The frustrated Lewis pair induced formation of a pentafulvene [6 + 4] cycloaddition product

Article abstract of DOI:10.1039/c0cc04567e

The frustrated Lewis pair Mes₂P-CH₂CH ₂-B(C₆F₅)₂ reacts with excess 6,6-dimethylpentafulvene to yield a P/B-Lewis pair addition product to an elusive pentafulvene [6 + 4] cycloaddition dime

Full text of DOI:10.1039/c0cc04567e

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COMMUNICATION
The frustrated Lewis pair induced formation of a pentafulvene [6 + 4]
cycloaddition productw
¨
¨
Cornelia M. Momming, Gerald Kehr, Roland Frohlichz and Gerhard Erker*
Received 22nd October 2010, Accepted 14th December 2010
DOI: 10.1039/c0cc04567e
The frustrated Lewis pair Mes2P–CH2CH2–B(C6F5)2 reacts
with excess 6,6-dimethylpentafulvene to yield a P/B-Lewis pair
addition product to an elusive pentafulvene [6 + 4] cycloaddition
dimer. This observation may open a new field of utilization of
frustrated Lewis pair chemistry.
[m + n] Cycloaddition reactions are of great synthetic
importance. Their detailed study has significantly contributed
to the understanding of mechanistic routes taken in organic
transformations.1 The most frequently employed reaction of
this type is the [4 + 2] cycloaddition, i.e. the Diels–Alder-
reaction and its variants. Higher order cycloadditions such as
the [6 + 4] or [8 + 2] cycloaddition have been studied2 and in
some cases utilized,3 but to a much lesser extent than many of
Scheme 1 Dimerization reactions of fulvene derivatives.
their lower congeners.
Fulvenes should be prime candidates for e.g. [6 + 4] cyclo-
addition reactions, but they mostly react by the preferred
[4 + 2] reaction alternative.4 The much more reactive iso-
benzofulvene 1, however, undergoes rapid dimerization to
form the respective [6 + 4] cycloaddition product 2.5 To the
best of our knowledge, information about the [6 + 4] dimer-
ization of simple pentafulvenes is scarce. Neuenschwander
suggested the potential occurrence of a [6 + 4] intermediate
4 in the trimerization reaction of 6,6-dimethylfulvene 3
(Scheme 1). We have now found a unique way to generate
the framework of the 6,6-dimethylpentafulvene [6 + 4] dimer
with the aid of frustrated Lewis pair chemistry.
When the intramolecular frustrated Lewis pair 5 was stirred
with a ca. 10 fold excess of 6,6-dimethylfulvene 3 in pentane at
room temperature a slow reaction occurred to give a colorless
precipitate. It was collected after a total of 7 days reaction time
and isolated in 43% yield as an off-white solid. Single crystals
suited for the X-ray crystal structure analysis were obtained
from benzene/heptane by the diffusion method. It showed that
we had isolated the formal addition product (6) to the endo-
[6 + 4] dimer (4) of 6,6-dimethylpentafulvene 3 (see Fig. 1 and
Scheme 2). The core of the structure contains the tricyclic ring
system formally derived from the pentafulvene [6 + 4] dimer-
ization. It features
a bridgehead CQC double bond
Frustrated Lewis pairs have been known to undergo a
number of small molecule activation reactions.6 We had
shown that the P(o-tolyl)3/B(C6F5)3 Lewis pair can add to
non-conjugated di-acetylenes to induce additional carbon–
carbon coupling.7 The very reactive intramolecular Lewis pair
58 undergoes 1,2-addition reactions to olefins or acetylenes9
but also is able to undergo 1,4-addition reactions to conju-
gated enynes and diynes.10
(C7–C8: 1.318(4) A), the endocyclic C3–C4 double bond
(1.313(5) A) and the exocyclic CQCMe2 bond (C11–C12:
1.319(4) A). The (C6F5)2B–CH2–CH2–PMes2 system 5 is found
1,2-attached at the five-membered ring (C9–P: 1.882(3) A,
C10–B: 1.651(4) A). The resulting P/B containing heterocycle
attains a rigid boat conformation (see Fig. 1). The overall
structure can formally be described as the regioselective
exo-1,2-addition product of the P/B Lewis pair 5 to the
C9QC10 carbon–carbon double bond of the endo-[6 + 4]
dimer of 6,6-dimethylfulvene.
Organisch-Chemisches Institut der Universitat Munster,
¨
¨
Corrensstrasse 40, 48149 Munster, Germany.
In solution we have also found a single diastereoisomer of 6.
1
It shows the H NMR signals of two pairs of methyl groups
¨
E-mail: erker@uni-muenster.de; Fax: +49 251-83 36503
w Electronic supplementary information (ESI) available: Additional
experimental and spectroscopic data. CCDC 798300. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc04567e
0
(C12–CH3/CH3 ; d 1.48/1.03) and three olefinic CH signals
(8-H: d 4.65; 3-H/4-H; d 5.91/5.98). There is a phosphonium
