Thermochemistry and photochemistry of spiroketals derived from indan-2-one: Stepwise processes versus coarctate fragmentations

Article abstract of DOI:10.3762/bjoc.9.191

Coarctate reactions are defined as reactions that include atoms at which two bonds are made and two bonds are broken simultaneously. In the pursuit of the discovery of new coarctate reactions we investigate the fragmentation reactions of cyclic ketals. Three ketals with different ring sizes derived from indan-2-one were decomposed by photolysis and pyrolysis. Particularly clean is the photolysis of the indan-2-one ketal 1, which gives o-quinodimethane, carbon dioxide and ethylene. The mechanism formally corresponds to a photochemically allowed coarctate fragmentation. Pyrolysis of the five-ring ketal yields a number of products. This is in agreement with the fact that coarctate fragmentation observed upon irradiation would be thermochemically forbidden, although this exclusion principle does not hold for chelotropic reactions. In contrast, fragmentation of the seven-ring ketal 3 is thermochemically allowed and photochemically forbidden. Upon pyrolysis of 3 several products were isolated that could be explained by a coarctate fragmentation. However, the reaction is less clean and stepwise mechanisms may compete.

Full text of DOI:10.3762/bjoc.9.191

Thermochemistry and photochemistry of spiroketals
derived from indan-2-one: Stepwise processes
versus coarctate fragmentations
Götz Bucher*1,2, Gernot Heitmann3 and Rainer Herges*3
Full Research Paper
Open Access
Address:
Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum,
Beilstein J. Org. Chem. 2013, 9, 1668–1676.
1
doi:10.3762/bjoc.9.191
Universitätsstr. 150, D-44801 Bochum, Germany, 2WestCHEM,
School of Chemistry, University of Glasgow, Joseph-Black-Building,
University Avenue, Glasgow G12 8QQ, United Kingdom and
Received: 01 March 2013
Accepted: 22 July 2013
Published: 15 August 2013
3Otto-Diels-Institut für Organische Chemie, Universität Kiel,
Otto-Hahn-Platz 4, D-24098 Kiel, Germany
This article is part of the Thematic Series "New reactive intermediates in
organic chemistry".
Email:
Götz Bucher* - goebu@chem.gla.ac.uk;
Rainer Herges* - rherges@oc.uni-kiel.de
Associate Editor: M. S. Sherburn
*
Corresponding author
© 2013 Bucher et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
coarctate reaction; fragmentation; matrix isolation; photolysis;
pyrolysis; spiroketal
Abstract
Coarctate reactions are defined as reactions that include atoms at which two bonds are made and two bonds are broken simulta-
neously. In the pursuit of the discovery of new coarctate reactions we investigate the fragmentation reactions of cyclic ketals. Three
ketals with different ring sizes derived from indan-2-one were decomposed by photolysis and pyrolysis. Particularly clean is the
photolysis of the indan-2-one ketal 1, which gives o-quinodimethane, carbon dioxide and ethylene. The mechanism formally corre-
sponds to a photochemically allowed coarctate fragmentation. Pyrolysis of the five-ring ketal yields a number of products. This is in
agreement with the fact that coarctate fragmentation observed upon irradiation would be thermochemically forbidden, although this
exclusion principle does not hold for chelotropic reactions. In contrast, fragmentation of the seven-ring ketal 3 is thermochemically
allowed and photochemically forbidden. Upon pyrolysis of 3 several products were isolated that could be explained by a coarctate
fragmentation. However, the reaction is less clean and stepwise mechanisms may compete.
Introduction
Pericyclic reactions, according to the original definition, are is made and one bond is broken. However, there are a number
characterized by a cyclic array of bond making and bond of reactions that include a linear system of atoms, or at least one
breaking [1-3]. At each atom, involved in the reaction, one bond atom, at which two bonds are made and two bonds broken sim-
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Beilstein J. Org. Chem. 2013, 9, 1668–1676.
ultaneously. Nevertheless, their transition states exhibit a cyclic
overlap of basis orbitals. The orbital basis can be derived from
the orbital basis of pericyclic transition states by constriction
(
coarctation, Figure 1). Hence, these reactions have been coined
“
coarctate” reactions [4,5].
