2D 1H and 13C NMR evidences of the [2+2] autodimerization of 2-benzyl-5-benzylidene cyclopentanone yielding two different diphenyl, dispiro cyclobutane derivatives

Article abstract of DOI:10.1016/S1386-1425(01)00727-2

The ene-ene [2+2] cycloaddition of 2-benzyl-5-benzylidenecyclopentanone proceeds smoothly and spontaneously in benzene-d₆ or deuteriochloroform solution to give two different diphenyl dispiro [4.1.4.1] dodecan-4,11-diones. Detailed ¹

Full text of DOI:10.1016/S1386-1425(01)00727-2

Spectrochimica Acta Part A 58 (2002) 2079–2087
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1
13
2D H and C NMR evidences of the [2+2]
autodimerization of 2-benzyl-5-benzylidene cyclopentanone
yielding two different diphenyl, dispiro cyclobutane
derivatives
E. D´ıaz a,*, H. Barrios a, D. Corona a, A. Guzma´n a, R. D´ıaz b, A. Fuentes b
a Instituto de Qu´ımica, Uni6ersidad Nacional Auto´noma de Me´xico, Circuito Exterior, C.U. Coyoaca´n 04510, Me´xico D.F., Mexico
b Facultad de Qu´ımica, UAEM, Toluca 50000, Toluca, E. de Me´xico, Mexico
Received 19 September 2001; accepted 8 October 2001
Abstract
The ene-ene [2+2] cycloaddition of 2-benzyl-5-benzylidenecyclopentanone proceeds smoothly and spontaneously
in benzene-d6 or deuteriochloroform solution to give two different diphenyl dispiro [4.1.4.1] dodecan-4,11-diones.
1
Detailed H and 13C 2DNMR spectroscopy (COSY, HMQC, HMBC) were performed in order to prove the existence
in solution of two cyclobutane derivatives, one a previously described photodimer obtained by the UV irradiation of
crystals of 2-benzyl-5-benzylidenecyclopentanone. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: 1H and 13C NMR; [2+2] Autodimerization; Dispirocyclobutane derivatives
reaction results from overlaps of the orbital of
one molecule with the orbital of the other.
On the other hand, the 1,4 cycloaddition [1,2]
reactions have been known for many years. In the
cases of 1,4 cycloaddition in benzylidenecy-
cloalkanones, they yield dimers with structures
well established and different from those of the
[2+2] cycloaddition reaction.
During work on the stereochemistry of cyclic
systems Cornubert et al. [3] investigated stereoiso-
merism in substituted cyclopentanones and cyclo-
hexanones, preparing the 2-benzyl-5-benzylidene
cyclopentanone (1). Compound 1 was unstable in
light and upon irradiation Cornubert reported the
formation of a phototrimer, however years later,
1. Introduction
It is well known that [2+2] cycloaddition are
not easy under thermal conditions. Under vigor-
ous conditions, cycloaddition may occur by steps
via diradicals and not in a concerted fashion.
Likewise, photochemical [2+2] cycloaddition
are very common, although some of these proceed
also by a stepwise sequence.
In cycloaddition, two new bonds are formed by
use of y electrons of the reactants. The concerted
* Corresponding author. Tel.: +52-616-25-76; fax: +52-
616-22-17
E-mail address: maudiaz@servidor.unam.mx (E. D´ıaz).
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(01)00727-2

