Topics in Current Chemistry (2020) 378:1
1 3
but-3-en-2-ones and isatines affording spirooxindole tetrahydropyranones was also
reported [119].
8 Enamine Catalysis in the Presence of Lewis Acids for Michael
Reactions
One of the most important C–C-bond-forming reactions in organic chemistry is the
Michael-type addition [120]. Although numerous theories were proposed to rationalize the behavior of different Michael acceptors, Mayr recently measured their
kinetics with different nucleophiles in order to quantify their electrophilic character [121]. On the basis of the Mayr equation Log(k) = S N (E + N) [15], new empirical electrophilicity parameters E for Michael acceptors were introduced in the Mayr
scale. DFT calculations were performed to confirm the suggested reaction mechanisms and to explain the origin of the electrophilic reactivities. Iminium Michael
acceptors are used in organocatalysis through the iminium activation mode [122].
As we have briefly discussed in the Introduction, electrophilicity of the chiral iminium derivatives were evaluated and the possibility to use different nucleophiles,
under the general Mayr equation, was rationalized. However, to enhance the possibility offered by the activation mode, and to expand the possible nucleophiles,
Lewis acids were employed also in the iminium activation mode. Wang, Xu, and
Scheme 20 NbCl 5 /Primary amine catalyzed Biginelli reaction
Reprinted from the journal
54
1 3
but-3-en-2-ones and isatines affording spirooxindole tetrahydropyranones was also
reported [119].
8 Enamine Catalysis in the Presence of Lewis Acids for Michael
Reactions
One of the most important C–C-bond-forming reactions in organic chemistry is the
Michael-type addition [120]. Although numerous theories were proposed to rationalize the behavior of different Michael acceptors, Mayr recently measured their
kinetics with different nucleophiles in order to quantify their electrophilic character [121]. On the basis of the Mayr equation Log(k) = S N (E + N) [15], new empirical electrophilicity parameters E for Michael acceptors were introduced in the Mayr
scale. DFT calculations were performed to confirm the suggested reaction mechanisms and to explain the origin of the electrophilic reactivities. Iminium Michael
acceptors are used in organocatalysis through the iminium activation mode [122].
As we have briefly discussed in the Introduction, electrophilicity of the chiral iminium derivatives were evaluated and the possibility to use different nucleophiles,
under the general Mayr equation, was rationalized. However, to enhance the possibility offered by the activation mode, and to expand the possible nucleophiles,
Lewis acids were employed also in the iminium activation mode. Wang, Xu, and
Scheme 20 NbCl 5 /Primary amine catalyzed Biginelli reaction
Reprinted from the journal
54