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

690
Scheme 84
with TMSCl yields the γ-adduct 477 despite the
formation of a highly strained cyclopropene ring,
while in the reaction with acetone a mixture of
cyclopropene 479 and methylenecyclopropane 480 is
obtained. Alkylation occurs at the R-position to give
478 (Scheme 84). 261
4. Cyclic Silanes
Cyclic silanes as 481 necessarily possess cisoid
stereochemistry. Polymerization to 482 occurs if a
methyl or phenyl group is the substituent on silicon.
This process could be suppressed by introducing an
electron-donating substituent on the phenyl ring
attached to silicon; 262 the regioselectivity of the attack
is now controlled by the steric demand of the electrophile.
Thus D2O yields exclusively R-adducts 484
while smaller electrophiles such as halomethanes
give predominantly R-products 484 beside minor
γ-attack to form 485. Larger molecules, e.g., propyl
bromide, yield a mixture of R- 484 and γ-adducts 485
favoring the latter 485 (Scheme 85). 263
Scheme 85
5. Pentadienylsilanes (CdC−CdC−C−SiR3)
ɛ-Product 490 is exclusively formed in reactions of
anion 486 with TMSCl, while upon alkylation the
γ-product 487 is produced. 264 Reactions of 486 with
carbonyl compounds produce a mixture of ɛ- 489 and
γ-adducts 488 (Scheme 86). The γ-selectivity is
Scheme 86
Scheme 87
enhanced with magnesium, boron, or copper reagents
265 and similar behavior is observed with
sterically less bulky carbonyl compounds and silanes
with substituents on silicon favoring the γ-product.
In the reaction of 1,5-disilyated pentadienyllithium
493 with carbonyl compounds, like acetone or cyclohexanone,
γ-addition occurs to give 492 while the
reaction with bulky ketones, e.g., 2,4-dimethyl-3pentanone,
yields products of terminal attack 496
which are immediately transformed into the conjugated
trienes 497 by Peterson elimination (Scheme
87). Conjugated addition to R,�-unsaturated enones
yields predominantly products of central γ-attack
494. 265 Furthermore, trimethylsilylation exclusively
takes place at the central carbon, a reaction which
is also observed in the alkylation with tert-butyl
bromide to give 495 and 491 respectively (Scheme
87). However, with sec-butyl, isobutyl, and n-butyl
bromides, a mixture of R- and γ-products is obtained.