Nitrene-carbene-carbene rearrangement. Photolysis and thermolysis of tetrazolo[5,1- a ]phthalazine with formation of 1-phthalazinylnitrene, o-cyanophenylcarbene, and phenylcyanocarbene

Article abstract of DOI:10.1021/jo402448u

1-Azidophthalazine 9A is generated in trace amount by mild FVT of tetrazolo[5,1-a]phthalazine 9T and is observable by its absorption at 2121 cm^-1 in the Ar matrix IR spectrum. Ar matrix photolysis of 9T/9A at 254 nm causes ring opening to gener

Full text of DOI:10.1021/jo402448u

Article
pubs.acs.org/joc
Nitrene-Carbene-Carbene Rearrangement. Photolysis and
Thermolysis of Tetrazolo[5,1‑a]phthalazine with Formation of
1‑Phthalazinylnitrene, o-Cyanophenylcarbene, and
Phenylcyanocarbene
Martin Høj, David Kvaskoff, and Curt Wentrup*
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia
S
* Supporting Information
ABSTRACT: 1-Azidophthalazine 9A is generated in trace amount by mild FVT of
tetrazolo[5,1-a]phthalazine 9T and is observable by its absorption at 2121 cm−1 in
the Ar matrix IR spectrum. Ar matrix photolysis of 9T/9A at 254 nm causes ring
opening to generate two conformers of (o-cyanophenyl)diazomethane 11 (2079
and 2075 cm−1), followed by (o-cyanophenyl)carbene 312, cyanocycloheptatetraene
13, and finally cyano(phenyl)carbene 314 as evaluated by IR spectroscopy. The two
3
3
carbenes 12 and 14 were observed by ESR spectroscopy (D|hc = 0.5078, E|hc =
0.0236 and D|hc = 0.6488, E|hc = 0.0195 cm−1, respectively). The rearrangement of
12 ⇄ 13 ⇄ 14 constitutes a carbene−carbene rearrangement. 1-Phthalazinylnitrene
310 is observed by means of its UV−vis spectrum in Ar matrix following FVT of 9
above 550 °C. Rearrangement to cyanophenylcarbenes also takes place on FVT of 9
as evidenced by observation of the products of ring contraction, viz., fulvenallenes and ethynylcyclopentadienes 16−18. Thus the
overall rearrangement 10 → 11 → 12 ⇄ 13 ⇄ 14 can be formulated.
azide 9A. The vast majority of the sample still exists in the
tetrazole form 9T. This is unsurprising in view of the calculated
energy difference, ΔE = 10 kcal/mol and ΔG(298 K) = 8.55
kcal/mol at the B3LYP/6-31+G** level. This would corre-
spond to only ca. 0.3% azide at equilibrium at 450 °C. In the
case of the parent compound, tetrazolo[1,5-a]pyridazine, the
calculated energy difference between azide and tetrazole is ca. 5
kcal/mol, and 1−2% of the azide valence tautomer was
observable by IR spectroscopy following FVT at 250−400
°C.1b,3 1-Azidophthalazines and 3-azidopyridazines were
previously unknown, but in the case of tetrazolo[1,5-
a]pyrimidine/2-azidopyrimidine it is known that although the
tetrazole is the stable form at room temperature, the positive
TΔS term causes increasing amounts of the azide form at
elevated temperatures.4
Photolysis of the matrix-isolated tetrazole 9T at 222 nm
causes a decrease in the IR absorptions of the tetrazole, but the
expected nitrene 10 is not observed. Instead, new bands
assigned to the s-Z and s-E conformers of diazo compound 11
develop at 2079 and 2075 cm−1 within 10 s of photolysis
(Scheme 2, Figure 1, and Figures S1−S4, Supporting
Information). The calculated absorptions of 11 are at 2122
and 2118 cm−1 (654 km/mol) at the B3LYP/6-31G** and the
absorption due to the CN group is ca. 20 times weaker (2247
cm−1, 36 km/mol) (Supporting Information). Further
photolysis of this matrix for 30 s at 222 nm causes a further
INTRODUCTION
■
The matrix photochemistry of several hetarylnitrenes has been
described recently.1 For example, 2-quinazolinylnitrene 1
undergoes ring opening to a diradical 4 and ring expansion
to a seven-membered ring carbodiimide 3 (best seen in the
phenyl derivative 3b), and the latter can be photochemically
interconverted with the isocyanophenyldiazo compound 5.2
Moreover, the formation of a second, minor nitrene was
observed by ESR spectroscopy. The D value of this nitrene was
in good agreement with the phenylnitrene derivative 6. 2-
Quinazolinylnitrene 1 and 3-phthalazinylnitrene 8 are poten-
tially connected via the intermediates 2 and 3 and the transition
state 7 as illustrated in Scheme 1, but no evidence for the
existence of 8 has been obtained previously.
