Page 899 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

883
X E a Degree of Asynchronicity
SECTION 10.3
N = O −1 4 0.309
1,3-Dipolar
C≡N +1 4 0.198
Cycloaddition Reactions
Cl +10 1 0.107
OH +13 2 0.154
+14 3 0.025
CH 3
Note that there is no barrier for the (hypothetical) reaction with dinitrosoethene.
For X = N=O and C≡N, shallow energy minima for prereaction complexes were
identified. The conclusion that can be drawn from this study is that stronger EWGs
in the dipolarophile lead to greater electrophile/nucleophile character and higher
reactivity. This is the same qualitative trend noted for D-A reactions (see p. 847). The
nitrile ylide is clearly an electron donor in these reactions, which is consistent with
the relatively high energy of the HOMO, as indicated in Figure 10.14.
Computations were also applied to representative 1,3-dipoles in reaction with
ethene. 125 The E and E for the reactions were calculated using CCSD(T)/6-311G**
a
energies at B3LYP/6-31G* structural minima.

1,3-Dipole E a E Leading bond (Å)
+
H−C≡N -CH − +6 9 −72 1 2.443
2
+
H−C≡N −NH − +6 9 −62 6 2.326
H−C≡N −O − +11 4 −45 9 2.235
+
N≡N −CH − +14 3 −37 0 2.225
+
2
N≡N −NH − +17 2 −26 2 2.143
+
N≡N −O − +23 5 −10 7 2.036
+
A number of factors appear to be involved here. There is an Bell-Evans-Polyani type
correlation with the most stable products (most negative E having the lowest E .
a
There is also a trend toward a later TS with the slower reactions, as indicated by the
length of the leading bond in the TS. This is consistent with the Hammond postulate,
with the smaller E correlating with an early TS. Within the 1,3-dipoles, there is a
a
negative correlation with electronegativity. The most electrophilic 1,3-dipoles are the
least reactive in this case. This trend suggests a dominant HOMO -LUMO
dipole dipolarophile
interaction, but it also may reflect the strength of the bonds being formed, which
decreases in the same direction.
In broad terms, there is similarity in the reactivity and regiochemistry relationships
for 1,3-DPCA and those of the D-A reaction. The most favorable reactions are those
with the most complementary electronic character, that is, high nucleophilicity in one
reactant with high electrophilicity in the other. Such reactions have high charge transfer
character, early TS, and lower TS energy. Bond formation is more advanced in the TS
between the most complementary pair of reaction centers and asynchronicity is high.
The best match between HOMO and LUMO predicts the preferred regiochemistry.
Relative reactivity trends should also be governed by these criteria, but as yet no broad
quantitative analyses of relative reactivity have been developed.
125
M.-D. Su, H.-L. Liao, W.-S. Chung, and S.-Y. Chu, J. Org. Chem., 64, 6710 (1999).