Chemistry of the Alkaline Earth
Metal Enolates 10
Aldehydes, ketones, carboxylic esters, carboxylic amides, imines and N,N -
B
disubstituted hydrazones react as electrophiles at their sp2-hybridized carbon
atoms. These com- pounds also become nucleophiles, if they contain an H atom
in the a position relative to their C“O or C“N bonds. This is because they are C,H-
acidic at that position, that is, the H atom in the a position can be removed with a
base (Figure 10.1). The depro- tonation forms the conjugate bases of these
substrates, which are called enolates. De- pending on the origins of these enolates,
they may be called aldehyde enolates, ketone enolates, ester enolates, or amide
enolates. The conjugate bases of imines and hydra- zones are called aza-enolates.
The reactions discussed in this chapter all proceed via enolates.
O O M
M ba + H C C –baH
C C
X X
(M = Li , Na , K ) X
H Aldehyde enolate1
alkyl, aryl Ketone enolate1
Oalkyl, Oaryl Ester enolate
NR1R2 Amide enolate
1
Also called simply enolate.
NR NR Li
Li iPr2N + H C C – iPr2NH C C
X X
Fig. 10.1. Formation of
X = H, alkyl enolates from different
R = alkyl, N(alkyl)2 aza-
enolate C,H acids.
10.1 Basic Considerations
10.1.1 Notation and Structure of Enolates
In valence bond theory, every enolate can be described by two resonance forms.
B
,The negative formal charge is located at a C atom in one of these resonance
forms and at an O or an N atom in the other resonance form. In the following,
we refer to these
,
, 374 10 Chemistry of the Alkaline Earth Metal Enolates
resonance forms as the carbanion and the enolate resonance forms, respectively.
Only the enolate resonance form is shown in Figure 10.1 because this resonance
form has the higher weight according to resonance theory. The enolate
resonance form places negative charge on the more electronegative heteroatom
O or N. These heteroatoms stabilize the negative charge better than the less
electronegative C atom in the car- banion resonance form.
In Figure 10.1 the enolate structures are shown with the charge on the
heteroatom and with the heteroatom in association with a metal ion. The metal
ion stems from the reagent used in the enolate formation. In the majority of the
reactions in Chapter 10, the enolate is generated by deprotonation of C,H acids.
The commonly employed bases contain the metal ions Li , Na , or K . Therefore,
in Chapter 10, we will essentially consider the chemistry of lithium, sodium, and
potassium enolates.
It is known that the chemistry of enolates depends on the nature of the metal.
A
More- over, the metals are an integral part of the structures of enolates. Lithium
enolates are most frequently employed, and in the solid state the lithium cations
definitely are as- sociated with the heteroatoms rather than with the carbanionic
C atoms. Presumably the same is true in solution. The bonding between the
heteroatom and the lithium may be regarded as ionic or polar covalent. However,
the heteroatom is not the only bond- ing partner of the lithium cation irrespective
of the nature of the bond between lithium and the heteroatom:
• Assuming ionic Li O or Li NR interactions, it may be appropriate to draw a
parallel between the structures of enolates and ionic crystals of the Li Cl or Li H
types. In the latter structures, every lithium is coordinated by six neighboring
anions.
• From the viewpoint of polar, yet covalent Li¬O and Li¬N bonds, lithium would
be unable to reach a valence electron octet in the absence of bonding partners
in addition to the heteroatom. The lithium thus has to surround itself by other
donors in much the same way as has been seen in the case of the organolithium
compounds (cf. Section 8.1).
Be this as it may, lithium attempts to bind to several bonding partners, and the
struc- tural consequences for the enolates of a ketone, an ester, and an amide are
shown in Figure 10.2: In contrast to the usual notation, these enolates are not
monomers at all! The heteroatom that carries the negative charge in the enolate
resonance form is an excellent bonding partner, such that several such
heteroatoms are connected to every lithium atom. Lithium enolates often result
in “tetramers” if they are crystallized in the absence of other lithium salts and in
the absence of other suitable neutral donors. The lithium enolate of tert-butyl
methyl ketone, for example, crystallizes from THF in the form shown in Figure
10.3.
