Page 553 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Table 6.2. 1,3-Dipolar Compounds 527

+ .. – + – SECTION 6.2
.. N .. N CR Diazoalkane
N CR N .. 2
.. 2
+ .. .. – + .. – 1,3-Dipolar
N NR N NR
.. N .. N Azide Cycloaddition Reactions
..
..
+ – + –
RC N .. RC N CR Nitrile ylide
..
.. 2
CR 2
+ .. .. – + .. – Nitrile imine
RC N NR RC N .. NR
..
+ .. – + .. –
RC N O .. RC N O .. Nitrile oxide
..
..
..
–
+ .. – R C + N CR
R C N CR 2 2 R .. 2 Azomethine ylide
..
2
+ .. .. – + .. –
R 2 C N .. .. O R 2 C N O .. Nitrone
..
R R
+ .. .. – .. O + .. – .. O Carbonyl oxide
..
..
R 2 C O .. R 2 C O ..
+ C X C X C X
:B + δ+ :B δ– :B
A – A A
C X C X C X
+ δ+
:B + :B δ– :B
A – A A
The other reactant in a dipolar cycloaddition, usually an alkene or alkyne, is
referred to as the dipolarophile. Other multiply bonded functional groups such as
imine, azo, and nitroso can also act as dipolarophiles. The 1,3-dipolar cycloadditions
involve four electrons from the 1,3-dipole and two from the dipolarophile. As in the
D-A reaction, the reactants approach one another in parallel planes to permit interaction
between the and orbitals.
∗

Mechanistic studies have shown that the TSs for 1,3-dipolar cycloadditions (1,3-
DCA) are not very polar, the rate of reaction is not strongly sensitive to solvent
polarity, and in most cases the reaction is a concerted 2 + 4 cycloaddition. 137
s
s
The destruction of charge separation that is implied is more apparent than real because

137
P. K. Kadaba, Tetrahedron, 25, 3053 (1969); R. Huisgen, G. Szeimes, and L. Mobius, Chem. Ber., 100,
2494 (1967); P. Scheiner, J. H. Schomaker, S. Deming, W. J. Libbey, and G. P. Nowack, J. Am. Chem.
Soc., 87, 306 (1965).