Correlation between carbon-carbon bond length and the ease of retro Diels-Alder reaction

Article abstract of DOI:10.1007/s12039-014-0709-6

The bond length between C8-C9 in (1′ R,4′ S,4a′ R,8a′ S)-6′,7′-dimethyl-1′,4′,4a′,8a′-tetrahydrospiro [cyclopropane-1,9′-[1,4]methanonaphthalene]-5′,8′-dione is 1.571 (2) ? and between C7-C12 is 1.567 (2) ? which are longer than the corresponding bond length for saturated bicyclic systems (1.531-1.535 ?). This paper reports the correlation between bond length and the ease of retro Diels -Alder reaction.

Full text of DOI:10.1007/s12039-014-0709-6

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J. Chem. Sci. Vol. 126, No. 5, September 2014, pp. 1369–1371. ꢀ Indian Academy of Sciences.
Correlation between carbon–carbon bond length and the ease of retro
Diels–Alder reaction
SAMBASIVARAO KOTHA∗, SHAIBAL BANERJEE and MOBIN SHAIKH
Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai,
Maharashtra, 400 076, India
e-mail: srk@chem.iitb.ac.in
MS received 28 April 2014; revised 22 August 2014; accepted 1 September 2014
Abstract. The bond length between C8-C9 in (1ꢁR,4ꢁS,4aꢁR,8aꢁS)-6ꢁ,7ꢁ-dimethyl-1ꢁ,4ꢁ,4aꢁ,8aꢁ-tetrahydrospiro
[cyclopropane-1,9ꢁ-[1,4]methanonaphthalene]-5ꢁ,8ꢁ-dione is 1.571 (2) Å and between C7-C12 is 1.567 (2) Å
which are longer than the corresponding bond length for saturated bicyclic systems (1.531-1.535Å). This paper
reports the correlation between bond length and the ease of retro Diels−Alder reaction.
Keywords. Crystal structures; retro Diels−Alder reaction; spirocyclic dienes.
1. Introduction
system 2. Therefore, the adduct 1 is more prone to rDA
reaction. White and co-workers discussed the effect of
orbital interactions on rDA reaction.
The Diels–Alder (DA) reaction is a versatile protocol
for C–C bond formation and also compatible with the
wide range of functional groups.1 However, its reverse
reaction i.e., retro Diels–Alder (rDA), has been rela-
tively less explored owing to the high energy require-
ment to cross the activation barrier.2 Despite existing
problems, rDA reaction has evolved as a useful tool
and remains to be a preferred method for the prepara-
tion of several reactive olefins or metastable molecular
entities.3,4 The rate of rDA reaction depends on several
factors like the geometry of the adducts, hetero atoms,
nature of substituents tagged with the adducts, catalysts
used, etc. It is well known that trimethylsilyl group on
a bicyclic DA adduct exerts a positive effect on the
rate of rDA reaction.5 Similarly, Kotha and co-workers
reported that cyclopropane ring containing norbornane
also exerts a positive effect on the rDA reaction due to
stabilization of the transition state.6 In addition to these
factors, studies conducted by White and co-workers on
the rDA reaction of bicyclic adducts have pointed out an
interesting correlation between the bond distance of the
carbon atoms involved in the bond breaking process and
the rate of the rDA reaction.7,8 To this end, they have
examined bond distance between the unsaturated nor-
bornene and its saturated counterpart by x-ray crystal-
lographic data (figure 1). It was observed that the bond
length between C1-C10 and C4-C5 has been elongated
in unsaturated system 1 as compared to the saturated
In the present manuscript, we examined the crys-
tal structure of (1ꢁR,4ꢁS,4aꢁR,8aꢁS)-6ꢁ,7ꢁ-dimethyl-
1ꢁ,4ꢁ,4aꢁ,8aꢁ-tetrahydrospiro[cyclopropane-1,9ꢁ -[1,4]me-
thanonaphthalene]-5ꢁ ,8ꢁ-dione and found that the
bond length between atoms connecting diene and the
dienophile is somewhat longer than the normal C−C
bond length. The increased bond length is responsible
for the facile rDA reaction of 5.
2. Experimental
The detailed experimental method for the synthesis of 5
has previously been reported by us.6 The product 5 was
recrystallized using hexane-ether solvent.
3. Results and Discussion
We have synthesized several cyclopropane contain-
ing spirocyclic dienes under mild reaction condi-
tions. The spiro-diene 3 was reacted with 2,3-dime-
thylbenzoquinone 4 to deliver spirocyclic adduct 5
(scheme 1). The detailed experimental method has been
reported in our earlier manuscript.6 The retro Diels–
Alder reaction for this bicyclic system 5 has been
achieved by refluxing in toluene for 28 h with 35%
yield.
After repeated attempts of crystallization from
hexane-ether mixture, we were successful in getting the
triclinic crystals of the DA adduct 5, whose structure
∗For correspondence
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1370
Sambasivarao Kotha et al.
groups flanked away from the norbornane ring, cf.
figure 2. It was observed for compound 5 that the dis-
tance for C8-C9 bond is 1.571 (2) Å and that for C7-
C12 bond is 1.567 (2) Å (figure 2). The bond dis-
tances observed for the carbon atoms of 5 involved
in the bond breaking process are greater than the
average C−C bond distance generally found in a
saturated bicyclic systems which is 1.531–1.535 Å.
The cyclopropane bond, (C13-C14) is aligned in a
near anti-periplanar conformation to the C8-C9 bond,
which undergoes cleavage during the rDA reaction.
The angles between C6-C7-C12 and C1-C8-C9 were
found to be 112.81 (13)◦ and 113.03 (13)◦, respec-
tively; clearly, the deviation from the normal angle
of 109◦ is significant. The greater bond lengths and
strained conformation in the adduct 5 are possible rea-
sons for the facile nature of the adduct to undergo rDA
reaction.
Figure 1. The molecular system examined by White and
co-workers7,8 for correlation of bond distance with rDA
reaction.
We have also prepared a spirocyclic dione 6 and
found no rDA reaction under the above mentioned con-
ditions. It clearly indicates the role of cyclopropane
ring in rDA reaction (scheme 2).9 Unfortunately, our
unsuccessful efforts thus far in obtaining crystals of the
adduct 6 for x-ray structure determination preclude cor-
relation, analogous to that of compound 5 of the bond
lengths with reactivity.
Scheme 1. The Diels–Alder and the retro Diels–Alder
approaches to the norbornene system 5.
was refined to an R factor of 4.24%. The crystal data
and the details of refinement are collected in table 1.
The adduct 5 has endo conformation with two methyl
Table 1. Crystal data and structure refinement details for compound 5.
Empirical formula
Formula weight
Temperature
C15 H16 O2
228.28
150(2) K
Wavelength
0.71073 Å
Triclinic, P -1
Crystal system, space group
Unit cell dimensions
a = 7.1512(9) A
b = 9.114(2) A
c = 10.183(3) A
V/Å3
alpha = 64.04(3) deg.
beta = 88.829(16) deg
gamma = 80.669(16) deg
587.9(2)
Z, Calculated density
Absorption coefficient (mm−1
F(000)
Crystal size
Theta range for data collection
2, 1.290 Mg/m3
0.084
244
)
0.34 × 0.26 × 0.21 mm
3.50 to 24.99 deg
Limiting indices
−8 <=h<=8, −10<=k <=10, −11<=l<=12
4962/2058 [R(int) = 0.0188]
99.5%
Reflections collected/unique
Completeness to theta = 24.99
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Semi-empirical from equivalents
0.9825 and 0.9719
Full-matrix least-squares on F2
2058/0/156
1.091
Final R indices [I>2sigma(I)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.0424, wR2 = 0.1054
R1 = 0.0491, wR2 = 0.1098
0.273 and -0.225 e.A−3

