Structural manifestations of the cheletropic reaction

Article abstract of DOI:10.1039/b417538g

Examination of selected cyclopentenone derivatives which are fixed into the boat conformation reveals structural deviations from 'normal' C-C(O) and C=O bond distances consistent with the early stages of the cheletropic extrusion of carbon monoxide. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417538g

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C O M M U N I C A T I O N
Structural manifestations of the cheletropic reaction†
Goh Yit Wooi and Jonathan M. White*
School of Chemistry, The University of Melbourne, Parkville, VIC 3010, Melbourne, Australia
Received 22nd November 2004, Accepted 31st January 2005
First published as an Advance Article on the web 14th February 2005
Examination of selected cyclopentenone derivatives which
are fixed into the boat conformation reveals structural
=
deviations from ‘normal’ C–C(O) and C O bond distances
consistent with the early stages of the cheletropic extrusion
of carbon monoxide.
Scheme 2
Introduction
Structural changes which occur along a reaction coordinate
can appear in the ground state of the reactant as measurable
deviations of bond distances and angles from ‘normal values’
along the reaction coordinate and are believed to represent the
early stages of the reaction.1 This is the structure-correlation
principle and it applies when the molecule in question exists in
a ground state geometry, which is similar to the transition state
geometry for the reaction. A classic example of this principle is
provided by the series of structures of 2-oxygenated derivatives of
tetrahydropyran 1 (Scheme 1).2 The structures are characterised
by short endocyclic C–O bonds, and long exocyclic C–OR
bonds, consistent with progress along the reaction coordinate
towards the oxonium ion 2.
The cheletropic decarbonylation of cyclopentenones is a
disrotatory process,7,8 thus a ground state geometry which most
closely resembles the transition state for this reaction can be
achieved in the bicyclic fragment 7. A survey of the Cambridge
Crystallographic Database9 for molecules containing the bi-
cyclic cyclopentenone fragment 7 (which are potential substrates
for the thermal cheletropic extrusion of carbon monoxide) was
carried out. For comparison purposes the saturated bicyclic
fragment 8 which cannot undergo this reaction was also
extracted from the CSD. Only those structures with R factors
<5% and with esd values on the C–C distances between 0.001
˚
and 0.005 A were retained. This small set of structures shows
that the C–C bonds which break during the cheletropic loss of
CO in 7 are indeed longer than in the corresponding fragment
8 which cannot undergo this reaction, furthermore there is a
=
slight decrease in the C O bond which develops some triple-
bond character during the reaction.
Scheme 1
Thermal pericyclic fragmentation reactions such as the retro
Diels–Alder reaction are ideal candidates for study using the
structure-correlation principle; the precursor molecules can be
readily prepared, and readily constrained into the reactive boat
conformation. For example, in the benzoquinone cycloadduct
3 the carbon-carbon bonds which break in the retro Diels–
˚
To further examine the structural features associated with the
cheletropic reaction we prepared and determined the low tem-
perature crystal structures of the three cycloadducts 9, 10, and 11
(Scheme 3).‡ Compound 9, an adduct of the cyclopentadienone
12 and benzoquinone, undergoes a further [2+2] cycloaddition
during crystallisation, this process occurred in dull light. This
compound cannot formally undergo the cheletropic reaction
and is therefore ideal for comparison purposes. Compounds 10
and 11 can both undergo cheletropic decarbonylation, however
decarbonylation of 11 generates a new aromatic ring and has
been reported to occur cleanly at relatively low temperatures.10
Alder reaction are lengthened by ca. 0.03 A compared to the
corresponding saturated adduct 4 which cannot undergo the
retro Diels–Alder reaction.3,4
Recently, structural effects consistent with the onset of the
planar pseudopericyclic decarbonylation5 have been observed
in molecules containing the pyrroledione 5 and furandione 6
substructures.6
We were interested to determine whether the structural
changes associated with the thermal cheletropic decarbonyla-
tion of cyclopentenones7 would appear in the ground state
(Scheme 2). These effects include lengthening of bonds a and
b which are broken in the reaction and smaller effects on the
=
remaining C–C bond distances and the C O bond distance.
† Electronic supplementary information (ESI) available: Summary of
the CCD search results for fragments 7 and 8. See http://www.rsc.org/
suppdata/ob/b4/b417538g/
Scheme 3
9 7 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 9 7 2 – 9 7 4
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Fig. 1 Thermal ellipsoid plots for compounds 9–11.
Thermal ellipsoid plots for 9–11 are presented in Fig. 1 and Phencyclone 12
relevant structural parameters are presented in Table 1.
