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

694
Scheme 96
Scheme 97
Lüning et al. 293 found that the regioselectivity of
the protonation of 545 by water is highly dependent
on the age of the allyllithium solution with R/γ-(546/
547) ratios varying from 2.7:1 to 1:3.5 (Scheme 97).
Reactions of dithio-substituted crotyllithium 549
with aldehydes proceed at the γ-position to give 551
in a highly regio- and stereoselective manner. 294,295
A chairlike transition state 550 is proposed to account
for the γ- and threo-selectivities (Scheme 98). 296 The
regioselectivity of the reaction with ketones depends
on the nature of the ketone. Hence, ketones with
small substituents yield R-adducts 552, while with
bulky and unsaturated ketones, except 2-cyclopentenones,
γ-products 548 are generated. 295 For example,
the γ-1,4-product 548 is obtained with 2-methyl-2-cyclopentenone.
297 Fang 295 explained the regioselectivity
of the reaction with ketones in terms of
the HSAB principle. 276,277 Comparison of aldehydes
and ketones following the rules of the HSAB principle
reveals that, e.g., the R-addition of 3-pentanone to
give 552 is much faster than the γ-addition of
propanal producing 551. The γ-adducts 551 are prone
to cyclization, giving the corresponding spirodithianes
553 which can be converted into the γ-lactones 554
(Scheme 98). 298 In the reaction of cinnamyllithium
with carbonyl compounds without the mediation of
BF3‚Et2O (furnishing R-product 552), no regio- and
stereoselectivity was found with various carbonyl
compounds except for benzophenone. 299
A side reaction of the deprotonation of 2-alkenyl-
555 and 2-benzyl-1,3-dithiolanes is the cycloelimination
of ethylene which furnishes unsaturated dithiocarboxylate
anions. 300 Thus, base-induced ring fragmentation
produces the dithiocrotonate anion 556
which, depending on the nucleophilicity of the lithium
reagent, gives conjugate addition to 557 (with moderately
nucleophilic n BuLi, phenyllithium) or enola-
Scheme 98
Scheme 99
tion to 558 (with less nucleophilic t BuLi, LDA, LTMP,
LHDS). Subsequent alkylation resulted in formation
of 1,1-bis(alkylthio)alk-1-enes 559 and 1,1-bis(alkylthio)alka-1,3-dienes
560, respectively (Scheme 99).
The dithiosubstituted crotyllithium 561 reacts at
the γ-terminus to give 564 with aliphatic aldimines
562. In the presence of BF3 the reaction occurs
predominantly at the R-site and forms 563 (Scheme
100). This was also interpreted on the basis of the
HSAB concept. 301