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

Scheme 100
In reactions of the allyl anion of 565 with three- to
six-membered cyclic ethers the R-products 566 are
formed in the presence of BF3 (Scheme 101). 302
Scheme 101
2. 2-(2-Propenyl)-1,3-dithiane 1-Oxide
(CdC−C(SR)(SOR))
The anion of 2-(2-propenyl)-1,3-dithiane 1-oxide
567 reacts with aldehydes at the R-site and predominantly
on the face syn to the sulfinyl group affording
568 (Scheme 102) 298 in contrast to the dithiosubstituted
crotyllithium (see section III.C.1). 294,295
Scheme 102
3. 2-Alkenyl-1,1-disulfones (CdC−C(SO2R)2)
Allylic 1,1-disulfones 569 undergo R-alkylation to
570. Upon reduction of 570 allylic sulfones 572 are
produced which can be converted into olefins 571 by
lithium in ethylamine (Scheme 103). 303
Scheme 103
D. General Discussion
The bissulfone [CdC-C(SO2R)2] undergoes exclusive
R-substitution, as does the monosulfide-monosulfoxide
[CdC-C(SOR)SR].
For dithioketals [CdC-C(SR)2], and for dichloro
compounds [CdC-CCl2, reactions often take place
preferentially at the R-position, but more bulky
electrophiles such as Me3SnCl and carbonyl compounds
react at least in part at the γ-postion.
The unstable ketal [CdC-C(OR)2] reacts exclusively
at the R-position.
An attempted rationalization of these results is
presented in section IX.
IV. Allylic Anions Stabilized by Two Different
Gem Heteroatoms (CdC−CXY)
Various examples of allylic anions substituted by
two geminal heteroatoms are considered. Besides
those substituted by silicon and halogen, especially
oxygen- and/or nitrogen-containing reagents are reviewed.
Furthermore, the cyano function is treated
here as a heterogroup and therefore cyanonitriles are
also discussed.
A. Silicon with Halogen, Oxygen, Nitrogen,
Sulfur, or Phosphorus
1. 1-Chloro-1-(trimethylsilyl)-2-propenes
(CdC−C(Cl)(SiR3))
Lithiated 1-chloro-1-(trimethylsilyl)-2-propenes 574
are prepared by the transmetalation reaction of
n BuLi with the corresponding lead compound 573
(Scheme 104). 304 The coupling reaction of 574 with
trimethylchlorosilane and trimethyltin chloride yields
the γ-products 576 and 577, respectively, while the
alkylation with MeI gives R-products 578. Seyferth
and Mammarella explain the attack by carbonyl
compounds in terms of the HSAB approach: the
carbon atom substituted with chlorine and trimethylsilyl
substituents should have greater negative
charge delocalization hence be the “soft” terminus,
while CH2 represents the “hard” terminus. Thus, the
reaction with benzaldehyde or 1,1,1-trifluoroacetone
results in the formation of γ-adduct 575 and 581,
respectively, while the reaction with “softer” dialkyl
ketones including cyclohexanone which is not sufficiently
selective gives a mixture of R- and γ-adducts
579 and 580 (Scheme 104) in contrast to the formation
of R-product 526 (Scheme 93) with gem-dichloroallyllithium.
Since the trimethylsilyl group is not
expected to be as efficient an electron-withdrawing
group as the electronegative chlorine substituent the
Scheme 104
695