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

Scheme 162. Allyl Anions Stabilized by Three
Heteroatoms
groups. This outcome of the reaction is favored by
the SPh group in the γ-position to silicon and also
by the SPh R to the silicon because this impedes
attack at the R-carbon in 801 (Scheme 153), since it
is already substituted twice. However, if the SPh
group in the γ-position to the silicon is replaced by a
methoxy function, the R-carbon related to silicon and
sulfur is attacked (Scheme 154). This is in agreement
with the directing effects of oxygen and sulfur (see
section V.14). 2-(Diethylamino)-4-(phenylthio)-2-butenonitrile
809 is alkylated at the carbon R to the thio
group. This regioselectivity is enhanced by the cyano
and the amino group in the γ-position (Scheme 155).
Opposed directing effects contribute to the outcome
of the reactions in the other cases discussed.
VII. Results Available on the Structure of
Organometallic Intermediates and Their
Non-metalated Precursors
Though various crystal structures of non-metalated
substituted allylic structures have been reported,
there is still a dearth of information about the related
allylmetals.
A. Crystallographic Data on Allyllithiums
1. Sulfur Derivatives
The X-ray structure of an allyl sulfoxide/cyclopentenone
adduct 843 (Scheme 163) demonstrates the
steric demand of modified biphenyl substituents at
a sulfur atom107 and the course of the highly regioand
diastereoselective reaction by which 843 is
formed. Compound 843 results from the combination
of the corresponding allyllithium with cyclopentenone
in THF at -78 °C and presents an example for the
utility of modified lithiated allyl sulfones.
Scheme 163
The crystal structures of several lithiated allylic
sulfoximines have also been reported. 151a-c Exclusive
1,4-R-addition of acyclic enones to lithiated N-p-tosyl-
S-phenyl-S-prop-2-enyl sulfoximine proceeds in high
yield and high diastereoselectivity to give the �-substituted
γ-keto sulfoximine 846 (Scheme 164). 151a
Scheme 164
709
Cyclic enones such as cyclopentenone and cyclohexenone
add to give mixtures of the four possible
diastereoisomers. The X-ray structure of one of the
cyclopentenone adducts, 844 (Scheme 163), is shown.
Benzylideneacetophenone reacted with the lithiated
allyl sulfoximine to give predominantly one product,
the 3R*,4R*,5S* structure 845 (Scheme 163). 151a The
acyclic adducts 846 can easily be reduced with
DIBAL-H to give the corresponding alcohols 847 with
surprising diastereoselectivity. Scheme 164 shows
significant characteristics of two relevant X-ray
structures. 149c,151c Solvent separated contact ion pairs
of [Li(12-crown-4)2] + , and the sulfoximine anions
showed the X-ray structure of a lithiated allylic
sulfoximines/12-crown-4 complexes. 151c
Stability problems of sulfur-substituted allyllithiums
are apparent from investigations by Seebach et
al.: the TMEDA complexed lithiated 2-butenyl tertbutyl
sulfide 848 (X-ray structure: Scheme 165) was
stable only up to -20 °C. 403 The Li + is coordinated
at the TMEDA nitrogen atoms and the double bond,
which acts as an additional ligand. These structural