Regioselectivity of the Reactions of Heteroatom-Stabilized Allyl ...

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also been devoted to find synthetic transformations
in which the normal chemical reactivity of functional
groups is reversed. Reversed polarity synthons of
carbonyl compounds function as acyl anions or
“masked” acyl anions. In gem-diheterostabilized allyl
anions (sections III and IV), R-attack provides acyl
anion equivalents whereas γ-attack can lead to
homoenolate equivalents, synthons which are capable
of transforming aldehydes and ketones into γ-lactols
and γ-lactones.
If an allylic anion reacts with an aldehyde or
unsymmetrical ketone, the question of the stereochemistry
of the stereogenic center thus formed is
raised. Additionally, if a γ-product 3 is obtained, the
stereochemistry can be either E or Z. Utilization of
chiral substrates or reagents further requires consideration
of enantiomer and diastereomer separation.
Initially, interest was focused on the regioselectivity
of the reactions of unsymmetrical allylic
anions to form carbon-carbon bonds; with the production
of chiral homoallylic alcohols, the stereoselectivity
of carbon-carbon bond formations has received
increasing attention.
The scope of the present review is limited to
heteroatom-stabilized allyl anions, i.e., to those where
one (as in Scheme 1) or more heteroatoms are directly
attached to the three-carbon systems of the anion.
Such heterosubstitution generally considerably stabilizes
the allyl anion. We discuss first allyl anions
stabilized by a single heterosubstituent, considering
successively halogen substitution, followed by O-, S-,
Se-, N-, P-, Si-, and B-linked allylic anions. Second,
allylic anions stabilized by two heteroatoms are
considered, subdivided into those substituted by two
identical geminal heteroatoms, two different geminal
heteroatoms and two heteroatoms at the 1,2- or 1,3position.
Next, examples of three heteroatom-stabilized
allylic anions are presented. Structures of the
various classes of allylic anions are described in the
light of NMR and crystallographic data. While the
review is mainly concerned with lithium as the
counterion; transmetalation with other metal ions is
briefly discussed if appropriate, and summarized in
the final section. No previous review has dealt with
this subject matter in the manner described here, but
many reviews have provided valuable overviews of
much of this information: some important works are
found in ref 4a-d.
II. Allylic Anions Stabilized by One Heteroatom
(CdC−C−X)
Extensive work has been carried out on monoheterosubstituted
allyl anions, covering a variety of O-,
S-, N-, P-, and Si-linked derivatives in various
oxidation states. Selenium- and boron-substituted
allyl anions are less explored. From the synthetic
point of view, sulfur- and phosphorus-stabilized allyl
anions are probably the most important, but many
other anions of this class are also of significant
synthetic interest.
A. Halogen
Among the monohalogen-substituted allyl anions,
most of the work has been carried out on the chloro
derivatives together with some on bromine-stabilized
anions. Fluoroallyllithium, while as yet unreported
due to its structural instability, has been treated
theoretically. 5
1. Allyl Chlorides (CdC−C−Cl)
Kharasch’s examination of halogen-substituted allyl
anions in 1939 provided the first examples of
heterosubstituted allylic species to be studied. 6 Analogous
to the corresponding bromides, allyl chlorides
4 undergo R-alkylation to 7 with aliphatic halides 7
and TMSCl (Scheme 2). 7,8 Such sequences were, for
example, used for the preparation of lavanduol 9 and
pheromones. 10 Similarly, 1-chloro-1-methylallyllithium
19 reacts with aldimines and ketimines via
attack at the R-carbon to produce N-substituted
ethylenic aziridines 20. 11 However, mainly γ-attack
to give 9 occurs with iminium salts (Scheme 2) 12 and
upon stannylation to give 10. 7,13 γ-Attack also occurs
with aromatic and mixed ketones, aliphatic aldehydes
and aromatic aldehydes with or without electron-donating
substituents to produce γ-chlorinated
�-ethylenic alcohols 13. 7,14 Cyclic ketones cause formation
of R-adducts while for aliphatic ketones and
for aromatic aldehydes bearing an electron-attracting
substituent, R- 12 and γ-products 13 are both observed.
14 In general, with aliphatic and aromatic
esters R-attack is observed to give R-chlorinated
�-ethylenic ketones 18 while the analogous products
Scheme 2