Figure 2: Stereochemistry of coarctate reactions derived from a
Hückel (top) and a Möbius band (bottom). The terminator loops T and
T’ are coplanar in the coarctate Hückel system and orthogonal in the
coarctate Möbius transition state.
Following the above principle we developed a number of novel
coarctate reactions [7-11], several of which provide synthetic
access to a broad range of heterocycles [12-21]. Synthetically
probably less useful, but suitable to check the coarctate stereo-
chemical rules, is a peculiar fragmentation reaction that we
discovered 15 years ago (Scheme 1) [9,10].
Figure 1: Formal, topological approach to derive coarctate reactions
from pericyclic reactions; p, q: number of atoms or basis orbitals in the
terminator groups T and T’; n: number of orthogonal pairs of basis
orbitals in the transition state of coarctate reactions.
The reaction proceeds spontaneously at temperatures below
−
80 °C. Quantum chemical calculations of the parent reaction
predict an activation barrier of 11.3 kcal/mol and a concerted
mechanism. This is in agreement with the stereochemical rule
Similar to pericyclic reactions [6], rules were derived to predict that a coarctate reaction with eight (4n) electrons should
their stereochemistry, and whether they would be thermochemi- proceed via a Möbius transition state, with the two terminating
cally or photochemically allowed. The atom (or the linear groups orthogonal with respect to each other. The orthogonal
system of atoms) at which two bonds are made and broken, arrangement is provided by the spiro connection of the three-
each contribute two basis orbitals to the transition state and the five-membered rings. Following these rules, a Möbius
(
Figure 1, bottom, right). A cyclic array of orbitals is attained if type coarctate fragmentation with two five-ring terminators
the linear system of orbital overlap at each end is bound by (10 electrons) should proceed as a photochemical reaction, and
terminating groups, e.g. a lone pair, or two atoms to form a a corresponding fragmentation with a seven- and a five-ring
three-ring, or four atoms to a five-ring, etc. Similar to peri- (12 electrons) should be thermochemically activated
cyclic reactions, thermochemical coarctate reactions proceed via (Scheme 2) [22].
Hückel transition states, if the number of delocalized electrons
in the transition state is 4n + 2, and they exhibit Möbius tran- To test the above hypothesis, we now investigate the thermo-
sition states with 4n electrons. If in a formal, topological trans- chemistry and the photochemistry of the ketals 1 and 3, derived
formation a closed ribbon is transformed into a coarctate band, from indan-2-one and ethylene glycol, and cis-2-butene-1,4-diol
the two loops T and T’ that are formed are coplanar. The analo- (Scheme 3). The ketal 2, derived from 1,3-propanediol, was
gous transformation of a Möbius ribbon leads to a band whose chosen as a reference system that cannot undergo a coarctate
loops are orthogonal with respect to each other (Figure 2).
fragmentation.
Scheme 1: Coarctate fragmentation of the spiroozonide derived from methylenecyclopropane.
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Scheme 2: Photochemically and thermally allowed coarctate fragmentations of spiroketals.
Scheme 3: Precursors used in this study.
Results
1467.4, 873.1, 776.2, 738.7 cm−1) [23], ethylene (ET,
Photolysis and pyrolysis of the ketals. Photolysis (λexc = ν = 1438.1, 953.8 cm−1) and indan-2-one (IN, ν = 1761.0
54 nm, Hg low-pressure lamp) of indan-2-one ethylene ketal cm−1). Some weak product bands could not be assigned. A
2
(
(
1), matrix-isolated in Ar at 10 K, leads to the formation of CO2 difference IR spectrum (product bands at a very early stage of
vs, ν = 2342.1 cm−1), o-xylylene (XY, ν = 1550.4, 1470.8, the photolysis minus precursor bands) is given in Figure 3.
