Chemistry Reference
In-Depth Information
unsaturated alcohols have the similar structures to enamides and thus, are most fre-
quently studied in hydrogenation. However, much less success has been achieved, prob-
ably due to the weaker coordinating ability of the ester groups compared with that of
the amide in enamides. A few catalysts that have shown superior performance for the
hydrogenation of enamides have also provided good selectivities and activities for unsat-
urated alcohols. For example, Rh complexes of BPE [164], DuPhos [164-166],
93
[168] ,
TangPhos [169], and
12
[134] as well as Ru complex of BINAP [170] can produce over
95% ee for
-(acyloxyl) acrylates. The selectivities achieved from BINAP and Et-
DuPhos also did not compromise for the
E/Z
mixture of the
α
- substituted substrates.
Some examples on the standard substrates are summarized in Table 7.5.
Rh complexes of BPE, DuPhos [172],
t
Bu - BisP * ,
t
Bu - MiniPhos [173] , and
13
[174]
are also studied for the hydrogenation of enol phosphate (Fig. 7.21). Up to 99.8% ee
was achieved from a series of
o
-benzoyl phosphates. Enol phosphinates are structural
analogues of enol acetate, and the hydrogenation product can be easily converted into
the corresponding alcohols. Andersson and others studied the Ir - P,N - ligand - catalyzed
asymmetric hydrogenation of this type of substrates [175]. A series of di- and trisubsti-
tuted enol phosphinates have been hydrogenated in excellent enantioselectivities. A few
purely alkyl-substituted enol phosphinates were also reduced with over 90% ee, provid-
ing an alternative solution to the chiral alcohol synthesis in addition to alkyl ketone
reduction. Asymmetric hydrogenation of a cyclic enol acetate was examined using Rh-
PennPhos as the catalyst [157a]. High enantioselectivity was achieved under relatively
mild conditions. Another example of fi ve-membered cyclic enol ester was hydrogenated
using the Ru - BINAP catalyst producing 4 - methylene -
β
γ
- butyrolactone in 94% ee [176] .
COOEt
COOEt
OBz
O
O
O
P
H
3
CO
P
P
P
Ph
Ph
Ph
H
3
CO
O
O
O
O
Ph
Ph
Ph
>99% ee
>99% ee
>99% ee
Up to 99.8% ee
OAc
O
O
94% ee
99.1% ee
O
O
Ru-(
S
)-di-
t
Bu-MeO-BINAP
O
O
HBF
4
,
i
PrOH, 60 atm H
2
n-C
6
H
13
OH
n-C
6
H
13
OH
97% ee
Figure 7.21.
Asymmetric hydrogenation of enol phosphate, enol phosphinates, and cylcic
substrates.