type 31P NMR resonance at d 33.2 and a borate 11B NMR
signal at d ꢀ14. Due to the chirality of the framework the
z X-Ray crystal structure analyses.
c
2006 Chem. Commun., 2011, 47, 2006–2007
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tentatively assign the structure of the 6,6-dimethylfulvene
[6 + 4] dimer isomer 7. This is likely formed from the dimer
4 by means of a rapidly proceeding thermally induced
1,5-hydrogen migration inside the cyclopentadiene moiety of
the dimer framework. Compound 7 features a total of four
olefinic 1H NMR signals at d 6.13 (3-H), 6.02 (4-H), 5.96
(8-H), and 5.76 (10-H). Then there are the 1H/13C NMR
signals of four CH3 substituents. The 9-H/H0 resonances were
found at d 2.69/2.68.
The mechanism of the formation of the formal frustrated
Lewis pair adduct 6 of the 6,6-dimethylpentafulvene [6 + 4]
dimer still needs to be elucidated. It is well conceivable that the
formation of the dimeric framework might have been induced
by the strong boron Lewis acid part of 5, following a cationic
reaction sequence with an eventual cooperative trapping by
the intramolecular phosphorus Lewis base. Alternatively, it is
conceivable that the frustrated P/B Lewis pair has effectively
added to the fulvene dimer 4 from an unfavorable dimer/
monomer equilibrium situation.11 Whatever the detailed
mechanism might be, the remarkable formation of product 6
from the Lewis pair 5 and 6,6-dimethylfulvene, indicates that
frustrated Lewis pair chemistry might actually have a much
wider implication and application potential than previously
thought.
Financial support by the Deutsche Forschungsgemeinschaft
is gratefully acknowledged.
Notes and references
1 R. B. Woodward and R. Hoffmann, Angew. Chem., Int. Ed. Engl.,
1969, 8, 781 (Angew. Chem., 1969, 81, 797) and references cited
thereinK. N. Houk, Acc. Chem. Res., 1975, 8, 361.
2 K. N. Houk and R. Woodward, J. Am. Chem. Soc., 1970, 92, 4145;
R. W. Alder, J. N. Harvey, G. C. Lloyd-Jones and J. M. Oliva,
J. Am. Chem. Soc., 2010, 132, 8325.
Fig. 1 Two projections of compound 6 (the lower one only contains
the core atoms for clarity).
3 Review: J. H. Rigby, Org. React. (Hoboken, NJ, U.S.), 1997, 49,
331.
4 B. Uebersax and M. Neuenschwander, Chimia, 1981, 35, 400 and
references cited therein.
5 H. Tanida, T. Irie and K. Tori, Bull. Chem. Soc. Jpn., 1972, 45,
1999; M. N. Paddon-Row, P. L. Watson and R. N. Warrener,
Tetrahedron Lett., 1973, 14, 1033; M. N. Paddon-Row, K. Gell and
R. N. Warrener, Tetrahedron Lett., 1975, 16, 1975;
B. M. Gatehouse, P. Leverett and D. Taylor, Aust. J. Chem.,
1979, 32, 211–216; R. N. Warrener, M. N. Paddon-Row,
R. A. Russell and P. L. Watson, Aust. J. Chem., 1981, 34, 397;
R. N. Warrener, M. L. A. Hammond and D. N. Butler, Synth.
Commun., 2001, 31, 1167.
6 D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46
(Angew. Chem., 2010, 122, 50).
Scheme 2 Synthesis of compounds 6 and 7.
7 C. Chen, R. Frohlich, G. Kehr and G. Erker, Chem. Commun.,
¨
mesityl groups at P and the C6F5 substituents at B are pairwise
diastereotopic. Moreover, at 253 K the rotation of both the
P-mesityl and B-C6F5 groups is frozen on the NMR time scale,
so that all their core atoms feature separate individual NMR
signals. Thus, we observe a total of 10 well resolved 19F NMR
resonances of the adduct 6 (four ortho, two para and four meta
19F NMR signals).
2010, 46, 3580.
8 P. Spies, G. Erker, G. Kehr, K. Bergander, R. Frohlich, S. Grimme
¨
and D. W. Stephan, Chem. Commun., 2007, 5072; P. Spies,
S. Schwendemann, S. Lange, G. Kehr, R. Frohlich and
¨
G. Erker, Angew. Chem., Int. Ed., 2008, 47, 7543 (Angew. Chem.,
2008, 120, 7654).
9 C. M. Momming, S. Fromel, G. Kehr, R. Frohlich, S. Grimme and
¨
¨
¨
G. Erker, J. Am. Chem. Soc., 2009, 131, 12280.
10 C. M. Momming, G. Kehr, B. Wibbeling, R. Frohlich,
¨
¨
Heating of the adduct 6 in d6-benzene solution at 85 1C (6 h)
resulted in a cleavage of the P/B Lewis pair from the organic
framework. Admixed with some pentane, from which it was
hard to separate due to its marked volatility, we isolated
a hydrocarbon product in low yield (ca. 35%) which we
B. Schirmer, S. Grimme and G. Erker, Angew. Chem., Int. Ed.,
2010, 49, 2414 (Angew. Chem., 2010, 122, 2464).
11 Secondary orbital interactions should favor the formation of the
exo-[6 + 4] fulvene dimer. However, the endo-[6 + 4] product is
found to be the major product observed in the iso-benzofulvene
dimerization reaction (see ref. 5).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2006–2007 2007