Figure 3: Difference infrared spectrum, showing the changes in the IR spectrum after photolysis (λexc = 254 nm, 20 min) of 1 in Ar matrix. Except for
the water band, all IR bands pointing downwards belong to 1. Product bands pointing upwards are labelled according to their assignment (XY =
o-xylylene, ET = ethylene, IN = indan-2-one). The intense band pointing upwards at 1109 cm−1 is an artefact due to a subtraction error of a very
intense precursor band.
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Beilstein J. Org. Chem. 2013, 9, 1668–1676.
Figure 3 clearly shows that at least at this early stage of photo- [CO2] = 1.0, the concentrations of the other pyrolysis products
lysis, practically no CO (ν = 2138.4 cm−1) is formed. At a later are as follows: [CO] = 27.2, [IN] = 3.8, [ET] = 15.9,
stage of photolysis (20 h, λ = 254 nm), CO is also detected, and [BC] = 11.4, [XY] = 1.3, [AA] = 3.9 [25]. Formaldehyde is
the relative integrals of the CO2 and CO bands yield an esti- formed as a minor product only.
mated ratio of CO2 and CO of 2.5:1. It is noted, however, that
the formation of CO may well be due to photolysis of CO2 due The photochemistry of ketals 2 and 3 was investigated as well
to hard UV radiation (λ = 185 nm) also emitted by the Hg low- by matrix isolation spectroscopy. Unfortunately no product
pressure lamp used [24].
could be unambiguously identified. The FVP of 2 yielded the
product spectrum shown in Figure 5. Again, both carbon
In contrast to the very clean photochemistry of 1, flash vacuum monoxide and carbon dioxide were formed along with the
pyrolysis FVP (T = 870 °C) of 1, followed by trapping of the organic products. Carbon monoxide was formed in large excess
reaction products in solid argon, yielded a variety of products over carbon dioxide (CO/CO2 = 13.5:1). Organic products
whose identity could only partially be elucidated. The ratio of include mostly FA, ET, and IN, as well as BC and XY, but
CO2/CO being formed in the pyrolysis reaction was different many peaks have to remain unassigned. Propene was not
from the photochemical decomposition of 1. Based on the formed. Compared to the FVP of 1, the FVP of 2 yields signifi-
integrals of the CO2 and CO bands, it can be estimated as 1:27. cantly increased amounts of 2-indanone and formaldehyde.
Figure 4 shows an infrared spectrum of the pyrolysis products.
Relative to [CO2] = 1.0, the concentrations of the other pyro-
lysis products are as follows: [CO] = 13.5, [IN] = 9.1,
The organic products include formaldehyde (FA), acetaldehyde [ET] = 15.4, [BC] = 19.4, [XY] = 1.2, [FA] = 12.2.
AA), ethene (ET), o-xylylene (XY), benzocyclobutene (BC),
(
styrene (ST), and indan-2-one (IN). Some peaks could not be Flash vacuum pyrolysis of indan-2-one cis-2-butene-1,4-diol
assigned. The assignment of the pyrolysis products is based on ketal (3) again gave rise to a complex mixture of products
a comparison with literature data (XY) [21], as well as refer- (Figure 6). Among them, the two conformers of 1,3-butadiene
ence spectra of authentic samples (ET, BC, IN, AA, FA, ST). (tBD and cBD) could be assigned based on a comparison with
By calibrating IR band integrals to calculated (B3LYP/6- literature data [26]. Further products include o-xylylene (XY),
3
1G(d,p)) IR band intensities of selected bands, a crude benzocyclobutene (BC), indan-2-one (IN), and formaldehyde
measure of product ratios could be obtained. Relative to (FA). Again, a number of IR peaks have to remain unassigned.
Figure 4: Infrared spectrum obtained upon FVP of 1 at T = 1143 K and trapping the pyrolysate in solid argon at T = 10 K.
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Beilstein J. Org. Chem. 2013, 9, 1668–1676.
Figure 5: Infrared spectrum obtained upon FVP of 2 at T = 963 K and trapping the pyrolysate in solid argon at T = 10 K.
