature value,33a 112-113 °C; literature value,33b
115-116 °C); sublimes at 80-100 °C (10-3 mbar). The sub-
stance is extremely light sensitive and should be kept in the
1
dark. H NMR (CDCl3) δ 8.65 (dd, J ) 9.3 and 1.1 Hz, 1H),
8.62-8.60 (m, 1H), 8.18 (dd, J ) 9.2 and 1.2 Hz, 1H), 7.82.-
7.80 (m, 1H), 7.70 (ddd, J ) 15.3, 8.4 and 1.4 Hz, 1H), 7.64-
7.57 (m, 3H), 7.46 (s, 1H); 13C NMR (CDCl3) δ 135.1, 131.6,
131.1, 128.2, 127.5, 127.3, 127.1, 126.7, 126.5, 126.1, 126.0,
123.1, 122.6, 113.4 ppm; IR (KBr) 2217(w), 2157(w), 2114,
2103(s), 2070(m), 1395(m), 1314(m), 1267(s), 764(m), 747(m),
725(s) cm-1; UV (CH3CN) λmax 318 (sh), 310, 262, 256 (sh), 246
nm (cf. ref 33b). Anal. Calcd for C14H9N3: C, 76.70; H, 4.14;
N, 19.16. Found: C, 76.71; H, 4.05; N, 19.18.
Fluorene-9-carbonitrile 12.6b,28 This compound was pre-
pared by literature methods; mp 152.5-153 °C (literature
value,6b,28 152-153°C); GC/MS Rt 10.94 min; m/z 191.
Fluorene-2-carbonitrile.29 This compound was prepared
by literature methods; mp 90 °C (literature value,29 92 °C);
GC/MS Rt 11.63 min; m/z 191.
6-(5-Tetrazolyl)phenanthridine 18. A mixture of 1 g (5
mmol) of 6-cyanophenanthridine, 3.25 g (50 mmol) of NaN3,
and 2.65 g (50 mmol) of NH4Cl was suspended in 50 mL of
DMF and refluxed for 6 h. The solvent vas removed in vacuo,
and the residue was taken up in 50 mL of H2O and brought to
pH 2 with concentrated HCl. The product thus precipitated
was filtered and recrystallized from EtOH to give 0.7 g (63%)
of 18 as white needles, mp 203-206 °C. 13C NMR (DMSO-d6)
δ 154.8, 144.2, 142.3, 132.8, 131.7, 129.8, 129.4, 128.8, 128.4,
127.5, 124.0, 123.3, 122.8, 122.6 ppm. Anal. Calcd for
C14H9N5: C, 68.01; H, 3.67; N, 28.32. Found: C, 67.60; H, 3.53;
N, 28.36.
[1,2,3]Triazolo[1,5-f]phenanthridine 19. Phenanthridine-
6-carboxaldehyde34 (3.0 g; 14.2.mmol) was dissolved in 250 mL
of warm ethanol (60 °C). Concentrated sulfuric acid (6 mL)
and hydrazine hydrate (36 mL) were added with stirring. The
mixture was allowed to stir at 60 °C for 3 h, cooled to room
temperature, and evaporated in vacuo. Water (50 mL) was
added, and the solution was extracted with 400 mL of CHCl3.
Drying over MgSO4, filtering, and removal of the solvent in
vacuo afforded 2.5 g (79%) of the hydrazone, mp 119-121 °C,
which was used immediately by dissolving it in 125 mL of
CHCl3 and adding 20 g of activated MnO2 in small portions
while stirring. After stirring for 3 h, the mixture was filtered
over Celite, and the filtrate was concentrated in vacuo to yield
Fluorene-4-carbonitrile 13. To fluorene-4-carboxylic acid
(50 mg; tech grade, 90%, 0.21 mmol) in dry CH2Cl2 (15 mL)
was added chlorosulfonyl isocyanate30 (23 mL, 0.26 mmol)
during 5 min at 0 °C, and the mixture was then stirred at 20
°C for 15 h. Triethylamine (34 mL, 0.25 mmol) was then added
at 0 °C over 5 min, and the resulting mixture was stirred for
3 h at room temperature. The reaction mixture was poured
into water (10 mL), and the product was extracted with CH2-
Cl2 (10 mL). The extract was dried over MgSO4, filtered, and
evaporated to yield a crude solid (66 mg). This was purified
by flash silica gel chromatography (eluent CH2Cl2) to afford a
white solid (30 mg, 73%; or 95% based on recovered fluorene-
4-carboxylic acid (10 mg, 22%)); mp 77 °C (literature value,31
77-78 °C); GC/MS Rt 11.18 min; m/z 191, 163, 95, 81; 1H NMR
(200 MHz, CDCl3) 3.89 (s, 2H), 7.30-7.46 (m, 3H), 7.58 (d,
1H, J ) 7.8 Hz), 7.62 (d, 1H, J ) 7.8 Hz), 7.72 (d, 1H, J ) 7.6
Hz), 8.42 (d, 1H, J ) 7.6 Hz).