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G.C. Forward [4] established the correct structure
of the irradiation product as the photodimer (2)
and demonstrated a head-tail dimerization.
ene at lA=3.34, dd, J=5.0, −14.0; lM=2.51,
dd, J=9.0; −14.0; lX=2.26 m and the remaining
protons for the methylene groups of the cyclopen-
tanone moiety at l=2.45, m, 2.16, m; 1.63 and
1.15, m (Fig. 1a).
1
The low resolution H NMR spectrum did not
mention detailed features (lppm 7.37, 20H, aro-
matic; 4.28, 2H, cyclobutane protons and 0.8–3.0
multiplet, 14H, methylene). With new high resolu-
tion NMR spectra currently available, we under-
took this work in order to demonstrate that the
photodimer 2 described by Forward and Whiting
[4] already exist in the NMR tube solution of
2-benzyl-5-benzylidene cyclopentanone 1.
Surprisingly, the 1D 1H NMR spectrum
recorded in CDCl3 and C6D6 of compound 1 was
time depending due to the formation of two new
products detected through the new protons signals
displayed (Figs. 2 and 3).
1
2.1. 2D H NMR
At first, we should point out that the ratio
between product 1 and the new products formed in
solution was dependent upon the chosen solvent.
In this way, compound 1 after 1 week in DMSO-d6
solution at room temperature, displayed in its
proton NMR spectrum only traces of the com-
pound 2, easily recognized by the small singlet at
l=4.19, as well as the small multiples at l=3.06,
2.37, 2.21, 1.92, 1.67, 1.37 and 0.91. When the
NMR solvent was C6D6 and recorded immediately
after dissolving, only traces of the product 2 were
initially detected (see Fig. 1). After 12 h, the proton
NMR spectrum displayed signals that enabled the
assignment of the ratio 1:2 of :8:2, respectively.
Nevertheless, when the solvent was CDCl3 and
after 12 h at room temperature (see Fig. 2), a
mixture of 1, 2 and 6 whose ratio was calculated
as 16:61:23, respectively was detected.
2. Results and discussion
In the course of our search of the reaction
between 2,5-dibenzylidenecyclopentanone 3 with
6-amino-1,3-dimethyl uracil (4) using phase trans-
fer conditions, we were able to isolate in good
yields and identify by spectroscopic methods
(NMR, MS, IR) the oxidized monoadduct 5, as
well as a reduced derivative whose structure was
established as 2-benzyl-5-benzylidene cyclopen-
tanone 1 in good yields. Thereby, we undertake,
using 2D high resolution NMR spectra, the struc-
tural identification of compound (5) as well as of
compounds (2) and (6), formed in solution at the
expense of (1).
Compound 5 showed at MS a molecular ion m/z
M+ 395 which match well with the molecular
formula C25H21N3O2. The 1H NMR spectrum
showed a triplet l=7.67, 4J=2.7 Hz for the vinyl
proton; two NMe groups at l=3.86, s and 3.32,
s, as well as ten aromatic protons between l=7.6
and 7.1 assigned to the phenyl groups. At l=3.13
and 2.78 four protons were observed correspond-
ing to the methylenes of the five member ring.
Using these data and the knowledge of the reaction
mechanism previously proposed by us [5–8], we
assigned the structure 5 for this adduct.
In order to establish unambiguously that the
new signals of the products formed in solution
corresponded to the photoadducts 2 and 6, we
prepared 2 through the UV irradiation of 2-benzyl-
5-benzylidene cyclopentanone 1 (see Scheme 1)
using the procedures described previously by For-
ward and Whiting [4].
The high resolution 1H NMR spectrum
(CDCl3, 500 MHz) of compound 2 (see Fig. 3
Scheme 2) showed at l=6.86, 2Hortho, 7.10,
1Hpara and 7.17, 2Hmeta for the phenyl ring of the
benzyl moiety. In addition, we observed the aro-
matic protons of the phenyls attached to the
cyclobutane moiety at l=7.35, 2Hortho, 7.32,
2Hmeta and 7.23, 1Hpara (the assignment was made
through a HMBC experiment), a sharp singlet at
l=4.25, assigned to both methine of the cyclobu-
On the other hand, product 1, isolated as pale
yellowish crystals, showed at MS a molecular ion
m/z M+ 262 which matched well with molecular
1
formula C19H18O. The H NMR (C6D6) showed a
4
triplet at l=7.66, J=2.7 Hz and the usual ten
aromatic protons between l=7.3 and 7.0. We also
observed an AMX pattern of the benzylic methyl-

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2081
tane ring and an AMX pattern at lA=2.59, dd,
J=4.5; −14.0; lM=2.10, m and lX=1.08, dd,
J=11.0; −14.0, assigned to the benzylic methyl-
ene and the methine where this was attached. The
larger chemical shift difference between both pro-
tons of each benzylic methylene (Dl=1.51) sug-
gested that the benzylic proton at l=1.08
suffered a symmetrical diamagnetic shielding for
only one proton of each methylene through the
magnetic anisotropy of the phenyl rings [9–11]
attached to the cyclobutane moiety, as is shown in
Fig. 4. This spectroscopic behaviour matched well
with the structure previously established via X-ray
crystallographic studies [12].
On the other hand, using the vertical enhance-
ment of the signal at l=4.25, we were able to
observe two additional singlets at l=4.23 and
4.18, that we assigned to traces of a new and
non-described asymmetric photoadduct 6. Thus,
the two cyclobutane hydrogens with the previ-
ously mentioned chemical shifts were magnetically
non-equivalent and were not observable coupled.
As was observed through the COSY spectrum [13]
(see Fig. 2, insert) of the mixture of 1, 2 and 6,
which displayed cross peaks for the compound 6;
these signals enabled us to detect evidence of the
chemical shifts from the protons of both benzylic
methylene at l=1.14, dd, J=11.0; −14.0 and
Fig. 1. (a) 1H NMR 500 MHz (C6D6) of compound 1 immediately as solved. (b) 1H NMR 500 MHz (C6D6) of compound 1 after
12 h. (c) COSY spectrum of compound 1 after 12 h.

