A ground state tri-π-methane rearrangement

Article abstract of DOI:10.1016/s0040-4039(00)01705-6

An example of a ground state tri-π-methane rearrangement is described. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/s0040-4039(00)01705-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9585–9587
A ground state tri-p-methane rearrangement†
Howard E. Zimmerman,* Vladim´ır C´ırkva and Lei Jiang
Contribution from the Chemistry Department of the University of Wisconsin, Madison, WI 53706, USA
Received 28 August 2000; accepted 26 September 2000
Abstract
An example of a ground state tri-p-methane rearrangement is described. © 2000 Elsevier Science Ltd.
All rights reserved.
1. Introduction
The di-p-methane rearrangement has been an intensively studied photochemical reaction.2
Recently, the tri-p-methane rearrangement has been reported as a reaction which begins with the
same initial p–p bonding of the excited state.1b Now we report a curious rearrangement of a
tri-p-methane system in the ground state. The reaction mechanism, while reminiscent of the
photochemical counterpart, is quite different. The synthesis of the two reactants is outlined in
Scheme 1.
Scheme 1. Synthesis of tri-p-methane reactant
* Corresponding author.
†
This is Paper 258 of our general series. For Paper 257 see Ref. 1a, for Paper 256 see Ref. 1b
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01705-6

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2. Ground state tri-p-methane rearrangements
The tri-p-methane systems 4a and 4b were found3 to afford five-membered ring products 5a
and 5b on boron trifluoride catalysis. The structure of 5a was established by X-ray analysis and
that of 5b derived from NMR which paralleled that of 5a. This rearrangement is shown in Eq.
(1).
(1)
The mechanism is quite different from that of the photochemical di-p-methane and tri-p-
methane counterparts and is shown in Scheme 2. The boron trifluoride coordination at the
carbonyl oxygen leads to a Prins type bonding with the diphenylvinyl moiety to afford
zwitterions 7. This cyclopropylcarbinyl species then undergoes opening of the three-membered
ring to afford zwitterion 8. This much of the mechanism is equivalent to a 1,2-diphenylvinyl
shift. However, the mechanism of a 1,2-vinyl shift involves the p system as outlined here rather
than being a rearrangement using only the sigma bond to the vinyl group. Finally, bonding of
the enolate and diphenylallylic allyl cation moieties of species 8 leads to the final five-membered
ring products 9a and 9b. We note that the interconversion of species 6, 7 and 8 may well be
reversible but driven forward by the energetics of the final ring closure.
Scheme 2. Mechanism of the BF3 catalyzed tri-p-methane rearrangement

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This is our first example of the reaction. Both the regioselectivity, wherein the diarylvinyl
groups bear different substitution and also the success of five-ring formation where other
carbonyl groups are utilized are of interest and presently under investigation.
Acknowledgements
Support of this research by the National Science Foundation is especially appreciated for its
support of basic research. We thank Mr. Lei Jiang for initiating this project and first uncovering
the reaction. The first author was, as an undergraduate at Yale, the first of Harry Wasserman’s
research students, and wishes to express appreciation of this experience which was a beginning.
References
1. (a) Zimmerman, H. E.; Alabugin, I. V.; Smolenskaya, V. N. Tetrahedron Symposium-in-Print, 2000, 6821–6831;
(b) Zimmerman, H. E.; C´ırkva, V. Org. Lett. 2000, 2, 2365–2367.
2. (a) Zimmerman, H. E.; Paufler, R. J. Am. Chem. Soc. 1960, 82, 1514–1515. (b) Zimmerman, H. E.; Grunewald,
G. L. J. Am. Chem. Soc. 1966 88, 183–184. (c) Zimmerman, H. E.; Binkley, R. W.; Givens, R. S.; Sherwin, M.
A. J. Am. Chem. Soc. 1967, 89, 3932–3933. For a recent review see Zimmerman, H. E.; Armesto, D. Chem. Rev.
1996, 96, 3065–3112.
3. Rearrangement detail. To 28 mg (0.20 mmol) of boron trifluoride etherate in 3 ml of ether was added a 106 mg
(0.20 mmol) portion of methyl 2-carbomethoxy-4-methyl-4-(2,2-diphenylvinyl)-6,6-diphenyl-2,5 hexadienoate in
1.0 ml of ether. Stirring for 24 h at room temperature was followed by filtering through silica gel with ether,
concentration and chromatography on silica get with 1:3 ether–hexane. Crystallization from ether–methanol
afforded 80 mg (76%) of 1-methyl-3,3-diphenyl-4,4-dicarbomethoxy-5-(2,2-diphenylvinyl)-1-cyclopentene (30a) as
colorless crystals, mp 175–177°C. Similarly, to 28 mg (0.20 mmol) of boron trifluoride etherate in 3.0 ml of ether
was added 121 mg (0.20 mmol) of methyl 2-carbomethoxy-4-benzyl-4-(2,2- diphenylvinyl)-6,6-diphenyl-2,5-hexa-
dienoate in 1.0 ml of ether. After stirring for 24 h at room temperature and workup as in the methyl analog above,
crystallization from ether–methanol afforded 98 mg (81%) of 1-benzyl-3,3-diphenyl-4,4-dicarbomethoxy-5-(2,2-
diphenylvinyl)-1-cyclopentene (30b) as colorless crystals, mp 226–228°C.
.