Chemistry Reference
In-Depth Information
Castillón [11] elucidated the influence of the 1,2-acetonide and the nature of the
substituent at C5 in ligands
12
(Figure 9.2). Replacement of the 1,2-acetonide in
12c
with a 2-
O
-alkyl ether resulted in
gluco
-ligand
17c,
which gave 83% ee under
optimized conditions, and thus remained below that of
12c,
while introduction of
an isopropoxy group at C5, as in ligand
16c,
resulted in an obvious loss of stereose-
lectivity. The
xylo
-configured ligand
18c
gave slightly higher ee than its acetonide-
containing counterpart
4c.
Replacement of the optimized biphenyl residue of the
phosphite with a methylene-bridged bis-phenol led to a complete loss of catalytic
activity in almost all cases.
An especially interesting application of carbohydrate diphosphites was reported
by Diéguez and Claver in 2005, when they employed ligands
4b
and
12b
in the
asymmetric hydroformylation of heterocyclic olefins [12]. 2,5-Dihydrofuran (
31
) is
a challenging substrate as hydroformylation does not exclusively yield the expected
3-tetrahydrofuran carbaldehyde
32
but may also gives rise to the undesired 2-
regioisomer
33
(Scheme 9.4). This side product is formed from 2,3-dihydrofuran
34,
which in turn results from isomerization of 2,5-dhydrofuran (
31
) mediated by
the rhodium-based hydroformylation catalyst [13]. When 2,3-dihydrofuran (
34
)
is employed as substrate, again 3- and the 2-carbaldehydes
32
and
33
may be
obtained as products (Scheme 9.4). The mechanistic rationale behind the hydro-
formylation of substrates
31
and
34
is shown in Scheme 9.5. Thus, to obtain good
results for
31
in this transformation and to avoid product mixtures, the isomeriza-
tion of
31
to
34
via
-hydride elimination from intermediate
35
has to be
suppressed. For the reaction of
34,
a good control of the regiochemistry of
the hydrorhodation step, leading to intermediates
35
and
36,
respectively, is
essential.
β
2,5-dihydrofuran
CHO
*
CO, H
2
+
+
*
CHO
O
Rh(I) +
ligand*
O
O
O
31
32
33
34
(undesired)
(undesired)
2,3-dihydrofuran
CHO
*
CO, H
2
*
CHO
O
Rh(I) +
ligand*
O
O
34
32
33
(undesired)
Scheme 9.4
Products and side products of the asymmetric hydroformylation of 2,5- and
2,3-dihydrofuran.
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