Figure 6: Infrared spectrum obtained upon FVP of 3 at T = 1043 K and trapping the pyrolysate in solid argon at T = 10 K.
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Beilstein J. Org. Chem. 2013, 9, 1668–1676.
CO2 and CO are formed in a ratio of 1:2.6 in this pyrolysis reac- ring-opening reaction to yield an ester biradical 7, which can
tion. Relative to [CO2] = 1.0, the concentrations of the pyro- either cleave into ethylene, carbon dioxide and o-xylylene
lysis products are as follows: [CO] = 2.6, [IN] = 1.0, [tBD] = (XY), or eliminate acetaldehyde (AA) to yield an acyl-benzyl
7
.4, [cBD] = 1.5, [BC] = 6.3, [XY] = 0.4.
diradical 9 [27]. The latter can then either undergo ring closure
to form indan-2-one (IN), or decarbonylate to give o-xylylene
(XY). The equilibrium of XY and benzocyclobutene (BC) is
Discussion
Flash vacuum pyrolysis of 1 yields a complex mixture, which established in the literature [28], as well as the formation of
contains a variety of fragmentation products derived from both styrene ST from BC [29]. An alternative mechanism, the
sides of the spiroketal linkage. The relative ratio of carbon chelotropic elimination of 1,3-dioxolan-2-ylidene is not likely.
dioxide and carbon monoxide being formed (CO2/CO = 1:27) This carbene has been generated from a norbornadiene spiro
indicates that a coarctate fragmentation of 1 can play a minor ketal, and it cleanly fragmented into CO2 and ethylene [30].
role only, if any. The composition of the product mixture is best Theoretical calculations support the low barrier for fragmenta-
rationalized by a series of stepwise processes, which starts with tion [31]. We explain the different reaction behaviour of our
either a C–C cleavage (pathway A, more favourable) or a C–O spiroketal 1 by the fact that two energetically unfavourable
cleavage (pathways B or C, less favourable). In principle, a products would have to be formed (a quinodimethane and a
chelotropic elimination of 1,3-dioxol-2-ylidene is also conceiv- carbene), whereas the fragmentation of the norbornadiene ketal
able (pathway D). Scheme 4 shows a possible mechanistic gives benzene and a carbene. The mechanism for the thermal
scenario. While it is questionable whether the biradical inter- decomposition of 2 is likely to be similar. The high yield of
mediates shown in Scheme 4 are in fact true minima or not, we formaldehyde in the pyrolysis of 2 is readily explained by the
note that they will be exceedingly short-lived at T = 1143 K in fact that the ester biradical 13 formed can lose one equivalent of
any event.
ethene and formaldehyde to yield the acyl-benzyl type biradical
5 (Scheme 5).
1
As the C–C bond being broken in mechanism A is significantly
weaker than the C–O bonds that need to be cleaved in mecha- In the FVP of 1 and 2, CO is formed in large excess over CO2.
nisms B and C, pathway A is expected to be the most facile This excess is far less pronounced in the FVP of 3. This could
decay mechanism for 1. The primarily formed benzyl- possibly indicate that a coarctate fragmentation of 3 (which
dialkoxymethyl biradical 4 should undergo a very facile would be a concerted version of pathway C in Scheme 3) could
Scheme 4: Possible fragmentation pathways in the FVP of 1.
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Beilstein J. Org. Chem. 2013, 9, 1668–1676.
Scheme 5: Possible fragmentation pathways in the FVP of 2.
possibly also contribute to the product distribution (Scheme 6). Conclusion
A chelotropic reaction forming 4,7-dihydro-1,3-dioxepine-2- In agreement with predictions, spiroketals derived from indan-
ylidene as discussed in the fragmentation of 5-ring spiroketal 1 2-one undergo photochemical coarctate fragmentation, if both
cannot be excluded. It is known that the sulfur analogue terminators are 5-membered rings, and thermal coarctate frag-
4
,7-dihydro-1,3-dithiepine-2-yldidene cleanly fragments into mentation, if both a 5-ring and a 7-ring terminator are present.
carbon disulfide and butadiene [32]; however, 4,7-dihydro-1,3- In the latter case, the experimental evidence suggests that the
dioxepine-2-ylidene does not give carbon dioxide and buta- thermal coarctate fragmentation competes with stepwise
diene [33].
processes.