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1
Fig. 2. (a) H NMR 500 MHz (CDCl3) of compound 1, 2 and 6 after 12 h. (b) Vertical enhancement of the 4.18–4.26 ppm chemical
shift range of the spectrum (a). (c) COSY of the mixture of compounds 1, 2 and 6.
2.60 (overlapped) and l=2.91, dd J=4.5; −14.0
and l=2.30, dd, J=10.0, −14.0, respectively.
These chemical shifts suggested an asymmetric
structure, where only one proton (l=1.14) from
both methylenes was shielded for one of the
phenyl rings attached to the cyclobutane moiety
and displaying the remaining benzylic methylene
protons, the usual chemical shifts between l=
2.91 and 2.30. Thus, the two benzylic units were
unambiguously cis to each other (preserving the
centre of symmetry as in compound 2) (see Fig.
4).
2.1.1. 2D 13CNMR
The photodimer 2 displayed at 13C NMR a
signal at l=218.1 assigned to the carbonyls of
cyclopentanone. Two signals of non-protonated
carbons at l=139.9 and 138.0 for both ipso
carbons of the different phenyl rings. In addition,
we observed the signals for the methines of the
phenyl rings (DEPT) at l=130.7, 128.5, 128.3
and 128.4 (methines ortho and meta) and finally,
the signals assigned to the para carbons at l=
127.5 and 126.0.
At higher field, we observed a non-protonated

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2083
The proton triplet at l=7.23 correlated with the
carbon signal at l=127.5 and the protons at
l=7.35 and 7.32 with the carbons at l=130.7 and
128.4, respectively.
carbon at l=59.6, the methine at l=54.7 and
49.7, as well as the three methylene carbon signals
(DEPT) [8] at l=35.2, 34.7 and 25.4.
The HMQC [14] experiment enabled us to assign
the proton singlet at l=4.25, which correlates with
the carbon signal at l=54.7 and the multiplet at
l=2.10 with the methine at l=49.7. The benzylic
methylene protons displaying the higher chemical
shift difference between both protons at l=1.08
and 2.59 correlated with the signal at l=34.7. Con-
sequently, the methylene protons at l=1.40 and
1.82 correlated with the carbon signal at l=25.4.
At lower field, we were able to assign the proton
doublet at l=6.86 with the carbon signal at
l=128.5; the proton at l=7.10 with the signal at
l=126.0 and the proton triplet at l=7.17 with the
carbon at l=128.2.
The HMBC [15] experiment allowed us to assign
the aromatic non-protonated and protonated car-
bons. For example, the proton singlet at l=4.25
correlated through 3s bond with the carbonyl signal
at l=218.1, with the ipso carbon at l=138.0 and
with the methine carbon at l=130.7. This exper-
iment enabled us to confirm the chemical shifts of
the phenyl group carbons attached to the cyclobu-
tane ring. Additional correlations through this
methine signal were those observed at higher field
with the non-protonated carbon at l=59.6 and the
methylene carbon at l=35.2. Using the benzylic
protons at l=1.08 and 2.60, we observed the 2|
Fig. 3. (a) 1H NMR 500 MHz (CDCl3) of compound 2. (b) COSY spectrum of compound 2.

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E. D´ıaz et al. / Spectrochimica Acta Part A 58 (2002) 2079–2087
Scheme 1.
and 3| bond correlation with the carbonyl car-
bon at l=218.1, the ipso carbon at l=139.9
and the methine at l=128.5, as well as with the
methine carbon at l=49.7 and the methylene
carbon at l=25.4. The triplet at l=7.17 corre-
lated with the ipso carbon at l=139.9; the
triplet at l=7.32 was correlated with the ipso
carbon at l=138.0 and finally, the doublet at
l=6.86 correlated with the carbon signal at
l=128.5. The methylene protons at l=1.60
and 2.75 correlated with the non-protonated car-
bon at l=59.6, as well as the methines at
l=54.7 and 49.7 and the methylene at l=25.4.
The methylene protons at l=1.40 and 1.82 cor-
related with the non-protonated carbon at
l=59.6, the methine at l=49.7 and the meth-
ylene at l=35.2.
In addition, because the 13C NMR experiment
has the longer recording consuming time and in
order to avoid the dimerization of compound 1,
we chose as solvent DMSO-d6, which allowed us
to detect exclusively the signals due to the com-
pound 1. In this way, using the noise decoupling
experiment and HMQC and HMBC experi-
ments, we completed the assignments of protons
and carbons of this compound.

E. D´ıaz et al. / Spectrochimica Acta Part A 58 (2002) 2079–2087
2085
3. Conclusions
photodimer 2, as well as a new centroasymmetric
photodimer 6.
It was reported in 1969 that the UV irradiation
of 2-benzyl-5-benzylidene cyclopentanone yielded
only one of four possible photodimer, the cen-
trosymmetric photodimer.
4. Experimental
In light of modern NMR spectroscopy, we were
able to observe the interesting behaviour of 2-ben-
zyl-5-benzylidene cyclopentanone and deduce the
existence in solution of the well known
Melting points were determined with a Kofler
Hot Stage apparatus and were not corrected. The
1H and 13C NMR spectra were recorded using a
Varian Unity 300 spectrometer operating at ob-
Scheme 2.

