Chemistry Reference
In-Depth Information
[M]
[M]
Nu
+
X
Nu
Nu
[M]
Scheme 8B.1.
Allylic substitution via allyl-metal complexes as intermediates.
Union Carbide), who demonstrated in 1970 that phosphines accelerate the reaction and
palladium can be employed in catalytic amount [3,4]. The fi eld was soon dominated by
Trost and co-workers, who started their crusade in 1973 [5]. They were the fi rst to
achieve enantioselectivity in an allylic substitution, in stoichiometric (1973) [6] as well
as catalytic (1977) reaction mode [7].
Thus, the stage was set for our topic, asymmetric catalysis. For a considerable time,
progress was slow; however, in the 1990s, dramatic improvements were achieved for the
Pd-catalyzed allylic substitution. The progress was periodically documented in excellent
reviews [8-13]. In the present series of topics, the fi eld was last reviewed by Trost and
Lee in 2000 [14]. At that time, Pd catalysis dominated the fi eld. In the meantime, enor-
mous advances have been made with Mo, Ir, Ru, Ni, and Cu catalysts, which have offered
new opportunities, for example, for reactions with monosubstituted allylic substrates.
An exhaustive coverage of the whole fi eld by Lu and Ma, with emphasis on chiral
ligands, was published in January 2008 [15].
In this chapter, we will follow Trost and Lee in the general ordering and will, in the
fi rst part, update the fi eld of Pd-catalyzed reactions reported after ca. 2000 with prefer-
ence for preparative applicability rather than benchmark examples, which have been
fully presented by Lu and Ma. The emphasis will be placed, in the second part, on the
reactions promoted by non-Pd catalysts as cited above. There are many examples of
stereospecifi c rather than enantioselective reactions, which are typical for Fe and Rh
complexes. These reactions are not covered in detail here.
8B.2. PALLADIUM - CATALYZED ENANTIOSELECTIVE
ALLYLIC ALKYLATIONS
8B.2.1. Fundamentals, Mechanistic Aspects
Many aspects discussed in this chapter are valid for transition metal complexes in
general. The corresponding reaction schemes are presented using the letter M, short for
transition metal. Those facts that are specifi c for palladium are formulated with the
element symbol Pd.
8B.2.1.1. Classifi cation of Allylic Substitutions
An allylic substitution begins with
coordination of a low valent metal species to the double bond of an allylic system. Sub-
sequent oxidative addition by expulsion of the leaving group X yields a π - allyl complex
as intermediate (Scheme 8B.2). This intermediate can be a neutral or, presumed to be
more reactive, a cationic complex, depending on the nature of the ligands and the coun-
