J. Am. Chem. Soc. 1996, 118, 1209-1210
1209
Scheme 1
New Radical Allylation Reaction
Be´atrice Quiclet-Sire† and Samir Z. Zard*,†,‡
Institut de Chimie des Substances Naturelles, CNRS
91198 Gif-sur-YVette, France
Laboratoire de Synthe`se Organique associe´ au CNRS
Ecole Polytechnique, 91128 Palaiseau, France
ReceiVed July 10, 1995
The utility of organotin hydrides, which have dominated the
synthetic radical chemistry scene for the past two or three
decades,1 has been significantly increased by the development
of allylstannanes and other related tin derivatives. These allow
a functional group to be introduced in the last propagation step
instead of the usual hydrogen atom transfer.2 However, the
toxicity of organotin residues as well as the perennial purifica-
tion problems commonly encountered with stannane derivatives3
has generally precluded large-scale application of stannane-based
methodology. We now describe a new allylation process which
does not involve heavy metals and which can lead to a number
of interesting synthetic transformations.
Our approach relies on the fact that sulfonyl radicals add
reversibly to olefins and the little-appreciated observation that
alkyl radicals can react in a reversible manner with sulfur
dioxide.4 In general, the equilibrium favors the alkylsulfonyl
radical, but if the alkyl group represents a stabilized radical (e.g.,
a benzyl), then extrusion of sulfur dioxide occurs readily.5
Practically all the synthetic applications of sulfonyl radicals have
involved arylsulfonyls which do not undergo R-scission,6 so that
this aspect of their chemistry has on the whole been overlooked
in the past.4c
With these considerations in mind, we contrived a system
where a succession of reversible or degenerate steps force the
process in the desired direction, even in cases where the
extrusion of sulfur dioxide from the alkylsulfonyl radical is not
particularly favored. Our conception is outlined in the reaction
manifold displayed in Scheme 1. Addition of a radical to the
terminal olefin of alkyl allyl sulfone 1 generates an alkylsulfonyl
radical 2 (path A) which can only react in a redundant manner
with its precursor 1 (path B). The only way for the system to
evolve is by extrusion of sulfur dioxide (path C) to give alkyl
radical R•, which can now propagate the chain by addition to 1
to give the desired product 37 via path A. This scheme is still
flawed, because the product 3 can also serve as substrate for
radical additions. As 3 accumulates in the medium, it competes
for radical R• (path D) to give adduct 4 and so on, bringing the
system out of control. Since the product 3 is expected to have
a reactivity similar to that of the starting allyl sulfone 1, one
solution would be to operate at low conversion, a very serious
limitation from a synthetic standpoint. To avoid such an
untoward situation, we envisaged the possibility of adding a
relay allylating reagent to overwhelm unwanted path D. In
principle, this could be accomplished by adding an excess of
an aryl allyl sulfone 5. This second allyl transfer reagent, by
a sheer concentration effect, will continuously scavenge R• (path
E) to provide the same product 3 and an arylsulfonyl radical 6
which cannot loose a sulfur dioxide molecule. The only
alternative then is a reversible reaction (path F) with the first
allyl sulfone 1 thus regenerating the relay sulfone 5 and
producing alkylsulfonyl radical 2 to propagate the chain. Under
such conditions, the formation of the desired product 3 will
therefore occur principally through path E rather than path A
and formation of side products by route D will be largely
circumvented.
† Institut de Chimie des Substances Naturelles, CNRS.
‡ Laboratoire de Synthe`se Organique associe´ au CNRS.
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Pereyre, M.; Quintard, J.-P.; Rahm, A. Tin in Organic Synthesis; Butter-
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290. (c) It is amusing to note that, in the latter review, the fragmentation of
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The following examples demonstrate the validity of the above
analysis. Upon heating, a solution of 1a-d with a 5-fold excess
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0002-7863/96/1518-1209$12.00/0 © 1996 American Chemical Society