During our efforts to generate phthalazinylnitrene 8a we have
discovered a novel type of nitrene-carbene-carbene rearrange-
ment and are pleased to report the results herein.
RESULTS AND DISCUSSION
■
1. Photolysis. Tetrazolo[5,1-a]phthalazine exists exclusively
in the tetrazole form 9T in the solid state and in solution. Also
sublimation of the tetrazole through a hot tube up to 250 °C
with isolation of the material in an Ar matrix at 20 K gives no
indication of the formation of the azide valence tautomer 9A in
the IR spectrum (Figure S1, Supporting Information). However
mild flash vacuum thermolysis (FVT) at 450 °C, which is just
below the temperature where decomposition of the tetrazole
starts, causes the appearance of a very weak peak at 2121 cm−1
in the Ar matrix IR spectrum, which may be ascribed to the
Received: November 3, 2013
Published: December 4, 2013
© 2013 American Chemical Society
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Scheme 1. 2-Quinazolinylnitrene and Its Isomers
Figure 1. (a) Calculated IR spectrum of triplet carbene 12 (A). (b)
Calculated IR spectrum of diazo compound 11 (B) at the B3LYP/6-
31+G** level (wavenumbers scaled by 0.9613). (c) IR difference
spectrum after 40-s photolysis of matrix isolated 9T at 222 nm.
Positive peaks A (2230, 744 cm−1) and B (2079, 2075 cm−1) are due
to the products formed. Negative bands are due to the disappearing
9T. W = water. Ordinates: absorbance in arbitrary units. A larger
version of this spectrum is available in the Supporting Information
(Figure S3).
Scheme 2. Azidophthalazine-Tetrazolophthalazine-(o-
Cyanophenyl)diazomethane-Cyanophenylcarbene-
a
Cyanocycloheptatetraene Rearrangement
Supporting Information). The intensities of the diazo peaks at
2075−2079 cm−1 decrease slightly (on irradiation >300 nm) or
increase slightly (on broadband photolysis); therefore, they
barely appear in the difference spectra shown in Figure 2.
Furthermore, new bands appear more slowly, especially at 788,
747, and 728 cm−1 (Figure 2).
The data indicate the formation of four different species: first,
diazo compound 11 (2075−2079 cm−1, Figure 1); second,
cyanophenylcarbene 12 (2230 and 744 cm−1, Figure 1 and
Supplementary Figure S4); and third, cyanocycloheptatetraene
13 (2236, 788, 747, and 728 cm−1, Figure 2). As in the case of
the parent cycloheptatetraene,5,6 the intensity of the allenic
stretching vibration of 13 is low (1802 cm−1, 13 km/mol at the
B3LYP/6-31G** level), and a very weak band near 1800 cm−1
may be due to this species. Most interestingly, the photolysis at
λ >300 nm produces three more peaks at 1383, 672, and 727
cm−1 (the latter barely resolved from the peak at 728 cm−1 due
to 13) (Figure 2d and Supplementary Figure S5), which are
indicative of the fourth product, phenylcyanocarbene 14
(Scheme 1 and Figure 2). This species (14) is also formed
on photolysis at 222 nm and on broadband photolysis, but the
best results are obtained at λ > 300 nm.