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E. D´ıaz et al. / Spectrochimica Acta Part A 58 (2002) 2079–2087
Fig. 4.
servation frequency of 300.0 MHz for 1H and 75.0
MHz for 13C.
methane and Triton B were purchased from
Aldrich Chemical and used as received.
1
The H and 13C chemical shifts (l) are given in
4.1. General procedures for the synthesis of the
adduct 5
ppm relative to tetramethyl silane (TMS).
High resolution NMR spectra were recorded on
a Varian Unity 500 operating at 500.3 MHz for
1H and 125.0 MHz for 13C. The experiments were
performed using an inverse detection 5 mm probe.
The COSY, HMQC and HMBC spectra were
recorded by the usual Varian Unity software.
Mass spectra were recorded on instruments
JEOL-JMS AX505HA and JEOL-JMS 10217 us-
ing standard FAB or CI/EI sources in glycerol or
with CH4 gas, respectively. The IR spectra were
performed on Nicolet FX-SX and Nicolet 55-X in
solution or KBr mode.
To a suspension of 2 mM of 6-amino-1,3-
dimethyl uracil in 3 ml of dichloromethane, 1 ml
of Triton B (methanol solution) and 1 ml of
distilled water were added, followed by 1 mM of
2,5-phenylidencyclopentanone in
5
ml of
dichloromethane. The mixture was stirred at
room temperature until the starting material was
consumed (reaction was monitored by TLC).
Usual work up followed by flash column chro-
matography (silica gel, Kieselgel 60, 230–400
mesh, Merck) or preparative thin layer chro-
matography (silica gel, Kieselgel GF-254, plates
20×20, 2.0 mm thick) yielded the adduct 5 and
The 2,5-dibenzylidene cyclopentanone used as a
starting material was prepared following the pro-
cedure described by Quilico [16].
the
reduced
2-benzyl-5-benzylidenecyclopen-
The 2-benzyl-5-benzylidene cyclopentanone
used for the UV irradiation was prepared by the
method described by Forward and Whiting [4].
The 6-amino-1,3-dimethyl uracil, dichloro-
tanone 1, which were recrystallized from a mix-
ture of dichloromethane/hexane.
Adduct
5
showed the followed physical
constants:

E. D´ıaz et al. / Spectrochimica Acta Part A 58 (2002) 2079–2087
2087
Leo´n, A. Fuentes, Spectrochim. Acta Part A 54 (1998)
567.
Yellow solid m.p. 264–266 °C, MW 395
(C25H21N3O2); MS EI, M+ m/z 395, 394 318, 304.
IR wmax (KBr); 3025, 2931, 1702, 1658, 1554, 1492,
[7] E. D´ıaz, H. Barrios, J.L. Nava, M.I. Chavez, A. Guzma´n,
J.F. Fuentes, A. Fuentes, Spectrosc. Lett. 31 (1998) 51.
[8] E. D´ıaz, H. Barrios, J.L. Nava, R.G. Enriquez, A. Guz-
ma´n, L. Leo´n, J.F. Fuentes, A. Fuentes, A. Quintero,
J.D. Solano, J. Heterocyc. Chem. 34 (1997) 1037.
[9] C.E. Johnson, F.A. Bovey, J. Chem. Phys. 29 (1958)
1012.
[10] E. D´ıaz, H. Barrios, F. Yuste, J.L. Aguilera, W.F.
Reynolds, M.R. van Calsteren, C.K. Jankowski, J. Hete-
rocyc. Chem. 30 (1993) 97.
[11] E. D´ıaz, E. Galeazzi, J.L. Nava, J.M. Muchowski, A.
Guzma´n, M.R. van Calsteren, C.K. Jankowski, Mag.
Res. Chem. 33 (1995) 581.
[12] (a) C.R. Theocharis, W. Jones, Stud. Org. Sol. State
Chem. 32 (1987) 47;
1448 and 1358 cm−1
.
4.1.1. Compound 2
The diphenyldispiro [4.1.4.1] dodecan-4,11-
dione (2) was prepared using the method de-
scribed by Forward and Whiting [4]. The
photodimer crystallized from dichloromethane
gave colorless needles m.p. 242–243 °C (lit [3]
m.p. 240–241 °C). IR wmax 1730, 700 cm−1. MS
m/z 524 M+, 262 (100%).
(b) D.A. Whiting, J. Chem. Soc. C, Part III, (1971) 3396.
[13] A. Derome, Modern Techniques for Chemistry Research,
Pergamon Press, New York, 1987, p. 240.
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