Radical allylation with α-branched allyl sulfones

Article abstract of DOI:10.1002/anie.200804298

(Chemical Equation Presented) Branching out with sulfones: The use of allyl isopropyl sulfones solves the problem of premature isomerization and allows the radical addition of xanthates to α-branched allyl sulfones (see scheme; DCE=1,2-dichloroethane, TMS

Full text of DOI:10.1002/anie.200804298

Angewandte
Chemie
DOI: 10.1002/anie.200804298
Radical Reactions
Radical Allylation with a-Branched Allyl Sulfones**
Nicolas Charrier and Samir Z. Zard*
Radical allylations, especially using allylstannanes, have
emerged as powerful synthetic tools as evident by the
numerous synthetic applications on a wide range of sub-
strates.[1,2] One of the chief limitations of this technology is
with respect to the substitution pattern around the allyl group.
a-Substituted allylstannanes, such as 1, rapidly rearrange
under the reaction conditions into their more stable g-
substituted isomers (e.g. 2), and these were found to react
mostly through abstraction of the allylic hydrogen atom
rather than by the desired addition–fragmentation process
(Scheme 1).[2e,3,4] Other approaches using allylcobalt,[5] allyl-
peroxide-mediated reaction of xanthate 7 with a,a-dimethyl-
allyl ethyl sulfone 8 in 1,2-dichloroethane (DCE) at reflux was
sluggish, requiring 70 mol% peroxide and giving only 27% of
the desired product 9 (Scheme 2). Considerable quantities of
Scheme 2. Allylation with a-substituted allyl sulfone reagents.
Scheme 1. Side reactions during allylation with a-substituted allyl
triorganotin reagents.
the unwanted rearranged sulfone 11 were formed by addi-
tion–fragmentation of the intermediate ethylsulfonyl radicals
with a,a-dimethylallyl ethyl sulfone 8. Unlike the case of b-
substituted allyl sulfones 4, which can not be isomerized by
reaction with ethylsulfonyl radicals (because the addition–
fragmentation is degenerate), the persistence of the ethyl-
sulfonyl radicals in the medium in the case of a-substituted
allyl sulfones such as 8 (and more generally 6) turns out to be
a serious problem. Ethylsulfonyl radicals can not be removed
through reaction with xanthate 7, as the addition to the
thiocarbonyl leads to an intermediate 12 with a quite weak
gallium,[6] allylhalogen,[7] or allylsulfur[8] derivatives have
been explored in an attempt to overcome these shortcomings,
but most still require a stoichiometric amount of a tin-based
promoter[9] (or other metal-based reagents) and have usually
been applied to the introduction of simple allyl moieties.[10]
A
combination of radical and ionic sequence have also been
devised.[11]
We recently described a tin-free allylation of iodides and
xanthates 3 based on the use of allyl ethyl sulfones of general
structure 4 and leading to terminal alkenes 5 (Scheme 1).[12]
Unfortunately, when we tried to expand its scope to
encompass a-substituted allyl sulfones 6, we encountered
the same problems of undesired isomerization. Thus, lauroyl-
À
SO2 S bond. This intermediate forms readily but only
collapses back to the starting components and does not
therefore propagate the chain. The result is a poor yield of
product 9 and the need for a relatively large amount of
initiator in addition to an excess of the allylating agent 8, as
much of it is wasted through isomerization into g,g-dimethyl-
allyl ethyl sulfone 11.
[*] N. Charrier, Prof. S. Z. Zard
Laboratoire de Synthꢀse Organique associꢁ au CNRS
Ecole Polytechnique
The first resolution of this difficulty was to increase the
temperature of the reaction. The unimolecular extrusion of
sulfur dioxide from the ethylsulfonyl radical has a high
positive entropy component and is consequently much more
sensitive to temperature than the bimolecular addition to a,a-
dimethylallyl ethyl sulfone 8a, the first step in the unwanted
isomerization sequence. The faster loss of sulfur dioxide
would generate highly reactive ethyl radicals, and these
91128 Palaiseau Cedex (France)
Fax: (+33)169335972
E-mail: zard@poly.polytechnique.fr
[**] N.C. thanks the French Ministꢀre de l’Education Nationale, de la
Recherche et de la Technologie for a fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804298.
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should preferentially attack the more radicophilic thiocar-
bonyl group of xanthate 7 to give key intermediate 13.
Collapse of the latter can now proceed in the desired direction
and propagate the chain through the formation of starting
carbon radical 10. Indeed, when the reaction was performed
in refluxing chlorobenzene, the yield of 9 increased to 60%
