Isomerization of tricyclo[3.2.2.02,4]nona-2(4),6-diene to the anti- Bredt compound 8-methylenebicyclo[3.2.1]octa-1,6-diene

Full text of DOI:10.1021/ja983199c

5328
J. Am. Chem. Soc. 1999, 121, 5328-5329
Communications to the Editor
Isomerization of Tricyclo[3.2.2.02,4]nona-2(4),6-diene
to the Anti-Bredt Compound
The twisting distortion of π bonds was not given much attention
until the early part of this century when J. Bredt studied it.10
Although his conclusions were aimed at bicyclic systems of the
camphane and pinene series, Bredt’s Rules came to imply a
complete prohibition of bridgehead double bonds.4a,11 On the other
hand, bicyclo[3.2.2]non-1-ene (6) and bicyclo[3.2.2]non-1(7)-ene
(7), whose largest ring containing the double bond is trans-
cycloheptene, have been prepared by Hofmann elimination of
bicyclo[3.2.2]non-1-yl trimethylammonium hydroxide.12 4-Methyl-
bicyclo[3.2.1]octa-1,3-diene (8) was synthesized by the Wittig
reaction of cyclopenten-3-one with butenylidenetriphenylphos-
phorane.13 The smaller system, bicyclo[2.2.2]oct-1-ene (9), a
trans-cyclohexene derivative, has been generated both by insertion
of bicyclo[2.2.1]hept-1-yl carbene and elimination of 2-bromo-
1-ethoxybicyclo[2.2.2]octane.14 When 1-iodo-2-chlorobicyclo-
[2.2.1]heptane or 1-iodo-2-bromobicyclo[2.2.1]heptane was treated
with butyllithium, the bicyclo[2.2.1]hept-1-ene (10) was formed.15
Adamantene (11), which can be regarded as a methylene-bridged
bicyclo[3.3.1]non-1(9)-ene containing the trans-cyclohexene skel-
eton, was synthesized by thermally induced fragmentation of the
di-tert-butyl ester of 1,2-adamantanediperoxycarboxylic acid, by
elimination of 1,2-dihaloadamantane and by irradiation of 1-ada-
mantyl phenylacetate, 2-adamantyl phenylacetate, or protoada-
mantan-4-one hydrazone.16 In this paper, we utilize the unusual
properties of cyclopropenes to synthesize the anti-Bredt compound
8-methylenebicyclo[3.2.1]-octa-1,6-diene (12) which was formed
from the isomerization of 5.
8-Methylenebicyclo[3.2.1]octa-1,6-diene
Gon-Ann Lee,* Yu-Hsien Lin, Ai Ni Huang, Yi Ching Li,
Yih-Chyn Jann, and Chi-Sheng Chen
Department of Chemistry, Fu Jen Catholic UniVersity
Hsinchuang, Taipei 24205
Taiwan, ROC
ReceiVed September 8, 1998
Although the existence of cyclopropenes has been known for
over a century, and the first authenticated synthesis of cyclopro-
pene was reported by Dem’yanov and Doyarenko in 1922,1 the
cyclopropenes continue to fascinate both theoretical and experi-
mental chemists because of their unique structure, high degree
of ring strain, and difficult synthesis.2 Cyclopropenes undergo
many unusual processes such as ring opening reactions to form
vinyl carbenes and [2 + 2] cycloadditions to give tricyclo-
[3.1.0.02,4]hexanes in order to release strain energy.3 Fused bicyclic
cyclopropenes, which are more energetic than cyclopropene itself,
have been well studied.3,4 Four 1,2-fused tricyclics with a
cyclopropene fused to a bicyclic ring skeleton, 1,5,6 2,5,7 3,8 and
4,9 have been synthesized and trapped. However, the chemistry
of 1, 2, 3, and 4 is unknown. In these compounds, the strain
energies of compounds 1 and 2 are very high, and therefore, their
isomerization becomes complicated. The stereochemistry of the
Diels-Alder reactions of 1 and 2 with diphenylisobenzofuran
(DPIBF) is different, but only exo addition (from the view of
bicyclic systems) adducts are formed, and the effects of CH2-
CH2 and CHdCH bridges in these reactions are not clear. To
better understand these effects in this type of Diels-Alder
reaction, we have synthesized tricyclo[3.2.2.02,4]nona-2(4),6-diene
(5) and trapped it with DPIBF.
The synthesis of the starting material 2-bromo-4-chlorotricyclo-
[3.2.2.02,4]non-6-ene (13) is illustrated in Scheme 1. Compound
13 is synthesized by the reaction of cyclohexa-1,3-diene with
1-bromo-2-chlorocyclopropene which was generated by the
fluoride-induced elimination of 1-bromo-2,2-dichloro-1-trimeth-
ylsilylcyclopropane in dichloromethane.5 Elimination of 13 with
methyllithium in ether solution at 0 °C yielded the desired
compound, tricyclo[3.2.2.02,4]nona-2(4),6-diene (5), which was
trapped with DPIBF. Theoretically, four isomers are possible in
this Diels-Alder reaction, but only one isomer 14 was generated.
* To whom correspondence should be addressed. E-mail: chem1010@
fujens.fju.edu.tw.
(1) Dem’yanov, N. Y.; Doyarenko, M. N. Bull. Acad. Sci. Russ. 1922, 16,
297.
