Page 120 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

468 Catalysis, Homogeneous

FIGURE 30 Chiral diphosphine ligands.
FIGURE 32 Simplified allylic isomerization scheme.

The chirality resides neither on the backbone nor on the
phosphorus atom, but on the cyclic substituents. Modifi- 2. Allylic Mechanism
cation of the R-groups leads to highly efficient catalysts.
A second mechanism that has been brought forward in-
C 1 -chiral ligands can also be very selective as has been
volves the formation of allylic intermediates. The pres-
shown by Togni who developed a range of ligands, here
ence of a hydride on the metal complex is not required in
exemplified by Josiphos. The ligand contains “two” chiral
this mechanism which can best be described as an oxida-
centers, one at the carbon atom and the other involves fa-
tive addition of an “activated” C-H bond (i.e. an allylic
cial chirality of the cyclopentadienyl plane. Derivatives of
hydrogen) to the metal. The allyl group can recollect its
this type are used for the commercial production of chi-
hydrogen at the other end of the allyl group and the result
ral pharmaceuticals and agrochemicals. Another way to
is also a 1,3 shift of hydrogen (Fig. 32).
achieve chirality is by hindered rotation around molecular
axes (“axial” chirality) as will be shown in the next section
3. Asymmetric Isomerization
where BINAP will be introduced. Application DIOP?
An important application of an isomerization is found in
B. Isomerization the Takasago process for the commercial production of
(−)menthol from myrcene. The catalyst used is a rhodium
1. Insertion and β-Elimination complex of BINAP. The BINAP complex is an asym-
A catalytic cycle which involves only one type of ele- metric ligand based on the atropisomerism of substituted
mentary reaction must be a very facile process. Isomer- dinaphthyl (Fig. 33). It was first introduced by Noyori.
ization is such a process since only migratory insertion Atropisomers of diphenyl and the like are formed when
and its counterpart β-hydride elimination are required. ortho substituents do not allow rotation around the cen-
Hence the metal complex can be optimized to do exactly tral carbon-carbon bond. As a result two enantiomers are
this reaction as fast as possible. The actual situation is formed.
slightly more complex due to the necessity of vacant sites, For asymmetric hydrogenation, transfer hydrogenation,
which have to be created for alkene complexation and for and isomerization of double bonds using both ruthenium
β-elimination. As expected, many unsaturated transition and rhodium complexes BINAP has been extensively
metal hydride complexes catalyze isomerization. Exam- used. The synthesis of menthol is given in the reaction
ples include monohydrides of Rh(I), Pd(II), Ni(II), Pt(II), scheme, Fig. 34. The key reaction is the enantioselective
and Zr(IV). The general scheme for alkene isomerization isomerization of the allylamine to the asymmetric enam-
is very simple; for instance it may read as shown in Fig. 31. ine. It is proposed that this reaction proceeds via an allylic
intermediate.
This is the only step that needs to be steered to the cor-
rect enantiomer, since the other two are produced in the

FIGURE 31 Simplified isomerization scheme involving β-hydride
elimination. FIGURE 33 The two enantiomers of BINAP.