The UV−vis spectra of matrices obtained by photolysis of 9T
feature prominent vibrational progressions at 390−450 nm
(Figure 3) very similar to the spectrum of phenylcarbene (λmax
= 430 nm)6 but red-shifted by ca. 20 nm in accord with the
extra conjugation with the CN group. Cyanocarbene, H−C−
CN, features a similar vibrational progression in the range 250−
295 nm.7 The spectrum in Figure 2 may be due to either (o-
cyanophenyl)carbene 12 (s-Z or s-E) or phenylcyanocarbene
14, or a mixture of both. The longest-wavelength transitions
calculated for 12 and 14 at the TD-B3LYP/6-31+G** level are
418 and 397 nm, respectively. Thus, the agreement is best for
a
See text for the description of 14.
decrease of the bands belonging to tetrazole 9T and
development of a new nitrile band at 2230 cm−1 together
with a band at 744 cm−1, both ascribed to the carbene 12
(calculated at 2230 cm−1 (32 km/mol) and 828 cm−1 (970 km/
mol)) (Figure 1). Both 11 and 12 have a nitrile band at 2230
cm−1, but it is very weak for the diazo compound and is
essentially due to carbene 12 at this stage. Carbene 12 was also
obtained on photolysis at λ > 300 nm or by using the
broadband irradiation of a high pressure Hg/Xe lamp, but diazo
compund 11 was best observed by using the 222 nm
irradiation.
The rearranged carbene 14 described below is not predicted
to have an observable nitrile group frequency in the IR (see
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12, but it cannot be excluded that both are present, or that the
long-lived carbene 14 is the major species, especially since the
strongest absorptions are obtained on irradiation at >300 nm
(Figure 3). This is in agreement with the ESR data reported
below. Our calculated longest-wavelength transition for phenyl-
carbene at the same level is 55 nm too low. A calculated error of
−20 nm at the CASPT2 level has also been reported for the
longest-wavelength UV absorption of phenylcarbene.6c
ESR spectroscopy confirmed the formation of carbenes 12
and 14. Photolysis of the Ar matrix of 9T/9A at 20 K causes
rapid appearance of typical arylcarbene signals assigned to 12
(D|hc = 0.5078, E|hc = 0.0236). This is followed by signals
arising from the known phenylcyanocarbene8 14 (D|hc =
0.6488, E|hc = 0.0195 cm−1), which becomes the dominant
species (Scheme 2 and Figure 4).
The rearrangement of 12 to 14 constitutes a carbene−
carbene rearrangement via cyanocycloheptatetraene 13. It is
interesting to note the rather high D value of 14; phenyl-
carbenes usually have D values around 0.5, and extra
conjugation should lower, not increase the D value if it were
Figure 2. (a) Calculated spectrum of triplet carbene 14 (D). (b)
Calculated spectrum of cycloheptatetraene 13 (C) at the B3LYP/6-
31+G** level (wavenumbers scaled by 0.9613). (c) IR difference
spectrum after broadband photolysis of matrix isolated 9T for 14 min
(high pressure Hg-Xe lamp). (d) IR difference spectrum after
photolysis of matrix isolated 9T for 46 min at λ > 300 nm. Negative
peaks are due to 9T being consumed. Positive bands are ascribed to
cyanocycloheptatetraene 13 (bands marked C at 2236, 788, 747, and
728 and perhaps 1800 cm−1) or phenylcyanocarbene 14 (bands
marked D at 672, 727, and 1383 cm−1). Note very weak diazo bands at
2075−2079 cm−1, marked B, either slightly negative or slightly
positive, i.e., the diazo compound 11 is being produced and consumed
at about equal rates in these photolyses. Ordinates: absorbance in
arbitrary units. See Supplementary Figures S3−S5 for spectra of 11
and 12 and for a larger scale version of Figure 1d. See text for carbene
structures.
Figure 3. UV−vis spectra (Ar, 20 K) of the products of photolyses of
9T. The inset shows an expansion of the visible region. The bands in
the UV region are due largely to 9T. For the UV spectrum of 9T itself
see Supplementary Figure S2. Note: IR and UV−vis spectra were
obtained for the same matrices, i.e., by using CsI and KBr windows,
which absorb strongly between 200 and 250 nm. Thus the spectra are
easily saturated in this region, thereby making background subtraction
difficult.
