Pyrolysis and UV photoelectron spectroscopy of bicyclo[3.2.0]hept-6-en-2-one; preparation and detection of cyclohepta-2(Z),4(E)-dien-1-one

Article abstract of DOI:10.1039/b111602a

Flash vacuum pyrolysis of bicyclo[3.2.0]hept-6-en-2-one (1) in the source chamber of a UV photoelectron (PE) spectrometer using a CW CO₂ laser as a directed heat source facilitated an electrocyclic ring expansion to yield the transient species

Full text of DOI:10.1039/b111602a

Pyrolysis and UV photoelectron spectroscopy of
bicyclo[3.2.0]hept-6-en-2-one; preparation and detection of
cyclohepta-2(Z),4(E)-dien-1-one
Tom Bajorek and Nick H. Werstiuk*
Department of Chemistry, McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada.
E-mail: werstiuk@mcmaster.ca; Fax: +1 (905) 522-2509; Tel: +1 (905) 525-9140
Received (in Corvallis, OR) 19th December 2001, Accepted 4th February 2002
First published as an Advance Article on the web 25th February 2002
Flash vacuum pyrolysis of bicyclo[3.2.0]hept-6-en-2-one (1)
in the source chamber of a UV photoelectron (PE) spectrom-
1 is shown in Fig. 1(d). Although the calculated and experi-
v
mental IP s differ slightly, the agreement in the features of the
eter using a CW CO
2
laser as a directed heat source
experimental and simulated spectra is excellent.
facilitated an electrocyclic ring expansion to yield the
transient species cyclohepta-2(Z),4(E)-dien-1-one (2), the PE
spectrum of which was compared to that of an authentic
sample of cyclohepta-2(Z),4(Z)-dien-1-one (4) and con-
firmed a conrotatory ring opening of 1 that obeys the
Woodward–Hoffmann rules.
The pyrolysis of 1 in the PE spectrometer allowed us to
record the PE spectrum of 2 as shown in Fig. 1(b). This thermal
reaction of 1 follows a conrotatory ring opening of the
cyclobutene moiety. A disrotatory ring opening, which is
thermally forbidden, would generate cyclohepta-2(Z),4(Z)-
1
0
dien-1-one (4). We synthesized an authentic sample of 4 and
Ultraviolet photoelectron spectroscopy in conjunction with ab
initio calculation† of molecular orbital eigenvalues and ei-
genvectors provides information about the electronic structure
and bonding of stable and transient species that is provided by
no other experimental technique. Our ultraviolet PE spectrome-
Table 1 Experimental and calculated vertical IPs (eV) of 1, 2 and 4
1
2
4
Becke3LYP
Exp.
Becke3LYP
Exp.
Becke3LYP
Exp.
IP
IP
IP
IP
IP
v
v
v
v
v
8.73
8.98
8.65
8.36
8.84
9.78
10.14
9.08
9.06
8.96
—
—
10.19
10.18
10.93
11.16
ter is interfaced with a CW CO
2
laser that has been successfully
used as a directed heat source in gas phase pyrolysis
experiments.†1
d,e,2–4
This unique experimental setup allows the
IPv
9.36
generation of transients and strained molecules directly in the
source chamber of the spectrometer. The laser is focused on the
tip of a quartz nozzle generating a 1 mm hot zone that is 10 mm
away from the ionization beam and eliminates the need for an
external electric furnace that is normally several centimeters in
length. In this way, strained molecules and transient species
may be generated and detected in real time.
The electrocyclic ring opening of cyclobutene has received
considerable attention, mostly directed at photochemical study.5
The Woodward–Hoffmann rules predict that the thermal
reaction undergoes a conrotatory ring opening while the
photochemical process exhibits a disrotatory ring opening. If
the substituents on the cyclobutene ring are another ring, that
joins the two saturated carbons of the cyclobutene, a thermal
reaction should generate a trans double bond in the newly
formed ring. In the case of bicyclo[6.2.0]deca-9-ene the ring is
large enough to accommodate a trans double bond in the
6
thermal product, cycloocta-1(Z),3(E)-diene. When the second
ring joining the two bridge heads of the cyclobutene is small,
such as in the case of bicyclo[3.2.0]hept-6-ene, the product of
the thermal ring expansion is cyclohepta-1(Z),3(Z)-diene which
indicates that this process unexpectedly appears to follow the
thermally forbidden disrotatory pathway.7 We decided to
investigate the thermal ring opening of bicyclo[3.2.0]hept-6-en-
-one (1)8 using PE spectroscopy, which in principle can yield
,9
2
dienones 2, 3, and 4.
We found that, bicyclo[3.2.0]hept-6-en-1-one (1), when
pyrolysed, undergoes a ring opening to the transient dienone,
cyclohepta-2(Z),4(E)-dien-1-one (2), and not 3 or 4. The first
and the second IP bands of 1 at 8.98 and 10.14 eV and are only
0
.25 and 0.36 eV higher in energy then the predicted
Becke3LYP values as shown in Table 1. The PE spectrum of
ketone 1 is shown in Fig. 1(a) and the simulated PE spectrum of
Fig. 1 Experimental PE spectra: (a) 1, (b) pyrolysis of 1, (c) 4. Simulated
Becke3LYP/6-31+G(d) PE spectra: (d) 1, (e) 2 and (f) 4.
6
48
CHEM. COMMUN., 2002, 648–649
This journal is © The Royal Society of Chemistry 2002

