Chemistry Reference
In-Depth Information
R
2
1.
N
Cycloaddition
N
N
R
1
-C
CH
+
N
3
R
2
a.
1,3-dipolar cycloaddition
R
1
R
2
R
2
+
b.
Diels-Alder cycloaddition
R
1
R
1
2.
Nucleophilic substitution involving ring-opening reactions
of strained heterocyclic electrophiles
HX
X
H
+
+
SR,
+
NR
2
X
=
O,
NR,
N
u
Nu
3.
Reactions (non-aldol) involving carbonyl group
R
3
X-NH
2
N-XR
3
O
hydrazone/
formation
oxime
ether
R
1
R
2
X
=
O,
NR
R
1
R
2
O
amide/isourea
formation
R
3
-NH
2
R
1
NHR
3
4.
Addition reactions to carbon-carbon multiple bonds
X
X
a.
Formation of three-membered rings
R
1
R
2
R
1
R
2
+
SR,
+
NR
2
X
=
O,
NR,
h
R
1
R
R
1
b.
Thiol-ene reaction
R-SH
+
S
O
nucleophile
R-SH
+
O
R
S
R
1
R
1
SCHEME 1.2
Click reactions according to Carvalho, Field, et al. [4].
click reaction product with the “open square.” Figure 1.4 shows clearly that both
functionalities involved in a click process should not be attached to a sacrificial unit.
In such a case, the molecule of a sacrificial unit will react with another molecule of a
sacrificial unit to form a polymer (Scheme 1.4a). Even if the same functionalities are
attached to the sacrificial units, certain amounts of undesired symmetrical products
will be formed (Scheme 1.4b).
Employing two different click reactions solves the problem (Scheme 1.5).
What chemical functionalities can act as sacrificial functionalities? The answer
depends mainly on the structure of molecular units that are to be coupled. If one (or
more) of the molecular units to be connected is a (poly)saccharide, such functionalities
as acetals and esters should be rather avoided since polysaccharides usually have an
abundance of such groups.
Let us take a look at a few applicable examples of click chemistry reactions. It has
been already more than 10 years since the concept was introduced [8]. Since then,
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