(24) Wentrup, C.; Blanch, R.; Briehl, H.; Gross, G. J. Am. Chem.
Soc. 1988, 110, 1874.
(25) (a) Kuhn, A.; Plu¨g, C.; Wentrup, C. J. Am. Chem. Soc. 2000,
122, 1945. (b) Kappe, C. O.; Wong, M. W.; Wentrup, C. J. Org. Chem.
1995, 60, 1686.
(26) Schmidt, J.; Strobel, M. Ber. Dtsch. Chem. Ges. 1903, 36, 2513.
(27) (a) Fife, W. K. J. Org. Chem. 1983, 48, 1375. (b) Taylor, E. C.;
Furth, B.; Pfau, M. J. Am. Chem. Soc. 1965, 87, 1400.
(28) Vorla¨nder, D. Ber. Dtsch. Chem. Ges. 1911, 44, 2466.
(29) Ray, F. E.; Quinlin, P. J. Org. Chem. 1948, 13, 652.
(30) Cf. Vorbru¨ggen, H. Tetrahedron Lett. 1968, 1631-1634.
(31) Bachmann, W. E.; Brockway, C. E. J. Org. Chem. 1948, 13, 4.
(32) (a) Cf. Gillman, H.; Cook, T. H. J. Am. Chem. Soc. 1940, 62,
2813. (b) Smith, P. A. S.; Rowe, C. D.; Brunner, L. B. J. Org. Chem.
1969, 34, 3430. (c) Spagnolo, P.; Zanirato, P. J. Org. Chem. 1978, 43,
3539. (d) Gies, A.-E.; Pfeiffer, M.; Sirlin, C.; Spencer, J. Eur. J. Org.
Chem. 1943, 8, 1957.
(33) (a) Shudo, K.; Okamoto, T.; Chem. Pharm. Bull, 1976, 24, 1013.
(b) Denis, J. N.; Krief, A. Tetrahedron 1979, 35, 2901.
(34) Lion, C.; Boukou-Koba, J.-P.; Charvy, C. Bull. Soc. Chim. Belg.
1991, 100, 169-174.
7954 J. Org. Chem., Vol. 70, No. 20, 2005

Photolysis and Thermolysis of Nitrenes and Carbenes
1750 mg (70%) of colorless crystals of 19, mp 185-186 °C
(literature value,35 187-188 °C); sublimes at 90 °C (10-3 mbar).
Preparative FVT of 9-Azidophenanthrene 8. A 150 mg
(0.6 mmol) portion of 8 was sublimed at 80 °C and thermolyzed
at 450 °C at a vacuum of 10-4-10-3 mbar in the course of 3 h.
The products were isolated on a coldfinger cooled with liquid
nitrogen. After the end of the thermolysis, the products were
taken up in acetone and separated by chromatography on silica
gel and eluted with CHCl3 to afford phenanthrene (30%),
9-cyanofluorene 12 (40%), 9,9′-azophenanthrene 11 (10%), and
9-aminophenanthrene 10 (ca. 10%), all of which were identified
by spectroscopic comparison with authentic materials. For GC/
MS analysis similar FVT was carried out at 500 and 600 °C
at a vacuum of 10-5-10-4. The following results are derived
from uncorrected GC integrations of the products of FVT at
500 (600) °C: fluorene 0.5 (0.5)%, phenanthrene 7 (5)%,
9-cyanofluorene 12 71 (74)%; 4-cyanofluorene 13 2.7 (6.4)%,
1-cyanofluorene 16 0.1 (1.5)%, and 9-aminophenanthrene 10
11 (5)%
Preparative FVT of 9-Cyanofluorene 12. 9-Cyanofluo-
rene (60 mg) was subjected to preparative FVT at 600-1000
°C at a vacuum of 10-5 mbar, and the products were examined
by GC/MS. At 600 (800) °C 12 was recovered unchanged in
99.9 (99.0)% yield, together with a 0.1 (1.0)% yield of fluorene.
At 1000 °C the following results were obtained: fluorene, 1.2%;
9-cyanofluorene, 95.9%; 1-cyanofluorene, 2.9% (uncorrected GC
integration). 1-Cyanofluorene was isolated by sublimation (85
°C, 10-3 mbar) and recrystallization from petroleum ether
(bp 60-80 °C) and had a mp of 92-94 °C (literature value,31
94-94.5 °C), and its IR and UV spectra were identical with
those reported.31 The 2-, 3- and 4-cyanofluorenes were not
detectable in this experiment.
Preparative FVT of 6-(5-Tetrazolyl)phenanthridine
18. A 0.1 g (0.4 mmol) portion of 13 was sublimed at 180 °C
and thermolyzed at 600 °C/10-3 mbar in the course of 5 h. The
product was isolated on a liquid nitrogen coldfinger. TLC of
the product indicated one major compound and only very minor
additional spots. The major component was isolated by column
chromatography (silica gel/CHCl3) and identified as 9-cyano-
fluorene 12 (52 mg; 68%). In some of the FVT reactions
6-cyanophenanthridine 17 was isolated in 3-10% yield.
