Page 136 - 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

484 Catalysis, Homogeneous

out with hundreds of C 2 chiral diphosphines and specific
substrates only.

FIGURE 64 Disparlure.
I. Terephthalic Acid
1. Introduction
alkenes are prochiral, or when the faces are enantiotopic).
Terephthalic acid (1,4-benzenedicarboxylic acid) is used
Three important reactions should be mentioned in this
for the production of polyesters with aliphatic diols as the
context the:
comonomer. The polymer is a high-melting, crystalline
material forming very strong fibers. It is the largest volume
1. Katsuki-Sharpless epoxidation of allylic alcohols synthetic fiber and the production of terephthalic acid is
2. Sharpless asymmetric hydroxylation of alkenes with the largest scale operated process based on a homogeneous
osmium tetroxide catalyst. More recently the packaging applications (PET,
3. Jacobsen asymmetric epoxidation of alkenes the recyclable copolymer with ethylene glycol) have also
gained importance. Terephthalic acid is produced from
All three reactions find wide application in organic syn- p-xylene by oxidation with oxygen. The reaction is car-
thesis. Here we will only discuss the Sharpless epoxida- ried out in acetic acid and the catalyst used is cobalt (or
tion of allylic alcohols. This reaction finds industrial ap- manganese) acetate and bromide. Phthalic anhydride is
plication in Arco’s synthesis of glycidol, the epoxidation made from naphthalene or o-xylene by air oxidation over
product of allyl alcohol, and Upjohn’s synthesis of dispar- a heterogeneous catalyst. The main application of phthalic
lure (Fig. 64), a sex pheromone for the gypsy moth. The anhydride is in the dialkylesters used as plasticizers (soft-
synthesis of disparlure starts with a C 10 allyl alcohol in eners) in PVC. The alcohols used are, for instance, 2-
which the alcohol is replaced by the other carbon chain ethylhexanol obtained from butanal, a hydroformylation
after asymmetric epoxidation. Perhaps today the Jacobsen product.
method can be used directly on a suitable alkene, although
the steric differences between both ends of the molecules
2. The Chemistry
are extremely small.
The overall reaction reads as follows (Fig. 66):
The reaction produces two moles of water per mole
2. Katsuki-Sharpless Asymmetric Epoxidation
of terephthalic acid. The oxidation reaction is a radical
The reaction uses allylic alcohols and hydroperoxides. process. The key step of the reaction scheme involves
The catalyst is a chiral Ti(IV) catalyst. The chirality the cobalt(III)-bromide-catalyzed H-abstraction to give a
is introduced in the catalyst by reacting titaniumtetra- benzylic radical, hydrogen bromide, and a divalent cobalt
isopropoxide with one mole of a simple tartrate diester. species (Fig. 67). This is the initiation reaction of the rad-
The reaction is shown in Fig. 65. ical chain reaction. Without cobalt or manganese and bro-
The enantioselectivity is not very sensitive to the nature mide the reaction would be very slow.
of the allylic alcohol. By contrast, titanium and tartrates When one methyl group has been oxidized 4-toluic acid
are essential to the success. Note the difference with the is formed. This intermediate is soluble in the solvent used,
L-dopa asymmetric hydrogenation, which can be carried acetic acid. In the next step it is oxidized to terephthalic

FIGURE 65 Katsuki-Sharpless asymmetric epoxidation.