and required only 30 mol% of lauroyl peroxide (Scheme 3).
Scheme 3. Resolutions of premature allyl sulfone isomerization.
DLP=dilauroyl peroxide.
Only little of isomerized sulfone 11 was produced. In the same
manner, xanthate 14 was converted into 16 in 78% yield using
sulfone 15a as the allylating agent (Scheme 3). This simple
expedient opened the route to the introduction of more
substituted allyl side chains, at least in the case of robust
substrates able to withstand the relatively high temperature.
For more complex or fragile structures, a milder and more
general procedure was required. To avoid the problematic
isomerization of the allyl sulfone, we postulated that replacing
the ethyl group by an isopropyl group would lead to the
formation of isopropylsulfonyl radicals, and these should
extrude sulfur dioxide at a significantly faster rate as
compared with ethylsulfonyl congeners. Even though no
kinetic measurements for the extrusion process were avail-
able, we hoped that the enhancement in the rate of
fragmentation would be sufficient for our purposes, allowing
us to operate at a much lower temperature and without the
problem of isomerization of the allyl sulfone reagent.
Indeed, using isopropyl sulfones 8b and 15b as the
allylating agent, the same allylated products 9 and 16 were
obtained from xanthates 7 and 14, respectively, in almost
identical yield and at the much lower temperature of DCE at
reflux. The mildness of the reaction conditions permits the
introduction of more functionalized side chains, as illustrated
by the examples in Scheme 4. Thus, citronellal-derived
sulfone 18 reacted with acetoacetyl xanthate 17 to furnish
diene 19 in 52% yield, without the need to protect the allylic
alcohol. The synthesis of the delicate skipped dienes 22 and 25
was accomplished in 69% and 66% yield starting from
xanthates 20 and 23, and sulfones 21 and 24, respectively. The
fact that there is little interference from the easily abstract-
able doubly allylic hydrogen atoms is quite remarkable.
Finally, the possibility of accessing the even more interesting
skipped enynes is highlighted by the clean formation of
Scheme 4. Radical additions on functionalized allyl sulfones. TMS=tri-
methylsilyl, Ts=4-toluenesulfonyl.
compound 27 in 73% yield from xanthate 14 and sulfone
26.
Although access to the series of allylated derivatives
displayed in Scheme 3 would be beyond most of the previous
radical allylation reactions, the fact that the precursors are
xanthates provides a concise and modular route to consid-
erably more complex structures, by combining the now well-
established xanthate transfer technology[13] with the present
allylation process. This approach is exemplified by the
transformations in Scheme 5.
Addition of cyclopropylacetonyl xanthate 28 to vinyl
pivalate provides adduct 29 in 89% yield. Allylation with
sulfones 8b and 26 under the usual reaction conditions
furnishes allylated compounds 30 and 31 in 71% and 62%
yield, respectively. The reaction of trimethoxybenzyl xanthate
32 with N-phenylmaleimide gives a high yield of the trans
adduct 33, and the xanthate group in the latter can be
replaced efficiently by an enyne side chain through a radical
addition–fragmentation process to allyl sulfone 26. The yield
of the resulting product 34 is 73% and, as would be expected,
À
the C C bond formation takes place from the opposite side to
the trimethoxybenzyl side chain with net overall retention of
configuration. Another variation is illustrated by the last
sequence starting with chloroketone xanthate 35. This highly
versatile xanthate[14] undergoes reaction with methallyl ace-
tate to give 36, from which the xanthate group can be
reductively removed by further reaction with lauroyl peroxide
in isopropyl alcohol. Displacement of the chloride in 37 now
gives rise to another xanthate 38, which can be allylated in
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Experimental Section
Typical procedure for the radical allylation: Lauroyl peroxide
(0.1 mmol) was added every hour to a solution of the xanthate
(1.0 mmol) and allyl sulfone (2.0 mmol) in degassed DCE at reflux
(1.0 mL) under a nitrogen atmosphere, until complete consumption
of the starting xanthate. The reaction mixture was then cooled to
room temperature, concentrated in vacuo, and purified by flash
column chromatography.
Received: August 31, 2008
Published online: October 31, 2008
Keywords: b elimination · allyl sulfones · allylation ·
.
radical reactions · xanthates
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