Chemistry Reference
In-Depth Information
8D
ASYMMETRIC ENE REACTIONS
AND CYCLOADDITIONS
KOICHI MIKAMI AND KOHSUKE AIKAWA
Department of Applied Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo
152 - 8552, Japan
8D.1. INTRODUCTION
The Diels-Alder (D-A) diene (Scheme 8D.1a) and Alder ene (Scheme 8D.1b) reactions
[1] were found by Diels and/or Alder, hence the name [2]. The D-A and ene reactions
are mechanistically related. These reactions are defi ned as six-electron pericyclic pro-
cesses between a “diene” or an alkene bearing an allylic hydrogen (an “ene”) and an
electron- defi cient multiple bond (a “dienophile” or an “enophile”) to form two
σ
- bonds
with migration of the
π
-bond. In the ene reaction, the two electrons (HOMO) of the
- electrons (
T1
) of the diene. Therefore, the
activation energy is greater to break the C-H single bond, and higher temperatures are
necessary in the ene reaction. Lewis acid-promoted reactions have been developed to
lower the (di)enophile LUMO.
This chapter covers the scope of the asymmetric catalysis of ene- and D-A-like reac-
tions, which were reported after publication of the second edition in 2000, catalyzed by
a chiral Lewis acid of which the complexation modes are fi rst summarized.
allylic C- H
σ
- bond (
T2
) replace the two
π
8D.2. MECHANISTIC CONSIDERATIONS
8D.2.1. Transition State Conformations
The transition state structures (cf. Scheme 8D.1) have been reported for the thermal
D-A and ene reactions of ethylene with butadiene and propene, respectively [3]. The
transition structure of the D-A reaction is highly symmetric [4]. In contrast, the ene
