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

700
Scheme 123
Scheme 124
E. Nitrogen and Phosphorus
1. 1-(Diethoxyphosphoryl)-1-(dimethylamino)-1-propenes
(C−CdC(NMe2)(PO(OEt)2)
1-(Diethoxyphosphoryl)-1-(dimethylamino)allyl anion
674 undergoes γ-alkylation and silylation to give
675 which can be alkylated again to give 678, 680,
and 682 and yield upon hydrolysis the corresponding
carboxylic acid 679, 681, and 683 alkylated at C-3.
Reaction with benzaldehyde gives low yields of the
δ-hydroxyallyl compound 677, similar to the reaction
with isobutenoxide producing 676 (Scheme 125). 354
F. General Discussion
The orientation of reactions with electrophiles of
allyl anions stabilized by two different gem-heteroatoms
are summarized in Scheme 126.
Geminal benzotriazole/alkoxy substituents (Bt-C-
OR) direct substitution exclusively to the R-position
for all electrophiles studied except very sterically
crowed carbonyl groups where the regioselectivity
favors γ-substitution. A rather similar pattern is
shown for cyano/alkoxy (NC-C-OR) except that here
reactions with carbonyl electrophiles are probably
reversible and kinetically favor R-substition at low
temperature but thermodynamically favor γ-substitution
at high temperature. For cyano/diallylamino
(NC-C-NR2), conjugate addition occurs R but allylation
gives R/γ mixtures.
Another set of geminal substituents shows essentially
exclusive reaction at the γ-position. This
includes P(O)R2 or PO(OEt)2 paired with OMe,
OSiMe3, NR2, and SiMe3. The SiMe3/SR pair also
Scheme 125
belongs this class as does the ionized N-triazolyl/
ethoxy derivative.
The SiR3/Cl and SiR3/OR pairs fall into an intermediate
position showing a mixture of R- and γ-substitution.
V. Allyl Anions Stabilized by Two Non-Gem-
Heteroatoms (XCdC−CY and CdCX−C−Y)
In addition to allyl anions stabilized by two identical
heteroatoms, allylic systems bearing two different
heteroatoms have been studied particularly those
containing oxygen or sulfur.
A. Two Identical Heteroatoms
Symmetrical allylic systems substituted in the 1and
3-position by two identical heteroatom-linked
substituents are a unique class of compounds, since
attachment of an electrophile to either terminal
carbon yields the same product. We also discuss
compounds 1,3-disubstituted by two different functional
groups linked by the same heteroatom.
1. 1,3-Bis(alkylthio)-1-propenes (RS−CdC−C−SR)
The 1,3-bis(methylthio)allyl anion is readily synthesized
in a three-step sequence from epichlorhydrin.
Although probably intrinsically unstable, it
undergoes a clean reaction with electrophiles to give
a protected form of the conjugated aldehyde RCHd