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

Protonation with tert-butyl alcohol proceeds at the
terminal carbon. 264
1,10-Bis(trimethylsilyl) tetraene anion 498 shows
R-selective addition of carbonyl compounds to give
499 followed by double-Peterson elimination furnishing
the polyene 500 (Scheme 88). 265
Scheme 88
H. Boron
A review of allylboron chemistry up to 1972 is given
by Mikhailov. 266 Boron compounds usually serve as
auxiliaries due to the formation of “ate” complexes
with anions stabilized by another heteroatom (see
Scheme 3). The extent of the present section is
limited to a discussion of allyldimesitylboranes and
disiamyloct-1-enylborane.
1. Allyldimesitylboranes (CdC−C−B(Mes)2)
Allyldimesitylboranes 501, after deprotonation, are
γ-alkylated producing (E)-vinylboranes 502 which
can be oxidized to aldehydes. Treatment of 501 with
benzaldehyde also yields the γ-product 503, giving
on oxidation a γ-lactol 504 (Scheme 89). 267
Scheme 89
2. Disiamyloct-1-enylborane (CdC−C−B(Siam)2)
The regioselectivity of disiamyloct-1-enylborane
anion 505 reactions is electrophile dependent. While
with MeI and H2O the R-carbon is attacked to give
507, 268 upon deprotonation with lithium 2,2,6,6tetramethylpiperidide
followed by trimethylsilylation,
γ-product 506 is obtained, owing to the steric repul-
Scheme 90
sion between the bulkier siamyl substituent and the
trimethylsilyl group (Scheme 90). Similarly, treatment
of 505 with acetone yields γ-adduct 508. By
replacing the siamyl substituent with the less bulky
9-borabicyclo[3.3.1]nonan (9-BBN) group as in 509
R-attack is observed upon trimethylsilylation to give
510 (Scheme 90). 269
I. General Discussion
The orientation of reactions with electrophiles of
allyl anions stabilized by a heteroatom (cf. Scheme
91) can be generalized as follows (however, for silicon
species, there are numerous exceptions depending on
substituents or for steric causes):
Whether the reaction of an allyl anion 512 with
an electrophile gives an R-product 513, aγ-product
511, or a mixture of 511 and 513 depends on the
following factors: (i) nature of heterosubstituent(s)
X; (ii) nature of electrophile; (iii) nature of gegen
cation M + ; (iv) reaction conditions including solvent
and temperature; and (v) other substitution in starting
allyl anion.
Of these factors i-iii are the most important. On
the basis of factors i-iii, the heterosubstituents
groups X can be divided into various classes:
a. One group of heterosubstituents causes exclusive
R-substitution, regardless of the other factors: sulfone
(SO2R), benzotriazole (Bt), carbamoyl (NRCOR),
and sulfinamide (SONR2).
b. Another group of heterosubstituents generally
shows R-substitution, but with these the amount of
γ-substitution can be increased by increasing bulk
attached to the heteroatom or increasing bulk of the
Scheme 91
691