Figure 4. ESR spectra of cyanophenylcarbenes as a function of
photolysis time at 222 nm. (a) Initial spectrum ascribed to triplet (o-
3
cyanophenyl)carbene, 12 (D|hc = 0.5078, E|hc = 0.0236), observed
within 10 s of photolysis. (b) Mixture of 312 and 314. (c)
3
Phenylcyanocarbene, 14 (D|hc = 0.6488, E|hc = 0.0195 cm−1), with
3
a small amount of 12 remaining. Ordinates: derivative intensities in
arbitrary units.
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a true carbene. The high D value suggests that triplet 14 may
have significant spin density on N as expressed in the canonical
3
3
structures 14a and 14b in Chart 1.
Chart 1. Structures of Triplet and Singlet
Phenylcyanocarbene 14
The IR spectrum calculated at the B3LYP/6-31G** level
Figure 5. UV−vis spectra (Ar matrix) after FVT of 9T at 550, 600,
and 700 °C. The inset shows an expansion of the visible region.
Absorbance in arbitrary units. See caption to Figure 3.
does not indicate the presence of a CN group in triplet
3
phenylcyanocarbene, 14, nor is one observed experimentally
(Figure 2). A shallow Ph−C−C angle of ca. 160°, and
in the visible spectra of many arylcarbenes and arylnitrenes,16
and red-shifting of the nitrene spectra relative to the carbenes
has been reported.16d,e The absorption range in Figure 5 is
similar to that of 3-quinolylnitrene.16a,c While the singlet
nitrene 110 may readily undergo ring opening to diazo
equidistant CC and CN bonds in conformity with
3
1
structure 14a are predicted. In contrast, the singlet, 14, has
a distinctly bent carbene-like structure (Chart 1) with a Ph−
C−C angle of ca. 118°, C−C single bonds, and a CN triple
bond (see Supporting Information). Phenylcyanocarbene has
been generated from different precursors in solution and
undergoes cyclopropanation of alkenes,9 but the fact that the
reaction is nonstereospecific led to the suggestion that both an
initially formed singlet and a ground state triplet might be
involved.9d Attempts to generate phenylethynylnitrene, Ph−
CC−N, led instead to chemistry apparently derived from
phenylcyanocarbene.10 The structures (carbene vs nitrene) of
cyanocarbene11 and phenylcyanocarbene12 have been subjects
of much discussion and controversy. The best experimental and
theoretical data currently agree that triplet cyanocarbene
HCCN is a quasi-linear molecule with a barrier to linearity of
the order of 200−300 cm−1, a HCC angle of 146−154°, and a
high degree of ketenimine biradical structure, H−C•C
N•.11 The ESR spectrum of this species is known, and it has a
high D value of 0.8629 cm−1 (E ≈ 0).13 The corresponding all-
carbon species triplet propargylene, H−C•CC•−H, has
been shown to have a symmetric (C2 or C2v) structure and a
zero-field D value of 0.64 in Ar, i.e., very similar to that of 14.14
Given these results, determination of the ground state, the
singlet−triplet splitting, and the structures of the singlet and the
triplet phenylcyanocarbene 14, which has been recently
described as a bent singlet arylcarbene,12 seems worthy of
further investigation.
3
compound 11 (see Schemes 2 and 4), the triplet 10 may
not have any easily available rearrangement paths; hence it can
survive the FVT experiment. The signal intensities in the visible
spectrum decrease with increasing FVT temperature (Figure 5),
as the nitrene rearranged, presumably on the singlet energy
surface, to yield the final products described next (Scheme 3).
FVT of 9T/9A at 550−700 °C with isolation of the
thermolyzate in Ar matrices causes diminution of the peaks due
to tetrazole 9T (and disappearance of the very weak peak due
to azide 9A at 2121 cm−1) and the appearance of new peaks at
3306, 3296 (acetylenic C−H), 2236 (CN), 1966 and 1949
cm−1 (allenic CCC) (Supplementary Figures S6 and S7).
