PtCl2-catalyzed rearrangement of methylenecyclopropanes

Article abstract of DOI:10.1021/ja061392y

Alkylidenecyclopropanes readily convert into cyclobutene derivatives on treatment with catalytic amounts of PtCl₂. The reaction is strongly accelerated when performed under an atmosphere of CO (1 atm). The resulting cyclobutenes are isolated in

Full text of DOI:10.1021/ja061392y

Published on Web 04/26/2006
PtCl2-Catalyzed Rearrangement of Methylenecyclopropanes
Alois Fu¨rstner* and Christophe A¨ıssa
Max-Planck-Institut fu¨r Kohlenforschung, D-45470 Mu¨lheim/Ruhr, Germany
Received February 28, 2006; E-mail: fuerstner@mpi-muelheim.mpg.de
Scheme 1. PtCl2-Catalyzed Cyclobutene Formation
Activation of π-bonds with catalytic amounts of carbophilic
transition metal cations, most notably PtII, AuI, or AuIII, constitutes
a formidable trigger for a host of skeletal rearrangement reactions.1
Transformations of this type are simple, safe, and convenient to
perform and usually result in a significant increase in structural
complexity. While the reactivity of alkynes and enynes in the
presence of such catalysts has already been investigated in
considerable detail,1 the behavior of alkenes is far less understood.
Various recent examples of Pt- or Au-catalyzed addition reactions
to unactivated olefins, however, provide encouraging leads for
further investigations.2
As part of our ongoing studies in this field,3 we identified
alkylidenecyclopropanes4 as a potentially useful class of substrates.
It was anticipated that coordination of a soft cation to their double
bond might engender productive isomerizations driven by the
release of ring strain. Although many different reactions of
alkylidenecyclopropanes induced by noble metal- or Lewis-acid
catalysts are already known in the literature,5 most notably their
conversion into homoallylic products, it was hoped that other
conceivable scenarios might be realized that have little or no
precedence.6
Table 1. Cyclobutenes by PtCl2-Catalyzed Rearrangement of
Alkylidenecyclopropanesa
We were pleased to see that treatment of compound 3 with
catalytic amounts of PtCl2 in toluene resulted in the clean formation
of cyclobutene 4 (Scheme 1).7 In line with previous findings from
our group, the reaction was significantly accelerated when per-
formed under an atmosphere of CO.8 Under these conditions, the
catalyst loading can be reduced to 1 mol %, providing product 4
in 77% isolated yield. Table 1 shows the scope of this transforma-
tion which is applicable to alkylidenecyclopropanes with either
aliphatic or aromatic substituents at the double bond. Electron-
withdrawing as well as electron-donating substituents are well
accommodated. It is also worth mentioning that the substrates were
conveniently formed by a new variant of the Julia-Kocienski
olefination,9 simply on exposure of the corresponding aldehyde to
cyclopropyl sulfone 2 and Cs2CO3 in THF/DMF at 70 °C. Since
the olefination occurs under Barbier conditions and no separate
deprotonation step is necessary,10 this procedure is highly user
friendly. For details, consult the Supporting Information.
a All reactions were performed with PtCl2 (5 mol %) in toluene (0.1 M)
at 80 °C under CO (1 atm) unless stated otherwise. b c ) 0.02 M.
Scheme 2. Proposed Mechanism
A tentative mechanism for the observed cyclobutene formation
is depicted in Scheme 2. One of the possible resonance structures
of the complex formed by coordination of Pt(2+) to the double
bond of the substrate constitutes a stabilized cyclopropylmethyl
cation.1 This “nonclassical” species is prone to rearrange to the
corresponding cyclobutenyl cation complex which likely has some
carbene character11 and evolves by 1,2-hydrogen shift to the final
product. This interpretation gains credence by the deuterium-
labeling experiment depicted in Scheme 3. In line with the proposed
mechanism, product 4-D is exclusively labeled at the 2-position,
with a deuterium incorporation of g97% (NMR).
CH2Cl2 to the crude mixture formed upon PtCl2-catalyzed re-
arrangement of substrate 17 bearing an allyl ether entity led to
chromene 19 in good overall yield. This product originates from
an efficient ROM/RCM cascade13 of cyclobutene 18 initially formed
(Scheme 4).
The novel cyclobutene formation can be linked to further catalytic
transformations. Thus, addition of Grubbs catalyst (5 mol %)12 and
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10.1021/ja061392y CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Deuterium-Labeling Experiment
observation that substrate 25 provides the dimeric olefin 26 which
likely originates from the same type of zwitterionic intermediate.
Since the arene in 25 is somewhat less activated, elimination by
loss of proton outperforms the Friedel-Crafts pathway and leads
to the observed product 26.
In summary, we have shown that alkylidenecyclopropanes, on
activation with catalytic amounts of PtCl2 or, preferentially, PtCl2/
CO (1 atm), undergo previously unknown ring expansions, thus
opening a convenient new entry into variously substituted cy-
clobutenes and derivatives thereof. Further investigations on this
and related types of noble metal-catalyzed rearrangements are
ongoing and will be reported in due course.
Scheme 4. Cyclobutene Formation/ROM/RCM Cascade
Acknowledgment. Financial support by the MPG and the Fonds
der Chemischen Industrie is gratefully acknowledged. We thank
Umicore, Hanau, for a generous gift of noble metal salts and B.
Gabor and R. Ettl for their help with the NMR investigations.
Scheme 5. PtCl2-Catalyzed Dimerization
Supporting Information Available: Experimental details, including
the formation of alkylidenecyclopropanes by Julia-Kocienski olefi-
nation. This material is available free of charge via the Internet at http://
pubs.acs.org
References
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Scheme 6. Cyclodimerizations of Electron-Rich Substrates
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Yet other possibilities were encountered when the PtCl2-catalyzed
rearrangement was applied to substrates bearing very electron-rich
arene substituents. Even though compound 11 provides the corre-
sponding cyclobutene 12 in reasonable yield (cf. Table 1, entry 4),
the dimeric product 21 is obtained when the reaction is performed
at higher concentrations (Scheme 5). The unusual structure of this
compound was unambiguously elucidated by extensive NMR
investigations (cf. Supporting Information). Its formation is readily
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yield (Scheme 6). This mechanistic proposal also accounts for the
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