Page 962 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

946 Table 10.9. DFT Global Electrophilicity of Representative Dienes and Dienophiles a

CHAPTER 10 Primarily Electrophilic Balanced Primarily Nucleophilic
Concerted Pericyclic 0.77
NC 2 C=C CN 2 5.96 CH 3 CO 2 CH = CHCH=CH 2 1.10 CH 3 OCH=CHCH=CH 2
Reactions
Maleic anhydride 3.24 CH 2 =CHCH=CH 2 1.05 CH 2 =CH 2 0.73
+ − 0.73
CH 2 =CHCH=O B H 3 3.20 CH 2 =CCH=CH 2 0.94 TMSOCH=CHCH=CH 2

2.82 Furan 0.59
CH 2 =C CN 2 CH 3
0.93 CH 3 2 NCH=CHCH=CH 2 0.57
CH 2 =CHNO 2 2.61 CH 3 CH=CHCH=CH 2
0.88 HC≡CH 0.54
CH 3 O 2 CC≡CCO 2 CH 3 2.27 H 2 C=CCH=CH 2

1.93 OTMS 2-Methylfuran 0.52
CH 2 =C CO 2 CH 3 2
CH 2 =CHCH=O 1.84 CH 3 2 C=CHCH=CH 2 0.86
CH 2 =CHCN 1.74 Cyclopentadiene 0.83 CH 3 OCH=CH 0.42
1.65 Pyrrole 0.31
CH 2 =CHCOCH 3
1.52 0.27
HC≡CCO 2 CH 3 CH 3 2 NCH=CH 2
1.50
CH 2 =CHCO 2 CH 3
a. From L. R. Domingo, M. Aurell, P. Perez, and R. Contreras, Tetrahedron, 58, 4417 (2002).
Domingo investigated the global electrophilic parameter, , as an indicator of
relative reactivity. 341 Table 10.9 gives the value of this parameter calculated for a
number of dienes and dienophiles.
This parameter gives an ordering that is in good qualitative agreement with the
reactivity trends that would be expected on the basis of polar and resonance substituent
interactions, although the parent molecules, ethene and ethyne, are somewhat more
toward the nucleophilic side of the scale than might have been anticipated. The
electrophilic group includes the traditional dienophiles such as acrolein and acryloni-
trile. Dienes with donor substituents, such as 1-methoxy-1,3-butadiene, exhibit the
anticipated nucleophilic characteristics. Note that a 1-ERG seems to have a stronger
effect than a 2-ERG (compare the isomeric trimethylsiloxy-1,3-butadienes). This is
consistent with the greater reactivity of 1-methoxy-1,3-butadiene than the 2-isomer
(see Table 10.2). Methoxyethene and dimethylaminoethene are among the most nucle-
ophilic dienophiles in the list. It should be noted that is a global parameter; that is, it
pertains to the molecule as a whole. Thus, it gives no indication of the regioselectivity
of the reaction, but is an indicator of the direction and extent of electron transfer
between the reactants. The idea that increased charge transfer increases reactivity
suggests that mutual reactivity will be highest for compounds that have the largest
difference in . This is equivalent to the FMO concept that the strongest donors and
strongest acceptors will have the highest mutual reactivity.
The issue of regiochemistry can be addressed by identifying sites of local
electrophilicity and local nucleophilicity. This was done by calculation of a local
−
electrophilicity index. 342 The index of nucleophilicity can be taken as f , the local
Fukui function for electrophilic attack. The regiochemistry is then predicted by
matching the highest local electrophilicity in the electrophilic component with the
largest f − for the nucleophilic component. Table 10.10 gives some values of repre-
sentative dienes and dienophiles.
–
We see that a terminal ERG on the diene leads to f – >f , that is, the carbon at
1 4
the end of the conjugated system is the most nucleophilic. For example, compare the
f – values for the substituted (4) and unsubstituted (1) atoms for 1,3-pentadiene, and
341 L. R. Domingo, M. J. Aurell, P. Perez, and R. Contreras, Tetrahedron, 58, 4417 (2002).
342
L. R. Domingo, M. J. Aurell, P. Perez, and R. Contreras, J. Phys. Chem. A, 106, 6871 (2002).