Page 964 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

948 the local softness of reacting positions is matched. 343 This proposal has been explored in
a quantitative way. 344 Local softness values were calculated for a number of substituted
CHAPTER 10
dienes and dienophiles. Then the alternative regioisomers were evaluated by comparing
Concerted Pericyclic the alternative pairs:
Reactions
2
Ortho regioisomer = s − −s + + s − −s + 2 (10.4)
1 1 4 2

2
Meta regioisomer = s − −s + + s − −s + 2 (10.5)
1 2 4 1
–
where s and s are the local softness values for nucleophilic and electrophilic
+
reactivity, as appropriate for the reaction of a nucleophilic diene with and electrophilic
dienophile.
ERG ERG
ERG ERG
1 EWG EWG 1 2′
2 2
1′
1′
3 2′ 3 EWG EWG
4 para – product 4
meta – product
Table 10.11 gives the value of the local softness for some representative diene
and dienophiles.
Qualitatively comparing these numbers to the traditional polar and resonance
substituent effects, we see that for a donor substituent s − 4 >s . EWG substituents
−
1
+
make s + 2 >s . However, the trend that dominates the sum of the differences in
1
Table 10.11. Local Softness Parameters for Dienes and Dienophiles a
Dienes
+
+
−
−
1-Substituent s 1 s 1 s 4 s 4
0 266 0 277 0 309 0 282
CH 3
CH 3 O 0 363 0 193 0 293 0 201
C 2 H 5 O 0 369 0 177 0 294 0 198
CH 3 2 N 0 413 −0 137 0 321 0 193
C 2 H 5 2 N 0 396 −0 298 0 281 0 103
CO 2 H 0 271 0 118 0 224 0 131
CN 0 416 0 354 0 264 0 319
Dienophiles
−
+
−
+
1-Substituent s 1 s 1 s 2 s 2
CO 2 H 0 167 0 073 0 306 0 150
0 166 0 077 0 300 0 139
CO 2 CH 3
CH = O 0 030 0 024 0 309 0 276
CH 3 CO 0 038 0 101 0 327 0 188
CN 0 296 −0 014 0 278 0 140
−0 53 −0 073 0 291 0 258
NO 2
a. From J. Damoun, G. Van de Woude, F. Mendez, and P. Geerlings, J. Phys. Chem. A, 101,
886 (1997).
343 J. L. Gazquez and F. Mendez, J. Phys. Chem., 98, 4591 (1994).
344
J. Damoun, G. Van de Woude, F. Mendez, and P. Geerlings, J. Phys. Chem. A, 101, 886 (1997).