recorded its PE spectrum for comparison as shown in Fig.
(c).
Although one could expect two possible products from the
Woodward–Hoffmann rules and follows the conrotatory path.
This generated the transient dienone 2 and its photoelectron
spectrum was recorded for the first time. The product of the
pyrolysis was confirmed by recording an authentic PE spectrum
of 4 which was also recorded for the first time and has allowed
us to differentiate between the two isomers. The possibility of 3
as the product of the pyrolysis was eliminated based on
transition state calculations, which have shown that the
transition state that links 1 and 3 is 54.0 kJ mol higher in
energy than the transition state linking 1 and 2. Work is
currently underway in our laboratory to re-examine the gas
phase pyrolysis of bicyclo[3.2.0]hept-6-ene.
1
conrotatory ring opening, namely dienones 2 or 3, transition
state calculations at Becke3LYP/6-31+G(d) indicate that 2
should be the product. A transition state, which would lead to 4,
could not be located. The PE spectra of 2 and 4 are very distinct.
The photoelectron spectrum of the pyrolysate can clearly be
assigned to the cyclohepta-2(Z),4(E)-dien-1-one (2) based on
the calculated Becke3LYP IPs (Table 1) as well as the simulated
spectrum (Fig. 1e). The first and second bands in the PE
spectrum of the transient dienone 2 are separated by 0.7 eV,
while the second and third bands are separated by 1.1 eV. The
three IP bands in 2 are observed at 8.36, 9.06, and 10.18 eV
while the predicted Becke3LYP IPs are at 8.65, 9.08, and 10.19
eV. The difference between the first experimental and calcu-
2
1
Notes and references
v v
lated IP is only 0.29 eV, while the second and third IP s show
†
3
9
Becke3LYP1 calculations were performed on IBM RS/6000 model 39H,
a
a difference of 0.02 and 0.01 eV, confirming an excellent
correlation between experiment and theory. Remarkable corre-
lation is also observed between the experimental and simulated
spectra of 4, which are shown in Fig. 1(c) and 1(f). A single
large IP band is observed in the experimental as well as in the
simulated spectrum at 9 eV.
50, 350, SGI Octane and Intel dual processor PC computers with Gaussian
1b
1c
4
and Gaussian 98. Becke3LYP/6-31+G(d) optimized geometrical
structures were used in single point radical cation calculations performed at
the same level of theory and with the same bases set. The interpretation of
1
d,e
PE spectra was based on a recently developed
and successfully
applied1 routine based on Becke3LYP theory. The simulated PE spectra
of all compounds were obtained from MO results using a Fortran program
PESPEC.1 Gaussian line shape with a full width at half height (FWHH) and
a temperature set to 300 K was used to simulate the partial PE spectra with
ten eigenvalues (HOMO to HOMO-9, B3LYP/6-31+ G(d)).
f,g
At the Becke3LYP/6-31+G(d) level the conrotatory ring
h
opening of 1 has a barrier of 131.9 kJ mol21. The transition state
was confirmed by a frequency calculation, which had one
negative eigenvalue with a single negative frequency. Visual-
ization of this vibrational mode showed that the newly forming
double bond at position C4 in the product is the trans double
