Origin of Enantioselectivity: A Model Study of Sharpless …
443
The basal plane of this fused octahedron is constructed by tartrate ligand. To avoid
steric interaction to maximum extent, two tartrate ligands approach the basal plane
from diametrically opposite side. Tartrate ligand has two different ligating sites of
which alkoxy oxygen satisfies both covalent and co-ordinate valency and carbonyl
oxygen of ester moiety initiates only co-ordinate linkage. In order to make the structure intact, alkoxy oxygens of tartrate ligand satisfy two consecutive covalent valencies of Ti(IV) making a five-membered chelate. Between two alkoxy oxygen atoms,
one is positioned at fused corner. These oxygens show special bonding features by
linking one Ti(IV) with covalent bond and connecting another Ti(IV) by coordinate linkage. In this way, three valencies of hexavalent Ti(IV) are fulfilled by chiral
auxiliary. Remaining valency on equatorial plane is satisfied by initially attached
isopropoxide ligand. One axial site is also linked with isopropoxide ligand by a
covalent bond and its opposite axial site has a provision of linking with carbonyl
oxygen by co-ordinate linkage. In this way, both primary and secondary valencies
of Ti (IV) are satisfied. The role of ester moiety is vital since experiment carried out
with ligand having no group capable of maintaining co-ordinate valency at axial site,
losses enantioselectivity. In epoxidation experiment, optically pure tartrate is used
so orientation of ester groups forces carbonyl groups to align in anti-parallel fashion.
Carbonyl oxygen of ester group initiated co-ordinate linkage with Ti(IV) ion and as a
consequence locks this site until and unless another ligand with co-ordinate bonding
ability approaches. If the catalyst–chiral auxiliary aggregate is closely scrutinized,
then each Ti(IV) nucleus is found associated with two isopropoxide ligands, one is
in equatorial plane and other is in axial plane. In the one-pot synthesis, next step
is the loading of allyl alcohol which under experimental condition easily generates
alkoxide ligand which attempts to substitute similarly behaved isopropoxide ligand
from catalyst surface. Two options are left with incoming allyl alkoxide ligand,
either to replaced axially linked isopropoxide or equatorially attached isopropoxide.
Among two possibilities, only axially anchored allyl alkoxide ion has the potential to generate specific epoxide. Final step of the synthesis is associated with the
addition of peroxidating agent. TBHP has two vicinal oxygen atoms in which end
oxygen has the ability to produce oxide linkage while intervening oxygen being
attached to electron releasing tertiary butyl group has the ability of developing coordinate bond. These features together add a property within TBHP by which it
can simultaneously shed two ligands from catalyst–chiral auxiliary substrate trio
surface. End oxygen of peroxide gets connected with Ti(IV) using covalent linkage
thereby displace isopropoxide ion from equatorial position while intervening oxygen
is in a geometrically feasible position to initiate co-ordinate linkage with Ti(IV).
Ester group linked with Ti(IV), carried out rotation around C1–C2 bond of tartrate
ligand, deblocks the axial site of catalyst surface. This allows nice attachment of
oxidant using one covalent and one co-ordinate bond. In short, two vicinal oxygen
atoms of peroxide, anchored itself along one edge of the octahedron and this specific
attachment suggests octahedron arrangement around one Ti(IV) has Z-in compressed
geometry. This compression tendency is justified in Scheme 2. In this way, four chemicals together develop a chiral surface, which maintains a high degree of enantioselectivity during interaction between alkene moieties with peroxide. This geometry
443
The basal plane of this fused octahedron is constructed by tartrate ligand. To avoid
steric interaction to maximum extent, two tartrate ligands approach the basal plane
from diametrically opposite side. Tartrate ligand has two different ligating sites of
which alkoxy oxygen satisfies both covalent and co-ordinate valency and carbonyl
oxygen of ester moiety initiates only co-ordinate linkage. In order to make the structure intact, alkoxy oxygens of tartrate ligand satisfy two consecutive covalent valencies of Ti(IV) making a five-membered chelate. Between two alkoxy oxygen atoms,
one is positioned at fused corner. These oxygens show special bonding features by
linking one Ti(IV) with covalent bond and connecting another Ti(IV) by coordinate linkage. In this way, three valencies of hexavalent Ti(IV) are fulfilled by chiral
auxiliary. Remaining valency on equatorial plane is satisfied by initially attached
isopropoxide ligand. One axial site is also linked with isopropoxide ligand by a
covalent bond and its opposite axial site has a provision of linking with carbonyl
oxygen by co-ordinate linkage. In this way, both primary and secondary valencies
of Ti (IV) are satisfied. The role of ester moiety is vital since experiment carried out
with ligand having no group capable of maintaining co-ordinate valency at axial site,
losses enantioselectivity. In epoxidation experiment, optically pure tartrate is used
so orientation of ester groups forces carbonyl groups to align in anti-parallel fashion.
Carbonyl oxygen of ester group initiated co-ordinate linkage with Ti(IV) ion and as a
consequence locks this site until and unless another ligand with co-ordinate bonding
ability approaches. If the catalyst–chiral auxiliary aggregate is closely scrutinized,
then each Ti(IV) nucleus is found associated with two isopropoxide ligands, one is
in equatorial plane and other is in axial plane. In the one-pot synthesis, next step
is the loading of allyl alcohol which under experimental condition easily generates
alkoxide ligand which attempts to substitute similarly behaved isopropoxide ligand
from catalyst surface. Two options are left with incoming allyl alkoxide ligand,
either to replaced axially linked isopropoxide or equatorially attached isopropoxide.
Among two possibilities, only axially anchored allyl alkoxide ion has the potential to generate specific epoxide. Final step of the synthesis is associated with the
addition of peroxidating agent. TBHP has two vicinal oxygen atoms in which end
oxygen has the ability to produce oxide linkage while intervening oxygen being
attached to electron releasing tertiary butyl group has the ability of developing coordinate bond. These features together add a property within TBHP by which it
can simultaneously shed two ligands from catalyst–chiral auxiliary substrate trio
surface. End oxygen of peroxide gets connected with Ti(IV) using covalent linkage
thereby displace isopropoxide ion from equatorial position while intervening oxygen
is in a geometrically feasible position to initiate co-ordinate linkage with Ti(IV).
Ester group linked with Ti(IV), carried out rotation around C1–C2 bond of tartrate
ligand, deblocks the axial site of catalyst surface. This allows nice attachment of
oxidant using one covalent and one co-ordinate bond. In short, two vicinal oxygen
atoms of peroxide, anchored itself along one edge of the octahedron and this specific
attachment suggests octahedron arrangement around one Ti(IV) has Z-in compressed
geometry. This compression tendency is justified in Scheme 2. In this way, four chemicals together develop a chiral surface, which maintains a high degree of enantioselectivity during interaction between alkene moieties with peroxide. This geometry