Phenanthrene-9,10-quinone (0.75 g, 3.6 mmol) was added to
1,3-diphenyl-2-propanone (0.80 g, 3.8 mmol) in dry methanol
(45 ml) in a two-necked round bottom flask fitted with a water-
cooled condenser and a dropping funnel containing a solution
of methanolic potassium hydroxide (0.2 g KOH in 0.8 ml) in dry
methanol. The reaction mixture was stirred while heating. The
methanolic potassium hydroxide solution was added dropwise
once the methanol began to reflux. The reaction was further
refluxed for 15 min. The black powder formed upon chilling was
filtered and washed with cold methanol, dried on vacuum to
Compound 10 lies on a crystallographic plane of symmetry
while 9 and 11 show good agreement between bond distances
and angles related by an approximate non-crystallographic plane
of symmetry. The mean C–C(O) bond distances in 9 (which
˚
cannot undergo cheletropic decarbonylation) are 1.530(3) A.
The corresponding distance in the reactive benzyne adduct 11
˚
is 1.559(2) A which is significantly lengthened. The mean C–
C(O) distance in the less reactive adduct 10 is 1.541(2) which is
=
intermediate between these two values. The C O bond distances
in 10 and 11 are slightly shorter than in 9, consistent with the
development of some triple bond character; it is interesting to
note that the infrared stretching frequencies (mmax) for 10 and 11,
which are 1796 and 1806 cm−1, respectively, differ significantly
from 9 (1755 cm−1), also indicating some triple bond character.
These results clearly demonstrate that the cheletropic decar-
bonylation does manifest in the ground state as measurable
deviations of bond distances along the reaction coordinate.
Furthermore a qualitative relationship between structure and
reactivity has been established.
yield the product (1.20 g, 87%), mp 226–228 ◦C; IR (KBr) 1700
1
=
(C O); H NMR (CDCl3) d 6.92 (t, 2H), 7.24 (d, 2H), 7.35–
7.43 (m, 10H), 7.53 (d, 2H), 7.78 (d, 2H);13C NMR (CDCl3)
d 123.1, 124 4, 128.1, 128.2, 128.4, 128.6, 129.0, 129.9, 131.4,
132.2, 133.5, 148.2, 200.2.
Phencyclone benzoquinone cycloadduct 9
Phencyclone (0.38 g, 1.0 mmol) in 10 ml dry benzene was added
to benzoquinone (0.16 g, 1.5 mmol) in 10 ml dry benzene.
The reaction was stirred at 80 ◦C until the reaction mixture
change from black to bright yellow. The solvent was removed
under vacuum. Recrystallisation from chloroform yielded a pale
yellow powder. (0.40 g, 82%) Recrystallisation from benzene–
ethanol gave ve◦ry fine yellowish block crystals, mp 279–282 ◦C
Experimental
General experimental procedures have been published pre-
viously.11
Table 1 Selected structural parameters for compounds 9–11
◦
(darken at 245 C and shrink at 257 C). IR (KBr) 1679, 1791
1
=
(C O); H NMR (CDCl3) d 4.60 (s, 2H), 5.75 (s, 2H), 7.08
9
10
1.194(2) A
11
1.197(2) A
(d, 2H), 7.18 (d, 2H), 7.25 (t, 2H), 7.34 (t, 2H), 7.46 (t, 2H), 7.56
(t, 2H), 7.66 (t, 2H), 8.27 (d, 2H), 8.68 (d, 2H);13C NMR (CDCl3)
d 48.1, 65.5, 123.22, 126.0, 126.3, 126.7, 127.2, 128.2, 128.3,
128.9, 131.2, 133.2, 134.0, 140.8, 141.5, 194.7, 198.1. Attempted
recrystallisation by dissolving the yellowish powder in minimum
amount of ethyl acetate, with heating and under the exposure of
sunlight, led to the formation of the fully saturated cycloadduct
˚
1.204(2) A
˚
˚
˚
˚
˚
C12–O1
C11–C12
C12–C13
C9–C13–C14
C10–C11–C15
C–C(O)–C
˚
1.533(3) A
1.541(2) A
1.557(2) A
˚
˚
1.528(3) A
1.541(2) A
1.560(2) A
100.1(2)◦
100.3(2)◦
101.1(2)◦
107.0(1)◦
107.0(1)◦
100.6(1)◦
104.4(1)◦
103.9(1)◦
99.5(1)◦
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 9 7 2 – 9 7 4
9 7 3

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9 as colourless blocks, mp 335–338 ◦C (darken at 333 ◦C); mmax
0.0676, GOF = 0.803, max. and min. difference peak and hole
3
(KBr) 1755 (C O); 1H NMR (CDCl3) d 3.29 (d, 2H), 3.81
0.189 and −0.185 e A , respectively.
˚
=
(d, 2H), 5.66 (d, 2H), 6.66 (t, 2H), 6.88 (broad s, 2H), 7.16–
7.28 (m, 10H), 7.86 (d, 2H);13C NMR (CDCl3) d 51.3, 51.9,
54.1, 61.1, 122.8, 127.3, 128.0, 128.6, 129.4, 129.5, 130.1, 131.3,
204.7, 208.1.
