Hydrogenation Reactions - Carbohydrates: Tools for Stereoselective Synthesis - page 161

Phosphinite xylose derivatives ( 23, 24 ) (Figure 8.3), have been used as ligands
in iridium-catalyzed hydrogenation of imines although they provide only moderate
ee; N -(phenylethylidene)aniline 5a and N -(phenylethylidene)benzylamine 5b
(Scheme 8.2) were used as model substrates. The important fact is that the enan-
tioselectivity depends on the fine tuning of the structural parameters of the ligand.
These catalytic systems were active at 50 bar of hydrogen and 25 °C. In the asym-
metric hydrogenation of N- arylimines, results were poorly reproducible. However,
in the hydrogenation of 5a (Scheme 8.2) using the complex [Ir(COD)( 23 )]BF 4 as
catalyst precursor, enantioselectivities up to 57% were achieved. The best enanti-
oselectivity was obtained at 10 bar [20].
One of the advantages of the diphosphinite ligands is their modular nature,
which allows different backbones as well as different substituents groups.
Diphosphinites 25e-h (Figure 8.3), modified with different electron-donating or
electron-withdrawing groups on the aryl residue, have been used in combination
with [Ir(COD)Cl] 2 or as cationic iridium complexes formed from [Ir(COD) 2 ]BF 4 .
Cationic iridium complexes gave rise to catalytic systems that were more active
than the neutral iridium complexes, promoting the hydrogenation of
N- (phenylethylidene)benzylamine 5b with conversions of between 70% and 100%.
The use of additives was, in general, detrimental to both the conversion and the
enantioselectivity. The system based on 25f (Ar

= 4-OMe-C 6 H 4 ), which is a stronger

electron donor than 25g (Ar 4-CF 3 -C 6 H 4 ), gave lower conversion than the systems

based on ligand 25g. Concerning the enantioselectivity, however, the best results
were with ligand 25h (Ar

=

3,5-Me 2 -C 6 H 3 ) (76% ee) in the hydrogenation of
N- (phenylethylidene)aniline ( 6a ), and in the hydrogenation of N- (phenylethylidene)
benzylamine ( 6b ) (entries 12 and 13, Table 8.2) the results were best with ligand
25f (Ar

=

= 4-OMe-C 6 H 4 ), (70% ee) [21].

8.3.2
Phosphite Ligands

While diphosphinites derived from carbohydrates were successfully applied in
metal-catalyzed asymmetric hydrogenation, the early application of carbohydrate
phosphite ligands only provided low-to-moderate enantioselectivities. The use of
diphosphite ligands in asymmetric catalysis was first reported by Brunner [22] and
Wink [23]. These authors used diphosphite ligands derived from d-mannitol and
tartaric acid in the Rh-catalyzed hydrogenation of enamides and obtained low
enantioselectivities (1-34% ee).
In 1999, Selke et al . reported the use of a series of diphosphite ligands 28 with
a glucopyranoside backbone (Figure 8.4) in the Rh-catalyzed hydrogenation of
methyl (Z) - N- acetamidocinnamate ( 1b ), showing rather low enantioselectivities
(ees up to 13%) [24]. This contrasts with the excellent results obtained with the
related diphosphinite ligands 8 (Figure 8.1).
Diphosphite ligands 29-34 derived from d-( )-glucose with a

furanoside backbone (Figure 8.4), developed by Claver et al . , have proved to be
highly efficient for the Rh-catalyzed hydrogenation [25]. The new ligands were

+ )-xylose and d-( +