Scheme 3. Ring Contraction Products Resulting from FVT
of Tetrazolo[5,1-a]phthalazine 9T
2. Flash Vacuum Thermolysis. The UV−vis spectra of the
Ar matrix isolated pyrolyzates from the 550−700 °C FVT
reactions also feature overlapping vibrational progressions
similar to the one described for the photolysis reaction above
(λmax up to 450 nm) but red-shifted with λmax up to 470 nm
(Figure 5). The longest wavelength absorption cannot
therefore be due to carbene 14 (see calculated transitions
above), and our general experience is that arylcarbenes do not
survive FVT experiments, but nitrenes do.1,15 Therefore, the
visible spectrum (Figure 4) can also not be due to carbene 12
3
but can be ascribed to the triplet nitrene 10 and is in fact in
excellent agreement with the calculated electronic transitions
for 310 (λmax (calcd) 396, 443, and 483 nm at the TD-B3LYP/
6-31+G** level). Vibrational progressions have been observed
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These characteristics are indicative of rearrangement to the two
principal cyanofulvenallenes 15 and 16 and the two
corresponding ethynylcyclopentadienes 17 and 18 in analogy
with the rearrangement and ring contraction of phenylcarbene
itself (Scheme 3).17,18 The fulvenallenes have substantially
lower calculated energies than the ethynylcyclopentadienes (by
ca. 8 kcal/mol; see Supporting Information). It is not excluded
that other isomers may be present, as they may interconvert via
a series of sigmatropic 1,5-shifts of H and CN in the
ethynylcyclopentadienes, and the latter may be formed in
wall-induced tautomerizations of the fulvenallenes under high
temperature FVT conditions.
3. Mechanism. DFT calculations of the energies of ground
states and key transition state structures were calculated at the
B3LYP/6-31+G** level of theory. The energies of the open-
shell singlet nitrene (1A″) was estimated using the Cramer−
Ziegler method.19 The data are presented in Scheme 4. The
As for the diazo compund 11, once formed, it readily loses
N2 to generate carbene 12, which is observed in the photolysis
experiments. The carbene−carbene rearrangement 12 → 13 →
14 is completely analogous to the phenylcarbene rearrange-
ment.21 The more highly conjugated phenylcyanocarbene 14 is
calculated to have the lowest energy of these three species on
both singlet and triplet surfaces (Scheme 4). Thus, singlet 14
lies17 kcal/mol below 12, but just 1 kcal/mol below 13. Triplet
14 lies 15 kcal/mol below triplet 12 and 8 kcal/mol below
singlet 13 (Scheme 4). The rearrangement to fulvenallenes and
ethynylcyclopentadienes (Scheme 4) is assumed to take place
in the same way as the ring contraction of phenylcarbene.18
The calculated activation barrier is high (50−55 kcal/mol for
phenylcarbene), and the reaction only takes place on high-
temperature FVT. The reaction is not complete, and there are
still appreciable amounts of unchanged tetrazole present in the
matrix IR spectra of the pyrolyzates (Supplementary Figures S5
and S6).
Ring expansions of 1-phthalazinylnitrene 10 to the
triazabenzocycloheptatetraenes 20 and 22 (Scheme 5) is
Scheme 4. Reaction Scheme with Energy Values at the
B3LYP/6-31+G** Level (Numbers in Normal Font) in kcal/
mol
Scheme 5. Potential but Unobserved Intermediates and
Reactions (Energy Values at the B3LYP/6-31+G** Level
(Numbers in Normal Font) in kcal/mol)
unlikely. These are predicted to be relatively high energy
processes, and there is no evidence for their involvement.
Compound 20 would have provided a route to 2-
quinazolinylnitrene, but no products of this nitrene were
recorded (cf. Scheme 1). The high energy 2,3,4-triazacyclo-
heptatetraene would have to exist as the ylidic structure 22 in
order to retain an aromatic benzene ring.1,16d It would be
possible for this species to ring-open to diazo compound 11
with a transition state energy of 39 kcal/mol. In view of the
much lower energies required for the formation of diazo
compund 11 from azide 9A, tetrazole 9T, or nitrene 11
(Scheme 4), the involvement of 22 is unlikely.