(2) Halton, B.; Banwell, M. B. Cyclopropenes. In The Chemistry of the
Cyclopropyl Group; Rappoport, Z., Ed.; Wiley: New York, 1987; Chapter
21.
(3) Billups, W. E.; Lee, G.-A.; Arney, B. E., Jr.; Whitmire, K. H. J. Am.
Chem. Soc. 1991, 113, 7980.
(4) (a) Billups W. E.; Haley, M. M.; Lee, G.-A. Chem. ReV. 1989, 89,
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113, 3, 7969. (c) Billups, E. W.; Luo, M.; Lee, G.-A.; Chee, J.; Arney, B. E.,
Jr.; Wiberg, K. B.; Artis, D. R. J. Org. Chem. 1996, 61, 764.
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26, 1199.
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1972, 11, 518.
(16) (a) Alberts, A. H.; Strating, J.; Wynberg, H. Tetrahedron Lett. 1973,
3047. (b) Lenoir, D. Tetrahedron Lett. 1972, 1186. (c) Gano, J. E.; Eizenberg,
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10.1021/ja983199c CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/25/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 22, 1999 5329
Scheme 1
Scheme 2
Its structure was determined by single-crystal X-ray analysis. The
stereochemistry of 14 shows that the Diels-Alder reaction occurs
via exo addition from the view of cyclopropene and endo addition
from the view of bicyclo[2.2.2]oct-2,5-diene (exo-endo adduct).
This finding leads us to conclude that the repulsion of H2C-
CH2 and DPIBF is larger than HCdCH and DPIBF and that the
repulsion between HCdCH and the oxygen atom is greater than
that between HCdCH and the benzene ring at DPIBF. The result
is different from that of the Diels-Alder cycloadditions of
tricyclo[5.2.2.02,6]undeca-2,5,8-triene with dienophiles.17
When compound 13 was reacted with methyllithium at 0 °C
for 30 min and the solution of DPIBF in ether was then added to
the mixture, two compounds, 14 and 15, were obtained. To define
the origin of compound 15, compound 14 was subjected to the
reaction conditions but underwent no change. The structure of
15 was shown by single-crystal X-ray analysis. Thus, 15 was
formed by DPIBF with 8-methylenebicyclo[3.2.1]octa-1,6-diene
(12) which was produced by the isomerization of 5.
According to the literature, cyclopropenes can undergo [2 +
2] cycloaddition to give dimers via a stepwise diradical18 and
cyclopropene/vinylcarbene rearrangement19 to release the strain
energy. There are two possible mechanisms in this isomerizations
the diradical mechanism and the vinyl carbene mechanism. In
the diradical mechanism, compound 5 undergoes electrocyclic
opening of the cyclopropyl radical 17, formed from 16, to give a
new 1,4-diradical 18 which is transformed to 12 by breaking the
2,3-bond, or the cyclopropyl radical 17 undergoes a coupling
reaction to give pentacyclo[3.2.2.0.2,40.2,704,6]nonane (19) followed
by ring opening rearrangement to form 12. In the vinyl carbene
mechanism, compound 5 isomerizes to 12 via the cyclopropene/
vinyl carbene rearrangement to yield carbene 20 followed by
intramolecular insertion. (Scheme 2)
To understand the mechanism of this isomerization, we used
UV light to irradiate the reaction mixture, causing the yield of
15 to be higher than in the control reaction. As a result, we
propose that this reaction proceeds via a diradical mechanism.
Furthermore, theoretical calculations show that the heat of
formation of compound 19 (178 kcal/mol) is higher than that of
compound 5 (129 kcal/mol).20 Therefore, it is proposed that
compound 5 rearranges to 1,3-diradical 17 followed by electro-
cyclic opening to generate 1,4-diradical 18 which converts to 12.
In conclusion, we have demonstrated a facile route to synthesize
the highly strained compound 5, which forms a sole adduct 14
with DPIBF. Furthermore, compound 5 rearranges to another
highly strained anti-Bredt compound 12, via diradicals 17 and
18. This methodology may open up an avenue for the synthesis
of bicyclic [n.2.1] systems containing bridgehead double bonds.
Acknowledgment. Financial support from the National Science
Council of the Republic of China (NSC 88-2113-M-030-006) is gratefully
acknowledged.
Supporting Information Available: Experimental procedures, spec-
tral data of the new compounds, and crystal structures of 14 and 15.
This material is available free of charge via the Internet at http://pubs.acs.org.
(17) (a) Paquette, L. A.; Carr, R. V. C.; Bo¨hm, M. C.; Gleiter, R. J. Am.
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2797. (c) Zimmermam, H. E.; Bunce, R. E. J. Org. Chem. 1982, 47, 3377.
(19) (a) Franck-Neumann, M.; Lohmann, J. J. Tetrahedron Lett. 1979, 2397.
Franck-Newmann, M.; Lohmann, J. J. Angew. Chem., Int. Ed. Engl. 1977,
16, 323. (c) Baird, M. S.; Buxton, S. R.; Whitley, J. S. Tetrahedron Lett.
1984, 25, 5385.
JA983199C
(20) Using HyperChem, single point, SemiEmpirical, molecule, AM1,
convergence limit ) 0.01, iteration limit ) 50, accelerate convergence ) YES,
RHF calculation ) singlet state calculation, number of electrons ) 46, number
of double occupied levels ) 23, charge on the system ) 0, total orbitals )
46.