bond. This thermal reaction, in principle, could lead to two
different products, where both would be the result of a
conrotatory ring opening. In the first case, a transition state that
leads to 2 was found. The transition state that would link 1 and
1 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) M. J. Frisch, G. W.
Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R.
Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K.
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Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S.
Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C.
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Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B.
Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G.
Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L.
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M. Muchall, N. H. Werstiuk and B. Choudhury, Can. J. Chem., 1998,
3
was also found at the Becke3LYP/6-31+G(d) level of theory
2
1
with the energy barrier being 185.9 kJ mol . Therefore it is
clear that the PE spectrum observed upon the pyrolysis of 1 is
that of 2. The computational and experimental evidence shows
that neither 3 nor 4 can be generated in the thermal
2
1
rearrangement of 1. The difference of 54.0 kJ mol in the
activation energy between the two processes clearly indicates
that the rearrangement of 1 to 2 is favored. The calculations also
predict that 2 is 24.7 kJ mol more thermodynamically stable
than 3. The calculated total and relative energies of 1–4, radical
cations (RC) and transition states Ta and Tb are shown in Table
2
1
2
. Calculated first vertical ionization potentials were obtained
1d
by the calculational method of Muchall and Werstiuk.
Our pyrolysis experiments on 1 have shown that this bicyclic
enone undergoes a thermal ring opening that obeys the
7
6, 221; (e) H. M. Muchall, N. H. Werstiuk, B. Choudhury, J. Ma, J.
Warkentin and J. P. Pezacki, Can. J. Chem., 1998, 76, 238; (f) N. H.
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76, 1162; (g) H. M. Muchall and P. Rademacher, J. Mol. Struc., 1997,
Table 2 Becke3LYP/6-31+G(d) total and relative energies of 1–4, radical
cations (RC) and transition states Ta and Tb
4
35, 157; (h) N. H. Werstiuk, G. Timmins, J. Ma and T. A. Wildman,
Relative energy/kJ
mol
Ionization
potential/eV
Can. J. Chem., 1992, 70, 1971.
2 N. H. Werstiuk, J. Ma, C. D. Roy, A. J. Kresge and E. Jefferson, Can.
J. Chem., 1995, 73, 1738.
2
1
T
E /hartrees
1
1
Ta
2
2
3
3
Tb
4
2346.733 976
2346.413 253
2346.683 808
2346.705 326
2346.387 364
2346.695 937
2346.695 937
2346.663 182
2346.763 673
2346.763 673
0.0
8.73
3 N. H. Werstiuk, C. D. Roy and J. Ma, Can. J. Chem., 1995, 73, 146.
4 N. H. Werstiuk, C. D. Roy and J. Ma, Can. J. Chem., 1994, 72, 2537.
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9 C. G. Scouten, F. E. Barton, Jr., J. R. Burgess, P. R. Story and J. F. Garst,
Chem. Comm., 1969, 78.
–RC
—
a
131.9
75.4
8.65
8.75
–RC
—
100.1
–RC
—
a
185.9
277.9
—
8.84
4
a
–RC
Ta-transition state between 1 and 2. Tb-transition state between 1 and 3
1
0 K. E. Hine and R. F. Childs, J. Am. Chem. Soc., 1973, 95, 3289.
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