It should be noted that the classical nitrene-carbene
rearrangement is usually strongly in favor of the nitrene
because nitrenes are thermodynamically lower in energy than
isomeric carbenes.22 In the present case we are not dealing with
isomeric carbenes and nitrenes, and once the system 12 ⇄ 13
⇄ 14 has been formed, the reaction becomes effectively
irreversible.
tetrazole 9T lies ca. 10 kcal/mol below the azide, thus
explaining why it is not possible to generate appreciable
quantities of 9A. Temperatures above 400 °C are required to
generate 0.3% azide at equilibrium, but azides start to
decompose at this temperature. The activation barriers for
denitrogenation of aryl and pyridyl azides are ca. 30 kcal/mol.20
A corresponding process generates the nitrene 10 from azide
9A. In addition, a direct path from the tetrazole to the diazo
compound 11 with a transition state energy only 20 kcal/mol
above 9T was found computationally. This value is lower than
1
the energy of the open-shell singlet nitrene, 10 (S1), but not
3
below the triplet, 10 (To). Nonetheless, as described above, a
detectable amount of azide is formed on mild FVT (2121
cm−1). Thermal loss of N2 from this azide then generates
enough nitrene to be detected in the UV−vis spectrum (Figure
5). Carbenes 12 and 14 will be formed at the same time, but
since they do not survive FVT experiments, they remain unseen
under these conditions.
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Commercially available tetrazolo[5,1-a]phthalazine 9 was purified
by sublimation at 140 °C in high vacuum and had mp = 215−216 °C
(lit.26 214−216 °C).
CONCLUSION
■
Tetrazolo[5,1-a]phthalazine 9T exists exclusively in the
tetrazole form in the solid state and in solution. A small
amount of the azide form 9A is obtained by mild flash vacuum
thermolysis of 9T at 450 °C and is observed by IR spectroscopy
(2121 cm−1). Irradiation of 9T in Ar matrix at 20 K initially
produces (o-cyanophenyl)diazomethane 11 (2079 and 2075
cm−1), which further eliminates nitrogen to give (o-
cyanophenyl)carbene 312, as observed by IR and ESR
spectroscopy (D/hc = 0.5078, E|hc = 0.0236 cm−1). Further
photolysis leads to cyanocycloheptatetraene 13, which is
observed by IR spectroscopy. Finally, phenylcyanocarbene 14
forms, especially on irradiation at λ > 300 nm. 314 is
characterized by IR and ESR spectroscopy (D|hc = 0.6488
cm−1, E|hc = 0.0195 cm−1). The ESR spectrum as well as the
B3LYP calculated structure indicate that triplet 14 has
significant spin density on N and is not well described as a
carbene. Both singlet and triplet 14 are of lower calculated
energy than 12 and 13.
Specific Experimental Conditions and IR Peak Listings.
Matrix Isolation of Tetrazolo[5,1-a]phthalazine 9T. Sublimation:
120−130 °C. FVT: 170 °C. Carrier gas Ar, 400 hPa. Temperature:
20−21 K. Wavenumbers (relative intensity) assigned to 9T: 3455
(0.03), 3075 (0.03), 1982 (0.02), 1952 (0.02), 1848 (0.03), 1835
(0.03), 1724 (0.04), 1590 (0.22), 1565 (0.07), 1527 (0.55), 1515
(0.27), 1478 (0.22), 1455 (0.52), 1377 (0.33), 1353 (0.39), 1321
(0.37), 1303 (0.39), 1238 (0.11), 1221 (0.27), 1112 (0.14), 1100
(0.51), 965 (0.42), 925 (0.18), 900 (0.28), 783 (0.08), 763 (1.00), 716
(0.50), 593 (0.58), 509 (0.15), 480 (0.07) cm−1.
Photolysis of Matrix Isolated 9T at 222 nm. Continued from
previous matrix isolation. Photolysis for 10 and 30 s at 222 nm.
Wavenumbers from difference spectra (relative intensity) assigned to
diazo compound 11: 2079 (0.93), 2075 (1.00). Assigned to carbene
12: 2230(0.12), 744 (0.59) cm−1.
Further Photolysis of Matrix Isolated 9T at 222 nm.
Continued from previous photolysis experiment. Photolysis for 2, 5,
and 15 min at 222 nm. Wavenumbers (relative intensity) from
difference spectra assigned to diazo compound 11: 2080 (0.29), 2075
(0.28). Assigned to carbene 12: 2230 (0.20), 744 (0.87), 554 (0.24).
Assigned to cyanocycloheptatetraene 13: 787 (0.22), 747 (1.00), 728
(0.26). Assigned to carbene 14: 672 (0.18) cm−1.
