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

688
Scheme 75
Scheme 76
Scheme 77
the γ-adduct 431 upon treatment with electrophiles
(Scheme 75). 237 In the absence of internal chelation
a substituent in the γ-position of γ-substituted allylsilyl
anion from 432 tends to favor R-alkylation to
form 433 (Scheme 76).
Reaction of 434 with epoxides 435 produces a
mixture of the R- 436 and (E)-γ-isomers 437; ethylene
oxide (435, R 1 ) R 2 ) R 3 ) H) and monosubstituted
derivatives (435, R 1 or R 2 or R 3 * H) afford mainly
R-adducts 436 while 1,2-di- (435, R 1 and R 2 * H, R 3
) H) and 1,1,1-trisubstituted derivatives (435,R 1 ,R 2 ,
R 3 * H) yield γ-adducts 437. 238 The reaction of
epoxysilanes 438 with lithiated allylsilane 434 gives
predominantly the (E)-γ-product 439 (Scheme 77). 239
Reactions of allyltrialkylsilane anions 440 with
carbonyl compounds furnish preferentially (E)-γ-
Scheme 78
addition products 441 227,240-242 which can lead upon
further transformations to the corresponding epoxides
443, γ-lactols 442, γ-lactones 445, and 1-silyl-
1,3-dienes 444 (Scheme 78). 243,244
In general, metalation with organoaluminum, 41,42,245
organoboron, 246 organotitanium, 247-249 or organozinc
250,251 compounds directs the reaction of allyltrialkylsilanes
446 with aldehydes to the R-position to
give 448 and 450 (see also section IX.D). The reaction
of the analogous allylgermanes -CdC-CH2-GePh3s
which usually show behavior similar to allylsilaness
proceeds less regioselectively. 250 Furthermore, addition
of magnesium bromide is effective in directing
R-regioselectivity by complexation with the carbonyl
compound and thus rendering it a more reactive
electrophile. 233 These reactions proceed via a chair
transition states 447. Similarly, the (trimethylsilyl)allylchromium
reagent affords with carbonyl compounds
anti-configurated R-products via the chair
transition state 447 (in Mg, Zn, or Cr cases). 252,253 The
titanium reagent 456, obtained by transmetalation
of the lithiated allyltriphenylsilane 455, undergoes
a regioselective and steroselective reaction to give 1,3dienes
457, 254 whereas the copper reagent 451 obtained
from allyltrimethylsilane 446 gives mainly or
exclusively the γ-products 449 with carbonyl compounds
as well as with alkyl and acyl halides.
Reaction of R,�-unsaturated esters and ketones produces
1,4-γ-adducts 452 and 454, while with R,�unsaturated
aldehydes 1,2-γ-adducts 453 are afforded
(Scheme 79). 255
2. Functionalized Allyltrialkylsilanes (CdC−C−SiR3)
The substituents on silicon exert considerable
influence on the regioselectivity and also the stereoselectivity
of these reactions. 256
a. Alkoxysilyl-Substituted Allylanions. The
R-regioselectivity of alkylation reactions with the
anions of alkoxysilyl-substituted allyl derivatives 458
is improved compared with that of simple alkylsubstituted
anions due to the chelation of the lithium
counterion by the alkoxy group in proximity to silicon
(Scheme 80), 230 an effect which is also observed for
alkoxy-substituted 1-silyl-2-methylallyl carbanions
generated with the Lochmann-Schlosser’s base.