Scheme 6: Possible fragmentation pathways in the FVP of 3.
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Beilstein J. Org. Chem. 2013, 9, 1668–1676.
Experimental
1592.5 (vw), 1572.9 (vw), 1488.3 (m), 1477.1 (w), 1464.8 (w),
General: Matrix-isolation experiments were performed using 1433.4 (w), 1425.9 (w), 1381.5 (w), 1369.9 (w), 1333.7 (m),
standard matrix-isolation techniques [34]. For sample deposi- 1305.7 (vw), 1298.5 (w), 1284.5 (vs), 1254.1 (m), 1237.4 (w),
tion, the slow-spray-on technique was used. Sample tempera- 1215.2 (w), 1170.3 (vw), 1150.9 (vs), 1135.6 (vs), 1133.0 (vs),
tures for deposition were ambient temperature (1), ca. 40 °C (2), 1117.7 (s), 1111.0 (vs), 1081.2 (w), 1045.9 (s), 1032.9 (s),
and ca. 60 °C (3). The argon used was of 99.999% purity. In 1024.0 (m), 966.3 (w), 937.6 (w), 868.8 (w), 734.8 (s), 678.9
pyrolysis experiments, the length of the pyrolysis zone was ca. (vw), 607.2 (vw), 594.3 (vw), 555.4 (vw) cm−1; EIMS m/z: M+
5
cm. Reference IR spectra of benzocyclobutene, styrene, 190 (100), 176, 161, 132 (68), 104 (80), 91, 77, 51; Anal. calcd
indan-2-one, acetaldehyde, formaldehyde, ethene and propene for C12H14O2: C, 75.8; H, 7.4; found: C, 75.3; H, 7.2.
in Ar matrices were independently measured. IR spectra were
recorded with a resolution of 0.5 cm−1. The outer matrix Indan-2-one cis-2-butene-1,4-diol ketal (3) was prepared analo-
window used for photolysis was from Suprasil quartz specified gously. Due to the limited thermal stability of 3, benzene had to
for transmission down to λ = 190 nm.
be used as solvent, and the product could not be distilled.
Instead, a sample of the solid dark brown crude product was
Ketal 1 was synthesized according to a published procedure purified by sublimation in ultra-high vacuum (10−6 mbar), using
35]. Ketals 2 and 3 were prepared analogously, starting from matrix-isolation equipment. Colourless crystals, mp 78 °C;
indan-2-one and propane-1,3-diol and cis-2-butene-1,4-diol, 1H NMR (CDCl3, 400 MHz) δ 7.16 (m, 4H), 5.73 (t, J = 1.5 Hz,
respectively. 2H), 4.31 (d, J = 1.5 Hz, 4H), 3.25 (s, 4H) ppm; 13C NMR
CDCl3, 100 MHz) δ 139.84, 129.53, 126.63, 124.64, 113.31,
Indan-2-one ethylene ketal (1): IR (Ar, 10 K) ν: 3109.2 (vw), 62.93, 44.07 ppm; IR (Ar, 10 K) ν: 3078.6 (vw), 3058.5 (vw),
[
(
3
2
2
084.4 (vw), 3057.7 (vw), 3034.8 (w), 2993.3 (w), 2989.6 (w), 3046.1 (w), 2986.4 (vw), 2966.8 (vw), 2952.9 (w), 2949.1 (w),
962.3 (w), 2928.9 (vw), 2898.5 (w), 2894.8 (w), 2878.1 (w), 2945.8 (w), 2926.2 (w), 2920.9 (w), 2912.3 (w), 2908.0 (w),
834.1 (vw), 1598.6 (vw), 1485.5 (m), 1472.5 (vw), 1465.1 2865.0 (w), 2838.8 (vw), 2718.3 (vw), 1622.9 (w), 1612.3 (vw),
