Page 108 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 13 ELEMENTS
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Subsequently, borylcopper reagents were also synthesized, as shown below. As softer
nucleophiles than boryllithiums, borylcopper reagents exhibit significantly different
reactivity, undergoing, for instance, conjugate additions with , -unsaturated carbonyl
compounds:

O O CuCI, O
LiCI, KOAc
B B B Cu
DMF
O O O
O
(3.43)
O
B O
O

REVIEW PROBLEM 3.13
Suggest a rationale for the apparent inaccessibility and instability of boryl anions
−
(BR ). Why, for instance, can you not obtain a boryl anion by deprotonating a
2
dialkylborane with n-butyllithium?

3.7 INDIUM-MEDIATED ALLYLATIONS

Indium-mediated allylations of carbonyl compounds have proved to be an important appli-
cation of the metal. In a typical procedure, a carbonyl compound, an allyl halide, and indium
powder are all stirred together in a solvent such as THF or DMF or, remarkably enough,
water:
HO
X In(s), RCHO
(3.44)
THF, DMF, H 2 O
R
H
Under these conditions, trivalent allylindium species analogous to Grignard reagents are
generated in situ:

Br
Br
3 + 2 In In + In
Br Br
(3.45)
The reaction between the allylindium intermediates and the carbonyl substrate, however,
does not occur “directly,” but rather via a cyclic six-membered transition state. Evidence for