Chemistry Reference
In-Depth Information
O
O
P
PPh
2
NH
N
N
t
Bu
t
Bu
t
Bu
t
Bu
O
N
N
O
Zn
Zn
N
N
O
O
Bu
t
t
Bu
t
Bu
t
Bu
Figure 10.12.
Templated - heterobidentate ligand
60
.
Ethyl acetate and acetic acid can also be found as by-products. One of the potential
applications of the vinyl acetate hydroformylation is the production of enantiopure
propane 1,2-diol. For the vinyl acetate hydroformylation, it has been proposed that the
insertion of the alkene into the Rh-H at the branched carbon atom is stabilized by a
fi ve-membered ring intermediate, which has been observed by NMR spectroscopy [26].
Diphosphite and Phosphine-Phosphite Ligands
In 1993, Takaya et al. published the
fi rst important results in the asymmetric hydroformylation of vinyl acetate with chiral
diphosphites, in which they have achieved an ee up to 52% [27]. Later, in 1994, the
(
R
,
S
) - BINAPHOS ligand
33
(Fig. 10.5) was found to deliver high regioselectivity in
branched aldehyde, excellent yields, and and ee of up to 92% [28]. Its polymer-supported
catalysts were also successfully applied [20].
In 2004, a multisubstrate screening approach has been used in the Dow Chemical
Company to identify the best catalyst for hydroformylation of vinyl acetate. The chiral
phosphite Kelliphite
23p
(Fig. 10.4) gave up 88% ee and excellent branched selectivity
in vinyl acetate hydroformylation [15j]. The same authors have recently extended their
work to eight new biaryl-bridged diphosphite, related to Kelliphite, that allowed to them
study the effect of the bridging biaryl moiety in enantioselectivity. They found that
smaller dihedral angles increased regio- and enantioselectivities. However, none of the
new modifi cations provided higher enantioselectivities than those obtained with
23p
[15i] .
More recently, Zhang an coworkers have found that the binol-based diphosphite
61
(Fig. 10.13) provided regioselectivities up to 98% and enantioselectivities up to 80% [29].
