Polysulfones: Catalysts for alkene isomerization

Article abstract of DOI:10.1002/anie.200453897

A radical intermediate is generated when methylidenecyclopentane (1) is isomerized by SO₂ into 1-methylcyclopentene (3) through the formation of a polysulfone polymer (PS), which first abstracts a hydrogen atom from the alkene. The allyl radical intermediate 2 abstracts a hydrogen atom from another alkene molecule 1, to yield the isomerized alkene and regenerating the allyl radical. Polysulfones are organic catalysts.

Full text of DOI:10.1002/anie.200453897

Communications
Alkene Isomerization
Polysulfones: Catalysts for Alkene
Isomerization**
´
Dean Markovic and Pierre Vogel*
It has long been known that alkenes can be isomerized by
sulfur dioxide. The process is generally explained by invoking
an ene reaction between an SO2 molecule and the alkene to
give a b,g-unsaturated sulfinic acid intermediate (e.g. 1 +
SO2!2; Scheme 1), which then undergoes a [1,3] sigmatropic
shift to form an isomeric b,g-unsaturated sulfinic acid (2!3),
followed by a retro-ene reaction that eliminates SO2 (3!4 +
SO2).[1]
Scheme 1. General mechanism for SO2-catalyzed isomerization of
methylidenecyclopentane (1).
When we treated methylidenecyclopentane (1) with
various amounts of SO2 in CD2Cl2 at 08C, isomerization
into 1-methylcyclopentene (4) occurred within a few hours.
However, in the presence of a small amount of CH2I2, the
reaction was totally inhibited. As 1,1-diodoalkanes in the
presence of SO2 are suspected to generate iodine radicals,[2]
this striking observation suggests that the SO2-promoted
isomerization 1!4 does not follow the generally accepted
mechanism 1!2!3!4, but rather another mechanism that
involves radical intermediates. This hypothesis was confirmed
by the observation that TEMPO (2,2,6,6-tetramethyl-1-piper-
idinyloxy free radical) and Bu3SnH both efficiently inhibit the
SO2-promoted isomerization 1!4. In the absence of radical-
scavenging agents, the kinetics of the SO2-promoted isomer-
ization 1!4 showed an induction period, the duration of
which increased upon decreasing the SO2 concentration. This
period was followed by a zeroth-order reaction until about
70% conversion. During the induction period a white
precipitate formed, the amount of which increased upon
´
[*] D. Markovic, Prof. Dr. P. Vogel
Institute of Chemical Sciences and Engineering,
Swiss Federal Institute of Technology (EPFL),
BCH, 1015 Lausanne (Switzerland)
Fax: (+41)21-693-9355
E-mail: pierre.vogel@epfl.ch
[**] This work was supported by the Swiss National Science Foundation
(Grant No. 2000-20100002/1) and the Office FØdØral de L'Education
et de la Science (OFES) (Grant No. COI.0071, COST D13/010/01).
We also thank Mr. Martial Rey for technical help and Professor L.
Forro (EPFL) for access to ESR instruments.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200453897
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Angewandte
Chemie
increasing the proportion of SO2. Elemental analysis and IR
spectra confirmed that the precipitate is a 1:1 copolymer of 1
and SO2, a polysulfone (PS). Titration of this polymer with
aqueous NaOH (0.1m) neutralized the free sulfinic acid
moieties and allowed us to evaluate an average molecular
mass of 230000.
The ESR spectrum of the polymer PS taken during its
formation showed typical signals for carbon-centered (ter-
tiary) and sulfonyl radicals.[3] At 298 K, their estimated
concentrations amount to approximately 10ꢀ6 m.
When freshly prepared PS or neutralized PS (with
aqueous NaOH) was added to a solution of 1 in CD2Cl2 an
immediate zeroth-order isomerization 1!4 occurred at 08C.
The reaction was inhibited efficiently with TEMPO and
Bu3SnH. Polysulfones are known to equilibrate with radicals
upon heating (ceiling temperature of polysulfone forma-
tion).[4,5] These radicals can abstract a hydrogen atom from 1
to give the allyl radical intermediate 5 (initiation step,
Scheme 2.). Intermediate 5 then reacts with 1 to produce 4
during which allyl radical 5 is regenerated (propagation step).
Isomerization of 1!4 was also induced upon irradiation
or heating (1208C) of 1 in the presence of a catalytic quantity
of (PhSO2)2 (diphenyldisulfone);[6] however, polymeric mate-
rial was also formed. Therefore the PhSO2C radical generated
under these conditions is not as selective a catalyst as PS for
the alkene isomerization, perhaps because of its ability to add
to the alkene concurrently with hydrogen abstraction. All the
experiments reported above are consistent with the radical
chain mechanism shown in Scheme 2. Further proof was
obtained in the following way.
Figure 1. Partial 2H{1H}-NMR spectrum of 1-methylcyclopentene
(methyl region) arising from the reaction 1+6!4 (CD2Cl2, 08C).
deuterated allyl radical 5. At the end of the isomerization (20
1
more half-lives), the H-and 2H{1H}-NMR spectra did not
change, showing that the exchange between positions C3,4
and the methyl groups in 1-methylcyclopentenes is much
slower than the isomerization 1!4.
The polysulfone PS catalyzed the isomerization of the
methylidenecyclohexane into 1-methylcyclohexene, 2-meth-
ylbut-1-ene into 2-methylbut-2-ene, 2-ethylbut-1-ene into 2-
ethylbut-2-ene, and 2-methylhept-1-ene into 2-methylhept-2-
ene in CD2Cl2 at room temperature. All reactions were high-
yielding, with no formation of polymeric material (1H NMR
spectroscopic analysis). This was not the case with the SO2-
induced isomerizations, which were all accompanied by the
formation of polymers.
In summary, we have uncovered a new mechanism for
alkene isomerization induced by sulfur dioxide: The poly-
sulfone polymers resulting from their co-condensation are
organic catalysts for the alkene isomerization.
Received: January 30, 2004 [Z53897]
Keywords: alkenes · isomerization · organocatalysts ·
.
polysulfones · radicals
Scheme 2. Radical mechanism for the polysulfone-catalyzed isomeriza-
tion of alkenes.
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a
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Communications
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