(
vw), 1421.1 (vw), 1342.4 (vw), 1331.9 (m), 1305.9 (vw), 1607.9 (vw), 1592.9 (vw), 1589.4 (vw), 1573.0 (vw), 1487.9
1
1
1
7
292.9 (s), 1233.4 (m), 1222.9 (w), 1200.0 (w), 1160.9 (vw), (m), 1465.8 (w), 1449.2 (w), 1425.2 (w), 1390.2 (w), 1363.3
136.8 (m), 1110.3 (vs), 1074.2 (w), 1051.9 (vw), 1033.3 (s), (w), 1330.3 (m), 1306.4 (w), 1284.9 (s), 1227.6 (m), 1220.9
027.8 (m), 1019.7 (w), 944.4 (w), 867.0 (vw), 785.9 (vw), (w), 1201.7 (m), 1167.6 (w), 1155.8 (w), 1123.8 (vs), 1115.4
41.7 (s), 715.7 (m), 597.7 (vw), 590.5 (vw), 536.8 (vw) cm−1. (w), 1102.2 (vw), 1090.2 (s), 1078.3 (m), 1044.6 (s), 1026.9
(
m), 1009.9 (m), 951.9 (vw), 946.6 (vw), 920.8 (vw), 879.4
Synthesis of 2: Indan-2-one propane-1,3-diol ketal (2) was (vw), 872.8 (vw), 817.4 (vw), 732.5 (s), 684.8 (vw), 667.8 (vw),
prepared as described for the synthesis of 1, with the exception 641.8 (m), 619.4 (m), 617.4 (m), 595.8 (w), 559.1 (vw), 527.5
of the use of toluene rather than benzene as solvent. Indan-2- (vw) cm−1; EIMS m/z: M+ 202 (55), 176, 161, 149, 148, 147,
one (2.0 g, 0.015 mol) and 1,3-propanediol (1.4 g, 0.018 mol) 132, 104 (100), 91, 78, 54 (95), 51, 39; Anal. calcd for for
were heated under reflux in 100 mL toluene together with C13H14O2: C, 77.2; H, 7.0; found: C, 77.5; H, 7.0; HRMS–ESI
2
0 mg p-toluenesulfonic acid. The mixture was heated under (m/z): [M]+ calcd for C13H14O2Na, 225.0898; found, 225.0891.
reflux for 12 h, during which time the water formed was
distilled off as an azeotrope with toluene. The toluenic solution Acknowledgements
was then washed twice with aq NaHCO3 and once with water. Financial support by the Deutsche Forschungsgemeinschaft and
After drying over anhydrous Na2SO4, the toluene was removed EPSRC (Glasgow Centre for Physical Organic Chemistry as
on a rotary evaporator. Purification of the crude product thus part of WestCHEM) is gratefully acknowledged. The authors
obtained was achieved by distillation in high vacuum. Yield thank W. Sander for access to matrix isolation equipment.
1
4
5
.2 g (42%) after distillation. bp 93–97 °C (0.01 mbar); mp
4 °C; 1H NMR (CDCl3, 400 MHz) δ 7.14 (m, 4H), 3.97 (t, J = References
.5 Hz, 4H), 3.28 (s, 4H), 1.78 (m, 2H) ppm; 13C NMR
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4
3
2
2
2
2
2.54, 25.56 ppm; IR (Ar, 10 K) ν: 3111.7 (vw), 3084.8 (vw),
076.7 (vw), 3060.8 (vw), 3050.1 (w), 3039.8 (w), 3035.7 (w),
993.7 (w), 2983.8 (m), 2980.1 (m), 2972.7 (m), 2962.7 (m),
949.8 (m), 2943.1 (m), 2930.7 (m), 2908.2 (w), 2899.4 (w),
891.9 (w), 2884.6 (m), 2879.0 (m), 2876.1 (m), 2858.9 (m),
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