FVT of 9 with Ar matrix isolation of the product affords a
highly structured UV−vis spectrum with absorptions up to 475
nm, which is ascribed to triplet 1-phthalazinylnitrene, 10, as
3
Photolysis of Matrix Isolated 9T > 300 nm. Continued from
photolysis at 222 nm. Photolysis for 30 s and 5, 10, and 30 min.
Compound 11 has disappeared. Wavenumbers (relative intensity)
cm−1 assigned to 12: 744 (0.36), 554 (0.07). Assigned to
cyanocycloheptatetraene 13: 2236 (0.12), 1624 (0.23), 1422 (0.13),
1383 (0.37), 1164 (0.11), 921 (0.11), 788 (0.65), 747 (0.47), 742
(0.34), 728 (part of 1.00), 624 (0.17), 587 (0.13), 539 (0.21), 405
(0.17). Assigned to 14: 1383, 728 (part of 1.00), 672 (0.35) cm−1.
Broadband Photolysis of Matrix Isolated 9T. Sublimation:
120−130 °C. FVT: 200 °C. Carrier gas Ar, 120 hPa. Photolysis for 30
s, 1, 2, 5, and another 5 min. Compound 11 not identified.
Wavenumbers (relative intensity) assigned to 12: 744 (0.49), 554
(0.21). Assigned to 13: 2236 (0.31), 1624 (0.20), 1420 (0.08), 1380
(0.15), 1162 (0.23), 1106 (0.23), 921 (0.27), 788 (0.60), 746 (0.46),
728 (1.00), 718 (0.29), 672 (0.24), 624 (0.24), 540 (0.17), 405 (0.13).
Assigned to 14: 672 (0.24) cm−1.
FVT of 9T at 550 °C and Matrix Isolation. Sublimation: 120−
140 °C. FVT: 550 °C. Carrier gas Ar, 240 hPa. IR wavenumbers
(relative intensity) assigned to 15: 2236 (0.11), 1966 (0.32), 1949
(0.29), 1082 (0.10), 876 (0.42), 777 (0.28), 600 (0.27). Assigned to
16: 3306 (0.24), 1590 (0.11), 1494 (0.14), 1454 (0.10), 1446 (0.09),
1342 (0.18), 763 (0.32), 737 (0.25), 721 (0.59), 675 (0.14), 695
(0.11), 564 (0.10). 9T: 756 (1.00) cm−1.
the only reactive intermediate expected to survive the FVT
conditions. FVT of 9 causes ring contraction, presumably of
cyanocycloheptatetraene 13, to yield cyanofulvenallenes and
cyano(ethynyl)cyclopentadienes 15−18. Taken together, the
results indicate the occurrence of a nitrene-carbene-carbene
rearrangement, 10 → 11 → 12 ⇄ 13 ⇄ 14.
COMPUTATIONAL METHOD
■
Energies, vibrational frequencies, and electronic transitions
were calculated at the (TD)-B3LYP/6-31+G** level of theory
using the Gaussian 03 suite of programs.23 Reported energies
include zero-point vibrational energy corrections. The vibra-
tional frequencies were scaled by a factor 0.9613.24 For the
compounds that have lost one or two molecules of nitrogen,
the calculated absolute energy (including ZPVE) of the N2
molecule was added once or twice as appropriate. Transition
states were verified by intrinsic reaction coordinate calculations.
Energies of open shell species were calculated by the sum
method.19 Geometries, energies, and imaginary frequencies are
listed in the Supporting Information.
FVT of 9T at 600 °C and Matrix Isolation. Sublimation: 120−
140 °C. FVT: 600 °C. Carrier gas Ar, 100 hPa. IR wave numbers
(relative intensity) assigned to 15: 2236 (0.11), 1966 (0.33), 1948
(0.30), 1082 (0.11), 876 (0.45), 777 (0.22), 600 (0.31). Assigned to
16: 3306 (0.23), 3296 (0.08), 1590 (0.21), 1494 (0.15), 1457 (0.07),
1342 (0.16), 763 (0.28), 737 (0.12), 721 (0.53), 675 (0.15). 9T: 756
(1.00) cm−1.
FVT of 9T at 700 °C and Matrix Isolation. For this experiment a
tube insert filled with quartz wool was placed in the back of the oven
to prevent the compound from subliming backward. The sublimation
oven temperature was 250 °C. FVT: 700 °C. Carrier gas Ar, 180 hPa.
