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

Scheme 79
Scheme 80
b. Aminosilyl-Substituted Allylanions. While a
dialkylamino group on silicon favors γ-alkylation, 257
aminomethyl substituents on the silicon atom as in
460 are effective ligands for the lithium ion for
direction R-orientation. For example, 100% regioselectivity
is achieved for the methylation of 461 (R )
CH2CH2OEt) to give 462 (Scheme 81). 230 The course
of the reaction is dependent on the solvent; for
example, the regioselectivity in ether is much higher
compared with that in THF and dependent on the
size of the alkylating agent. 231 Silylallyl carbanions
463 bearing a chiral lithium complexing substituent
remote from silicon are similarly R-alkylated by small
electrophiles in nonpolar solvents to give 464 and
γ-attacked by secondary halides to form 465. Both
types of reaction proceed stereoselectively (Scheme
81). 258
The reactions of aminomethyl-substituted silylallyl
anions with carbonyl compounds give a mixture of
the R- and γ-products whereby the γ-selectivity is
increased with the decrease of the steric bulkiness
Scheme 81
Scheme 82
Scheme 83
of the amino group. If the silyl moiety in 466 contains
a group L which is capable of internal chelation (467),
the reactions with carbonyl compounds give preferentially
γ-addition products such as 468 and 469
(Scheme 82) analogous to allyltrialkylsilanes. 259
The addition of copper cyanide to form 471 also
improves the γ-selectivity to give 472, similar to that
of allyltrialkylsilanes (see also Scheme 79), while
after transmetalation of aminosilyl-substituted allyl
anion 470 with zinc the R-adduct 474 is obtained
(Scheme 83). 260 The use of magnesium bromide in
reactions with carbonyl compounds also causes the
formation of R-product.
3. Silylated Methylenecyclopropanes
689
The reaction of silylated methylenecyclopropane
anions 476 with benzaldehyde followed by quenching