Chemistry Reference
In-Depth Information
COOCH
3
COOCH
3
H
COOCH
3
NH
2
NH
2
NH
2
H
O
H
2
N
OH
2
O
-H
+
H
2
N
+H
+
H
2
N
O
O
Co
Co
Co
H
2
N
H
2
N
H
2
N
O
H
NH
2
NH
2
NH
2
O
O
O
O
H
O
H
O
H
H
H
H
H
N
N
N
H
H
+H
+
-H
+
H
2
N
O
O
H
2
N
O
O
H
2
N
O
O
Co
Co
Co
H
O
H
2
N
H
H
2
N
H
2
N
NH
2
NH
2
NH
2
O
O
Figure 6.12
Hydration of a pendant alkene by coordinated hydroxide (top), displaying specificity in site of attack,
with the five-membered chelate ring only formed, and stereospecific attack of a deprotonated amine
nucleophile (bottom) at the alkene of a chelate maleate ion.
ring formation. With the transition state apparently important, it is difficult to be certain
why there is exclusive formation of the five-membered ring, but which
CH
centre is
more electron-deficient is likely to be involved.
Whereas the maleate monoester met above can coordinate as a monodentate ligand
through the one free carboxylic acid group, the diacid can employ both acid groups to bind
as a didentate chelate, forming a seven-membered chelate ring. Chelated maleate dianion
bound to Co(III) can also undergo intramolecular attack, but in this case the nucleophile is a
deprotonated amine group of an adjacent chelated 1,2-ethanediamine (Figure 6.12, bottom),
as no coordinated hydroxide is present. The reaction has a choice of two sites for attack
(at either end of the C C), but again there is stereospecificity observed. This specificity in
ring formation may arise if we require in the transition state one carboxylate to be coplanar
and thus conjugated with the diene. This leads to two discrete conformations, depending
on which of the two carboxylate groups is coplanar. From examining models, it appears
substantially more favourable for nucleophilic addition to occur at the 'front' CH group,
as a result of the spatial orientation of the lone pair and the preferred conformation adopted
by the chelate ring leading to a closer and appropriately directed approach at that site,
depicted in Figure 6.12.
It is notable that reaction does not occur between free maleate ion and free 1,2-
ethanediamine, although it does occur (but only very slowly) with maleate diester and
1,2-ethanediamine. Acceleration of the reaction of the maleate resulting from coordination
to a metal ion is significant, and lies in the range 10
6
-10
10
. Accelerations of this size are
common for reactions of coordinated nucleophiles. The observation of stereospecificity and
large accelerations suggests that these types of reactions may be relevant to the modes of ac-
tion of certain metalloenzymes, where reactions are very rapid, specific and stereoselective.
Such reactions are met in Chapter 8.
6.6
Organometallic Synthesis
Compounds with metal-carbon bonds usually require specialized approaches to their syn-
thesis that differ from those discussed above for traditional Werner-type coordination