IR wavenumbers (relative intensity) assigned to 15: 2236 (0.13), 1966
(0.39), 1948 (0.35), 1082 (0.11), 876 (0.43), 777 (0.22), 600 (0.24).
Assigned to 16: 3306 (0.15), 3296 (0.04), 1590 (0.20), 1494 (0.13),
1342 (0.15), 763 (0.21), 721 (0.40), 675 (0.09). 9T: 756 (1.00) cm−1.
EXPERIMENTAL SECTION
■
Matrix isolation experiments employed an apparatus consisting of an
FVT oven containing a 10 cm long, 0.7 cm i.d. electrically heated
quartz tube suspended in a vacuum chamber directly flanged to the
cold head of a closed cycle liquid He cryostat with a wall-free flight
path of ca. 3 cm between the exit of the quartz tube and the cold
target. The cold target was KBr or CsI for IR and UV spectroscopy
and a Cu rod for ESR spectroscopy. The system vacuum was ca. 10−5
mbar. The precursor 9 (typically 5−10 mg) was sublimed at 120−140
°C in a stream of Ar, typically 10 hPa/min, passing the vapor through
the FVT oven before depositing the material on the cold target for IR,
UV−vis, or ESR spectroscopy, usually at 20 K, to form a matrix.
Drawings and photographs of the apparatus have been published.25
Afterward, the KBr, CSI, or Cu target was cooled to 10 K, and IR
spectra were recorded with 1 cm−1 resolution. Photolysis was carried
out using a 1000 W high pressure Hg/Xe lamp equipped with a water
filter to absorb IR radiation, monochromator, and appropriate cutoff
filters, a 75 W low pressure Hg lamp (254 nm), or an excimer lamp
operating at 222 nm (25 mW/cm2). In FVT experiments, the
tetrazole/azide was pyrolyzed at 550−700 °C, and the products were
isolated in Ar matrix at 20 K for IR or UV spectroscopy.25
ASSOCIATED CONTENT
■
S
* Supporting Information
Ar matrix UV and IR spectra. Cartesian coordinates, calculated
energies and vibrational frequencies, and where relevant
electronic transitions of intermediates and transition states.
312
dx.doi.org/10.1021/jo402448u | J. Org. Chem. 2014, 79, 307−313

The Journal of Organic Chemistry
Article
115, 855. (c) Bonvallet, P. A.; Todd, E. M.; Kim, Y. S.; McMahon, R. J.
J. Org. Chem. 2002, 67, 9031. (d) Maltsev, A.; Bally, T.; Tsao, M.-L.;
Platz, M. S.; Kuhn, A.; Vosswinkel, M.; Wentrup, C. J. Am. Chem. Soc.
2004, 126, 237. (e) Vosswinkel, M.; Kvaskoff, D.; Wentrup., C. J. Am.
Chem. Soc. 2011, 133, 5413.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wentrup@uq.edu.au.
■
(17) Schissel, P. C.; Kent, M. E.; McAdoo, D. J.; Hedaya, E. J. Am.
Chem. Soc. 1973, 92, 2147−2149.
Notes
(18) Wentrup, C.; Wentrup-Byrne, E.; Muller, P. Chem. Commun.
̈
1977, 210−212.
The authors declare no competing financial interest.
(19) Johnson, W. T. G.; Sullivan, M. B.; Cramer, C. Int. J. Quantum
Chem. 2001, 85, 492−508.
ACKNOWLEDGMENTS
■
(20) Dyall, L. K. Aust. J. Chem. 1975, 28, 2147. Dyall, L. K.; Smith, P.
A. S. Aust. J. Chem. 1990, 43, 997. Dyall, L. K.; Wong, M. W. Aust. J.
Chem. 1985, 38, 1045.
This research was supported by the Australian Research
Council, the Centre for Computational Molecular Science at
The University of Queensland, and the National Computing
Infrastructure facility supported by the Australian Government
(MAS project g01). M.H. was a study-abroad student from the
University of Copenhagen, Denmark. We thank Prof. Peter
Matyus, Semmelweis University, Budapest, for an initial sample
of tetrazole 9.
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