Crystal data for 10. C39H24O3, M = 540.58, Hexagonal,
˚
P6(3)/m, a = 15.3580(4), b = 15.3580(4), c = 20.1237(12) A,
3
V = 4110.6(3) A , Z = 6, Dcalc = 1.310 Mg m−3, l(Mo Ka) =
˚
0.082 mm−1, 25909 XXX measured (2hmax = 27.5◦), 3233 unique
(Rint = 0.098), 1982 having I > 2r(I). Full matrix least squares on
F2, R1 = 0.0497, wR2 = 0.0772, GOF = 0.887, max. and min.
Phencyclone napthaquinone cycloadduct 10
3
˚
difference peak and hole 0.24 and −0.21 e A , respectively.
Phencyclone (0.38 g, 1.0 mmol) in 10 ml dry benzene was added
to napthaquinone (0.16 g, 1.5 mmol) in 10 ml dry benzene. The
reaction was stirred at 80 ◦C until the reaction mixture changed
from black to bright orange. The solvent was removed under
vacuum and the residue was recrystallised from chloro◦form to
Crystal data for 11. C35 H22 O1.5 (CH3CN)1.5, M = 520.11,
monoclinic, C2/c, a = 20.7668(18), b = 10.7728(8), c =
3
˚
˚
25.1640(18 A), b = 104.376(2), V = 5453.3(7) A , Z = 8,
Dcalc = 1.267 Mg m−3, l(Mo Ka) = 0.076 mm−1, 14041 measured
(2hmax = 25.0◦), 4789 unique (Rint = 0.094), 4195 having I > 2r(I).
Full matrix least squares on F2, R1 = 0.0481, wR2 = 0.1329,
GOF = 1.072, max. and min. difference peak and hole 0.30 and
give orange block crystals (0.46 g, 85%), mp 253–255 C; mmax
1
=
(KBr) 1691, 1796 (C O); H NMR (CDCl3) d 4.80 (s, 2H),
6.66–6.76 (m, 4H), 6.98 (s, 2H), 7.13–7.78 (m, 10H), 8.08–8.11
(m, 2H), 8.34 (d, 2H), 8.46 (d, 2H);13C NMR (CDCl3) d 49.1,
65.6, 122.8, 124.4, 125.6, 126.4, 126.7, 126.9, 128.1, 128.4, 129.1,
130.7, 131.2, 132.3, 133.5, 134.0, 136.3, 194.6, 198.5.
3
˚
−0.22 e A , respectively.
Intensity data were collected with a Bruker SMART Apex
CCD detector using Mo Ka radiation (graphite crystal
monochromator k = 0.71073). Data were reduced using the
program SAINT.12 The structure was solved by direct methods
and difference Fourier synthesis.
Phencyclone benzyne cycloadduct 11
Phencyclone (0.50 g, 1.3 mmol) was dissolved in 1,2
dichloroethane (20 ml) in a three-necked round bottom flask
which was fitted with two dropping funnel and a water cooled
condenser. Anthranilic acid (0.18 g, 1.3 mmol) in 15 ml 1,2-
dichloroethane was placed in one of the dropping funnels and
the other was charged with isoamyl nitrile (0.35 ml, 2.6 mmol)
in 15 ml of 1,2-dichloroethane. The reaction mixture was
heated to reflux and solutions from the dropping funnels were
simultaneously added dropwise to the refluxing mixture over a
period of 20 min. After the addition was completed, the dark
green solution changed to pale yellow. The mixture was further
refluxed for 60 min and solvent was removed under reduced
pressure. The residue was recrystallised from acetonitrile to
give colorless block crystals (0.52 g, 90%). Air bubbles were
observed to evolve from the product at 93–95 ◦C during the
mp determination, and they left behind a white compound
Acknowledgements
Our gratitude goes to the University of Melbourne for MIRS
and MIFRS scholarships to Goh Yit Wooi.
Notes and references
‡ CCDC reference numbers 256554–256556. See http://www.rsc.org/
suppdata/ob/b4/b417538g/ for crystallographic data in .cif format.
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which melted at 285–289 ◦C. mmax (KBr) 1806 (C O); H NMR
1
=
(CDCl3) d 8.71 (d, 2H), 7.21–8.15 (m, 20H); 13C NMR (CDCl3) d
66.8, 123.5, 125.8, 126.0, 126.1, 126.5, 126.9, 127.8, 128.5, 130.4,
132.0, 132.5, 142.1, 144.6, 195.0.
X-ray crystallography‡
Crystal data for 9. C35 H22 O3, M = 490.53, triclinic, P-1, a =
˚
10.0236(11), b = 10.4368(11), c = 12.1152(13) A, V = 1155.4(2)
◦
◦
◦
3
˚
A , a = 109.755(2) , b = 103.475(2) , c = 90.167(2) , Z = 2,
Dcalc = 1.410 Mg m−3, l(Mo Ka) = 0.089 mm−1, 6182 reflections
measured (2hmax = 25◦), 4034 unique (Rint = 0.0390), 2534 having
I > 2r(I). Full matrix least squares on F2, R1 = 0.0456, wR2 =
9 7 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 9 7 2 – 9 7 4