“Tetramers” like the one in Figure 10.3 contain cube skeletons in which the
corners are occupied in an alternating fashion by lithium atoms and enolate
Metal Enolates 10
Aldehydes, ketones, carboxylic esters, carboxylic amides, imines and N,N -
B
disubstituted hydrazones react as electrophiles at their sp2-hybridized carbon
atoms. These com- pounds also become nucleophiles, if they contain an H atom
in the a position relative to their C“O or C“N bonds. This is because they are C,H-
acidic at that position, that is, the H atom in the a position can be removed with a
base (Figure 10.1). The depro- tonation forms the conjugate bases of these
substrates, which are called enolates. De- pending on the origins of these enolates,
they may be called aldehyde enolates, ketone enolates, ester enolates, or amide
enolates. The conjugate bases of imines and hydra- zones are called aza-enolates.
The reactions discussed in this chapter all proceed via enolates.
O O M
M ba + H C C –baH
C C
X X
(M = Li , Na , K ) X
H Aldehyde enolate1
alkyl, aryl Ketone enolate1
Oalkyl, Oaryl Ester enolate
NR1R2 Amide enolate
1
Also called simply enolate.
NR NR Li
Li iPr2N + H C C – iPr2NH C C
X X
Fig. 10.1. Formation of
X = H, alkyl enolates from different
R = alkyl, N(alkyl)2 aza-
enolate C,H acids.
10.1 Basic Considerations
10.1.1 Notation and Structure of Enolates
In valence bond theory, every enolate can be described by two resonance forms.
B
,The negative formal charge is located at a C atom in one of these resonance
forms and at an O or an N atom in the other resonance form. In the following,
we refer to these
,
, 374 10 Chemistry of the Alkaline Earth Metal Enolates
resonance forms as the carbanion and the enolate resonance forms, respectively.
Only the enolate resonance form is shown in Figure 10.1 because this resonance
form has the higher weight according to resonance theory. The enolate
resonance form places negative charge on the more electronegative heteroatom
O or N. These heteroatoms stabilize the negative charge better than the less
electronegative C atom in the car- banion resonance form.
In Figure 10.1 the enolate structures are shown with the charge on the
heteroatom and with the heteroatom in association with a metal ion. The metal
ion stems from the reagent used in the enolate formation. In the majority of the
reactions in Chapter 10, the enolate is generated by deprotonation of C,H acids.
The commonly employed bases contain the metal ions Li , Na , or K . Therefore,
in Chapter 10, we will essentially consider the chemistry of lithium, sodium, and
potassium enolates.
It is known that the chemistry of enolates depends on the nature of the metal.
A
More- over, the metals are an integral part of the structures of enolates. Lithium
enolates are most frequently employed, and in the solid state the lithium cations
definitely are as- sociated with the heteroatoms rather than with the carbanionic
C atoms. Presumably the same is true in solution. The bonding between the
heteroatom and the lithium may be regarded as ionic or polar covalent. However,
the heteroatom is not the only bond- ing partner of the lithium cation irrespective
of the nature of the bond between lithium and the heteroatom:
• Assuming ionic Li O or Li NR interactions, it may be appropriate to draw a
parallel between the structures of enolates and ionic crystals of the Li Cl or Li H
types. In the latter structures, every lithium is coordinated by six neighboring
anions.
• From the viewpoint of polar, yet covalent Li¬O and Li¬N bonds, lithium would
be unable to reach a valence electron octet in the absence of bonding partners
in addition to the heteroatom. The lithium thus has to surround itself by other
donors in much the same way as has been seen in the case of the organolithium
compounds (cf. Section 8.1).
Be this as it may, lithium attempts to bind to several bonding partners, and the
struc- tural consequences for the enolates of a ketone, an ester, and an amide are
shown in Figure 10.2: In contrast to the usual notation, these enolates are not
monomers at all! The heteroatom that carries the negative charge in the enolate
resonance form is an excellent bonding partner, such that several such
heteroatoms are connected to every lithium atom. Lithium enolates often result
in “tetramers” if they are crystallized in the absence of other lithium salts and in
the absence of other suitable neutral donors. The lithium enolate of tert-butyl
methyl ketone, for example, crystallizes from THF in the form shown in Figure
10.3.
“Tetramers” like the one in Figure 10.3 contain cube skeletons in which the
corners are occupied in an alternating fashion by lithium atoms and enolate