Retro Diels–Alder reaction of spirocyclic adduct
Supplementary Information
1371
X-ray diffraction data were collected on an OXFORD
XCALIBUR-S CCD single-crystal X-ray diffractome-
ter. The structure was solved and refined by full-matrix
least-squares techniques on F2 using the SHELX-
97 program (Sheldrick, 1997). Crystallographic data
(excluding structure factors) for the structures reported
in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publi-
cation Compound 5 CCDC 958630. Copies of available
material can be obtained, free of charge, on application
to the CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (e-mail: deposit@ccdc.cam.ac.uk).
Figure 2. ORTEP perspective diagram of the structure of
compound 5.
Acknowledgements
We would like to acknowledge the DST, New Delhi
for the financial support. We also thank, SAIF-Mumbai
for recording the spectral data. S K thanks DST, New
Delhi for the award of a J C Bose fellowship. S K
thanks Darshan Mhatre for his help regarding X-ray
data collection.
References
Scheme 2. Attempted retro Diels–Alder reaction of the
norbornene system 6.
1. Carruthers W 1990 In Cycloaddition Reactions in
Organic Synthesis (Oxford: Pergamon press)
2. Seybold G 1977 Angew. Chem. Int. Ed. Engl. 89 377
3. Kotha S and Banerjee S 2013 RSC Adv. 3 7642
4. Kashinath K, Swaroop P S and Reddy D S 2012 RSC Adv.
2 3596
4. Conclusion
While the occurrence of Diels–Alder reactions are gen-
erally reconciled based on overlap of the frontier molec- 5. Magnus P, Cairns P M and Moursounidis J 1987 J. Am.
Chem. Soc. 109 2469
6. Kotha S, Banerjee S, Patil M P and Sunoj R B 2006 Org.
Biomol. Chem. 4 1854
7. White J M, Birney D, Lim T K, Koh J H and Pool B R
2002 J. Am. Chem. Soc. 124 5091
8. White J M and Pool B R 2000 Org. Lett. 2 3505
9. Kotha S, Manivannan E and Sreenivasachary N 1999 J.
Chem. Soc. Perkin Trans. 1 19 2845
ular orbitals, experimental results of retro Diels-Alder
reactions have rarely been examined from the point
of structural considerations. We have shown that the
increase in the bond lengths of pertinent C-C bonds, as
revealed by X-ray crystal structure analysis, is respon-
sible for the observed facile retro Diels–Alder reaction
of compound 5.