Hydrogenation Reactions - Carbohydrates: Tools for Stereoselective Synthesis - page 172

genation of a series of unsaturated carboxylic acid derivatives. It is remarkable

that these phosphite-phosphoroamidite ligands showed a much higher degree of
enantioselectivity and higher reaction rates than their corresponding diphosphite
analogues under similar reaction conditions [38].

α , β

8.4
P-N Donors

Although O-S, N,S and P,N ligands have been synthesized and used in other
asymmetric reactions [3a], heterodonor ligands derived from carbohydrates have
been scarcely used in asymmetric hydrogenation, with the exception of P,N
ligands. The most important application of P,N ligands in asymmetric hydrogena-
tion is in the asymmetric hydrogenation of C=N and C=C unfunctionalized sub-
strates. In this context, Pfaltz and others have used phosphine-oxazoline ligands
as chiral mimics of Crabtree's catalyst. They have been successfully used for the
asymmetric hydrogenation of imines [39] and a limited range of alkenes [40]. Later,
iridium complexes with chiral P,N ligands were found to be efficient catalysts for
the hydrogenation of unfunctionalized olefins [41].
Carbohydrate phosphite-oxazolines developed by Diéguez et al . have been
shown to be a new class of highly versatile ligands appropriate for asymmetric
catalysis [42]. A phosphite-oxazoline ligand library 64-67 (Figure 8.12) can easily
be prepared in a few steps from commercial d-glucosamine (Scheme 8.9) and
applied in the iridium-catalyzed hydrogenation of unfunctionalized alkenes [43].

HO
HO
HO

Ph Ph

O

O (a-c) O

O
HO

(d-e )

O

O

O
AcO

OH

OAc

NH 2 HCl

NHCOR

N

O

D-glucosamine
hydrochloride

R

Ph O

O

O

O
X

64
65
66
67

R= Ph
R= i Pr
R= t Bu
R= Me

O

N

R

Scheme 8.9 Synthesis of pyranoside phosphite/phosphinite-oxazoline ligands 64-67:
(a) (RCO) 2 O/NaOMe/MeOH or RCOCl/NaHCO 3 (aq.); (b) PhCHO/ZnCl 2 ; (c) Ac 2 O/Py;
(d) SnCl 4 /CH 2 Cl 2 ; (e) K 2 CO 3 /MeOH.

These ligands can easily be tuned in the oxazoline and biaryl phosphite moieties
so that their effect on catalytic performance can be explored. By carefully selecting
the ligand components, excellent activities and enantioselectivities (92- > 99%) are