For GC/MS analysis FVT was carried out at 450 (550) °C
at a vacuum of 10-6 mbar with Ar matrix isolation on a Cu
rod for ESR spectroscopy. After the end of the ESR experiment
the products was rinsed from the Cu rod with CH2Cl2 and
examined by GC/MS with the following results: fluorene 0
(0.5)%, phenanthrene 0 (1.0)%, phenanthridine 0.9 (9.0)%,
9-cyanofluorene 12 0.5 (7.6)%, 4-cyanofluorene 13 0 (4.2)%,
1-cyanofluorene 16 0 (0.5)%, 6-methylphenanthridine 22 0
(13.5)%, 6-cyanophenanthridine 17 3.4 (8.4)%, 9-aminophenan-
threne 0 (6.1)%, and triazolophenanthridine 19 80 (36)%.
Preparative FVT of triazolophenanthridine 19. For
GC/MS analysis FVT was carried out at 750 °C at a vacuum
of 10-5 mbar with the following results: fluorene 0.1%,
phenanthrene 2.3%, phenanthridine 10.8%, 9-cyanofluorene
12 40.2%, 4-cyanofluorene 13 6.3%, 1-cyanofluorene 16 12.4%,
6-methylphenanthridine 22 6.6%, and 9-aminophenanthrene
2.7%.
ESR Spectra. These were recorded on an X-band spec-
trometer. Zero-field splitting parameters D and E were evalu-
ated using Wasserman’s method.23 For the observation of
thermally produced nitrenes, 10 mg of the precursor was
subjected to FVT in the internal oven described above, in a
vacuum of ca. 10-6 mbar, and the products were codeposited
with Ar on a Cu rod (7 cm long, 1.5 mm i.d.) at 15 K.
9-Azidophenanthrene 8 was sublimed at 60 °C, codeposited
with Ar, and photolyzed at 308 nm for 2 min at 15 K using
the excimer lamp. The ESR spectrum of the nitrene 9 was
recorded at a microwave frequency of 9.728230 GHz (H0 3471.4
G). X2 ) 6436.5 G, Y2 ) 6543.4 G. D ) 0.8098 cm-1; E e 0.0023
cm-1
.
9-Azidophenanthrene 8 was sublimed at 60 °C and ther-
molyzed at 550 °C. The ESR spectrum of the nitrene 9 was
recorded at a microwave frequency of 9.728229 GHz (H0 3471.3
G). X2 ) 6461.0 G, Y2 ) 6551.5 G. D ) 0.8108 cm-1; E e 0.0020
cm-1
.
Tetrazolylphenanthridine 18 was sublimed over the tem-
perature interval 70-250 °C and thermolyzed at 550 °C. The
ESR spectrum of the nitrene 9 was recorded at a microwave
frequency of 9.727944 GHz (H0 3471.2 G). X2 ) 6434.1 G, Y2
) 6541.1 G. D ) 0.8093 cm-1; E e 0.0024 cm-1
.
Triazolophenanthridine 19 was sublimed through the in-
ternal oven at 110 °C, codeposited with Ar, and photolyzed at
308 nm using the excimer lamp. The ESR spectrum of carbene
21 was recorded at a microwave frequency of 9.728155 GHz
(H0 3471.2 G). Resonance fields: Z1 ) 2035 G, X2 ) 5043 G,
Y2 ) 6100 G, Z2 ) 8986 G. D ) 0.5161 cm-1; E ) 0.0257 cm-1
(Figure 4a).
Triazolophenanthridine 19 was sublimed at 110 °C and
thermolyzed at 550 °C. The ESR spectrum of the nitrene 9
was recorded at a microwave frequency of 9.728211 GHz (H0
3471.3 G). X2 ) 6434.4 G, Y2 ) 6538.9 G. D ) 0.8090 cm-1; E
e 0.0023 cm-1 (Figure 4b).
Acknowledgment. This work was supported by the
Australian Research Council, the APAC national su-
percomputing facility (merit allocation scheme), and the
Centre for Computational Molecular Science at The
University of Queensland. We thank Dr. Michael Voss-
winkel for early assistance with the calculations of
structures, energies, and IR spectra of some of the
intermediates at the B3LYP/6-31G* and B3LYP/6-
31+G* levels of theory and Adelheid Lukosch for some
preliminary experiments.
Supporting Information Available: IR spectra of the
FVT products from 18; ESR spectra of nitrene 9T formed from
8, 18, and 19; Cartesian coordinates, absolute energies,
frequencies, and structures of compounds and transition states
calculated at the (U)B3LYP/6-31G* level of theory (for nitrene
9, also at the CASPT2/CASSCF(8,8) level); time-dependent
B3LYP/6-31G* calculations of UV-vis spectra; UB3LYP/EPR-
II optimized geometries and spin densities for nitrene 9T and
carbene 21T. This material is available free of charge via the
Internet at http://pubs.acs.org.
(35) Abramovitch, R. A.; Takaya, T. J. Org. Chem. 1972, 37, 2022-
2029.
JO050898G
J. Org. Chem, Vol. 70, No. 20, 2005 7955