Chemistry Reference
In-Depth Information
AlH
3
Li
O
O
OH
H
+
H
−
and R
−
are poor leaving
groups, reaction irreversible
and get protonation
R
H
R
H
R
H
H
H
Li
HAlH
3
AlH
3
Li
O
O
O
H
+
good leaving group,
carbonyl reforms and is
subsequently reduced
RCH
2
OH
R
L
R
L
R H
aldehyde
primary alcohol
H
HAlH
3
Li
Amides
seem to behave differently, with complex metal hydride reduction giving an
amine
, effectively
converting the carbonyl group to a methylene (see Section 7.11).
Li
AlH
3
H
O
O
H
H
aluminate is good leaving
group, forms iminium cation
that is subsequently reduced
R
NHR
′
R
NHR
′
R
NHR′
R
NHR′
H
amine
H
AlH
3
Li
HAlH
3
Li
This behaviour results from initial formation of an intermediate with two potential leaving groups, an amide
anion R
2
N
−
and the aluminate anion (OAlH
3
)
2
−
. Aluminate is the better leaving group, and its loss produces an
iminium cation that is also subject to further reduction. This gives us the
amine
product.
Although at first glance the behaviour of some of these carbonyl compounds towards nucleophiles might seem
anomalous, closer consideration shows there is a logical explanation for the reactions observed. Furthermore, if
we understand the underlying mechanisms, these reactions become predictable.
7.12 Carbon as a nucleophile: Grignard
reagents
such as
Grignard reagents
are conveniently regarded
as sources of carbanion equivalents, and these add to
the carbonyl, followed by loss of the leaving group.
As with other examples, a tetrahedral anionic com-
plex with the metal is likely to be produced. Regen-
eration of the carbonyl with loss of the leaving group
produces an intermediate
ketone
.
The reaction of carbon nucleophiles derived from
organometallics with carboxylic acid derivatives fol-
lows closely the reactions we have already encoun-
tered in Sections 6.3.2 and 7.6.2. Organometallics
O
MgX
O
O
OH
R′Mg
X
R
L
R
L
R
R
′
R
R
′
R
′
R′
tertiary
alcohol
ketone
R′
MgX
